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

1. Intracellular recordings were made from L-type horizontal cells in the retina of the turtle Pseudemys scripta elegans. The responses were evoked by 500 msec pulses of 'white' light. 2. L-type horizontal cells were classified as either, 'small receptive field' s.r.f. or 'large receptive field' l.r.f. based upon (1) receptive field size and (2) kinetics of responses to test flashes covering small and big spots. 3. 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. 4. 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. 5. 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 state. 6. The photoresponses of red cones and s.r.f. horizontal cells were compared under similar states of adaptation. In the dark-adapted state of the gain, expressed as the millivolt change in the horizontal cell per millivolt change in the cone, was not linearly related to cone potential, and was highest for dim stimuli. Light-adaption modified the synaptic transmission to make the horizontal cells most sensitive to light modulation around the background illumination. 7. The mechanisms by which signal transmission can be modified by light-adaptation are discussed in terms of transmitter release by the presynaptic terminals and its binding to post-synaptic sites.