Yeung Jacky Y T, Canning Kevin J, Zhu Guoyun, Pennefather Peter, MacDonald John F, Orser Beverley A
Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada.
Mol Pharmacol. 2003 Jan;63(1):2-8. doi: 10.1124/mol.63.1.2.
In the hippocampus, two distinct forms of GABAergic inhibition have been identified, phasic inhibitory postsynaptic currents that are the consequence of the vesicular release of GABA and a tonic conductance that is activated by low ambient concentrations of extracellular GABA. It is not known what accounts for the distinct properties of receptors that mediate the phasic and tonic inhibitory conductances. Moreover, the physiological role of the tonic inhibitory conductance remains uncertain because pharmacological tools that clearly distinguish tonic and phasic receptors are lacking. Here, we demonstrate that GABAA receptors that generate a tonic conductance in cultured hippocampal neurons from embryonic mice have different pharmacological properties than those in cerebellar granule neurons or pyramidal neurons in the dentate gyrus. The tonic conductance in cultured hippocampal neurons is enhanced by the benzodiazepine, midazolam, and is insensitive to the inhibitory effects of the competitive antagonist, gabazine (< or =10 microM). We also identify penicillin as an uncompetitive antagonist that selectively inhibits the synaptic but not tonic conductance. GABA was applied to hippocampal neurons to investigate the properties of synaptic and extrasynaptic receptors. GABA-evoked current was composed of two components: a rapidly desensitizing current that was blocked by penicillin and a nondesensitizing current that was insensitive to penicillin blockade. The potency of GABA was greater for the penicillin-insensitive nondesensitizing current. Single-channel studies show that the gabazine-insensitive GABAA receptors have a lower unitary conductance (12 pS) than that estimated for synaptic receptors. Thus, specialized GABAA receptors with an apparent higher affinity for GABA that do not readily desensitize mediate the persistent tonic conductance in hippocampal neurons. The receptors underlying tonic and phasic inhibitory conductances in hippocampal neurons are pharmacologically and biophysically distinct, suggesting that they serve different physiological roles.
在海马体中，已鉴定出两种不同形式的GABA能抑制，即由GABA囊泡释放引起的相位性抑制性突触后电流和由细胞外低环境浓度GABA激活的持续性电导。介导相位性和持续性抑制性电导的受体的不同特性的原因尚不清楚。此外，由于缺乏能明确区分持续性和相位性受体的药理学工具，持续性抑制性电导的生理作用仍不确定。在这里，我们证明，在来自胚胎小鼠的培养海马神经元中产生持续性电导的GABAA受体具有与小脑颗粒神经元或齿状回锥体神经元中不同的药理学特性。培养海马神经元中的持续性电导被苯二氮䓬类药物咪达唑仑增强，并且对竞争性拮抗剂gabazine（≤10 microM）的抑制作用不敏感。我们还确定青霉素是一种非竞争性拮抗剂，它选择性抑制突触传导而非持续性电导。将GABA应用于海马神经元以研究突触和突触外受体的特性。GABA诱发的电流由两个成分组成：一个被青霉素阻断的快速脱敏电流和一个对青霉素阻断不敏感的非脱敏电流。对于对青霉素不敏感的非脱敏电流，GABA的效力更大。单通道研究表明，对gabazine不敏感的GABAA受体的单位电导（12 pS）低于突触受体的估计值。因此，对GABA具有明显更高亲和力且不易脱敏的特殊GABAA受体介导海马神经元中的持续性电导。海马神经元中持续性和相位性抑制性电导的基础受体在药理学和生物物理学上是不同的，这表明它们发挥不同的生理作用。
