Glycine uptake in rat central nervous system slices and homogenates: evidence for different uptake systems in spinal cord and cerebral cortex

— Evidence is presented that glycine is taken up by two different transport systems in rat CNS tissue slices; one system has relatively low affinity for glycine (Km= 300 μm) and predominates in cerebral cortex, cerebellum and mid-brain, the other has a higher affinity for glycine (Km= 40 μm) and is detectable only in spinal cord, medulla and pons. The low affinity transport system appears to be shared by other small neutral amino acids, whereas the high affinity system is very specific for glycine. Both transport systems were shown to be present in particles in homogenates of CNS tissue by incubation with glycine in vitro, and subcellular fractionation studies suggested that synaptosomes were partly responsible for such uptake. Various substances were tested as inhibitors of the high affinity uptake system for glycine in spinal cord slices; the most potent inhibitors were p-chloro-mercuriphenylsulphonate, N-ethylmaleimide, chlorpromazine, imipramine, desipramine, hydrazinoacetic acid and haloperidol. No competitive inhibitors of the high affinity glycine uptake were found. It is suggested that the high affinity transport system is associated with inhibitory synapses where glycine is a transmitter.