The Membrane Components of Crustacean Neuromuscular Systems: I. Immunity of different electrogenic components to tetrodotoxin and saxitoxin

Axon spikes in crayfish and lobster neuromuscular preparations were blocked by tetrodotoxin or saxitoxin (concentration 10⁻⁹ to 10⁻⁸ g/ml). Responses evoked in the excitatory synaptic membrane by ionophoretically applied glutamate, or in the inhibitory by GABA were unaffected by concentrations of the poisons up to 10⁻⁵ g/ml. These confirm other findings that the poisons do not affect electrically inexcitable membrane components. “Miniature” p.s.p.’s, which indicate local secretory activity in the presynaptic terminals were unaffected by the poisons. Electrical stimuli applied to the axon terminals elicited localized p.s.p.’s after spike electrogenesis of the axons was blocked. Thus, persistence of secretory activity may be linked to persistence of depolarizing K activation in the axons. Spikes induced in the muscle fibers by procaine were not affected by the poisons. In correlation with other data this finding indicates that the depolarizing electrogenic element, which does not depend upon Na activation in the normally gradedly responsive muscles, differs chemically from the Na activation component which is present in the conductile membrane of various cells. Three other varieties of electrically excitable response which are present in crayfish muscle fibers (hyperpolarizing Cl activation, depolarizing K inactivation, and K activation) were, likewise, immune to the toxin.