Neostigmine-induced alterations at the mammalian neuromuscular junction. I. Muscle contraction and electrophysiology

The effects of single and repetitive injections of neostigmine on neuromuscular physiology were examined in rat extensor digitorum longus muscles. The characteristic facilitation of neuromuscular transmission associated with acute anticholinesterase treatment was accompanied by significant pre- and postsynaptic alterations in neuromuscular transmission. Three days of neostigmine treatment caused a decrease in indirectly and directly elicited muscle contraction. Miniature end-plate potential amplitude and frequency, end-plate potential amplitude, junctional acetylcholine sensitivity and quantal content of nerve-evoked end-plate potentials were also decreased by this treatment. By 22 to 25 days of continued treatment, the decreased rate of transmitter release had returned almost to normal, whereas the alterations of the postsynaptic membrane persisted for as long as 106 days. Alterations were also found in the muscle action potential and in certain passive electrical properties of the extrajunctional muscle membrane. In addition, many of the physiological changes were correlated directly with the morphological changes observed in rats treated similarly. We conclude that neostigmine treatment in rats in therapeutic doses has deleterious effects on neuromuscular physiology and neuromuscular ultrastructure. Although the pattern of these changes is not identical with that seen in rabbit and human myasthenia gravis, the neostigmine treatment used in patients with myasthenia gravis may contribute in part to the neuromuscular alterations observed in this disease.