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Review Article

Pituitary Hormones as Neurotrophic Signals: Update on Hypothalamic Differentiation in Genetic Models of Altered Feedback²

Carol J. Phelps^*,1 and David L. Hurley

^* Department of Anatomy, Tulane University School of Medicine, New Orleans, Louisiana 70112; and
Department of Cell and Molecular Biology, Tulane University, New Orleans, Louisiana 70118

	Abstract

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 
Studies of mutant mice that are growth hormone (GH)- and prolactin (PRL)-deficient have provided evidence that these pituitary hormones have trophic, as well as dynamic, feedback effects on the hypothalamic neurons that regulate GH and PRL secretion (1). This review examines further evidence, from those animals and from recent transgenic models, for GH and PRL effects on neuronal differentiation. Characterization of the Ames dwarf (Prop-1df) mutation and discovery of other genes important to pituitary differentiation reveal an expression sequence of transcription factors, Hesx1 (Rpx) to P-Lim to Prop-1 to Pit-1, that heralds influence on hypothalamic differentiation. Occasional expression of GH and PRL in the Ames dwarf pituitary may result from the "partial loss of function" nature of the Ames Prop-1 mutation. In transgenic mice with moderately or extremely elevated GH levels, neurons that regulate GH exhibit respective maximum and minimum expression and cell number in inhibitory somatostatin (SRIH) and in stimulatory GH-releasing hormone (GHRH). The phenomenon is inverted in GH-lacking dwarfs, and patterns of SRIH underexpression and GHRH overexpression are established early in postnatal development. The differentiation of PRL-inhibiting dopaminergic (DA) neurons is supported not only by PRL, but by human GH, which is lactogenic in rodents. Transgenic mice with peripherally expressed hGH have increased numbers of DA neurons, as opposed to the decreased DA population in PRL-deficient dwarf mice. Rats engineered to express hGH in GHRH neurons do not show this increase, whereas spontaneously GH-deficient dwarf rats show increased DA neuron number. These findings may be explained by feedback on neurons that co-express GHRH and DA. Current studies suggest that Snell (Pit-1dw) dwarf mice show a more severe and earlier DA neuron deficiency than Ames dwarfs, and that PRL feedback must occur prior to 20 days of postnatal age to maintain the DA neuronal phenotype. Insights into the mechanisms of GH and PRL effects on hypophysiotropic neurons include receptor localization on identified neuronal phenotypes, including intermediate neurons that mediate dynamic feedback, and elucidation of signal transduction pathways for GH and PRL in peripheral cell types. New transgenic models of altered GH, PRL, or receptor expression offer further study of neurotrophic effects.

	Introduction

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 
This article provides an update of a minireview published in 1994 (1), the purpose of which was to examine a theory that "the effect of pituitary hormones on hypophysiotropic [pituitary-regulating] neurons extends beyond dynamic feedback, influencing developmental differentiation, survival, and connectivity of these cells." The studies described in that review were restricted largely to those that used as models mice bearing spontaneous mutations that result in dwarfism, through failure of the pituitary to produce growth hormone (GH), prolactin (PRL), and thyroid stimulating hormone (TSH). Subsequent work in this and other laboratories has extended examination of the theory using the same dwarf mice, spontaneous dwarf rats, and additional transgenic models of GH or PRL under- or overexpression. The new findings give further support to the classification of anterior pituitary hormones as neurotrophic factors for their respective regulatory neurons. Evidence from another hypothalamic-pituitary system, gonadotropin-releasing hormone neurons and target luteinizing hormone–producing cells, also supports this theory in terms of axonal guidance. Insights into the mechanisms of this feedback/neurotrophic effect have been reported, in localization of GH and PRL receptors and neuronal activation by GH in the hypothalamus. New transgenic, including "knockout," rodent models of altered GH or PRL expression or function have been developed very recently; examination of hypophysiotrophic neuron differentiation in these animals will shed more light on whether pituitary hormones are neurotrophic.

	Developmental Differentiation

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 
The trophic influence of a factor upon differentiation of specific cell types may be deduced initially by assessing the adult condition in an animal that genetically lacks or overproduces that factor, because the condition is lifelong. Assessment of the adult condition of GH- and PRL- regulating hypothalamic neurons in rodent models that either fail to produce or overexpress GH or PRL has been accomplished for an increasing number of spontaneous or transgenically engineered animals. An overview of these assessments is given in Table I, for GH-inhibiting somatostatin (somatotropin release–inhibiting hormone, SRIH), GH-releasing hormone (GHRH), and PRL-inhibiting tuberoinfundibular dopamine (TIDA).

Two types of dwarf mouse, Snell [dw (37); dw^j (38)] and Ames [df (39)], with absent GH and PRL (40), were used to elucidate physiological roles of GH and PRL long before the nature of the spontaneous mutations was known. With recent characterization as mutations in pituitary transcription factors, as discussed below, the nomenclature for the genetic designations is being changed from the traditional dw for Snell and df for Ames; both mutations are simple recessives, and dwarfism is manifested in the homozygous condition. "Little" (lit/lit) mice (41) have GH levels that are 5%–10% of those in normal (LIT/?) mice (41, 42); lit was characterized in 1993 as a point mutation in the GHRH receptor (43, 44). In rats, two spontaneous genetic dwarfs have been identified. The dw dwarf rat (45) has severely reduced (to 5%) GH levels; the mutation has not been characterized. The dr mutation in the Sprague-Dawley strain (46) produces no GH (47) and is a single base deletion in the GH structural gene (48). Production of PRL is not reduced in either dwarf rat type (19, 47).

As opposed to spontaneous mutations, transgenic alteration of GH includes giants as well as dwarfs. Transgenic dwarf mice were produced by ablating somatotrophs through expression of the diphtheria toxin gene driven by the GH promoter (49). Other types of transgenic dwarfs have been produced (50, 51), but hypophysiotropic neurons have been studied only in the "GH-DT" [Tg (GH, DT-A + Mt, GHRH) Bri78] dwarf. Transgenic mice that express human GHRH (52, 53) produce very high levels of endogenous (mouse) GH. Mice engineered to produce heterologous (bovine or human) GH (54) show high circulating levels, depending on gene variant type, copy number, and promoter construct (55); as discussed in a later section, hGH also has PRL-like effects in these animals. In contrast to the giantism resulting from widespread peripheral production of heterologous GH when expression is driven by a ubiquitous promoter such as metallothionein, hGH expression restricted to GHRH neurons suppresses endogenous GHRH production, resulting in reduced GH levels and dwarfism, in the recently described "transgenic growth-retarded" (Tgr) rat (26). All of these genetic models of altered GH and PRL production represent lifelong conditions, whereby the hormone(s) could affect differentiation during critical periods in the development of hypothalamic neurons. Direct assessment of hypophysiotropic neuron development has been accomplished in the spontaneous dwarf mice. These dwarf models also have been used to elucidate anterior pituitary differentiation.

Anterior Pituitary.
The spontaneous genetic mutations that result in 1) failure of the pituitary to produce GH, PRL, and TSH, and 2) phenotypic dwarfism have provided models for assessing not only similar human deficits (56-62), but differentiation per se of this complex organ of diverse and largely independent cell types. The adenohypophysis serves as a model of progressive restriction in gene expression through development and has attracted the attention of numerous research groups recently; the defects in dwarf pituitaries have provided the means and impetus for identifying genes responsible for the differentiation sequence.

The Snell dwarf mutations, dw and dw^j, were characterized in 1990 (63) as, respectively, a base substitution and a sequence rearrangement in the pituitary-specific transcription factor Pit-1 (64, 65), which is necessary for the expression of GH, PRL, and TSH. Because the Ames dwarf (df/df) pituitary was phenotypically identical to that of the Snell dwarf, but df localized to chromosome 11 (66, 67), rather than 16, it was likely that df preceded dw in the pituitary differentiation sequence (68, 69).

Characterization of the Ames dwarf mutation.
Using positional cloning and additional amplification methods, Sornson et al. (70) determined that a novel homeobox-containing protein is encoded at the df map position on chromosome 11. The protein is a member of the "paired" class of homeobox proteins and is expressed only in the pituitary. The protein binds to DNA via the homeodomain in the promoter region of the Pit-1 gene, suggesting involvement in the activation of Pit-1 gene transcription. Because the protein thus foretells the production of Pit-1, it was named Prophet of Pit-1, abbreviated Prop-1 (70).

A mutation of Prop-1 that prevents the proper function of the protein was shown to be present at the df allele. Due to a T-to-C transition mutation, the serine residue at codon 83 is changed to proline (S83P), resulting in reduction in the DNA binding affinity of Prop-1 protein by at least 8-fold (70), and leading to failure to properly activate Pit-1 transcription. The loss of Pit-1 expression due to mutant Prop-1 would lead to the GH-, PRL-, and TSH-deficient phenotype. It is important to note, however, that there may be some residual Prop-1 function in Ames dwarf pituitary because the S83P Prop-1 protein retains some DNA binding capacity (70, 71). Recently, analysis of patients with multiple pituitary hormone deficiency has revealed Prop-1 mutations. Although the equivalent of the S83P mutation has not been found, the mutations that were characterized resulted in truncation of the Prop-1 protein, due to a 2-base pair deletion (A301G302), or a C-to-T transition leading to a cysteine substitution for arginine at codon 120 (72).

The genetic nomenclature for the mouse dwarf mutations has been changed officially to reflect characterization of the genes in which they occur, from dw to Pit1dw, dw^j to Pit1dwJ, lit to Ghrhrlit, and df to Prop1df (73).

Expression sequence of pituitary differentiation genes.
Beyond the characterization of Prop-1 as the factor responsible for initiating Pit-1 expression, several genes that function earlier during anterior pituitary development have been characterized (reviewed in Ref. 71). In temporal order of activation, the first of these is named Rpx, for Rathke's pouch homeobox (74), and is also known as Hesx1 (75). As the name implies, Rpx/Hesx1 expression is eventually restricted to Rathke's pouch, the earliest identifiable precursor of the pituitary. However, Rpx/Hesx1 expression initiates during gastrulation at embryonic day (e)7 in the endodermal prechordal plate, then appears in the ectoderm that will become the cephalic neural plate. By e9, restriction of Rpx/Hesx1 expression occurs such that transcripts are found exclusively in the ectodermal cells fated to give rise to Rathke's pouch, and expression increases at e11.5, the time of definitive pouch formation. Eventually, by e15.5, after the appearance of some differentiated cell types in the anterior pituitary, Rpx/Hesx1 expression is no longer detectable (74). Like Prop-1, Rpx/Hesx1 is a member of the paired class of homeobox genes, and the loss of Rpx expression by el5.5 depends upon the proper function of Prop-1; in adult Ames dwarf mice, Rpx expression continues (76). Mutation of the Hesx1 gene has been shown to be responsible for familial septo-optic dysplasia (77).

Two other important genes for pituitary development are members of the LIM family of homeobox genes, named Lhx3 and Lhx4 (78, 79). Lhx3, also named P-Lim (80) and mLIM3 (81), has been shown to be expressed in the pituitary throughout development and in the adult. Transgenic site-directed mutagenesis, or "knock-out," mice lacking Lhx3 have been shown to initiate but not complete the formation of Rathke's pouch, and to fail to maintain Rpx/Hesx1 expression beyond el2.5 (78). Pituitaries from these Lhx3-deficient mice contain only one differentiated cell type, the corticotroph (78). Lhx4 is expressed by e9.5 in the developing pituitary, becomes restricted to the future anterior lobe by el2.5, and then decreases to low levels that are maintained through adulthood in the anterior and intermediate lobes (79). Mice in which Lhx4 has been deleted transgenically show Rathke's pouch formation and elaboration of a hypoplastic anterior pituitary containing a greatly reduced number of differentiated, hormone-producing cells. Thus, Lhx4 is likely to be a step in the genetic program of pituitary development that functions after Lhx3 activity, leading to the proliferation of distinct cell types after differentiation from precursors (79).

