EFFECTS OF m-AMINO BENZOIC ACID ETHYLESTER ON NEUROMUSCULAR TRANSMISSION IN THE FROG

¹ Department of Physiology, University of Minnesota Medical School, Minneapolis, Minnesota

Effects of m-amino benzoic acid ethylester (MS-222) were investigated on the sartorius muscle of the frog, Rana pipiens, with intracellular microelectrodes. It was found that this drug has specific effects on the transmitter releasing mechanism of the motor nerve ending. 10^-5 MS-222 significantly reduced mean quantal content of end-plate potentials (e.p.p.'s) without any change in stimulation threshold of the motor nerve terminal. Lack of effect of MS-222 on the average quantal size of the e.p.p. indicated that this drug has little or no effect on the postsynaptic sites of the neuromuscular junction. 10^-5 MS-222 showed little effect on the electrical activity of the muscle fiber. However, higher concentrations of MS-222 depolarized the membrane of the muscle fiber and presumably of the motor nerve terminal, and eventually abolished the excitability of the former.