Changes in spontaneous and evoked release of transmitter induced by the crotoxin complex and its component phospholipase A 2 at the frog neuromuscular junction

Abstract

The time course and characteristics of the changes induced by the action of the crotoxin complex and of its component phospholipase A 2 on transmission at the frog neuromuscular junction was studied in vitro at single endplates using intracellular recording techniques. Both the crotoxin complex and phospholipase A 2 induced variable changes in spontaneous transmitter release in which bursts of miniature endplate potentials alternated with periods when the frequency of miniature endplate potentials was stable but their amplitudes were markedly heterogeneous. The bursts of miniature endplate potentials were of sudden onset and of a high initial frequency, and their duration and incidence were highly variable both at one endplate and between endplates. These changes in the frequency and amplitude of miniature potentials occurred both before and after the abolition of neurally evoked endplate potentials and in the presence of tetrodotoxin. At a late stage in crotoxin intoxication, potassium depolarization of the motor nerve terminals failed to increase spontaneous transmitter release. Both the crotoxin complex and phospholipase A 2 reduced the amplitude of endplate potentials although phospholipase A 2 was less effective. The course of depression of the quantal content of evoked transmitter release induced by the crotoxin complex was occasionally interrupted by a brief facilitation of response; these irregularities in the rate of depression were more clearly seen when endplate potentials were recorded at partially curarized endplates exposed to either the crotoxin complex or to phospholipase A 2. At the stage of abolition of endplate potentials an increase in the external concentration of Ca 2 + enabled the evoked response to be restored temporarily. The crotoxin complex and phospholipase A 2 had no effect on the input resistance of the sarcolemmal membrane, but raised the threshold stimulus required to initiate an action potential at concentrations which affected transmitter release. Crotapotin did not affect any of the measured parameters even at high concentrations. The diversity of changes observed in the properties of spontaneous and evoked transmitter release, suggests that the crotoxin complex and phospholipase A 2 may induce two kinds of alterations in the structure of the presynaptic membrane as a result of the action of phospholipase A 2, viz ( a) the formation of transient instabilities with an increased Ca 2 + inflow and ( b) prolonged disturbance of the calcium channel and/or the synchronized release of subunits at the active zone.