Regulation of melanocortin-4-receptor (MC4R) expression by SNP rs17066842 is dependent on glucose concentration

Melanocortin-4-receptor (MC4R) gene codes for a G-protein-coupled receptor that is highly expressed in the hypothalamus and involved in the regulation of appetite. Single-nucleotide polymorphisms (SNPs) in the MC4R gene region have been associated with obesity, type 2-diabetes (T2D) and with antipsychotic-induced weight gain. Of these, rs17066842 (G>A) in the MC4R promoter region is the top variant associated with obesity and diabetes. In this study, we investigated the effect of rs17066842 on MC4R expression at various glucose concentrations using reporter gene expression in the SH-SY5Y cell line and regulation of MC4R expression in human cerebral organoids. We observed that higher glucose concentrations significantly reduced MC4R mRNA expression in SH-SY5Y cells. In addition, at high glucose concentrations, the luciferase reporter plasmid containing the MC4R promoter insert with the G-allele of rs170066842 showed significantly reduced activity compared to the A-allele carrying plasmid. The immediate early gene product, early growth-response 1 (EGR-1), was identified to bind to the sequence containing the G-allele at rs17066842 but not to the A-allele-containing sequence. Interestingly, in human induced pluripotent stem cell (hiPSC)-derived cerebral organoids, we observed increased MC4R expression in response to high glucose exposure. These opposite observations might suggest that glucose regulation is complex and may be cell-specific. This study provides evidence that rs17066842 regulates MC4R gene expression through binding of EGR-1 and that this process is influenced by glucose concentration.

Ultraviolet B stimulates proopiomelanocortin signalling in the arcuate nucleus of the hypothalamus in mice

We previously found that ultraviolet B (UVB) could stimulate the paraventricular nucleus (PVN) with activation the systemic hypothalamic-pituitary- adrenal (HPA) axis. To investigate whether UVB can also stimulate other hypothalamic nuclei, we tested its effect on the proopiomelanocortin (POMC) related signalling system in the arcuate nucleus (ARC) of female C57BL/6 and FVB albino mice. The shaved back skin of the mice was irradiated with either 100 or 400 mJ/cm2 of UVB. After 1, 3, 6 and 12 h, blood and hypothalamus were collected and processed for gene and protein expression, and measurement of α-MSH and β-endorphin (β-END) levels. An in situ immunohistochemical examination was performed for melanocortin receptor 4 (MC4R) and POMC-derived α-MSH. The expression of Pomc and MC4R mRNAs was stimulated, whereas that of AgRP was inhibited after exposure to UVB. It was accompanied by an increased number of both α-MSH- and MC4R-immunoreactive neurons in the ARC, and by increased levels of α-MSH and β-END (both found in the hypothalamus and plasma). This surprising discovery of UVB stimulating the POMC system in the ARC, accompanied by the increased plasma levels of α-MSH and β-END, paves the way for exciting areas of research on the communication between the skin and the brain, as well as is suggesting a new role for UVB in regulation of body metabolism.

Maternal and post-weaning high-fat, high-sucrose diet modulates glucose homeostasis and hypothalamic POMC promoter methylation in mouse offspring

Substantial evidence demonstrated that maternal dietary nutrients can significantly determine the susceptibility to developing metabolic disorders in the offspring. Therefore, we aimed to investigate the later-life effects of maternal and postweaning diets interaction on epigenetic modification of the central nervous system in the offspring. We examined the effects of dams fed a high-fat, high-sucrose (FS) diet during pregnancy and lactation and weaned to FS diet continuously until 32 weeks of age. Then, DNA methylation and gene expressions of hypothalamic proopiomelanocortin (POMC) and melanocortin receptor 4 (MC4R) were determined in the offspring. Offspring of FS diet had heavier body weight, impaired glucose tolerance, decreased insulin sensitivity and higher serum leptin level at 32-week age (p < 0.05). The expression of POMC and MC4R genes were significantly increased in offspring exposed to FS diet during gestation, lactation and into 32-week age (p < 0.05). Consistently, hypomethylation of POMC promoter in the hypothalamus occurred in the FS diet offspring (p < 0.05), compared with the C group. However, no methylation was detected of MC4R promoter in both the two groups. Furthermore, POMC-specific methylation (%) was negatively associated with glucose response to a glucose load (r = -0.273, p = 0.039). Maternal and post-weaning high-fat diet predisposes the offspring for obesity, glucose intolerance and insulin resistance in later life. Our findings can advance our thinking around the DNA methylation status of the promoter of the POMC and MC4R genes between long-term high-fat, high-sucrose diet and glucose homeostasis in mouse.

