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.
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