Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198-5840, USA.
J Neurophysiol. 2011 Dec;106(6):2922-35. doi: 10.1152/jn.00634.2011. Epub 2011 Aug 31.
Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation. Ca(2+) channel number and single-channel current amplitude were calculated by mean-variance analysis of I(Ca). Two different comparisons-one comparing average numbers of release events to average I(Ca) amplitude and the other involving deconvolution of both EPSCs and simultaneously recorded cone I(Ca)-suggested that fewer than three Ca(2+) channel openings accompanied fusion of each vesicle at the peak of release during the first few milliseconds of stimulation. Opening fewer Ca(2+) channels did not enhance fusion efficiency, suggesting that few unnecessary channel openings occurred during strong depolarization. We simulated release at the cone synapse, using empirically determined synaptic dimensions, vesicle pool size, Ca(2+) dependence of release, Ca(2+) channel number, and Ca(2+) channel properties. The model replicated observations when a barrier was added to slow Ca(2+) diffusion. Consistent with the presence of a diffusion barrier, dialyzing cones with diffusible Ca(2+) buffers did not affect release efficiency. The tight clustering of Ca(2+) channels, along with a high-Ca(2+) affinity release mechanism and diffusion barrier, promotes a linear coupling between Ca(2+) influx and vesicle fusion. This may improve detection of small light decrements when cones are hyperpolarized by bright light.
光使视锥细胞超极化，导致突触电压门控 Ca(2+)通道很少打开。为了在这些条件下理解神经递质传递，我们确定了在视锥带突触处囊泡融合所需的 L 型 Ca(2+)通道开放的数量。在电压钳制的视锥细胞中激活 Ca(2+)电流（I(Ca)），并从蝾螈视网膜切片制备中的水平细胞记录兴奋性突触后电流（EPSC）。通过 I(Ca)的均值-方差分析计算 Ca(2+)通道数量和单通道电流幅度。两种不同的比较——一种是比较平均释放事件数与平均 I(Ca)幅度，另一种是涉及 EPSC 和同时记录的视锥细胞 I(Ca)的解卷积——表明在刺激的最初几毫秒内，每个囊泡融合伴随着不到三个 Ca(2+)通道的开放。开放较少的 Ca(2+)通道并没有提高融合效率，这表明在强去极化期间很少发生不必要的通道开放。我们使用经验确定的突触尺寸、囊泡池大小、释放的 Ca(2+)依赖性、Ca(2+)通道数量和 Ca(2+)通道特性，模拟视锥突触的释放。当在 Ca(2+)扩散中添加一个障碍时，该模型复制了观察结果。与存在扩散障碍一致，用可扩散的 Ca(2+)缓冲液透析视锥细胞不会影响释放效率。Ca(2+)通道的紧密聚集，加上高 Ca(2+)亲和力的释放机制和扩散障碍，促进了 Ca(2+)内流和囊泡融合之间的线性耦合。这可能会提高当视锥细胞被强光超极化时对小光衰减的检测能力。