Inhibiting neuropeptide Y Y1 receptor modulates melanocortin receptor- and NF-κB-mediated feeding behavior in phenylpropanolamine-treated rats

Neuropeptide Y (NPY) and nuclear factor-kappa B (NF-κB) are involved in regulating anorexia elicited by phenylpropanolamine (PPA), a sympathomimetic drug. This study explored whether NPY Y1 receptor (Y1R) is involved in this process, and a potential role for the proopiomelanocortin system was identified. Rats were given PPA once a day for 4days. Changes in the hypothalamic expression of the NPY, Y1R, NF-κB, and melanocortin receptor 4 (MC4R) levels were assessed and compared. The results indicated that food intake and NPY expression decreased, with the largest reductions observed on Day 2 (approximately 50% and 45%, respectively), whereas NF-κB, MC4R, and Y1R increased, achieving maximums on Day 2 (160%, 200%, and 280%, respectively). To determine the role of Y1R, rats were pretreated with Y1R antisense or a Y1R antagonist via intracerebroventricular injection 1h before the daily PPA dose. Y1R knockdown and inhibition reduced PPA anorexia and partially restored the normal expression of NPY, MC4R, and NF-κB. The data suggest that hypothalamic Y1R participates in the appetite-suppression from PPA by regulating MC4R and NF-κB. The results of this study increase our understanding of the molecular mechanisms in PPA-induced anorexia.

Leptin resistance and desensitization of hypophagia during prolonged inflammatory challenge

Acute exposure to bacterial lipopolysaccharide (LPS) is a potent inducer of immune response as well as hypophagia. Nevertheless, desensitization of responses to LPS occurs during long-term exposure to endotoxin. We induced endotoxin tolerance, injecting repeated (6LPS) LPS doses compared with single (1LPS) treatment. 1LPS, but not 6LPS group, showed decreased food intake and body weight, which was associated with an increased plasma leptin and higher mRNA expression of OB-Rb, MC4R, and SOCS3 in the hypothalamus. Hypophagia induced by 1LPS was associated with lower levels of 2-arachidonoylglycerol (2-AG), increased number of p-STAT3 neurons, and decreased AMP-activated protein kinase (AMPK) activity. Desensitization of hypophagia in the 6LPS group was related to high 2-AG, with no changes in p-STAT3 or increased p-AMPK. Leptin decreased food intake, body weight, 2-AG levels, and AMPK activity and enhanced p-STAT3 in control rats. However, leptin had no effects on 2-AG, p-STAT3, or p-AMPK in the 1LPS and 6LPS groups. Rats treated with HFD to induce leptin resistance showed neither hypophagia nor changes in p-STAT3 after 1LPS, suggesting that leptin and LPS recruit a common signaling pathway in the hypothalamus to modulate food intake reduction. Desensitization of hypophagia in response to repeated exposure to endotoxin is related to an inability of leptin to inhibit AMPK phosphorylation and 2-AG production and activate STAT3. SOCS3 is unlikely to underlie this resistance to leptin signaling in the endotoxin tolerance. The present model of prolonged inflammatory challenge may contribute to further investigations on mechanisms of leptin resistance.

