Glycine removal from both glycinergic and glutamatergic synapses into astrocytic processes by glycine transporter 1 in the adult mouse spinal cord.

Glycine (Gly) and γ-aminobutyric acid (GABA) are inhibitory neurotransmitters in the spinal cord. Released Gly is removed by two Gly transporters (GlyTs): GlyT1 and GlyT2. We have previously demonstrated the localization of GlyT2, which reuptakes Gly into presynaptic terminals. In the present study, we focused on GlyT1, which terminates glycinergic transmission by uptaking Gly into astrocytes. GlyT1 may also regulate glutamatergic transmission by controlling the concentration of Gly, which is a co-agonist of N-methyl D-aspartate receptors in the forebrain, where glycinergic synapses are sparse or absent. The immunohistochemical localization of GlyT1 was examined using a newly established GlyT1 antibody in the adult spinal cord; glycinergic, GABAergic, and glutamatergic synapses were intermingled in this region. GlyT1 was localized not only near peri-synaptic sites but also throughout astrocytic processes in the gray matter, including laminae I and II, where glycinergic terminals were sparse. GlyT1-positive honeycomb-like structures often contained GlyT2-positive glycinergic terminals. Immuno-electron microscopic analysis revealed that GlyT1-expressing astrocytic processes filled the entire interneuronal space and sealed not only inhibitory but also excitatory synapses; however, GlyT1 was not expressed in axon terminals. In addition, both GlyT1 and astrocytic GABA transporter, GAT-3, were similarly localized in the astrocytic processes. Together, these results suggest that GlyT1 expression is unrelated to glycinergic terminal density. Gly may be removed by GlyT1 from the entire interneuronal space-including glycinergic, GABAergic, and glutamatergic synaptic clefts-into astrocytic processes. Moreover, GlyT1 may regulate glutamatergic transmission via N-methyl D-aspartate receptors and might be involved in neural plasticity in the spinal cord.