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GLYCINE: SYNTHESIS AND UPTAKE
Glycine is the major inhibitory neurotransmitter in the brainstem and spinal cord, where it participates in a variety of motor and sensory functions. Glycine is also present in the forebrain, where it has recently been shown to function as a coagonist at the
N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. In the latter, context glycine promotes the actions of glutamate, the major excitatory neurotransmitter (for a discussion of glycine's role as a coagonist of the NMDA receptor, see
Excitatory Amino Acid Neurotransmission). Thus, glycine subserves both inhibitory and excitatory functions within the CNS.
Glycine is formed from serine by the enzyme serine hydroxymethyltransferase (SHMT). Glycine, like GABA, is released from nerve endings in a Ca2+-dependent fashion. The actions of glycine are terminated primarily by reuptake via Na+/Cl--dependent, high-affinity glycine transporters. The specific uptake of glycine has been demonstrated in the brainstem and spinal cord in regions where there are also high densities of inhibitory glycine receptors.