Akasu T, Koketsu K
J Physiol. 1986 Nov;380:93-109. doi: 10.1113/jphysiol.1986.sp016274.
The post-synaptic effects of 5-hydroxytryptamine (5-HT) were examined in neurones of bull-frog sympathetic ganglia with intracellular micro-electrode and voltage-clamp recording techniques. Atropine (1 microM) was used to block the muscarinic cholinoceptors. 5-HT reduced the amplitude of the fast excitatory post-synaptic potential (fast e.p.s.p.). 5-HT also reduced the mean amplitude of the miniature excitatory post-synaptic potentials (m.e.p.s.p.s) without affecting their frequency. Voltage-clamp studies showed that 5-HT decreased in a dose-dependent manner the amplitude of the acetylcholine (ACh) current produced by ionophoretic application of ACh to sympathetic neurones. The relationship between the log of the ACh dose, applied ionophoretically, and the peak ACh current (the dose-response curve) was examined in voltage-clamped neurones. 5-HT caused a parallel shift to the right of the dose-response curve for ACh. Analysis using a double reciprocal plot (Lineweaver-Burk plot) revealed that 5-HT increased the apparent dissociation constant (Km) of ACh for the receptor without changing the maximum ACh current (Vmax), suggesting a competitive antagonism. The relationship between the 5-HT dose and the magnitude of inhibition of the ACh current was obtained using two different amplitudes for the ACh response. The dose-response curve of 5-HT-induced inhibition using a relatively high amplitude ACh current, S1, was parallel with that for a relatively low amplitude ACh current, S2. The Dixon plot of these two curves yielded an apparent inhibition constant (Ki) of 42 microM. Both fast excitatory post-synaptic currents (fast e.p.s.c.s) and miniature excitatory post-synaptic currents (m.e.p.s.c.s) had single-exponential decay time courses. The time constants of fast e.p.s.c. decay (tau e) and m.e.p.s.c. decay (tau m) were not altered by 5-HT, suggesting that 5-HT does not change the kinetics of opening and closing of the ionic channel associated with the nicotinic receptor. 5-HT did not alter the reversal potential of the fast e.p.s.c. These results suggest that 5-HT decreases the sensitivity of the nicotinic receptor of sympathetic neurones, by interfering with ACh binding at the active site on the receptor-ionic-channel complex. 5-HT may physiologically inhibit cholinergic transmission as it is an endogenous substance which antagonizes the nicotinic receptor in post-ganglionic neurones of bull-frog sympathetic ganglia.
采用细胞内微电极和电压钳记录技术，在牛蛙交感神经节神经元中研究了5-羟色胺（5-HT）的突触后效应。使用阿托品（1微摩尔）阻断毒蕈碱型胆碱能受体。5-HT降低了快速兴奋性突触后电位（快速e.p.s.p.）的幅度。5-HT还降低了微小兴奋性突触后电位（m.e.p.s.p.s）的平均幅度，而不影响其频率。电压钳研究表明，5-HT以剂量依赖性方式降低了通过离子电泳向交感神经元施加乙酰胆碱（ACh）所产生的ACh电流的幅度。在电压钳制的神经元中，研究了离子电泳施加的ACh剂量的对数与ACh峰值电流之间的关系（剂量-反应曲线）。5-HT导致ACh的剂量-反应曲线向右平行移动。使用双倒数图（Lineweaver-Burk图）分析表明，5-HT增加了ACh与受体的表观解离常数（Km），而不改变最大ACh电流（Vmax），提示为竞争性拮抗作用。使用两种不同幅度的ACh反应获得了5-HT剂量与ACh电流抑制幅度之间的关系。使用相对高幅度的ACh电流S1时，5-HT诱导抑制的剂量-反应曲线与使用相对低幅度的ACh电流S2时的曲线平行。这两条曲线的Dixon图得出表观抑制常数（Ki）为42微摩尔。快速兴奋性突触后电流（快速e.p.s.c.s）和微小兴奋性突触后电流（m.e.p.s.c.s）均具有单指数衰减时间过程。5-HT未改变快速e.p.s.c.衰减的时间常数（tau e）和m.e.p.s.c.衰减的时间常数（tau m），提示5-HT不改变与烟碱型受体相关的离子通道的开闭动力学。5-HT未改变快速e.p.s.c.的反转电位。这些结果表明，5-HT通过干扰ACh在受体-离子通道复合物活性位点的结合，降低了交感神经元烟碱型受体的敏感性。5-HT可能在生理上抑制胆碱能传递，因为它是一种内源性物质，可拮抗牛蛙交感神经节后神经元中的烟碱型受体。
