The role of central melanocortin receptors in the activation of the hypothalamus-pituitary-adrenal-axis and the induction of excessive grooming

Role of brain NUCB2/nesfatin-1 in stress and stress-related gastrointestinal disorders

Since the discovery of NUCB2/nesfatin-1 as a novel anorexigenic factor, the expanding function of this peptide has been elucidated in recent years. Increasing evidence suggests that NUCB2/nesfatin-1 is also involved in the regulation of stress and stress-related gastrointestinal disorders. Therefore, we investigated the relationship between NUCB2/nesfatin-1, stress and stress-related gastrointestinal disorders and summarized the results of these studies. Different stressors and duration of stress activate different NUCB2/nesfatin-1-associated brain regions and have different effects on serum corticosterone levels. Central and peripheral NUCB2/nesfatin-1 mediates stress-related gastrointestinal disorders but appears to be protective against inflammatory bowel disease. NUCB2/nesfatin-1 plays an important role in mediating the brain-gut crosstalk, but precise clarification is still needed to gain more insight into these complex relationships.

The Melanocortin System: A Promising Target for the Development of New Antidepressant Drugs

Major depression is one of the most prevalent mental disorders, causing significant human suffering and socioeconomic loss. Since conventional antidepressants are not sufficiently effective, there is an urgent need to develop new antidepressant medications. Despite marked advances in the neurobiology of depression, the etiology and pathophysiology of this disease remain poorly understood. Classical and newer hypotheses of depression suggest that an imbalance of brain monoamines, dysregulation of the hypothalamic-pituitary-adrenal axis (HPAA) and immune system, or impaired hippocampal neurogenesis and neurotrophic factors pathways are cause of depression. It is assumed that conventional antidepressants improve these closely related disturbances. The purpose of this review was to discuss the possibility of affecting these disturbances by targeting the melanocortin system, which includes adrenocorticotropic hormone-activated receptors and their peptide ligands (melanocortins). The melanocortin system is involved in the regulation of various processes in the brain and periphery. Melanocortins, including peripherally administered non-corticotropic agonists, regulate HPAA activity, exhibit anti-inflammatory effects, stimulate the levels of neurotrophic factors, and enhance hippocampal neurogenesis and neurotransmission. Therefore, endogenous melanocortins and their analogs are able to complexly affect the functioning of those body’s systems that are closely related to depression and the effects of antidepressants, thereby demonstrating a promising antidepressant potential.

The Melanocortin System behind the Dysfunctional Eating Behaviors

The dysfunction of melanocortin signaling has been associated with obesity, given the important role in the regulation of energy homeostasis, food intake, satiety and body weight. In the hypothalamus, the melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) contribute to the stability of these processes, but MC3R and MC4R are also localized in the mesolimbic dopamine system, the region that responds to the reinforcing properties of highly palatable food (HPF) and where these two receptors seem to affect food reward and motivation. Loss of function of the MC4R, resulting from genetic mutations, leads to overeating in humans, but to date, a clear understanding of the underlying mechanisms and behaviors that promote overconsumption of caloric foods remains unknown. Moreover, the MC4R demonstrated to be a crucial modulator of the stress response, factor that is known to be strictly related to binge eating behavior. In this review, we will explore the preclinical and clinical studies, and the controversies regarding the involvement of melanocortin system in altered eating patterns, especially binge eating behavior, food reward and motivation.

A limbic circuitry involved in emotional stress-induced grooming

Prolonged exposure to negative stressors could be harmful if a subject cannot respond appropriately. Strategies evolved to respond to stress, including repetitive displacement behaviours, are important in maintaining behavioural homoeostasis. In rodents, self-grooming is a frequently observed repetitive behaviour believed to contribute to post-stress de-arousal with adaptive value. Here we identified a rat limbic di-synaptic circuit that regulates stress-induced self-grooming with positive affective valence. This circuit links hippocampal ventral subiculum to ventral lateral septum (LSv) and then lateral hypothalamus tuberal nucleus. Optogenetic activation of this circuit triggers delayed but robust excessive grooming with patterns closely resembling those evoked by emotional stress. Consistently, the neural activity of LSv reaches a peak before emotional stress-induced grooming while inhibition of this circuit significantly suppresses grooming triggered by emotional stress. Our results uncover a previously unknown limbic circuitry involved in regulating stress-induced self-grooming and pinpoint a critical role of LSv in this ethologically important behaviour.

Self-grooming is a frequently observed repetitive behaviour in rodents that is believed to contribute to post-stress de-arousal. The authors identified a previously unknown limbic circuit that includes the ventral lateral septum in rats and is involved in regulating stress-induced self-grooming.

Depression increases the genetic susceptibility to high body mass index: Evidence from UK Biobank

BACKGROUND

This study aimed to explore the association between depression and body mass index (BMI), and to investigate whether genetic susceptibility to high BMI is different among individuals with or without depression.

METHODS

We used data on 251,125 individuals of white British ancestry from the UK Biobank. We conducted Mendelian randomization (MR) analysis to test for a causal association between depression and BMI using a major depressive disorder (MDD)-related genetic risk score (GRSMDD ) as an instrument for depression. We also examined whether depression modifies genetic susceptibility to high BMI, by investigating the interaction between depression and the BMI-related GRSBMI .

RESULTS

We found observational and genetic evidence for an association between depression and BMI (MR beta: 0.09, 95% confidence interval [CI] 0.04-0.13). Further, the contribution of genetic risk to high BMI was higher among individuals with depression compared to controls. Carrying 10 additional BMI increasing alleles was associated with 0.24 standard deviation (SD; 95%CI 0.23-0.25) higher BMI among depressed individuals compared to 0.20 SD (95%CI 0.19-0.21) higher in controls, which corresponds to 3.4 kg and 2.8 kg extra weight for an individual of average height. Amongst the individual loci, the evidence for interaction was most notable for a variant near MC4R, a gene known to affect both appetite regulation and the hypothalamic-pituitary adrenal axis (pinteraction  = 5.7 × 10-5 ).

CONCLUSION

Genetic predisposition to high BMI was higher among depressed than to nondepressed individuals. This study provides support for a possible role of MC4R in the link between depression and obesity.

Expression of Melanocortin-4 Receptor mRNA in Male Rat Hypothalamus During Chronic Stress

The effects of chronic stress and glucocorticoids receptor antagonist (RU486) on expression of melanocortin 4 receptor (MC4R) mRNA in arcuate nucleus (ARC) of male rats were evaluated. In this study, adult male Sprague Dawley rats were placed into four groups (n=6/group); stress, RU486, stress/RU486, and control groups. In stress group, the rats were restrained, 1 h/day, for 12 days. In RU486 group, the rats were injected RU486 for 12 days. In stress/RU486 group, the rats were injected RU486 1 h before the stress process for 12 days. Relative expression of MC4R mRNA was determined using real-time PCR. Relative expression of MC4R mRNA in the stress group was higher than that of the control rats (P<0.05). Relative expressions of MC4R mRNA were not different between the stress, RU486 and stress/RU486 groups (P>0.05). Chronic restraint stress causes increase in mRNA expression of MC4R in ARC and blockade of glucocorticoid receptors has no effect on this up-regulation.

