Regulation of photoreceptor neurotransmission

描述（由申请人提供）：与大多数其他中枢神经系统神经元不同，光感受器在黑暗中具有相对去极化的静息电位，对光表现出渐变的超极化反应，并且通常不产生动作电位。为了适应视杆细胞和视锥细胞的分级反应，有人提出视杆细胞和视锥细胞的突触传递机制也不同于其他中枢神经系统神经元。在大多数中枢神经系统神经元中，突触后反应反映了独立的微型兴奋性突触后电流（mEPSCs）的综合作用，这些电流由单个突触囊泡融合产生，可以短暂地将紧邻的突触间隙中的谷氨酸升高到高浓度（bbb1mm）。相反，有研究表明，光感受器突触的突触后反应可能由突触间隙中谷氨酸的空间整合水平决定。该提案通过同时使用来自光感受器和突触后神经元的全细胞记录以及用于测量胞吐的电容技术来区分这两种可能性。除了确定胞外分泌与光感受器突触突触后反应之间的关系外，本实验还将分析突触间隙中的谷氨酸水平及其对谷氨酸受体脱敏的影响，OFF型双极细胞和杆状细胞与锥体接触的水平细胞中谷氨酸受体的差异，以及水平和OFF双极细胞中mepsc的特性。如果发现囊泡突触后的量子行为在很大程度上决定了二级视网膜神经元的反应，那么所诱发的突触后电流将被解卷积到它们的单个量子中，并用于确定不同生理条件下（如光照水平变化）的释放参数。除了了解视觉信息如何在视觉通路的第一突触中转换之外，关于光感受器调节谷氨酸释放的实验对于理解视网膜的病理生理学也很重要，因为谷氨酸释放增加（例如伴随缺血）可能对突触后神经元产生兴奋毒性后果。

DESCRIPTION (provided by applicant): Unlike most other CNS neurons, photoreceptors have a relatively depolarized resting potential in darkness, exhibit graded hyperpolarizing responses to light, and do not usually generate action potentials. To accommodate the graded responses of rods and cones it has been suggested that mechanisms of synaptic transmission from rods and cones also differ from those at other CNS neurons. At most CNS neurons, post-synaptic responses reflect the summed actions of independent miniature excitatory post-synaptic currents (mEPSCs) each arising from fusion of single synaptic vesicles that briefly elevate glutamate in the immediately adjoining synaptic cleft to high concentrations (>1 mM). In contrast, it has been suggested that post-synaptic responses at the photoreceptor synapse may be determined by spatially integrated levels of glutamate in the synaptic cleft. The proposal distinguishes between these two possibilities using simultaneous whole cell recordings from photoreceptors and post-synaptic neurons as well as capacitance techniques for measuring exocytosis. In addition to defining the relationship between exocytosis and postsynaptic responses at the photoreceptor synapse, the proposed experiments will analyze glutamate levels in the synaptic cleft and the impact of these levels on glutamate receptor desensitization, differences between glutamate receptors in OFF-type bipolar cells and horizontal cells contacted by rods vs. cones, and properties of mEPSCs in horizontal and OFF bipolar cells. If the quantal post-synaptic actions of vesicles are found to largely determine responses of second order retinal neurons, then evoked post-synaptic currents will be deconvolved into their individual quanta and used to determine release parameters under different physiological conditions such as changing levels of illumination. In addition to understanding how visual information is transformed across the first synapse in the visual pathway, the proposed experiments on the regulation of glutamate release by photoreceptors are also important for understanding pathophysiology in the retina since increased glutamate release (e.g., accompanying ischemia) can have excitotoxic consequences on post-synaptic neurons.