Functional characteristics of lateral interactions between rods in the retina of the snapping turtle.

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Abstract
Intracellular recordings were made of the slow hyperpolarizing light responses of single rods in the retina of the snapping turtle [Chelydra serpentina and C. osceola]. Physiological criteria used to identify rods were verified by intracellular injections of Procion Yellow. The amplitudes of the responses elicited by fixed intensity flashes increased as the stimulus was enlarged to a diameter of 300 .mu.m. Scattered light was incapable of accounting for this effect, which must result from summative interaction of rods with neighboring receptors. Effects of summative interaction were observed even at stimulus intensities that produced maximal responses. Enlarging the diameter of the higher intensity stimuli from 100-300 .mu.m increased the peak response amplitude by almost 50%; it also produced a distinct initial peak of the response which is termed the overshoot. The amplitude of this overshoot was graded with stimulus size. Complete intensity-response relationships were determined using stimulus diameters of 100 and 750 .mu.m for each rod. With the smaller stimulus the intensity response range was 4.5 log units, and with the larger stimulus this was increased to 5.0 log units. For intensities below about 60 quanta/.mu.m2 per flash (514 nm) the amplitudes elicited by the large stimulus followed a sigmoid-shaped curve. At higher intensities an additional lobe appeared on the intensity-response relationship. The appearance of this lobe correlated with the emergence of the overshoot on the response wave form. Determinations of rod flash sensitivity (mV/quantum per .mu.m2) showed that it increased with stimulus size up to a stimulus diameter of about 300 .mu.m. With diameters between 50-150 .mu.m, a linear relationship existed between the flash sensitivity and stimulus area. Absolute quantal sensitivities increased with stimulus area by a factor of 26, from a value of 28 .mu.V/photoisomerization per rod with a stimulus 25 .mu.m in diameter, to 720 .mu.V/photoisomerization per rod with a stimulus 300 .mu.m in diameter. The spectral sensitivity of the dark-adapted rod response closely followed the difference spectrum of the rod photopigment for wavelengths > 450 nm. This was true throughout the intensity range of the response, including low intensities where response averaging was necessary. The summative interaction observed in this work is apparently almost exclusively from rod to rod. Any given rod sums signals originating from other rods within a radius of about 150 .mu.m. This summative interaction affects the sensitivity, maximum amplitude and wave form of the light response. Procion Yellow injections revealed fine processes extending laterally for up to 35 .mu.m from the rod''s synaptic terminal in the outer plexiform layer. These processes exhibit en passage and terminal swellings. This may be a possible pathway for summative interaction between distant rods.