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Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana 70808–4124
Prior experiments have shown that the adipocyte hormone leptin can advance puberty in mice. We hypothesized that it would also stimulate gonadotrophin secretion in adults. Since the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) is drastically affected by estrogen, we hypothesized that leptin might have different actions dependent on the dose of estrogen. Consequently in these experiments, we tested the effect of injection of leptin into the third cerebral ventricle of ovariectomized animals injected with either the oil diluent, 10 µg or 50 µg of estradiol benzoate 72 hr prior to the experiment. The animals were ovariectomized 3–4 weeks prior to implantation of a cannula into the third ventricle 1 week before the experiments. The day after implantation of an external jugular catheter, blood samples (0.3 ml) were collected just before and every 10 min for 2 hr after 3V injection of 5 µl of diluent or 10 µg of leptin. Both doses of estradiol benzoate equally decreased plasma LH concentrations and pulse amplitude, but there was a graded decrease in pulse frequency. In contrast, only the 50-µg dose of estradiol benzoate significantly decreased mean plasma FSH concentrations without significantly changing other parameters of FSH release. The number of LH pulses alone and pulses of both hormones together decreased as the dose of estrogen was increased, whereas the number of pulses of FSH alone significantly increased with the higher dose of estradiol benzoate, demonstrating differential control of LH and FSH secretion by estrogen, consistent with alterations in release of luteinizing hormone releasing hormone (LHRH) and the putative FSH-releasing factor (FSHRF), respectively.
The effects of intraventricularly injected leptin were drastically altered by increasing doses of estradiol benzoate. There was no significant effect of intraventricular injection of leptin (10 µg) on the various parameters of either FSH or LH secretion in ovariectomized, oil-injected rats, whereas in those injected with 10 µg of estradiol benzoate there was an increase in the first hr in mean plasma concentration, area under the curve, pulse amplitude, and maximum increase of LH above the starting value (
max) on comparison with the results in the diluent-injected animals in which there was no alteration of these parameters during the 2 hr following injection. The pattern of FSH release was opposite to that of LH and had a different time-course. In the diluent-injected animals, probably because of the stress of injection and frequent blood sampling, there was an initial significant decline in plasma FSH at 20 min after injection, followed by a progressive increase with a significant elevation above the control values at 110 and 120 min. In the leptin-injected animals, mean plasma FSH was nearly constant during the entire experiment, coupled with a significant decrease below values in diluent-injected rats, beginning at 30 min after injection and progressing to a maximal difference at 120 min. Area under the curve, pulse amplitude, and max of FSH was also decreased in the second hour compared to values in diluent-injected rats. In contrast to the stimulatory effects of intraventricular injection of leptin on pulsatile LH release manifest during the first hour after injection, there was a diametrically opposite, delayed significant decrease in pulsatile FSH release. This differential effect of leptin on FSH and LH release was consistent with differential effects of leptin on LHRH and FSHRF release. Finally, the higher dose of E2 (50 µg) suppressed release of both FSH and LH, but there was little effect of leptin under these conditions, the only effect being a slight (P < 0.04) increase in pulse amplitude of LH in this group of rats. The results indicate that the central effects of leptin on gonadotropin release are strongly dependent on plasma estradiol levels. These effects are consistent with differential effects of estrogen on the release of LHRH and the putative FSHRF.
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