Postnatal Sim1 deficiency causes hyperphagic obesity and reduced Mc4r and oxytocin expression

Single-minded 1 (SIM1) mutations are one of the few known causes of nonsyndromic monogenic obesity in both humans and mice. Although the role of Sim1 in the formation of the hypothalamus has been described, its postdevelopmental, physiological functions have not been well established. Here we demonstrate that postnatal CNS deficiency of Sim1 is sufficient to cause hyperphagic obesity. We conditionally deleted Sim1 after birth using CaMKII-Cre (alpha-calcium/calmodulin-dependent protein kinase II-Cre) lines to recombine a floxed Sim1 allele. Conditional Sim1 heterozygotes phenocopied germ line Sim1 heterozygotes, displaying hyperphagic obesity and increased length. We also generated viable conditional Sim1 homozygotes, demonstrating that adult Sim1 expression is not essential for mouse or neuron survival and revealing a dosage-dependent effect of Sim1 on obesity. Using stereological cell counting, we showed that the phenotype of both germ line heterozygotes and conditional Sim1 homozygotes was not attributable to global hypocellularity of the paraventricular nucleus (PVN) of the hypothalamus. We also used retrograde tract tracing to demonstrate that the PVN of germ line heterozygous mice projects normally to the dorsal vagal complex and the median eminence. Finally, we showed that conditional Sim1 homozygotes and germ line Sim1 heterozygotes exhibit a remarkable decrease in hypothalamic oxytocin (Oxt) and PVN melanocortin 4 receptor (Mc4r) mRNA. These results demonstrate that the role of Sim1 in feeding regulation is not limited to formation of the PVN or its projections and that the hyperphagic obesity in Sim1-deficient mice may be attributable to changes in the leptin-melanocortin-oxytocin pathway.

Melanocortin-4 receptor activation inhibits c-Jun N-terminal kinase activity and promotes insulin signaling

The melanocortin system is crucial to regulation of energy homeostasis. The melanocortin receptor type 4 (MC4R) modulates insulin signaling via effects on c-Jun N-terminal kinase (JNK). The melanocortin agonist NDP-MSH dose-dependently inhibited JNK activity in HEK293 cells stably expressing the human MC4R; effects were reversed by melanocortin receptor antagonist. NDP-MSH time- and dose-dependently inhibited IRS-1(ser307) phosphorylation, effects also reversed by a specific melanocortin receptor antagonist. NDP-MSH augmented insulin-stimulated AKT phosphorylation in vitro. The melanocortin agonist melanotan II increased insulin-stimulated AKT phosphorylation in the rat hypothalamus in vivo. NDP-MSH increased insulin-stimulated glucose uptake in hypothalamic GT1-1 cells. The current study shows that the melanocortinergic system interacts with insulin signaling via novel effects on JNK activity.

Downregulation of melanocortin-4 receptor during refeeding and its modulation by adrenalectomy in rats

Melanocortin system and corticotropin releasing hormone (CRH) are implicated in the control of feeding behavior. Besides its anorexigenic effect on food intake, CRH is one of the most important regulators of hypothalamic-pituitary-adrenal (HPA) axis activity. Therefore, there could be an interplay between HPA axis activity and melanocortin system. We investigated the expression of melanocortin-4 receptor (MC4-R) mRNA in the hypothalamus of rats after 14 days of food restriction or after a fasting-refeeding regimen, in sham or adrenalectomized rats. Male Wistar rats were subjected to free access to food or food ingestion restricted for 2 h a day (8-10 AM) during 14 d, when plasma corticosterone, ACTH, insulin, leptin concentrations, and MC4-R mRNA expression were determined before and after refeeding. Another set of rats was fasted for 48 h, followed by refeeding during 2 or 4 h on the seventh day after adrenalectomy (ADX) or sham surgery. On the day of the experiment, rats were anesthetized and perfused and the brain processed for MC4-R mRNA by in situ hybridization. Long-term reduction of food intake, either secondary to food restriction or adrenalectomy, reduced body weight gain and also leptin and insulin plasma concentrations. Food ingestion reduced MC4-R expression in the paraventricular nucleus in naive rats subjected to food restriction and also in sham rats fasted for 48 h. However, after ADX, MC4-R expression was not changed by refeeding. In conclusion, the present data indicate that MC4-R expression is downregulated by food ingestion and this response could be modulated by glucocorticoid withdrawal.

