Signal transmission from red cones to horizontal cells in the turtle retina.

Publisher Website
Google Scholar
Abstract
Intracellular recordings were made from L-type horizontal cells in the retina of the turtle Pseudemys scripta elegans. The responses were evoked by 500 ms pulses of white light. L-type horizontal cells were classified as either, small receptive field (s.r.f.) or large receptive field (l.r.f.) based upon receptive field size and kinetics of responses to test flashes covering small and big spots. Constant illumination of the entire receptive field, with any intensity studied, evoked a response that reached a peak and then slowly sagged back to a steady-state level that was about half the peak response. Termination of backgrounds resulted in a very fast recovery of the membrane potential that overshot the dark-adapted potential. This off response had faster kinetics in horizontal cells than in red cones. The intensity-response curve measured around any background intensity was shifted along the log intensity axis toward higher test intensities. The curves obtained under light-adapted conditions were sharper than the curve measured in the dark-adapted stage. The photoresponse of red cones and s.r.f. horizontal cells were compared under similar states of adaptation. The gain in the dark-adapted state, expressed as the mV change in the horizontal cell per mV change in the cone, was not linearly related to cone potential, and was highest for dim stimuli. Light-adaptation modified the synaptic transmission to make the horizontal cells most sensitive to light modulation around the background illumination. The mechanisms by which signal transmission can be modified by light-adaptation were discussed in terms of transmitter release by the presynaptic terminals and its binding to post-synaptic sites.