PITUITARY ADENYLATE-CYCLASE ACTIVATING PEPTIDE-38 (PACAP-38), PACAP-27, AND PACAP RELATED PEPTIDE (PRP) IN THE RAT MEDIAN-EMINENCE AND PITUITARY

Pituitary adenylate cyclase activating peptide (PACAP) is a member of the vasoactive intestinal peptide-like peptide family. It is found in the hypothalamus, where the PACAP precursor is processed to form PACAP -38, the C-terminal truncated PACAP-27 and PACAP related peptide (PRP) . Both PACAPs are potent stimulators of anterior pituitary adenylate c yclase activity, but the physiologically relevant anatomical sources o f PACAP and possible importance of PRP in this regard are poorly under stood, Using immunocytochemistry with epitope-specific antisera, we no w show that PACAP38-, PACAP27- and PRP-positive nerve fibres are ail p resent in the rat median eminence, The major immunoreactive species pr esent was PACAP38. Numerous PACAP38-immmunoreactive nerve fibres were observed in the internal layer and a few were present in the posterior pituitary lobe, The external layer of the median eminence contained a few PACAP-38-immunoreactive fibres and PACAP-38-positive nerve termin als were rarely seen in the perivascular portal spaces. Surprisingly, delicate PACAP-38-positive nerve fibres were identified in the anterio r pituitary lobe intermingled between the pituitary cells although non e of the secretory pituitary cells contained immunoreactive PACAP38, P ACAP27 or PRP and preproPACAP mRNA was not detected in the gland by No rthern blotting or in situ hybridization. PACAP-27- and PRP-immunoreac tive nerve fibres and terminals were found in the same locations as PA CAP-38 although generally in lower numbers, Specific radioimmunoassays and HPLC revealed that PACAP-38 accounts for the vast majority of the adenohypophyseal PACAP-immunoreactivity, whereas PACAP-27 and PRP wer e found in low to undetectable concentrations. In primary cultures of rat pituitary cells and in the clonal gonadotrope-derived aT3-1 cell l ine, PACAP-27 and PACAP-38 were equipotent stimulators of cAMP accumul ation, whereas PRP Was ineffective. We conclude that the distribution of PACAP-immunoreactive nerve fibres in the hypothalamus of the adult male rat is not that expected for a classic releasing factor suggestin g that other sources of PACAP are relevant for stimulation of anterior pituitary cells or that the hypothalamic PACAP system is activated un der specific endocrine or developmental conditions.