Amino acid neurotransmitters in the CNS

Abstract

D-Aspartate exchange, defined as amino acid-stimulated D-[³H]aspartate efflux, was investigated in a preparation of rat brain synaptosomes. The efflux of radiolabelled D-aspartate was found to be enhanced by micromolar concentrations of externally added D- and L-aspartate, L-glutamate, L-cysteate.and L-cysteinesulphinate. The stimulation of release by external amino acids followed Michaelis-Menten kinetics; the apparent K _m values (in μM) were: 14.65 ± 0.98 for D-aspartate; 8.00±1.5 for L-aspartate; 22.31 ± 1.62 for L-glutamate; 6.76 ± 0.3 for L-cysteate and 7.89± 1.23 for L-cysteinesulphinate. The ν_max values for efflux were 2.16–4.06 per mg protein. The exchange process was found to require external NaCl but was very little affected by increase in the external [K⁺]. The demonstration of exchange as a part of the transport process provides support for the suggestion that in synaptosomal preparations a substantial portion of influx and efflux of amino acid neurotransmitters occurs via a reversible membrane carrier.