Zhou Chengwen, Ding Li, Deel M Elizabeth, Ferrick Elizabeth A, Emeson Ronald B, Gallagher Martin J
Department of Neurology, Vanderbilt University School of Medicine, Nashville, TN 37232 USA.
Department of Molecular Physiology & Biophysics, Vanderbilt University School of Medicine, USA.
Neurobiol Dis. 2015 Jan;73:407-17. doi: 10.1016/j.nbd.2014.10.021. Epub 2014 Nov 4.
We previously demonstrated that heterozygous deletion of Gabra1, the mouse homolog of the human absence epilepsy gene that encodes the GABAA receptor (GABAAR) α1 subunit, causes absence seizures. We showed that cortex partially compensates for this deletion by increasing the cell surface expression of residual α1 subunit and by increasing α3 subunit expression. Absence seizures also involve two thalamic nuclei: the ventrobasal (VB) nucleus, which expresses only the α1 and α4 subtypes of GABAAR α subunits, and the reticular (nRT) nucleus, which expresses only the α3 subunit subtype. Here, we found that, unlike cortex, VB exhibited significantly reduced total and synaptic α1 subunit expression. In addition, heterozygous α1 subunit deletion substantially reduced miniature inhibitory postsynaptic current (mIPSC) peak amplitudes and frequency in VB. However, there was no change in the expression of the extrasynaptic α4 or δ subunits in VB and, unlike other models of absence epilepsy, no change in tonic GABAAR currents. Although heterozygous α1 subunit knockout increased α3 subunit expression in medial thalamic nuclei, it did not alter α3 subunit expression in nRT. However, it did enlarge the presynaptic vesicular inhibitory amino acid transporter puncta and lengthen the time constant of mIPSC decay in nRT. We conclude that increased tonic GABAA currents are not necessary for absence seizures. In addition, heterozygous loss of α1 subunit disinhibits VB by substantially reducing phasic GABAergic currents and surprisingly, it also increases nRT inhibition by prolonging phasic currents. The increased inhibition in nRT likely represents a partial compensation that helps reduce absence seizures.
我们之前证明，Gabra1基因的杂合缺失会导致失神发作，Gabra1是人类失神癫痫基因的小鼠同源基因，编码GABAA受体（GABAAR）α1亚基。我们发现，大脑皮层通过增加残余α1亚基的细胞表面表达以及增加α3亚基表达，部分补偿了这种缺失。失神发作还涉及两个丘脑核团：腹侧基底（VB）核，仅表达GABAARα亚基的α1和α4亚型；网状（nRT）核，仅表达α3亚基亚型。在这里，我们发现，与大脑皮层不同，VB中α1亚基的总表达和突触表达显著降低。此外，α1亚基的杂合缺失显著降低了VB中微小抑制性突触后电流（mIPSC）的峰值幅度和频率。然而，VB中突触外α4或δ亚基的表达没有变化，并且与其他失神癫痫模型不同，强直GABAAR电流没有变化。虽然α1亚基的杂合敲除增加了内侧丘脑核团中α3亚基的表达，但并未改变nRT中α3亚基的表达。然而，它确实扩大了nRT中突触前囊泡抑制性氨基酸转运体的斑点，并延长了mIPSC衰减的时间常数。我们得出结论，增加的强直GABAA电流对于失神发作并非必要。此外，α1亚基的杂合缺失通过大幅降低相位性GABA能电流使VB去抑制，令人惊讶的是，它还通过延长相位性电流增加了nRT的抑制作用。nRT中增加的抑制作用可能代表了一种部分补偿，有助于减少失神发作。
