ACh recognition and modulation of the inhibitory nicotinic ACh receptor

海马胆碱能神经元的放电在合颌运动神经元中产生ipsp，在合颌肌纤维中产生ijp。我们先前证明产生这些抑制反应的ACh受体可能是烟碱性的，肌肉受体可能与Cl^-通道有关。已知Lophotoxin和三种类似物（bippinatin-A，B，C）在受体的ACh结合位点与Tyr^<190>共价反应并抑制其功能。ACh结合位点的双分子反应常数按bippinatin-B>A>C的顺序递减。本研究探讨了这些类似物对ijp或ipsp的影响，并将其与对兴奋性反应的影响进行了比较，以了解抑制性乙酰胆碱受体对乙酰胆碱的识别。Bippinatin-B在很大程度上减少了ijp的大小，而bippinatin-A和-C则增加了ijp的大小。JC神经元的电压钳实验表明，当Cl^-注入JC神经元时，突触电流反应的逆转电位向更正的方向移动，这表明JC神经元中的ACh受体也可能与Cl^-通道有关。Bippinatin-B显著降低ipsp的大小，而bippinatin-A最初增加ipsp的大小（约25%），后来又降低ipsp的大小。bippinatin-C的作用更为复杂。最初，bippinatin-C显著增加IPSP大小（约200%），然后降低。在此之后，MA的放电倾向于产生较大的去极化反应，最终所有反应停止。这些结果表明，抑制性受体的ACh结合位点可能与兴奋性受体的ACh结合位点相似，但结合位点周围的精细分子结构可能不同。

Firing of Aplysia cholinergic MA neurons produces IPSPs in the jaw-closing (JC) motor neurons and IJPs in the JC muscle fibers. We previously demonstrated that the ACh receptors producing these inhibitory responses may be nicotinic, and the muscle receptors may be associated with Cl^- channel. Lophotoxin and the three analogues (bippinatin-A,B,C)are known to react covalently with Tyr^<190> in the ACh binding site of the receptor and inhibit the function. Bimolecular reaction constants decreased in the order bippinatin-B>A>C for the ACh binding site. In the present study the effects of these analogues on the IJPs or IPSPs were explored and compared with those on the excitatory responses to know the ACh recognition of the inhibitory ACh receptors. Bippinatin-B largely reduced the size of the IJPs, while bippinatin-A and -C increased it. The voltage clamp experiments of the JC neurons showed that the reversal potential of the synaptic current response was shifted toward more positive when the Cl^- was imjected into the JC neurons, suggesting that the ACh receptor in the JC neurons also may be associated with Cl^- channel. Bippinatin-B largely reduced the size of the IPSPs while bippinatin-A initially increased it (ca.25%) and reduced it later. The effect of bippinatin-C was more complex. Initially bippinatin-C largely increased the IPSP size (ca.200%) and then reduced it. After that the MA firing tended to produce the large depolarizing response and finally all responses ceased. These results suggest that the ACh binding site of the inhibitory receptor may be similar to that of the excitatory ACh receptor but the fine molecular structure around the binding site may be different between them.