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

Jan 1, 1987Experimental biology

GABA and glycine change how rod and cone signals combine in horizontal cells of the frog retina

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Abstract

Under mesopic conditions, the response to a weak red flash can increase up to four-fold when presented against a green background.

Horizontal cells in the Xenopus retina exhibit non-linear interactions between synaptic inputs from rod and cone photoreceptors.
Enhancement occurs only when a red flash is combined with a green background, and not when the colors are reversed or identical.
The strength of the horizontal cell response is not affected by membrane polarization up to +/- 30 mV.
Under photopic conditions, enhancement is not observed with any combination of wavelengths.
GABA superfusion leads to hyperpolarization and increases rod input, while picrotoxin causes depolarization and favors cone input.
Glycine selectively reduces cone input, while its antagonist strychnine increases rod input, suggesting modulation by retinal circuits.

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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.

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

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