From the Department of Biology, The Huck Institutes of Life Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802 and.
From the Department of Biology, The Huck Institutes of Life Sciences, The Pennsylvania State University, University Park, Pennsylvania 16802 and; the Institutes of Brain Science and State Key Laboratory for Medical Neurobiology, Fudan University, Shanghai 200032, China.
J Biol Chem. 2013 Aug 30;288(35):25053-25065. doi: 10.1074/jbc.M113.491464. Epub 2013 Jul 9.
The GABAA receptors are the major inhibitory receptors in the brain and are localized at both synaptic and extrasynaptic membranes. Synaptic GABAA receptors mediate phasic inhibition, whereas extrasynaptic GABAA receptors mediate tonic inhibition. Both phasic and tonic inhibitions regulate neuronal activity, but whether they regulate each other is not very clear. Here, we investigated the functional interaction between synaptic and extrasynaptic GABAA receptors through various molecular manipulations. Overexpression of extrasynaptic α6β3δ-GABAA receptors in mouse hippocampal pyramidal neurons significantly increased tonic currents. Surprisingly, the increase of tonic inhibition was accompanied by a dramatic reduction of the phasic inhibition, suggesting a possible homeostatic regulation of the total inhibition. Overexpressing the α6 subunit alone induced an up-regulation of δ subunit expression and suppressed phasic inhibition similar to overexpressing the α6β3δ subunits. Interestingly, blocking all GABAA receptors after overexpressing α6β3δ receptors could not restore the synaptic GABAergic transmission, suggesting that receptor activation is not required for the homeostatic interplay. Furthermore, insertion of a gephyrin-binding-site (GBS) into the α6 and δ subunits recruited α6(GBS)β3δ(GBS) receptors to postsynaptic sites but failed to rescue synaptic GABAergic transmission. Thus, it is not the positional effect of extrasynaptic α6β3δ receptors that causes the down-regulation of phasic inhibition. Overexpressing α5β3γ2 subunits similarly reduced synaptic GABAergic transmission. We propose a working model that both synaptic and extrasynaptic GABAA receptors may compete for limited receptor slots on the plasma membrane to maintain a homeostatic range of the total inhibition.
GABAA 受体是大脑中的主要抑制性受体，位于突触和突触外膜上。突触 GABAA 受体介导相性抑制，而突触外 GABAA 受体介导紧张性抑制。相性和紧张性抑制都调节神经元活动，但它们之间是否相互调节尚不清楚。在这里，我们通过各种分子操作研究了突触和突触外 GABAA 受体之间的功能相互作用。在小鼠海马锥体神经元中过度表达突触外 α6β3δ-GABAA 受体可显著增加紧张性电流。令人惊讶的是，紧张性抑制的增加伴随着相性抑制的显著减少，这表明总抑制可能存在一种自动调节。单独过表达 α6 亚基会诱导 δ 亚基表达上调，并抑制相性抑制，类似于过表达 α6β3δ 亚基。有趣的是，在过表达 α6β3δ 受体后阻断所有 GABAA 受体不能恢复突触 GABA 能传递，表明受体激活对于自动调节的相互作用不是必需的。此外，将 GABAA 受体的 gephyrin 结合位点（GBS）插入 α6 和 δ 亚基中，将 α6(GBS)β3δ(GBS) 受体募集到突触后位点，但不能挽救突触 GABA 能传递。因此，不是突触外 α6β3δ 受体的位置效应导致相性抑制的下调。过表达 α5β3γ2 亚基也同样降低了突触 GABA 能传递。我们提出了一个工作模型，即突触和突触外 GABAA 受体可能争夺质膜上有限的受体槽，以维持总抑制的自动调节范围。