Synaptic changes induced by melanocortin signalling

The melanocortin system has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its involvement in memory, nociception, mood disorders and addiction. In this Review, we focus on the role of the melanocortin 4 receptor and provide an integrative view of the molecular mechanisms that lead to melanocortin-induced changes in synaptic plasticity within these diverse physiological systems. We also highlight the importance of melanocortin peptides and receptors in chronic pain syndromes, memory impairments, depression and drug abuse, and the possibility of targeting them for therapeutic purposes.

Melanocortin MC(4) receptor-mediated feeding and grooming in rodents

Decades ago it was recognized that the pharmacological profile of melanocortin ligands that stimulated grooming behavior in rats was strikingly similar to that of Xenopus laevis melanophore pigment dispersion. After cloning of the melanocortin MC1 receptor, expressed in melanocytes, and the melanocortin MC4 receptor, expressed mainly in brain, the pharmacological profiles of these receptors appeared to be very similar and it was demonstrated that these receptors mediate melanocortin-induced pigmentation and grooming respectively. Grooming is a low priority behavior that is concerned with care of body surface. Activation of central melanocortin MC4 receptors is also associated with meal termination, and continued postprandial stimulation of melanocortin MC4 receptors may stimulate natural postprandial grooming behavior as part of the behavioral satiety sequence. Indeed, melanocortins fail to suppress food intake or induce grooming behavior in melanocortin MC4 receptor-deficient rats. This review will focus on how melanocortins affect grooming behavior through the melanocortin MC4 receptor, and how melanocortin MC4 receptors mediate feeding behavior. This review also illustrates how melanocortins were the most likely candidates to mediate grooming and feeding based on the natural behaviors they induced.

Intranasal infusion of melanocortin receptor four (MC4R) antagonist to rats ameliorates development of depression and anxiety related symptoms induced by single prolonged stress

Brain melanocortinergic systems and specifically melanocortin receptor four (MC4R) are implicated in modulation of anxiety- and depressive-like behavior induced by mild or moderate stress. Here we examine whether blockage of central MC4Rs with HS014 before severe traumatic stress may protect against development of anxiety and depression co-morbid with post-traumatic stress disorder (PTSD). Male rats were treated intranasally (IN) with vehicle or varied doses of HS014, 30min prior to single prolonged stress (SPS) animal model of PTSD. IN administration of 100μg HS014 pre-SPS improved despair behavior in forced swim (FS) immediately after immobilization stress part of SPS protocol. During all 4 intervals of 20min FS these rats spent less time immobile than rats given vehicle or 3.5ng HS014. This dose of HS014 also had a long-term beneficial effect manifested as reduction of immobility time in forced swim test performed after SPS. However, both HS014 doses were effective in ameliorating development of anxiety-like behavior after traumatic stress. Thus, rats given IN HS014 prior to SPS exhibited less open arms (OA) visits in elevated plus maze (EPM), spent longer time in OA and less in closed arms, had lower anxiety index, higher risk assessment and more head dips over borders in OA. They also spent longer time in the center of the open field and defecated less. Reduced grooming behavior in EPM was observed with 100μg HS014. This is the first study revealing pronounced resilience effects of HS014 on development of behavioral symptoms co-morbid with PTSD.

Melanocortin-4 receptor in the medial amygdala regulates emotional stress-induced anxiety-like behaviour, anorexia and corticosterone secretion

The central melanocortin system has been implicated in emotional stress-induced anxiety, anorexia and activation of the hypothalamo-pituitary-adrenal (HPA) axis. However, the underlying neural substrates have not been identified. The medial amygdala (MeA) is highly sensitive to emotional stress and expresses high levels of the melanocortin-4 receptor (MC4R). This study investigated the effects of activation and blockade of MC4R in the MeA on anxiety-like behaviour, food intake and corticosterone secretion. We demonstrate that MC4R-expressing neurons in the MeA were activated by acute restraint stress, as indicated by induction of c-fos mRNA expression. Infusion of a selective MC4R agonist into the MeA elicited anxiogenic-like effects in the elevated plus-maze test and decreased food intake. In contrast, local MeA infusion of SHU 9119, a MC4R antagonist, blocked restraint stress-induced anxiogenic and anorectic effects. Moreover, plasma corticosterone levels were increased by intra-MeA infusion of the MC4R agonist under non-stressed conditions and restraint stress-induced elevation of plasma corticosterone levels was attenuated by pretreatment with SHU 9119 in the MeA. Thus, stimulating MC4R in the MeA induces stress-like anxiogenic and anorectic effects as well as activation of the HPA axis, whereas antagonizing MC4R in this region blocks such effects induced by restraint stress. Together, our results implicate MC4R signalling in the MeA in behavioural and endocrine responses to stress.

Increased morning adrenocorticotrophin hormone (ACTH) levels in women with postpartum thoughts of harming the infant

INTRODUCTION

Some postpartum women experience intrusive thoughts of harming the infant. The hypothalamic-pituitary-adrenal (HPA) axis, which has been linked to postpartum depression, may play a role in the aetiology of postpartum thoughts of harming the infant. We aimed to study whether HPA axis hormones measured early postpartum are related to postpartum intrusive thoughts.

METHOD

132 women who delivered a child at a university hospital participated in a follow-up study with visits at 2-3 days postpartum and 8th week postpartum. Participants were assessed for trait anxiety, social support, peripartum or postpartum anxiety or depression, stressful life events and obstetric variables including perinatal complications and lactation. Postpartum thoughts of harming the infant were assessed with a semi-structured interview. Serum cortisol, and plasma CRH and ACTH levels were measured within 48 h postpartum at 8-9 AM. A logistic regression was performed to explore the relationship between clinical variables, hormonal measures and postpartum intrusive thoughts.

RESULTS

Patients with postpartum thoughts of harming the infant had, when compared to those women without intrusive thoughts, higher ACTH levels (7.59 pmol/L vs 5.09 pmol/L, p<0.05) without significant differences in CRH or cortisol levels. In the logistic regression analysis, adjusted for breast-feeding and psychopathological status, only ln ACTH was associated with the presence of postpartum thoughts of harming the infant (OR=5.2, CI 95% 1.2-22.6, p=0.029). No other clinical variables were associated with postpartum intrusive thoughts.

CONCLUSIONS

Our study suggests that a dysregulation of the hypothalamic-pituitary-adrenal axis may play a role in the aetiology of postpartum thoughts of harming the infant.

