Selective stimulatory actions of corticotropin-releasing factor ligands on correlates of energy balance

Sex-specific effects of relaxin-3 on food intake and brain expression of corticotropin-releasing factor in rats

This study compared the effects of relaxin-3 (RLN3) on food intake, plasma corticosterone, and the expression of corticotropin-releasing factor (CRF) in male and female rats. RLN3 was injected into the lateral ventricle at 25, 200, and 800 pmol concentrations. RLN3 at 25 pmol increased food intake (grams) at 30 and 60 minutes after injection in female but not male rats. Female rats also showed higher increase in relative to body weight (BW) food intake (mg/g BW) for all RLN3 concentrations at 30 minutes and for 800 pmol of RLN3 at 60 minutes. Moreover, RLN3 at 800 pmol significantly increased 24-hour BW gain in female but not male rats. At 60 minutes after administration, 800 pmol of RLN3 produced a significant increase in plasma corticosterone and in the expression of CRF and c-fos mRNAs in the parvocellular paraventricular hypothalamic nucleus (PVN) in male but not female rats. The levels of c-fos mRNA in the magnocellular PVN were increased by RLN3 but did not differ between the sexes. Conversely, expression of CRF mRNA in the medial preoptic area was increased in female rats but was not sensitive to 800 pmol of RLN3. In the bed nucleus of the stria terminalis, 800 pmol of RLN3 significantly increased CRF mRNA expression in female but not male rats. Therefore, female rats showed more sensitivity and stronger food intake increase in response to RLN3. The differential effects of RLN3 on CRF expression in the PVN and bed nucleus of the stria terminalis may contribute to the sex-specific difference in the behavioral response.

Central and peripheral effects of ghrelin on energy balance, food intake and lipid metabolism in teleost fish

Ghrelin was first identified and characterized from rat stomach as an endogenous ligand for the growth hormone secretagogue receptor. Ghrelin and its receptor system are present not only in peripheral tissues such as stomach and intestine, but also in the central nervous system of mammals. Interestingly, administration of ghrelin induces an orexigenic effect and also modifies locomotor activity, suggesting its involvement in feeding control and the regulation of energy balance, in addition to the regulation of growth hormone release. Information about ghrelin in non-mammals, such as teleost fish, has also been increasing, and important data have been obtained. An understanding of the evolutionary background of the energy regulation system and the central and peripheral roles of ghrelin in teleost fish could provide indications as to their roles in mammals, particularly humans. In this review, we overview the central and peripheral effects of ghrelin on energy balance, locomotor activity, and lipid metabolism in teleost fish.

Feeding microstructure in diet-induced obesity susceptible versus resistant rats: central effects of urocortin 2

With one billion people overweight worldwide, the need to identify risk factors and treatments for obesity is urgent. The present study determined whether rats genetically prone to diet-induced obesity (DIO) show preexisting differences in meal microstructure and are sensitive to central anorectic effects of corticotropin-releasing factor type 2 (CRF(2)) receptor stimulation. Male, selectively bred DIO rats and their diet resistant (DR) counterparts (n = 9/genotype) were weaned onto low-fat chow and compared as young adults for spontaneous or intracerebroventricular urocortin 2 administration-induced (0, 0.3, 1, 3 microg) differences in ingestion. DIO rats were hyperphagic selectively at the dark cycle onset, showing shorter latencies to initiate feeding, faster returns to eating following meal completion, and a lower satiety ratio than DR rats. At other times, DIO rats had briefer postmeal intervals, but ate smaller and briefer meals, resulting in normal intake. DIO rats also ate faster than DR rats. Urocortin 2 was less potent in DIO rats, ineffective at the 0.3 microg dose, but produced CRF(2) antagonist-reversible anorexia at higher doses. Though heavier, chow-maintained DIO rats were proportionately as or more lean than DR rats. Thus, DIO rats showed signs of a preexisting, heritable deficit in the maintenance of postmeal satiety and a reduced sensitivity to anorectic CRF(2) agonist stimulation. The meal patterns of DIO rats temporally resemble human 'snacking' behaviour, which predicts adult obesity. Because central CRF(2) stimulation retains full anorectic efficacy at higher doses in the DIO model, manipulating this neuropeptidergic system might yield new therapeutic approaches for diet-induced obesity.

