EFFECT OF VINBLASTINE AND COLCHICINE ON UPTAKE AND RELEASE OF PUTATIVE TRANSMITTERS BY SYNAPTOSOMES AND ON BRAIN ACTOMYOSIN‐LIKE PROTEIN

Abstract— The effects of several inhibitors, including vinblastine and colchicine, on the accumulation of a number of putative transmitters by a rat brain synaptosomal preparation and their subsequent release by excess K⁺ was examined. In addition, the effect of the alkaloids on the ATPase activity of the actomyosin‐like protein, neurostenin, isolated from the synaptosomal preparation, was studied. The uptakes of radioactive glutamate, GABA, dopamine and norepinephrine were energy‐dependent, as evidenced by their susceptibility to 0.01 mM carbonyl cyanide m‐chlorophenylhydrazone (Cl‐CCP), 01 mM ouabain and temperature. The active accumulations of GABA, dopamine and norepinephrine were also greatly inhibited by 1 mM6‐hydroxydopamine (6‐OHDA), 01 mM mersalyl, 0.05–0.25mM vinblastine and 0.1–1.0 mM colchicine. Vinblastine was approximately 10‐fold more potent (K₁, −0.1 mM) than colchicine as an inhibitor. The release of actively accumulated dopamine or norepinephrine by excess K⁺ (increasing the [K⁺] from 5 to 30 mM) was inhibited somewhat when vinblastine was present during the entire incubation period. If the synaptosomes were preloaded with the radioactive compounds prior to addition of vinblastine, there was no discernible effect on the relative amount of material released by excess K⁺. However, the addition of inhibitor under the latter conditions caused a leakage of radioactivity into the medium even without excess K⁺ being present. Glutamate accumulation was somewhat different from that of GABA, dopamine or norepinephrine. Although it required energy for uptake, 6‐OHDA, mersalyl, vinblastine or colchicine were not inhibitory. Studies of the oxidative metabolism of glutamate and GABA by this synaptosomal preparation indicated that the mechanisms of inhibition by vinblastine was not attributable to a metabolic effect. Both vinblastine and colchicine inhibited the Mg²⁺‐stimulated, but not the Ca²⁺‐activated ATPase of neurostenin. This effect was probably attributable to an interaction of the vinblastine with the neurin moiety of this actomyosin‐like protein. We suggest that the inhibitory phenomena exhibited by vinblastine and colchicine in this synaptosomal preparation arose from the effect of these alkaloids on the neurin associated with the synaptic membrane.