The effect of neonatal capsaicin on the c-Fos-like immunoreactivity induced in subnucleus oralis neurons by noxious intraoral stimulation

The noxious stimulus-dependent induction of c-Fos-like immunoreactivity (Fos-LI) in neurons in the subnucleus oralis and the medullary dorsal horn (MDH) was significantly suppressed by the selective destruction of unmyelinated primary neurons. The induction of Fos-LI by topical capsaicin application to the lingual mucosal stimulation was almost completely suppressed by neonatal capsaicin treatment. Fos-LI induction by the tooth pulp stimulation and by formalin injection to the lingual mucosa were only partially reduced. These results provide an evidence that the noxious signals from the intraoral structures are transmitted by both unmyelinated and myelinated nociceptors to the subnucleus oralis as well as the MDH.