CNS melanocortin system involvement in the regulation of food intake

Activation of central melanocortin-4 receptor suppresses lipopolysaccharide-induced fever in rats

Activation of central melanocortin receptors (MCR) inhibits fever, but the identity of the MCR subtype(s) mediating this antipyretic effect is unknown. To determine whether selective central melanocortin receptor-4 (MC4R) activation produces antipyretic effects, the MC4R selective agonist MRLOB-0001 (CO-His-d-Phe-Arg-Trp-Dab-NH(2)) was administered intracerebroventricularly to rats treated with Escherichia coli lipopolysaccharide (LPS, 30 microg/kg ip). Treatment with MRLOB-0001 (150 ng icv) did not lower core body temperature (T(c)) in afebrile rats but did suppress LPS-induced increases in T(c) and associated decreases in tail skin temperature (T(sk)), an indicator of vasomotor thermoeffector function. In contrast, systemic treatment with MRLOB-0001 (150 ng iv) did not produce similar antipyretic effects. Coadministration of the selective MC4R antagonist HS014 (1 microg icv) blocked the antipyretic effects of MRLOB-0001. HS014 alone (1 microg icv) had no significant effect on LPS-induced increases in T(c) or decreases in T(sk) and in afebrile rats had no significant effects on T(c) or T(sk). We conclude that pharmacological activation of central MC4R suppresses febrile increases in T(c) and that inhibition of heat conservation pathways may contribute to this effect. These findings suggest that the central MC4R may mediate the long-recognized antipyretic effects of centrally administered melanocortins.

Ontogeny of the hypothalamic neuropeptide Y system

Early onset obesity and type II diabetes is rapidly becoming an epidemic, especially within the United States. This dramatic increase is likely due to many factors including both prenatal and postnatal environmental cues. The purpose of this review is to highlight some of the recent advances in our knowledge of the development of the hypothalamic circuits involved in the regulation of energy balance, with a focus on the neuropeptide Y (NPY) system. Unlike the adult rat, during the postnatal period NPY is transiently expressed in several hypothalamic regions, along with the expected expression within the arcuate nucleus (ARH). These transient populations of NPY neurons during the postnatal period may provide local NPY production to sustain the necessary energy intake during this critical growth phase. This may be physiologically important since ARH-NPY projections do not fully develop until the 3rd postnatal week. The significance of this ontogeny is that many peripheral metabolic signals have little effect of feeding prior to the development of the ARH projections. The essential questions now are whether prenatal and/or postnatal exposure to high levels of insulin or leptin during development can cause permanent changes in the function of hypothalamic circuits. It is vital to understand not only the natural development of the hypothalamic circuits that regulate energy homeostasis, but also their abnormal development caused by maternal and postnatal environmental cues. This will be pivotal for designing intervention and therapeutics to treat early onset obesity/type II diabetes, which may very well need to be different from those designed to prevent/treat adult onset obesity/type II diabetes.

Assessment of the aversive consequences of acute and chronic administration of the melanocortin agonist, MTII

BACKGROUND

The synthetic melanocortin (MC) agonist, melanotan-II (MTII), reduces food intake and body weight for hours to days after administration. One early report on the effect of MTII suggested that part of its anorexic action may be mediated by aversive consequences. In that experiment, MTII was found to support a mild conditioned taste aversion (CTA).

OBJECTIVE

The present experiments replicate and extend those findings in two additional CTA paradigms to further characterize the aversive effects of MTII in rats.

METHODS

Experiment 1 simultaneously assessed the ability of MTII to support CTA and reduce food intake, using a small oral infusion of a novel taste as the conditioned stimulus. Experiment 2 assessed the aversive consequences of chronic MTII administration. To accomplish this, we paired implantation of lithium chloride (LiCl)-, MTII- or saline-containing osmotic minipumps with a constantly available novel flavor. After 7 days, rats received a choice test between the minipump-paired flavor and a previously available neutral flavor.

RESULTS

Rats with saline minipumps exhibited no preference for either flavor. By contrast, rats in both the LiCl and MTII minipump groups significantly preferred the neutral flavor, indicating the development of a CTA. Additionally, CTA produced by administration of MTII was found to be more resistant to extinction than that produced by LiCl.

CONCLUSIONS

The reduction in food intake caused by MTII is accompanied by aversive consequences regardless of route of administration. These results present difficulties for the development of MCs-based therapies for obesity.

