PACAP-induced formation of cyclic AMP in the chicken brain: regional variations and the effect of melatonin

We have studied the effects of pituitary adenylate cyclase-activating polyp eptide (PACAP(27) and PACAP(38)) on cyclic AMP formation in chick brain, an d the action of melatonin upon the PACAP-evoked effects. PACAP stimulated c yclic AMP production in the hypothalamus > cerebral cortex > pineal gland > optic lobes. In the hypothalamus and cerebral cortex, the rank-order of bo th PACAP forms and VIP in evoking the cyclic AMP response was: PACAP(38) ap proximate to PACAP(27) much greater than VIP, suggesting the presence in th e tested tissues of PAC, receptors. Melatonin suppressed (IC50 = 19.8 nM) t he PACAP(27) (0.1 mu M)-induced cyclic AMP response in the hypothalamus, bu t not in the cerebral cortex. Melatonin also suppressed the hypothalamal cy clic AMP synthesis stimulated by forskolin, but not that evoked by histamin e or isoprenaline. Our observations show that PACAP is capable of potently stimulating cyclic AMP formation in some regions of the chick brain, and su ggest that the hypothalamus may be a site for a functional interaction betw een PACAP and the pineal hormone melatonin. (C) 1999 Elsevier Science B.V. All rights reserved.