Effects of fluoxetine administration on hypothalamic melanocortin system in obese Zucker rats

The aim of the present work was to study the potential involvement of melanocortin system in the anorectic mechanism of fluoxetine, a selective serotonin reuptake inhibitors, in obese Zucker rats. Male obese Zucker (fa/fa) rats were administered fluoxetine (10 mg/kg; i.p.) daily for two weeks. The control group was given 0.9% NaCl solution. RT-PCR for pro-opiomelanocortin (POMC), Agouti gene related peptide (AgRP) and melanocortin receptor 4 (MC4-R) in the hypothalamus, as well as regional immunostaining for alpha-melanocyte stimulating hormone (alpha-MSH) and MC4-R were carried out. Fluoxetine administration increased POMC expression and reduced MC4-R expression in the hypothalamus, without changes in AgRP mRNA levels. Moreover, an increase in the numbers of alpha-MSH positively immunostained neural cells in the hypothalamic arcuate nucleus (ARC), as well as a significant decrease in the numbers of neural cells positively immunostained for MC4-R in the paraventricular nucleus (PVN), without changes in lateral hypothalamic area (LHA), were observed. These results suggest the involvement of alpha-MSH in central fluoxetine anorectic action.

Divergence of melanocortin pathways in the control of food intake and energy expenditure

Activation of melanocortin-4-receptors (MC4Rs) reduces body fat stores by decreasing food intake and increasing energy expenditure. MC4Rs are expressed in multiple CNS sites, any number of which could mediate these effects. To identify the functionally relevant sites of MC4R expression, we generated a loxP-modified, null Mc4r allele (loxTB Mc4r) that can be reactivated by Cre-recombinase. Mice homozygous for the loxTB Mc4r allele do not express MC4Rs and are markedly obese. Restoration of MC4R expression in the paraventricular hypothalamus (PVH) and a subpopulation of amygdala neurons, using Sim1-Cre transgenic mice, prevented 60% of the obesity. Of note, increased food intake, typical of Mc4r null mice, was completely rescued while reduced energy expenditure was unaffected. These findings demonstrate that MC4Rs in the PVH and/or the amygdala control food intake but that MC4Rs elsewhere control energy expenditure. Disassociation of food intake and energy expenditure reveals unexpected divergence in melanocortin pathways controlling energy balance.

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.

Expression of the human melanocortin-2 receptor in different eukaryotic cells

The human melanocortin-2 receptor (hMC2R) is mainly present in the adrenal cortex and has been difficult to express in heterologous cells. The hMC2R fused to the EGFP at its C-terminus has been stably transfected in the murine M3 melanoma and HEK293 cells. In the M3 cells, the hMC2R-EGFP was well-addressed to the cell membrane and functional whereas in the HEK293 cells, the hMC2R-EGFP was retained intracellularly. These results suggest that some specific factors, missing in cells, which do not express any melanocortin receptor, are involved in the correct addressing of the hMC2R to the cell membrane.

1 kb of 5' flanking sequence from mouse MC4R gene is sufficient for tissue specific expression in a transgenic mouse

The melanocortin 4 receptor (MC4R) plays a critical role in the regulation of energy homeostasis, and the MC4R knockout mouse and humans with MC4R defective mutations in only one allele indicate that there is a gene dosage effect. Alterations in gene expression levels for MC4R could, therefore, have significant effects on energy homeostasis. To begin to develop a mouse model for studies on MC4R promoter in situ we used approximately 1 kb mouse MC4R promoter together with 426 bp MC4R 5' UTR, previously shown to support basal expression of reporter gene transcription in cell lines with endogenous MC4R mRNA, and fused this DNA to a nuclear localized LacZ reporter gene. The construct was injected into pronuclei from FVB mice. Five transgenic lines were identified as carrying autosomal transgene insertions; three of these had significant beta-galactosidase staining in brain and in a few cells in the heart but not in kidney, liver, lung, gonadal fat or testis. The pattern of transgene expression in the brain differed markedly for the three lines, and in one of these lines was remarkably similar to endogenous MC4R mRNA expression observed using in situ hybridisation. In conclusion, approximately 1 kb mouse MC4R promoter is sufficient to direct gene expression to the brain including regions that express endogenous MC4R mRNA.

