Intracellular mechanisms underlying presynaptic modulations

我们直接记录了大鼠脑干切片中一个巨大的突触前终末（称为Held萼）和一个位于梯形体内侧核的突触后靶细胞。代谢性谷氨酸受体（mGluR）激动剂L-AP4可逆性抑制MNTB神经元记录的EPSCs。类似地，GABA_B受体激动剂巴氯芬抑制EPSCs。两种激动剂均抑制从突触前端记录的钙电流，而对突触前电压依赖性钾电流没有影响。L-AP4或巴氯芬对EPSCs的减少完全可以通过突触前Ca的减少来解释。我们得出结论，mGluR或GABA_B受体激动剂抑制突触前Ca通道，从而减少递质释放。该突触前ca通道经药理学鉴定为p型。突触前p型Ca通道在短时间高频去极化脉冲和长时间去极化脉冲的刺激下表现出明显的失活。这种失活依赖于外部Ca浓度，当Ca电流最大时失活最大。细胞内装载BAPTA或用Sr或Ba取代载流子Ca后，失活作用不明显，但当外部二价阳离子完全被Na取代时，失活作用被消除。因此，失活机制依赖于二价阳离子电流，这表明阻滞位点靠近或在Ca通道孔内。当EPSCs被短暂去极化脉冲诱导的突触前Ca电流（IpCa）激活时，破伤风刺激产生深度突触抑制，随后逐渐恢复。在恢复过程中，IpCa也从失活中恢复过来，EPSCs的恢复完全可以用IpCa的恢复来定量解释。我们得出结论，突触前Ca电流失活有助于破伤风后突触抑制。

We made direct recordings from a giant presynaptic terminal called as the calyx of Held and a postsynaptic target cell in the medial nucleus of the trapezoid body (MNTB) in rat brain stem slice. The metabotropic glutamate receptor (mGluR) agonist L-AP4 reversibly suppressed EPSCs recorded from the MNTB neurons. Similarly the GABA_B receptor agonist baclofen suppressed EPSCs. Both agonists were found to suppress Ca current recorded from the preterminal, whereas had no effect on the presynaptic voltage-dependent potassium current. The reduction of EPSCs by L-AP4 or baclofen could be fully explained by the reduction of presynaptic Ca. We conclude that mGluR or GABA_B receptor agonists suppress presynaptic Ca channel thereby reducing transmitter release.This presynaptic ca channel was pharmacologically identified as P-type. The presynaptic P-type Ca channel showed a marked inactivation when it was evoked by a brief depolarizing pulse at high frequency or when a depolarization was prolonged. This inactivation was dependent upon external Ca concentration and was largest when Ca current was largest. The inactivation was not much attenuated by intracellular loading of BAPTA or replacement of charge carrier Ca by Sr or Ba, but was abolished when external divalent cations were entirely replaced by Na. Thus, the inactivation mechanism is divalent cation current dependent, suggesting that the locus of block is close to or inside the Ca channel pore. When EPSCs were evoked by presynaptic Ca current (IpCa) induced by a brief depolarizing pulse, tetanic stimulation produced a profound synaptic depression followed by a gradual recovery. During the recovery, IpCa also recovered from inactivation and the recovery of EPSCs was fully explicable quantitatively by the recovery of IpCa. We conclude that presynaptic Ca current inactivation contributes to post-tetanic synaptic depression.