Delayed satiety-like actions and altered feeding microstructure by a selective type 2 corticotropin-releasing factor agonist in rats: intra-hypothalamic urocortin 3 administration reduces food intake by prolonging the post-meal interval

Brain corticotropin-releasing factor/urocortin (CRF/Ucn) systems are hypothesized to control feeding, with central administration of 'type 2' urocortins producing delayed anorexia. The present study sought to identify the receptor subtype, brain site, and behavioral mode of action through which Ucn 3 reduces nocturnal food intake in rats. Non-food-deprived male Wistar rats (n=176) were administered Ucn 3 into the lateral (LV) or fourth ventricle, or into the ventromedial or paraventricular nuclei of the hypothalamus (VMN, PVN) or the medial amygdala (MeA), regions in which Ucn 3 is expressed in proximity to CRF(2) receptors. LV Ucn 3 suppressed ingestion during the third-fourth post-injection hours. LV Ucn 3 anorexia was reversed by cotreatment with astressin(2)-B, a selective CRF(2) antagonist and not observed following equimole subcutaneous or fourth ventricle administration. Bilateral intra-VMN and intra-PVN infusion, more potently than LV infusion, reduced the quantity (57-73%) and duration of ingestion (32-68%) during the third-fourth post-infusion hours. LV, intra-PVN and intra-VMN infusion of Ucn 3 slowed the eating rate and reduced intake by prolonging the post-meal interval. Intra-VMN Ucn 3 reduced feeding bout size, and intra-PVN Ucn 3 reduced the regularity of eating from pellet to pellet. Ucn 3 effects were behaviorally specific, because minimal effective anorectic Ucn 3 doses did not alter drinking rate or promote a conditioned taste aversion, and site-specific, because intra-MeA Ucn 3 produced a nibbling pattern of more, but smaller meals without altering total intake. The results implicate the VMN and PVN of the hypothalamus as sites for Ucn 3-CRF(2) control of food intake.

Vital functions of corticotropin-releasing factor (CRF) pathways in maintenance and regulation of energy homeostasis

Regulation of energy homeostasis is a vital function of the CNS requiring adaptive responses to maintain and support life after stress perturbations. The mechanisms whereby these processes occur are, however, only partially understood. A major determinate of these responses is corticotropin-releasing factor (CRF). Receptors for CRF, CRFR1 and CRFR2, have been hypothesized to play distinct roles in the alterations necessary for homeostatic maintenance. The function of CRFR2, in particular, has remained elusive despite its presence in both the CNS and periphery. In this work, we have used complimentary gene deletion and pharmacological approaches to elucidate the crucial role CRFR2 plays in the regulation of regional tissue thermogenesis and adaptive physiology. Analyses of interscapular brown adipose tissue (IBAT) thermogenesis by thermal signature analysis and the concordant biochemical changes in key sympathetic components in mice deficient for CRFR2 revealed significantly elevated basal IBAT thermogenesis and prolonged adrenergic responsivity of IBAT in older mice. Measurement of metabolic rates by indirect calorimetry after chronic high-fat diet challenge and treatment with the CRFR1 antagonist NBI-27914 revealed a decreased respiratory exchange ratio of these mice that was normalized with NBI-27914. Further, as a definitive measure for physiological pathology, mice examined in a behavioral model of differential temperature selection showed a predilection for warmer external temperatures, supporting a loss of body heat in these mice. These studies provide physiological, biochemical, and behavioral evidence for the critical participation of CRF pathways in the maintenance and adaptive responses necessary for regulation of energy homeostasis.

Increased glucocorticoid response to a novel stress in rats that have been restrained

Rats exposed to repeated restraint stress (3 h of restraint on each of 3 days) lose weight during stress and do not return to the weight of nonstressed controls once stress ends. Others have reported that chronic stress raises the daily nadir of corticosterone release and increases the adrenal response to subsequent stress; therefore, we examined glucocorticoid release in rats that had been exposed to repeated restraint. Repeated restraint had no effect on the diurnal pattern of corticosterone or insulin release, measured 12 days after restraint had ended, indicating that the reduced weight of the rats is not associated with an elevated corticosterone-insulin ratio. In contrast, rats that had been exposed to repeated restraint, 12 days previously, showed a blunted corticosterone release during a second restraint stress, a normal response to the novel physiological stress of 2-deoxy glucose (2-DG) injection, but an exaggerated corticosterone response to the novel mild stress (MS) of either placement in a unfamiliar environment or an intraperitoneal injection of saline. Mice exposed to repeated restraint showed a similar hyperresponsiveness to novel MS, suggesting that repeated restraint lowers the threshold for stress-induced activation of the adrenal gland. MS caused a small, but significant, degree of hypophagia in rats that had been exposed to repeated restraint stress. Therefore, multiple aspects of the stress response may be exaggerated in these animals and contribute to the chronic reduction in body weight.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Chronic exercise lowers the defended body weight gain and adiposity in diet-induced obese rats

