Glycinergic control of [Leu 5]enkephalin levels in chicken retina

Abstract

Retinal levels of [Leu 5]enkephalin-like immunoreactivity (LE-LI) increase during the light and decrease during darkness, in vivo 15. 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 light 2. Superfusing isolated retinas with physiological buffer containing picrotoxin (100 μM), GABA (50 mM), or the GABA agonists muscimol (100 μM), (+)-baclofen (200 μM), 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.