Botta Paolo, Mameli Manuel, Floyd Kirsten L, Radcliffe Richard A, Valenzuela C Fernando
Department of Neurosciences, University of New Mexico Health Sciences Center Albuquerque, New Mexico 87131-0001, USA.
J Pharmacol Exp Ther. 2007 Nov;323(2):684-91. doi: 10.1124/jpet.107.127894. Epub 2007 Aug 17.
Cerebellar granule neurons (CGNs) extrasynaptically express GABA(A) receptors containing alpha(6)beta(x)delta subunits, which mediate tonic inhibitory currents. Although it has been shown that the function of these receptors is potently and directly enhanced by ethanol, this finding has not been reproducible across different laboratories. In outbred Sprague-Dawley rats, a naturally occurring arginine (R) to glutamine (Q) mutation in position 100 of the alpha(6) subunit was reported to increase the ethanol sensitivity of these receptors. However, we did not detect an action of this mutation in selectively bred rats (alcohol-tolerant and alcohol-nontolerant). Consequently, we reexamined the effect of the mutation on ethanol sensitivity in Sprague-Dawley rats. Using patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that 25 mM ethanol increases the tonic current amplitude, tonic current noise, and spontaneous inhibitory postsynaptic current (sIPSC) frequency to a similar extent in alpha(6)-100R/100R and alpha(6)-100Q/100Q CGNs. Exposure to 80 mM ethanol increased the tonic current amplitude to a significantly greater extent in alpha(6)-100R/100R than in alpha(6)-100Q/100Q CGNs; however, the effects of 80 mM ethanol on the tonic current noise and sIPSC frequency were not significantly different between these groups. In the presence of tetrodo-toxin, a non-N-methyl-d-aspartate receptor antagonist, exogenous GABA, and a GABA transporter inhibitor, neither 8 nor 40 mM ethanol consistently affected tonic current amplitude or noise in alpha(6)-100R/100R or alpha(6)-100Q/100Q CGNs. Thus, the alpha(6)-R100Q GABA(A) receptor subunit polymorphism does not in-crease the acute ethanol sensitivity of extrasynaptic receptors, lending further support to the hypothesis that ethanol modulates these currents indirectly via a presynaptic mechanism.
小脑颗粒神经元（CGNs）在突触外表达含有α(6)β(x)δ亚基的GABA(A)受体，这些受体介导紧张性抑制电流。尽管已经表明这些受体的功能会被乙醇有效且直接增强，但这一发现并未在不同实验室得到重复验证。在远交群斯普拉格-道利大鼠中，据报道α(6)亚基第100位天然存在的精氨酸（R）到谷氨酰胺（Q）的突变会增加这些受体对乙醇的敏感性。然而，我们在选择性育种大鼠（耐酒精和不耐酒精）中未检测到这种突变的作用。因此，我们重新研究了该突变对斯普拉格-道利大鼠乙醇敏感性的影响。利用小脑蚓部矢状旁切片中的膜片钳电生理技术，我们发现25 mM乙醇在α(6)-100R/100R和α(6)-100Q/100Q CGNs中对紧张性电流幅度、紧张性电流噪声和自发性抑制性突触后电流（sIPSC）频率的增加程度相似。暴露于80 mM乙醇时，α(6)-100R/100R中紧张性电流幅度的增加程度明显大于α(6)-100Q/100Q CGNs；然而，80 mM乙醇对紧张性电流噪声和sIPSC频率的影响在这些组之间没有显著差异。在存在河豚毒素（一种非N-甲基-D-天冬氨酸受体拮抗剂）、外源性GABA和GABA转运体抑制剂的情况下，8 mM或40 mM乙醇均未持续影响α(6)-100R/100R或α(6)-100Q/100Q CGNs中的紧张性电流幅度或噪声。因此，α(6)-R100Q GABA(A)受体亚基多态性不会增加突触外受体对急性乙醇的敏感性，这进一步支持了乙醇通过突触前机制间接调节这些电流的假说。
