Research ReportSerotonin 2C receptor signaling in a diffuse neuronal network is necessary for LPS anorexia
Introduction
Anorexia is a prominent component of the innate immune system response that protects against bacterial infection, often modeled with the gram-negative bacterial toxin lipopolysaccharide (LPS), and other immune challenges. Peripheral infection initiates a complex cascade of immune events. A key initial element of this is the release of pro-inflammatory cytokines and chemokines from macrophages and other immune cells; these initiate both local and systemic responses, collectively called the acute phase response. The brain also reacts to this peripheral immune response, leading to anorexia, fever and further brain-mediated elements of the acute phase response. A principal mechanism through which peripheral immune signaling affects the brain is thought to be via the release of prostaglandins from blood brain barrier endothelial cells. Prostaglandins then act on receptors on neurons and glial cells in the brain. The sites where such responses are initiated, however, remain in doubt. It is thought that LPS anorexia arises from altered information processing within the same neural networks that control normal eating (Asarian and Langhans, 2005, Langhans, 2007).
The role of the brain neurotransmitter serotonin in the control of both normal eating (Blundell, 1984, Heisler et al., 2003, Simansky, 1996, Tecott, 2007) and LPS anorexia (Langhans, 2007) has been intensively investigated. In healthy animals, agonists of serotonin 1B and serotonin 2C receptors reduce food intake, and antagonists to these receptors attenuate the anorectic effect of the serotonin agonist d-fenfluramine (Vickers et al., 1996). Peripheral administration of 2C-receptor antagonists also attenuated LPS anorexia (von Meyenburg et al., 2003a), implicating serotonin, and in particular signaling via 2C receptors, in infection anorexia. LPS anorexia was also attenuated by direct injection of the serotonin 1A-receptor agonist 8-OH-DPAT into the midbrain raphe (Hrupka and Langhans, 2001). Because most serotonin 1A receptors are somatodendritic autoreceptors with a negative-feedback function, these findings suggest that increased activity of serotonin neurons in the midbrain raphe is involved in LPS anorexia.
The present studies were undertaken to further investigate the role of serotonin 2C receptor signaling in LPS anorexia in rats using the specific 2C-receptor antagonist SB 242084. In particular, we wished to focus on the role of serotonin 2C receptors in the initial stage of LPS anorexia. This is important because LPS appears to elicit a cascade of immune (Asarian and Langhans, 2005, Langhans, 2007) and neural responses, such that more brain areas are involved as the acute phase response progresses (Elmquist et al., 1996, Rivest and Laflamme, 1995). In previous studies of the influence of SB 242084 on LPS anorexia (von Meyenburg et al., 2003a, von Meyenburg et al., 2003b), SB 242084 first attenuated anorexia starting 4 h after LPS injection, perhaps because the antagonist was injected simultaneously with LPS. Therefore, in order to determine if serotonin 2C receptors contribute to the initial phase of LPS anorexia, we injected SB 242084 prior to LPS. First, we characterized the effects of SB 242084 pretreatment on LPS anorexia. Second, we used immunocytochemical techniques to assay the numbers of cells expressing c-Fos, the product of the immediate early gene c-fos, in brain sites where altered neuronal activity, either in serotonin neurons or in their projection sites, might cause anorexia.
To identify sites potentially involved in the development of LPS anorexia and perhaps necessary for its subsequent maintenance, we collected brain tissue 90 min after LPS injection. We examined c-Fos expression in two midbrain sites rich in rostrally projecting serotonin neurons, the dorsal (DR) and median (MnR) raphe nuclei, and three forebrain sites that are rich in serotonin 2C receptors, the hypothalamic paraventricular (PVN) and arcuate (Arc) nuclei and the central nucleus of the amygdala (CeA). We also examined c-Fos expression in two hindbrain sites rich in serotonin neurons, the raphe magnus (RMg) and raphe pallidus (RPa) nuclei. All these raphe nuclei are interconnected, and each expresses serotonin 2C receptors (Hoffman and Mezey, 1989, Mengod et al., 1990, Molineaux et al., 1989, Wright et al., 1995). Finally, we examined the nucleus tractus solitarii (NTS) and the A1 noradrenergic area of the ventrolateral medulla (A1) because several studies implicate these areas in LPS anorexia (Laflamme et al., 1999, Lacroix and Rivest, 1997, Rivest and Laflamme, 1995, Valles et al., 2005).
