Expression of neuropeptides (CGRP, substance P) during and after orthodontic tooth movement in the rat

The present study was designed to investigate the changes in patterns of calcitonin gene-related peptide (CGRP) and substance P (SP) innervation during buccally-directed orthodontic tooth movement (OTM) of the first maxillary molar in the rat. In addition, the CGRP- and SP-innervation was examined 14 and 28 days after the orthodontic force was discontinued. Comparisons were made with the innervation found in control animals. Orthodontic appliances produced a buccally directed force in the range of 20-50 g on the upper right first molar for either 24 hours or 3 days. When the orthodontic appliances were removed one group of animals were killed immediately, whereas the other groups were killed after a healing period of 14 days or 28 days. After fixation the upper jaws were decalcified, frozen, and sectioned in the frontal plane at the level of the first and second molars. The sections were processed for CGRP, SP, and routine morphology. An increase was found in the number of nerve fibres showing CGRP-like immunoreactivity (LI) in the pulp, periodontal ligament (PDL) and marginal gingivae with the fibres showing a higher intensity of immunolabelling after 24 hours or 3 days of OTM. The contralateral first upper molar showed an increase in the expression of CGRP. Fourteen days after removal of the appliances, there was still a marked CGRP-innervation of the PDL in test teeth as well as in contralateral teeth. After 28 days, a decrease in CGRP-innervation was observed, but this innervation was still more pronounced than that found in the controls. The pattern of SP-innervation changed similarly to that of the CGRP-innervation, however, the nerve fibres showing SP-LI were considerably fewer than those showing CGRP-LI at all stages and in all areas. This study suggests that CGRP- and SP-changes not only occur in the tooth exposed to buccally directed OTM, but also in the contralateral tooth and that the changes are evident for a considerable time after terminating OTM.