GlyT1 and GlyT2 in brain astrocytes: expression, distribution and function

GlyT1 and GlyT2 are the transporters responsible for glycine uptake from the synaptic cleft. The expression and function of these two glycine transporters in rat cortical cultured astrocytes over several maturation stages (10, 18 and 24 days in vitro) were herein investigated. Quantitative PCR and western blot showed that both GlyT1 and GlyT2 transcripts and protein were expressed in astrocytes in the examined maturation stages. Double detection of Glial fibrillary acidic protein (GFAP) and GlyT1/GlyT2 revealed that both transporters were detected in the cell body and in the processes of astrocytes. Furthermore, the double immunofluorescence analysis carried out in P21 rat brain slices corroborated the presence of both transporters in cortical and hippocampal astrocytes. The functional characterization of GlyT1 and GlyT2 in cultured astrocytes performed by [³H]glycine uptake experiments revealed that both transporters take up glycine in a concentration-dependent way, but with a very distinct affinity. Kinetic analysis revealed a K _m of 51.15 ± 4.96 μM and a V _max of 379.30 ± 10.31 pmol/min/mg for GlyT1 and a K _m of 1,801 ± 148.9 μM and a V _max of 5,730 ± 200.2 pmol/min/mg for GlyT2. It is concluded that astrocytes express functional GlyT2, which challenge previous findings that those cells would express only GlyT1, whereas GlyT2 was supposed to be restricted to the glycinergic nerve terminals. Therefore, the work herein reported provides new insights about glycinergic neurotransmission in the brain.