Targeting the safety factor for neuromuscular transmission to treat myasthenia gravis

In myasthenia gravis autoantibodies attack the postsynaptic membrane of the neuromuscular junction causing fatiguing weakness that can wax and wane. Weakness occurs when the safety factor for neuromuscular transmission becomes marginal, meaning that the (postsynaptic) endplate potential is no longer sufficient to reliably trigger action potentials in the muscle fiber. Cholinesterase inhibitor drugs provide temporary relief by increasing the endplate potential amplitude, but additional symptomatic treatment options are needed. Here we discuss our recent experience in early preclinical testing of candidate compounds. Using an ex vivo mouse nerve-muscle contraction assay, followed up by endplate potential recordings, we examined the effects of cannabinoids. Our findings highlighted the potentially confounding effects of dimethylsulfoxide (DMSO) when used as a solubilizing agent. DMSO produced a dose-dependent restoration of force to curarized muscle and enhanced miniature endplate potential amplitude, thus enhancing the safety factor for neuromuscular transmission at concentrations as low as 0.1% v/v. When examined in the absence of curare, the DMSO-induced increase in quantal amplitude was opposed by a homeostatic reduction in the number of quanta of acetylcholine released by the nerve terminal. In contrast to DMSO, cannabinoids appear to work via cannabinoid receptor type 1 to reduce quantal number, thereby weakening the safety factor. Our results highlight the need to consider the effects of solubilizing agents per se when screening new therapeutics for neurological diseases. They also demonstrate the need to take synaptic homeostasis into account, which can otherwise distort or mask the effects of bioactive agents upon neurotransmission.