GH, PRL, TSH, Pit-1 cells in df/df anterior pituitary.
Gage et al. (69) reported that, in Ames dwarf pituitary, GH was detected by immunocytochemistry (ICC) in approximately 100 cells/gland, estimated to be 0.05% the normal number of GH-producing cells (82). Subsequently, the same group reported an analysis of three to four df/df pituitaries that showed cells containing immunoreactive GH, PRL, and TSH; immunoreactive Pit-1 was detected in all these cells (83). No GH-, PRL-, or TSH-positive cells were detected in Snell dwarf pituitaries (83). Although cells with ultrastructural features common to somatotrophs and lactotrophs have been observed in the adult Snell dwarf pituitary (84), these "ambiguous" cells do not contain immunoreactive GH, PRL, or TSH (85, 86), supporting the prediction that the Snell Pit-1 mutation would completely block expression of these hormones (63). These ultrastructural studies show maintenance of a possible precursor cell type that remains undifferentiated in the Snell dwarf pituitary (86).

In the studies of Gage et al. (83), hormone-positive cells were in "clusters," suggesting clonal origin (69, 83). In another study, Andersen et al. (68) reported that no GH-positive cells could be detected by ICC in Ames dwarf pituitary sections, but that GH was detected in 10 to 20 cells per pituitary in dispersed glands (68). Routine pituitary section ICC in this laboratory has also detected occasional GH- and PRL-positive cells in Ames dwarf pituitaries (Phelps, unpublished). The frequency of pituitaries showing "clones" (approximately 20% of over 200 df/df glands) has been less than that reported by Gage and colleagues (69, 83), perhaps due to colony differences (69) or to the larger sample. Figure 1 shows examples of GH immunostaining in two adult df/df pituitaries that showed "clones" compared with the abundant GH in a DF/df gland, and typical absence of GH in a Snell dwarf pituitary. Cell clusters containing PRL also have been found, usually in the same pituitaries in which GH was detected. It is assumed that these cells express Pit-1. Although these findings appear to be at odds with the previous reports of undetectable GH or PRL in the dwarf pituitary (40, 42, 87), it is important to note, as does the study by Slabaugh et al. (40), that biochemical assays of GH or PRL have a sensitivity of 0.05%, a level of GH cell occurrence that was reached only in the Gage et al. (69) study. There are reports that mRNAs for GH, PRL, TSH or Pit-1 are not detectable in Ames dwarf mice, assayed by either RT-PCR (88) or in situ hybridization (68). The presence of these clonal cells remains unexplained, but may relate to the fact that the Prop-1 mutation does not completely prevent DNA binding by the protein. Thus, partial function may initiate occasional cellular differentiation in the Ames dwarf pituitary (68, 83). Also, it is likely that other "paired" homeodomain proteins form heterodimers with Prop-1, so it may be important to identify such products and their expression patterns to explain the presence of hormone-positive cells in the Ames dwarf pituitary.

View larger version (159K): [in this window] [in a new window]   Figure 1.    Photomicrographs of GH immunoreactivity in Ames dwarf (df/df) anterior pituitary (upper panels), compared with GH in normal mouse (DF/df) and Snell dwarf (dw/dw) pituitaries (lower left and right panels, respectively). All were horizontal 5-µm sections, counterstained with hematoxylin, and photographed at 20x original objective magnification. The neurointermediate lobe appears in the upper left area of each panel.

SRIH and GHRH expression in GH-excess transgenic mice.
Several GH-transgenic mouse types were used to evaluate the feedback effect of differing levels of heterologous GH on SRIH expression (24). The models were mice with the metallothionein (MT) promoter directing expression of the bovine (b) GH gene (MT-bGH), mice with MT promoter controlling the human (h) GH gene (MT-hGH), and mice with the bGH gene fused to the phosphoenolpyruvate carboxykinase (PEPCK) promoter (PEPCK-bGH). Because these constructs direct GH expression in multiple peripheral sites, and lack upstream SRIH and GHRH regulatory regions, GH production is unregulated and excessive. Circulating levels of heterologous GH ranged from a mean of 19.9 ng/ml in MT-hGH, to 30 ng/ml in MT-bGH, to 4381 ng/ml in PEPCK-bGH mice. Body weights were increased in proportion to GH levels. SRIH expression was examined by both peptide ICC and mRNA in situ hybridization, in hypophysiotropic PeN and nonhypophysiotropic medial basal hypothalamus (MBH). All three transgenic groups showed a two-fold increase in numbers of immunostained cells in PeN and more robust staining intensity in ME terminals, compared with normal littermates. SRIH mRNA expression, measured on x-ray autoradiographs for total hybridization intensity, did not differ in the MBH among normal and transgenic mice, but PeN SRIH mRNA was increased in all the transgenic groups to 200%–240% that of normals. These results are shown graphically in the upper panel of Figure 2, compared with normal, dwarf and hGHRH transgenic mice. Hypophysiotropic SRIH expression increased in the presence of elevated GH, but not in proportion to circulating GH levels, as was the case for body weight. Previously, PeN SRIH mRNA was found to increase to ~200% of normal in hGHRH transgenic mice with circulating mGH levels ranging from 166–1095 ng/ml (21). Taken together, these findings suggest that there is a maximal level of hypophysiotropic SRIH expression that cannot be exceeded despite increasingly elevated GH levels. The maximal value may be due to properties of the signal transduction pathway for GH to affect transcription in SRIH or intermediate neurons, or may be a limitation of the transcriptional machinery itself.

View larger version (50K): [in this window] [in a new window]   Figure 2.    Mean total levels, as assessed by in situ hybridization, of hypophysiotropic SRIH (top panel) or GHRH (bottom panel) mRNA in control, GH-deficient and GH-excess spontaneous or transgenic mice. Expression for each experimental model has been normalized after setting control (normal sibling) values equal to 100% for each analysis using data presented separately (2–4, 21, 22, 24, and Hurley, unpublished data). Vertical bars on each column represent the standard error of the mean (SEM) for each group, also normalized to percentage. tg = transgenic, GH-DT = GH-diphtheria toxin [Tg (GH,DT-A + Mt, GHRH) Bri 78], hGHRH = human GH-releasing hormone [Tg (Mt, GHRH) Bri 11], MT-bGH = metallothionein promoter - bovine GH, PEPCK-bGH = phosphoenolpyruvate carboxykinase promoter - bovine GH, MT-hGH = MT - human GH.

SRIH neuronal development in dwarfs.
To determine the developmental pattern that gives rise to the adult state of severely reduced number of detectable hypophysiotropic SRIH neurons (14), SRIH mRNA levels (2) and cell numbers (8) were determined from postnatal day 1 (day of birth) through 90 days of age in Ames dwarf and normal mice.

Analysis of SRIH mRNA expression was accomplished using both total hybridization intensity in the entire PeN or ARC, and grain counts indicating SRIH mRNA per neuron. Although total SRIH mRNA increased from day 3 through adulthood in the PeN of normal mice, there was no increase until 60 days in Ames dwarf mice, resulting in a reduction to 40% in adult dwarfs compared with normal littermates; data are graphed in Figure 3A. Thus, the reduced total hypophysiotropic SRIH mRNA in GH-deficient Ames dwarf mice appears developmentally shortly after initial detectability of SRIH in the PeN, because SRIH mRNA levels fail to increase during postnatal development. However, these studies also showed that there was no difference between dwarf and normal SRIH expression levels per neuron, indicating that extant SRIH neurons in Ames dwarf mice produce normal levels of SRIH mRNA (2).

View larger version (30K): [in this window] [in a new window]   Figure 3.    Developmental patterns of PeN SRIH (left panels) mRNA (A) and cell number (B), GHRH mRNA (C) and TIDA neuron number (D) in Ames dwarf versus normal mice. Each point represents the mean, and each vertical bar the SEM. In this and subsequent figures, * = P < 0.05, ** = P < 0.01, *** = P < 0.001, compared with normals of the same age.

GHRH development in dwarfs.
Studies of adult Ames dwarf mice showed that GHRH mRNA and peptide are overexpressed in the ARC (3). To examine the developmental time course of GHRH overexpression, GHRH mRNA was quantified in dwarf and normal mice at 1, 3, 7, 14, 21, and 60 postnatal days by in situ hybridization (4). This was the first study of the developmental pattern of GHRH expression in mice. Both total mRNA and mRNA expression per neuron were quantified. As shown in Figure 3C, total GHRH mRNA was the same in dwarf and normal mice at 1, 3, and 7 days, then GHRH mRNA in dwarf increased at day 14 to 240% of that in normal littermates, and remained 200% through 60 days, although total GHRH mRNA increased in both dwarfs and normals during this period. GHRH mRNA per neuron is shown in Figure 4, plotted with PeN SRIH mRNA per cell in Ames dwarf (top panel) and normal mice (bottom panel). GHRH mRNA per neuron was the same in normal and dwarf mice at 1 day, increased in dwarfs to 190% of that in normals at 3 days, and rose to 300% of normal levels by 7 days and beyond. In normal mice (lower panel, Fig. 4), neither GHRH nor SRIH per neuron changed during the postnatal period examined. Thus the increase in GHRH mRNA per neuron in Ames dwarf mice that is first detectable at 3 days occurs 7 days after the failure to initiate GH production, which occurs normally at el7.5. This onset of GHRH overexpression (i.e., expression levels per cell) occurs earlier than the failure of SRIH to increase in total (Fig. 3A) or per cell (Fig. 4), which may be pertinent to the influence of SRIH on GHRH. Subsequent to the early per-cell increase in GHRH mRNA in response to absent inhibition by GH, long-term GH absence may lead to recruitment of neurons to a GHRH-expressing phenotype to increase the total production of GHRH.

View larger version (18K): [in this window] [in a new window]   Figure 4.    Quantification during development of GHRH and PeN SRIH mRNA per neuron in Ames dwarf (upper panel) and normal mice (lower panel). Each point represents the mean, and vertical bars represent the SEM.

Effect of peripheral hGH on TIDA neurons.
Transgenic mice developed to produce heterologous (human or bovine) GH driven by either MT or PEPCK promoters (54, 93, 94) have been used to study the effects of excessive GH on reproduction as well as growth and metabolism in both sexes (55). The effects of hGH and bGH on reproduction in these mice differ, probably because hGH has lactogenic, as well as somatogenic, properties in rodents (95, 96). An investigation of the possible differential effects of PRL-like hGH versus only-somatogenic bGH on TIDA neurons (25) showed that both catecholamine fluorescence and numbers of neurons detectable by ICC for the catecholamine synthetic enzyme tyrosine hydroxylase (TH) were increased significantly in mice bearing either MT-hGH or PEPCK-hGH, but not bGH, transgenes, as shown in Figure 5. In the particular lines of mice assessed, hGH levels were higher in PEPCK-hGH than in MT-hGH transgenics; numbers of TIDA neurons were 134% of normal in MT-hGH transgenics, and 147% of normal in the PEPCK-hGH mice. These findings indicate that not only is the stimulatory feedback signal necessary for a normal complement of PRL-regulating TIDA neurons, but that a larger population results if an excess PRL stimulus is present during TIDA development. Because the high circulating levels of hGH ("PRL") are the result of peripheral (liver, gut, kidney) production, the studies support the role of circulating, rather than retrogradely transported or hypothalamically produced, PRL in TIDA development.

View larger version (65K): [in this window] [in a new window]   Figure 5.    TIDA neuron numbers, as assessed by TH immunocytochemistry, in transgenic hGH-expressing mice, compared with cell numbers in normal littermates. Columns represent the mean, and vertical bars the SEM.

