GLYCINERGIC CONTROL OF [LEU5]ENKEPHALIN LEVELS IN CHICKEN RETINA

Retinal levels of [Leu5]enkephalin-like immunoreactivity (LE-LI) increase during the light and decrease during darkness, in vivo15. Intravitreal injection of the GABA antagonist picrotoxin had no effect on the accumulation of LE-LI during the light, suggesting the absence of significant GABAergic control over LE-LI cells. However, injection of the glycine antagonist strychnine, prevented the light-induced increase of retinal levels of LE-LI during 6 h exposure to light, indicating the presence of glycinergic control over the LE-LI neurons. When applied during the dark, strychnine increased the depletion of LE-LI by 34% compared to vehicle-injected eyes, suggesting that the LE-LI neurons receive some glycinergic input during the dark as well. The release of LE-LI from retinas superfused in vitro is depressed by exposing the preparation to light2. Superfusing isolated retinas with physiological buffer containing picrotoxin (100-mu-M), GABA (50 mM), or the GABA agonists muscimol (100-mu-M), (+)-baclofen (200-mu-M), or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) (100 mM), had no effect on the efflux of LE-LI. Strychnine (100 mM) however increased the efflux of LE-LI by 64%, compared to the spontaneous efflux during the light. Glycine (15 and 50 mM) decreased the spontaneous efflux of LE-LI from retinas superfused in darkness by 44-48% and by 31% at 5 mM. These data are consistent with the results from pharmacological manipulations in vivo. We conclude that the LE-LI amacrine cells are under inhibitory control from glycinergic but not from GABAergic neurons. A change in degree of glycinergic inhibition according ambient lighting may contribute to, but does not account entirely for, the light-driven variation in activity of LE-LI neurons in chicken retina.