Amino Acid Transport in Spinal and Sympathetic Ganglia
It has long been considered that, in common with monoamine neurotransmitters, the postsynaptic actions of the amino acids glycine, GABA, taurine, glutamate, aspartate,and proline are terminated by their rapid removal from the vicinity of the receptors by specific transport processes associated with the presynaptic nerve terminals. In tissue slices or synaptosomes, these transport processes possess a prominent high affinity component, which is absolutely dependent on the presence of extracellular Na+. Although other, non-transmitter amino acids appear also to be transported by systems of both low and high affinities8, their high-affinity components do not share the same requirement for sodium; neither are these systems localized preferentially in those areas of the CNS utilizing these substances as transmitters. Evidence from non- equilibrium density gradient centrifugation studies8 and electron microscopic autoradiography18 has demonstrated that accumulation is primarily into the nerve terminal regions of tissue preparations. However, it has been argued on the basis of the compartmentation of amino acid metabolism6,7 that the glial cell is likely to be of major importance in transmitter inactivation and, in vivo, the autoradiographic evidence20, 26 supports this suggestion. A number of preparations, including physically separated glial elements (see Haber, this book) and the isolated retina (see Neal, this book), have been classically utilized to demonstrate that glial cells will specifically accumulate a variety of neurotransmitters.
KeywordsDopamine Glutamate2 Glycine Retina Proline
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