Electrophysiological and ultrastructural analysis of the neuromuscular blockade and miotoxicity induced by the Micrurus nigrocinctus snake venom.

Micrurus nigrocinctus is the most abundant coral snake in Central America. The venom of this specie induced a concentration-dependent (10-20 micrograms/ml) depolarization in the isolated mouse phrenic nerve-diaphragm preparations incubated at 37 degrees C. d-Tubocurarine (10 micrograms/ml) and (alpha beta ungarotoxin (3-5 micrograms/ml) were able to partially protect against the depolarization induced by the venom (10 micrograms/ml), suggesting the involvement of subsynaptic cholinergic receptors. This venom (10 micrograms/ml) also increased the frequency and amplitude of miniature end-plate potentials (mepps) during the first 10-20 min of incubation. Subsequently, the mepps progressively decreased and disappeared after 60 min. These responses were accompanied by ultrastructural changes involving the nerve terminals, the subsynaptic junctional folds and the muscle mitochondria. The synaptic gutter was shallow and, very often, "shrunken" terminals with omega-shaped axolemmal indentations and a decreased number of synaptic vesicles were present. A common finding was the presence of numerous finger-like, membrane-bounded bodies interposed between the terminal and the Schwann cells or postsynaptic sarcolemma. The preincubation of the venom with specific antivenom or the incubation of the preparations at room temperature (24-26 degrees C) reduced the number and intensity of the ultrastructural alterations. The last finding suggests the involvement of a enzymatic process, probably a phospholipase A2, present in the venom. There was a good correlation between the electrophysiological and ultrastructural effects induced by the venom which allow us to conclude that M. nigrocinctus venom has a presynaptic action in the initial stages of intoxication followed by sub- and postsynaptic effects, the last being the most important cause of neuromuscular blockade. A direct action of the venom on muscle fibers may also contributes to the irreversible blockade.