Parnas I, Rashkovan G, Ong J, Kerr D I
The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, Department of Neurobiology, The Hebrew University, Jerusalem, Israel.
J Neurophysiol. 1999 Mar;81(3):1184-91. doi: 10.1152/jn.1999.81.3.1184.
Release of excitatory transmitter from boutons on crayfish nerve terminals was inhibited by (R,S)-baclofen, an agonist at GABAB receptors. Baclofen had no postsynaptic actions as it reduced quantal content without affecting quantal amplitude. The effect of baclofen increased with concentration producing 18% inhibition at 10 microM; EC50, 50% inhibition at 30 microM; maximal inhibition, 85% at 100 microM and higher. There was no desensitization, even with 200 or 320 microM baclofen. Phaclofen, an antagonist at GABAB receptors, competitively antagonized the inhibitory action of baclofen (KD = 50 microM, equivalent to a pA2 = 4.3 +/- 0.1). Phaclofen on its own at concentrations below 200 microM had no effect on release, whereas at 200 microM phaclofen itself increased the control level of release by 60%, as did 2-hydroxy-saclofen (200 microM), another antagonist at GABAB receptors. This increase was evidently due to antagonism of a persistent level of GABA in the synaptic cleft, since the effect was abolished by destruction of the presynaptic inhibitory fiber, using intra-axonal pronase. We conclude that presynaptic GABAB receptors, with a pharmacological profile similar to that of mammalian GABAB receptors, are involved in the control of transmitter release at the crayfish neuromuscular junction.
(R,S)-巴氯芬是一种GABAB受体激动剂，它能抑制小龙虾神经末梢上终扣释放兴奋性递质。巴氯芬没有突触后作用，因为它降低了量子含量，而不影响量子幅度。巴氯芬的作用随浓度增加而增强，在10微摩尔时产生18%的抑制作用；半数有效浓度（EC50）为30微摩尔时产生50%的抑制作用；最大抑制作用在100微摩尔及更高浓度时为85%。即使使用200或320微摩尔的巴氯芬，也没有脱敏现象。GABAB受体拮抗剂法氯芬竞争性拮抗巴氯芬的抑制作用（解离常数KD = 50微摩尔，相当于亲和力常数pA2 = 4.3 ± 0.1）。浓度低于200微摩尔的法氯芬本身对释放没有影响，而在200微摩尔时，法氯芬本身使释放的对照水平增加了60%，GABAB受体的另一种拮抗剂2-羟基-巴氯芬（200微摩尔）也是如此。这种增加显然是由于对突触间隙中持续存在的γ-氨基丁酸（GABA）水平的拮抗作用，因为使用轴突内链霉蛋白酶破坏突触前抑制纤维后，这种作用就消失了。我们得出结论，小龙虾神经肌肉接头处的递质释放控制涉及到突触前GABAB受体，其药理学特性与哺乳动物的GABAB受体相似。
