Melanocortin MC4R receptor is required for energy expenditure but not blood pressure effects of angiotensin II within the mouse brain.

The brain renin-angiotensin system (RAS) is implicated in control of blood pressure (BP), fluid intake, and energy expenditure (EE). Angiotensin II (ANG II) within the arcuate nucleus of the hypothalamus contributes to control of resting metabolic rate (RMR) and thereby EE through its actions on Agouti-related peptide (AgRP) neurons, which also contribute to EE control by leptin. First, we determined that although leptin stimulates EE in control littermates, mice with transgenic activation of the brain RAS (sRA) exhibit increased EE and leptin has no additive effect to exaggerate EE in these mice. These findings led us to hypothesize that leptin and ANG II in the brain stimulate EE through a shared mechanism. Because AgRP signaling to the melanocortin MC4R receptor contributes to the metabolic effects of leptin, we performed a series of studies examining RMR, fluid intake, and BP responses to ANG II in mice rendered deficient for expression of MC4R via a transcriptional block (Mc4r-TB). These mice were resistant to stimulation of RMR in response to activation of the endogenous brain RAS via chronic deoxycorticosterone acetate (DOCA)-salt treatment, whereas fluid and electrolyte effects remained intact. These mice were also resistant to stimulation of RMR via acute intracerebroventricular (ICV) injection of ANG II, whereas BP responses to ICV ANG II remained intact. Collectively, these data demonstrate that the effects of ANG II within the brain to control RMR and EE are dependent on MC4R signaling, whereas fluid homeostasis and BP responses are independent of MC4R signaling.

Assessment of T-2 toxin effect and its metabolite HT-2 toxin combined with insulin-like growth factor I, leptin and ghrelin on progesterone secretion by rabbit ovarian fragments.

Assessment of A-trichothecene mycotoxins (T-2 and HT-2 toxins) effect combined with growth factor IGF-I, and the metabolic hormones leptin and ghrelin on progesterone secretion by rabbit ovarian fragments was studied. Rabbit ovarian fragments were incubated without (control group) or with T-2/HT-2 toxin, or their combinations with insulin-like growth factor I (IGF-I), leptin or ghrelin at various concentrations for 24 h. Secretion of progesterone was determined by ELISA. First, T-2 toxin and HT-2 toxins at all doses used (0.01, 0.1, 1, 10, and 100 ng mL(-1)) were not shown to be potential regulators of progesterone secretion in rabbit ovarian fragments. Second, T-2 toxin but not HT-2 toxin combined with IGF-I was shown to be potential regulator of progesterone secretion in rabbit ovarian fragments. T-2 toxin at all doses used (0.01; 0.1; 1; 10; and 100 ng mL(-1)) combined with IGF-I (at dose 100 ng mL(-1)) significantly (P < 0.05) decreased progesterone secretion by rabbit ovarian fragments. Third, T-2 toxin and HT-2 toxin at all doses used in the study (0.01, 0.1, 1, 10, and 100 ng mL(-1)) combined with leptin (at dose 1000 ng mL(-1)) were not shown to be potential regulators of progesterone secretion in rabbit ovarian fragments. Furthermore, T-2 toxin and HT-2 toxin at all doses used in the study (0.01, 0.1, 1, 10, and 100 ng mL(-1)) combined with ghrelin (500 ng mL(-1)) were not shown to be potential regulators of progesterone secretion in rabbit ovarian fragments. Results in this study showed that trichothecene as T-2 toxin combined with IGF-I but not HT-2 toxin was able to decrease progesterone secretion in rabbit ovarian fragments in vitro. Experimental results of T-2 and HT-2 toxins combined with leptin and ghrelin did not confirm ability to modulate progesterone secretion by ovarian fragments in rabbits.

AMPK and the neuroendocrine regulation of appetite and energy expenditure.

This review highlights recent advances in the hormonal control of hypothalamic AMPK activity and the impact on appetite and energy metabolism. AMPK is an intracellular energy sensor that switches off ATP-consuming pathways and switches on ATP-producing pathways such as glucose uptake and fatty acid oxidation. In this regard, it is well positioned to respond to dynamic changes in metabolic state and nutritional over- or under-supply. Within the hypothalamus, AMPK responds to peripheral hormones that convey metabolic information based on increased plasma concentrations. For example, negative energy balance increases plasma ghrelin concentrations, increases hypothalamic AMPK and drives food intake. Conversely, plasma leptin concentrations are secreted in proportion to adipose levels and leptin suppresses hypothalamic AMPK activity and restricts food intake. This review explains that hypothalamic AMPK mediates neuroendocrine feedback control of energy metabolism. A current working model suggests that endocrine feedback influences hypothalamic AMPK via a number of mechanisms designed to shift an organism from negative to neutral energy balance. These mechanisms include (1) ghrelin stimulation of AMPK in NPY/AgRP in the arcuate nucleus (2) ghrelin stimulation of AMPK in the ventromedial hypothalamic nucleus, (3) a novel ghrelin-stimulated AMPK-dependent presynaptic mechanism that sustains AgRP neuron firing via a local synaptic memory system, (4) adiponectin stimulation of hypothalamic AMPK and (5) hypothalamic AMPK control of energy expenditure by thyroid hormone or leptin. The number of diverse mechanisms ensures hypothalamic AMPK drives the shift from negative to neutral energy balance and underscores the fundamental importance of hypothalamic AMPK to maintain neutral energy balance.

