DIFFERENTIAL c-FOS EXPRESSION IN NUCLEI OF THE CAUDAL MEDULLA OBLONGATA FOLLOWING NOXIOUS COLORECTAL DISTENSION IN ANAESTHETIZED RATS

It has been suggested that noxious information from the lower bowel may reach the medulla oblongata via several spinal pathways terminating in the ventrolateral reticular area, the nuclei of the solitary tract and medullar dorsal column nuclei correspondingly. The contribution of these pathways to visceral pain processing in the medulla is not established. In this study, performed in urethane-anaesthetized rats, we used c-Fos immunohistochemistry to quantitatively characterize and compare neuronal responses in the gracile (Gr) and cuneate (Cu) nuclei, nuclei of the solitary tract (NTS), lateral reticular nucleus (LRt) and ventrolateral reticular area (VLM) of the medulla to a repeated noxious colorectal distension (CRD). When compared with sham distension, after CRD, the most pronounced induction of c-Fos expression was observed in the NTS, while a modest increase was found in the Gr. Simultaneously, the CRD produced a distinct decrease in the c-Fos immunoreactivity in the VLM. Changes in the c-Fos expression observed in Cu and LRt were insignificant. The present results suggest that under anaesthesia the transmission of nociceptive information from the colorectal region to the medulla is mainly accomplished via excitatory spinosolitary afferents and ventrolateral spinal pathway recruiting inhibitory mechanisms in the medullar reticular formation. The excitatory spinal inputs to the dorsal column nuclei seem to be modestly involved.