PHOTORECEPTOR COUPLING IN TURTLE RETINA

Photoreceptors in the isolated turtle retina of two species of turtle, Chelydra serpentina and Pseudemus scripta elegans, were penetrated wi th double-barrel electrodes. Physiological responses were recorded thr ough one barrel and Neurobiotin tracer was injected from the other. In tracellular injection of Neurobiotin revealed patterns of tracer-coupl ed photoreceptors. Both the patterns of tracer coupling and the electr ophysiology suggest a high degree of specificity of connections. Rods seem to be coupled only to rods and green and red cones seem to be cou pled to cones of the same spectral type. Receptive-field profiles, mea sured with a thin, sharply focused slit of Light, often had well-defin ed peaks and troughs in sensitivity. We have taken advantage of this o bservation and used the position of a peak in sensitivity to locate th e position on the retina of a coupled cell. In one rod, it was possibl e to correlate physiological and morphological data and to show that t he peaks in the physiological receptive field occurred at positions on the retina where there were dye-coupled cells. This provides direct e vidence that gap junctions produce the physiological coupling between rods.