The phosphorylated
Extracellular Signal-regulated Kinase (pERK) and Fos expression and masticatory muscle activity were analyzed in rats with
capsaicin-induced acute
inflammation of the tooth pulp in order to clarify the role of the spinal trigeminal nucleus and upper cervical spinal cord in tooth pulp
pain. Digastric and masseteric muscle activities were significantly increased following
capsaicin injection into the molar tooth pulp but not after vehicle treatment. The pERK-like immunoreactive (LI) neurons were observed in the subnuclei interpolaris-caudalis transition (Vi/Vc) zone, the paratrigeminal nucleus (Pa5) and the superficial laminae of the caudal Vc/C2 zone. The pERK expression was detected as early
as 2 min and peaked at 5 min after
capsaicin or vehicle injection. The pERK expression in the Vi/Vc zone and Pa5 was bilateral, whereas it was predominantly ipsilateral in the caudal Vc/C2 zone. The
capsaicin treatment of the whisker pad produced pERK expression in the rostro-caudal middle portion of the ipsilateral Vc, but small number of pERK-LI cells were observed after vehicle treatment. The pERK expression was similar in the Vi/Vc zone following
capsaicin injection into the upper or lower molar tooth pulp, whereas the pERK expression was in the lateral portion of the caudal Vc/C2 zone after upper molar injection and restricted to the medial portion of the Vc/C2 zone after the lower molar
capsaicin. These data were confirmed with Western blots. There were differences in the distribution of Fos
protein-like immunoreactive (LI) cells and pERK-LI cells following tooth pulp stimulation. After
capsaicin application into the upper molar tooth pulp, no pERK-LI cells were observed in the ventral part of the Vi/Vc zone, whereas many Fos
protein-LI cells were expressed in this region. The difference in the distribution pattern of pERK- and Fos
protein-LI cells in the Vi/Vc zone suggests their differential temporal expression profiles after
capsaicin. The present findings suggest that tooth-pulp-driven neurons in the spinal trigeminal nucleus are involved in tooth pulp
pain through activation of the intracellular signal transduction pathway that involves earlier ERK phosphorylation and subsequent Fos expression.