PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE TYPE-I RECEPTORS MEDIATE CYCLIC-AMP-DEPENDENT ENHANCEMENT OF NEURONAL ACETYLCHOLINE SENSITIVITY

Nicotinic acetylcholine (ACh) receptors (AChRs) on ciliary ganglion ne urons are positively regulated by elevated cAMP levels. Vasoactive int estinal peptide (VIP) can act as a first messenger in the regulation, because application of 1 mu M VIP rapidly increases both neuronal cAMP levels and ACh sensitivity. We now report that high affinity receptor s for a close VIP relative, pituitary adenylate cyclase-activating pol ypeptide (PACAP), are present on ciliary ganglion neurons and mediate the cAMP-dependent modulation of AChRs. Consistent with the presence o f PACAP type 1 receptors, binding studies revealed sites on the neuron s having approximate to 1000-fold higher affinity for the 38- and 27-a mino acid forms of PACAP than for VIP, and cAMP radioimmunoassays demo nstrated that PACAP38 and PACAP27 are approximate to 600-fold more pot ent agonists for mobilizing neuronal cAMP than is VIP. In accord with their higher affinity and potency, PACAP38 and -27 (both at 10 nM) inc reased neuronal ACh sensitivity by approximate to 50% within 10 min, w hereas VIP at the same low concentration was ineffective. The increase d ACh sensitivity induced by 10 nm PACAP38 or PACAP27 or 1 mu M VIP de pends on coincident increases in cAMP levels, because treatment of neu rons with adenylate cyclase inhibitors blocked both effects. The findi ngs demonstrate the presence of functional PACAP type I receptors on c iliary ganglion neurons that preferentially recognize PACAP38 and -27 over VIP and act via adenylate cyclase to initiate cAMP-dependent enha ncement of AChR function. Finally, we detected PACAP38-like material i n ciliary ganglia, suggesting a role for the peptide in modulating neu ronal AChRs in vivo.