It would be possible for latrotoxin to counteract effects of Botox under certain conditions. They would both have to affect the release of the same transmitter, and on a time scale conducive to antagonism to occur. Also, the doses/concentrations would have to be such that efficacy was similar for both. That is to say, you wouldn't want to have to use a dose of latrotoxin that was several orders of magnitude greater than the effective dose of Botox. As far as a mechanistic explanation goes, there could be several, but the simplest would be that both Botox and latrotoxin act at the same site but produce opposite effects (they are inverse agonists). Each would tend to counteract the effects of the other. Another explanation is that Botox and latrotoxin act (bind) at different site to affect calcium influx in an opposite manner. Botox might inhibit calcium influx (thus inhibiting transmitter release) and latrotoxin might do just the opposite. Again, there could be several other mechanistic explanations. One could write an entire chapter on this subject.