Galvez-Ruano E, Aprison M H, Robertson D H, Lipkowitz K B
Department of Psychiatry, Indiana University School of Medicine, Indianapolis, USA.
J Neurosci Res. 1995 Dec;42(5):666-73. doi: 10.1002/jnr.490420509.
Based on our molecular modeling investigations of the glycinergic receptor, we expanded our studies to similarly investigate the GABAergic receptor. New data suggest there may exist a slightly different agonistic mechanism for the molecules described herein as compared to glycine. The origin of this is undoubtedly the fact that, while glycine has a positive and two negative binding sites, it is significantly shorter than GABA and the other GABA agonists. Clearly, discovery of more glycine agonists is needed to further clarify this point. Moreover, we find a remarkedly different antagonistic mechanism exists for this phylogenetically newer inhibitory system in the central nervous system (CNS) than recently reported for strychnine and eight weaker glycine antagonists. We used GABA and six agonists (muscimol, dihydromuscimol, THIP, isoguvacine, trans-3-aminocyclopentane-1-carboxylic acid, piperidine-4-sulfonic acid) and five antagonists (bicuculline-N15-methobromide, R5135, pitrazepin, iso-THAZ and securinine) to derive our conclusions. We found that each of the agonists have three clearly defined atoms that can serve as attachment points at the GABAA receptor site. One of the three attachment atoms includes a carbonyl or carboxylate oxygen. The role of the carbonyl or carboxylate atom is very important. First, we theorize that a rapid two-point attachment occurs (one from the positive end and one from one of the other two negative atoms on the ligand) at the recognition site in the receptor where GABA or a GABAergic agonist binds. The positive end of the agonist perhaps associates through hydrogen bonding to a beta-carboxyl group in one of the aspartate molecules in the polypeptide. The negative attachment points perhaps bind through hydrogen bonding to arginine molecules in this polypeptide. The second negative site in the agonist immediately triggers a conformational change by pulling together the aforementioned groups by electrostatic attraction, and hence opening the chloride channel. We propose the carbonyl oxygen is partly responsible for triggering the opening by formation of a double hydrogen bond to arginine. We postulate that this attraction is the first step inducing the conformational change. In the case of the GABA antagonists investigated, a fourth attachment site was not found. In fact only two sites have been identified similar to the group II glycine antagonists. Our data support a hypothesis for GABAergic antagonist activity which suggests that the antagonist simply binds to the recognition site and blocks the neurotransmitter, GABA, from entering this site thereby preventing the opening of the chloride channel; it just stays closed. This mechanism is different from the mechanism proposed for the large number of Group I glycine antagonists (Aprison et al.: J Neurosci Res 41: 259-269, 1995).
基于我们对甘氨酸能受体的分子建模研究，我们扩展了研究范围，以类似方式研究GABA能受体。新数据表明，与甘氨酸相比，本文所述分子可能存在略有不同的激动机制。其根源无疑在于，虽然甘氨酸有一个正性结合位点和两个负性结合位点，但它比GABA和其他GABA激动剂短得多。显然，需要发现更多的甘氨酸激动剂来进一步阐明这一点。此外，我们发现，与最近报道的士的宁和八种较弱的甘氨酸拮抗剂相比，中枢神经系统（CNS）中这种系统发育上更新的抑制系统存在明显不同的拮抗机制。我们使用GABA和六种激动剂（蝇蕈醇、二氢蝇蕈醇、THIP、异鹅膏蕈氨酸、反式-３-氨基环戊烷-１-羧酸、哌啶-4-磺酸）以及五种拮抗剂（荷包牡丹碱-N15-甲溴化物、R5135、匹拉西泮、异-THAZ和一叶萩碱）得出我们的结论。我们发现，每种激动剂都有三个明确的原子，可作为在GABAA受体位点的附着点。三个附着原子中的一个包括羰基或羧酸盐氧。羰基或羧酸盐原子的作用非常重要。首先，我们推测在GABA或GABA能激动剂结合的受体识别位点会发生快速的两点附着（一个来自正端，一个来自配体上另外两个负性原子中的一个）。激动剂的正端可能通过氢键与多肽中天冬氨酸分子之一的β-羧基结合。负性附着点可能通过氢键与该多肽中的精氨酸分子结合。激动剂中的第二个负性位点通过静电吸引将上述基团拉到一起，从而立即引发构象变化，进而打开氯离子通道。我们认为羰基氧通过与精氨酸形成双氢键，部分负责触发通道开放。我们假定这种吸引力是诱导构象变化的第一步。在所研究的GABA拮抗剂的情况下，未发现第四个附着位点。实际上，只确定了两个与II组甘氨酸拮抗剂类似的位点。我们的数据支持一种关于GABA能拮抗剂活性的假说，该假说表明拮抗剂只是简单地与识别位点结合，阻止神经递质GABA进入该位点，从而防止氯离子通道打开；通道就一直保持关闭状态。这种机制与大量I组甘氨酸拮抗剂所提出的机制不同（Aprison等人：《神经科学研究杂志》41: 259 - 269, 1995）。
