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. For this purpose, a single subcutaneous injection of botulinum toxin (1 IU; DL50) on to the surface of the mouse levator auris longus muscle was performed. The muscles were removed at several time-points after injection (i.e. at one, two, three, four, five, six and 12 weeks). Using electrophysiological techniques, we studied the effect of different types of calcium channel blockers (nitrendipine, omega-conotoxin-GVIA and omega-agatoxin-IVA) on the quantal content of synaptic transmission elicited by nerve stimulation. Morphological analysis using the conventional silver impregnation technique was also made. During the first four weeks after intoxication, sprouts were found at 80% of motor nerve terminals, while at 12 weeks their number was decreased and the nerve terminals were enlarged. The L-type channel blocker nitrendipine (1 microM) inhibited neurotransmitter release by 80% and 30% at two and five weeks, respectively, while no effects were found at later times. The N-type channel blocker omega-conotoxin-GVIA (1 microM) inhibited neurotransmitter release by 50-70% in muscles studied at two to six weeks, respectively, and had no effect 12 weeks after intoxication. The P-type channel blocker omega-agatoxin-IVA (100 nM) strongly reduced nerve-evoked transmitter release (>90%) at all the time-points studied. Identified motor nerve terminals were also sensitive to both nitrendipine and omega-conotoxin-GVIA. This study shows that multiple voltage-dependent calcium channels were coupled to transmitter release during the period of sprouting and consolidation, suggesting that they may be involved in the nerve ending functional recovery process.