Overexpression and functional characterisation of the human melanocortin 4 receptor in Sf9 cells

The human melanocortin 4 receptor (MC4r) was successfully expressed in Sf9 cells using the baculovirus infection system. N- and C-terminally His-tagged receptors generated B(max) values of 14 and 23 pmol receptor/mg membrane protein, respectively. The highest expression level obtained with the C-terminally His-tagged MC4r corresponded to 0.25mg active receptor/litre culture volume. Addition of a viral signal peptide at the N-terminus of the His-tagged MC4r did not improve the expression level. Confocal laser microscopy studies revealed that both the N- and C-terminally tagged MC4r did not accumulate intracellularly and were mainly located in the plasma membrane. The recombinant receptors showed similar affinity for the agonist NDP-MSH (Kd = 11 nM) as to MC4r expressed in mammalian cells. Functional coupling of the highest expressed C-terminal tagged receptor to endogenous Galpha protein was demonstrated through GTPgammaS binding upon agonist stimulation of the receptor. Ki values for the ligands MTII, HS014, alpha-, beta-, and gamma-MSH are comparable to the values obtained for MC4r expressed in mammalian cells.

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.

Expression of melanocortin MC3 and MC4 receptor mRNAs by neuropeptide Y neurons in the rat arcuate nucleus

Neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (alpha-MSH), two neuropeptides that are synthesized in neurons of the arcuate nucleus of the hypothalamus, exert opposite actions on food intake and body weight. NPY is orexigenic and decreases energy expenditure whereas alpha-MSH reduces food consumption and stimulates catabolism. alpha-MSH is an endogenous ligand for the central melanocortin receptors, MC3-R and MC4-R. In order to determine whether alpha-MSH may act directly on NPY neurons in the arcuate nucleus, we have investigated the possible occurrence of MC3-R and MC4-R mRNA in NPY-expressing cell bodies in the rat hypothalamus. Double-labeling in situ hybridization histochemistry using (35)S-labeled (MC3-R or MC4-R) and digoxigenin-labeled (NPY) riboprobes revealed that 38 +/- 1% of the NPY mRNA-positive perikarya expressed MC3-R mRNA while only 9 +/- 2% of the NPY-producing neurons contained MC4-R mRNA. The proportions of NPY neurons that express MC3-R mRNA or MC4-R mRNA were not significatively different in the anterior and posterior aspects of the arcuate nucleus. The present study shows that a large proportion of NPY neurons in the rat hypothalamus express MC3-R mRNA while a much lower number of NPY neurons express MC4-R mRNA, suggesting that melanocortins may directly modulate the activity of the hypothalamic NPY system, mainly through activation of MC3-R. These data provide additional evidence for the complex interactions between the stimulatory (NPY) and inhibitory (alpha-MSH) pathways controlling feeding behavior and energy homeostasis.

Proopiomelanocortin and melanocortin receptors in the adult rat retino-tectal system and their regulation after optic nerve transection

The aim of this study was to characterise the expression of the melanocortin system in the normal and injured rat visual system. Using real-time polymerase chain reaction and immunohistochemistry, we detected melanocortin MC(3), MC(4) and MC(5) receptors and proopiomelanocortin in adult retina and superior colliculus. Melanocortin MC(4) receptor mRNA was the most abundant receptor. Melanocortin MC(3), MC(4) and MC(5) receptors were localised to the ganglion cell and inner nuclear layers and the melanocortin MC(3) and MC(4) receptors were localised to retinal ganglion cells. Transection of the optic nerve leads to ganglion cell death and both melanocortin receptor and proopiomelanocortin expression decreased in superior colliculus after transection whereas the expression was unchanged or even increased in the retina. alpha-Melanocyte-stimulating hormone elicited neurite outgrowth from embryonic retinal explants. Together, these data implicate a role for the melanocortin system in the adult rat retina and that melanocortins can stimulate neurite growth from retinal neurons.

