Ultrastructural effects of pyridostigmine on neuromuscular junctions in rat diaphragm

Pyridostigmine bromide, a quaternary carbamate, is widely used in treatment of myasthenia gravis and has been suggested for use in prophylaxis against intoxication with irreversible cholinesterase inhibitors. Since there are virtually no anatomical data concerning the neuromuscular toxicity of the drug, this study was undertaken to evaluate the effects of acute and subacute doses of pyridostigmine on the ultrastrucutre of nerve terminals in rat diaphragm neuromuscular junctions (NMJs). Pyridostigmine in a Mestinon-equivalent buffer was administered by single, subcutaneous injection (acute exposure; 10-30 min) or by a subcutaneously implanted Alzet osmotic minipump (subacute exposure; 2, 7 or 14 days). Acute exposure doses ranged from 0.0036 mg/kg to 3.6 mg/kg (0.001-1.0 LD50), while subacute exposure doses ranged from 0.43 to 20 mg of the drug. Both acute and subacute exposures resulted in dose dependent alterations of presynaptic elements in diaphragmatic NMJs, which included disruption of organelles in the axon terminal, regional or total withdrawal of the nerve terminal from postsynaptic junctional folds, and invasion of Schwann cell fingers into the synaptic cleft. These ultrastructural observations suggest that the normal cell-to-cell interactions at diaphramatic NMJs are altered by this "reversible," cholinesterase inhibiting drug.