A comparison of the inhibitory actions of glycine and GABA on acetylcholine release from the rabbit retina

Abstract

The inhibition of [³H]acetylcholine (ACh) release from cholinergic amacrine cells by glycine and GABA was studied using an in vivo eyecup preparation in anesthetized rabbits. Glycine (1 mM) had no effect on basal ACh release, but completely blocked the light-evoked release of ACh. Glycine also blocked the strong potentiating effects of picrotoxin (20 μM) normally observed on basal and light-evoked release. Strychnine (20 μM) increased basal release, albeit less than picrotoxin, but partially inhibited and altered the shape of light-evoked responses. Co-perfusion of picrotoxin and strychnine after strychnine application resulted in a larger additional basal increase. However, light-evoked responses were not restored to a control shape and magnitude, or to potentiated levels as with picrotoxin alone, but remained altered and partially inhibited. These results support the concept of a sustained GABA-mediated inhibition of the cholinergic pathway in the intact retina. In contrast, glycine-mediated inhibition of the cholinergic pathway differs, with the present results indicating a significantly smaller sustained inhibition of basal release and a temporal inhibition of light-evoked release. The lack of effect of any of these compounds on kainate-evoked responses indicates that these effects are predominately indirect, possibly on the presynaptic bipolar cell.