Ultrastructural localization of acetylcholine receptor at the motor endplate: myasthenia gravis and other neuromuscular diseases.

Peroxidase-conjugated alpha-bungarotoxin (P-BGT) was used for the ultrastructural localization of the acetylcholine receptor in motor endplates. Brachial biceps muscle specimens were obtained from six patients with myasthenia gravis (MG) (two ocular and four generalized), five other patients with neuromuscular diseases (limb-girdle dystrophy, polymyositis, and amyotrophic lateral sclerosis) and two controls. In all patients with generalized MG, most of the endplates showed a marked decrease in P-BGT binding. In one of two patients with ocular MG, the amount and distribution of P-BGT binding appeared normal, whereas the other patient showed a slight decrease in P-BGT binding. There was a loose correlation between clinical severity of MG and acetylcholine receptor index or antiacetylcholine receptor antibodies. On the other hand, the amount and distribution of acetylcholine receptor in other neuromuscular diseases was well preserved, even at the endplates denuded of their nerve terminals in amyotrophic lateral sclerosis (ALS) cases.