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 neuromuscular 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 extracted from one upper quadrant of 10 mice. The side where teeth were removed was functionally disused (D), since occlusion could not be achieved. Similarly, the intact contralateral (CL) side experienced overload. Morphological measurements were taken of zinc iodide osmium (ZIO)-stained NMJs from control animals, and treated animals at 7 days (7D) and 4 months (4M) post-extraction. Data was analyzed with repeated measures multivariate analysis of covariance (MANCOVA). Test animals did not lose weight, which indicated that sufficient food was consumed after tooth extraction. Fiber diameters from the 7D group were smaller than the controls, while those from the 4M animals were larger. In both the 7D and 4M animals, the CL fibers were larger than the D fibers. There was a trend towards larger NMJs in the 4M animals than in control animals of the same age. NMJs from the CL side of the 4M animals had 14% larger nerve terminal areas (p<0. 1), while on the 4M D side they were 6% larger (p<0.10). NMJs from the 4M D group had 5% longer nerve terminals (p<0.07), while the 4M CL NMJs were 6% longer (p<0.05). NMJs from the D side of the 7D animals were 5% smaller in area while on the CL side they 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 extraction were probably due to differences in degeneration -regeneration of nerve terminals. Acute disuse and overload probably produced greater nerve terminal retraction, whereas chronic changes consisted of a recovery characterized by NMJ plasticity similar to that observed in exercise, disuse and aging studies.