Immunoreactive corticotropin-releasing hormone levels in the hypothalamus of female Wistar fatty rats

Corticotropin-releasing factor decreases meal size by decreasing cluster number in Koletsky (LA/N) rats with and without a null mutation of the leptin receptor

The homozygous mutant Koletsky rat is a monogenic form of obesity and hyperphagia due to a null mutation of the leptin receptor (lepr(fak)). To investigate if the lack of leptin action on the brain of homozygous mutants affected the inhibitory potency of corticotropin-releasing factor (CRF) on meal size, artificial cerebrospinal fluid or one of five doses of CRF was administered through third ventricular cannulas in 8 +/+, 10 +/fa(k), and 8 fa(k)/fa(k) rats 15 min before access to 20% sucrose in lickometer tubes for 30 min. CRF had equivalent inhibitory potency in fa(k)/fa(k) and +/+ rats. Thus, the complete lack of leptin action in fa(k)/fa(k) rats did not change the inhibitory potency of CRF. CRF was significantly more potent, however, in +/fa(k) rats than in the other two genotypes. Thus, the heterozygote condition of this mutation did not function as a classical recessive mutation for this behavioral phenotype. Despite these differences in potency, microstructural analysis revealed that CRF decreased intakes in all three genotypes by decreasing the number of clusters of licking without changing the size of clusters.

Neuropeptides and obesity

This review focuses on the expression, content, and release of neuropeptides and on their role in the development of obesity in animal models with single-gene mutations. The balance between neuropeptides that contribute to the control of feeding behavior is profoundly and variously altered in these models, supporting the concept of the existence of several types of obesity. The hypothalamic neuropeptide Y (NPY) and the pro-opiomelanocortin (POMC) systems are the networks most studied in relation to energy intake. Both receive information about the nutritional status and the level of energy storage through insulin and leptin signaling mediated by specific receptors located on POMC and NPY neurons present predominantly in the arcuate nucleus (ARC). When leptin signaling is defective, through a defect in either the receptor (Zucker fa/fa rat, cp/cp rat, and db/db mouse) or in the peptide itself (ob/ob mouse), the NPY system is upregulated as shown by mRNA overexpression and increased peptide release, whereas the content and/or release of some inhibitory peptides (neurotensin, cholecystokinin) are diminished. For the POMC system, there is a complex interaction between the tonic inhibition of food intake exerted by alpha-melanocyte-stimulating hormone (alpha-MSH) and the Agouti-related protein at the level of the type 4 melanocortin receptor. The latter peptide is coexpressed with NPY in the ARC. Corticotropin-releasing factor (CRF) is the link between food intake and environmental factors. It not only inhibits food intake and prevents weight gain, likely through hypothalamic effects, but also activates the hypothalamo-pituitary axis and therefore contributes to energy storage in adipose tissue. The factors that prod the CRF system toward the hypothalamic or hypothalamo-pituitary axis system remain to be more clearly defined (comodulators, connections between limbic system and ARC, cellular location, and type of receptors, etc. ). The pathways used by all of these neuromodulators include numerous brain areas, but some interest has returned to the classic ones such as the ventromedial and lateral hypothalamic areas because of the recent discovery of some peptides (orexins and melanin-concentrating hormone for the lateral hypothalamus) and receptors (CRF type 2 in the ventromedial hypothalamus). All of these pathways are redundant and function in a coordinated manner and sometimes by the novel expression of a peptide in an unusual area. The importance of such a phenomenon in obesity remains to be determined. Even if single-gene mutations are exceptions in human obesity, the study of genetic animal models of obesity has greatly contributed to the understanding of the regulation of feeding behavior and will allow researchers to develop new drug treatments for obesity that have to be associated with drastic changes in lifestyle (feeding, work habits, and physical activity) for a complete efficiency.

Neuropeptide Y and corticotropin-releasing hormone concentrations within specific hypothalamic regions of lean but not ob/ob mice respond to food-deprivation and refeeding

Leptin is proposed to control food intake at least in part by regulating hypothalamic neuropeptide Y (NPY), a stimulator of food intake, and corticotropin-releasing hormone (CRH), an inhibitor of food intake. Ob/ob mice are leptin-deficient and would thus be expected to exhibit alterations in hypothalamic NPY and CRH. We therefore measured concentrations of NPY and CRH in discrete regions of the hypothalamus (i.e., ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial nucleus; DMH, dorsomedial nucleus; and SCN, suprachiasmatic nucleus) of 6.5-7-wk-old ob/ob and lean mice with free access to stock diet, 24 h after food deprivation, and 1 h after refeeding. Fed ob/ob mice had 55-75% higher concentrations of NPY in the ARC, VMH and SCN than lean mice. Food deprivation increased NPY concentrations approximately 70% in the ARC, PVN and VMH of lean mice, and refeeding lowered NPY concentrations approximately 70% in the PVN of these mice. NPY in these hypothalamic regions of ob/ob mice was unresponsive to food deprivation or refeeding. The most pronounced change in CRH concentrations within the regions examined (i.e., ARC, PVN and VMH) occurred in the ARC of lean mice where refeeding lowered CRH concentrations by 75% without influencing ARC CRH concentrations in ob/ob mice. The hypothalamic concentrations of two neuropeptides involved in body weight regulation (i.e., NPY and CRH) in leptin-deficient ob/ob mice respond abnormally to abrupt changes in nutritional status.

