Furthermore, one of the genes most highly enriched in non-telencephalic astrocytes was Slc6a9 (Figure 3G), encoding the glycine reuptake transporter GLYT1. Glycine is a widely used inhibitory neurotransmitter only in the caudal parts of the brain and in the spinal cord. This suggests a specific role for non-telencephalic astrocytes in clearing glycine from the synaptic cleft. We note that since GLYT1 is a reversible glycine transporter (Supplisson and Roux, 2002), if it were expressed in telencephalic astrocytes, then those cells would potentially secrete glycine into the synaptic cleft instead of absorbing it. Glycine is not only an inhibitory neurotransmitter, but also a co-ligand for the NMDA glutamate receptor involved in coincidence detection. This may explain the need for distinct types of astrocytes in glycine-rich and glycine-poor regions of the brain.