Melanocortin 4 receptor is expressed in the dorsal root ganglions and down-regulated in neuropathic rats

Recent reports have demonstrated effectiveness of melanocortin antagonists as potent analgesics, and have suggested that the spinal melanocortin 4 receptor (MC4-R) mediates their effects on pain transmission. These findings prompted us to investigate the changes in MC4-R mRNA level in the spinal cord and dorsal root ganglia (DRG) of neuropathic animals at different time points after sciatic nerve injury by quantitative real-time PCR. The spinal MC4-R mRNA level was not affected by sciatic nerve injury. In contrast, down-regulation of MC4-R mRNA in DRG developed 2 weeks after the injury and was parallel with the attenuated effectiveness of MC4-R ligands in neuropathic animals. The MC4-R adaptation in DRG observed in neuropathic rats indicates their important role in presynaptic modulation of activity of the primary afferents in neuropathic pain.

Altered levels of POMC, AgRP and MC4-R mRNA expression in the hypothalamus and other parts of the limbic system of mice prone or resistant to chronic high-energy diet-induced obesity

The melanocortinergic system plays an important role in promoting negative energy balance and preventing excessive fat deposition. This study has investigated the levels of mRNA expression of proopiomelanocortin (POMC), agouti-related protein (AgRP) and the melanocortin-4 receptor (MC4-R) in diet-induced obese (DIO) and diet-resistant (DR) mice. Thirty C57 mice were used in this study. Twenty-four mice were fed with a high-fat diet (HF: 40% of calories from fat, 20% from saturated fat) for 4 weeks and then classified as DIO and DR according to their body weight gain. Six mice were placed on a low-fat diet (LF: 10% of calories from fat, 1% from saturated fat) and were used as controls. After 22 weeks of feeding, visceral fat deposits were more than twice as heavy in the DIO mice as in the DR and LF mice, while the latter two groups had no significant difference. Using quantitative in situ hybridization techniques, this study found that the DIO mice had a significantly lower level of Arc POMC (-29%) and AgRP (-31%) mRNA expression than the DR and LF mice, respectively. The mice on high-fat diets had higher levels of AgRP mRNA expression in the bed nucleus of stria terminalis (BST), and ventral part of the lateral septal nucleus (LSV) than the LF mice. Furthermore, the DIO mice had a 40% higher level of MC4-R mRNA expression in the ventromedial hypothalamic nucleus (VMH) and posterodorsal part of the medial amygdaloid nucleus (MePD) than the LF mice. In conclusion, this study has demonstrated that differential expression of POMC, AgRP and MC4-R mRNA levels exists in DIO, DR and LF mice. These differences were shown to occur in the specific nuclei of the hypothalamus and other parts of the limbic system. These findings may assist in understanding the involvement of the melanocortinergic system in the regulation of body weight via the autonomic and limbic systems.

The effect of morphine on MC4 and CRF receptor mRNAs in the rat amygdala and attenuation of tolerance after their blockade

The relationships between the CRF, which enhances the proopiomelanocortin (POMC) biosynthesis, and POMC-derived peptides (opioids and melanocortins) might be a new target for rational treatment of morphine tolerance. In the present study, we investigated the effect of acute and chronic morphine administration on the level of CRF1 and melanocortin 4 receptor (MC4-R) mRNAs in the rat amygdala by quantitative real-time PCR method. Moreover, we investigated the effect of antagonists of melanocortin and CRF receptors, SHU9119 and alpha-helical CRF (alphah-CRF), respectively, administered bilaterally into the central nucleus of the amygdala, on morphine tolerance using tail-flick and paw withdrawal tests. Our study demonstrated that acute morphine administration decreased the level of MC4-R mRNA in the rat amygdala. This decrease was attenuated following chronic morphine administration, and mRNA level of MC4 receptors was gradually increased and, on 9th day of morphine administration, i.e. in the period when morphine tolerance already developed, the level was significantly increased in comparison with control and with the effect after single morphine dose. In contrast, morphine did not affect the CRF receptor. In behavioral study, we demonstrated that SHU9119 and alphah-CRF significantly increased the antinociceptive effect of morphine, when they were injected into the amygdala prior to morphine administration in tolerant rats. We have shown for the first time the contribution of amygdalar melanocortin receptors to morphine tolerance, and we conclude that the altered melanocortin receptor function may play an important role in the development of morphine-induced tolerance. CRF and melanocortin peptides can modulate the phenomena in the same direction, in opposition to opioids. Therefore, antagonists of melanocortin receptors may be regarded as possible therapeutic modulators of morphine tolerance.