Distribution and function of melanocortin receptors within the brain

Biological responses to pro-opiomelanocortin (POMC)-derived peptides administered in the brain were documented in the 1950s but their molecular mechanisms of action only began to be resolved with the mapping of melanocortin receptor subtypes to specific brain regions in the 1990s. Out of the five melanocortin receptor subtypes, MC3R and MC4R are widely recognised as 'neural' melanocortin receptors. In situ hybridization anatomical mapping of these receptor subtypes to distinct hypothalamic nuclei first indicated their roles in energy homeostasis, roles that were later confirmed with the obese phenotypes exhibited by Mc3R and Mc4R knockout mice. It is perhaps less well known however, that all five melanocortin receptor subtypes have been detected in developing and/or adult brains of various species. This chapter provides a comprehensive summary of the detection and mapping of each melanocortin receptor subtype in mammalian, chicken and fish brains and relates the sites of expression to functions that are either known or proposed for each receptor subtype.

Mitogen-activated protein kinase contributes to lipopolysaccharide-induced activation of corticotropin-releasing hormone synthesizing neurons in the hypothalamic paraventricular nucleus

To determine whether the p44/p42 MAPK (ERK1/2) signaling pathway is involved in the activation of CRH-containing neurons in the hypothalamic paraventricular nucleus (PVN) after bacterial lipopolysaccharide (LPS) administration, Sprague Dawley rats were injected with LPS, and studied after 2, 6, 9, and 12 h. In saline-treated controls, isolated weak phosphorylated (phospho)ERK1/2 immunoreactive neurons were observed in the PVN. However, a dramatic increase in phospho-ERK1/2 immunoreactivity was apparent in the PVN 2 h after LPS administration, and gradually declined to baseline levels 9-12 h after injection. By double-labeling immunofluorescence, all CRH-containing neurons in the PVN contained phospho-ERK1/2 2 h after LPS. Intracerebroventricular administration of the MAPK inhibitor, PD98059, prevented LPS-induced ERK1/2 phosphorylation, c-fos activation, and the increase of CRH gene expression in the PVN but had no effect on c-fos activation in brainstem A2-C1/C2 regions. We conclude that LPS rapidly increases the phospho-ERK1/2 in CRH-containing neurons in the PVN and that activation of MAPKs is necessary for LPS-induced activation of the hypothalamic-pituitary-adrenal axis.

Effects of melanotan II, a melanocortin agonist, on grooming and exploration in rats after repeated restraint/immobilization

2mg/kg melanotan II (MTII, administered i.p.), a cyclic peptide analog of alpha-melanocyte stimulating hormone, at a single dose increased grooming in naive rats placed in an unfamiliar open-field device without changing locomotion or rearing. Male rats exposed to restraint/immobilization stress (IS) for 1h on three consecutive days displayed increased grooming after the second stressor exposure, compared to pre-stress levels. MTII, administered to the rats after IS, enhanced the grooming response compared both to the pre- and post-stress values. The increase was greatest after the first dose and declined over the following two applications. As to the locomotion of rats in the entire experimental space, IS reduced the distance moved only after the first two stressor exposures; MTII did not influence these alterations. Locomotion in the central part of arena was not reduced by the stressor or by MTII, on the contrary, there was an increase in both groups after the third intervention. The only observed change in rearing was an increase in the MTII group after the third restraint exposure. Thus, MTII selectively increased grooming without markedly affecting the spatio-temporal structure of locomotor behavior in the open-field. The decline of MTII enhanced grooming over the three test days may be interpreted in terms of adaptation to the stressor and of the developing tolerance to the peptide.

Central administration of selective melanocortin 4 receptor antagonist HS014 prevents morphine tolerance and withdrawal hyperalgesia

Major problem involved in treatment of chronic pain with morphine is the development of tolerance and dependence. Previous studies have demonstrated the participation of melanocortin (MC) system in the development of tolerance to antinociceptive effect of morphine. However, the impact of supraspinal MC4 receptors (MC4 R) modulation on this phenomenon and morphine withdrawal hyperalgesia remained unexplored. We investigated the role of central MC4 R in acute, chronic effects and withdrawal reactions of morphine using tail flick test. Acute intracerebroventricular (icv) administration of morphine (2-20 microg/rat) exhibited antinociceptive activity, which was antagonized by subeffective dose of nonselective MC R agonist NDP-MSH (0.04 ng/rat, icv), and potentiated by subeffective dose of MC4 R antagonist HS014 (0.008 ng/rat, icv). Isobolographic analysis revealed antagonistic interaction between NDP-MSH and morphine, and additive interaction between HS014 and morphine combinations. While chronic icv infusion of morphine (20 ng/microl/h) via osmotic pump for 7 days developed tolerance to its antinociceptive effect, its discontinuation produced hyperalgesia. Co-administration of HS014 (0.008 ng/rat, icv) with chronic morphine not only delayed the development of tolerance but also prevented withdrawal hyperalgesia. Furthermore, acute treatment with HS014 (0.008 and 0.04 ng/rat, icv) dose dependently attenuated the withdrawal hyperalgesia. This suggests the involvement of central MC4 R in the mechanism of development of tolerance and dependence following chronic morphine administration. We speculate that targeting this receptor may be a novel strategy to improve the effectiveness of morphine in the treatment of chronic pain.

Identification of arginine analogues as antagonists and agonists for the melanocortin-4 receptor

In the present study, conducted to explore potent and small molecular melanocortin-4 (MC4) receptor ligands, we found that tripeptide 3a, containing a D-Phe-Arg-2-Nal (Nal; naphthylalanine) sequence, exhibited a moderate affinity for the MC4 receptor. Structural optimization led to the identification of a compound with a high affinity for the MC4 receptor, namely, tripeptide 3e, which showed a 70-fold higher affinity for the MC4 receptor than the lead compound 3a. Moreover, in an effort to further reduce the peptidic characters of tripeptide 3e, we found that dipeptide 3g exhibited a relatively high affinity for the MC4 receptor. Furthermore, in these analogues, the substituted position (1' vs. 2') of the naphthyl ring of Nal residue at position 7 was found to be important for the differentiation of agonist and antagonist activity. The synthesis and structure-activity relationships of the arginine analogues as MC4 receptor ligands were described in this paper.

Novel piperazines: Potent melanocortin-4 receptor antagonists with anxiolytic-like activity

In the present study, we found that a novel piperazine compound, 11a, showed a moderate affinity (IC(50)=333nM) for the MC4 receptor. We developed the new type of piperazine compounds and found that mono-piperazine 11b exhibited a high-affinity (IC(50)=40.3nM) for the MC4 receptor. We also found that a series of biphenyl analogues exhibited a high-affinity for the receptor, and in particular, compound 11j exhibited the highest affinity for the MC4 receptor with an IC(50) value of 14.5nM. Furthermore, some of these compounds, when administered orally, significantly reversed the stress-induced anxiety-like behavior in rats. In this paper, we report the synthesis, structure-activity relationships, and oral activity of the novel mono-piperazines as MC4 receptor antagonists.

