Intracellular mechanisms underlying presynaptic modulations

我们在大鼠脑干切片上直接记录了一个巨大的突触前终末（称为Held的萼）和一个位于斜方体内侧核（MNTB）的突触后靶细胞。代谢型谷氨酸受体（mGluR）激动剂L-AP 4可逆地抑制从MNTB神经元记录的EPSC。GABA_B受体激动剂巴氯芬同样抑制EPSC。这两种激动剂被发现抑制钙电流记录从preterminal，而突触前电压依赖性钾电流没有影响。L-AP 4或巴氯芬对EPSCs的抑制作用可以通过突触前Ca ~（2+）的减少来解释。我们认为mGluR或GABA_B受体激动剂抑制突触前钙通道，从而减少递质释放，这种突触前钙通道被初步鉴定为P型。突触前P-型钙通道在高频短去极化脉冲或延长去极化脉冲时表现出明显的失活。这种失活是依赖于外部的钙浓度，是最大的钙电流最大时。的失活是没有太大的衰减由细胞内加载的BAPTA或更换的电荷载体钙Sr或Ba，但被废除时，外部二价阳离子完全取代钠。因此，失活机制是二价阳离子电流依赖性的，表明阻断位点靠近或位于Ca通道孔内。当突触前钙电流（IpCa）诱发短暂的去极化脉冲，强直刺激产生了深刻的突触抑制后逐渐恢复。在恢复过程中，IpCa也从失活中恢复，EPSC的恢复完全可以通过IpCa的恢复来定量解释。我们的结论是突触前钙电流失活有助于强直后突触抑制。

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.