Asc-1/SLC7A10 regulates NMDA receptor co-agonist availability at hypoglossal motoneurons

The Asc-1/SLC7A10 transporter regulates extracellular levels of the NMDA receptor co-agonists glycine and D-serine, yet its role at excitatory synapses in the brainstem remains poorly understood. We investigated how pharmacological Asc-1 blockade modulates NMDA receptor-mediated synaptic transmission at hypoglossal motoneurons and whether glycine or D-serine is the functionally dominant co-agonist at these synapses. Whole-cell patch-clamp recordings were performed in Mg²⁺-free ACSF from hypoglossal motoneurons in brainstem slices of neonatal mice. Bath application of the selective Asc-1 inhibitor BMS-466442 (10 µM) significantly increased NMDA receptor-mediated mEPSC amplitude and tonic holding current, without affecting mEPSC frequency. Exogenous glycine (1 mM) robustly potentiated mEPSC amplitude and frequency both in the absence and presence of BMS-466442, whereas D-serine selectively increased tonic holding current in an Asc-1-dependent manner. Three-way ANOVA confirmed a significant co-agonist type × condition interaction, with glycine more effective than D-serine at potentiating phasic synaptic responses. These findings identify Asc-1 as a regulator of NMDA receptor co-agonist availability at brainstem excitatory synapses and suggest that glycine, rather than D-serine, is the dominant synaptic co-agonist at mouse hypoglossal motoneurons.