DISTRIBUTION, CHARACTERIZATION, AND GROWTH HORMONE-RELEASING ACTIVITYOF PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE IN THE EUROPEANEEL, ANGUILLA-ANGUILLA

The 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. Howev er, to date, the detailed distribution of PACAP immunoreactivity has n ever been investigated in the fish brain. In the present study, we hav e determined the localization of PACAP-immunoreactive neurons in the c entral nervous system of a primitive teleost fish, the European eel An guilla anguilla, using an antiserum raised against PACAP27. PACAP-posi tive perikarya were exclusively observed in the diencephalon, i.e. in the preoptic nucleus of the hypothalamus 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 medul la oblongata. In addition, a dense accumulation of PACAP-containing ne rve terminals was found in the pars distalis of the pituitary. The PAC AP-like immunoreactivity contained in the eel brain was characterized by HPLC analysis combined with RIA quantification. The major form of P ACAP-immunoreactive material coeluted with mammalian PACAP38. Molecula r cloning of the PACAP precursor has previously shown that in fish, PA CAP and GI-I-releasing hormone (GHRH) originate from the same precurso r. We have thus investigated the effects of PACAP and GHRH on GH secre tion from eel pituitary cells in primary culture. Dose-response experi ments revealed that PACAP27 and PACAP38 possessed the same efficacy, b ut. PACAP38 was 12 times more potent than PACAP27 in stimulating GH re lease (ED50 = 4.3 x 10(-10) and 3.5 x 10(-9) M, respectively). In cont rast, 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 dienceph alon and send numerous projections in the pars distalis; and 2)PACAP, but not GHRH, dose dependently stimulates GH secretion from cultured e el pituitary cells.