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JF Margiotta and D Pardi
Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, New York 10029, USA.
Nicotinic acetylcholine (ACh) receptors (AChRs) on ciliary ganglion neurons are positively regulated by elevated cAMP levels. Vasoactive intestinal peptide (VIP) can act as a first messenger in the regulation, because application of 1 microM VIP rapidly increases both neuronal cAMP levels and ACh sensitivity. We now report that high affinity receptors for a close VIP relative, pituitary adenylate cyclase-activating polypeptide (PACAP), are present on ciliary ganglion neurons and mediate the cAMP-dependent modulation of AChRs. Consistent with the presence of PACAP type I receptors, binding studies revealed sites on the neurons having approximately 1000-fold higher affinity for the 38- and 27-amino acid forms of PACAP than for VIP, and cAMP radio- immunoassays demonstrated that PACAP38 and PACAP27 are approximately 600-fold more potent agonists for mobilizing neuronal cAMP than is VIP. In accord with their higher affinity and potency, PACAP38 and -27 (both at 10 nM) increased neuronal ACh sensitivity by approximately 50% within 10 min, whereas VIP at the same low concentration was ineffective. The increased ACh sensitivity induced by 10 nM PACAP38 or PACAP27 or 1 microM VIP depends on coincident increases in cAMP levels, because treatment of neurons with adenylate cyclase inhibitors blocked both effects. The findings demonstrate the presence of functional PACAP type I receptors on ciliary ganglion neurons that preferentially recognize PACAP38 and -27 over VIP and act via adenylate cyclase to initiate cAMP-dependent enhancement of AChR function. Finally, we detected PACAP38-like material in ciliary ganglia, suggesting a role for the peptide in modulating neuronal AChRs in vivo.
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