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ORIGINAL RESEARCH ARTICLE

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) Regulates the Expression of PACAP in Cultured Tilapia Astrocytes

Institute of Biotechnology, National Dong Hwa University, Hualien 974, Taiwan

To whom requests for reprints should be addressed at ¹ 1, sec. 2, Da-Hsueh Rd., Shou-Feng, Hualien 974, Taiwan. E-mail: cfweng{at}mail.ndhu.edu.tw

Neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic hormone that is involved in numerous physiologic functions. The present study examines the presence and the functional significance of PACAP and its receptor in the brain and astrocytes of tilapia (Oreochromis mossambicus). This is the first demonstration of the full-length nucleotide sequence of tPACAP gene in tilapia pituitary, brain, and cultured astrocytes. Two cDNA variants of the growth hormone–releasing hormone (GHRH)–PACAP gene were identified in tilapia pituitary, brain, and cultured astrocytes as a result of exon skipping with a long form (271 bp) encoding both tPACAP₃₈ and tGHRH and a short form (166 bp) encoding only tPACAP₃₈. The short form was found to be more abundant in astrocytes. Addition of ovine PACAP₃₈ (1 nM) to cultured astrocytes significantly stimulated the expression of tPACAP₃₈ at 4 hrs, but the effect dropped after 8 hrs of treatment. By contrast, the expression of PACAP type I receptor (PAC₁-R) mRNA in the astrocytes was not responsive to PACAP₃₈ treatment. The tPACAP₃₈ expression also was activated by the cAMP analog, dibutyryl-cAMP, in a dose-dependent manner. Adding high salinity (170 mM NaCl, 500 mOsm/kg osmolarity) to cultured medium substantially increased astroglial tPACAP₃₈ expression over 4 hrs to a level that was maintained for 16 hrs. This observation was not found when mannitol (270 mM) was supplemented as an osmolarity-enhancing agent (500 mOsm/ kg). Taken together, tPACAP expression in tilapia astrocytes was well regulated by exogenous PACAP, cAMP, and salinity and might be involved in the adaptation to high salinity when the fish is in a seawater environment.

Key Words: PACAP • PACAP type I receptor • autocrine/paracrine • astrocyte • salinity • tilapia • Oreochromis mossambicus

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