TXNIP in Agrp neurons regulates adiposity, energy expenditure, and central leptin sensitivity.

Thioredoxin interacting protein (TXNIP) has recently been described as a key regulator of energy metabolism through pleiotropic actions that include nutrient sensing in the mediobasal hypothalamus (MBH). However, the role of TXNIP in neurochemically specific hypothalamic subpopulations and the circuits downstream from MBH TXNIP engaged to regulate energy homeostasis remain unexplored. To evaluate the metabolic role of TXNIP activity specifically within arcuate Agrp neurons, we generated Agrp-specific TXNIP gain-of-function and loss-of-function mouse models using Agrp-Ires-cre mice, TXNIP (flox/flox) mice, and a lentivector expressing the human TXNIP isoform conditionally in the presence of Cre recombinase. Overexpression of TXNIP in Agrp neurons predisposed to diet-induced obesity and adipose tissue storage by decreasing energy expenditure and spontaneous locomotion, without affecting food intake. Conversely, Agrp neuronal TXNIP deletion protected against diet-induced obesity and adipose tissue storage by increasing energy expenditure and spontaneous locomotion, also without affecting food intake. TXNIP overexpression in Agrp neurons did not primarily affect glycemic control, whereas deletion of TXNIP in Agrp neurons improved fasting glucose levels and glucose tolerance independently of its effects on body weight and adiposity. Bidirectional manipulation of TXNIP expression induced reciprocal changes in central leptin sensitivity and the neural regulation of lipolysis. Together, these results identify a critical role for TXNIP in Agrp neurons in mediating diet-induced obesity through the regulation of energy expenditure and adipose tissue metabolism, independently of food intake. They also reveal a previously unidentified role for Agrp neurons in the brain-adipose axis.

The nucleus tractus solitarius: a portal for visceral afferent signal processing, energy status assessment and integration of their combined effects on food intake.

For humans and animal models alike there is general agreement that the central nervous system processing of gastrointestinal (GI) signals arising from ingested food provides the principal determinant of the size of meals and their frequency. Despite this, relatively few studies are aimed at delineating the brain circuits, neurochemical pathways and intracellular signals that mediate GI-stimulation-induced intake inhibition. Two additional motivations to pursue these circuits and signals have recently arisen. First, the success of gastric-bypass surgery in obesity treatment is highlighting roles for GI signals such as glucagon-like peptide-1 (GLP-1) in intake and energy balance control. Second, accumulating data suggest that the intake-reducing effects of leptin may be mediated through an amplification of the intake-inhibitory effects of GI signals. Experiments reviewed show that: (1) the intake-suppressive effects of a peripherally administered GLP-1 receptor agonist is mediated by caudal brainstem neurons and that forebrain-hypothalamic neural processing is not necessary for this effect; (2) a population of medial nucleus tractus solitarius (NTS) neurons that are responsive to gastric distention is also driven by leptin; (3) caudal brainstem-targeted leptin amplifies the food-intake-inhibitory effects of gastric distention and intestinal nutrient stimulation; (4) adenosine monophosphate-activated protein kinase (AMPK) activity in NTS-enriched brain lysates is elevated by food deprivation and reduced by refeeding and (5) the intake-suppressive effect of hindbrain-directed leptin is reversed by elevating hindbrain AMPK activity. Overall, data support the view that the NTS and circuits within the hindbrain mediate the intake inhibition of GI signals, and that the effects of leptin on food intake result from the amplification of GI signal processing.

Assessment by c-Fos immunostaining of changes in brain neural activity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and leptin in rats.

The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes multiple effects in laboratory animals. One of these is a wasting syndrome (a dramatic loss of body weight over 2-5 weeks) whose mechanism is still largely unknown. We exploited the over 1000 times difference in TCDD sensitivity between Long-Evans (Turku/AB); (L-E) and Han/Wistar (Kuopio); (H/W) rats to reveal brain areas that might be activated by a single dose of TCDD (50 microg/kg) given 24 hr previously. Leptin (1.3 mg/kg intraperitoneally 2 hr before tissue harvest) was used as a reference compound, as its neural pathway for decreasing food intake in the control of energy homeostasis is fairly well known. Serial sections of the brains were immunostained with an antibody for the activity marker c-Fos, and selected areas -- primarily in the hypothalamus -- were analysed with a computer-assisted microscope. Given alone, TCDD did not elicit any major alterations in c-Fos protein levels in the hypothalamic nuclei at the early time-point studied (24 hr after administration), neither in pooled data nor in individual strains. The control substance leptin proved that the method is valid as it increased the number of c-Fos-immunopositive cells in the hypothalamic ventromedial and arcuate nuclei. Although the present findings are not suggestive of a primary role for the hypothalamus in the wasting syndrome, a time-course study covering also the feeding-active dark hours is warranted for their verification.

