“Myotrophic” influences on motoneurons of normal and dystrophic mice in parabiosis

Abstract

The electrophysiological demonstration of less-than-normal number and size of motor units in solei of dystrophic mice raises the question whether such defects are neurogenic or myogenic. Dystrophic and normal littermates were parabiosed in pairs at 20 to 23 days ex utero to facilitate a cross of the tibial nerve of one partner onto the soleus muscle of the other. If the dystrophic muscle exerts detrimental influences on the nerve of normal genotype, this preparation should show a decrease in the number and the size of motor units compared to the normal muscle reinnervated by a nerve of dystrophic genotype. This is implied electrophysiologically in 10 parabiotic pairs examined 5 to 6 months postoperatively. Dystrophic solei reinnervated by nerves of normal genotype showed a 23% reduction in number (17 ± 2, mean ± sd) and a 50% reduction in size (78 ± 17 mg, mean ± sd) of motor units as compared to the normal solei reinnervated by nerves of dystrophic geotype (22 ± 1, 154 ± 38 mg, mean ± sd). The former preparations also showed abnormal end-plates compared to the latter. These endplates were irregular in size and shape and often exhibited decreased acetylcholinesterase activity. The possibility that a detrimental myotrophic influence is exerted on motoneurons in dystrophic mice is discussed.