Neurons in the rostral raphe pallidus (rRP) have been proposed to mediate experimental stress-induced
tachycardia and
fever in rats, and projections from the dorsomedial hypothalamus (
DMH) may signal their activation in these settings. Thus, we examined c-fos expression evoked by air jet/restraint stress and restraint stress or by systemic administration of
lipopolysaccharide (10 microg/kg and 100 microg/kg) as well as the distribution of the
neuronal nitric oxide synthase (nNOS) in neurons retrogradely labeled from the raphe with
cholera toxin B in key hypothalamic regions. Many neurons in the medial preoptic area and the dorsal area of the
DMH were retrogradely labeled, and approximately half of those in the medial preoptic area and moderate numbers in the dorsal
DMH were also positive for nNOS. Either stress paradigm or dose of
lipopolysaccharide increased the number of c-fos-positive neurons and nNOS/c-fos double-labeled neurons in all regions examined. However, retrogradely labeled neurons positive for c-fos were increased only in the dorsal
DMH and adjoining region in both stressed and
lipopolysaccharide-treated groups, and triple-labeled neurons were found only in this area in rats subjected to either stress paradigm. Thus, hypothalamic neurons that project to the rRP and express c-fos in response to either experimental stress or systemic
inflammation are found only in the dorsal
DMH, and many of those activated by stress contain nNOS, suggesting that
nitric oxide may play a role in signaling in this pathway.