It was hypothesized that the intrahypothalamic production of hGH in Tgr rats would have a PRL-like stimulatory effect on TIDA neurons, as had circulating hGH in MT- or PEPCK-hGH transgenic mice, especially within neurons that express both GHRH and DA, a subpopulation of the ARC that has been demonstrated in rats (97). This background and hypothesis are illustrated by the cartoons of Figure 6. Brains of Tgr, normal (AS parent strain), and dw/dw rats prepared by paraformaldehyde/glutaraldehyde (98) perfusion in the laboratory of Dr. I. C. A. F. Robinson at the NIMR, London, were shipped to New Orleans (in a significant adventure with transatlantic couriers and the USDA) for assessing TIDA catecholamine fluorescence and neuron number by TH ICC. Although increased DA fluorescence in Tgr individual TIDA neurons (as shown in Fig. 7) indicated stimulation, TH-immunoreactive neuron numbers did not differ between Tgr and AS controls. However, the TIDA neuronal population was increased significantly in dw/dw rats. To test whether the 37% increase in TIDA neurons resulted from increased GHRH expression, a group of dw/dw rats was treated with GH to reduce GHRH (99). The treatment did not decrease TIDA neuron numbers (i.e., TH production) significantly, suggesting that a permanent developmental effect had occurred. The results have been interpreted to indicate that, in the dw/dw rat, with isolated pituitary GH deficiency, ongoing PRL secretion stimulates differentiation of a normal population of neurons that produce DA only, whereas the greatly decreased GH negative feedback results in an increased population of GHRH-producing neurons, a portion of which also produce DA, resulting in an increase in total numbers of TIDA neurons. In the Tgr rat hypothalamus, hGH appears to exert a developmental suppression of the GHRH population, a portion of which are DA-producing. It is likely that the hGH also exerted a stimulatory effect on TIDA (DA-only) neuronal number, or the resulting TH-immunoreactive population in Tgr rats would have been reduced compared with normal AS rats, rather than comparable. The latter interpretation is supported both by the increased DA fluorescence in extant TIDA neurons in Tgr, and by reduced pituitary levels of PRL that would result from increased DA secretion. However, GHRH and DA/TH expression have been analyzed only independently in Tgr rats, and confirmation of this interpretation awaits quantification of the neuronal population that expresses both GHRH and DA/TH. The interpretation/revised hypothesis (19) is illustrated in Figure 6.

View larger version (21K): [in this window] [in a new window]   Figure 6.    The precedent, hypothesis, and outcome of studies of TIDA neurons in Tgr and dw dwarf rats.

View larger version (129K): [in this window] [in a new window]   Figure 7.    Paraformaldehyde-induced catecholamine histofluorescence in normal (parent strain AS) rat (top panel) and Tgr rat (bottom panel) basal hypothalamus. Sections were coronal, 30 µm thick, and photographed at original objective magnification of 10x.

View larger version (102K): [in this window] [in a new window]   Figure 8.    Photomicrographs of TH immunostaining in Snell dwarf (dw/dw) and normal (DW/?) hypothalamus, at 14 days postnatally (top panels) and as adults (bottom panels). The sections were coronal, 30 µm, and photographed at 10x original objective magnification.

View larger version (29K): [in this window] [in a new window]   Figure 9.    The pattern of TIDA neuron development and loss, compared for Snell and Ames dwarf mice and with normal counterparts.

	Mechanisms of Neurotrophic Feedback

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 
GH and PRL Receptors in Hypothalamus.
A direct trophic effect of GH and PRL on respective hypophysiotropic neurons would require the presence of hormone receptors in those specific neuron populations. It is remarkable that hypothalamic GH and PRL receptor localization has been only recent, despite considerable information on the nature of these receptors (100-102).

In the case of CNS GH receptor (GHR), localization to both PeN and ARC predominates (103). However, subsequent studies showed that, whereas GHR is expressed by SRIH neurons in PeN, the majority of GHR-expressing neurons in ARC are not GHRH-producing (104), but have a neuropeptide Y (NPY) phenotype (105, 106), consistent with a role established earlier for NPY in GH regulation (107). The dw dwarf rat has been shown to express GHR mRNA in ARC at markedly reduced levels, which could be reversed by peripheral GH treatment (108). Studies of GHR in the CNS of mouse models are lacking completely.

In the case of PRL receptor (PRLR), expression in the adult hypothalamus is very low (109, 110), and regional mapping (111-114) has not indicated particularly greater expression in ARC, compared with other hypothalamic or extrahypothalamic areas. Indeed, a uniform finding in all the PRLR localization studies is the remarkable abundance of PRLR mRNA or protein in the ventricular choroid plexus, which may both indicate an access route of PRL to the brain and emphasize that the feedback route may, at least in part, be indirect. Until very recently, morphologic in situ localization studies omitted neuronal phenotype identification. In 1997, Arbogast and Voogt (115) reported co-localization of PRLR and TH in fetal rat hypothalamic neurons in vitro. In August, 1998, Lerant and Freeman (116) published a thorough report of PRLR ICC in subdivisions of TH-immunoreactive hypothalamic neurons, as influenced by the ovarian steroid milieu. Expression of the PRLR in hypophysiotropic DA neurons appeared to correlate with both estrogen and PRL levels in the circulation, at least in certain regions. Frequency of PRLR localization was quantified within counts of 50 TH-positive cells in each region; percentage co-localization was rather low (<20%) in ovariectomized rats, but rose to 80% in dorsal TIDA neurons after estrogen. It was not addressed whether a significant population of nondopaminergic PRLR neurons exists. Studies of PRLR localization in mouse have not been reported.

Thus, it appears that the substrates exist for direct influence of PRL on TIDA, and GH on SRIH, but not GHRH, neurons, suggesting that signaling among these hypophysiotropic populations, as well as from other, indirect, pathways and molecules, may be important in the feedback effects of GH and PRL.

Mediators of GH and PRL Feedback.
Related to studies of GHR localization were concurrent reports of c-fos expression in hypothalamic neurons after acute GH treatment; activation occurred in both PeN and ARC (104, 117). Kamegai et al. (118) reported that c-fos expression induced by exogenous GH in PeN was in SRIH neurons, but that NPY-producing cells in ARC were activated by GH. Subsequently, Chan et al. (119) showed that hypothalamic NPY expression reduced by hypophysectomy was restored by treatment with GH. Total ARC levels of NPY mRNA are reduced significantly in the dw dwarf rat, and are elevated after peripheral treatment with bGH (120). Suzuki et al. (121) reported that intracerebroventricular administration of agonists specific for NPY receptors Y1 and Y2 were as effective as NPY in suppressing pulsatile GH secretion in conscious male rats. In the hypothalamus, both receptor subtypes are expressed in ARC, and the Y2 receptor is also found in PeN (122, 123), where synaptic connections of NPY axons on SRIH neurons have been demonstrated (124). In the study by Suzuki et al. (121), knifecut anterolateral deafferentation of ARC abolished the inhibitory effect of NPY on GH release, indicating that the effect of NPY on GH is mediated by SRIH, as had been postulated before (125).

NPY may be involved in PRL feedback mediation as well. In most studies of GH stimulation of c-fos expression in hypothalamus (104, 106, 117, 119), recombinant human GH was used. Because hGH is lactogenic in rodents, including binding to PRLR (126) and trophic effects on TIDA neurons (19, 25), NPY neuron activation may result from PRL stimulation. Increased hypothalamic NPY expression has been measured during lactation (127-130), and acutely after suckling (131). Circulating PRL levels are elevated after administration of antiserum to NPY (132), suggesting a stimulation of DA by NPY. Numbers of NPY-immunoreactive neurons are reduced in the hypothalamus of Ames and Snell dwarf mice (Phelps, unpublished data).

Galanin-producing hypothalamic neurons may also be involved in GH feedback to GHRH neurons; hypothalamic galanin expression is reduced in Ames dwarf mice (133) and dw rats (134), and galanin expression reduced by hypophysectomy, like NPY, can be restored by peripheral GH treatment (134). Also similar to NPY, galanin receptor is expressed on PeN SRIH, but not in GHRH, neurons (134). There is evidence that galanin is involved directly in regulating PRL secretion because mice engineered to lack functional galanin have reduced PRL production, including response to estrogen, and fail to lactate (135).

Other endogenous GH secretagogues (GHS) may be involved in GH effects in hypothalamus. The recent characterization (136, 137) of a specific receptor (GHSR) for synthetic GH-releasing peptides such as GHRP-6 (138), and demonstration that GHS acts in the hypothalamus (139) fostered study of GHSR expression in the CNS as influenced by GH (120). GHSR is expressed in arcuate and ventromedial nuclei, but not PeN; GHSR mRNA levels are increased markedly in dw/dw rats and reduced after bGH treatment to normal or lower levels (120).

It would be logical to suggest that feedback effects of GH in the hypothalamus are mediated by peripheral or local production of IGF-I stimulated by GH, because IGF-I has been shown to mediate the biological effects of GH via both endocrine (production of IGF-I in liver) and paracrine (local IGF-I production) routes. IGF-I receptor has been localized to the hypothalamus in several species using radiolabeled ligand-binding techniques (140-142), and in rat CNS using nucleotide probes and antisera (143). Circulating levels of IGF-I are extremely low in dwarf mice, but rise in response to GH treatment (144). Although one report exists of IGF-I ICC in mouse brain, including reduction in Snell dwarf (145), this laboratory has been unable to detect IGF-I in brains of adult normal or dwarf mice, using several polyclonal and monoclonal antisera (unpublished results). It might be valuable to assess IGF-I receptor expression in brains of GH (and PRL)-deficient models. However, Minami et al. (146) found that injection of rat GH, but not human IGF-I, into PeN or ARC inhibited duration and amplitude of GH secretory pulses.

In all these studies, there is evidence that the feedback effects of GH are upon SRIH directly, including SRIH to GHRH, even when NPY or galanin is involved. There is abundant anatomical evidence for SRIH terminals on GHRH neurons (147-150), and the pattern of GH release after intracerebroventricular NPY, compared to that after ARC deafferentation, suggests effects on GHRH that are mediated by anterior neuronal elements (121). Although the effect could result from a number of possible transmitters, specific inhibition of SRIH has been shown to result in increased GHRH expression (151). Thus, while GH feedback is effected by many peptide transmitters, SRIH may be the final common mediator of this feedback (119, 152) for GH secretory patterns (153, 154) and GH trophic effects.

An established CNS neurotrophin that might mediate the effects of PRL on TIDA neurons is glial cell line–derived neurotrophic factor, GDNF. Originally shown to promote survival and differentiation of cultured midbrain DA neurons (155), GDNF has been shown to protect and stimulate the nigrostriatal (155) and other motor pathways (156) in several experimental paradigms. Intranigral injection of GDNF in adult rats has been shown to protect against neuron loss after axotomy (157), 6-hydroxydopamine (158, 159), or methamphetamine (160), and to restore functional deficits and DA levels after MPTP in mice (161) and monkeys (162). GDNF promotes fiber outgrowth of fetal midbrain DA grafts in oculo (163) and in situ (164). Mice engineered to bear a null mutation in the GDNF locus (165) showed no deficits in brainstem catecholamine neuron groups, but GDNF-null mice die within 12–24 hr of birth, so examination of the late-developing A12 DA neurons was not possible. Choi-Lundberg and Bohn (166) found low GDNF levels in diencephalon at P0 and P10, but the potential role of GDNF in development of hypothalamic DA neurons is yet to be explored.

