| The dental pulp is a favorable model for studies of interactions between nociceptive sensory neurons and their peripheral target tissues. In the present study, we retrogradely labeled pulpal afferent neurons with an improved method that permits monitoring of changes in neuronal phenotype in response to controlled tooth injuries. The capacity of retrograde neuronal tracers to diffuse through dentinal tubules was exploited, thereby avoiding the severe injury to the pulp associated with previous tracer application methods. The strategy was to apply the durable fluorescent tracer, Fluoro-gold (FG), to exposed dentin in the floor of shallow cavities in molars in order to pre-label pulpal neurons in trigeminal ganglia of young adult Sprague-Dawley rats. A high percentage of pupal afferent neurons was retrogradely labeled by application of FG to exposed dentin. The FG fluorescent signal persisted in most labeled neurons for at least eight weeks, and these neurons maintained their peripheral projection to the dental pulp. Following tracer application to dentin, the pulp tissue appeared normal histologically, with the exception that a layer of reactive dentin was deposited at the pulp-dentin border beneath the shallow cavities. Thus, labeling of trigeminal neurons via dentinal tubules provides the basis for a useful model for precisely assessing properties of pulpal afferents in both quiescent and activated states. This new experimental system was used to demonstrate that the pulpal sensory innervation includes a population of NGF-responsive neurons that upregulate expression of CGRP and BDNF in response to tooth injury. |
