Modulation of Neurotransmitter Transport by the Activity of the Action Potential Sodium Ion Channel in Membrane Vesicles from Rat Brain

(1) Transport of the neurotransmitters γ-amino-butyric acid (GABA) and L-glutamic acid was measured in isolated membrane vesicles derived from rat brain, with an artificially imposed electrochemical gradient of sodium ions (out > in) as a major driving force. Both transport processes were strongly inhibited by the alkaloid neurotoxin veratridine. Tetrodotoxin, which by itself had a slight stimulatory effect, completely reversed the inhibition by veratridine. (2) The degree of inhibition of neurotransmitter transport by veratridine was strongly dependent on the nature of the internal cation. With internal Tris or lithium ions inhibition by the neurotoxin was relatively poor as compared to the case when internal potassium was used. The parameter was not dependent on the nature of either the internal or the external anion. (3) The membrane vesicles catalyzed the uptake of ²²Na⁺ which was 3-1 CMold enhanced by veratridine. This effect was completely reversed by tetrodotoxin. The veratridine-stimulated sodium ion uptake obeyed Michaelis-Menten kinetics [apparent K_m = 11m. M, V_max = 150 nmol/(min mg of protein)]. Vesicles which had been allowed to accumulate ²²Na⁺ rapidly lost their radioactivity upon dilution into sodium-containing media, provided nigericin was present during dilution. (4) Veratridine-dependent sodium ion accumulation was also highly dependent on the nature of the internal cation, Tris and lithium being relatively poor in comparison with potassium. The nature of either the internal or the external anion was not important. (5) The concentration dependence of the inhibiting effect of veratridine on GABA transport paralleled that of its stimulatory effect on ²²Na⁺uptake (the half-maximal effects ranged from 20 to 30 µM). A similar parallel was found between the reversal of the effects of veratridine on both processes by tetrodotoxin (half-maximal effects between 10 and 20 nM). (6) It is concluded that in the isolated membrane vesicles functionality of the action potential Na⁺ channels is preserved and that, in rat brain, the sodium-coupled neurotransmitter high-affinity uptake systems for GABA and L-glutamic acid in the vesicles originate predominantly from the presynaptic plasma membrane.