Botulinum toxin-induced focal paresis in mice is unaffected by muscle activity.

INTRODUCTION: To test the hypothesis that the efficacy of botulinum toxin depends on the activity of the neuromuscular junction, we developed an in vivo paradigm to determine the degree and duration of low-dose botulinum toxin-induced focal paresis in mice. METHODS: We combined an automated wheel-running paradigm with low-dose botulinum toxin injections into the calf muscles of wild-type mice. Half of the mice were injected either before the nightly running or before the daily resting period. RESULTS: After botulinum toxin injections, running distance and maximum velocity decreased dose-dependently. The degree and duration of decrease between the respective groups with regard to the time-points of injection were identical. CONCLUSIONS: This in vivo paradigm quantifies the degree of otherwise clinically inapparent botulinum toxin-induced focal calf muscle paresis. Increased muscle activity after low-dose injections does not influence the efficacy of botulinum toxin in normal muscles.