Glycine transporter-1 antagonist provides 1 neuroprotection in vivo

Abstract Glycine fulfills several roles in biology including protein synthesis, inhibitory transmission via glycine receptor activation and excitatory transmission through glutamate-sensitive N-methyl-D-aspartate receptors (NMDARs). Low glycine doses enhance NMDAR function while high doses trigger glycine-induced NMDAR internalization (GINI) in vitro. The physiological relevance of GINI has been questioned given that the high-affinity glycine transporter type 1 (GlyT1), located on astrocytes and neurons, maintains synaptic glycine concentrations far below the level that would saturate the glycine binding site (GBS) on NMDARs. Here, we report evidence that GINI occurs also in vivo and is neuroprotective following ischemic insult. Mice pre-treated with a GlyT1 antagonist (GlyT1-A), which increased glycine levels, exhibited smaller stroke volume, reduced cell death, and minimized behavioural deficits following stroke induction by either photothrombosis or endothelin-1. We demonstrate that in a modified in vitro ischemic paradigm, glycine is released at levels surpassing what occurs during ischemia alone. Therefore, glycine accumulates in the synaptic cleft, enhances occupancy of GBS and reaches the set point to trigger GINI. We report that GINI is observed during stroke, in vivo, only in the presence of a GlyT1-A. Moreover, we show evidence of a protective effect on the vasculature in the peri-infarct area. Therefore, these data strongly suggest that GlyT1 is a therapeutic target to prevent cell death following an ischemic event.