L. Asarian

Divergent effects of estradiol and the estrogen receptor-α agonist PPT on eating and activation of PVN CRH neurons in ovariectomized rats and mice.

Eating is modulated by estradiol in females of many species and in women. To further investigate the estrogen receptor mechanism mediating this effect, ovariectomized rats and mice were treated with estradiol benzoate or the estrogen receptor-alpha (ER-alpha)-selective agonist PPT. PPT inhibited eating in rats much more rapidly than estradiol (approximately 2-6 h versus >24 h). In contrast, the latencies to vaginal estrus after PPT and estradiol were similar (>24 h). PPT also inhibited eating within a few hours in wild-type mice, but failed to inhibit eating in transgenic mice deficient in ER-alpha (ERalphaKO mice). PPT, but not estradiol, induced the expression of c-Fos in corticotrophin-releasing hormone (CRH)-expressing cells of the paraventricular nucleus (PVN) of the hypothalamus within 90-180 min in rats. Both PPT and estradiol reduced c-Fos expression in an ER-alpha-containing area of the nucleus of the solitary tract. The anomalously rapid eating-inhibitory effect of PPT suggests that PPTs neuropharmacological effect differs from estradiols, perhaps because PPT differentially activates membrane versus nuclear ER-alpha or because PPT activates non-ER-alpha membrane estrogen receptors in addition to ER-alpha. The failure of PPT to inhibit eating in ERalphaKO mice, however, indicates that ER-alpha is necessary for PPTs eating-inhibitory action and that any PPT-induced activation of non-ER-alpha estrogen receptors is not sufficient to inhibit eating. Finally, the rapid induction of c-Fos in CRH-expressing cells in the PVN by PPT suggests that PPT elicits a neural response that is similar to that elicited by stress or aversive emotional stimuli.

Hepatic‐Portal Vein Infusions of Glucagon‐Like Peptide‐1 Reduce Meal Size and Increase c‐Fos Expression in the Nucleus Tractus Solitarii, Area Postrema and Central Nucleus of the Amygdala in Rats

We recently reported that brief, remotely controlled intrameal hepatic‐portal vein infusions of glucagon‐like peptide‐1 (GLP‐1) reduced spontaneous meal size in rats. To investigate the neurobehavioural correlates of this effect, we equipped male Sprague‐Dawley rats with hepatic‐portal vein catheters and assessed (i) the effect on eating of remotely triggered infusions of GLP‐1 (1 nmol/kg, 5 min) or vehicle during the first nocturnal meal after 3 h of food deprivation and (ii) the effect of identical infusions performed at dark onset on c‐Fos expression in several brain areas involved in the control of eating. GLP‐1 reduced (P < 0.05) the size of the first nocturnal meal and increased its satiety ratio. Also, GLP‐1 increased (P < 0.05) the number of c‐Fos‐expressing cells in the nucleus tractus solitarii, the area postrema and the central nucleus of the amygdala, but not in the arcuate or paraventricular hypothalamic nuclei. These data suggest that the nucleus tractus solitarii, the area postrema and the central nucleus of the amygdala play a role in the eating‐inhibitory actions of GLP‐1 infused into the hepatic‐portal vein; it remains to be established whether activation of these brain nuclei reflect satiation, aversion, or both.

A new look on brain mechanisms of acute illness anorexia.

