Kelm M Katherine, Criswell Hugh E, Breese George R
Department of Pharmacology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7178, USA.
J Neurophysiol. 2008 Dec;100(6):3417-28. doi: 10.1152/jn.90970.2008. Epub 2008 Oct 22.
Ethanol increases miniature inhibitory postsynaptic current frequency and decreases the paired-pulse ratio, which suggests that ethanol increases both spontaneous and evoked GABA release, respectively. We have shown previously that ethanol increases GABA release at the rat interneuron-Purkinje cell synapse and that this ethanol effect involves calcium release from internal stores; however, further exploration of the mechanism responsible for ethanol-enhanced GABA release was needed. We found that a cannabinoid receptor 1 (CB1) agonist, WIN-55212, and a GABA(B) receptor agonist, baclofen, decreased baseline spontaneous GABA release and prevented ethanol from increasing spontaneous GABA release. The CB1 receptor and GABA(B) receptor are Galpha i-linked G protein-coupled receptors with common downstream messengers that include adenylate cyclase and protein kinase A (PKA). Adenylate cyclase and PKA antagonists blocked ethanol from increasing spontaneous GABA release, whereas a PKA antagonist limited to the postsynaptic neuron did not block ethanol from increasing spontaneous GABA release. These results suggest that presynaptic PKA plays an essential role in ethanol-enhanced spontaneous GABA release. Similar to ethanol, we found that the mechanism of the cannabinoid-mediated decrease in spontaneous GABA release involves internal calcium stores and PKA. A PKA antagonist decreased baseline spontaneous GABA release. This effect was reduced after incubating the slice with a calcium chelator, BAPTA-AM, but was unaffected when BAPTA was limited to the postsynaptic neuron. This suggests that the PKA antagonist is acting through a presynaptic, calcium-dependent mechanism to decrease spontaneous GABA release. Overall, these results suggest that PKA activation is necessary for ethanol to increase spontaneous GABA release.
乙醇可增加微小抑制性突触后电流频率并降低双脉冲比率，这分别表明乙醇增加了自发性和诱发性GABA释放。我们之前已经表明，乙醇可增加大鼠中间神经元-浦肯野细胞突触处的GABA释放，且这种乙醇效应涉及从内部储存库释放钙；然而，需要进一步探索负责乙醇增强GABA释放的机制。我们发现，大麻素受体1（CB1）激动剂WIN-55212和GABA(B)受体激动剂巴氯芬可降低基线自发性GABA释放，并阻止乙醇增加自发性GABA释放。CB1受体和GABA(B)受体是与Gαi偶联的G蛋白偶联受体，具有包括腺苷酸环化酶和蛋白激酶A（PKA）在内的共同下游信使。腺苷酸环化酶和PKA拮抗剂可阻止乙醇增加自发性GABA释放，而仅限于突触后神经元的PKA拮抗剂则不能阻止乙醇增加自发性GABA释放。这些结果表明，突触前PKA在乙醇增强的自发性GABA释放中起重要作用。与乙醇类似，我们发现大麻素介导的自发性GABA释放减少的机制涉及内部钙储存和PKA。PKA拮抗剂可降低基线自发性GABA释放。在用钙螯合剂BAPTA-AM孵育切片后，这种效应减弱，但当BAPTA仅限于突触后神经元时则不受影响。这表明PKA拮抗剂是通过突触前钙依赖性机制来减少自发性GABA释放的。总体而言，这些结果表明PKA激活是乙醇增加自发性GABA释放所必需的。
