Infusion of gamma 2-MSH produce a conditioned taste aversion in morphine-dependent rats

Effects of melanocortin receptor ligands on ethanol intake and opioid peptide levels in alcohol-preferring AA rats

Melanocortin (MC) peptides are suggested to play a role in opiate dependence, where they antagonise the addictive properties of opiates. To further study the involvement of the MCs in drug dependence, we analysed the effects of the MC(4)-receptor antagonist HS014 (1 nmol/rat), and the non-selective MC-receptor agonist MTII (1 nmol/rat), using i.c.v. administration, on ethanol intake in alcohol-preferring AA rats. The rats had access to ethanol during 40 days, resulting in a mean ethanol intake of 6.6 g/kg/day, before treatment. One group received only artificial cerebrospinal fluid solution. MTII caused a reduction in ethanol intake and ethanol preference, whereas HS014 was without effect. No effect on water intake was observed. A decrease in food intake was detected after MTII, whereas HS014 induced an increase in food intake. Analysis of dynorphin B and Met-enkephalin-Arg(6)Phe(7) immunoreactive levels revealed that MTII and HS014 altered opioid peptide levels in several brain areas and the pituitary gland of the rats with an established ethanol intake. This is the first report showing that manipulation of the MC-receptor system changes ethanol intake in chronically ethanol-drinking AA rats. In addition, manipulation of the MC system modulates ethanol-induced changes in opioid peptide levels.

Differential effects of alpha-, beta- and gamma(2)-melanocyte-stimulating hormones on hypothalamic neuronal activation and feeding in the fasted rat

Hypothalamic pro-opiomelanocortin neurones have an established role in the control of feeding. While pro-opiomelanocortin is the precursor for at least three melanocortin peptides, alpha-, beta- and gamma-melanocyte-stimulating hormone (MSH), it has been widely assumed that alpha-MSH is the predominant ligand involved. We compared the effects of centrally administered alpha-, beta- and gamma(2)-MSH on hypothalamic neuronal activation and on food intake in rats fasted for 48 h. Significant reductions in food intake were seen with alpha-MSH (first hour) and gamma(2)-MSH (second hour) but not with beta-MSH. The pattern of neuronal activation, assessed by the detection of early growth response factor-1 protein, showed considerable overlap; all three melanocortins activated cells in the arcuate, ventromedial, paraventricular, periventricular and supraoptic nuclei, as well as the preoptic area. alpha-MSH and beta-MSH produced activation in the dorsomedial nuclei while gamma(2)-MSH was only weakly active here. Retrograde labelling by systemic Fluorogold injection revealed that many cells activated by MSH compounds in the arcuate, paraventricular, periventricular and supraoptic nuclei (but not dorsomedial or ventromedial) project outside the blood-brain barrier and are therefore likely to include neuroendocrine cells. Desacetyl-alpha-MSH, which has previously been reported to lack effects on feeding, produced no discernible neuronal activation in the hypothalamus. Our finding that both the pattern of neuronal activation and the distribution of neuroendocrine cells activated in response to these closely related peptides show only partial overlap suggests that, in addition to common pathways, there may exist distinct hypothalamic circuits activated by different pro-opiomelanocortin products. The slower time course of gamma(2)-MSH- versus alpha-MSH-induced suppression of feeding provides further support for the notion that the biological responses to individual melanocortin peptides may involve distinct neuronal mechanisms.

Role of melanocortins in the central control of feeding

The injection of a melanocortin peptide or of melanocortin peptide analogues into the cerebrospinal fluid or into the ventromedial hypothalamus in nanomolar or subnanomolar doses induces a long-lasting inhibition of food intake. The effect keeps significant for up to 9 h and has been observed in all animal species so far tested, the most susceptible being the rabbit. The anorectic effect of these peptides is a primary one, not secondary to the shift towards other components of the complex melanocortin-induced behavioral syndrome, in particular grooming. The site of action is in the brain, and the effect is not adrenal-mediated because it is fully exhibited also by adrenalectomized animals. It is a very strong effect, because the degree of feeding inhibition is not reduced in conditions of hunger, either induced by 24 h starvation, or by insulin-induced hypoglycemia, or by stimulation of gamma-aminobutyric acid (GABA), noradrenergic or opioid systems. The microstructural analysis of feeding behavior suggests that melanocortins act as satiety-inducing agents, because they do not significantly modify the latencies to start eating, but shorten the latencies to stop eating. The mechanism of action involves the activation of melanocortin MC(4) receptors, because selective melanocortin MC(4) receptor antagonists inhibit the anorectic effect of melanocortins, while inducing per se a strong stimulation of food intake and a significant increase in body weight. Melanocortins seem to play an important role in stress-induced anorexia, because such condition, in rats, is significantly attenuated by the blockage of melanocortin MC(4) receptors; such a role is not secondary to an increased release of corticotropin-releasing factor (CRF), because, on the other hand, the CRF-induced anorexia is not affected at all by the blockage of melanocortin MC(4) receptors. The physiological meaning of the feeding inhibitory effect of melanocortins, and, by consequence, the physiological role of melanocortins in the complex machinery responsible for body weight homeostasis, is testified by the hyperphagia/obesity syndromes caused by mutations in the pro-opiomelanocortin (POMC) gene, or in the melanocortin MC(4) receptor gene, or in the agouti locus. Finally, recent evidences suggest that melanocortins could be involved in mediating the effects of leptin, and in controlling the expression of neuropeptide Y (NPY).

Chronic exposure to antibodies directed against anti-opiate peptides alter delta-opioid receptor levels

The development of addictive states in response to chronic opioid use may be regulated partially by the release of endogenous peptides. These anti-opiate peptides (AOP) are secreted or released into the CNS and produce diverse actions that counterbalance the effects of prolonged opiate exposure. Though the mechanism(s) by which these peptides exert their physiological properties remain largely unknown, there is some indication that AOP's modulate opioid receptor levels. In this study, we investigated the effects of chronically infused alpha-melanocyte stimulating hormone (alpha-MSH), dynorphin(1-8) (DYN(1-8)), dynorphin A (DYNA), and NPFF antibodies on delta-opioid receptor expression in rat brains. Quantitative autoradiographic experiments revealed that antibodies directed against alpha-MSH and DYNA produced significant increases in delta receptor levels in the caudate, claustrum, and cingulate cortex of the rat brain. Conversely, NPFF monoclonal antibodies caused significant decreases in the caudate, nucleus accumbens, olfactory tubercle, and cingulate cortex. These results suggest that the density of delta-opioid receptors is affected by changes in the levels of the anti-opioid peptides in the extracelluar fluid in the rat brain.

Melanocortin receptors: perspectives for novel drugs

The cloning of five different subtypes of melanocortin receptor subtypes have recently opened up new possibilities for the development of drugs. The physiological roles of the five melanocortin receptors have started to become understood, and compounds with selective actions on some of the five subtypes have become available. Presently, most clinically promising application for drugs active on melanocortin receptors are for control of feeding homeostasis and body weight and for treatment of inflammatory diseases. I review here the cloning, localisation, function and structure of the melanocortin receptors, in relation to the possibilities to develop selective drugs for these receptors.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.