NEUROPEPTIDE-Y INHIBITS (3)[H]NORADRENALINE RELEASE IN THE RAT VAS-DEFERENS INDEPENDENTLY OF CAMP LEVELS

The purpose of the present investigation was to ascertain the function al significance of the reduction in cyclic AMP (cAMP) levels in the in hibitory action of neuropeptide Y (NPY) on [H-3]noradrenaline ([H-3]NA ) release, as well as to further characterize the subtype(s) of NPY re ceptors involved in the peptide's actions in the rat vas deferens. We studied the effects of NPY, carboxyterminal fragments of this peptide and the NPY analog [Leu(31),Pro(34)]-NPY on three functional responses , namely, the release of [H-3]NA and the associated muscle contraction s evoked by electrical stimulation, and the accumulation of cAMP stimu lated by forskolin. NPY, a known inhibitor of the electrically-evoked [H-3]NA release and neurogenic contractions is also a potent inhibitor of the forskolin-stimulated cAMP synthesis in the prostatic portion o f the rat vas deferens. However, the ability of NPY to inhibit cAMP ac cumulation is lost upon tissue denervation, suggesting that this is li kely to be a prejunctional effect. Elevation of cAMP levels by the use of the cell permeant analog of cAMP, 8-(p-chlorophenylthio)-cAMP (8pC PTcAMP) increases the electrically-evoked release of [H-3]NA. However, the inhibition of [H-3]NA release by NPY is not prevented by 8pCPTcAM P. Structure-activity relationship studies reveal that NPY and related peptides inhibit the release of [H-3]NA, the muscle contractions and the synthesis of cAMP with a similar pharmacological profile. NPY is t he most potent inhibitory agent, whereas [Leu(31),Pro(34)]-NPY and NPY 13-36, the respective Y-1 and Y-2 selective agonists, display similar potencies to inhibit the three responses. It is concluded that NPY inh ibits neurotransmission in the rat vas deferens through the activation of a peptide receptor different from the known NPY-Y-1 or NPY-Y-2 rec eptor subtypes. NPY receptor activation in the vas deferens is negativ ely coupled to adenylyl cyclase activity. This intracellular signallin g pathway is, however, not likely to mediate the peptide effects on th e prejunctional regulation of noradrenaline release.