Disuse and functional overload induced by tooth extraction alters neuromuscular morphology in C57BL/6J mice

The effects of functional disuse and overload on the morphology of neuromus cular junctions (NMJ) in masseter muscles of 8 month old C57BL/6J mice were studied. In order to stimulate disuse caused by tooth loss, teeth were ext racted from one upper quadrant of 10 mice. The side where teeth were remove d was functionally disused (D), since occlusion could not be achieved. Simi larly, the intact contralateral (CL) side experienced overload. Morphologic al measurements were taken of zinc iodide osmium (ZIO)-stained NMJs from co ntrol animals, and treated animals at 7 days (7D) and 4 months (4M) post-ex traction. Data was analyzed with repeated measures multivariate analysis of covariance (MANCOVA). Test animals did not lose weight, which indicated th at sufficient food was consumed after tooth extraction. Fiber diameters fro m the 7D group were smaller than the controls, while those from the 4M anim als were larger. In both the 7D and 4M animals, the CL fibers were larger t han the D fibers. There was a trend towards larger NMJs in the 4M animals t han in control animals of the same age. NMJs from the CL side of the 4M ani mals had 14% larger nerve terminal areas (p<0.1), while on the 4M D side th ey were 6% larger (p<0.10). NMJs from the 4M D group had 5% longer nerve te rminals (p<0.07), while the 4M CL NMJs were 6% longer (p<0.05). NMJs from t he D side of the 7D animals were 5% smaller in area while on the CL side th ey were 11% smaller (p<0.1). Nerve terminals in 7D animals were 6% longer ( p<0.05) than in controls. Changes in NMJ morphology following tooth extract ion were probably due to differences in degeneration - regeneration of nerv e terminals. Acute disuse and overload probably produced greater nerve term inal retraction, whereas chronic changes consisted of a recovery characteri zed by NMJ plasticity similar to that observed in exercise, disuse and agin g studies.