Search

Under conditions in which vasoactive intestinal peptide (VIP) induces somatostatin release from cortical and diencephalic neuronal cultures, VIP causes large increases in intracellular cyclic AMP. Both the release of somatostatin and the increase in cyclic AMP elicited by VIP require exogenous calcium, can be blocked by cobalt ion, and can be qualitatively mimicked by depolarizating concentrations of exogenous potassium ion. Direct activation of adenylate cyclase by forskolin causes large increases in cyclic AMP content but does not induce somatostatin release. In the absence of VIP, the calcium ionophore, ionomycin, and the phorbol ester, phorbol 12-myristate-13-acetate, also stimulate somatostatin release. These results indicate that VIP-stimulation of cyclic AMP formation and VIP-stimulation of somatostatin release are calcium-dependent and that the two phenomena are dissociatable. Cyclic AMP formation is not a necessary condition for VIP-induced somatostatin release. Nucleotide formation may be a sufficient condition for release or, possibly in association with calcium influx, it may be an event unrelated to the release process.

The effects of the active phorbol ester 12-myristate, 13-acetate (PMA), the inactive ester 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), and the synthetic diacylglycerol 1-oleoyl-2-acetyl-glycerol (OAG) on cyclic AMP production were examined in rat cerebral cortical and diencephalic cells. With the aid of a prelabeling technique for measuring cyclic AMP accumulation in the cells, it was found that neither PMA nor OAG significantly increased cyclic AMP formation in either type of cell. In contrast, PMA enhanced the cyclic AMP response to vasoactive intestinal peptide (VIP) and forskolin in cerebral cortical and diencephalic cells, whereas 4 alpha-PDD was inactive. A 15-min preincubation was used to obtain maximal enhancement. The concentration dependence of PMA on VIP-stimulated cyclic AMP accumulation was determined in cortical cells (EC50 = 6.2 x 10(-8) M). OAG was also able to potentiate VIP-induced cyclic AMP formation in cortical and diencephalic cells. However, its potentiating effect was weaker than that observed with PMA treatment. The data show, at an early stage of development (primary cultures, 8-10 days), a modulation of VIP- or forskolin-cyclic AMP response by the activators of protein kinase C, i.e., PMA and OAG, in two different structures of the central nervous system: the cerebral cortex and the diencephalon. To our knowledge, this is the first demonstration of such a potentiation within the diencephalon.