Mechanisms of synaptic transmission in the retina

Albert Einstein College Of Medicine, Bronx NY

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Abstract

In the retina, the sign-inverting synapse between photoreceptors and ON bipolar cells is the foundation for the ON pathway in vision. The synapse is sign-inverting because glutamate, released in darkness by rod and cone photoreceptors, hyperpolarizes the membrane of ON bipolar cells. Hyperpolarization arises when binding of glutamate to the metabotropic receptor mGluR6 on the dendrites of the ON bipolar cell activates a G protein (Go), which then shuts off a synaptic transduction current, most likely through a membrane-delimited pathway. The channel, whose molecular identity is currently unknown, allows a mixture of cations to flow through it; as a result of this ion selectivity, the ON bipolar cell to depolarizes when light shuts off transmitter release from photoreceptors and allows these channels to open. Recent evidence from ours and other labs demonstrates that Ca2+ strongly inhibits the transduction current, contributing to the conversion of sustained to transient light responses in the retina. The goal of this proposal is to functionally identify the transduction channel(s) in mouse ON bipolar cells, and to elucidate the mechanism of regulation by Ca2+. The identity of the channel is currently unknown, but our recent findings suggest that it is a member of the TRP channel family in particular, and the vanilloid subgroup of the TRP family, TRPV1 in particular. TRPV1 agonist such as capsaicin and anandamide appear to open the ON bipolar cell transduction channel as well. Furthermore, TRPV1 channels are known to be regulated by a variety of intracellular messengers such as PIP2, Ca2+, and cAMP, many of which are also known to modulate the transduction channel in ON bipolar cells. Aims 1 and 2 of this proposal will examine the possibility that one or more TRPV1 agonist may serve as the endogenous activator of the transduction channel in bipolar cells of the mouse retina, as well as the mechanisms by which synthesis of these endogenous compounds are regulated. Aim III will follow up on our recent studies on Ca2+ regulation of transduction channels. The goal of this aim is to determine the mechanism by which Ca2+ depresses the transduction current, and it will also address the possibility that transduction channels expressed in rod and cone bipolar cells are differentially regulated by Ca2+. Results from these studies will provide insight into the fundamental mechanisms that regulate sensitivity and dynamic range in the visual system.