Palmer Mary J, Hull Court, Vigh Jozsef, von Gersdorff Henrique
The Vollum Institute, Oregon Health and Science University, Portland, Oregon 97239, USA.
J Neurosci. 2003 Dec 10;23(36):11332-41. doi: 10.1523/JNEUROSCI.23-36-11332.2003.
The release of vesicular protons during exocytosis causes a feedback inhibition of Ca2+ channels in photoreceptor terminals; however, the effect of this inhibition on subsequent exocytosis has not been studied. Here we show that a similar L-type Ca2+ channel inhibition occurs in bipolar cell terminals in slices of goldfish retina, and we investigate the effect that this has on subsequent exocytosis with membrane capacitance measurements. We find that transient Ca2+ current inhibition is correlated with exocytosis and modulated by the concentration of extracellular pH buffer. Ca2+ current inhibition is negligible in acutely dissociated terminals, demonstrating the importance of an intact synaptic cleft. The sensitivity of bipolar cell Ca2+ currents to extracellular pH was assessed: channel conductance is reduced and activation is shifted to more positive potentials by acidification. The effect of Ca2+ current inhibition on subsequent exocytosis was investigated by measuring paired-pulse depression. Under conditions in which there is a large amount of inhibition of Ca2+ influx, the degree of paired-pulse depression is significantly reduced. Finally, we show that under physiological (bicarbonate) buffering conditions, pronounced Ca2+ current inhibition occurs after exocytosis ( approximately 60% peak inhibition), which can decrease subsequent exocytosis during single depolarizations. We estimate that exocytosis is accompanied by a transient change in synaptic cleft pH from 7.5 to approximately 6.9. We suggest that this effect serves as an activity-dependent modulator of exocytosis at ribbon-type synapses where a large and compact coterie of vesicles can fuse at each active zone.
胞吐过程中囊泡质子的释放会对光感受器终末的Ca2+通道产生反馈抑制；然而，这种抑制对后续胞吐作用的影响尚未得到研究。在此我们表明，在金鱼视网膜切片的双极细胞终末也会发生类似的L型Ca2+通道抑制，并且我们通过膜电容测量研究了其对后续胞吐作用的影响。我们发现瞬时Ca2+电流抑制与胞吐作用相关，并受细胞外pH缓冲液浓度的调节。在急性解离的终末中，Ca2+电流抑制可忽略不计，这表明完整突触间隙的重要性。评估了双极细胞Ca2+电流对细胞外pH的敏感性：酸化会降低通道电导并使激活向更正的电位偏移。通过测量双脉冲抑制来研究Ca2+电流抑制对后续胞吐作用的影响。在Ca2+内流受到大量抑制的条件下，双脉冲抑制程度显著降低。最后，我们表明在生理（碳酸氢盐）缓冲条件下，胞吐作用后会发生明显的Ca2+电流抑制（峰值抑制约60%），这可在单次去极化期间减少后续胞吐作用。我们估计胞吐作用伴随着突触间隙pH从7.5瞬时变化至约6.9。我们认为这种效应在带状突触处作为一种依赖于活动的胞吐作用调节剂，在每个活动区有大量紧密排列的囊泡能够融合。
