Noradrenergic neurons of the area postrema mediate amylin's hypophagic action.

Circulating amylin inhibits food intake via activation of the area postrema (AP). The aim of this study was to identify the neurochemical phenotype of the neurons mediating amylin's hypophagic action by immunohistochemical and feeding studies in rats. Expression of c-Fos protein was used as a marker for neuronal activation and dopamine-beta-hydroxylase (DBH), the enzyme-catalyzing noradrenaline synthesis, as a marker for noradrenergic neurons. We found that approximately 50% of amylin-activated AP neurons are noradrenergic. To clarify the functional role of these neurons in amylin's effect on eating, noradrenaline-containing neurons in the AP were lesioned using a saporin conjugated to an antibody against DBH. Amylin (5 or 20 microg/kg s.c.)-induced anorexia was observed in sham-lesioned rats with both amylin doses. Rats with a lesion of > 50% of the noradrenaline neurons were unresponsive to the low dose of amylin (5 microg/kg) and only displayed a reduction in food intake 60 min after injection of the high amylin dose (20 microg/kg). In a terminal experiment, the same rats received amylin (20 microg/kg) or saline. The AP and nucleus of the solitary tract (NTS) were stained for DBH to assess noradrenaline lesion success and for c-Fos expression to evaluate amylin-induced neuronal activation. In contrast to sham-lesioned animals, noradrenaline-lesioned rats did not show a significant increase in amylin-induced c-Fos expression in the AP and NTS. We conclude that the noradrenergic neurons in the AP mediate at least part of amylin's hypophagic effect.

Mechanisms of amylin/leptin synergy in rodent models.

The present studies aimed to identify mechanisms contributing to amylin/leptin synergy in reducing body weight and adiposity. We reasoned that if amylin/leptin harnessed complementary neuronal pathways, then in the leptin-sensitive state, amylin should augment leptin signaling/binding and that in the absence of endogenous amylin, leptin signaling should be diminished. Amylin (50 microg/kg, ip) amplified low-dose leptin-stimulated (15 microg/kg, ip) phosphorylated signal transducer and activator of transcription-3 signaling within the arcuate nucleus (ARC) in lean rats. Amylin (50 microg/kg x d) or leptin (125 microg/kg x d) infusion to lean rats decreased 28-d food intake (14 and 10%, respectively), body weight (amylin by 4.3%, leptin by 4.9%), and epididymal fat (amylin by 19%, leptin by 37%). Amylin/leptin co-infusion additively decreased food intake (by 26%) and reduced body weight (by 15%) and epididymal fat (by 78%; all P < 0.05 vs. all groups) in a greater than mathematically additive manner, consistent with synergy. Amylin increased leptin binding within the ventromedial hypothalamus (VMN) by 35% and dorsomedial hypothalamus by 47% (both P < 0.05 vs. vehicle). Amylin/leptin similarly increased leptin binding in the VMN by 40% and ARC by 70% (P < 0.05 vs. vehicle). In amylin-deficient mice, hypothalamic leptin receptor mRNA expression was reduced by 50%, leptin-stimulated phosphorylated signal transducer and activator of transcription-3 within ARC and VMN was reduced by 40%, and responsiveness to leptin's (1 mg/kg x d for 28 d) weight-reducing effects was attenuated (all P < 0.05 vs. wild-type controls). We suggest that amylin/leptin's marked weight- and fat-reducing effects are due to activation of intrinsic synergistic neuronal signaling pathways and further point to the integrated neurohormonal therapeutic potential of amylin/leptin agonism in obesity.

Exenatide improves hypertension in a rat model of the metabolic syndrome.

