Melatonin modulation of presynaptic nicotinic acetylcholine receptors located on short noradrenergic neurons of the rat vas deferens: a pharmacological characterization

Abstract

Melatonin, the pineal hormone produced during the dark phase of the light-dark cycle, modulates neuronal acetylcholine receptors located presynaptically on nerve terminals of the rat vas deferens. Recently we showed the presence of high affinity nicotine-binding sites during the light phase, and low and high affinity binding sites during the dark phase. The appearance of the low affinity binding sites was due to the nocturnal melatonin surge and could be mimicked by exposure to melatonin in vitro. The aim of the present research was to identify the receptor subtypes responsible for the functional response during the light and the dark phase. The rank order of potency of agonists was dimethylphenylpiperazinium (DMPP) = cytisine > nicotine > carbachol and DMPP = nicotine = cytisine > carbachol, during the light and dark phase, respectively, due to an increase in apparent affinity for nicotine. Mecamylamine similarly blocked the DMPP response during the light and the dark phase, while the response to nicotine was more efficiently blocked during the light phase. In contrast, methyllycaconitine inhibited the nicotine-induced response only at 21:00 h.Since a7 nicotinic acetylcholine receptors (nAChRs) have low affinity for nicotine in binding assays, we suggest that a mixed population composed of a3ß4 -plus a7-bearing nAChR subtypes is present at night. This plasticity in receptor subtypes is probably driven by melatonin since nicotine-induced contraction in organs from animals sacrificed at 15:00 h and incubated with melatonin (100 pg/ml, 4 h) is not totally blocked by mecamylamine. Thus melatonin, by acting directly on the short adrenergic neurons that innervate the rat vas deferens, induces the appearance of the low affinity binding site, probably an a7 nAChR subtype.

Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells

“…The data obtained with the embryo retinal cells in culture are somewhat similar to those observed in rat sympathetic nerve terminals and in the cerebellum (11,12). In all cases melatonin only interferes with nAChRs sensitive to α-bungarotoxin.…”

“…Therefore, it was not a surprise that the reduction in the number of α-bungarotoxin binding sites was accompanied by the appearance of the functional response between DIV4 and DIV5 retinal cultures. Melatonin has been shown to modulate the expression and the activity of α7 nAChRs in sympathetic nerve terminals of rat vas deferens (10,12,13) and cerebellum (11). The release of noradrenaline in the vas deferens or of glutamate in the cerebellum is increased by melatonin.…”

“…Acetylcholine-induced release of neurotransmitter from sympathetic nerve terminals of the vas deferens and cerebellum slices of the rat is higher in the dark phase than in the light phase of the day. In both cases the effect of acetylcholine is mediated by nAChRs sensitive to α-bungarotoxin (11,12). Melatonin, the hormone released by the pineal gland during the dark phase of the day, is responsible for the diurnal variation of nAChR-mediated responses.…”

Melatonin Nocturnal Surge Modulates Nicotinic Receptors and Nicotine-Induced [3H]Glutamate Release in Rat Cerebellum Slices

“…Therefore, the ␣7-nAChRs are down-regulated during the dark phase, when compared with the end of the light phase. This result was similar to that obtained in hypothalamus (Morley and Garner, 1990) but was different from the one obtained in peripheral tissue, where ␣7-nAChRs were shown to be up-regulated during the dark phase in preterminal noradrenergic neurons, which innervate the rat vas deferens (Markus et al, 1996;Zago and Markus, 1999). It is interesting to consider that even the clock genes present out-of-phase expression when different tissues are compared.…”

“…␣7-nicotinic receptors located in sympathetic nerve terminals (Markus et al, 1996;Zago and Markus, 1999). To evaluate whether these receptors, in the cerebellum, could be modified by environmental illumination, binding parameters (B max and K d ) of cerebellum membranes from rats killed 3 h before, or 6 h after, lights off were analyzed.…”