The actions of gamma-aminobutyric acid, glycine and their antagonists upon horizontal cells of the Xenopus retina.

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RESUMO

We examined the effects of gamma-aminobutyric acid (GABA) and glycine and their respective antagonists, picrotoxin and strychnine, upon the membrane potential and light-evoked responses of the type H1 horizontal cell of the Xenopus retina. This horizontal cell receives mixed input from rod and cone receptors. Under control conditions the mean membrane potential was -37.8 +/- 9.7 mV. Addition of 5 mM-GABA to the superfusate hyperpolarized the cell by 4.0 +/- 2.6 mV within 3-5 min; addition of 0.5 mM-picrotoxin depolarized the cell by 4.3 +/- 2.1 mV. Prolonged (greater than 15 min) exposures to the drugs elicited more pronounced changes in membrane potential. GABA and picrotoxin affected primarily the cone-dependent input to the H1 horizontal cell. Under dark-adapted conditions, response wave forms were essentially unaltered by the drugs, but when the horizontal cell was moderately or fully light adapted, GABA reduced and picrotoxin enhanced the cone-dependent component of its response to light. Long-term (greater than 15 min) exposures to GABA and picrotoxin elicited changes in response kinetics usually associated with dark and light adaptation, respectively. Glycine, at bath concentrations of 0.6 mM or greater, depolarized horizontal cells by 21 mV on average and reduced or abolished their light response. This action did not occur in the presence of 0.1 mM-strychnine. When all light-evoked activity was blocked by 20-40 mM-magnesium, the depolarizing action of glycine still occurred. Thus, glycine appears to act directly upon the horizontal cell membrane. Neither GABA nor glycine, nor their respective antagonists, affected the spatial extent of the horizontal cell receptive field.

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