Wang D, Cui L-N, Renaud L P
Neuroscience Program, Ottawa Health Research Institute and University of Ottawa, 725 Parkdale Avenue, Ontario, K1Y 4K9, Ottawa, Canada
Neuroscience. 2003;118(1):49-58. doi: 10.1016/s0306-4522(02)00906-5.
The suprachiasmatic nucleus (SCN), the dominant circadian pacemaker in mammalian brain, sends axonal projections to the hypothalamic paraventricular nucleus (PVN), a composite of magno- and parvocellular neurons. This neural network likely offers SCN output neurons a means to entrain diurnal rhythmicity in various autonomic and neuroendocrine functions. Earlier investigations using patch-clamp recordings in slice preparations have suggested differential innervation by SCN efferents to magnocellular versus parvocellular PVN cells. In magnocellular PVN, cells respond to focal electrical stimulation in SCN with a GABA(A) receptor-mediated postsynaptic inhibition whose magnitude can be modulated by presynaptic GABA(B) receptors. By contrast, SCN-evoked responses in parvocellular PVN neurons typically involve both GABA(A)- and glutamate-receptor-mediated components. In the present patch-clamp study, 69/85 periventricular parvocellular PVN cells displayed SCN-evoked inhibitory and/or excitatory postsynaptic currents (IPSCs; EPSCs). In the presence of selective receptor antagonists, we sought evidence for their modulation by GABA acting at pre- and/or postsynaptic GABA(B) receptors. Cells responded to bath-applied baclofen (5-10 microM) with a tetrodotoxin-resistant membrane hyperpolarization associated with a reduction in input resistance and/or outward current, due to increase in a potassium conductance, blockable with 2-hydroxysaclofen (300 microM). At 1 microM where baclofen had no significant postsynaptic effect, evidence of activation of presynaptic GABA(B) receptors included reduction in SCN-evoked IPSCs and EPSCs with no change in their kinetics, and paired-pulse depression that was sensitive to both baclofen and saclofen. Baclofen also induced significant reductions in frequency but not amplitudes of miniature IPSCs and EPSCs. These observations suggest that levels of synaptically released GABA from the terminals of SCN output neurons can influence the relative contribution of pre- versus postsynaptic GABA(B) receptors in modulating both excitatory and inhibitory SCN innervation to parvocellular PVN neurons.
视交叉上核（SCN）是哺乳动物大脑中主要的昼夜节律起搏器，它向含有大细胞和小细胞神经元的下丘脑室旁核（PVN）发出轴突投射。这个神经网络可能为SCN输出神经元提供了一种在各种自主神经和神经内分泌功能中调节昼夜节律的方式。早期在脑片制备中使用膜片钳记录的研究表明，SCN传出纤维对大细胞与小细胞PVN细胞的支配存在差异。在大细胞PVN中，细胞对SCN中的局部电刺激产生GABA(A)受体介导的突触后抑制反应，其幅度可由突触前GABA(B)受体调节。相比之下，SCN诱发的小细胞PVN神经元反应通常涉及GABA(A)受体和谷氨酸受体介导的成分。在本膜片钳研究中，85个室周小细胞PVN细胞中有69个表现出SCN诱发的抑制性和/或兴奋性突触后电流（IPSCs；EPSCs）。在存在选择性受体拮抗剂的情况下，我们寻找证据证明GABA通过作用于突触前和/或突触后GABA(B)受体对其进行调节。细胞对浴加巴氯芬（5 - 10 microM）的反应是产生一种对河豚毒素耐药的膜超极化，伴有输入电阻降低和/或外向电流减少，这是由于钾电导增加所致，可被2 - 羟基巴氯芬（300 microM）阻断。在1 microM时，巴氯芬没有明显的突触后效应，突触前GABA(B)受体激活的证据包括SCN诱发的IPSCs和EPSCs减少，其动力学无变化，以及对巴氯芬和沙氯芬均敏感的双脉冲抑制。巴氯芬还显著降低了微小IPSCs和EPSCs的频率，但不影响其幅度。这些观察结果表明，SCN输出神经元终末突触释放的GABA水平可影响突触前和突触后GABA(B)受体在调节SCN对小细胞PVN神经元的兴奋性和抑制性支配中的相对作用。
