Expression of Fos immunoreactivity in the rat supraspinal regions following noxious visceral stimulation

We used immunohistochemical detection of the Fos protein to study the neuro nal activation in the brain of methoxyfluorane-anesthetized rats after noxi ous deep somatic or visceral stimulation. The anesthesia was effective in t riggering gene induction in many brain regions. Nevertheless, Fos appeared de novo in several brain nuclei following noxious stimulation in anesthetiz ed animals. This could be of clinical relevance, as it suggests that the ga s anesthetic does not suppress noxious stimulus-evoked reactivity in brain neurons. Two types of visceronociceptive stimuli were used to compare the e ffects of a diffuse visceral inflammation (peritoneal inflammation) with th ose of a more restricted inflammation (urinary bladder inflammation). In th e same supraspinal areas, there were very few immunostained neurons in unst imulated controls, whereas Fos-positive cells were slightly more numerous i n anesthetized controls and significantly more numerous after noxious stimu lation. The peritoneal inflammation induced more Fos-labeled neurons than t he restricted visceral stimulation. Labeled cells were found in these cases mainly in the ventrolateral medulla, parabrachial complex, dorsal raphe nu cleus, periaqueductal gray, several hypothalamic and thalamic nuclei, amygd aloid complex, and cortex. Altogether these findings indicated that somatic and visceral inputs generally activate the same neuronal groups. However, a separation between the activation of somatic and visceral pathways was fo und in some brain nuclei, such as the parabrachial complex, hypothalamic, a nd thalamic nuclei. (C) 1998 Elsevier Science Inc.