Dynamic role of GlyT1 as glycine sink or source: pharmacological implications for the gain control of NMDA receptors

Glycine transporter 1 (GlyT1) mediates termination of inhibitory glycinergic receptors signaling in the spinal cord and brainstem, and is also diffusely present in the forebrain. Here, it regulates the ambient glycine concentration influencing the ‘glycine’-site occupancy of N -methyl-d-aspartate (NMDARs). GlyT1 is a reversible transporter with a substantial, but not excessive, sodium-motive force for uphill transport. This study examines its potential role as a glycine source, either by reversed-uptake or by heteroexchange. I explored how glycine accumulation triggers its release, facilitating the activation of NMDARs by glutamate applied alone. Indeed, glutamate evokes no current in “naive” oocytes coexpressing GluN1/GluN2A and GlyT1, a previously characterized cellular model, but now using GlyT1 as the only potential source of coagonist for NMDAR activation. After glycine uptake, however, glutamate evokes large currents, blocked by ALX-5407 and potentiated by sarcosine, a specific inhibitor and substrate of GlyT1, respectively. These results suggest higher occupancy of the co-agonist site when GlyT1 functions as a glycine source either by reversed-uptake or by heteroexchange. A difference between these two glycine-release mechanisms occurs at hyperpolarized potentials, which induce an apparent voltage-dependent block of NMDAR currents, whereas heteroexchange preserves NMDAR activation at these potentials. Together, these results confirm GlyT1-mediated efflux as a positive regulator of NMDAR co-agonist site occupancy, and demonstrate sarcosine heteroexchange effectiveness in enhancing coagonist site occupancy. Depending on its actual mode of transport, GlyT1-inhibitors and sarcosine may have distinct effects on ambient glycine and NMDAR facilitation, and be a source of variation in reversing NMDAR hypofunction in schizophrenia.