Acute and chronic effects of botulinum neurotoxin a on the mammalian neuromuscular junction

Abstract

Introduction: Botulinum neurotoxin A (BoNT/A) cleaves SNAP-25 and inhibits acetylcholine (ACh) release at the neuromuscular junctions (NMJ) to cause neuroparalysis. Previous reports indicate a dyssynchrony between the inhibitory effect of BoNT/A on ACh release and SNAP-25 cleavage.

Methods: We tested the in vitro (acute; 90 min) and in vivo (chronic; 12 h) effects of BoNT/A on stimulus-evoked ACh release (SEAR), twitch tension, and SNAP-25 cleavage in isolated extensor digitorum longus (EDL) nerve-muscle preparations (NMP).

Results: In vitro or in vivo BoNT/A poisoning inhibited SEAR and twitch tension. Conversely, SNAP-25 cleavage and inhibition of spontaneous release frequency were observed only in NMP poisoned with BoNT/A in vivo. Moreover, chronic treatment of BoNT/A inhibited ionomycin stimulated Ca(2+) signals in Neuro 2a cells.

Conclusions: These results demonstrate that the inhibition of SEAR precedes SNAP-25 cleavage and suggest involvement of a more complex mechanism for the inhibitory effect of BoNT/A at the NMJ.

Keywords: SNAP-25; acetylcholine release; botulinum neurotoxin; end plate currents; spontaneous release; twitch tension.