Serotonin 2C receptor signaling in a diffuse neuronal network is necessary for LPS anorexia

LPS, a potent activator of the innate immune system, is commonly used to investigate the acute phase response to infection, including anorexia. Serotonin 2C-receptor signaling has been shown to be involved in the mediation of LPS anorexia. Here we used the selective, potent and brain-penetrant serotonin 2C-receptor antagonist SB 242084 to identify the brain sites involved in LPS anorexia. Male Long-Evans rats received 1 ml/kg intraperitoneal injections of 0 or 0.3 mg/kg SB 242084 and intraperitoneal injections of 0 or 100 μg/kg LPS 1 h later, at dark onset. Food intake was measured in one set of rats and c-Fos immunoreactivity in another, unfed, group 90 min after LPS injection. SB 242084 markedly attenuated the LPS-induced reduction in food intake, with no anorexia evident for the first 2 h after LPS. SB 242084 also completely blocked the LPS-induced increases in c-Fos expression in the paraventricular nucleus, central nucleus of the amygdala, nucleus tractus solitarii, median raphe nucleus and dorsal raphe nucleus and partially blocked it in the A1 noradrenergic area of the ventrolateral medulla and the raphe pallidus nucleus. SB 242084 did not significantly alter the c-Fos response in the arcuate nucleus or the raphe magnus nucleus. These data indicate that 2C receptor signaling activates a diffuse neural network, presumably mediating anorexia and other responses to LPS; they also suggest that the arcuate and the raphe magnus neurons that express c-Fos after LPS are not necessary for LPS anorexia.