One of the important features of visual information processing in the retina is the conversion of relatively sustained light responses in the outer retina to more diversified responses, including the formation of the transient response, in the inner retina. Increasing evidence suggests that bipolar cells play an active role in retinal processing and that the diversified responses in the inner retina originate, in part, by means of the different forms of transmitter release from bipolar cells. By what mechanism(s) the different forms of transmitter release are generated and to what extent bipolar cell processing contributes to the overall retinal processing remain to be elucidated. The long-term objective of this proposal is to understand the roles of voltage-dependent membrane channels in bipolar cell signal processing. Mammalian retinal bipolar cells express a variety of voltage-dependent channels, including low-voltage-activated (LVA) and high-voltage-activated (HVA) Ca2+ channels at the axon terminals and voltage-gated Na+ channels in a subset of cone bipolar cells. The physiological roles of multiple voltage-activated Ca2+ channels and voltage-gated Na+ channels in bipolar cell signal processing will be investigated. The first part of this proposal is to study the roles of LVA and L-type HVA Ca2+ channels in bipolar cell transmitter release.
Specific aim 1 will determine the Ca2+ channel type(s) located at the axon terminals and involved in transmitter release of different subtypes of bipolar cells.
Specific aim 2 will determine the temporal properties of transmitter release from bipolar cells during the activation of LVA and L-type Ca2+ channels. The second part of this application is to study the roles of voltage-dependent membrane channels in shaping bipolar cell response waveforms.
Specific aim 3 will determine the roles of different voltage-dependent channels in the spontaneous and evoked response waveforms in isolated bipolar cells.
Specific aim 4 will determine the subtypes of bipolar cells expressing Na+ channels and the contribution of Na+ currents in bipolar cell light response waveforms. The studies will be carried out with isolated bipolar cells and with bipolar cells in retinal slice preparations of mammalian retinas. Patch-clamp recording and optical Ca2+ imaging methods will be employed. The knowledge we gain from these studies will lead to a better understanding of basic visual information processing in the mammalian retina. The understanding of the role of voltage-activated Ca2+ channels in synaptic transmission may also provide insight for identifying the mechanism of diseases caused by synaptic dysfunction.
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