GABA and glycine modify the balance of rod and cone inputs to horizontal cells in the Xenopus retina.

Under mesopic conditions, the light-evoked waveform of horizontal cells in Xenopus retina reflects synaptic input from both rod and cone photoreceptors. These inputs interact non-linearly: the response to a weak red (cone-effective) flash is increased up to four-fold when the red stimulus falls on a green (rod-effective) background by reference to the response elicited by the same red flash on a dark field. We refer to this phenomenon as enhancement. It was not observed either when stimulus and field wavelengths were reversed or were of the same color. Enhancement was not altered by polarization of the horizontal cell membrane up to +/- 30 mV with extrinsic current. Enhancement could not be elicited with any combination of test and background wavelengths under photopic conditions. Superfusion of the retina with GABA resulted in a hyperpolarization of the horizontal cell membrane and an emphasis of the rod input to the horizontal cell light-evoked response. Picrotoxin depolarized the cell and favored the cone input. Enhancement was altered in predictable ways by these drugs. Glycine reduced selectively the cone input to the horizontal cell, whereas its antagonist, strychnine, increased the rod input. The results are interpreted to indicate that the excitability of the horizontal cell is modified both by distal and proximal retinal circuits; a glycinergic interplexiform cell probably plays a role in the latter pathway.