	Current and Indicated Directions

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 
Connectivity.
It had been noted by this and other laboratories (5, 8, 9, 13, 91, 92) that the levels or appearance of GH- and PRL-regulating factors in the dwarf mouse hypothalamic ME were uniformly low, even for GHRH that is overexpressed (3, 4). This observation prompted a hypothesis that hypophysiotropic neurons in the dwarfs may not terminate appropriately near the hypophysial portal vasculature. Such a hypothesis may be tested using retrograde transport of peripherally injected fluorogold (FG) combined with immunocytochemical identification of neuron phenotypes (27, 28). FG is taken up by axon terminals not protected by the blood-brain barrier in areas such as the ME, and is subsequently sequestered in neuronal perikarya, where it is visible using either ultraviolet illumination or immunostaining (167).

As shown in Table III, the reduction of SRIH in the dwarf ME appears to be proportional to the reduction in detectable PeN perikarya, because 83% of the PeN SRIH neurons in dwarfs were labeled with FG, compared with 87% of the larger population in normal mice. Also, 73% of the large (>2x normal) dwarf GHRH-producing population showed FG labeling, compared with 76% of the lower normal population. The low GHRH in the dwarf ME suggests increased GHRH release. It must be cautioned that SRIH or GHRH perikarya that sequestered peripherally administered FG may terminate in internal layers of ME or in ventral ARC because these areas also lack blood-brain barrier (168). Among TIDA neurons, the percentage of cells showing FG label appears to be lower in dwarf (~50%) than in normal mice (70%–90%), such that a smaller proportion of neurons, in a population that is already reduced in size, terminate appropriately in the ME.

Tanycytic morphology has been shown to affect hypophysiotropic axon contacts with ME and change with hormonal milieu (e.g., GnRH terminals in ME are surrounded by tanycytic end feet, and do not reach the pericapillary basal lamina, except during the preovulatory LH surge) (175, 176). Lerant and Freeman (116) found that tanycytes show immunoreactivity for the ligand-binding portion of the PRL receptor, and suggested that PRL may act on tanycytes to regulate DA release from ME terminals.

Signal Transduction by GH and PRL in Other Systems.
Intracellular signaling by GH and PRL has been studied in detail in the liver and lymphocytes (reviewed in Refs. 100-102). GH and PRL each bind to specific receptor proteins that are members of the cytokine receptor superfamily (177). Ligand binding leads to receptor dimerization and resulting alteration of the intracellular portions of the receptor molecule, enabling signaling from the receptors via receptor-associated protein kinases (178). The Janus Associated Kinase (JAK)-1 and -2 proteins have been shown to be receptor-associated enzymes that become activated by GH or PRL binding to receptors (102, 179, 180). The JAK kinases, while bound to the activated receptor, act as protein tyrosine kinases on cytosolic regions of the receptor and the src homology (SH)-2 domains of selected Signal Transducers and Activators of Transcription (STAT) protein subtypes. Phosphorylation of the SH-2 domains of the STAT proteins promotes interactions forming homo- or heterodimers of the STATs, which then translocate to the nucleus, bind to cognate DNA response elements, and regulate transcription (178). The specificity of signaling by this mechanism is inherently subject to multiple levels of control, including selectivity of cell types expressing receptor, receptor-JAK complex preference for certain STAT members as preferred substrates, the possibility of homo- or heterodimeric STAT combinations, and sequence-selective DNA binding for a subset of gene targets (178). In the liver, the STAT subtypes that are activated in response to GH are 3 and 5b, and analysis of the DNA elements bound by these specific STATs is being used to identify potential genomic targets that are activated (179-182).

It is likely that JAK/STAT signaling pathways also are activated by GH and PRL in hypothalamic neurons. Signaling through another cytokine receptor ObR in hypothalamus has been shown by STAT3 activation after leptin treatment in mice and rats (183, 184). Activation was restricted to STAT3 as shown by DNA binding assays (183) and by immunodetection of phosphorylated STAT3 (184). Darnell (178) mentioned an unpublished finding that GH treatment resulted instead in hypothalamic STAT5 activation, emphasizing the selectivity and specificity of cytokine signaling pathways.

New Transgenic Models.
Although animal models will always be required for studying certain aspects of neuronal differentiation such as cytoarchitecture and connectivity, transgenic technology is providing the means to manipulate the neuroendocrine milieu in vivo tantamount to that in vitro. Like spontaneous mutations in specific transcription factors or structural genes, factors that are transgenically overexpressed or subjected to disruption (knock-out) have lifelong, thus developmental, effects. Several new models pertinent to the neurotrophic feedback of GH or PRL have been reported recently, in terms of general phenotypic effects.

GH receptor knock-out.
Unlike the availability of rodent models for panhypopituitary dwarfism such as Snell and Ames, and for isolated GH deficiency as in the little mouse or dwarf rats, human Laron dwarfism, the result of GHR dysfunction, knows no spontaneous animal counterpart. A mouse model for the syndrome has been produced by targeted disruption of the GHR binding protein gene (185). The mice exhibit severe growth retardation with proportional dwarfism, very low circulating IGF-I levels, and elevated GH. These mice are hyperprolactinemic, show diminished responses to GnRH and to LH, and reduced fertility in males (186). Whether these additional deficits are the result of low IGF-I is not known.

Disruption of PRL and PRLR.
Mice with transgenic null mutations in PRLR (187) and ligand (188) do not show identical phenotypes. Female PRLR knockout mice are sterile, showing multiple reproductive abnormalities such as irregular cyclicity, lack of pseudopregnancy, and defective implantation. Lactation and maternal behavior were compromised in females heterozygous for the mutation, and half of the PRLR-null male mice showed reduced or absent fertility (187). In PRL-null mice, reproductive deficits were less severe; whereas females were infertile, with absent post-pubertal mammary development, they displayed maternal behavior, and males were fertile (188). Further study of neuroendocrine function in the PRL knock-out mice showed that reduction in gonadotropin secretion did not compromise male fertility, and that testosterone secretion was unaffected (189). Preliminary morphological studies in this laboratory (190) indicate that TIDA cell numbers are unaffected in PRL-null mice of either sex. These findings are surprising, considering that PRL can repair infertility (191, 192) and maintain TIDA neurons (91) in spontaneous dwarf mice. The animals may provide a model for redundancy in feedback, and further studies are warranted.

Other models.
Transgenic mice that overexpress rat PRL have been reported and show markedly enlarged prostate (193). Interestingly, circulating IGF-I levels in those animals were elevated to the same extent as in bGH-transgenic mice. Disruption of the NPY gene in leptin-deficient mice has been shown to attenuate obesity (194), whereas NPY receptor Y5- or Y1-null mice become obese, suggesting that these receptors are not essential to NPY effects (195, 196).

In summary, recent studies of additional spontaneous and transgenic models of lifelong altered production of GH or PRL have shown that the differentiation of pertinent hypophysiotropic neurons in the hypothalamus is affected by the alterations, but not as consistently as would be predicted. The solutions to certain questions, such as the nature of the df mutation, were juxtaposed with new perplexities, such as GH expression in the df pituitary. Interesting contradictions surfaced, such as increased TIDA cells in the dwarf rat, opposed to profound decrease in dwarf mice; the endocrine milieu in the two dwarfs is not the same, and the feedback/neurotrophic results refuse neat categorization. Likewise, GH and PRL effects are not distinct, especially between species and when expression is altered experimentally. There seem to be limits in magnitude for the developmental neuronal response to altered GH and PRL. The findings have stimulated alternative interpretation and further study, the intricacy and redundancy in physiological signaling continuously revealing more complexity than predicted. New data and model systems in intracellular signaling and genetic engineering portend exciting novel approaches for addressing general feedback and specific trophic mechanisms in hypothalamic-pituitary interactions.

	Acknowledgments

 
The authors apologize to any investigators whose pertinent work was omitted, and would appreciate being alerted to such errors. Unpublished data that represent the work of Dr. Mario Romero while a student in Dr. Phelps' lab are included in this review; his permission to include these results is acknowledged gratefully. The contributions of collaborators Drs. Andrzej Bartke, Iain Robinson, Greg Thomas, and Beth Wee, and of technical associates Ms. Martha Romero, Ms. Cheryl Malcamp, and Ms. Shanna Joseph were essential to the authors' work. This article is dedicated to the memory of the author's mother, Helen Phelps; her lifelong love of learning was an inspiration.

	Footnotes

 
Studies by the authors were supported financially by PHS Grant NS25987 and NSF CAREER award IBN-9600805.

¹ To whom requests for reprints should be addressed at Department of Anatomy, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, LA 70112–2699. E-mail: cjphelps{at}mailhost.tcs.tulane.edu

² This Minireview is an update of a previously published paper (Proc Soc Exp Biol Med 206:6–23, 1994).

	References

Top Abstract Introduction Developmental Differentiation Mechanisms of Neurotrophic... Current and Indicated Directions References

 

