The effect of opiates and naloxone on food intake in virgin and lactating rats

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.

Lactation decreases mRNA levels of opioid peptides in the arcuate nucleus of the rat

The state of lactation results in increased food intake to compensate for the increased energy expenditure to produce nutrients supplied to the offspring. In this study, Sprague-Dawley female rats lactating for 10-16 days, and rats 7 days post-lactation were implanted with osmotic minipumps infusing either naltrexone (NTX) (70 microg/h) or saline (0.9%) over a 48 h period. mRNA levels of pro-dynorphin (proDYN), pro-opiomelanocortin (POMC) and pro-enkephalin (proENK) were measured in the arcuate nucleus (ARC) and whole pituitary of both groups. In both saline- and NTX-treated lactating subjects, food intake was higher than in post-lactating subjects (P < 0.01). In post-lactating subjects, NTX decreased food intake by 27% during the infusion period (P < 0.05). There were no significant differences in body weight between the treatment groups; however, naltrexone decreased body weight gain in both lactating and post-lactating subjects. In both saline and NTX-treated lactating subjects, ARC mRNA levels of proDYN, POMC and proENK were significantly decreased compared with the saline or NTX-treated post-lactating subjects (P < 0.01). NTX did not significantly influence gene expression of opioid peptides in the ARC in either the lactating or the post-lactating subjects. Neither the lactation condition nor NTX administration significantly changed mRNA levels of proDYN, POMC or proENK in whole pituitary. Thus, as has been noted in energy-deprived rats, opioid peptide gene expression is decreased in the ARC of lactating rats, a period during which rats have increased energy requirements.

Mu 1 opioid receptor involvement in maternal behavior

Previous studies have demonstrated that morphine inhibits the display of maternal behavior in lactating rats. Whether morphine exerts its actions specifically at the mu receptor has not yet been determined. The present study examined this possibility by evaluating whether naloxonazine, an irreversible and selective antagonist of the mu 1 opioid receptor subtype, is able to attenuate morphine's disruptive effect on maternal behavior in primiparous lactating rats. Experiment 1 compared the ability of naloxonazine (AZINE) and naloxone (NAL) to block the action of morphine (MOR) on maternal care. Virgin, Sprague-Dawley rats were mated in our colony and on day 3 postpartum (parturition, day 0) all rats received jugular catheters. On day 6 the mothers received one of the following treatments: MOR alone (10 mg/kg, SC, N = 10); MOR (10 mg/kg, SC) 24 hr after AZINE pretreatment (10 mg/kg, IV, N = 10); MOR (10 mg/kg, SC) 24 hr after NAL pretreatment (10 mg/kg, IV, N = 8); or MOR (10 mg/kg, SC) immediately after NAL (0.5 mg/kg, SC, N = 10). MOR alone completely disrupted maternal behavior (0% responded) which was blocked by prior NAL administration (100%). AZINE pretreatment 24 hr earlier partially blocked MOR disruption of MB (40% responded; significantly different from MOR alone). The response of rats pretreated 24 hr earlier with NAL did not differ from MOR alone. AZINE blocked MOR's effect on pup retrieval to an even greater degree (70% responded vs. 10% in MOR alone). Experiment 2 determined the ability of AZINE to interfere with varying doses of MOR on maternal behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Experimental manipulations of eating: advances in animal models for studying anorectic agents

The material set out in this text has been designed to show the wide range of procedures which have the capacity to modify eating behavior--to produce hyper- or hypophagia, to alter the profile of eating patterns, or to adjust dietary preferences and selection. Accordingly, in investigating anorectic drugs it seems necessary to observe the effects of drug actions in a variety of experimental models. This strategy will provide a more complete description of the effect of a drug, will throw light on the mechanism of action, and will provide a more realistic base for predicting the effects of drugs in man.

The effect of naloxone on nocturnal food intake in female and male rats

Previous studies have shown that estradiol and progesterone can alter the response of female rats to naloxone. For example, ovariectomized rats receiving estradiol were found to be less sensitive to the anorexic effect of naloxone than ovariectomized rats receiving oil (vehicle) or progesterone. In the present paper, we evaluated the effect of naloxone on nocturnal food intake in female rats during each stage of the estrous cycle to determine whether changing levels of gonadal hormones in intact female rats would affect their response to naloxone. To evaluate the role testosterone might play in modulating the male rat's feeding response to naloxone we studied the effect of peripherally administered naloxone (0.1, 1.0 and 10 mg/kg) on nocturnal food intake of intact, castrate and castrate + testosterone propionate male rats. During late metestrus, diestrus and proestrus, female rats decreased nocturnal food intake following the administration of naloxone (1.0 and 10 mg/kg) SC (p less than 0.05). During estrus, female rats failed to decrease food intake following any of the doses of naloxone administered. The male rat's response to naloxone does not appear to be altered by the presence or absence of testosterone. Thus, the level of estradiol and progesterone at different stages of the estrous cycle may affect the female rat's response to the satiety effect of naloxone.

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.