Methods: The 0.4mm-thick crowns were bonded on the occlusal surfaces of the first molars of rats. Crowns were removed at 2d, 3d, 4d, 5d, and 6d after wearing and mechanical nociceptive thresholds were tested with an electronic von-Frey anesthesiometer. At 3d, 5d, 7d, 14d, 21d, and 28d after 0.4mm occlusal alteration and removal of the crown at 4d, the brain stem sections of rats were obtained and processed for immunofluorescence staining for glial fibrillary acidic protein (GFAP). The activation of astrocytes was analyzed hemi-quantitatively.
Results: (1) Compared with sham-application controls, after removal of crowns at 6d, rats still exhibited decreased head withdrawal thresholds in masticatory muscles on both sides until the last day of the experiment (p<0.05); rats removed crowns at 2d, 3d, 4d, 5d exhibited the thresholds increasing and up to similar level of the controls. The later to remove the occlusal interference, the longer to recover the basal threshold level. (2) Vi/Vc astrocyte showed activation at 3d after occlusal interference, peaking at 14d, and sustaining a slight level at 28d. (3) After removal of the crownat 4d, Vi/Vc astrocytes maintained moderate activation to 7d, and returned to the normal from 14d.
Conclusions: Within this experiment, the mechanical hyperalgesia can be induced using occlusal interference and disappear after removing it at 5d or earlier. After removal of the crown at 4d, astrocyte activation decreases and returns to basal level. Persistent activation of astrocyte may involve in central mechanisms of the occlusal interference-induced chronic masticatory muscle pain.