Regulation of photoreceptor neurotransmission

描述（由申请人提供）：与大多数其他CNS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，对光表现出分级的超极化反应，并且通常不产生动作电位。为了适应杆和锥的分级反应，有人提出，从杆和锥的突触传递机制也不同于其他CNS神经元。在大多数CNS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（mEPSC）的总和作用，每个mEPSC由单个突触囊泡的融合产生，所述单个突触囊泡短暂地将紧邻突触间隙中的谷氨酸升高至高浓度（> ImM）。与此相反，有人认为，在感光突触的突触后反应可能是由空间整合水平的谷氨酸在突触间隙。该建议区分这两种可能性，同时使用全细胞记录光感受器和突触后神经元以及电容技术测量胞吐。除了定义胞吐作用和感光突触后反应之间的关系外，拟议的实验将分析突触间隙中的谷氨酸水平以及这些水平对谷氨酸受体脱敏的影响，OFF型双极细胞和水平细胞中谷氨酸受体之间的差异，以及水平和OFF双极细胞中mEPSC的特性。如果发现囊泡的量子突触后作用在很大程度上决定了二级视网膜神经元的响应，则诱发的突触后电流将被去卷积成它们各自的量子，并用于确定不同生理条件下的释放参数，例如改变照明水平。除了理解视觉信息如何在视觉通路中的第一个突触上转化之外，所提出的关于光感受器调节谷氨酸释放的实验对于理解视网膜中的病理生理学也很重要，因为增加的谷氨酸释放（例如，伴随局部缺血）可对突触后神经元具有兴奋性毒性后果。

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.