Markus R P, Zago W M, Carneiro R C
Department of Pharmacology, Instituto de Ciéncias Biomédicas, Universidade de Säo Paulo, Brazil.
J Pharmacol Exp Ther. 1996 Oct;279(1):18-22.
Melatonin, the pineal hormone produced during the dark phase of the light-dark cycle, modulates neuronal acetylcholine receptors located presynaptically on the sympathetic nerve terminals of the rat vas deferens. The pD2 values for nicotine were significantly higher at night (4.20 +/- 0.01) than in the afternoon (3.80 +/- 0.07). Exogenous melatonin shifted the concentration-response curves for nicotine to the left, mimicking the effect of darkness. Melatonin modifies both the displacement and the saturation curves of 3Hnicotine binding. In membranes from animals killed at 15:00 h, the binding of 3Hnicotine (5-6 nM) was first potentiated and then inhibited by sequential concentrations of (-)nicotine. Higher concentrations of 3Hnicotine (50-60 nM) were displaced by all concentrations of nonlabeled ligand. However, when membranes from tissues exposed to melatonin (exogenous or endogenous) were tested, the lower 3Hnicotine concentration was displaced progressively by increasing concentrations of nonlabeled ligand. Equilibrium binding studies show a single class of high-affinity nicotinic binding sites with an apparent Kd value of 16 nM and an average maximal number of binding sites of 66 fmol mg-1 protein when animals 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 (Kd = 36.7 nM; Bmax = 185.4 fmol/mg). Melatonin does not modify the functional response and the displacement of 3Hnicotine by dimethylphenylpiperazinium. The data suggest that nicotinic neuronal acetylcholine receptors stimulated by dimethylphenylpiperazinium do not change between the light and dark phases. Rather, the higher sensitivity to nicotine in prostatic portions incubated with exogenous melatonin, and in organs from animals killed at night, after the rise of endogenous melatonin, is probably due to the appearance of low-affinity neuronal nicotinic ACh binding sites.
褪黑素是在昼夜明暗循环的黑暗阶段由松果体分泌的一种激素，它可调节大鼠输精管交感神经末梢突触前的神经元乙酰胆碱受体。尼古丁的pD2值在夜间（4.20±0.01）显著高于下午（3.80±0.07）。外源性褪黑素使尼古丁的浓度-反应曲线左移，模拟了黑暗的作用。褪黑素改变了[3H]（-）尼古丁结合的置换曲线和饱和曲线。在15:00处死的动物的膜中，[3H]（-）尼古丁（5-6 nM）的结合先增强后被不同浓度的（-）尼古丁抑制。所有浓度的非标记配体均可置换较高浓度的[3H]（-）尼古丁（50-60 nM）。然而，当检测暴露于褪黑素（外源性或内源性）的组织的膜时，较低浓度的[3H]（-）尼古丁会被逐渐增加浓度的非标记配体置换。平衡结合研究表明，下午处死动物时，存在一类单一的高亲和力烟碱结合位点，其表观解离常数（Kd）值为16 nM，平均最大结合位点数为66 fmol/mg蛋白质。褪黑素虽然没有改变高亲和力结合位点的性质，但诱导出现了第二类表观亲和力较低（Kd = 36.7 nM；Bmax = 185.4 fmol/mg）的结合位点。褪黑素不改变二甲基苯基哌嗪对[3H]（-）尼古丁的功能反应和置换作用。数据表明，二甲基苯基哌嗪刺激的烟碱型神经元乙酰胆碱受体在明暗阶段之间没有变化。相反，在外源性褪黑素孵育的前列腺部分以及内源性褪黑素升高后夜间处死动物的器官中，对尼古丁的较高敏感性可能是由于低亲和力神经元烟碱型乙酰胆碱结合位点的出现。
