A fast rod photoreceptor signaling pathway in the mammalian retina

National Eye Institute

Investigators

Abstract

Rod photoreceptors mediate vision under conditions of dim illumination and achieve the remarkable feat of single photon detection. The great amplification required to respond to the absorption of a single photon necessarily implies that the kinetics of the rod photoresponse will be slow. Consistent with this, flicker fusion studies show that the rod system encodes light signals up to a flicker rate that is 5-10 slower than the cone system. Cones operate in bright lights and have photoresponses with faster kinetics than rods. Because the kinetics of the rod photoresponse is limiting, it is thought that transmission at the rod synapse would also be slow, which would have the benefit of filtering out rapid noise in the rod resulting from stochastic fluctuations in the transduction cascade and membrane conductance. Measurements of transmission at the rod synapse onto horizontal and Off bipolar cells in lower vertebrates are consistent with this view, and direct measurements of vesicle release from rods show that it is an unusually slow process.[unreadable] [unreadable] Mammalian retinas, in general, are rod dominant, and it is important to know whether the mammalian rod synapse, which differs in structure from the lower vertebrate rod synapse, also has slower kinetics. Unfortunately, in mammals, the predominant chemical synaptic output of rod photoreceptors is onto rod bipolar cells, which filter the rod input through a second messenger cascade that begins with a metabotropic glutamate receptor (mGluR6) and is consequently slow. Thus, while transmitter release and turnover at mammalian rods might be fast, the post-synaptic cell response is slow and thereby is not suitable for monitoring presynaptic release kinetics. In addition, capacitance measurements are difficult to perform due to limited patch pipette access onto such small cells. [unreadable] [unreadable] Recently, an alternative pathway for transmission at the rod synapse has been identified in some mammalian retinas, even though direct physiological evidence has not been reported. This pathway involves direct contacts between a rod terminal and a subtype of cone Off bipolar cell. Since Off bipolar cells contain AMPA/kainate receptors that can respond rapidly to transient changes in glutamate concentration, cell-pair recording in this pathway can provide the opportunity to characterize the kinetics of transmitter release and vesicle turnover in mammalian rods. [unreadable] [unreadable] We first used anatomical techniques to identify the types of cone bipolar cells that contacted rods. When injected with florescent tracer, the Off b2 bipolar cell, consistently contacted rod terminals within its dendritic field. Rod to b2 cell contacts were also observed when both cell types were simultaneously loaded with different tracers. b2 cell dendritic endings at rod terminals often had several branches, the ends of which were colocalized with GluR4 puncta. These results suggested that rods may signal directly to b2 Off bipolar cells at synapses that use AMPA-type receptors.[unreadable] [unreadable] We then examined the kinetics of the synapse between rods and b2 bipolar cells. We measured the transmission between a presynaptic rod and a postsynaptic b2 bipolar cell by simultaneously controlling their membrane voltages through whole cell patch pipettes. We found that although rod-initiated responses were smaller, when normalized, synaptic responses initiated by rods and cones had similar shapes and rise times. Rod synapses could still signal more sluggishly than cone synapses if transmission recovers more slowly after an initial epsc. Thus, we applied pair-pulse paradigm to measure response recovery and found that that ground squirrel rods and cones replenish their releasable pool of vesicles at similar rates.[unreadable] [unreadable] The fast kinetics of rod to b2 cone bipolar synapse indicates that this synapse may function to transmit high frequency signals under conditions where signaling in the other rod pathways is decreased by membrane and synaptic filtering. We are continuing our research on potential functions of this alternative rod pathway in the mammalian retina.