Alteration of nerve growth factor-receptor expression in the periodontal ligament of the rat during experimental tooth movement

Abstract

This pilot study deals with the initial responses of neural elements showing nerve growth factor-receptor (NGFR) immunoreactivity in the periodontal ligament of rats to orthodontic forces. The animals were killed at intervals of 1, 2, 6, 12 h and 1, 3, 5, 7 days after the insertion of elastic bands between the maxillary first and second molars. Serial frozen sections, prepared from each animal, were processed immunohistochemically to demonstrate NGFR, after which the periodontal ligament of the mesial root of the maxillary second molar was examined microscopically. In control sections, NGFR-positive neural elements were predominantly distributed at the apex of the bony socket on the distal side of the periodontal ligament. After 1 h of tooth movement, NGFR positively-stained nerve fibres tended to disappear slightly in both the intermediate and coronal regions of the distal periodontal ligament. By the third day of tooth movement, the periodontal ligament nerve fibres showed more intense NGFR-im-munoreactivity; thick, positively stained nerve fibres were recognized on the distal side in which active bone remodelling was occurring, and a few of these fibres were densely distributed around blood vessels or near Howship's lacunae. The distribution of NGFR-positive neural elements on the mesial side increased at 5 days of tooth movement. Finally, at 7 days of tooth movement, staining intensity for NGFR appeared to decrease. These findings demonstrate that an alteration in the distribution and the intensity of immunoreactive staining for NGFR in the periodontal ligament is associated with the bone remodelling induced by orthodontic tooth movement.