Synthesis of Diphenylmethyl Analogues and Their Affinity for the Melanocortin-4 Receptor and the Serotonin Transporter

While examining antagonists of the melanocortin-4 receptor (MC4 receptor), we found that compound 12b, containing a diphenylmethyl moiety, had a relatively high affinity for the MC4 receptor. When diphenylmethyl analogues were further examined, compounds 12c and 18 were also found to exhibit a high affinity for the MC4 receptor (IC(50)=46.7 nM and 33.2 nM, respectively). Furthermore, compound 12c was also found to show a high affinity for the serotonin transporter (IC(50)=10.7 nM). Here, we describe the synthesis and biological evaluation of various diphenylmethyl analogues in relation to their actions on the MC4 receptor and the serotonin transporter.

AgRP(83-132) and SHU9119 differently affect activity-based anorexia

Activity-based anorexia (ABA) mimics starvation and hyperactivity of anorexia nervosa patients in rats. Activation of the melanocortin (MC) system leads to hypophagia and increased energy expenditure in ad libitum fed rats. Therefore, activation of the MC system might underlie the development and propagation of ABA. Pro-opiomelanocortin (POMC) gene expression is normally decreased during negative energy balance. Strikingly, we found a transient up-regulation of POMC mRNA levels in the arcuate nucleus during the development of ABA, indicating a hyperactive MC system. However, wheel running and food intake were not influenced by treating ABA rats with the competitive antagonist SHU9119. This suggests that agonism of MC receptors by endogenous alpha-melanocyte-stimulating hormone (alpha-MSH) levels does not underlie ABA. Instead, treatment with the inverse agonist AgRP(83-132) did ameliorate signs of ABA. This implies that modulation of constitutive MC receptor activity rather than antagonizing putative alpha-MSH release contributes to the development and propagation of ABA.

Molecular basis of melanocortin-4 receptor for AGRP inverse agonism

We have investigated receptor structural components of the melanocortin-4 receptor (MC4R) responsible for ligand-dependent inverse agonism. We utilized agouti-related protein (AGRP), an inverse agonist which reduces MC4R basal cAMP production, as a tool to determine the molecular mechanism. We tested a series of chimeric receptors and utilized MC4R and MC1R as templates, in which AGRP is an inverse agonist for MC4R but not for MC1R. Our results indicate that replacements of the extracellular loops 1, 2 and 3 of MC4R with the corresponding regions of MC1R did not affect AGRP inverse agonist activity. However, replacement of the N terminus of MC4R with the same region of MC1R decreases AGRP inverse agonism. Replacement of transmembrane domains 3, 4, 5 and 6 of MC4R with the corresponding regions of MC1R did not affect AGRP inverse agonist activity but mutation of D90A in transmembrane 2 (TM2) and D298A in TM7 abolished AGRP inverse activity. Deletion of the distal MC4R C terminus fails to maintain AGRP mediated reduction in basal cAMP production although it maintains NDP-MSH mediated cAMP production. In conclusion, our results indicate that the N terminus and the distal C terminus of MC4R do appear to play important roles in AGRP inverse agonism but not NDP-MSH mediated receptor activation. Our results also indicate that the residues D90 in TM2 and D298 in TM7 of hMC4R are involved in not only NDP-MSH mediated receptor activation but also AGRP mediated inverse agonism.

Participation of alpha-melanocyte stimulating hormone in ethanol-induced anxiolysis and withdrawal anxiety in rats

Although recent reports underscore a close association between the ethanol consumption and the central melanocortin (MC) system in rats, neurobehavioral component of this association has not been explored. In this study, we investigated the role of alpha-melanocyte stimulating hormone (alpha-MSH) in ethanol (1.5-2 g/kg, i.p.) induced anxiolysis and anxiety-like behavior following withdrawal from prolonged ethanol (9% v/v ethanol, 15 days) consumption, using elevated plus maze (EPM) test in rats. While alpha-MSH (1-5 microg/rat, i.c.v.) showed dose-dependent anxiogenic-like effect, the MC4 receptor antagonist HS014 (1-10 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:500-1:50 dilution, 5 microl/rat, i.c.v.) failed to produce any effect in the EPM test. The anxiolytic-like effect of ethanol was suppressed by central administration of alpha-MSH (0.5 microg/rat, i.c.v.). On the other hand, pretreatment with either HS014 (5 nM/rat, i.c.v.) or antiserum against alpha-MSH (1:100 dilution, 5 microl/rat, i.c.v.) enhanced anxiolytic action of ethanol. Moreover, ethanol withdrawal anxiety was markedly blocked by HS014 (1-10 nM/rat, i.c.v.). These results suggest that alpha-MSH may be implicated in ethanol-induced anxiolysis and withdrawal anxiety. These findings also suggest MC4 receptors as possible therapeutic target for development of drugs to address the ethanol withdrawal-related conditions.

To eat or not to eat; regulation by the melanocortin system

The central melanocortin (MC) system is one of the best-characterized neuropeptidergic systems involved in the regulation of energy balance. This short review describes the role of the central MC system in feeding behavior. Pharmacological, anatomical and genetic studies show that activation of the MC system reduces meal size, whereas de-activation of the MC system increases meal size. Several brain regions, including distinct hypothalamic nuclei and the hindbrain, are involved in this process. Further dissection of MC pathways in feeding behavior is the subject of recent and probably future studies. As the MC system is involved in animal models of obesity and (possibly) anorexia, it appears that this is a target system for development of drugs for the treatment of disturbed human eating behavior.

MCL0042: a nonpeptidic MC4 receptor antagonist and serotonin reuptake inhibitor with anxiolytic- and antidepressant-like activity

In the present study, we examined the anxiolytic and antidepressant effects of MCL0042, a novel compound showing activity in both MC4 receptor antagonism and serotonin transporter inhibition. MCL0042 showed relatively high affinity for the MC4 receptor and serotonin reuptake site, as determined by receptor binding assays. MCL0042 attenuated [Nle(4),d-Phe(7)]alpha-MSH-increased cAMP formation in MC4 receptor expressing cells, and it inhibited [(3)H]serotonin uptake by rat brain synaptosomes; thus, MCL0042 is an MC4 receptor antagonist and serotonin transporter inhibitor. Subcutaneous administration of MCL0042 significantly increased the number of licks in a Vogel punished drinking test in rats, and it also significantly attenuated swim stress-induced reduction in time spent in open arms in an elevated plus-maze task in rats, showing the anxiolytic-like potential of MCL0042. Moreover, repeated administration of MCL0042 for 14 days attenuated olfactory bulbectomy-induced locomotor hyperactivity in rats, indicating antidepressant-like potential. These data show that MCL0042 has unique properties of both the MC4 receptor antagonist and serotonin transporter inhibitor, and produces anxiolytic and antidepressant activity in rats. Moreover, blockade of both the MC4 receptor and serotonin reuptake sites might represent a useful approach in the treatment of anxiety and depression.

