Central opioid receptor subtype mediation of isoproterenol-induced drinking in rats

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.

Blockade of central kappa-opioid receptors inhibits the antidipsogenic effect of interleukin-1beta

The objective of the present study was to investigate the role of brain kappa-opioid receptors (KOR) in the antidipsogenic effect promoted by third ventricle injections of interleukin-1beta (IL-1beta). Wistar male rats were submitted to three different, thirst-inducing, physiological conditions: dehydration induced by water deprivation, hyperosmolarity induced by salt-load and hypovolemia induced by polyethylene glycol subcutaneous injection. Third ventricle injections of IL-1beta significantly inhibited the increase in water intake observed in those situations. The pharmacological blockade of central KOR by the selective KOR antagonist nor-binaltorphimine (BNI) at different doses significantly inhibited the antidipsogenic effect induced by the central administration of IL-1beta in all conditions tested: dehydration, hypovolemia and hyperosmolarity. The central administration of IL-1beta failed to induce any locomotor deficit, as verified in an open field test. Stimulation of the central interleukinergic component did not result in any general suppression of ingestive behavior since no change in saccharin intake was recorded during a dessert test in animals receiving central injections of IL-1beta. Furthermore, the inhibitory effects of IL-1beta on water intake cannot be attributed to sickness-like effects induced by these compounds, since an aversion test excluded this possibility. In summary, the data shown in the present study clearly show that the antidipsogenic effect observed in rats following third ventricle injections of IL-1beta depend on the functional integrity of a brain kappa-opioid-dependent component.

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.

Characterization of the decline in alcohol consumption by aminopeptidase inhibition

Bestatin, an aminopeptidase inhibitor, prolongs the action of angiotensin and enkephalin. Previous studies have shown that bestatin can reduce alcohol intake in heterogeneous as well as genetically selected alcohol preferring 'P' rats, but more detailed studies of the characteristics of this effect have not yet been done. In experiment 1, daily injections of 10, 20 or 40 mg/kg bestatin for 12 days produced a uniform reduction in alcohol intake (approximately 40%) which did not recover following suspension of bestatin, but did recover following a 7 day period of morphine-stimulated alcohol drinking. In experiment 2, the lower end of the dose range was explored (2.5, 5, 10 mg/kg) and a dose-dependent effect was observed with a threshold dose of 5 mg/kg. Again, alcohol drinking did not recover following suspension of the 10 mg/kg dose of bestatin. Using a limited access procedure, bestatin reduced the intake of a preferred sodium chloride solution but not an avidly-consumed glucose solution indicating that while bestatin's effect was selective, it was not specific to alcohol. In the final experiment, the role of angiotensin and enkephalin was explored by pretreating animals with losartan (5, 10, 20 mg/kg), an angiotensin receptor blocker and naltrexone, (0.25, 0.5, 1 mg/kg) an opiate receptor blocker. Both agents produced significant but rather small attenuations in bestatin's effect suggesting that other factors are involved in its action. These findings further characterize the inhibitory effect of bestatin on alcohol intake in rodents. In addition, the novel finding of a carry-over inhibition of alcohol intake in the bestatin-free period adds further impetus to the recommendation that this drug, which has been safely used in humans, might be examined as a possible therapeutic agent for alcohol abuse.

Different central opioid receptor subtype antagonists modify maltose dextrin and deprivation-induced water intake in sham feeding and sham drinking rats