Bacterial lipopolysaccharide (LPS) and other microbial substances trigger the organisms acute phase response and cause acute illness anorexia. Pro-inflammatory cytokines are major endogenous mediators of acute illness anorexia, but how LPS or cytokines stimulate the brain to inhibit eating is not fully resolved. One emerging mechanism involves the activation of the enzyme cyclooxygenase-2 (COX-2) in blood-brain barrier endothelial cells and the subsequent release of prostaglandin E2 (PGE2). Serotonin neurons in the midbrain raphe are targets of PGE2, and serotonergic projections from the midbrain raphe to the hypothalamus appear to be crucial for LPS anorexia. That is, raphe projections activate (1) the corticotrophin-releasing hormone neurons in the paraventricular nucleus which then elicit the stress response and (2) the pro-opiomelanocortin neurons in the arcuate nucleus which then release alphaMSH and elicit anorexia. Here we review available data to support a role for this brain mechanism in acute illness anorexia by center staging PGE2 signaling pathways that converge on central neural circuits that control normal eating. In addition, we review interactions between gonadal hormones and immune function that lead to sex differences in acute illness anorexia. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Diet-genotype interactions in the early development of obesity and insulin resistance in mice with a genetic deficiency in tumor necrosis factor-α.

The onset of insulin resistance, the sites of action, and the mechanisms through which tumor necrosis factor-alpha (TNF-alpha) exacerbates the increase in adiposity and the development of insulin resistance in mice fed high-fat (HF) diet remain unclear. Here we investigated the effect of TNF-alpha deficiency on adiposity and insulin resistance during the initial 1 to 4 weeks of HF feeding. We examined body weight; the distribution of white adipose tissue (WAT); homeostasis model assessment; and levels of leptin, resistin, and adiponectin in the initial 4 weeks of HF feeding in TNF-alpha knockout (KO) mice and wild-type (WT) controls. Through 4 weeks of HF feeding, KO mice, unlike WT mice, maintained normal insulin sensitivity. Although WT-HF and KO-HF mice had similar levels of WAT at this time, KO-HF mice had more subcutaneous and less epididymal fat than WT-HF mice. The KO-HF mice also had less liver fat than the WT-HF mice. Finally, KO-HF mice had lower plasma levels of resistin than WT-HF mice. These data demonstrate that genetic lack of TNF-alpha protects insulin sensitivity during the early phase of HF feeding in the absence of altered total WAT. The data also suggest that the mechanism maintaining insulin sensitivity in the absence of TNF-alpha may involve redirection of the fat deposition to the metabolically more inert subcutaneous depot or decreases in circulating resistin and resultant decrease in liver fat deposition. The efficacy of therapeutic measures designed to counteract the effects of TNF-alpha may be increased during the early stages of obesity and insulin resistance.

Pharmacological, but not genetic, disruptions in 5-HT2C receptor function attenuate LPS anorexia in mice

Peripheral administration of bacterial lipopolysaccharide (LPS) elicits anorexia in several species, including rats and mice. There is strong evidence that antagonism of serotonergic activity at 2C receptors (5-HT(2C)R) attenuates LPS anorexia in rats. Here we used pharmacological and genetic approaches to examine the role of the 5-HT(2C)R in LPS anorexia in mice. In Experiment 1, SB 242084, a potent and selective 5-HT(2C) antagonist (0.3 mg/kg) was injected intraperitoneally 15 min before intraperitoneal LPS (2 microg/kg) injections just prior to dark onset in c57BL/6 mice. Food intake was recorded 1, 2 and 4 h after LPS administration. In Experiment 2, we recorded 2, 4 and 24 h food intake following dark onset intraperitoneal LPS (0.125, 0.25, 0.5, 1 and 2 microg/kg) injections in mice with a genetic deletion of 5-HT(2C)R and their WT controls. Our pharmacological results suggest that at least part of the anorexia following peripheral LPS administration is mediated by an increase in 5-HT-ergic activity at the 5-HT(2C)R. Our genetic data, in contrast, suggest that 5-HT(2C)R is not a necessary part of LPS anorexia.

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 microg/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.

Evidence that PGE2 in the dorsal and median raphe nuclei is involved in LPS-induced anorexia in rats.

