Abstract
Glycine, along with GABA, constitutes the major inhibitory neurotransmitter in the central nervous system. In the retina, glycinergic neurotransmission is primarily used by amacrine cells that are involved in the lateral processing of visual stimuli in the inner retina. We have previously shown that the high-affinity glycine transporter 1 (GlyT1), that is commonly used as a reliable marker for glycinergic amacrine cells in the retina, is essential for glycinergic neurotransmission by these cells. Abolishment of retinal GlyT1 expression results in a breakdown of glycinergic neurotransmission by AII amacrine cells, but most likely also by other glycinergic amacrine cell populations. However, the impact of loss of glycinergic neurotransmission on retinal signal processing and visually guided behavior, has not yet been elucidated. In this study, the effects of loss of retinal GlyT1 expression in glycinergic amacrine cells on the optomotor reflex and on the photopic and scotopic electroretinogram (ERG) responses were analyzed. We show that retinal GlyT1-deficient mice have normal optomotor responses to rotating black and white stripes. When stimuli with sawtooth luminance profiles were used, thereby differentially activating ON and OFF pathways, the GlyT1 deficient mice showed facilitated responses to ON preferring stimuli, whereas responses to OFF preferring stimuli were unchanged. These findings were corroborated by ERG recordings that showed undistinguishable responses after flash stimulation but revealed differences in the differential processing of ON and OFF preferring stimuli. To determine if the function of retinal GlyT1 is conserved in humans, we analyzed ERG recordings from a patient diagnosed with GlyT1 encephalopathy. We show that GlyT1 deficiency results in marked ERG changes, characterized by an almost complete loss of the "photopic hill" phenomenon, a hill-like appearance of the relationship between the b-wave amplitude and log light stimulus strength under background illumination conditions, and reductions in the ERG oscillatory potentials in the dark- and light-adapted states. Both findings are consistent with an altered interaction between ON- and OFF pathways in the retina. Taken together our data show that glycinergic neurotransmission in the retina has important functions in retinal ON and OFF processing both in mice and humans.