Pituitary adenylate cyclase activating polypeptide as a novel hypophysiotropic factor in fish

Abstract

Pituitary adenylate cyclase activating polypeptide (PACAP) is a novel member of the secretin-glucagon peptide family. In mammals, this peptide has been located in a wide range of tissues and is involved in a variety of biological functions. In lower vertebrates, especially fish, increasing evidence suggests that PACAP may function as a hypophysiotropic factor regulating pituitary hormone secretion. PACAP has been identified in the brain-pituitary axis of representative fish species. The molecular structure of fish PACAP is highly homologous to mammalian PACAP. The prepro-PACAP in fish, however, is distinct from that of mammals as it also contains the sequence of fish GHRH. In teleosts, the anterior pituitary is under direct innervation of the hypothalamus and PACAP nerve fibers have been identified in the pars distalis. Using the goldfish as a fish model, mRNA transcripts of PACAP receptors, namely the PAC1 and VPACI receptors, have been identified in the pituitary as well as in various brain areas. Consistent with the pituitary expression of PACAP receptors, PACAP analogs are effective in stimulating growth hormone (GH) and gonadotropin (GTH)-II secretion in the goldfish both in vivo and in vitro. The GH-releasing action of PACAP is mediated via pituitary PAC1 receptors coupled to the adenylate cyclase-cAMP-protein kinase A and phospholipase C-IP3-protein kinase C pathways. Subsequent stimulation of Ca2+ entry through voltage-sensitive Ca2+ channels followed by activation of Ca2+-calmodulin protein kinase II is likely the downstream mechanism mediating PACAP-stimulated GH release in goldfish. Although the PACAP receptor subtype(s) and the associated post-receptor signaling events responsible for PACAP-stimulated GTH-II release have not been characterized in goldfish, these findings support the hypothesis that PACAP is produced in the hypothalamus and delivered to the anterior pituitary to regulate GH and GTH-II release in fish.

Discovery of growth hormone-releasing hormones and receptors in nonmammalian vertebrates

“…Another interesting observation is the existence of PRPspecific receptors from fish (23)(24)(25) to chicken, suggesting that PRP potentially is a physiological regulator in these species. However, despite our efforts, we were unable to find the PRP-R gene in any of the mammalian genome databases (in total, 12 databases were analyzed), including the almost completed human and mouse genomes, indicating that PRP-R (but not PRP) was lost in the mammalian lineage.…”

“…Zebrafish and goldfish were purchased from local fish markets, and X. laevis was bought from Xenopus I. Peptides including X. laevis GHRH (xGHRH), zebrafish or goldfish GHRH (fish GHRH), goldfish PRPsalmon-like (previously gfGHRHsalmon-like) (24), and goldfish PRPcatfish-like (previously gfGHRHcatfish-like) (24) were synthesized by the Rockefeller University.…”

High-Level Expression, Purification, and Characterization of the Recombinant Grass Carp Pituitary Adenylate Cyclase-Activating Polypeptide

“…[3][4][5] In teleost fish, PACAP immunoreactivity can be detected in different parts of the pituitary. In goldfish, PACAP fibers are present in the pars distalis and neurointermediate lobe of the pituitary, 6) and more recently, nerve fibers with PACAP immunoreactivity were found to overlap with the distribution of somatotrophs in the grass carp pituitary. 7) The presence of a PACAP neuronal system within the pituitary is consistent with findings that PACAP can act as a potent secretagog for GH secretion in salmon, 8) goldfish, 6) European eel, 9) common carp, 10) and grass carp.…”

“…In goldfish, PACAP fibers are present in the pars distalis and neurointermediate lobe of the pituitary, 6) and more recently, nerve fibers with PACAP immunoreactivity were found to overlap with the distribution of somatotrophs in the grass carp pituitary. 7) The presence of a PACAP neuronal system within the pituitary is consistent with findings that PACAP can act as a potent secretagog for GH secretion in salmon, 8) goldfish, 6) European eel, 9) common carp, 10) and grass carp. 7) Notably in goldfish, both PACAP 27 and PACAP 38 were effective in stimulating GH secretion through activation of the adenylate cyclase-cAMP-protein kinase A and phospholipase C-IP3-protein kinase C pathways in cultured pituitary cells.…”

“…7) Notably in goldfish, both PACAP 27 and PACAP 38 were effective in stimulating GH secretion through activation of the adenylate cyclase-cAMP-protein kinase A and phospholipase C-IP3-protein kinase C pathways in cultured pituitary cells. 6) In another teleost grass carp, two forms of ovine PACAP not only trigger GH release but also enhance GH synthesis at the pituitary cell level. 7) In the same animal model, a full-length cDNA of PACAP has been cloned and confirmed to be a singlecopy gene in the genome.…”