Multiple types of calcium channels mediate transmitter release during functional recovery of botulinum toxin type A-poisoned mouse motor nerve terminals

The involvement of different types of voltage-dependent calcium channels in nerve-evoked release of neurotransmitter was studied during recovery from neuromuscular paralysis produced by botulinum toxin type A intoxication. Fo r this purpose, a single subcutaneous injection of botulinum toxin (1 IU; D L50) on to the surface of the mouse levator auris longus muscle was perform ed. The muscles were removed at several time-points after injection (i.e. a t one, two, three, four, five, six and 12 weeks). Using electrophysiologica l techniques, we studied the effect of different types of calcium channel b lockers (nitrendipine, omega-conotoxin-GVIA and omega-agatoxin-IVA) on the quantal content of synaptic transmission elicited by nerve stimulation. Mor phological analysis using the conventional silver impregnation technique wa s also made. During the first four weeks after intoxication, sprouts were f ound at 80% of motor nerve terminals, while at 12 weeks their number was de creased and the nerve terminals were enlarged. The L-type channel blocker n itrendipine (1 mu M) inhibited neurotransmitter release by 80% and 30% at t wo and five weeks, respectively, while no effects were found at later times . The N-type channel blocker omega-conotoxin-GVIA (1 mu M) inhibited neurot ransmitter release by 50-70% in muscles studied at two to six weeks, respec tively, and had no effect 12 weeks after intoxication. The P-type channel b locker omega-agatoxin-NA (100 nM) strongly reduced nerve-evoked transmitter release (>90%) at all the time-points studied. Identified motor nerve term inals were also sensitive to both nitrendipine and omega-conotoxin-GVIA. This study shows that multiple voltage-dependent calcium channels were coup led to transmitter release during the period of sprouting and consolidation , suggesting that they may be involved in the nerve ending functional recov ery process. (C) 1999 IBRO. Published by Elsevier Science Ltd.