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

M. Inoue, N. Fujishiro, K. Ogawa *, M. Muroi †, Y. Sakamoto, I. Imanaga and S. Shioda †

1. The role of pituitary adenylate cyclase-activating polypeptide (PACAP) in catecholamine secretion from dissociated adrenal chromaffin cells of the guinea-pig was investigated using amperometry, the patch clamp technique and 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 either the presence of external Ca²⁺ ions or nominally Ca²⁺-free solution, with 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 cation current, but did not affect the muscarine-induced outward current or nicotine-induced current.

4. PACAP-induced enhancement of nicotine- and muscarine-induced secretions was suppressed by the simultaneous application of PACAP and the protein kinase inhibitors 100 µM HA1004 or 2 µM H89.

5. Application of forskolin enhanced both muscarine- and nicotine-induced secretions, whereas application of a phorbol ester augmented the nicotine-induced secretion, but suppressed the muscarine-induced secretion in a reversible manner.

6. Immunohistochemical analysis of adrenal medullae revealed that PACAP-like immunoreactivity was present in nerve fibres surrounding putative chromaffin cells. PAC1R-like immunoreactivity was distributed diffusely in the plasma membrane, whereas nicotinic ACh receptor-like immunoreactivity was concentrated at the plasma membrane near the nucleus, where the synapses were mainly 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.

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