Naloxone and fluid consumption in rats: dose-response relationships for 15 days

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Peptides that regulate food intake: antagonism of opioid receptors reduces body fat in obese rats by decreasing food intake and stimulating lipid utilization

Agonists to opioid receptors induce a positive energy balance, whereas antagonists at these receptors reduce food intake and body weight in rodent models of obesity. An analog of 3,4-dimethyl-4-(3-hydroxyphenyl)piperidine, LY255582, is a potent non-morphinan antagonist for mu-, kappa-, and delta-receptors (K(i) of 0.4, 2.0, and 5.2 nM, respectively). In the present study, we examined the effects of oral LY255582 treatment on caloric intake, calorie expenditure, and body composition in dietary-induced obese rats. Acute oral treatment of LY255582 produced a dose-dependent decrease in energy intake and respiratory quotient (RQ), which correlated with the occupancy of central opioid receptors. Animals receiving chronic oral treatment with LY255582 for 14 days maintained a negative energy balance that was sustained by increased lipid use. Analysis of body composition revealed a reduction in fat mass accretion, with no change in lean body mass, in animals treated with LY255582. Therefore, chronic treatment with LY255582 reduces adipose tissue mass by reducing energy intake and stimulating lipid use.

Reductions in body weight following chronic central opioid receptor subtype antagonists during development of dietary obesity in rats

Acute administration of long-acting general opioid antagonists reduces body weight and food intake in rats. In contrast, chronic administration of short-acting general opioid antagonists produces transient effects. The present study evaluated whether chronic central administration of selective long-acting antagonists of mu (beta-funaltrexamine, BFNA, 20 micrograms), mu1 (naloxonazine, 50 micrograms), delta1 ([D-Ala2,Leu5,Cys6]-enkephalin, DALCE, 40 micrograms), delta2 (naltrindole isothiocyanate, NTII, 20 micrograms) or kappa (nor-binaltorphamine, NBNI, 20 micrograms) opioid receptor subtypes altered weight and intake of rats exposed to a palatable diet of pellets, fat, milk and water, relative to pellet-fed and diet-fed controls. Diet-fed rats receiving chronic vehicle injections significantly increased weight (7-10%) and intake over the 11-day time course. Weight was significantly reduced over the time course in rats administered either BFNA (9%), naloxonazine (12%), DALCE (7%) or NTII (6%). Initial weight reductions failed to persist following chronic NBNI. All antagonists chronically reduced fat intake, but did not systematically alter total intake, pellet intake or milk intake relative to the pattern of weight loss. These data indicate that central mu, mu1, delta1, delta2, and, to a lesser degree, kappa receptors mediate long-term opioid modulation of weight even in animals maintained on diets that ultimately result in dietary obesity.

Reward and reinforcement produced by drinking water: role of opioids and dopamine receptor subtypes

The conditioned place preference procedure was used to evaluate the reinforcing properties of drinking in water-deprived rats. Subjects were allowed to drink for 8 min and were then transferred to place preference cages. In Experiment 1, the effects of naloxone and pimozide on drinking-induced place preference were analyzed. Animals treated with naloxone, 16 mg/kg, before the conditioning sessions showed a place aversion instead of the place preference found in saline-treated animals. Naloxone also reduced drinking. It was proposed that naloxone induced a state of frustrative nonreward. Pimozide, 1 mg/kg, blocked place preference and somewhat reduced drinking. In Experiment 2, doses of 1 and 4 mg/kg naloxone were used. Both doses blocked place preference. A dose of 4 mg/kg had a marginal effect on drinking, while 1 mg/kg lacked effect on this behavior. Thus, naloxone may block the establishment of place preference without modifying drinking. The effects of the dopamine D1 antagonist SCH23390 and the D2 antagonist raclopride were studied in Experiment 3. SCH23390 blocked place preference and reduced drinking at doses of 0.25 and 0.125 mg/kg. A dose of 0.06 mg/kg did not affect drinking but inhibited place preference. Raclopride, 0.25 mg/kg, had the same effects as SCH23390 at the same dose while 0.125 mg/kg blocked place preference without affecting drinking. It appears that the effects of a D1 and a D2 antagonist are similar. Because the effects of these latter drugs also are similar to those obtained with naloxone, it is suggested that both dopamine and opioids are important for water-induced reinforcement. Possible interactions between these two neurotransmitter systems are discussed.

Naloxone persistently modifies water-intake

Rats, deprived of water for 19 hr, were given an opportunity to drink water for 30 min and for another 4.5 hr. Prior to drinking, rats were injected with saline or NX (2.5 mg/kg) to see if NX's effects persisted when given day after day. The findings confirmed that NX not only reduced intake but its effects showed no tolerance across consecutive administrations. Subsequently, when rats were deprived of water for 23.5 hr, NX produced a reliable decrease in intake. Intakes, however, were considerably greater than when subjects had an additional opportunity to drink.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.