Synaptic and extrasynaptic GABA_A receptor (GABA_AR)-mediated neurotransmission is a critical component of the suprachiasmatic nucleus (SCN) neuronal network. However, the properties of the GABA_A tonic current (I_tonic) and its origin remain unexplored. Spontaneous GABA_A postsynaptic currents (sGPSC) and I_tonic were recorded from SCN neurons using the whole-cell voltage-clamp technique at different times of the day. GABA_AR antagonists (bicuculline, gabazine, and picrotoxin) inhibited sGPSC and induced an outward shift of the holding current, which defined the I_tonic amplitude. The sGPSC frequency, synaptic charge transfer, and the I_tonic amplitude all demonstrated significant diurnal rhythms with peaks in the middle of the day (Zeitgeber Time, ZT8) and nadirs at night (ZT19). The I_tonic amplitude increased proportionally with the sGPSC frequency and synaptic charge transfer during the day and required action potential-mediated GABA release, which was confirmed by TTX application. The activation of presynaptic GABA_B receptors by baclofen did not significantly alter the I_tonic of neurons with low-frequency sGPSC. The equilibrium potential (Eq) for I_tonic was similar to the Eq for chloride and GABA_A receptor-activated currents. I_tonic showed outward rectification at membrane potentials over the range of -70 mV to -10 mV, then was linear at voltages greater than -10 mV. GABA_AR containing α4-, α5- and δ- subunits were expressed in SCN, and their contribution to I_tonic was confirmed by application of the GABA_AR agonist THIP and the GABA_AR inverse agonist L655,708. Thus, the I_tonic was mediated by extrasynaptic GABA_ARs activated predominantly by GABA diffusing out of GABAergic synapses.

中文翻译：

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视交叉上核神经元强直和突触 GABAA 受体介导电流的昼夜特性

突触和突触外 GABA _A受体 (GABA _A R) 介导的神经传递是视交叉上核 (SCN) 神经元网络的重要组成部分。然而，GABA _A补品电流 (I _tonic ) 的特性及其起源仍未探索。使用全细胞电压钳技术在一天中的不同时间从 SCN 神经元记录自发 GABA _A突触后电流 (sGPSC) 和 I_补品。GABA _A R 拮抗剂（荷包牡丹碱、gabazine 和印防己毒素）抑制 sGPSC 并诱导保持电流向外移动，这定义了 I_强直幅度。sGPSC 频率、突触电荷转移和 I_强直幅度都表现出显着的昼夜节律，在白天（Zeitgeber Time，ZT8）达到峰值，在夜间达到最低点（ZT19）。I_强直幅度与白天的 sGPSC 频率和突触电荷转移成比例增加，并且需要动作电位介导的 GABA 释放，这通过 TTX 应用得到证实。巴氯芬对突触前 GABA _B受体的激活并没有显着改变低频 sGPSC 神经元的 I_{强直。I tonic}的平衡电位 (Eq)与氯化物和 GABA _A受体激活电流的 Eq 相似。我_补品在 -70 mV 至 -10 mV 范围内的膜电位下显示出外向整流，然后在大于 -10 mV 的电压下呈线性。含有 α4-、α5- 和 δ- 亚基的GABA _A R 在 SCN 中表达，它们对 I_补品的贡献通过应用 GABA _A R 激动剂 THIP 和 GABA _A R 反向激动剂 L655,708 得到证实。因此，I_补品由突触外 GABA _A Rs 介导，主要由 GABA 扩散出 GABA 能突触激活。