Different central opioid receptor subtypes participate in the mediation of intakes of simple (sucrose: mu, kappa 1) and complex (maltose dextrin: mu) carbohydrates as well as deprivation-induced water intake (mu) under real-feeding and real-drinking conditions. An identical pattern of mu and kappa 1 mediation of sucrose intake was observed in sham-feeding rats as well, suggesting their actions on orosensory mechanisms supporting sucose intake. The present study examined whether centrally administered general (naltrexone: 1-50 micrograms), mu (beta-funaltrexamine: 1-20 micrograms), mu 1 (naloxonazine: 50 micrograms), kappa 1 (nor-binaltorphamine: 1-20 micrograms), delta 1 ([D-Ala2, Leu5, Cys6]-enkephalin: 10-40 micrograms) or delta 2 (naltrindole isothiocyanate: 20 micrograms) opioid subtype antagonists altered either maltose dextrin (10%) intake during sham feeding or deprivation (24 h)-induced water intake during sham drinking in rats with gastric fistulas. Sham feeding significantly increased maltose dextrin intake (180%) and sham drinking significantly increased deprivation-induced water intake (256%) over a 60 min time course. Naltrexone significantly and dose-dependently reduced maltose dextrin intake (78%) in sham feeding rats, and deprivation-induced water intake (51%) in sham drinking rats. Maltose dextrin intake in sham feeding rats was significantly reduced by either kappa 1 (69%) or delta 1 (59%) opioid antagonism, was significantly increased by mu 1 antagonism (43%), and was not significantly affected by either mu or delta 2 opioid antagonism. Deprivation-induced water intake in sham drinking rats was significantly reduced by either mu (41%), mu 1 (28%), delta 1 (48%) or delta 2 (28%) opioid antagonism, but was not significantly affected by kappa 1 opioid antagonism. The difference in opioid receptor subtype mediation of maltose dextrin intake in real feeding and sham feeding conditions suggest that kappa 1 and delta 1 receptors are involved in the orosensory mechanisms supporting maltose dextrin intake, while mu receptors are involved in the ingestive and post-ingestive mechanisms supporting maltose dextrin intake. The different patterns of opioid involvement in sucrose and maltose dextrin intake in sham feeding and real feeding conditions provide further support for the hypothesis that at least two different carbohydrate taste systems exist. The difference in opioid receptor subtype mediation of deprivation-induced water intake in real drinking and sham drinking conditions may reflect the removal in the sham drinking condition of a mu-mediated prerestorative satiety mechanism, and the unmasking of other opioid-mediated signalling mechanisms.

Antisense oligodeoxynucleotides against the MOR-1 clone alter weight and ingestive responses in rats

MOR-1 encodes a mu receptor. In an effort to establish the relationship of this cloned opioid receptor with ingestive behavior and analgesia in rats, the present study examined the actions of four antisense oligodeoxynucleotides aimed at exons 1 (AS1), 2 (AS2), 3 (AS3) and 4 (AS4) of the MOR-1 clone, as well as a mismatch antisense sequence (MS1). Rats were administered intracerebroventricular injections (10 micrograms/2 microliters) of each of the oligodeoxynucleotides on days 1, 3 and 5. Body weight and spontaneous food and water intake were monitored daily. In addition, 2-deoxy-D-glucose (2DG)-induced hyperphagia, central Angiotensin II (ANG-II) induced hyperdipsia and central morphine analgesia were examined 24 h following the last antisense injection. AS1, AS2, AS3 and AS4 each significantly reduced body weight (7-17 g), food intake (8-13 g) and water intake (11-23 ml), while the vehicle or MS1 conditions significantly increased weight (9-20 g) and produced smaller reductions (2-4 g) in food intake. None of the AS probes altered the magnitude of either 2DG-induced hyperphagia or ANG-II-induced hyperdipsia. Central morphine analgesia was reduced by pretreatment with AS1 and AS4, but not AS2, AS3 or MS1. The sensitivity of general feeding to all four exons suggest that the receptor responsible for this action is encoded by the MOR-1 clone. The differences between feeding and morphine analgesia raise the possibility that these two actions are mediated through different mu receptor subtypes. Our results also demonstrate the viability of the in vivo antisense technique in modulating opioid-mediated ingestive responses.

Analysis of central opioid receptor subtype antagonism of hypotonic and hypertonic saline intake in water-deprived rats

Intake of either hypotonic or hypertonic saline solutions is modulated in part by the endogenous opioid system. Morphine and selective mu and delta opioid agonists increase saline intake, while general opioid antagonists reduce saline intake in rats. The present study evaluated whether intracerebroventricular administration of general (naltrexone) and selective mu (beta-funaltrexamine, 5-20 micrograms), mu, (naloxonazine, 50 micrograms), kappa (nor-binaltorphamine, 5-20 micrograms), delta (naltrindole, 20 micrograms), or delta 1 (DALCE, 40 micrograms) opioid receptor subtype antagonists altered water intake and either hypotonic (0.6%) or hypertonic (1.7%) saline intake in water-deprived (24 h) rats over a 3-h time course in a two-bottle choice test. Whereas peripheral naltrexone (0.5-2.5 mg/kg) significantly reduced water intake and hypertonic saline intake, central naltrexone (1-50 micrograms) significantly reduced water intake and hypotonic saline intake. Water intake was significantly reduced following mu and kappa receptor antagonism, but not following mu 1, delta, or delta 1 receptor antagonism. In contrast, neither hypotonic nor hypertonic saline intake was significantly altered by any selective antagonist. These data are discussed in terms of opioid receptor subtype control over saline intake relative to the animal's hydrational state and the roles of palatability and/or salt appetite.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.