Involvement of melanocortin-4 receptor in anxiety and depression

The melanocortins, which are derived from proopiomelanocortin, have a variety of physiological functions mediated membrane surface receptors. To date, five subtypes have been cloned. With the cloning of melanocortin receptors, studies with genetic models, and development of selective compounds, the physiological roles of the five melanocortin receptors have begun to be understood. The melanocortin-4 receptor (MC4R), which is predominantly expressed in the central nervous system, has in particular become the focus of much attention in recent years because of the critical roles it plays in a wide range of functions, including feeding, sexual behavior, and stress. Recent development of selective antagonists for the MC4R has provided pharmacological evidence that blockade of MC4R could be a useful way of alleviating numerous conditions such as anxiety/depression, pain, and addiction to drugs of abuse.

alpha-MSH enhances activity-based anorexia

Activity-based anorexia (ABA) is considered an animal model of anorexia nervosa (AN). In ABA, scheduled feeding in combination with voluntary access to running wheels, results in hyperactivity, hypophagia, body weight loss and activation of the HPA axis. Since stimulation of the melanocortin (MC) system has similar effects, this system is a candidate system involved in ABA. Here it is shown that chronic alpha-MSH treatment enhances ABA by increasing running wheel activity (RWA), decreasing food intake and increasing HPA axis activation.

Synthesis of a novel potent cyclic peptide MC4-ligand by ring-closing metathesis

The synthesis of a novel potent cyclic peptide MC4-ligand by ring-closing metathesis (RCM) is described. Based on the Ac-Nle-Gly-Lys-D-Phe-Arg-Trp-Gly-NH2-MC4 ligand, Ac-Nle-Alg-Lys-D-Phe-Arg-Trp-Alg-NH2 was designed and synthesized followed by cyclization using RCM. Both compounds are high affinity and selective MC4-R-agonists. The cyclic RCM-peptide was more potent in a rat-grooming assay.

The role of the DRY motif of human MC4R for receptor activation

We constructed several mutant human MC4R cDNAs by site directed mutagenesis and expressed these receptors in COS-1 cells. The conserved DRY motif among GPCRs was mutated to generate eight mutants. While no MC4R ligand binding was detected in any of the mutants, one mutant, D146A, resulted in higher cAMP production in cells than the wild-type receptor without ligand stimulation.

Anxiolytic-like effect of a selective and non-peptidergic melanocortin 4 receptor antagonist, MCL0129, in a social interaction test

The social interaction test is an animal behavioral test of anxiety. Brain melanocortins such as alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropic hormone (ACTH) have anxiogenic effects in this test. Melanocortins have five receptor subtypes (MC1-MC5). Among them, MC3 and MC4 receptor are mainly expressed in the brain. We investigated the involvement of MC4 receptor in a social interaction test, using Ac-[Nle(4),Asp(5),D-Phe(7),Lys(10)]alpha-MSH-(4-10)-NH2 (MT II), an MC4 receptor agonist, and 1-[(S)-2-(4-fluorophenyl)-2-(4-isopropylpiperadin-1-yl)ethyl]-4-[4-(2-methoxynaphthalen-1-yl)butyl]piperazine (MCL0129), a selective and nonpeptide MC4 receptor antagonist. MT II dose-dependently and significantly reduced the time spent in social interaction. Acute administration of MCL0129 had no effect on the results of this test. In contrast, when given repeatedly for 1 week, MCL0129 significantly and dose-dependently increased the time spent in social interaction without affecting locomotor activity. These results suggest that MC4 receptor is involved in social interaction, and that MCL0129, an MC4 receptor antagonist, has an anxiolytic-like effect in this model.

Function of the hypothalamo-hypophyseal-adrenal system in mice with ectopic hyperproduction of the agouti protein

The agouti protein is known to compete with the melanocortin hormones (ACTH, melanocyte-stimulating hormone) at melanocortin receptors, which in turn are involved in controlling the central and peripheral components of the hypothalamo-hypophyseal-adrenal system. The aim of the present work was therefore to assess the effects of the dominant mutation Agouti yellow (A(y)/a), which induces ectopic hyperproduction of the agouti protein and yellow coat color, on the function of the hypothalamo-hypophyseal-adrenal system. Experiments were performed on male A(y)/a mice of the C57BI/6J line. Controls consisted of mice of the same line bearing the recessive mutation nonagouti (a/a), which leads to the absence of agouti protein and black coat color. The experimental results showed that mice with different agouti genotypes had identical basal corticosterone levels, though yellow mice, as compared with black mice, had increased corticosterone levels after restriction stress (p < 0.02), along with decreased stress reactivity after treatment with dexamethasone (p < 0.0007), and increased adrenal sensitivity to small doses of activity, both in vitro and in vivo.

Regulation of CRF, POMC and MC4R gene expression after electrical foot shock stress in the rat amygdala and hypothalamus

We investigated the effects of electrical foot shock stress on the melanocortin signaling cascade and the hypothalamus-pituitary-adrenal (HPA) system by observing levels of mRNA expression of corticotropin releasing factor (CRF), pro-opiomelanocortin (POMC), and melanocortin receptor subtype 4 (MC4R) in the rat amygdala and hypothalamus. When rats were exposed to electrical shock for 0.5 hr or 1 hr, plasma ACTH and corticosterone concentrations increased, indicating stress. The rats were then sacrificed to obtain RNA preparations from the brain tissue. In the amygdala, the expression of MC4R and POMC mRNA as well as CRF mRNA was significantly increased by electrical foot shock stress. In the hypothalamus, MC4R and POMC mRNA increased, but CRF mRNA remained unchanged. The duration of increased gene expression of MC4R and POMC in the amygdala was more sustained than in the hypothalamus. These results have provided the first evidence that exposure to stress increases expression of the MC4R system in the amygdala and hypothalamus.

α-MSH and γ-MSH inhibit IL-1β induced activation of the hypothalamic–pituitary–adrenal axis through central melanocortin receptors

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuroimmunomodulatory peptide that is involved in the control of host responses trough modulation of production and action of proinflammatory cytokines in inflammatory cells in the periphery and within the central nervous system (CNS). However, little is known about the receptors that mediate the modulatory effects of alpha-MSH in the CNS. The objective of the present study was to establish the specific melanocortin receptors involved in the inhibition by MSH peptides of IL-1beta-induced activation of the HPA. i.c.v. injection of 12.5 ng of IL-1beta caused significant changes in plasma corticosterone, as compared to basal levels. The treatment with gamma-MSH (1 microg), an MC3 receptor agonist, resulted in significant reduction of the IL-1beta-induced plasma corticosterone levels. Administration of the MC3/MC4 receptor antagonist SHU9119 blocked this effect. Besides, treatment with a high dose of alpha-MSH (1 microg) increased plasma corticosterone. When alpha-MSH was given at a lower dose (0.1 microg), it did not modify corticosterone levels but caused an inhibitory effect on the corticosterone release induced by IL-1beta. The administration of SHU9119 or a more selective MC4 receptor antagonist like HS014 blocked the effects of alpha-MSH. In conclusion, our results suggest that both alpha-MSH and gamma-MSH are capable of inhibiting the effect of the IL-1beta on the activation of HPA axis acting at the CNS, and that this effect is mediated by specific central melanocortin receptors.