BACKGROUND: Exenatide is a peptide incretin mimetic that has glucoregulatory actions associated with weight reduction. Previous reports demonstrated acute increases in blood pressure after systemic or intracerebroventricular administration of exenatide or glucagon like peptide 1 (GLP 1) in rats. However, there are limited studies testing the chronic effects of these peptides on arterial pressure and no reports showing the effects of these peptides to reverse hypertension in the context of the metabolic syndrome. METHODS: Thus, we examined the response to peripheral exenatide using telemetry in conscious, unrestrained rats under normotensive conditions and in a model of hypertension/metabolic syndrome induced by corticosterone. Rats were implanted with either corticosterone or wax (control) pellets, followed 14 days later by the additional implantation of pumps to deliver exenatide (1 microg/kg per day) or vehicle for 7 days. RESULTS: The 21-day corticosterone treatment produced hypertriglyceridemia, visceral fat deposition, hyperglycemia, insulin resistance, and an elevation of mean arterial blood pressure (MAP) by 14 +/- 1 mmHg. Exenatide significantly reversed corticosterone-induced increases in blood pressure and this normalization occurred independently from change in body weight. Additionally, exenatide reduced MAP by 5 +/- 3 mmHg in normotensive control rats. CONCLUSIONS: These results are the first demonstration of a durable antihypertensive effect of exenatide in a glucocorticoid-induced model of the metabolic syndrome.

Pharmacological actions of the peptide hormone amylin in the long-term regulation of food intake, food preference, and body weight.

The ability of amylin to reduce acute food intake in rodents is well established. Longer-term administration in rats (up to 24 days) shows a concomitant reduction in body weight, suggesting energy intake plays a significant role in mediating amylin-induced weight loss. The current set of experiments further explores the long-term effects of amylin (4-11 wk) on food preference, energy expenditure, and body weight and composition. Furthermore, we describe the acute effect of amylin on locomotor activity and kaolin consumption to test for possible nonhomeostatic mechanisms that could affect food intake. Four-week subcutaneous amylin infusion of high-fat fed rats (3-300 microg.kg(-1).day(-1)) dose dependently reduced food intake and body weight gain (ED(50) for body weight gain = 16.5 microg.kg(-1).day(-1)). The effect of amylin on body weight gain was durable for up to 11 wks and was associated with a specific loss of fat mass and increased metabolic rate. The body weight of rats withdrawn from amylin (100 microg.kg(-1).day(-1)) after 4 wks of infusion returned to control levels 2 wks after treatment cessation, but did not rebound above control levels. When self-selecting calories from a low- or high-fat diet during 11 wks of infusion, amylin-treated rats (300 microg.kg(-1).day(-1)) consistently chose a larger percentage of calories from the low-fat diet vs. controls. Amylin acutely had no effect on locomotor activity or kaolin consumption at doses that decreased food intake. These results demonstrate pharmacological actions of amylin in long-term body weight regulation in part through appetitive-related mechanisms and possibly via changes in food preference and energy expenditure.

Macrophage liver X receptor is required for antiatherogenic activity of LXR agonists.

OBJECTIVE: Complications of atherosclerotic cardiovascular disease due to elevated blood cholesterol levels are the major cause of death in the Western world. The liver X receptors, LXRalpha and LXRbeta (LXRs), are ligand-dependent transcription factors that act as cholesterol sensors and coordinately control transcription of genes involved in cholesterol and lipid homeostasis as well as macrophage inflammatory gene expression. LXRs regulate cholesterol balance through activation of ATP-binding cassette transporters that promote cholesterol transport and excretion from the liver, intestine, and macrophage. Although LXR agonists are known to delay progression of atherosclerosis in mouse models, their ability to abrogate preexisting cardiovascular disease by inducing regression and stabilization of established atherosclerotic lesions has not been addressed. METHODS AND RESULTS: We demonstrate that LXR agonist treatment increases ATP-binding cassette transporter expression within preexisting atherosclerotic lesions, resulting in regression of these lesions as well as remodeling from vulnerable to stable lesions and a reduction in macrophage content. Further, using macrophage-selective LXR-deficient mice created by bone marrow transplantation, we provide the first evidence that macrophage LXR expression is necessary for the atheroprotective actions of an LXR agonist. CONCLUSIONS: These data substantiate that drugs targeting macrophage LXR activity may offer therapeutic benefit in the treatment of atherosclerotic cardiovascular disease.