Serotoninergic neurons in the brainstem expressing FOS protein after orofacial noxious stimulation: an immunocytochemical double-labeling study.

Employing an immunocytochemical double-labeling technique, we investigated the co-localization of FOS protein, the expression product of c-fos proto-oncogene induced by orofacial noxious stimulation, and serotonin in the rat brainstem. About 3.2%-25.6% of serotonin-like immunoreactive neurons and 6.2%-46.7% of FOS-like immunoreactive neurons in the raphe nuclei, reticular formation and ventrolateral subdivision of the midbrain periaqueductal gray exhibit FOS-like immunoreactivity and serotonin-like immunoreactivity, respectively. The present results provide further morphological evidence for the involvement of serotoninergic neurons in modulating the transmission of noxious information.