Adult dystrophic (mdx) endplates exhibit reduced quantal size and enhanced quantal variation

Examination of miniature endplate potential (MEPP) distributions indicated that the average quantal size is significantly reduced by approximately 44% in young adult mdx mice (5-7 weeks). The average quantum declined further to approximately 37% of non-dystrophic levels in mature (6- to 24-month) mdx mice. Young adult non-dystrophic and mdx endplates and mature non-dystrophic endplates exhibited a linear relationship between the mean and variance of uniquantal MEPP amplitude distributions. Mature mdx endplates, however, exhibited a distinctly nonlinear relationship characterized by large increases in variance at larger mean MEPP amplitudes. These results indicate a reduced average density of functional acetylcholine (ACh) receptors (AChRs) at mdx endplates that, in mature preparations, is associated with greater temporal or spatial variability in the density of functional AChRs apposed to individual release sites.