Electrophysiological effects of melanocortin receptor ligands on neuronal activities of monoaminergic neurons in rats

The melanocortin-4 (MC4) receptor may possibly be involved in stress and stress-related behavior. In the present study, we examined effects of an intracerebroventricular injection of the MC4 receptor agonist, Ac-[Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH 4-10-NH2 (MT II), and the MC4 receptor inverse agonist, Agouti-related protein (AGRP), on dorsal raphe nucleus (DRN) serotonergic neurons and the locus coeruleus (LC) noradrenergic neurons, both of which are brain neuronal systems related to responses to stress. The firing rate of DRN serotonergic neurons was increased by MTII, while AGRP had a lack of effect on the firing rate of DRN serotonergic neurons. In comparison with the DRN, MTII significantly reduced the firing rate of LC noradrenergic neurons, while AGRP increased LC neuronal activity. These findings suggest that MC4 receptor ligands differently regulate serotonergic and noradrenergic neuronal systems. The MC4 receptor mediating multiple regulation on the monoaminergic neuronal system may, in part, relate to stress responses (anxiety and/or depressive behavior).

Interaction between alpha-melanocyte-stimulating hormone and corticotropin-releasing hormone in the regulation of feeding and hypothalamo-pituitary-adrenal responses

Both central alpha-melanocyte-stimulating hormone and corticotropin-releasing hormone (CRH) have been implicated in feeding and neuroendocrine mechanisms. The anatomical overlap and functional similarities between these two neurotransmitter systems led to the hypothesis that CRH might act as one of the mediators of the central actions of the melanocortin system. By double-labeling in situ hybridization, a subpopulation of CRH neurons in the paraventricular nucleus of the hypothalamus (PVN) were shown to contain the melanocortin-4 receptor (MC4R), concentrated in the ventromedial part of the parvicellular PVN (up to 33%). Intracerebroventricular injection of melanocortin agonist MTII to conscious and freely moving rats induced a rapid induction of CRH gene transcription in the PVN. This effect was accompanied by a rise in plasma corticosterone levels in a dose- and time-dependent manner, with the maximum response observed 30 min after MTII injection. MTII (0.5 nmol)-induced increase in plasma corticosterone was attenuated by the selective MC4R antagonist HS014 (0.25-1.0 nmol) and nonselective CRH receptor antagonist alpha-helical-CRH9-41 (0.125-0.5 nmol) in a dose-dependent manner. Moreover, the anorectic effect of MTII was evaluated at 1, 2, and 24 hr after intracerebroventricular injection. Approximately half of the inhibitory effect of MTII (0.5 nmol) on food intake was reversed by pretreatment with alpha-helical-CRH9-41 at 0.25 and 0.5 nmol doses. Collectively, these results provide evidence that CRH acts as a downstream mediator of melanocortin signaling and contributes to the mechanisms by which the central melanocortin system controls feeding and neuroendocrine responses.

Interaction between alpha-melanocyte-stimulating hormone and corticotropin-releasing hormone in the regulation of feeding and hypothalamo-pituitary-adrenal responses

Both central alpha-melanocyte-stimulating hormone and corticotropin-releasing hormone (CRH) have been implicated in feeding and neuroendocrine mechanisms. The anatomical overlap and functional similarities between these two neurotransmitter systems led to the hypothesis that CRH might act as one of the mediators of the central actions of the melanocortin system. By double-labeling in situ hybridization, a subpopulation of CRH neurons in the paraventricular nucleus of the hypothalamus (PVN) were shown to contain the melanocortin-4 receptor (MC4R), concentrated in the ventromedial part of the parvicellular PVN (up to 33%). Intracerebroventricular injection of melanocortin agonist MTII to conscious and freely moving rats induced a rapid induction of CRH gene transcription in the PVN. This effect was accompanied by a rise in plasma corticosterone levels in a dose- and time-dependent manner, with the maximum response observed 30 min after MTII injection. MTII (0.5 nmol)-induced increase in plasma corticosterone was attenuated by the selective MC4R antagonist HS014 (0.25-1.0 nmol) and nonselective CRH receptor antagonist alpha-helical-CRH9-41 (0.125-0.5 nmol) in a dose-dependent manner. Moreover, the anorectic effect of MTII was evaluated at 1, 2, and 24 hr after intracerebroventricular injection. Approximately half of the inhibitory effect of MTII (0.5 nmol) on food intake was reversed by pretreatment with alpha-helical-CRH9-41 at 0.25 and 0.5 nmol doses. Collectively, these results provide evidence that CRH acts as a downstream mediator of melanocortin signaling and contributes to the mechanisms by which the central melanocortin system controls feeding and neuroendocrine responses.

Involvement of the melanocortin MC4 receptor in stress-related behavior in rodents

The melanocortin subtype 4 (MC4) receptor has been postulated to be involved in stress and stress-related behavior. We made use of melanocortin MC4 receptor agonists and antagonist to investigate the relationship between the melanocortin MC4 receptor and stress related disorders. The nonspecific melanocortin receptor agonist alpha-melanocyte stimulating hormone (alpha-MSH) and the melanocortin MC4 receptor agonist, Ac-[Nle4,Asp5,D-Phe7,Lys10]alpha-MSH-(4-10)-NH2 (MT II) dose-dependently and significantly reduced the number of licking periods in the rat Vogel conflict test, suggesting that stimulation of the melanocortin MC4 receptor causes anxiogenic-like activity in rats. We synthesized a peptidemimetic melanocortin MC4 receptor selective antagonist, Ac-D-2Nal-Arg-2Nal-NH2 (MCL0020), which has high affinity for the melanocortin MC4 receptor with IC50 values of 11.63 +/- 1.48 nM, in contrast, the affinities for melanocortin MC1 and MC3 receptors were negligible. In addition, MCL0020 significantly attenuated the cAMP formation induced by alpha-MSH in COS-1 cells expressing the melanocortin MC4 receptor without affecting basal cAMP contents. Thus, we considered MCL0020 to be a selective melanocrotin MC4 receptor antagonist among melanocortin receptors. Restraint stress significantly reduced food intake in rats, and i.c.v. administration of MCL0020 dose-dependently and significantly attenuated restraint stress-induced anorexia without affecting food intake. Swim stress induced reduction in the time spent in the light area in the mouse light/dark exploration test, and MCL0020 significantly prevented it. Taken together our findings suggest that the melanocortin MC4 receptor might be related to stress-induced changes in behavior, and blockade of the melanocortin MC4 receptor may prevent stress-induced disorders such as anxiety.

