Pituitary adenylate cyclase-activating polypeptide may function as a neuromodulator in guinea-pig adrenal medulla

1. The role of pituitary adenylate cyclase-activating polypeptide (PACAP) i n catecholamine secretion from dissociated adrenal chromaffin cells of the guinea-pig was investigated using amperometry, the patch clamp technique an d immunochemistry. 2. Pretreatment of adrenal chromaffin cells with 0.3-10 nM PACAP for 2 min resulted in enhancement of nicotine- and muscarine-induced secretions in ei ther the presence of external Ca2+ ions or nominally Ca2+-free solution, wi th no change in basal secretion or the holding current at -60 mV in most of the cells tested. 3. Pretreatment with PACAP augmented the muscarine-induced non-selective ca tion current, but did not affect the muscarine-induced outward current or n icotine-induced current. 4. PACAP-induced enhancement of nicotine- and muscarine-induced secretions was suppressed by the simultaneous application of PACAP and the protein kin ase inhibitors 100 muM HA1004 or 2 muM H89. 5. Application of forskolin enhanced both muscarine- and nicotine-induced s ecretions, whereas application of a phorbol ester augmented the nicotine-in duced secretion, but suppressed the muscarine-induced secretion in a revers ible manner. 6. Immunohistochemical analysis of adrenal medullae revealed that PACAP-lik e immunoreactivity was present in nerve fibres surrounding putative chromaf fin cells. PAC1R-like immunoreactivity was distributed diffusely in the pla sma membrane, whereas nicotinic ACh receptor-like immunoreactivity was conc entrated at the plasma membrane near the nucleus, where the synapses were m ainly localized. 7. These observations suggest that PACAP in the guinea-pig adrenal medulla functions as a neuromodulator to facilitate ACh-induced secretion through a cAMP-protein kinase A dependent pathway.