1. Phelps CJ. Pituitary hormones as neurotrophic signals: Anomalous hypophysiotropic neuron differentiation in hypopituitary dwarf mice. Proc Soc Exp Biol Med 206:6–23, 1994.[Medline]
2. Hurley DL, Wee BE, Phelps CJ. Hypophysiotropic somatostatin expression during postnatal development in growth hormone–deficient. Ames dwarf mice: mRNA in situ hybridization. Neuroendocrinology 65:98–106, 1997.[Medline]
3. Phelps CJ, Dalcik H, Endo H, Talamantes F, Hurley DL. Growth hormone–releasing hormone peptide and mRNA are overexpressed in GH-deficient Ames dwarf mice. Endocrinology 133:3034–3037, 1993.[Abstract/Free Full Text]
4. Hurley DL, Wee BE, Phelps CJ. Growth hormone–releasing hormone expression during postnatal development in growth hormone–deficient Ames dwarf mice: mRNA in situ hybridization. Neuroendocrinology 68:201–209, 1998.[Medline]
5. Morgan WW, Bartke A, Pfiel K. Deficiency of dopamine in the median eminence of Snell dwarf mice. Endocrinology 109:2069–2075, 1981.[Abstract/Free Full Text]
6. Phelps CJ, Sladek Jr Jr, Morgan WW, Bartke A. Hypothalamic catecholamine histofluorescence in dwarf mice. Cell Tissue Res 240:19–25, 1985.[Medline]
7. Phelps CJ, Carlson SW, Vaccarella MY, Felten SY. Developmental assessment of hypothalamic tuberoinfundibular dopamine in prolactin-deficient dwarf mice. Endocrinology 132:2715–2722, 1993.[Abstract/Free Full Text]
8. Phelps CJ, Saleh MN, Romero MI. Hypophysiotropic somatostatin expression during postnatal development in growth hormone–deficient Ames dwarf mice: Peptide immunocytochemistry. Neuroendocrinology 64:364–378, 1996.[Medline]
9. Morgan WW, Besch KC. Effect of prolactin replacement on the number of tyrosine hydroxylase neurons in the arcuate nuclei of Ames dwarf and normal mice. Neuroendocrinology 52:70–74, 1990.[Medline]
10. Phelps CJ, Carlson SW, Vaccarella MY. Hypothalamic dopaminergic neurons in prolactin-deficient Ames dwarf mice: Localization and quantification of deficit by tyrosine hydroxylase immunocytochemistry. J Neuroendocrinol 6:145–152, 1994.[Medline]
11. O'Hara BF, Bendotti C, Reeves RH, Oster-Granite ML, Gearhart JD. Genetic mapping and analysis of somatostatin expression in Snell dwarf mice. Mol Brain Res 4:283–292, 1988.
12. Fuhrmann G, Scala-Guenot DD, Ebel A. Somatostatin levels in the central nervous system of the Snell dwarf mouse: Is somatostatin excess the primary molecular defect in the dw/dw dwarfism? Brain Res 328:161–164, 1985.[Medline]
13. Webb SM, Lewinski AK, Steger RW, Reiter RJ, Bartke A. Deficiency of immunoreactive somatostatin in the median eminence of Snell dwarf mice. Life Sci 36:1239–1245, 1985.[Medline]
14. Phelps CJ, Hoffman GE. Isolated deficiency of immunocytochemically detected somatostatin in Snell dwarf, but not in "Little" mice. Peptides 8:1127–1133, 1987.[Medline]
15. Phelps CJ. Isolated deficiency of tyrosine hydroxylase immunoreactivity in tuberoinfundibular neurons in pituitary prolactin–deficient Snell dwarf mice. Brain Res 416:354–358, 1987.[Medline]
16. Phelps CJ, Romero MA, Malcamp C, Joseph SR. TIDA neuron number in developing Snell dwarf mice (abstract). Soc Neurosci Abst 459:11, 1997.
17. Frohman MA, Downs TR, Chomczynski P, Frohman LA. Cloning and characterization of mouse growth hormone–releasing hormone (GRH) complementary DNA: Increased GRH messenger RNA levels in the growth hormone–deficient lit/lit mouse. Mol Endocrinol 3:1529–1536, 1989.[Abstract/Free Full Text]
18. Pellegrini E, Carmignac DF, Bluet-Pajot MT, Mounier F, Bennett P, Epelbaum J, Robinson IC. Intrahypothalamic growth hormone feedback: From dwarfism to acromegaly in the rat. Endocrinology 138:4543–4551, 1997.[Abstract/Free Full Text]
19. Thomas GB, Phelps CJ, Robinson IC. Differential regulation in hypothalamic tuberoinfundibular dopamine neurons in two dwarf rat models with contrasting changes in pituitary prolactin. J Neuroendocrinol 11:229–236, 1999.[Medline]
20. Sakuma S, Ishikawa H, Okuma S. The cell population of somatostatin and growth hormone–releasing factor using quantitative immunohistochemistry in the isolated GH-deficient dwarf rat. Brain Res 506:307–310, 1990.[Medline]
21. Hurley DL, Phelps CJ. Hypothalamic pre-prosomatostatin mRNA expression in mice transgenic for excess or deficient endogenous growth hormone. Endocrinology 130:1809–1815, 1992.[Abstract/Free Full Text]
22. Hurley DL, Phelps CJ. Altered hypothalamic growth hormone releasing hormone mRNA expression in mice transgenic for excess or deficient endogenous growth hormone. Mol Cell Neurosci 4:237–244, 1993.
23. Phelps CJ, Carlson SW, Hurley DL. Hypothalamic dopaminergic neurons in transgenic dwarf mice: Histofluorescence, immunocytochemical, and in situ hybridization studies. Anat Rec 231:446–456, 1991.[Medline]
24. Hurley DL, Bartke A, Wagner TE, Wee BE, Phelps CJ. Increased hypothalamic somatostatin expression in mice transgenic for bovine or human GH. J Neuroendocrinol 6:539–548, 1994.[Medline]
25. Phelps CJ, Bartke A. Stimulatory effect of human, but not bovine, growth hormone expression on numbers of tuberoinfundibular dopaminergic neurons in transgenic mice. Endocrinology 138:2849–2855, 1997.[Abstract/Free Full Text]
26. Flavell DM, Wells T, Wells SE, Carmignac DF, Thomas GB, Robinson ICAF. Dominant dwarfism in transgenic rats by targeting human growth hormone (GH) expression to hypothalamic GH–releasing factor neurons. EMBO J 15:3871–3879, 1996.[Medline]
27. Merchenthaler I, Setalo G, Csontos C, Petrusz P, Flerko B, Negro-Vilar A. Combined retrograde tracing and immunocytochemical identification of luteinizing hormone–releasing hormone- and somatostatin-containing neurons projecting to the median eminence of the rat. Endocrinology 125:2812–2821, 1989.[Abstract/Free Full Text]
28. Niimi M, Takahara J, Sato M, Kawanishi K. Identification of DA- and GHRF-containing neurons projecting to the median eminence of the rat by combined retrograde tracing and immunohistochemistry. Neuroendocrinology 55:92–96, 1992.[Medline]
29. Tannenbaum GS, Ling N. The interrelationship of growth hormone (GH)-releasing factor and somatostatin in generation of the ultradian rhythm of GH secretion. Endocrinology 115:1952–1957, 1984.[Abstract/Free Full Text]
30. Plotsky PM, Vale W. Patterns of growth hormone–releasing factor and somatostatin secretion into the hypophysial-portal circulation of the rat. Science 230:461–463, 1985.[Abstract/Free Full Text]
31. Maiter DM, Gabriel SM, Koenig JI, Russell WE, Martin JB. Sexual differentiation of growth hormone feedback effects on hypothalamic growth hormone–releasing hormone and somatostatin. Neuroendocrinology 51:174–180, 1990.[Medline]
32. Chomczynski P, Downs TR, Frohman LA. Feedback regulation of growth hormone (GH)–releasing hormone gene expression by GH in rat hypothalamus. Mol Endocrinol 2:236–241, 1988.[Abstract/Free Full Text]
33. de Gennaro Colonna V, Cattaneo E, Cocchi D, Müller EE, Maggi A. Growth hormone regulation of growth-releasing hormone gene expression. Peptides 9:985–988, 1988.[Medline]
34. MacLeod RM. Regulation of PRL secretion. In: Martini L, Ganong WF, Eds. Frontiers in Neuroendocrinology, Vol. 4. New York: Raven Press, pp169–194, 1976.
35. Ben-Jonathan N. Dopamine: A prolactin-inhibitory hormone. Endocr Rev 6:564–589, 1985.[Abstract/Free Full Text]
36. Arbogast LA, Voogt JL. Hyperprolactinemia increases and hypoprolactinemia decreases tyrosine hydroxylase messenger ribonucleic acid levels in the arcuate nuclei, but not the substantia nigra or zona incerta. Endocrinology 128:997–1005, 1991.[Abstract/Free Full Text]
37. Snell GD. Dwarf, a new Mendelian recessive character of the house mouse. Proc Natl Acad Sci U S A 15:733–734, 1929.[Free Full Text]
38. Eicher EM, Beamer WG. New mouse dw allele: Genetic location and effects on lifespan and growth hormone levels. J Hered 71:187–190, 1980.[Free Full Text]
39. Schaible R, Gowen JW. A new dwarf mouse. Genetics 46:896, 1961.
40. Slabaugh MB, Lieberman ME, Rutledge JJ, Gorski J. Growth hormone and prolactin synthesis in normal and homozygous Snell and Ames dwarf mice. Endocrinology 109:1040–1046, 1981.[Abstract/Free Full Text]
41. Eicher EM, Beamer WG. Inherited ateliotic dwarfism in mice. J Hered 67:87–91, 1976.[Free Full Text]
42. Cheng TC, Beamer WG, Phillips Ja III, Bartke A, Mallonee RL, Dowling C. Etiology of growth hormone deficiency in little, Ames, and Snell dwarf mice. Endocrinology 113:1669–1678, 1983.[Abstract/Free Full Text]
43. Godfrey P, Rahal JO, Beamer WG, Copeland NG, Jenkins NA, Mayo KE. GHRH receptor of little mice contains a missense mutation in the extracellular domain that disrupts receptor function. Nat Genet 4:227–232, 1993.[Medline]
44. Lin S-C, Lin CR, Gukovsky I, Lusis AJ, Sawchenko PE, Rosenfeld MG. Molecular basis of the little mouse phenotype and implications for cell type–specific growth. Nature 364:208–213, 1993.[Medline]
45. Charlton HM, Clark RG, Robinson IC, Goff AE, Cox BS, Bugnon C, Bloch BA. Growth hormone–deficient dwarfism in the rat: A new mutation. J Endocrinol 119:51–58, 1988.[Abstract/Free Full Text]
46. Okuma S, Kawashima S. Spontaneous dwarf rat. Exp Anim 29:301–304, 1980.
47. Nogami H, Takeuchi T, Suzuki K, Okuma S, Ishikawa H. Studies on prolactin and growth hormone gene expression in the pituitary gland of spontaneous dwarf rats. Endocrinology 125:964–970, 1989.[Abstract/Free Full Text]
48. Takeuchi T, Suzuki H, Sakurai S, Nogami H, Okuma S, Ishikawa H. Molecular mechanism of growth hormone (GH) deficiency in the spontaneous dwarf rat: Detection of abnormal splicing of GH messenger ribonucleic acid by the polymerase chain reaction. Endocrinology 126:31–38, 1990.[Abstract/Free Full Text]
49. Behringer RR, Mathews LS, Palmiter RD, Brinster RL. Dwarf mice produced by genetic ablation of growth hormone–expressing cells. Genes Dev 2:453–461, 1988.[Abstract/Free Full Text]
50. Borrelli E, Heyman RA, Arias C, Sawchenko PE, Evans RM. Transgenic mice with inducible dwarfism. Nature 339:538–541, 1989.[Medline]
51. Struthers RS, Vale WW, Arias C, Sawchenko PE, Montminy MR. Somatotroph hypoplasia and dwarfism in transgenic mice expressing a nonphosphorylatable CREB mutant. Nature 350:622–624, 1991.[Medline]
52. Hammer RE, Brinster RL, Rosenfeld MG, Evans RM, Mayo KE. Expression of human growth hormone–releasing factor in transgenic mice results in increased somatic growth. Nature 315:413–417, 1985.[Medline]