Hypothalamic NPY, AGRP, and POMC mRNA responses to leptin and refeeding in mice

Food deprivation (FD) increases hypothalamic neuropeptide Y (NPY) and agouti-related protein (AGRP) mRNA levels and decreases proopiomelanocortin (POMC) mRNA levels; refeeding restores these levels. We determined the time course of changes in hypothalamic NPY, AGRP, and POMC mRNA levels on refeeding after 24 h FD in C57BL mice by in situ hybridization. After 24 h deprivation, mice were refed with either chow or a palatable mash containing no calories or were injected with murine leptin (100 microg) without food. Mice were perfused 2 or 6 h after treatment. Food deprivation increased hypothalamic NPY mRNA (108 +/- 6%) and AGRP mRNA (78 +/- 7%) and decreased hypothalamic POMC mRNA (-15 +/- 1%). Refeeding for 6 h, but not 2 h, was sufficient to reduce (but not restore) NPY mRNA, did not affect AGRP mRNA, and restored POMC mRNA levels to ad libitum control levels. Intake of the noncaloric mash had no effect on mRNA levels, and leptin administration after deprivation (at a dose sufficient to reduce refeeding in FD mice) was not sufficient to affect mRNA levels. These results suggest that gradual postabsorptive events subsequent to refeeding are required for the restoration of peptide mRNA to baseline levels after food deprivation in mice.

Neuropeptides and amphibian prey-catching behavior

In mammals, a number of hypothalamic neuropeptides have been implicated in stress-induced feeding disorders. Recent studies in anurans suggest that stress-related neuropeptides may act on elemental aspects of visuomotor control to regulate feeding. Corticotropin-releasing hormone (CRH) and alpha-melanocyte-stimulating hormone, potent an orexic peptides in mammals, inhibit visually-guided prey-catching in toads. Neuropeptide Y (NPY), an orexic peptide in mammals, may be an important neuromodulator in inhibitory pre-tectal-tectal pathways involved in distinguishing predator and prey. Melanocortin, NPY and CRH neurons project onto key visuomotor structures within the amphibian brain, suggesting physiological roles in the modulation of prey-catching. Thus, neuropeptides involved in feeding behavior in mammals influence the efficacy of a visual stimulus in releasing prey-catching behavior. These neuropeptides may play an important role in how frogs and toads gather and process visual information, particularly during stress.

Central melanocortin receptors mediate changes in food intake in the rhesus macaque

In rodents, stimulation of melanocortin-3 and -4 receptor subtypes (MC3-R and MC4-R) causes a reduction in food intake, whereas antagonism of MC3-R and MC4-R increases food intake. This report describes the effects of the stable alphaMSH analog, NDP-MSH ([Nle(4), D-Phe(7)]alphaMSH), and the endogenous alphaMSH receptor antagonist, agouti-related protein, on feeding behavior in adult male rhesus macaques. Infusion of NDP-MSH into the lateral cerebral ventricle dose dependently suppressed intake of a normally scheduled meal without affecting nonfeeding behaviors. Conversely, infusion of agouti-related protein stimulated food intake during the scheduled afternoon meal. In addition to these physiological experiments, the effect of fasting on hypothalamic POMC gene expression was assessed by in situ hybridization. Missing a single meal or fasting for 48 h caused a similar reduction in POMC gene expression in the arcuate nucleus. These results demonstrate that in the primate, central melanocortin receptors can acutely regulate food intake and suggest that the central melanocortinergic system is a physiological regulator of energy balance in primate species.

Adiposity signals and the control of energy homeostasis

Recent technologic innovations have enabled probing the workings of individual cells and even molecules. As a result, our knowledge of the biological controls over eating and the regulation of body adiposity is increasing at a rapid pace. We review the evidence that food intake is controlled by separate but interacting groups of molecular signals. One group, termed satiety signals, are proportional to what is being consumed and help to determine meal size. Cholecystokinin is the best known of these, and its premeal administration causes a dose-dependent reduction of meal size. In and of itself, however, cholecystokinin (and other satiety signals) has little impact on body-fat stores. The second group, termed adiposity signals, circulate in proportion to body adiposity and enter the brain, where they interact with satiety signals in the brainstem and hypothalamus. Insulin and leptin are the best known of these adiposity signals, and the administration of either into the brain causes a dose-dependent reduction of both food intake and body weight. Within the brain, parallel but opposing pathways originating in the hypothalamic arcuate nuclei integrate adiposity signals with satiety signals. Those with a net anabolic effect increase food intake and reduce energy expenditure and are represented (among many such signals) by neuropeptide Y; those with a net catabolic effect decrease food intake and energy expenditure and are represented by brain melanocortins. This complex regulatory mechanism allows individuals to adapt their feeding schedule to idiosyncratic environmental constraints, eating whenever it is desirable or possible. Body-weight regulation occurs as adiposity signals alter the efficacy of meal-generated satiety signals.

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.