Effects of recombinant human leptin treatment as an adjunct of moderate energy restriction on body weight, resting energy expenditure and energy intake in obese humans.

We explored the effects of recombinant human leptin (rL) as an adjunct of mild energy restriction (2092 kJ/day less than needed) in the treatment of obese humans as part of a larger multicentre trial. In a double blind, randomised, placebo (P)-controlled design, the effects of 10 mg of rL once daily vs twice daily (rL OD/BID, by s.c. injection) upon body weight, resting energy expenditure (REE) and energy intake were compared. The study groups comprised 9 (P), 15 (rL OD) and 6 (rL BID) healthy subjects (body mass index 27.5-35 kg/m2). We observed in both groups treated with rL a decline of body weight. [2.8+/-1.1 kg (P), 5.2+/-0.9 kg (rL OD), 7.9+/-1.4 kg (rL BID), p < 0.035]. No significant effects of rL treatment upon energy intake or REE were observed. However, rL tended to reduce the decline of energy expenditure associated with energy restriction, whereas the tendency of energy intake to increase back to baseline levels in placebo-treated subjects was largely prevented in subjects treated with rL. Thus, rL appears to enhance the loss of body weight in obese humans in a dose-dependent fashion if prescribed as an adjunct of energy restriction. This effect might be mediated by rL ability to counteract the behavioural and metabolic adaptations that accompany weight loss attempts.

Low dose leptin administration reverses effects of sustained weight-reduction on energy expenditure and circulating concentrations of thyroid hormones.

Maintenance of a reduced body weight is associated with decreased 24-hour energy expenditure, and decreased circulating concentrations of leptin and thyroid hormones. To determine whether these adaptive metabolic and endocrine changes are partly leptin-mediated, we measured body composition, aspects of energy expenditure, and circulating concentrations of leptin and thyroid hormones in 4 subjects at 3 time points: 1.) Usual body weight; 2.) While stable at 10% reduced body weight; and 3.) During a 5-week period at 10% reduced body weight while receiving twice per day leptin injections that restored 8 AM circulating leptin concentrations to those seen at usual body weight. During maintenance of a 10% reduced body weight, circulating T3, T4, and leptin concentrations were decreased. All of these endocrine changes were reversed by administration of "replacement" doses of leptin (r-metHuLeptin). Indirect calorimetry, and subtle changes in body composition associated with leptin administration, were used to calculate the net change in stored calories and in 24-hour energy expenditure. Total energy expenditure increased in all subjects during r-metHuLeptin administration. These data indicate that decrease leptin concentrations resulting from loss of fat mass account for some aspects of the endocrine/metabolic phenotype associated with the weight-reduced state.

Hypothalamic neuronal histamine as a target of leptin in feeding behavior.

Leptin, an ob gene product, has been shown to suppress food intake by regulating hypothalamic neuromodulators. The present study was designed to examine the involvement of brain histamine in leptin-induced feeding suppression. A bolus infusion of 1.0 microg leptin into the rat third cerebroventricle (i3vt) elevated the turnover rate of hypothalamic neuronal histamine (P < 0.05) as assessed by pargyline-induced accumulation of tele-methylhistamine (t-MH), a major metabolite of histamine. No remarkable change in the mRNA expression of histidine decarboxylase (HDC), a histamine-synthesizing enzyme, was observed in the hypothalamus after i3vt infusion of leptin. These results indicate that leptin increases histamine turnover by affecting the posttranscriptional process of HDC formation or histamine release per se. As expected, concomitant suppression in 24-h cumulative food intake was also observed after infusion of leptin. Systemic depletion of brain histamine levels by pretreatment with an intraperitoneal injection of 224 micromol/kg alpha-fluoromethylhistidine (FMH), a suicide inhibitor of HDC, attenuated the leptin-induced feeding suppression by 50.7% (P < 0.05). This attenuation of feeding suppression was mimicked by the i3vt infusion of 2.24 micromol/kg FMH before leptin treatment (P < 0.05). In addition, concentrations of hypothalamic histamine and t-MH were lowered in diabetic (db/db) mice, which are known to be deficient in leptin receptors (P < 0.05 vs. lean littermates for each amine), although the amine levels were higher in diet-induced obese rats (P < 0.05 for each amine). Leptin-deficient obese mice (ob/ob) showed lower histamine turnover (P < 0.05 vs. lean littermates), which recovered after leptin infusion. Thus, a growing body of results points to an important role for the hypothalamic histamine neurons in the central regulation of feeding behavior controlled by leptin.