The effects of synaptic blocking agents on the antagonistic surround of the receptive field of cone photoreceptors were studied intracellular recording in the retina of hte turtle (Pseudemys scripta elegans) Illumination of a cone's receptive-field surround typically evoked a hybriid depolarizing response composed of two componests: (1) the graded synaptic feedback depolarization and (2) the prolonged depolarization a distinctive, intrinsic response of the cone. The locus of action of synaptic blocking agents was analyzed by comparing their effects on the light-evoked response of horizontal cells, the hybrid cone depolarization evoked by surround illumination, and the pure prolonged depolarization evoked by intracellular current injection.

The excitatory amino-acid antagonists, d-O-phosphoserine (DOS) and kynurenic acid (KynA), suppressed the light responses of horizontal cells and eliminated the surround-evoked, hybrid cone depolarization without affecting the prolonged depolarization evoked by current injection. Cobalt at 5–10 mM suppressed horizontal cell responses and thereby eliminated surround-evoked cone depolarizations. Cobalt (5–10 mM) also blocked the current-evoked prolonged depolarization, suggesting that the intrinsic cone mechanisms responsible for the prolonged depolarization are likely to be calcium-dependent.

Various GABA agonists and antagonists were found to have no effect on the surround-evoked depolarizations of cones. In contrast, a very low concentration of cobalt (0.5 mM) selectively suppressed the light-evoked feedback depolarization of cones without affecting horizontal cell responses or the current-evoked prolonged depolarization. Cobalt at 0.5 mM thus blocks the light-evoked action of the cone feedback synapse while sparing feedforward synaptic transmission from cones to horizontal cells. The implications of the present work for the possible neurotransmitters used at these synapses is discussed.

Intracellular recordings were made from rods in the superfused retina of the marine toad (Bufo marinus). It was found that injection of a brief depolarizing current pulse (0.04–1 nA) evoked a distinctive, long-lasting response, here called “the prolonged depolarization.” The response appears to be regenerative, has a stereotypical waveform, is typically about 6 mV in &litude and 3 s in duration, and has a relatively long recovery period (10–60 s). As a rule, the response cannot be directly evoked by light but the current-evoked response is significantly enhanced in the presence of steady illumination. The light-evoked hyperpolarization and the depolarizing spikes of the rod are both attenuated in the presence of the prolonged depolarization. The prolonged depolarization is not an altered manifestation of the depolarizing spikes of toad rods since both can be recorded simultaneously and steady illumination suppresses the spikes while enhancing the prolonged depolarization. The response is enhanced in chloride-free superfusate and also appears to be enhanced by the use of electrodes containing chloride. The response is markedly shortened in superfusates that lack calcium or contain 1–5 mM cobalt. On this and other evidence, it is suggested that the response may be generated by the sequential action of calcium channels and calcium-activated chloride channels. Although rarely evoked by light, the prolonged depolarization of toad rods is otherwise remarkably similar to the prolonged depolarization of turtle cones. It is proposed that the prolonged depolarization, in contrast to the feedback depolarization of cones, arises from mechanisms common to both rods and cones.