53. Mayo KE, Hammer RE, Swanson LW, Brinster RL, Rosenfeld MG, Evans RM. Dramatic pituitary hyperplasia in transgenic mice expressing a human growth hormone–releasing factor gene. Mol Endocrinol 2:606–612, 1988.[Abstract/Free Full Text]
54. Steger RW, Bartke A, Parkening TA, Collins T, Buonomo F, Tang K, Wagner TE, Yun JS. Effects of heterologous growth hormones on hypothalamic and pituitary function in transgenic mice. Neuroendocrinology 53:365–372, 1991.[Medline]
55. Bartke A, Cecim M, Tang K, Steger R, Chandrashekar V, Turyn D. Neuroendocrine and reproductive consequences of overexpression of growth hormone in transgenic mice. Proc Soc Exp Biol Med 206:345–359, 1994.[Medline]
56. Radovick S, Nations M, Du Y, Berg LA, Weintraub BD, Wondisford FE. A mutation in the POU-homeodomain of Pit-1 responsible for combined pituitary hormone deficiency. Science 257:1115–1118, 1992.[Abstract/Free Full Text]
57. Pfäffle RW, DiMattia GE, Parks JS, Brown MR, Wit JM, Jansen M, Van der Nat H, Van der Brande JL, Rosenfeld MG, Ingraham HA. Mutation of the POU-specific domain of Pit-1 and hypopituitarism without pituitary hypoplasia. Science 257:1118–1121, 1992.[Abstract/Free Full Text]
58. Ohta K, Nobukuni Y, Mitsubuchi H, Fujimoto S, Matsuo N, Inagaki H, Endo F, Matsuda I. Mutations in the Pit-1 gene in children with combined pituitary hormone deficiency. Biochem Biophys Res Commun 189:851–855, 1992.[Medline]
59. Parks JS, Pfäffle RW, Brown MR, Abdul-Latif H, Meacham LR. Growth hormone deficiency. In: Weintraub BD, Ed. Molecular Endocrinology: Basic Concepts and Clinical Correlations. New York: Raven Press, Ltd., pp473–490, 1995.
60. Wajnrajch MP, Gertner JM, Harbison MD, Chua Sc Jr, Leibel RL. Nonsense mutation in the human growth hormone–releasing hormone receptor causes growth failure analogous to the little (lit) mouse. Nat Genet 12:88–90, 1996.[Medline]
61. Baumann G, Maheshwari H. The dwarfs of Sindh: Severe growth hormone (GH) deficiency caused by a mutation in the GH-releasing hormone receptor gene. Acta Paediatr Suppl 423:33–38, 1997.[Medline]
62. Brown MR, Parks JS, Adess ME, Rich BH, Rosenthal IM, Voss TC, VanderHeyden TC, Hurley DL. Central hypothyroidism reveals compound heterozygous mutations in the Pit-1 gene. Horm Res 49:98–102, 1998.[Medline]
63. Li S, Crenshaw Eb III, Rawson EJ, Simmons DM, Swanson LW, Rosenfeld MG. Dwarf locus mutants lacking three pituitary cell types result from mutations in the POU-domain gene Pit-1. Nature 347:528–533, 1990.[Medline]
64. Rhodes SJ, Dimattia GE, Rosenfeld MG. Transcriptional mechanisms in anterior pituitary cell differentiation. Curr Opin Genet Dev 4:709–717, 1994.[Medline]
65. Theill LE, Karin M. Transcriptional control of GH expression and anterior pituitary development. Endocr Rev 14:670–689, 1993.[Abstract/Free Full Text]
66. Bartke A. Chromosomal location of the df mutation. Mouse Newsletter 32:52–53, 1965.
67. Buckwalter MS, Katz R, Camper SA. Localization of the panhypopituitary dwarf mutation (df) on mouse chromosome 11 in an intersubspecific backcross. Genomics 10:515–526, 1991.[Medline]
68. Andersen B, Pearse Rv II, Jenne K, Somson M, Lin S-C, Bartke A, Rosenfeld MG. The Ames dwarf gene is required for Pit-1 gene activation. Dev Biol 172:495–503, 1995.[Medline]
69. Gage PJ, Lossie AC, Scarlett LM, Lloyd RV, Camper SA. Ames dwarf mice exhibit somatotrope commitment but lack growth hormone–releasing factor response. Endocrinology 136:1161–1167, 1995.[Abstract]
70. Sornson MW, Wu W, Dasen JS, Flynn SE, Norman DJ, O'Connell SM, Gukovsky I, Carrière C, Ryan AK, Miller AP, Zuo L, Gleiberman AS, Andersen B, Beamer WG, Rosenfeld MG. Pituitary lineage determination by the Prophet of Pit-1 homeodomain factor defective in Ames dwarfism. Nature 384:327–333, 1996.[Medline]
71. Treier M, Rosenfeld MG. The hypothalamic-pituitary axis: Co-development of two organs. Curr Opin Cell Biol 8:833–843, 1996.[Medline]
72. Wu W, Cogan JD, Pfäffle RW, Dasen JS, Frisch H, O'Connell SM, Flynn SE, Brown MR, Mullis PE, Parks JS, Phillips Ja III, Rosenfeld MG. Mutations in PROP1 cause familial combined pituitary hormone deficiency. Nat Genet 18:147–149, 1998.[Medline]
73. Mouse Genome Informatics Resource. [On-line]. Available: http://www.informatics.jax.org. Mouse Genome Informatics. Bar Harbor, Maine: The Jackson Laboratory, 1998.
74. Hermesz E, Mackem S, Mahon KA. Rpx: A novel anterior restricted homeobox gene progressively activated in the prechordal plate, anterior neural plate and Rathke's pouch of the mouse embryo. Development 122:41–52, 1996.[Abstract]
75. Thomas PQ, Johnson BV, Rathjen J, Rathjen PD. Sequence, genomic organization, and expression of the novel homeobox gene Hesx1. J Biol Chem 270:3869–3875, 1995.[Abstract/Free Full Text]
76. Gage PJ, Brinkmeier ML, Scarlett LM, Knapp LT, Camper SA, Mahon KA. The Ames dwarf gene, df, is required early in pituitary ontogeny for the extinction of Rpx transcription and initiation of lineage-specific cell proliferation. Mol Endocrinol 10:1570–1581, 1996.[Abstract/Free Full Text]
77. Dattani MT, Martinez-Barbera IP, Thomas PQ, Brickman JM, Gupta R, Mårtensson IL, Toresson H, Fox M, Wales JK, Hindmarsh PC, Krauss S, Beddington RS, Robinson IC. Mutations in the homeobox gene HESX1/Hesx1 associated with septo-optic dysplasia in human and mouse. Nat Genet 19:125–133, 1998.[Medline]
78. Sheng HZ, Zhadanov AB, Mosinger B Jr, Fujii T, Bertuzzi S, Grinberg A, Lee EJ, Huang S-P, Mahon KA, Westphal H. Specification of pituitary cell lineages by the LIM homeobox gene Lhx3. Science 272:1004–1007, 1996.[Abstract]
79. Sheng HZ, Moriyama K, Yamashita T, Li H, Potter SS, Mahon KA, Westphal H. Multistep control of pituitary organogenesis. Science 278:1809–1812, 1997.[Abstract/Free Full Text]
80. Bach I, Rhodes SJ, Pearse Rv II, Heinzel T, Gloss B, Scully KM, Sawchenko PE, Rosenfeld MG. P-Lim, a LIM homeodomain factor, is expressed during pituitary organ and cell commitment and synergizes with Pit-1. Proc Natl Acad Sci U S A 92:2720–2724, 1995.[Abstract/Free Full Text]
81. Seidah NG, Barale J, Marcinkiewicz M, Mattei M, Day R, Chretien M. The mouse homeoprotein mLIM-3 is expressed early in cells derived from the neuroepithelium and persists in adult pituitary. DNA 13:1163–1180, 1994.
82. Sasaki F, Sano M. Role of the ovary in the sexual differentiation of prolactin and growth hormone cells in the mouse adenohypophysis: Stereological morphometric study by electron microscopy. J Endocrinol 9:117–121, 1982.
83. Gage PJ, Roller ML, Saunders TL, Scarlett LM, Camper SA. Anterior pituitary cells defective in the cell-autonomous factor, df, undergo cell lineage specification but not expansion. Development 122:151–160, 1996.[Abstract]
84. Wilson DB, Wyatt DP. Immunocytochemistry of TSH cells during development of the dwarf mutant mouse. Anat Embryol (Berl) 174:277–282, 1986.[Medline]
85. Wilson DB, Wyatt DP. Immunocytochemistry of ambiguous cells in adult and embryonic dwarf (dw) mouse pituitaries. Anat Rec 236:671–678, 1993.[Medline]
86. Wilson DB, Wyatt DP. Ultrastructural immunocytochemistry of somatotrophs and mammotrophs in embryos of the dwarf mutant mouse. Anat Rec 215:282–287, 1986.[Medline]
87. Slabaugh MB, Lieberman ME, Rutledge JJ, Gorski J. Ontogeny of growth hormone and prolactin gene expression in mice. Endocrinology 110:1489–1497, 1982.[Abstract/Free Full Text]
88. Hurley DL, Wojtkiewicz PW, Phelps CJ. Growth hormone and Pit-1 mRNA detection by reverse transcription: Polymerase chain reaction in adult and developing Ames dwarf mice. Rec Prog Horm Res 50:443–448, 1995.
89. Grumbach IM, Veh RW. The SA/rABC technique: A new ABC procedure for detection of antigens at increased sensitivity. J Histochem Cytochem 43:31–37, 1995.[Abstract]
90. Phelps CJ, Vaccarella MY, Romero MI, Hurley DL. Postnatal reduction in number of hypothalamic tuberoinfundibular dopaminergic neurons in prolactin-deficient dwarf mice. Neuroendocrinology 59:189–196, 1994.[Medline]
91. Romero MI, Phelps CJ. Prolactin replacement during development prevents dopaminergic deficit in hypothalamic arcuate nucleus of prolactin-deficient Ames dwarf mice. Endocrinology 133:1860–1870, 1993.[Abstract/Free Full Text]
92. Romero MI, Phelps CJ. Prolactin replacement in adult dwarf mice does not reverse the deficit in tuberoinfundibular dopaminergic neuron number. Endocrinology 136:3238–3246, 1995.[Abstract]
93. Palmiter RD, Norstedt G, Gelinas RE, Hammer RE, Brinster RL. Metallothionein–human GH fusion genes stimulate growth of mice. Science 222:809–814, 1983.[Abstract/Free Full Text]
94. McGrane MM, de Vente J, Yun J, Bloom J, Park E, Wynshaw-Boris A, Wagner T, Rottman FM, Hanson RW. Tissue-specific expression and dietary regulation of a chimeric phosphoenolpyruvate carboxykinase/growth hormone gene in transgenic mice. J Biol Chem 263:11443–11451, 1988.[Abstract/Free Full Text]
95. Milton S, Cecim M, Li YS, Yun JS, Wagner TE, Bartke A. Transgenic female mice with high human growth hormone levels are fertile and capable of normal lactation without having been pregnant. Endocrinology 131:536–538, 1992.[Abstract/Free Full Text]
96. Chandrashekar V, Bartke A, Wagner TE. Neuroendocrine function in adult female transgenic mice expressing the human growth hormone gene. Endocrinology 130:1802–1808, 1992.[Abstract/Free Full Text]
97. Meister B, Hökfelt T, Vale WW, Sawchenko PE, Swanson L, Goldstein M. Coexistence of tyrosine hydroxylase and growth hormone– releasing factor in a subpopulation of tuberoinfundibular neurons of the rat. Neuroendocrinology 42:237–247, 1986.[Medline]
98. Furness JB, Heath JW, Costa M. Aqueous aldehyde (Faglu) methods for the fluorescence histochemical localization of catecholamines for ultrastructural studies of central nervous tissue. Histochemistry 57:289–295, 1978.
99. Sato M, Frohman LA. Differential effects of central and peripheral administration of growth hormone (GH) and insulin-like growth factor on hypothalamic GH–releasing hormone and somatostatin gene expression in GH-deficient dwarf rats. Endocrinology 133:793–799, 1993.[Abstract/Free Full Text]
100. Kelly PA, Djiane J, Postel-Vinay M-C, Edery M. The prolactin/growth hormone receptor family. Endocr Rev 12:235–251, 1991.[Abstract/Free Full Text]
101. Kelly PA, Djiane J, Edery M. Different forms of the prolactin receptor: Insights into the mechanism of prolactin action. Trends Endocrinol Metab 3:54–59, 1992.[Medline]
