MELATONIN MODULATION OF PRESYNAPTIC NICOTINIC ACETYLCHOLINE-RECEPTORSIN THE RAT VAS-DEFERENS

Melatonin, the pineal hormone produced during the dark phase of the li ght-dark cycle, modulates neuronal acetycholine receptors located pres ynaptically on the sympathetic nerve terminals of the rat vas deferens . The pD(2) values for nicotine were significantly higher at night (4. 20 +/- 0.01) than in the afternoon (3.80 +/- 0.07). Exogenous melatoni n shifted the concentration-response curves for nicotine to the left, mimicking the effect of darkness. Melatonin modifies both the displace ment and the saturation curves of [H-3](-)nicotine binding. In membran es from animals killed at 15:00 h, the binding of [H-3](-)nicotine (5- 6 nM) was first potentiated and then inhibited by sequential concentra tions of (-)nicotine. Higher concentrations of [H-3](-)nicotine (50-60 nM) were displaced by all concentrations of nonlabeled ligand. Howeve r, when membranes from tissues exposed to melatonin (exogenous or endo genous) were tested, the lower [H-3](-)nicotine concentration was disp laced progressively by increasing concentrations of nonlabeled ligand. Equilibrium binding studies show a single class of high-affinity nico tinic binding sites with an apparent K-d value of 16 nM and an average maximal number of binding sites of 66 fmol mg(-1) protein when animal s were killed at the afternoon. Melatonin, although it did not change the properties of high-affinity binding sites, induced the appearance of a second population of lower apparent affinity (K-d = 36.7 nM;B-max = 185.4 fmol/ mg). Melatonin does not modify the functional response and the displacement of [H-3](-)nicotine by dimethylphenylpiperazinium . The data suggest that nicotinic neuronal acetylcholine receptors sti mulated by dimethylphenylpiperazinium do not change between the light and dark phases. Rather, the higher sensitivity to nicotine in prostat ic portions incubated with exogenous melatonin, and in organs from ani mals killed at night, after the rise of endogenous melatonin, is proba bly due to the appearance of low-affinity neuronal nicotinic ACh bindi ng sites.