Pituitary adenylate cyclase-activating polypeptide: control of rat pineal cyclic AMP and melatonin but not cyclic GMP.

In this study, the effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on cyclic nucleotide accumulation and melatonin (MT) production in dispersed rat pinealocytes were measured. Treatment with PACAP (10(-7) M) increased MT production 2.5-fold. PACAP (10(-7) M) also increased cyclic AMP accumulation four- to fivefold; this effect was potentiated two- to three-fold by alpha 1-adrenergic activation. This potentiation appears to involve protein kinase C (PKC) because alpha 1-adrenergic activation is known to translocate PKC and the PACAP-stimulated cyclic AMP accumulation was potentiated ninefold by a PKC activator, 4 beta-phorbol 12-myristate 13-acetate (PMA). Phenylephrine and PMA also potentiated the PACAP-stimulated MT accumulation. These results indicate that cyclic AMP is one second messenger of PACAP in the pineal gland and that the effects of PACAP on cyclic AMP and MT production can be potentiated by an alpha 1-adrenergic-->PKC mechanism. In addition to these findings, it was observed that PACAP treatment with or without phenylephrine or PMA did not alter cyclic GMP accumulation. This indicates that PACAP is the first ligand identified that increases cyclic AMP accumulation in the pineal gland without increasing cyclic GMP accumulation. That PACAP fails to activate the vasoactive intestinal peptide/cyclic GMP pathway suggests that the vasoactive intestinal peptide receptors present in the pineal may be distinct from the type II PACAP receptors.