Equipe Neurophysiologie de la Synapse Auditive, Unité Mixte de Recherche, Institut National de la Santé et de la Recherche Médicale U587 et Université Bordeaux Segalen, Institut des Neurosciences de Bordeaux, Centre Hospitalier Universitaire Pellegrin, Bordeaux, France.
J Neurophysiol. 2012 Dec;108(11):3116-23. doi: 10.1152/jn.00555.2012. Epub 2012 Sep 12.
During development, synaptic exocytosis by cochlear hair cells is first initiated by patterned spontaneous Ca(2+) spikes and, at the onset of hearing, by sound-driven graded depolarizing potentials. The molecular reorganization occurring in the hair cell synaptic machinery during this developmental transition still remains elusive. We characterized the changes in biophysical properties of voltage-gated Ca(2+) currents and exocytosis in developing auditory hair cells of a precocial animal, the domestic chick. We found that immature chick hair cells (embryonic days 10-12) use two types of Ca(2+) currents to control exocytosis: low-voltage-activating, rapidly inactivating (mibefradil sensitive) T-type Ca(2+) currents and high-voltage-activating, noninactivating (nifedipine sensitive) L-type currents. Exocytosis evoked by T-type Ca(2+) current displayed a fast release component (RRP) but lacked the slow sustained release component (SRP), suggesting an inefficient recruitment of distant synaptic vesicles by this transient Ca(2+) current. With maturation, the participation of L-type Ca(2+) currents to exocytosis largely increased, inducing a highly Ca(2+) efficient recruitment of an RRP and an SRP component. Notably, L-type-driven exocytosis in immature hair cells displayed higher Ca(2+) efficiency when triggered by prerecorded native action potentials than by voltage steps, whereas similar efficiency for both protocols was found in mature hair cells. This difference likely reflects a tighter coupling between release sites and Ca(2+) channels in mature hair cells. Overall, our results suggest that the temporal characteristics of Ca(2+) entry through T-type and L-type Ca(2+) channels greatly influence synaptic release by hair cells during cochlear development.
在发育过程中，耳蜗毛细胞的突触胞吐首先由模式化的自发 Ca(2+) 峰启动，在听觉开始时，由声音驱动的分级去极化电位启动。在这个发育转变过程中，毛细胞突触机制中发生的分子重组仍然难以捉摸。我们描述了在一种早熟动物，即家鸡的发育听觉毛细胞中，电压门控 Ca(2+) 电流和胞吐作用的生物物理特性的变化。我们发现，未成熟的小鸡毛细胞（胚胎第 10-12 天）使用两种类型的 Ca(2+) 电流来控制胞吐作用：低电压激活、快速失活（米贝地尔敏感）的 T 型 Ca(2+) 电流和高电压激活、非失活（硝苯地平敏感）的 L 型电流。由 T 型 Ca(2+) 电流引发的胞吐作用显示出快速释放成分（RRP），但缺乏缓慢持续释放成分（SRP），这表明这种短暂的 Ca(2+) 电流对远距离突触囊泡的募集效率低下。随着成熟，L 型 Ca(2+) 电流对胞吐作用的参与大大增加，诱导 RRP 和 SRP 成分的高度 Ca(2+) 有效募集。值得注意的是，在未成熟的毛细胞中，由 L 型 Ca(2+) 电流引发的胞吐作用，当由预先记录的天然动作电位触发时，比由电压步触发时具有更高的 Ca(2+) 效率，而在成熟的毛细胞中，两种方案的效率相似。这种差异可能反映了成熟毛细胞中释放位点和 Ca(2+) 通道之间的紧密耦合。总体而言，我们的结果表明，通过 T 型和 L 型 Ca(2+) 通道进入 Ca(2+) 的时间特征极大地影响了耳蜗发育过程中毛细胞的突触释放。
