Neuronal Uptake of Neurotransmitters and their Precursors: Studies with “Transmitter” Amino Acids and Choline

Abstract

Most neurotransmitters, except for acetylcholine, possess high affinity neuronal uptakes. With reasonably well established transmitter candidates such as catecholamines, serotonin and GABA, these uptake systems are studied with a view to characterizing the synaptic behavior of the transmitter in question. With more questionable transmitter candidates, such as the amino acids, before characterizing subtleties of their synaptic activities, it is crucial first to determine whether they are in fact neurotransmitters. In our laboratory, uptake studies have been employed to adduce evidence in support of certain amino acids as transmitters. Specifically, we have compared the synaptosomal accumulation of numerous amino acids and found that unlike most amino acids, glutamic and aspartic acids and glycine (in the spinal cord and brain stem) are transported by unique high affinity systems into distinct populations of synaptosomes. In this way neuronal uptake has provided powerful biochemical support for the proposition that these compounds are neurotransmitters in the mammalian central nervous system.