Pituitary adenylate cyclase-activating polypeptide, vasoactive intestinal polypeptide and their receptors: distribution and involvement in the secretion of Podarcis sicula adrenal gland

Abstract

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are regulatory neuropeptides of the hypothalamus-hypophyseal-adrenal axis, acting via the common receptors VPAC(1) and VPAC(2) and the selective PACAP receptor PAC(1). In the adrenal glands of the Italian wall lizard, Podarcis sicula, the presence of VIP in chromaffin cells, and the VIP-stimulated release of catecholamine and aldosterone in vivo, was previously shown. To examine the localization of both peptides and receptors and their mRNAs in the adrenal gland of P. sicula, immunohistochemistry and in situ hybridization were performed: PACAP and its mRNA were detected in chromaffin cells, VPAC(1) was found associated with steroidogenic tissue, VPAC(2) and PAC(1) with chromaffin tissue. Using 'far western blot' technique, we showed the presence of specific binding sites for VIP/PACAP in the adrenal glands of the lizard. The effects of both VIP and PACAP on the adrenal cells of the lizard were examined in vitro in adrenal cell co-cultures: both VIP and PACAP enhanced catecholamine, corticosterone and aldosterone release from adrenal cell co-culture in a time- and dose-dependent manner. The catecholamine release was inhibited by PAC(1) antagonist and in VPAC(2) immunoneutralized adrenal cells. The effects of VIP and PACAP on aldosterone secretion were counteracted by VPAC(1) antagonist administration in vitro. Corticosterone secretion elicited by VIP was not blocked by VPAC(1) antagonist, while the PACAP-induced release of corticosterone was blocked by the antagonist. Overall, our investigations indicate that these neuropeptides of the secretin superfamily can act not only as neurotransmitters but also as autocrine and paracrine regulators on chromaffin and cortical cells, being important mediators of the non-cholinergic system in the lizard adrenal gland.