The absence of spread of adaptation between rod photoreceptors in turtle retina

Abstract

Adaptation by weak backgrounds and the spatial spread of desensitization between rods was studied in the snapping turtle retina, Chelydra serpentina. Intracellular membrane potentials were recorded from these photoreceptors in an eyecup preparation. The kinetics and sensitivity of rod responses were changed significantly by large, very dim backgrounds. For the twenty-five most sensitive rods where the dark-adapted flash sensitivity, SDF, was greater than 1.0 mV/Rh*, Rh* being the number of effective photo-isomerizations per rod, the background intensity required to halve the amplitude of the linear range response averaged 0.21 Rh* s-1. The time-to-peak of the test responses was reduced up to 50% by these dim backgrounds. The desensitizing effects of full field backgrounds of various intensities on the responses to large test spots were measured. The dependence of incremental flash sensitivity, SF, on background intensity, IB, followed the form (FORMULA: SEE TEXT) where I0 is the background intensity which halved SDF. The same intensity dependence held for slit-shaped background fields that desensitized responses to small test spots. The desensitizing effects of large, very dim flashed and continuous backgrounds took several seconds to appear and decay to dark levels. This in conjunction with the sparsity of photons suggests, that the desensitization from a single photoisomerization can persist for several seconds. A comparison of the desensitizing effects of spot and annular backgrounds revealed that small spot backgrounds superimposed on the centered test spots desensitized rods more effectively than annular fields. This finding held true even when annular patterns produced a greater maintained hyperpolarization in the rods. Thus, there was no unique relationship between desensitization and the steady maintained hyperpolarization evoked by a background field. The dependence of adaptation on distance from the impaled rod was determined with slit-shaped background fields placed at different positions across the rod's receptive field. The desensitizing effect of displaced slit stimuli was found to decline much more rapidly with distance than excitation. Displacing the slit by 20 micron from the centre reduced its desensitizing effect by more than 1 log unit. In contrast, excitation fell to about 80% at the same distance (lambda ranging from 50 to 70 micron). The fall off of desensitization with distance matched the calculated fall off with distance of light scatter from a slit. No difference was noted in the kinetics of test responses in the presence of equally desensitizing, superimposed and displaced slits.(ABSTRACT TRUNCATED AT 400 WORDS)