Which opioid receptor mechanism modulates feeding?

Naltrexone and human eating behavior: a dose-ranging inpatient trial in moderately obese men

To investigate the effects of the long-acting opiate antagonist naltrexone on spontaneous human eating behavior, eight moderately obese male paid volunteers were housed in a hospital metabolic unit for 28 days and offered palatable foods ad lib by a platter service method. Under double-blind conditions, equally divided doses of 100, 200 and 300 mg naltrexone, or an acetaminophen placebo, were administered twice daily in tablet form for 3-day periods each, according to a Latin Square design. The doses of naltrexone resulted in decreases of daily caloric intake from placebo level, but these reductions were neither statistically significant nor dose-related. When the averaged effects of the doses were compared to placebo, five subjects showed intake reductions but the overall intake reduction of 301.5 +/- 198.1 kcal/day (mean +/- SEM) was not statistically significant. Naltrexone administration failed to selectively alter intakes of individual meals and snacks or macronutrient consumption patterns. During active drug periods, subjects lost 0.62 +/- 0.22 lb over 3 days, while during the placebo period, subjects gained 0.46 +/- 0.68 lb. However, there was no reliable change of basal metabolic rate as a function of naltrexone administration. The present results, which indicate that naltrexone administration is relatively ineffective in reducing food intake and inducing body weight loss in obese humans, are thus in contrast with reports that administration of opiate antagonist agents promote significant reductions of food intake and attenuations of body weight gain in experimental animals.

The kappa opioid receptor and food intake

Many studies have suggested a role of opioid receptors in the modulation of food intake. Several distinct classes of opioid receptors have been postulated. In an attempt to establish which opioid receptor(s) modulate feeding we studied the effect of the kappa agonist, bremazocine, on feeding and compared its effects to the preferential mu agonist, morphine, and the mixed kappa-sigma agonist, butorphanol and the kappa agonist, ethylketocyclazocine. Bremazocine increased feeding to the same extent as morphine and was less potent than the mixed agonist/antagonists. The bremazocine effect demonstrated a bell-shaped dose response curve. Daily administration of bremazocine or morphine enhances the effect on increasing food intake. However, this effect of daily injections on enhancing food intake is not present when animals receiving morphine are crossed over to bremazocine and vice versa. The bremazocine effect is enhanced by diprenorphine and not inhibited by naloxone. Low doses of the dopamine antagonist, haloperidol, enhance the bremazocine effect and higher doses inhibit it. Finally, using another kappa agonist, tifluadom, we showed that the effect on food intake is stereospecific. Our studies provided further evidence for a role for the kappa opioid receptor in feeding. However, they also suggest that more than one subpopulation of opioid receptors is involved in feeding modulation.

Selective attenuation of sweetened milk consumption by opiate receptor antagonists in male and female rats of the Roman strains

Male and female rats of the three Roman strains (Roman High-, Roman Low-, and Roman Control Avoidance; RHA, RLA and RCA, respectively) were familiarized with a highly palatable sweetened milk in a daily 30 min test. The animals were never food- or water-deprived prior to the test. Daily milk intake stabilised at a high level before drug tests were initiated. Effects of naloxone, diprenorphine, WIN 44,441-3, MR2266, MR2267, and ICI 154129 on milk consumption were investigated. Naloxone, diprenorphine and MR2266 each had comparable anorectic effects across strains and sexes. WIN 44,441-3 was relatively ineffective; MR2267 and ICI 154129 were without effect on milk consumption.

The effects of aging on day-night rhythms of kappa opiate-mediated feeding in the mouse

Day-night rhythms in feeding behavior and response to the specific kappa opioid agonist U-50,488H (0.10-10. mg/kg) were measured in young (1-2 months), mature (8-12 months) and old (24-30 months) male CF-1 mice. All the mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in old and mature animals. Young mice also displayed a significant, dose-related, nocturnal enhancement in U-50,488H-stimulated feeding. This day-night rhythm was reduced in mature animals and absent in old mice. In old mice, U-50,488H significantly stimulated feeding only after the high dose of 10 mg/kg. Additionally, old animals did not show the dose-dependent latency to initiation of feeding after administration which was observed in young mice and to a lesser extent in mature animals.

Peptides and feeding

This report reviews the hypothesis that peptides play a role in appetite modulation, stressing that the available evidence is predominantly pharmacological and thus caution needs to be taken in assigning physiological significance at this time. Two peptide systems have been postulated--a peripheral satiety system, typified by the gastrointestinal hormone cholecystokinin and a central feeding system driven by the opioid peptides and neuropeptide Y. This review also discusses the putative role of peptides in the anorexia of aging, drinking elicited by feeding and as mediators of the autonomic effects seen in association with ventromedial hypothalamic lesions.