Control of Ca channels mediating cortical synaptic transmission by transmitter-receptors and G-protein

为了确定介导中枢性突触传递的钙通道亚型，并研究其G蛋白介导的调节机制，用Nomarski光学显微镜对2~3周龄大鼠视皮层切片进行全细胞膜片钳记录。谷氨酸能兴奋性突触后电流(EPSCs)或GABA能抑制性突触后电流(IPSCs)分别由刺激相邻锥体神经元或中间神经元诱发。N-型钙通道的特异性阻断剂omega-conooxoxin GVIA和P/Q型钙通道阻断剂omega-agatoxin IVA在0.3-3um以剂量依赖的方式抑制EPSC，最大减幅为50%。Omega-agatoxin IVA(30 nM~(-1)um)可使IPSC的幅度降低70%，而Omega-Conooxin GVIA对IPSC无剂量依赖性抑制作用。同一突触前和突触后神经元对的EPSC在突触前被卡巴胆碱、去甲肾上腺素和血清素抑制，提示这3种胺受体共存于突触前终末，这些受体的激活汇聚到导致谷氨酸释放抑制的调节级联反应的某个步骤中。然后，我们观察了用欧米茄毒素GVLA或欧米茄毒素IVA阻断两种钙通道亚型中的一种后对EPSC的胺能调节。这些发现表明，这三种胺能重构体中的任何一种与任何一种钙通道亚型之间都不存在通过G蛋白的优势连锁。

In order to identify the Ca channel subtype mediating central synaptic transmission and to study the mechanisms of its G-proteins-mediated regulation, whole-cell patch-clamp recordings were performed from pyramidal neurons under observation with Nomarski optics in a slice preparation which was made from the visual cortex of 2- 3-weeks old rats. Glutamatergic excitatory postsynaptic currents (EPSCs) or GABA ergic inhibitory postsynaptic currents (IPSCs) were evoked by stimulation of neighboring pyramidal neurons or interneurons, respectively. EPSC thus evoked was inhibited by omega-conotoxin GVIA, a specific blocker of the N-type Ca channel, in a dose-dependent manner from 30 nM to 1 muM with a maximum reduction of 50%, or by omega-agatoxin IVA, a P/Q-type Ca channel blocker, at 0.3 - 3 muM with a similar maximum recbiction. The amplitude of IPSC was reduced by omega-agatoxin IVA (30 nM - 1 muM) by up to 70 %, whereas the dose-dependent inhibition of the IPSC was not observed by omega-conotoxin GVIA up tol muM The EPSC observed in the same presynaptic and postsynaptic neuronal pair was inhibited by carbachol, noradrenaline and sertonin presynaptically, suggesting that these 3 amine receptors coexist at the presynaptic terminal and that activation of these receptors converge into some step of the regulatory cascade leading to an inhibition of the glutamate release. We then examined aminergic modulation after blocking one of the two Ca channel subtypes by either omega-conotoxin GVLA or omega-agatoxin IVA.The inhibition of the EPSC by each amine was not significantly different between N-type mediated and P/Q-type mediated EPSCs. These findings suggest that there is no preferential linkage between any one of the three aminergic recepotrs and either Ca channel subtype via G-proteins.