Sign-preserving and sign-inverting synaptic interactions between rod and cone photoreceptors in the dark-adapted retina

摘要

Abstract We show that various types of rods and cones in the dark-adapted salamander retina are electrically coupled with linear and symmetrical junctional conductances Gj (40-223 pS) and a rank order: RodC-large single cone, rod-large single cone, rod-small single cone, rod-accessory double cone and rod-principal double cone. By systematically comparing the transjunctional current-voltage (Ij-Vj) relations and average Gj values of the five types of rod-cone pairs recorded at day and night times, our results suggest that the differences in Gj values among various types of rod-cone pairs are not caused by circadian differences, and the circadian-dependent changes in rod-cone coupling observed in the fish and rodent retinas are not present in the tiger salamander. In addition to rod-cone coupling, there is a sign-inverting, unidirectional rod→cone current IRC, and the IRC-VCone relations are linear, with a reversal potential near the chloride reversal potential ECl. IRC can be observed in rods and cones separated by at least 260 μm, and its waveform resembles that of the rod-elicited horizontal cell (HC) response IHC. A glutamate transporter-associated chloride channel blocker TBOA suppresses IRC but not IHC. These results suggest that IRC is largely mediated by HCs via a sign-inverting feedback chemical synapse associated with a chloride channel. IRC significantly reduced rod→cone coupling in the frequency range below 15 Hz, allowing better separation of rod and cone signals in the dark-adapted retina.