Melanocortin system and eating disorders

The melanocortin (MC) system is involved in the regulation of energy balance and in the development of obesity. Here we briefly review why we became interested in investigating whether the MC system - more particularly, the increased activity of the MC system - is also involved in disorders of negative energy balance. We provide evidence that suppression of increased MC receptor activity by treatment with the inverse agonist agouti-related peptide (AgRP) (83-132) rescues rats exposed to an animal model known as activity-based anorexia. Furthermore, we found a polymorphism, Ala67Thr AgRP, that was observed more frequently in anorexia nervosa.

Agouti-related protein prevents self-starvation

Food restriction leads to a paradoxical increase in physical activity and further suppression of food intake, such as observed in anorexia nervosa.(1,2) To understand this pathophysiological process, we induced physical hyperactivity and self-starvation in rats by restricting food in the presence of running wheels. Normally, decreased melanocortin receptor activity will prevent starvation.(3,4) However, we found that self-starvation increased melanocortin receptors in the ventral medial hypothalamus, a brain region involved in eating behavior.(5) Suppression of melanocortin receptor activity, via central infusion of Agouti-related protein (AgRP), increased survival rate in these rats by counteracting physical hyperactivity, food intake suppression as well as deregulated body temperature. We conclude that self-starvation may result from insufficient suppression of central melanocortin receptor activity.

Anxiolytic-like and antidepressant-like activities of MCL0129 (1-[(S)-2-(4-fluorophenyl)-2-(4-isopropylpiperadin-1-yl)ethyl]-4-[4-(2-methoxynaphthalen-1-yl)butyl]piperazine), a novel and potent nonpeptide antagonist of the melanocortin-4 receptor

We investigated the effects of a novel melanocortin-4 (MC4) receptor antagonist,1-[(S)-2-(4-fluorophenyl)-2-(4-isopropylpiperadin-1-yl)ethyl]-4-[4-(2-methoxynaphthalen-1-yl)butyl]piperazine (MCL0129) on anxiety and depression in various rodent models. MCL0129 inhibited [(125)I][Nle(4)-D-Phe(7)]-alpha-melanocyte-stimulating hormone (alpha-MSH) binding to MC4 receptor with a K(i) value of 7.9 nM, without showing affinity for MC1 and MC3 receptors. MCL0129 at 1 microM had no apparent affinity for other receptors, transporters, and ion channels related to anxiety and depression except for a moderate affinity for the sigma(1) receptor, serotonin transporter, and alpha(1)-adrenoceptor, which means that MCL0129 is selective for the MC4 receptor. MCL0129 attenuated the alpha-MSH-increased cAMP formation in COS-1 cells expressing the MC4 receptor, whereas MCL0129 did not affect basal cAMP levels, thereby indicating that MCL0129 acts as an antagonist at the MC4 receptor. Swim stress markedly induced anxiogenic-like effects in both the light/dark exploration task in mice and the elevated plus-maze task in rats, and MCL0129 reversed the stress-induced anxiogenic-like effects. Under nonstress conditions, MCL0129 prolonged time spent in the light area in the light/dark exploration task and suppressed marble-burying behavior. MCL0129 shortened immobility time in the forced swim test and reduced the number of escape failures in inescapable shocks in the learned helplessness test, thus indicating an antidepressant potential. In contrast, MCL0129 had negligible effects on spontaneous locomotor activity, Rotarod performance, and hexobarbital-induced anesthesia. These observations indicate that MCL0129 is a potent and selective MC4 antagonist with anxiolytic- and antidepressant-like activities in various rodent models. MC4 receptor antagonists may prove effective for treating subjects with stress-related disorders such as depression and/or anxiety.

Molecular and behavioral interactions between central melanocortins and cocaine

Behavioral and molecular studies have established a link between drugs of abuse and the central melanocortin system, particularly the melanocortin 4 receptor (MC4-R). The present study expands this line of investigation to characterization of the neurochemical and behavioral interactions between MC4-R and the psychomotor stimulant, cocaine. The results demonstrate that repeated, but not acute, cocaine administration up-regulates MC4-R mRNA expression in the striatum and hippocampus, but not cerebral cortex. Pharmacological studies indicate that the up-regulation of MC4-R expression occurs via dopamine D1 and D2 receptor-dependent mechanisms. The D1/D2 antagonist haloperidol and the D2-selective antagonist eticlopride mimic the effect of cocaine on MC4-R expression. In addition, coadministration of a D1-selective antagonist, SCH 23390 [R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine], completely blocks the up-regulation of MC4 mRNA by cocaine, demonstrating that D1 receptor activation is necessary for this response. Moreover, the results demonstrate that cocaine treatment increases behavioral responses (grooming and locomotor activity) to infusions of a melanocortin agonist, indicating that up-regulation of MC4-R expression results in functional consequences. These data further support a role for the melanocortin-MC4-R neuropeptide system in the biochemical and behavioral effects of cocaine.

Effects of melanocortins on cardiovascular regulation in rats

1. Of the melanocortin peptides, gamma(2)-melanocyte-stimulating hormone (MSH) has been attributed a cardiovascular effect, inducing an increase in blood pressure and heart rate. Although still controversial, this effect, based on pharmacological blockade experiments, is supposed to be mediated through sympathetic activation. 2. The aims of the present study were to identify the N-terminal pro-opiomelanocortin (N-POMC) fragments and melanocortins that influence blood pressure and heart rate and to investigate the real-time changes in baroreflex sensitivity and in sympathetic and vagal modulation underlying cardiovascular effects in conscious rats without the use of pharmacological blockade. 3. Intracerebroventricular administration of different melanocortins and N-POMC induced a long-lasting dose- dependent pressor response from 1 nmol onwards, with only a small initial bradycardic response with the highest dose. 4. Coinciding with this pressor response, an elicitation of the low-frequency (LF) component was observed in spectral analysis of both blood pressure variability (BPV) and heart rate variability (HRV), followed by the high-frequency (HF) component in at least BPV. Baroreflex sensitivity remained unchanged. 5. After intravenous administration, gamma(2)-MSH produced a short-lasting dose-dependent pressor and cardioaccelerator response with very rapid onset with concentrations from 1 nmol onwards. 6. Continuous infusion of gamma(2)-MSH depressed baroreflex sensitivity and simultaneously increased both components of BPV, with a radical reduction of the LF component and a preserved vagal HF component in HRV. 7. Of all the intravenously administered melanocortins, only gamma(2)-MSH was active. The central effect is likely to depend on an increase of (alpha-)sympathetic outflow. 8. For the peripheral effect, gamma(2)-MSH appeared to act as a baroreceptor reflex-blocking agent, being compatible with a role in the acute stress response.

