Absence of sympathetic innervation and sprouting in rat dentinal tubules

Objectives: This study was designed to determine if sympathetic nerve fibers exist in dentinal tubules in rat normal dental pulp, and if they sprout into the dentinal tubules in response to artificial cavity preparation in dentin. Methods: A total of 14 adult male Wistar rats were used. Normal dental pulp group (control) was no cavity preparation in 7 rats. Inflamed dental pulp group was cavity preparation in 7 rats. Sympathetic nerve fibers in rat molar dental pulp were labeled using an anterograde axonal transport technique involving injection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the left superior cervical ganglion (SCG) in the control group and 18 days after cavity preparation of left maxillary first molar in the experimental group. All rats were perfused through 4% paraformaldehyde in 0.1 mol/L phosphate buffer 3days after WGA-HRP injection. A pulp of left maxillary first molars were then observed using light and electron microscopes. Results: In control, scattered WGA-HRP reaction products were observed in unmyelinated nerve endings in the odontoblast layer and subodontoblastic region. In injured pulp 3 weeks after cavity preparation, reaction products were about 1.8-times more plentiful in the above areas (versus control pulp). However, no labeled nerve fibers were observed in the dentinal tubules in either control or injured dental pulp. Conclusions: These results indicate that although sympathetic nerve fibers do indeed sprout in rat dental pulp in response to cavity preparation, they do not penetrate into the dentinal tubules in which postganglionic nerve endings derived from the SCG were not originally present. This study was supported by a Grant-in -Aid for Scientific Research from the Japan Society for the Promotion of Science (17591984, 19791390) and by Highly-functional Interface Science: Innovation of Biomaterials with Highly-functional Interface to Host and Parasite.