The effects of running wheel exercise and caloric restriction on the regulation of body weight, adiposity, and hypothalamic neuropeptide expression were compared in diet-induced obese male rats over 6 wk. Compared with sedentary controls, exercising rats had reduced body weight gain (24%), visceral (4 fat pads; 36%) and carcass (leptin; 35%) adiposity but not insulin levels. Hypothalamic arcuate nucleus (ARC) proopiomelanocortin (POMC) mRNA expression was 25% lower, but ARC neuropeptide Y (NPY), agouti- related peptide, dorsomedial nucleus (DMN) NPY, and paraventricular nucleus (PVN) corticotropin- releasing hormone (CRH) expression was comparable to controls. Sedentary rats calorically restricted to 85% of control body weight reduced their visceral adiposity (24%), leptin (64%), and insulin (21%) levels. ARC NPY (23%) and DMN NPY (60%) were increased, while ARC POMC (40%) and PVN CRH (14%) were decreased. Calorically restricted exercising rats an half as much as ad libitum-fed exercising rats and had less visceral obesity than comparably restricted sedentary rats. When sedentary restricted rats were refed after 4 wk, they increased intake and regained the weight gain and adiposity of sedentary controls. While refed exercising rats and sedentary rats ate comparable amounts, refed exercising rats regained weight and adiposity only to the level of ad libitum-fed exercising rats. Thus exercise lowers the defended level of weight gain and adiposity without a compensatory increase in intake and with a very different profile of hypothalamic neuropeptide expression from calorically restricted rats. This may be due to exercise-related factors other than plasma insulin and leptin.

Differential involvement of amygdaloid CRH system(s) in the salience and valence of the stimuli

Anxiety is a heterogeneous term encompassing not only state or trait characteristics but also a wide range of pathologies such as generalized anxiety disorders, phobias, panic and obsessive-compulsive disorders, acute stress disorder, and posttraumatic stress disorder. Given that diverse forms of anxiety exist, numerous animal models have been developed, which are considered to be useful in identifying mechanisms underlying anxiety states. Examples of such animal models include paradigms that assess the behavioral response to neurogenic (or painful stimuli) or psychogenic stressors or to cues that had previously been associated with painful stimuli. The present report presents data regarding the impact of stressors on corticotropin-releasing hormone (CRH), and relates these to changes in anxiety-like states. Specifically, we demonstrate that (1) psychogenic stressors influence the in vivo release of CRH at the central nucleus of the amygdala (CeA); (2) although CRH changes within the CeA are exquisitely sensitive to stressors, they are also elicited by positive stimuli; and (3) while treatment with diazepam attenuates behavioral signs of anxiety, the CRH release associated with a stressor is unaffected by the treatment. The position is offered that although release of CRH within the CeA is increased under stressful conditions, it is not a necessary condition for the consequent behavioral expression of anxiety-like reactions, at least not in minimally threatening situations. We suggest that the CRH responses at the CeA may be involved in a preparatory capacity and, as such, may accompany a range of emotionally significant stimuli, be they appetitive or aversive.

Nonexercise muscle tension and behavioral fidgeting are positively correlated with food availability/palatability and body weight in rats

While certain measures of energy expenditure such as respiratory quotient and thermogenesis are readily quantifiable using existing animal models, the mechanism for and measurement of energy expenditure via nonexercise activity have not been thoroughly characterized. This low intensity form of physical exertion, associated with involuntary fidgeting and postural changes in man, was quantified in the present studies using passive measurement of muscle tension in rats. In particular, long-term weight loss and gain were induced using diet yoking and feeding of preferred foods in order to assess corresponding changes in locomotor activity and radiotelemetered measures of muscle tension, temperature and global activity. Hind limb muscle tension, but not body temperature, was increased 30-60% by enhancing the availability or palatability of food relative to the decreased muscle tension resulting from limited food availability. Enhancing food availability or palatability also produced a relative 5-15% increase in the amount of telemetered global activity. Importantly, neither diet yoking nor provision of a highly preferred diet altered a precise measure of behavioral locomotor activity. These results suggest that muscle tension and activity-in-place are positively correlated with weight change in the present studies and that these mechanisms of energy expenditure are mobilized by environmental changes in diet composition and meal pattern.

The corticotropin-releasing factor family of peptides and CRF receptors: their roles in the regulation of energy balance

The corticotropin-releasing factor (CRF) system could play a significant role in the regulation of energy balance. This system, which includes CRF, CRF-related peptides and CRF receptors, is part of a huge network of cells connected to central and peripheral pathways modulating energy metabolism. CRF and CRF-related peptides, which elicit their effects through G-protein-coupled receptors known in mammals as CRF(1) receptor and CRF(2) receptor, are capable of strong anorectic and thermogenic effects. Also supporting a role for the CRF system in the regulation of energy balance are findings demonstrating alterations in this system in obese and food-deprived animals that concur to facilitate energy deposition. In recent years, great progress has been made in understanding the specific physiological roles of the CRF system. In that respect, the discovery of urocortins II and III, two endogenous ligands of the CRF(2) receptor, and the development of selective and long-acting antagonists for the CRF receptors, have led to a better comprehension of the role of the CRF system in the regulation of energy balance. Although there are still important unresolved issues in the field of CRF research, the progress made recently warrants investigations aimed at evaluating the CRF system as a potential target for anti-obesity drugs.