Bartoletti Theodore M, Thoreson Wallace B
Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, NE 68198-5840, USA.
Mol Vis. 2011 Apr 12;17:920-31.
Vision is encoded at photoreceptor synapses by the number of released vesicles and size of the post-synaptic response. We hypothesized that elevating cytosolic glutamate could enhance quantal size by increasing glutamate in vesicles.
We introduced glutamate (10-40 mM) into cone terminals through a patch pipette and recorded excitatory post-synaptic currents (EPSCs) from horizontal or OFF bipolar cells in the Ambystoma tigrinum retinal slice preparation.
Elevating cytosolic glutamate in cone terminals enhanced EPSCs as well as quantal miniature EPSCs (mEPSCs). Enhancement was prevented by inhibiting vesicular glutamate transport with 1S,3R-1-aminocyclopentane-1,3-dicarboxylate in the patch pipette. A low affinity glutamate receptor antagonist, γD-glutamylglycine (1 mM), less effectively inhibited EPSCs evoked from cones loaded with glutamate than control cones indicating that release from cones with supplemental glutamate produced higher glutamate levels in the synaptic cleft. Raising presynaptic glutamate did not alter exocytotic capacitance responses and exocytosis was observed after inhibiting glutamate loading with the vesicular ATPase inhibitor, concanamycin A, suggesting that release capability is not restricted by low vesicular glutamate levels. Variance-mean analysis of currents evoked by flash photolysis of caged glutamate indicated that horizontal cell AMPA receptors have a single channel conductance of 10.1 pS suggesting that ~8.7 GluRs contribute to each mEPSC.
Quantal amplitude at the cone ribbon synapse is capable of adjustment by changes in cytosolic glutamate levels. The small number of channels contributing to each mEPSC suggests that stochastic variability in channel opening could be an important source of quantal variability.
视觉在光感受器突触处通过释放的囊泡数量和突触后反应的大小进行编码。我们推测，提高胞质谷氨酸水平可通过增加囊泡中的谷氨酸来增强量子大小。
我们通过膜片吸管将谷氨酸（10 - 40 mM）引入视锥细胞终末，并在虎纹钝口螈视网膜切片标本中记录水平细胞或 OFF 双极细胞的兴奋性突触后电流（EPSC）。
提高视锥细胞终末的胞质谷氨酸水平可增强 EPSC 以及量子微小 EPSC（mEPSC）。通过在膜片吸管中用 1S,3R - 1 - 氨基环戊烷 - 1,3 - 二羧酸抑制囊泡谷氨酸转运可阻止这种增强。一种低亲和力谷氨酸受体拮抗剂γD - 谷氨酰甘氨酸（1 mM）抑制从加载了谷氨酸的视锥细胞诱发的 EPSC 的效果不如对照视锥细胞，这表明从补充了谷氨酸的视锥细胞释放会在突触间隙产生更高的谷氨酸水平。提高突触前谷氨酸水平不会改变胞吐电容反应，并且在用囊泡 ATP 酶抑制剂 concanamycin A 抑制谷氨酸加载后仍观察到胞吐作用，这表明释放能力不受低囊泡谷氨酸水平的限制。对笼锁谷氨酸闪光光解诱发的电流进行方差 - 均值分析表明，水平细胞的 AMPA 受体单通道电导为 10.1 pS，这表明每个 mEPSC 约有 8.7 个谷氨酸受体通道起作用。
视锥细胞带状突触处的量子幅度能够通过胞质谷氨酸水平的变化进行调节。每个 mEPSC 中起作用的通道数量较少表明通道开放的随机变异性可能是量子变异性的一个重要来源。
