Use of pharmacologic antagonists to deduce commonalities of biologic activity among clostridial neurotoxins.

The pharmacologic activity of several clostridial neurotoxins was assayed on the mouse phrenic nerve-hemidiaphragm preparation. The substances that were assayed included botulinum neurotoxin types A, B, C and E and tetanus toxin. Experiments were done in the presence or absence of antagonists that inhibit either the internalization of toxins or intracellular expression of toxicity. Ammonium chloride and methylamine hydrochloride, agents that inhibit toxins that enter cells by receptor-mediated endocytosis, antagonized botulinum and tetanus neurotoxins. The magnitude of antagonism was substantial for all toxins. Calcium, 3,4-diaminopyridine and guanidine, agents that alter the intracellular expression of toxicity, produced a variable result. They were effective antagonists of botulinum neurotoxin type A, but they were less effective or inactive against the other neurotoxins. The ability of 3,4-diaminopyridine and guanidine to antagonize botulinum neurotoxin type A was highly calcium dependent. When ambient levels of the cation were reduced from 1.8 to 1.0 mM, the activity of the drugs was substantially reduced. The ability of these drugs to produce antagonism was also time dependent. When added simultaneously with toxin, they were maximally active; when added at later times, activity was diminished. A host of agents that alter intracellular levels of cyclic AMP, including theophylline, forskolin, isobutylmethylxanthine and cholera toxin, were evaluated as potential neurotoxin antagonists. Theophylline and isobutylmethylxanthine produced a transient increase in nerve-evoked muscle twitch. None of the drugs that alter tissue levels of cyclic AMP had a universal effect in antagonizing clostridial toxins. The data here have been compared with published data on drugs that antagonize binding of botulinum toxin and tetanus toxin.(ABSTRACT TRUNCATED AT 250 WORDS)