Detrimental effects of chronic hypothalamic-pituitary-adrenal axis activation. From obesity to memory deficits

Increasing evidence suggests that the detrimental effects of glucocorticoid (GC) hypersecretion occur by activation of the hypothalamic-pituitary-adrenal (HPA) axis in several human pathologies, including obesity, Alzheimer's disease, AIDS dementia, and depression. The different patterns of response by the HPA axis during chronic activation are an important consideration in selecting an animal model to assess HPA axis function in a particular disorder. This article will discuss how chronic HPA axis activation and GC hypersecretion affect hippocampal function and contribute to the development of obesity. In the brain, the hippocampus has the highest concentration of GC receptors. Chronic stress or corticosterone treatment induces neuropathological alterations, such as dendritic atrophy in hippocampal neurons, which are paralleled by cognitive deficits. Excitatory amino acid (EAA) neurotransmission has been implicated in chronic HPA axis activation. EAAs play a major role in neuroendocrine regulation. Hippocampal dendritic atrophy may involve alterations in EAA transporter function, and decreased EAA transporter function may also contribute to chronic HPA axis activation. Understanding the molecular mechanisms of HPA axis activation will likely advance the development of therapeutic interventions for conditions in which GC levels are chronically elevated.

Corticotropin-releasing factor and adrenocorticotrophic hormone as potential central mediators of OB effects

OB (leptin) has been identified as a factor that suppresses appetite and stimulates metabolism. Attention has focused on the hypothalamus as its potential site of action, but OB could also act on other brain regions. In addition, the paradox of high OB levels in obese humans remains unresolved. Here we show in mice that both the long and short form of the OB receptor are expressed not only in the hypothalamus but also in the amygdala and pituitary. Recombinant murine OB elicited the release of corticotropin-releasing factor from superfused brain slice preparations containing hypothalamus or amygdala. Because corticotropin-releasing factor inhibits appetite and stimulates metabolism, it may be a key mediator of central OB effects. Recombinant OB also induced pituitary release of adrenocorticotrophic hormone. Because adrenocorticotrophic hormone-induced elevation of plasma glucocorticoid levels can inhibit corticotropin-releasing factor release via negative feedback, the OB effects on pituitary adrenocorticotrophic hormone release may be pertinent to human obesity, which combines increased plasma glucocorticoid levels with elevated levels of OB. An imbalance between the effects of OB on corticotropin-releasing factor release from the hypothalamus and on adrenocorticotrophic hormone release from the pituitary could contribute to obesity.

Corticotropin-releasing factor-binding protein ligand inhibitor blunts excessive weight gain in genetically obese Zucker rats and rats during nicotine withdrawal

Elevation of the neuropeptide corticotropin-releasing factor (CRF) in the brain is associated with a reduction of food intake and body weight gain in normal and obese animals. A protein that binds CRF and the related peptide, urocortin, with high affinity, CRF-binding protein (CRF-BP), may play a role in energy homeostasis by inactivating members of this peptide family in ingestive and metabolic regulatory brain regions. Intracerebroventricular administration in rats of the high-affinity CRF-BP ligand inhibitor, rat/human CRF (6-33), which dissociates CRF or urocortin from CRF-BP and increases endogenous brain levels of "free" CRF or urocortin significantly blunted exaggerated weight gain in Zucker obese subjects and in animals withdrawn from chronic nicotine. Chronic administration of CRF suppressed weight gain nonselectively by 60% in both Zucker obese and lean control rats, whereas CRF-BP ligand inhibitor treatment significantly reduced weight gain in obese subjects, without altering weight gain in lean control subjects. Nicotine abstinent subjects, but not nicotine-naive controls, experienced a 35% appetite suppression and a 25% weight gain reduction following acute and chronic administration, respectively, of CRF-BP ligand inhibitor. In marked contrast to the effects of a CRF-receptor agonist, the CRF-BP ligand inhibitor did not stimulate adrenocorticotropic hormone secretion or elevate heart rate and blood pressure. These results provide support for the hypothesis that the CRF-BP may function within the brain to limit selected actions of CRF and/or urocortin. Furthermore, CRF-BP may represent a novel and functionally selective target for the symptomatic treatment of excessive weight gain associated with obesity of multiple etiology.

Immunoreactive corticotropin-releasing hormone levels in discrete hypothalamic nuclei of genetically obese Zucker rats

The levels of immunoreactive corticotropin-releasing hormone (ir-CRH) were measured in discrete hypothalamic nuclei and the median eminence of obese Zucker rats and their lean littermates. More than 90% of total hypothalamic ir-CRH was detected in the median eminence in both obese and lean rats. Though ir-CRH levels in the paraventricular nucleus of obese rats tended to be lower than those of their lean littermates, no significant difference of ir-CRH levels was observed in any hypothalamic nuclei studied between obese and lean rats. However, ir-CRH levels in the median eminence of obese rats were significantly lower than those of their lean littermates (5263 +/- 438 pg/tissue vs. 7050 +/- 473 pg/tissue, P < 0.05). These results suggest that the hypoactive hypothalamic CRH tonus would play some role in the phenotypic expression of obesity in the genetically obese Zucker rats.