102. Bole-Feysot C, Goffin V, Edery M, Binart N, Kelly PA. Prolactin (PRL) and its receptor: Actions, signal transduction pathways, and phenotypes observed in PRL receptor knockout mice. Endocr Rev 19:225–268, 1998.[Abstract/Free Full Text]
103. Burton KA, Kabigting EB, Clifton DK, Steiner RA. Growth hormone receptor messenger ribonucleic acid distribution in the adult male rat brain and its colocalization in hypothalamic somatostatin neurons. Endocrinology 131:958–963, 1992.[Abstract/Free Full Text]
104. Burton KA, Kabigting EB, Steiner RA, Clifton DK. Identification of target cells for growth hormone's action in the arcuate nucleus. Am J Physiol 269:E716–E722, 1995.[Abstract/Free Full Text]
105. Chan YY, Steiner RA, Clifton DK. Regulation of hypothalamic neuropeptide-Y neurons by growth hormone in the rat. Endocrinology 137:1319–1325, 1996.[Abstract]
106. Kamegai J, Minami S, Sugihara H, Hasegawa O, Higuchi H, Wakabayashi I. Growth hormone receptor gene is expressed in neuropeptide Y neurons in hypothalamic arcuate nucleus of rats. Endocrinology 137:2109–2112, 1996.[Abstract]
107. McDonald JK, Lumpkin MD, Samson WK, McCann SM. Neuropeptide Y affects secretion of luteinizing hormone and growth hormone in ovariectomized rats. Proc Natl Acad Sci U S A 82:561–564, 1985.[Abstract/Free Full Text]
108. Bennett PA, Levy A, Sophokleous S, Robinson IC, Lightman SL. Hypothalamic GH receptor gene expression in the rat: Effects of altered GH status. J Endocrinol 147:225–234, 1995.[Abstract/Free Full Text]
109. Chiu S, Koos RD, Wise PM. Detection of prolactin receptor (PRL-R) mRNA in the rat hypothalamus and pituitary gland. Endocrinology 130:1747–1749, 1992.[Abstract/Free Full Text]
110. Chiu S, Wise PM. Prolactin receptor mRNA localization in the hypothalamus by in situ hybridization. J Neuroendocrinol 6:191–199, 1994.[Medline]
111. Crumeyrolle-Arias M, Latouche J, Jammes H, Djiane J, Kelly PA, Reymond MJ, Haour F. Prolactin receptors in the rat hypothalamus: Autoradiographic localization and characterization. Neuroendocrinology 57:457–466, 1993.[Medline]
112. Roky R, Paut-Pagano L, Goffin V, Kitahama K, Valatx J-L, Kelly PA, Jouvet M. Distribution of prolactin receptors in the rat forebrain. Neuroendocrinology 63:422–429, 1996.[Medline]
113. Bakowska JC, Morrell JI. Atlas of the neurons that express mRNA for the long form of the prolactin receptor in the forebrain of the female rat. J Comp Neurol 386:161–177, 1997.[Medline]
114. Pi X-J, Grattan DR. Differential expression of the two forms of prolactin receptor mRNA within microdissected hypothalamic nuclei of the rat. Mol Brain Res 59:1–12, 1998.[Medline]
115. Arbogast LA, Voogt JL. Prolactin (PRL) receptors are colocalized in dopaminergic neurons in fetal hypothalamic cell cultures: Effect of PRL on tyrosine hydroxylase activity. Endocrinology 138:3016–3023, 1997.[Abstract/Free Full Text]
116. Lerant A, Freeman ME. Ovarian steroids differentially regulate the expression of PRL-R in neuroendocrine dopaminergic neuron populations: A double label confocal microscopic study. Brain Res 802:141–152, 1998.[Medline]
117. Minami S, Kamegai J, Sugihara H, Hasegawa O, Wakabayashi I. Systemic administration of recombinant human growth hormone induces expression of the c-fos gene in the hypothalamic arcuate and periventricular nuclei in hypophysectomized rats. Endocrinology 131:247–253, 1992.[Abstract/Free Full Text]
118. Kamegai J, Minami S, Sugihara H, Higuchi H, Wakabayashi I. Growth hormone induces expression of the c-fos gene in hypothalamic neuropeptide-Y and somatostatin neurons in hypophysectomized rats. Endocrinology 135:2765–2771, 1994.[Abstract]
119. Chan YY, Clifton DK, Steiner RA. Role of NPY neurones in GH-dependent feedback signalling to the brain. Horm Res 45(Suppl 1):12–14, 1996.
120. Bennett PA, Thomas GB, Howard AD, Feighner SD, van der Ploeg LH, Smith RG, Robinson IC. Hypothalamic growth hormone secretagogue-receptor (GHS-R) expression is regulated by growth hormone in the rat. Endocrinology 138:4552–4557, 1997.[Abstract/Free Full Text]
121. Suzuki N, Okada K, Minami S, Wakabayashi I. Inhibitory effect of neuropeptide Y on growth hormone secretion in rats is mediated by both Y1- and Y2-receptor subtypes and abolished after anterolateral deafferentiation of the medial basal hypothalamus. Regul Pept 65:145–151, 1996.[Medline]
122. Dumont Y, Fournier A, St-Pierre S, Quirion R. Comparative characterization and autoradiographic distribution of neuropeptide Y receptor subtypes in rat brain. J Neurosci 13:73–86, 1993.[Abstract]
123. Larsen PJ, Sheikh SP, Jakobsen CR, Schwartz TW, Mikkelsen JD. Regional distribution of putative NPY Y1 receptors and neurons expressing Y1 mRNA in forebrain areas of the rat central nervous system. Eur J Neurosci 5:1622–1637, 1993.[Medline]
124. Hisano S, Tsuruo Y, Katogani Y, Daikoku S, Chihara K. Immunohistochemical evidence for synaptic connections between neuropeptide Y-containing axons and periventricular somatostatin neurons in the anterior hypothalamus in rats. Brain Res 520:170–177, 1990.[Medline]
125. Rettori V, Milenkovic L, Aguila MC, McCann SM. Physiologically significant effect of neuropeptide Y to suppress growth hormone release by stimulating somatostatin discharge. Endocrinology 126:2296–2301, 1990.[Abstract/Free Full Text]
126. Aguilar RC, Fernandez HN, Dellacha JM, Calandra RS, Bartke A, Ghosh PK, Turyn D. Somatotropic and lactotropic receptors in transgenic mice expressing human or bovine growth hormone genes. Transgenic Res 1:221–227, 1992.[Medline]
127. Ciofi P, Fallon JH, Croix D, Polack JM, Tramu G. Expression of neuropeptide Y precursor-immunoreactivity in the hypothalamic dopaminergic tubero-infundibular system during lactation in rodents. Endocrinology 128:823–834, 1991.[Abstract/Free Full Text]
128. Malabu UH, Kilpatrick A, Ware M, Vernon RG, Williams G. Increased neuropeptide Y concentrations in specific hypothalamic regions of lactating rats: Possible relationship to hyperphagia and adaptive changes in energy balance. Peptides 15:83–87, 1994.[Medline]
129. Smith MS. Lactation alters neuropeptide-Y and proopiomelanocortin gene expression in the arcuate nucleus of the rat. Endocrinology 133:1258–1265, 1993.[Abstract/Free Full Text]
130. Pape J-R, Tramu G. Suckling-induced changes in neuropeptide Y and proopiomelanocortin gene expression in the arcuate nucleus of the rat: Evaluation of a putative intervention of prolactin. Neuroendocrinology 63:540–549, 1996.[Medline]
131. Li C, Chen P, Smith MS. The acute suckling stimulus induces expression of neuropeptide Y (NPY) in cells in the dorsomedial hypothalamus and increases NPY expression in the arcuate nucleus. Endocrinology 139:1645–1652, 1998.[Abstract/Free Full Text]
132. Ghosh PK, Debeljuk L, Wagner TE, Bartke A. Effect of immunoneutralization of neuropeptide Y on gonadotropin and prolactin secretion in normal mice and in transgenic mice bearing bovine growth hormone gene. Endocrinology 129:597–602, 1991.[Abstract/Free Full Text]
133. Hyde JF, Bartke A, Davis BM. Galanin gene expression in the hypothalamopituitary axis of the Ames dwarf mouse. Mol Cell Neurosci 4:298–303, 1993.
134. Chan YY, Grafstein-Dunn E, Delemarre-van de Waal HA, Burton KA, Clifton DK, Steiner RA. The role of galanin and its receptor in the feedback regulation of growth hormone secretion. Endocrinology 137:5303–5310, 1996.[Abstract]
135. Wynick D, Small CJ, Bacon A, Holmes FE, Norman M, Ormandy CJ, Kilic E, Kerr NC, Ghatei M, Talamantes F, Bloom SR, Pachnis V. Galanin regulates prolactin release and lactotroph proliferation. Proc Natl Acad Sci U S A 95:12671–12676, 1998.[Abstract/Free Full Text]
136. Howard AD, Feighner SD, Cully DF, Arena JP, Liberator PA, Rosenblum CI, Hamelin M, Hreniuk DL, Palyha OC, Anderson J, Paress PS, Diaz C, Chou M, Liu KK, McKee KK, Pong SS, Chaung LY, Elbrecht A, Dashkevicz M, Heavens R, Rigby M, Sirinathsinghji DJS, Dean DC, Melillo DG, Patchett AA, Nargund R, Griffin PR, DeMartino JA, Gupta SK, Schaeffer JM, Smith RG, Van der Ploeg LH. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science 273:974–977, 1996.[Abstract]
137. Pong S-S, Chaung L-HP, Dean DC, Nargund RP, Patchett AA, Smith RG. Identification of a new G-protein-linked receptor for growth hormone secretagogues. Mol Endocrinol 10:57–61, 1996.[Abstract/Free Full Text]
138. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology 114:1537–1545, 1984.[Abstract/Free Full Text]
139. Dickson SL, Doutrelant-Viltart O, Leng G. GH-deficient dw/dw rats and lit/lit mice show increased Fos expression in the hypothalamic arcuate nucleus following systemic injection of GH-releasing peptide-6. J Endocrinol 146:519–526, 1995.[Abstract/Free Full Text]
140. Sara VR, Uvnas-Moberg K, Uvnas B, Hall K, Wetterberg L, Posloncec B, Goiny M. The distribution of somatomedins in the nervous system of the cat and their release following neural stimulation. Acta Physiol Scand 115:467–470, 1982.[Medline]
141. Adem A, Jossan SS, d'Argy R, Gillberg PG, Nordberg A, Winblad B, Sara V. Insulin-like growth factor I (IGF-I) receptors in the human brain: Quantitative autoradiographic localization. Brain Res 503:299–303, 1989.[Medline]
142. Araujo DM, Lapchak PA, Collier B, Chabot J-G. Quirion R. Insulin-like growth factor I (somatomedin-C) receptors in the rat brain: Distribution and interaction with the hippocampal cholinergic system. Brain Res 484:130–138, 1989.[Medline]
143. Ocrant I, Valentino KL, Eng LF, Hintz RL, Wilson DM, Rosenfeld RG. Structural and immunohistochemical characterization of insulin-like growth factor I and II receptors in the murine central nervous system. Endocrinology 123:1023–1034, 1988.[Abstract/Free Full Text]
144. Nissley SP, Knazek RA, Wolff GL. Somatomedin activity in sera of genetically small mice. Horm Metab Res 12:158–164, 1980.[Medline]
145. Noguchi T, Kurata LM, Sugisaki T. Presence of a somatomedin-C-immunoreactive substance in the central nervous system: Immunohistochemical mapping studies. Neuroendocrinology 46:277–282, 1987.[Medline]
146. Minami S, Suzuki N, Sugihara H, Tamura H, Emoto N, Wakabayashi I. Microinjection of rat GH but not human IGF-1 into a defined area of the hypothalamus inhibits endogenous GH secretion in rats. J Endocrinol 153:283–290, 1997.[Abstract/Free Full Text]
147. McCarthy GF, Beaudet A, Tannenbaum GS. Colocalization of somatostatin receptors and growth hormone–releasing factor immunoreactivity in neurons of the rat arcuate nucleus. Neuroendocrinology 56:18–24, 1992.[Medline]
