Distribution, Characterization, and Growth Hormone- Releasing Activity of Pituitary Adenylate Cyclase- Activating Polypeptide in the European Eel, Anguilla anguilla*

International audienceThe complementary DNA encoding pituitary adenylate cyclase-activating polypeptide (PACAP) has been cloned from two species of teleost fishes, the Sockeye salmon and the Thai catfish, and the amino acid sequence of PACAP has been determined in another teleost, the stargazer. However, to date, the detailed distribution of PACAP im-munoreactivity has never been investigated in the fish brain. In the present study, we have determined the localization of PACAP-im-munoreactive neurons in the central nervous system of a primitive teleost fish, the European eel Anguilla anguilla, using an antiserum raised against PACAP27. PACAP-positive perikarya were exclusively observed in the diencephalon, i.e. in the preoptic nucleus of the hy-pothalamus and in the dorsal and ventral nuclei of the thalamus. PACAP-immunoreactive fibers were detected in various areas of the brain, notably in the ventral telencephalon, the diencephalon, the mesencephalon, the cerebellar valvula, and the medulla oblongata. In addition, a dense accumulation of PACAP-containing nerve terminals was found in the pars distalis of the pituitary. The PACAP-like im-munoreactivity contained in the eel brain was characterized by HPLC analysis combined with RIA quantification. The major form of PACAP-immunoreactive material coeluted with mammalian PACAP38. Molecular cloning of the PACAP precursor has previously shown that in fish, PACAP and GH-releasing hormone (GHRH) originate from the same precursor. We have thus investigated the effects of PACAP and GHRH on GH secretion from eel pituitary cells in primary culture. Dose-response experiments revealed that PACAP27 and PACAP38 possessed the same efficacy, but PACAP38 was 12 times more potent than PACAP27 in stimulating GH release (ED 50 4.3 10 10 and 3.5 10 9 M, respectively). In contrast, GHRH, even at a high concentration (10 6 M), had no effect on GH release. Taken together, these data indicate that in the eel, PACAP may play a significant role in the regulation of somatotrope cells: 1) PACAP-immunoreactive neurons are exclusively located in the diencephalon and send numerous projections in the pars distalis; and 2) PACAP, but not GHRH, dose dependently stimulates GH secretion from cultured eel pituitary cells. (Endocrinology 139: 4300 – 4310, 1998) P ITUITARY adenylate cyclase-activating polypeptide (PACAP) was first isolated from the ovine hypothal-amus on the basis of its ability to stimulate adenylyl cyclase activity in rat pituitary cells (1). The peptide was found to exist in two amidated forms with 38 (PACAP38) and 27 (PACAP27) amino acid residues (2). The N-terminal portion of PACAP shows 68% identity with vasoactive intestinal polypeptide (VIP), identifying PACAP as a member of the VIP/secretin/glucagon/GH-releasing hormone (GHRH) superfamily. The sequence of PACAP has been highly conserved during evolution from protochordates to mammals. In particular, the primary structure of PACAP38 is identical in all mammalian species studied to date (3), whereas tuni-cate, fish, amphibian, and avian PACAP27 exhibit more than 95% sequence similarity with their mammalian counterpart (4 –9) (Table 1). The distribution of PACAP has been investigated in the central nervous system of human and monkey (10), sheep (11), rat (12), frog (13), and newt (14). In both mammals and amphibians, PACAP-immunoreactive neurons are particularly abundant in hypothalamic hypophysiotropic nuclei, and a dense network of PACAP-containing fibers innervates the external zone of the median eminence. Concurrently, functional studies have shown that PACAP stimulates the secretory activity of adenohypophysial cells (15) and increases the intracellular calcium concentration in various types of pituitary cells in mammals and amphibians (16, 17)