PDL neuron-associated neurotransmitters in orthodontic tooth movement: identification and proposed mechanism of action

The remodeling of the PDL and alveolar bone that occurs during orthodontic tooth movement (OTM) has long been associated with an inflammatory process. The vascular and cellular changes that occur have been well documented, and several inflammatory mediators, growth factors and neuropeptides have been found in increased levels in the periodontal supporting tissues during orthodontic tooth movement. It recently was found that peripheral nerve fibers participate in the inflammatory process (neurogenic inflammation) by releasing various neuropeptides. Calcitonin gene-related peptide (CGRP) and substance P (SP), two PDL neuropeptides, play a role in inflammation by serving as vasodilators and inducing increased vascular permeability. They also stimulate plasma extravasation and proliferation of endothelial cells and fibroblasts; upregulate the expression of endothelial cell adhesion molecules; and act as neurotransmitters. The increase in concentration of these PDL neuropeptides during OTM indicates their importance to the process. This paper will review the current research and summarize the roles and proposed mechanism of these two main PDL neurotransmitters in OTM.