148. Bertherat J, Dournaud P, Bérod A, Normand E, Bloch B, Rosténe W, Kordon C, Epelbaum J. Growth hormone–releasing hormone-synthesizing neurons are a subpopulation of somatostatin receptor–labelled cells in the rat arcuate nucleus: A combined in situ hybridization and receptor light-microscopic radioautographic study. Neuroendocrinology 56:25–31, 1992.[Medline]
149. Liposits ZS, Merchenthaler I, Paull WK, Flerko B. Synaptic communication between somatostatinergic axons and growth hormone–releasing factor (GRF) synthesizing neurons in the arcuate nucleus of the rat. Histochemistry 89:247–252, 1988.[Medline]
150. Willoughby JO, Brogan M, Kapoor R. Hypothalamic interconnections of somatostatin and growth hormone releasing factor neurons. Neuroendocrinology 50:584–591, 1989.[Medline]
151. Tannenbaum GS, McCarthy GF, Zeitler P, Beaudet A. Cysteamine-induced enhancement of growth hormone–releasing factor (GRF) immunoreactivity in arcuate neurons: Morphological evidence for putative somatostatin/GRF interactions within hypothalamus. Endocrinology 127:2551–2560, 1990.[Abstract/Free Full Text]
152. Minami S, Kamegai J, Sugihara H, Suzuki N, Wakabayashi I. Growth hormone inhibits its own secretion by acting on the hypothalamus through its receptors on neuropeptide Y neurons in the arcuate nucleus and somatostatin neurons in the periventricular nucleus. Endocr J 45(Suppl):S19–S26, 1998.
153. Tannenbaum GS. Neuroendocrine control of growth hormone secretion. Acta Paediatr Suppl 372:5–16, 1991.
154. Robinson ICAF. Neuroendocrine control of GH secretion. Proc. 4th Intl. Cong. Neuroendocrinol. Folia Endocrinol Japon 74:401, 1998.
155. Lin LF, Doherty DH, Lile JD, Bektesh S, Collins F. GDNF: A glial cell line–derived neurotrophic factor for midbrain dopaminergic neurons. Science 260:1130–1132, 1993.[Abstract/Free Full Text]
156. Houenou LJ, Oppenheim RW, Li L, Lo AC, Prevette D. Regulation of spinal motoneuron survival by GDNF during development and following injury. Cell Tissue Res 286:219–223, 1996.[Medline]
157. Beck KI, Valverde J, Alexi T, Poulsen K, Moffat B, Vandlen RA, Rosenthal A, Hefti F. Mesencephalic dopaminergic neurons protected by GDNF from axotomy-induced degeneration in the adult brain. Nature 373:339–341, 1995.[Medline]
158. Hoffer BJ, Hoffman A, Bowenkamp K, Huetti P, Hudson J, Martin D, Lin LF, Gerhardt GA. Glial cell line–derived neurotrophic factor reverses toxin-induced injury to midbrain dopaminergic neurons in vivo. Neuroscience Lett 182:107–111, 1994.[Medline]
159. Sauer H, Rosenblad C, Bjorklund A. Glial cell line–derived neurotrophic factor but not transforming growth factor ß-3 prevents delayed degeneration of nigral dopaminergic neurons following striatal 6-hydroxydopamine lesion. Proc Natl Acad Sci U S A 92:8935–8939, 1995.[Abstract/Free Full Text]
160. Cass WA. GDNF selectively protects dopamine neurons over serotonin neurons against the neurotoxic effects of methamphetamine. J Neurosci 16:8132–8139, 1996.[Abstract/Free Full Text]
161. Tomac A, Widenfalk J, Lin LF, Kohno T, Ebendal T, Hoffer BJ, Olson L. Retrograde axonal transport of glial cell line–derived neurotrophic factor in the adult nigrostriatal system suggests a trophic role in the adult. Proc Natl Acad Sci U S A 92:8274–8278, 1995.[Abstract/Free Full Text]
162. Gash DM, Zhang Z, Ovadia A, Cass WA, Yi A, Simmerman L, Russell D, Martin D, Lapchak PA, Collins FA, Hoffer BJ, Gerhardt GA. Functional recovery in Parkinsonian monkeys treated with GDNF. Nature 380:252–256, 1996.[Medline]
163. Stromberg I, Bjorklund L, Johansson M, Tomac A, Collins F, Olson L, Hoffer B, Humpel C. Glial cell line–derived neurotrophic factor is expressed in the developing but not adult striatum and stimulates developing dopamine neurons in vivo. Exp Neurol 124:401–412, 1993.[Medline]
164. Wang Y, Tien LT, Lapchak PA, Hoffer BJ. GDNF triggers fiber outgrowth of fetal ventral mesencephalic grafts from nigra to striatum in 6-OHDA-lesioned rats. Cell Tissue Res 286:225–233, 1996.[Medline]
165. Sanchez MP, Silos-Santiago I, Friesen J, He B, Lira SA, Barbacid M. Renal agenesis and the absence of enteric neurons in mice lacking GDNF. Nature 382:70–73, 1996.[Medline]
166. Choi-Lundberg DL, Bohn MC. Ontogeny and distribution of glial cell line–derived neurotrophic factor (GDNF) mRNA in the rat. Brain Res Dev Brain Res 85:80–88, 1995.[Medline]
167. Chang HT, Kuo H, Whittaker JA, Cooper NG. Light and electron microscopic analysis of projection neurons retrogradely labeled with Fluoro-Gold: Notes on the application of antibodies to Fluoro-Gold. J Neurosci Methods 35:31–37, 1990.[Medline]
168. Romero MI, Phelps CJ. Identification of growth hormone–releasing hormone and somatostatin neurons projecting to the median eminence in normal and growth hormone–deficient Ames dwarf mice. Neuroendocrinology 65:107–116, 1997.[Medline]
169. Gibson MJ, Krieger DT, Charlton HM, Zimmerman EA, Silverman AJ, Perlow MJ. Mating and pregnancy can occur in genetically hypogonadal mice with preoptic area brain grafts. Science 225:949–951, 1984.[Abstract/Free Full Text]
170. Charlton HM, Wood MJ. Animal models for brain and pituitary gonadal disturbances. In: Swaab DF, Hofman MA, Mirmiran M, Ravid R, van Leeuwen FW, Eds. Progress in Brain Research. New York: Elsevier, Vol. 93: pp321–332, 1992.
171. Bruhn TO, Tresco PA, Jackson IM. Transplantation of fetal growth hormone–releasing factor-immunoreactive neurons into the ventricular system of adult MSG-treated rats. Peptides 12:957–961, 1991.[Medline]
172. Saitoh Y, Gibson MJ, Silverman AJ. Targeting of gonadotropin-releasing hormone axons from preoptic area grafts to the median eminence. J Neurosci Res 33:379–391, 1992.[Medline]
173. Wray S, Gähwiler BH, Gainer H. Slice cultures of LHRH neurons in the presence and absence of brainstem and pituitary. Peptides 9:1151–1175, 1988.[Medline]
174. Tobet SA, Hanna IK, Schwarting GA. Migration of neurons containing gonadotropin releasing hormone (GnRH) in slices from embryonic nasal compartment and forebrain. Brain Res Dev Brain Res 97:287–292, 1996.[Medline]
175. King JC, Rubin BS. Dynamic alterations in luteinizing hormone–releasing hormone (LHRH) neuronal cell bodies and terminals of adult rats. Cell Mol Neurobiol 15:89–106, 1995.[Medline]
176. King JC, Rubin BS. Neuronal glial interactions and regulation of LHRH secretion (abstract). Proc Endocrine Soc Mtg S23-1, 1997.
177. Horseman ND, Yu-Lee L. Transcriptional regulation by the helix bundle peptide hormones: Growth hormone, prolactin, and hematopoietic cytokines. Endocr Rev 15:627–649, 1994.[Abstract/Free Full Text]
178. Darnell Je Jr. STATs and gene regulation. Science 277:1630–1635, 1997.[Abstract/Free Full Text]
179. Moutoussamy S, Kelly PA, Finidori J. Growth-hormone-receptor and cytokine-receptor-family signaling. Eur J Biochem 255:1–11, 1998.[Medline]
180. Carter-Su C, Smit LS. Signaling via JAK tyrosine kinases: Growth hormone receptor as a model system. Rec Prog Horm Res 53:61–82, 1998.
181. Yi W, Kim S-O, Jiang J, Park S-H, Kraft AS, Waxman DJ, Frank SJ. Growth hormone receptor cytoplasmic domain differentially promotes tyrosine phosphorylation of signal transducers and activators of transcription 5b and 3 by activated JAK2 kinase. Mol Endocrinol 10:1425–1443, 1996.[Abstract/Free Full Text]
182. Gebert CA, Park S-H, Waxman DJ. Regulation of signal transducer and activator of transcription (STAT) 5b activation by the temporal pattern growth hormone stimulation. Mol Endocrinol 11:400–414, 1998.[Abstract/Free Full Text]
183. Vaisse C, Halaas JL, Horvath CM, Darnell Je Jr, Stoffel M, Friedman JM. Leptin activation of Stat3 in the hypothalamus of wild-type and ob/ob mice but not db/db mice. Nat Genet 14:95–97, 1996.[Medline]
184. McCowen KC, Chow JC, Smith RJ. Leptin signaling in the hypothalamus of normal rats in vivo. Endocrinology 139:4442–4447, 1998.[Abstract/Free Full Text]
185. Zhou Y, Xu BC, Maheshwari HG, He L, Reed M, Lozykowski M, Okada S, Cataldo L, Coschigamo K, Wagner TE, Baumann G, Kopchick JJ. A mammalian model for Laron syndrome produced by targeted disruption of the mouse growth hormone receptor/binding protein gene (the Laron mouse). Proc Natl Acad Sci U S A 94:13215–13220, 1997.[Abstract/Free Full Text]
186. Chandrashekar V, Bartke A, Coschigano KT, Kopchick JJ. Pituitary and testicular function in growth hormone receptor gene knockout mice. Endocrinology 140:1082–1088, 1999.[Abstract/Free Full Text]
187. Ormandy CJ, Camus A, Barra J, Damotte D, Lucas B, Buteau H, Edery M, Brousse N, Babinet C, Binart N, Kelly PA. Null mutation of the prolactin receptor gene produces multiple reproductive defects in the mouse. Genes Dev 11:167–178, 1997.[Abstract/Free Full Text]
188. Horseman ND, Zhao W, Montecino-Rodriguez E, Tanaka M, Nakahima K, Engle SJ, Smith F, Markoff E, Dorshkind K. Defective mammopoiesis, but normal hematopoiesis, in mice with a targeted disruption of the prolactin gene. EMBO J 16:6926–6935, 1997.[Medline]
189. Steger RW, Chandrashekar V, Zhao W, Bartke A, Horseman ND. Neuroendocrine and reproductive functions in male mice with targeted disruption of the prolactin gene. Endocrinology 139:3691–3695, 1998.[Abstract/Free Full Text]
190. Phelps CJ, Horseman ND. Hypophsiotropic dopamine neurons in PRL knockout mice (abstract). Proc Endocrine Soc Mtg P2-492, 1999.
191. Bartke A. Reproduction of female dwarf mice treated with prolactin. J Reprod Fertil 11:203–206, 1966.[Abstract/Free Full Text]
192. Bartke A, Goldman BD, Bex F, Dalterio S. Effects of prolactin (PRL) on pituitary and testicular function in mice with hereditary PRL deficiency. Endocrinology 101:1760–1766, 1977.[Abstract/Free Full Text]
193. Wennbo H, Kindblom J, Isaksson OG, Tornell J. Transgenic mice overexpressing the prolactin gene develop dramatic enlargement of the prostate gland. Endocrinology 138:4410–4415, 1997.[Abstract/Free Full Text]
194. Erickson JC, Hollopeter G, Palmiter RD. Attenuation of the obesity syndrome of ob/ob mice by the loss of neuropeptide Y. Science 274:1704–1707, 1996.[Abstract/Free Full Text]
195. Pedrazzini T, Seydoux J, Künster P, Aubert J-F, Grouzmann E, Beermann F, Brunner H-R. Cardiovascular response, feeding behavior, and locomotor activity in mice lacking the NPY Y1 receptor. Nat Med 4:722–726, 1998.[Medline]
196. Marsh DJ, Hollopeter G, Kafer KE, Palmiter RD. Role of the Y5 neuropeptide Y receptor in feeding and obesity. Nat Med 4:718–721, 1998.[Medline]

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