Congenital canine myasthenia gravis: I. Deficient junctional acetylcholine receptors.

The neuromuscular junction was studied during growth in two breeds of dog with a congenital familial form of myasthenia gravis (CMG): the Jack Russell terrier and the springer spaniel. Light microscopy revealed no difference in endplate size or nerve terminal morphology in age-matched CMG dogs and unaffected littermates. At all ages there was in individual muscle fibers of CMG dogs an approximately 75% lower postsynaptic membrane density of acetylcholine receptors (AChR). There was no evidence that this abnormality had an autoimmune basis. Measurements of miniature endplate potential amplitude in relation to fiber diameter revealed that the low density of AChR in the postsynaptic membrane of CMG dogs remained constant and did not change with the marked progression of muscle weakness during growth. Antigenic determinants of AChR were deficient in CMG muscle to the same extent as alpha-bungarotoxin (alpha BT) binding sites. Thus, a low density of AChR in the muscle's postsynaptic membrane appears to be the principal abnormality of CMG. The data suggest that the normal increase in number of acetylcholine (ACh) quanta released by nerve impulse may not fully compensate for the reduction in depolarization produced by a single quantum as the muscle fiber diameter increases. This could cause progression of weakness during growth. In addition to being a useful animal model of a rare form of CMG that resembles one form of human CMG, canine CMG offers a unique model for investigating events of synaptogenesis at the neuromuscular junction.