Effects of external ions on the synaptic transmission from photorecptors to horizontal cells in the carp retina.

1. Intracellular recordings were made from cones and horizontal cells of the isolated carp retina and the mechanisms of synaptic transmission from cones of horizontal cells were studied by changing the ionic composition of the external medium. 2. Cones were depolarized and their light responses enhanced in low Ca high-Mg medium. In the same medium, in which chemical transmission is suposed to be blocked, horizontal cells were hyperpolarized and their light responses disappeared. 3. When the synaptic input was removed, the membrane potential of horizontal cells agreed well with Ek. 4. In Na-free medium both cones and horizontal cells were hyperpolarized and response disappeared. On reaplication of normal Ringer, horizontal cells showed a trenaient membrane potential reversal (inside positive), indicating that the horizontal cell has a high Na permeability under the influence of the endogenous transmitter. 5. Application of La produced little change in cones, while it strongly depolarized horizontal cells...