Section snippets
Food intake
LPS significantly reduced cumulative food intake at 2, 4 and 23 h after intraperitoneal injection (Fig. 1). SB 242084 by itself had no detectable effect on food intake at any of these times but completely reversed LPS anorexia at each time. The statistical criteria for complete reversal were, first, that LPS alone reduce food intake (i.e., a significant difference between control-injected rats and LPS-injected rats); second, that the reversal of LPS anorexia by SB 242084 be significant (i.e., a
Discussion
In order to investigate the role of serotonin 2C receptor signaling in initial phase of the LPS-induced anorexia and associated changes in brain c-Fos expression, we pre-treated rats with the selective and potent serotonin 2C-receptor antagonist SB 242084, in contrast to previous studies in which either less specific antagonists were used or SB 242084 was administered after the onset of LPS anorexia (Hrupka and Langhans, 2001, von Meyenburg et al., 2003ab). The neural mechanism mediating the
Animals and housing
Male Long–Evans rats were bred on site using founders from Charles River Germany (Sulzfeld, Germany). At age of 5–6 weeks, they were housed individually in rooms maintained at 22 ± 2 °C with 12:12-h reversed light–dark cycles (lights on: 2300 h) and offered ground chow (Provimi Kliba NAFAG #3433, Kaiseraugst, Switzerland) and water ad libitum, except as noted below. All procedures were approved by the Veterinary Office of the Canton of Zurich.
LPS anorexia
Twenty-four rats (body weight range, 220–270 g) were
Acknowledgments
This work was supported by a grant from ETH-Zurich.
References (48)
- et al.
Localization of the 5-hydroxytryptamine2C receptor protein in human and rat brain using specific antisera
Neuropharmacology
(1995) - et al.
Pharmacological, but not genetic, disruptions in 5-HT(2C) receptor function attenuate LPS anorexia in mice
Pharmacol. Biochem. Behav.
(2007) Serotonin and appetite
Neuropharmacol
(1984)- et al.
Immunohistochemical localisation of the serotonin2C receptor protein in the rat CNS
Neuropharmacology
(2000) - et al.
c-Fos immunoreactivity induced by intraperitoneal LPS administration is reduced in the brain of mice lacking the microsomal prostaglandin E synthase-1 (mPGES-1)
Brain Behav. Immun.
(2007) - et al.
Estradiol-mediated increases in the anorexia induced by intraperitoneal injection of bacterial lipopolysaccharide in female rats
Physiol. Behav.
(2004) - et al.
Distribution of serotonin serotonin1C receptor mRNA in adult rat brain
FEBS Lett.
(1989) - et al.
A role for serotonin in lipopolysaccharide-induced anorexia in rats
Pharmacol. Biochem. Behav.
(2001) - et al.
Involvement of serotonergic pathways in mediating the neuronal activity and genetic transcription of neuroendocrine corticotropin-releasing factor in the brain of systemically endotoxin-challenged rats
Neuroscience
(1999) - et al.
Reversible inactivation of the dorsal vagal complex blocks lipopolysaccharide-induced social withdrawal and c-Fos expression in central autonomic nuclei
Brain Behav. Immun.
(2004)
The distribution and cellular localization of the serotonin 1C receptor mRNA in the rodent brain examined by in situ hybridization histochemistry. Comparison with receptor binding distribution
Neuroscience
Distribution of the serotonin serotonin2 receptor family mRNAs: comparison between serotonin2A and serotonin2C receptors
Brain Res. Mol. Brain Res.
Serotonin and the orchestration of energy balance
Cell. Metab.
Evidence that the anorexia induced by lipopolysaccharide is mediated by the serotonin2C receptor
Pharmacol. Biochem. Behav.
Evidence for a role of the serotonin2C receptor in central lipopolysaccharide-, interleukin-1 beta-, and leptin-induced anorexia
Pharmacol. Biochem. Behav.
Differential induction of c-Fos immunoreactivity in hypothalamus and brain stem nuclei following central and peripheral administration of endotoxin
Brain Res. Bull.
Anatomic and functional topography of the dorsal raphe nucleus
Ann. N.Y. Acad. Sci.
Loss of cholecystokinin and glucagon-like peptide-1 induced satiation in mice lacking serotonin 2C receptors
Am. J. Physiol.
Current perspectives on behavioural and cellular mechanisms of illness anorexia
Int. Rev. Psychiatry
Systemic administration of interleukin-1beta activates select populations of central amygdala afferents
J. Comp. Neurol.
Current and future approaches using genetically modified mice in endocrine research
Am. J. Physiol.
Endotoxin-induced activation of cerebral catecholamine and serotonin metabolism: comparison with interleukin-1
J. Pharmacol. Exp. Ther.
Distribution of c-Fos-like immunoreactivity in the rat brain following intravenous lipopolysaccharide administration
J. Comp. Neurol.
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