Anorexia is an element of the acute-phase immune response. Its mechanisms remain poorly understood. Activation of inducible cyclooxygenase-2 (COX-2) in blood-brain-barrier endothelial cells and subsequent release of prostaglandins (e.g., prostaglandin E2, PGE2) may be involved. Therefore, we sought to relate the effects of prostaglandins on the anorexia following gram-negative bacterial lipopolysaccharide treatment (LPS) to neural activity in the dorsal and median raphe nuclei (DRN and MnR) in rats. COX-2 antagonist (NS-398, 10mg/kg; IP) administration prior to LPS (100μg/kg; IP) prevented anorexia and reduced c-Fos expression the DRN, MnR, nucleus tractus solitarii and several related forebrain areas. These data indicate that COX-2-mediated prostaglandin synthesis is necessary for LPS anorexia and much of the initial LPS-induced neural activation. Injection of NS-398 into the DRN and MnR (1ng/site) attenuated LPS-induced anorexia to nearly the same extent as IP NS-398, suggesting that prostaglandin signaling in these areas is necessary for LPS anorexia. Because the DRN and MnR are sources of major serotonergic projections to the forebrain, these data suggest that serotonergic neurons originating in the midbrain raphe play an important role in acute-phase response anorexia.

Decreased adiposity and improved insulin sensitivity in CD14ko mice.

Low-grade, chronic, systemic inflammation supposedly contributes to the development of obesity and its co-morbidities, such as insulin resistance and type 2 diabetes. CD14 is a protein that facilitates binding of gram(+) and gram(−) bacterial pathogens to immune cells that initiate the inflammatory response. As CD14 may also bind circulating fatty acids, and as the absorption of bacterial pathogens may be increased with obesifying diets, CD14ko mice might develop less systemic inflammation than WT mice and might thus be protected against diet-induced obesity and its metabolic consequences. Nine-week-old CD14ko and WT mice (C57/Bl6) were fed chow (CH), HF (60% of the energy from lipid) or HS (CH+8% sucrose solution) diets. Food intake and body weight (BW) were measured periodically; intraperitoneal insulin sensitivity (IST) and body adiposity were measured 22–25 (CH, HF, HS) and 43–45 (CH, HF) weeks later. CD14ko mice ate less CH than WT mice and had lower BW, less adiposity, lower fasting insulin, and improved IST compared to WT mice on all diets. Thus, lack of CD14 improves insulin sensitivity and reduces adiposity regardless of the diet fed. Because CD14 is expressed on pancreatic beta cells, chronically reduced insulin release in CD14ko mice may explain all these effects.

Diet- and obesity-independent improvements in insulin sensitivity in mice genetically deficient in tumor necrosis factor-α (TNF-α KO).

High-fat (HF) feeding and obesity lead to progressive reductions in insulin sensitivity and type 2 diabetes mellitus. The cytokine TNF-α has been shown to contribute to this pathology in overweight HF-fed mice. Here we examined whether genetic deficiency of TNF-α protects against the development of insulin resistance independent of HF-feeding or obesity. Eight-wk old TNF-α KO and wild-type (WT) mice were fed chow (CH) or HF (60% energy from fat) diet. One and 4 week later plasma glucose and insulin levels were measured after 12-h fasts and 3-h re-feeding. Plasma leptin and resistin levels and body adiposity were measured after 4 week. KO mice ate more CH, but not more HF diet, than WT mice. BW and adiposity increased more in HF than CH mice, and TNF-α deficiency had no effect on either, despite the CH intake difference. Fasting glucose was lower after 1 week in KO HF than WT HF mice. Fasting insulin was lower in KO CH than WT CH mice. Fed glucose tended to be increased in KO mice in all tests, but was significantly so only in 4-week CH. Fed insulin was markedly lower in KO mice in all tests. Fasting leptin and resistin were increased by HF, with no genotype effect. Fed leptin (CH and HF) and resistin (HF) were reduced in KO mice. These data suggest that TNF-α deficiency improves insulin sensitivity even in the absence of HF-feeding or obesity, and that the mechanism may involve reduced circulating leptin.