The pineal gland and opiate-induced feeding

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Nutritional peculiarities and diet palatability in the cat

Cats have become the most popular companion animal in Western Europe. Unlike other domestic animals, cats are strict carnivores and this influences both their nutritional requirements and food preferences. Cats have very high protein requirements and their diet must contain some nutrients, such as arginine, taurine, niacin, vitamin A and arachidonic acid. Besides its nutritional value, a diet for cats must also be highly palatable. This paper offers a quick overview of feline nutritional peculiarities and the factors that influence food palatability in cats.

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.

Pineal involvement in the alimentary behavior and taste preferences in the rat

The purpose of this experiment was to explore the effects of pinealectomy or sham pinealectomy on circadian rhythms of taste preferences, food and fluid intake and body weight gain in rats. We compared the body weight gain, the amounts of food eaten and that of deionized water, total fluid, salt, sour, sweet and bitter near-threshold solutions consumed by rats before all surgical manipulations and after pinealectomy or sham pinealectomy. The results showed that the pineal gland does not exert a major influence on circadian organization of taste preferences, drinking, feeding and body weight modifications in the rat. The failure of pinealectomy to modify light/dark rhythms of taste preferences and other related alimentary behaviors may be explained by the fact that pinealectomy does not completely eliminate circulating melatonin.

Immunohistochemical evidence for the presence of peptides derived from proenkephalin, prodynorphin and proopiomelanocortin in the guinea pig pineal gland

By using a plethora of region-specific antisera, this light microscopic immunohistochemical study revealed that derivatives from the three opioid precursors, i.e. proenkephalin, prodynorphin and proopiomelanocortin are differentially distributed in the pineal gland of guinea pig. Various molecular forms of immunoreactive opioid peptides derived from proenkephalin or prodynorphin were present in a minority of pinealocytes as well as in nerves. In contrast to this dual distribution pattern of opioid-active peptides, the opioid-inactive derivative from proopiomelanocortin, alpha-melanocyte stimulating hormone, was exclusively present in a large proportion of pinealocytes. A multiple and differential origin and function of opioidergic pineal innervation involving sympathetic, parasympathetic and sensory components is suggested. alpha-MSH is proposed as a pineal hormone which may act in concert with melatonin to regulate pineal rhythms or may function like MSH of pituitary origin.

Food intake dependence on acute changes in light schedule

The mechanism of the diurnal eating pattern in the rat (eating much more food at night than in the day) is not entirely clear. To further study the influence of environmental lighting on food intake, male rats were maintained in constant 12:12 light-dark cycle (daytime from 0800 to 2000) and subjected to short-time acute changes in the light schedule. The results show: (1) Lights on during the nighttime (30-min from 2230 to 2300 or from 2258 to 2328) suppress short-term food intake; (2) Lights off during the daytime (2-hr from 1000 to 1200) increase short-term food intake. Both responses (1 and 2) are independent of the age of the rat. These results are consistent with acute regulatory adjustment of feeding to abrupt light shifts, possibly by generation through the retino-hypothalamic pathway to the suprachiasmatic nucleus (supporting its action as a pacemaker in the endogenous circadian feeding rhythm in the rat), although other control mechanisms are not excluded.

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.

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.

Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

Evidence from studies which utilise either opiate receptor agonists and antagonists strongly indicate a role for endorphinergic mechanisms in the control of feeding responses. Two means by which these compounds may exert an effect on feeding can be singled-out. Firstly, emerging evidence suggests that the process of achieving satiety (terminating a meal, or choice of a commodity) may be accelerated following treatments with opiate receptor antagonists. Secondly, the preference for highly palatable solutions (sweet solutions have received most attention) in two-bottle tests is blocked after injection of opiate receptor antagonists. This finding has been interpreted in terms of the abolition of the reward or incentive quality associated with the particularly attractive flavour. These two mechanisms of action may represent two aspects of a single, fundamental process. Following an introduction to rat urination model of in vivo kappa agonist activity, the consistent effect of several kappa agonists (including the highly selective U-50,488H) to stimulate food consumption is described. Recognising that members of the dynorphin group of endogenous opioid peptides are kappa receptor ligands, some with a high degree of selectivity, and the evidence the dynorphins and neo-endorphins produce hyperphagia in rats is particularly interesting. Such lines of evidence lead to the hypothesis that peptides of the dynorphin group may act endogenously to promote the expression of normal feeding behaviour.

Opioids and consummatory behavior

Since the second decade of this century it has been known that opiates can influence ingestive behaviors. Generally, opioid agents enhance feeding and opioid antagonists decrease feeding. The present paper reviews the responsiveness of different animal species to opiates in relation to ingestive behaviors, the opioid receptors involved in such consummatory behaviors, the site of action of opioid modulation of feeding, the role of glucose in opioid induced feeding, and endocrine effects on opioid feeding systems. We emphasize the finding that more than one opioid receptor is involved in the modulation of feeding. A large body of evidence indicates a major role for the dynorphin/alpha-neo-endorphin kappa opioid receptor as one of the receptors involved in feeding modulation. Opioids appear to exert their effect predominantly within the central nervous system, though peripheral effects on taste and gastrointestinal function may play a role in opioid-induced feeding. Although opioid blockade acutely blocks food intake, chronic administration of opiate antagonists to humans and laboratory animals has not proven to be an effective means of decreasing body weight. Chronic opiate administration decreases body weight and autosensitization of beta-endorphin increases body weight. Thus, although it is clear that opioids can effect food intake, it is not clear what effect chronic administration of opioids has no food intake or body weight.