O'Callaghan J F, Greenfield S A
University Department of Pharmacology, Oxford, UK.
J Neurosci Res. 1995 Sep 1;42(1):85-96. doi: 10.1002/jnr.490420110.
Quantitative autoradiography of brain tissue has revealed a high density of binding sites for the K-ATP channel antagonists, the sulphonylureas, and for sigma-ligands in the substantia nigra (SN). In view of the high density of the two binding sites in the SN the possibility has been investigated that the K-ATP channel and the sigma-binding site are functionally linked. The K-ATP channel-mediated membrane hyperpolarisation and decrease in input resistance produced by hypoxia and by the metabolic inhibitor, cyanide, in rostral substantia nigra pars compacta neurons are antagonised by the sigma-ligand BMS 181100. In addition, BMS 181100 antagonises activation of the K-ATP channel by diazoxide; cromakalim is found to be without effect in these neurons. Antagonism of the cyanide-induced hyperpolarisation is dose dependent and is observed at concentrations of the drug which have no observable effect on the resting membrane properties of the neurons. By contrast, the nonselective sigma ligands 1,3-di-O-tolylguanidine (10 microM) and (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine (100 microM), and the selective sigma 1-ligand (+)-pentazocine (5-10 microM) have no effect on the cyanide-induced hyperpolarisation. 5-HT (50-100 microM) and the selective 5-HT1A receptor agonist 8-OH-DPAT (50 microM) also fail to antagonise the cyanide-induced hyperpolarisation. The antagonism of the cyanide-induced hyperpolarisation by BMS 181100 persists in the presence of tetrodotoxin (1 microM) and in the presence of high concentrations of (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, but not under conditions of reduced calcium (0.1-0.2 mM) and raised magnesium (6 mM) concentrations, which block synaptic transmission. It is concluded that in substantia nigra phasic neurons the sigma-binding site does not regulate activation of the ATP-sensitive channel. However, BMS 181100 antagonises K-ATP channel activation in these neurons independently of sigma-binding sites and 5-HT receptors. This action of BMS 181100 is TTX insensitive and Ca2+ dependent.
脑组织的定量放射自显影显示，黑质（SN）中存在高密度的K-ATP通道拮抗剂、磺酰脲类以及σ配体的结合位点。鉴于SN中这两种结合位点的高密度，研究了K-ATP通道与σ结合位点在功能上是否存在联系。在黑质致密部吻侧神经元中，由缺氧和代谢抑制剂氰化物引起的K-ATP通道介导的膜超极化以及输入电阻降低，可被σ配体BMS 181100拮抗。此外，BMS 181100可拮抗二氮嗪对K-ATP通道的激活；发现克罗卡林对这些神经元无作用。氰化物诱导的超极化的拮抗作用呈剂量依赖性，在对神经元静息膜特性无明显影响的药物浓度下即可观察到。相比之下，非选择性σ配体1,3-二-O-甲苯基胍（10μM）和（+）-3-（3-羟苯基）-N-（1-丙基）哌啶（100μM），以及选择性σ1配体（+）-喷他佐辛（5-10μM）对氰化物诱导的超极化无作用。5-羟色胺（50-100μM）和选择性5-HT1A受体激动剂8-OH-DPAT（50μM）也不能拮抗氰化物诱导的超极化。在存在河豚毒素（1μM）和高浓度（+）-3-（3-羟苯基）-N-（1-丙基）哌啶的情况下，BMS 181100对氰化物诱导的超极化的拮抗作用仍然存在，但在钙浓度降低（0.1-0.2 mM）和镁浓度升高（6 mM）从而阻断突触传递的条件下则不存在。得出的结论是，在黑质的时相性神经元中，σ结合位点不调节ATP敏感性通道的激活。然而，BMS 181100在这些神经元中拮抗K-ATP通道的激活，与σ结合位点和5-羟色胺受体无关。BMS 181100的这一作用对河豚毒素不敏感且依赖于Ca2+。
