Singer Joshua H, Diamond Jeffrey S
Synaptic Physiology Unit, National Institutes of Health-National Institute of Neurological Disorders and Stroke, Bethesda, Maryland 20892-4066, USA.
J Neurosci. 2003 Nov 26;23(34):10923-33. doi: 10.1523/JNEUROSCI.23-34-10923.2003.
Night (scotopic) vision is mediated by a distinct retinal circuit in which the light responses of rod-driven neurons are faster than those of the rods themselves. To investigate the dynamics of synaptic transmission at the second synapse in the rod pathway, we made paired voltage-clamp recordings from rod bipolar cells (RBCs) and postsynaptic AII and A17 amacrine cells in rat retinal slices. Depolarization of RBCs from -60 mV elicited sustained Ca2+ currents and evoked AMPA receptor (AMPAR)-mediated EPSCs in synaptically coupled amacrine cells that exhibited large, rapidly rising initial peaks that decayed rapidly to smaller, steady-state levels. The transient component persisted in the absence of feedback inhibition to the RBC terminal and when postsynaptic AMPA receptor desensitization was blocked with cyclothiazide, indicating that it reflects a time-dependent decrease in the rate of exocytosis from the presynaptic terminal. The EPSC waveform was similar when RBCs were recorded in perforated-patch or whole-cell configurations, but asynchronous release from RBCs was enhanced when the intraterminal Ca2+ buffer capacity was reduced. When RBCs were depolarized from -100 mV, inactivating, low voltage-activated (T-type channel-mediated) Ca2+ currents were evident. Although Ca2+ influx through T-type channels boosted vesicle release, as reflected by larger EPSCs, it did not make the EPSCs faster, indicating that activation of T-type channels is not necessary to generate a transient phase of exocytosis. We conclude that the time course of vesicle release from RBCs is inherently transient and, together with the fast kinetics of postsynaptic AMPARs, speeds transmission at this synapse.
暗视觉由独特的视网膜回路介导，其中视杆驱动神经元的光反应比视杆本身的光反应更快。为了研究视杆通路中第二个突触处的突触传递动力学，我们在大鼠视网膜切片中对视杆双极细胞（RBC）以及突触后的AII和A17无长突细胞进行了双电极电压钳记录。将RBC从-60 mV去极化可引发持续的Ca2+电流，并在突触耦合的无长突细胞中诱发AMPA受体（AMPAR）介导的兴奋性突触后电流（EPSC），这些电流表现出大的、快速上升的初始峰值，并迅速衰减至较小的稳态水平。在没有对RBC终末的反馈抑制以及用环噻嗪阻断突触后AMPA受体脱敏的情况下，瞬态成分仍然存在，这表明它反映了突触前终末胞吐速率随时间的下降。当以穿孔膜片钳或全细胞模式记录RBC时，EPSC波形相似，但当终末内Ca2+缓冲能力降低时，RBC的异步释放增强。当RBC从-100 mV去极化时，可明显观察到失活的低电压激活（T型通道介导）Ca2+电流。尽管通过T型通道的Ca2+内流促进了囊泡释放，这表现为更大的EPSC，但它并没有使EPSC更快，这表明T型通道的激活对于产生胞吐的瞬态阶段不是必需的。我们得出结论，RBC囊泡释放的时间进程本质上是瞬态的，并且与突触后AMPAR的快速动力学一起，加快了该突触处的传递。