Effects of MCH/NEI in the striatum and interactions between them and with alpha-MSH on IP3 levels

In this study, we investigated: (a) the effect of melanocyte concentrating hormone (MCH) and neuropeptide glutamine (E)-isoleusine-(I) (NEI) on IP(3) production on an "in vitro" model using slices containing caudate putamen (CP) and accumbens nuclei (ACC); and (b) the interaction between these peptides and with alpha-melanocyte-stimulating-hormone (alpha-MSH) on the production of this second messenger. Only MCH at the highest dose studied (3.6 microM) increased the production of IP(3), whereas at the low concentration (0.6 microM) it did not affect IP(3) levels. NEI and alpha-MSH at both concentrations tested (0.6 and 3.6 microM), did not affect IP(3) production either. However, when NEI or alpha-MSH (at 3.6 microM) were added together with 3.6 microM MCH, the increase in the IP(3) content induced by this last peptide was blocked.

Drug target discovery by pharmacogenetics: mutations in the melanocortin system and eating disorders

The identification of the genetic defect underlying the obese phenotype of the viable yellow mouse, ectopic overexpression of the agouti protein which acts as antagonist at the melanocortin-4 receptor, together with the demonstration that the brain melanocortin system was one major downstream effector pathway of leptin signaling has put forward melanocortin receptors as drug targets for obesity. The lack of compounds acting as melanocortin receptor antagonists was the reason why pharmacological studies had not recognized melanocortin receptors as important drug targets earlier. Blockade of brain melanocortin receptors results in increased food intake and body weight, whereas stimulation of the brain melanocortin system results in decreased food intake and activation of the hypothalamo-pituitary-adrenal axis. Anorexia nervosa is characterized by decreased body weight and food intake accompanied by changes in neuroendocrine systems such as strong activation of the hypothalamo-pituitary-adrenal axis. Since agouti-related protein suppresses the activity of the melanocortin system, the AgRP gene was investigated as candidate gene in anorexia nervosa. One variant of the AgRP gene was associated with anorexia nervosa, thus putting forward melanocortin receptor blockade as putative pharmacotherapy. Investigating variations in candidate genes in disease populations appears to be a fruitful approach towards the identification of drug targets.

Association between an agouti-related protein gene polymorphism and anorexia nervosa

Anorexia nervosa (AN) is a life threatening disorder affecting mostly adolescent women. It is a dramatic psychiatric syndrome accompanied by severe weight loss, hyperactivity and neuroendocrine changes (reviewed in Refs 1 and 2). Several studies have shown a strong genetic component in AN (reviewed in Ref 3). Recent advances in unraveling the mechanisms of weight control point to a crucial role of the melanocortin-4 receptor (MC4-r) system in regulating body weight. The orexigenic neuropeptide agouti-related protein (AGRP), a MC4-r antagonist, plays a crucial role in maintaining body weight, by inducing food intake. The sequence of the coding region of the human AGRP gene (AGRP) was determined and the AGRP of 100 patients with AN was screened for variations. Three single nucleotide polymorphisms (SNPs) were identified and screened in a further 45 patients and 244 controls. Two alleles were in complete linkage disequilibrium and were significantly enriched in anorectic patients (11%; P = 0.015) compared to controls (4.5%). These data indicate that variations of AGRP are associated with susceptibility for AN. This is possibly caused by defective suppression of the MC4-r by the variant AGRP, leading to a decreased feeding signal, increasing the risk of developing AN. These results implicate that antagonism of the MC4-r might be considered as pharmacotherapy for patients with AN.

Further evidence for a significant participation of the melanocortin 4 receptor in the preovulatory prolactin surge in the rat

We previously reported that the melanocortin 4 receptor may play a significant role in mediating the preovulatory surges of luteinizing hormone and prolactin in the rat. In order to confirm this previous finding, in the present study we examined and compared the effects of intracerebroventricular administrations of 1.0 nmol of MT II (a non-selective melanocortin 3 and 4 receptor agonist) and 10 nmol of gamma(1)-melanocyte-stimulating hormone (a selective melanocortin 3 receptor agonist) on luteinizing hormone and prolactin surges in starved, gonadal steroid-primed ovariectomized female rats, which is a model deprived of inherent surges of the two hormones. MT II significantly recovered the surge of prolactin, but not of luteinizing hormone (although a tendency to increase was seen), and gamma(1)-melanocyte-stimulating hormone was without effect on both hormones. This study corroborated our previous report through a different and direct approach that the melanocortin 4 receptor, but not the melanocortin 3 receptor, plays a significant role in mediating the preovulatory prolactin surge in the rat.

Effects of chronic central nervous system administration of agouti-related protein in pair-fed animals

The melanocortin receptor (MC3-R and MC4-R) antagonist, agouti-related protein (AGRP), is a potent stimulant of food intake. We examined the effect of chronic intracerebroventricular (ICV) AGRP treatment on energy metabolism and pituitary function in ad libitum fed rats and rats administered AGRP and then pair-fed to a saline control group. Chronic ICV AGRP (83-132) administration (1 nmol/day for 7 days) significantly increased food intake and body weight in ad libitum fed animals compared with saline-treated controls (body weight on day 7: 272 +/- 6 [saline] vs. 319 +/- 8 g [AGRP ad libitum fed]; P < 0.001). A significant increase in the epididymal fat pad weight, interscapular brown adipose tissue (BAT) weight, and plasma leptin was also observed in the ad libitum fed group. In the AGRP pair-fed group, a significant increase in the epididymal fat pad weight, BAT weight, and plasma leptin was again observed, suggesting that AGRP caused metabolic changes independent of increased food intake. BAT uncoupling protein 1 (UCP-1) content was significantly decreased compared with saline controls in both the AGRP ad libitum fed (21 +/- 8% of saline control; P < 0.01) and AGRP pair-fed groups (24 +/- 7% of saline control; P < 0.01). Plasma thyroid-stimulating hormone (TSH) was significantly suppressed compared with saline controls in both the AGRP ad libitum fed and AGRP pair-fed groups (3.5 +/- 0.3 [saline] vs. 2.7 +/- 0.4 [AGRP ad libitum fed] vs. 2.1 +/- 0.2 ng/ml [AGRP pair-fed]; P < 0.01). This study demonstrates that independent of its orexigenic effects, chronic AGRP treatment decreased BAT UCP-1, suppressed plasma TSH, and increased fat mass and plasma leptin, suggesting that it may play a role in energy expenditure.

New aspects on the melanocortins and their receptors

Knowledge of melanocortins and their receptors has increased tremendously over the last few years. The cloning of five melanocortin receptors, and the discovery of two endogenous antagonists for these receptors, agouti and agouti-related peptide, have sparked intense interest in the field. Here we give a comprehensive review of the pharmacology, physiology and molecular biology of the melanocortins and their receptors. In particular, we review the roles of the melanocortins in the immune system, behaviour, feeding, the cardiovascular system and melanoma. Moreover, evidence is discussed suggesting that while many of the actions of the melanocortins are mediated via melanocortin receptors, some appear to be mediated via mechanisms distinct from melanocortin receptors.