The stimulation of food intake by selective agonists of mu, kappa and delta opioid receptors

Function and regulation of cholecystokinin octapeptide, β-endorp hin and gastrin in anorexic infantile rats treated with ErBao Granules

AIM: To study the role of cholecystokinin octapeptide (CCK-8), β-endorphin (β-EP), and gastrin in an anorexic infantile rat model and no subsequent regulation of nose peptides by the Yunpi complex prescription ErBao Granule.

METHODS: We fed infantile rats with special prepared forage. A liquid extract of ErBao Granule was administered to the rats daily for 3 weeks, CCK-8, β-EP, and gastrin concentrations in hypothalamus, gastric antrum, and plasma of the rats were measured by radioimmunoassay, and were compared with controls.

RESULTS: Treatment of rats with ErBao Granule inhibited CCK-8 secretion and increased β-EP and gastrin secretion. CCK-8 concentration in hypothalamus and plasma of model control group increased significantly and correlated negatively with food intake of models, respectively. β-EP concentration in gastric antrum and plasma of model control group decreased significantly and showed a positive correlation with food intake of models, respectively. Hypothalamus concentration of β-EP was similar in models and controls. Gastrin concentration in gastric antrum of models was lower than in the blank control group, and correlated positively to food intake of models. Finally, CCK-8 concentrations in plasma of rats showed a positive correlation with plasma β-EP (r = -0.68, P < 0.05).

CONCLUSION: The increased plasma and hypothalamus concentration of CCK-8, decreased gastric antrum and plasma level of β-EP, and decreased gastric antrum concentration of gastrin are associated significantly with the anorexia of infantile anorexic rat models produced by special forage. ErBao Granule can reverse these changes, which may be the major mechanisms of ErBao Granule simulating feeding.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

Opioids affect acquisition of LiCl-induced conditioned taste aversion: involvement of OT and VP systems

Aversive properties of lithium chloride (LiCl) are mediated via pathways comprising neurons of the nucleus of the solitary tract (NTS) and oxytocin (OT) and vasopressin (VP) cells in the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Because opioids act on brain regions that mediate effects of LiCl, we evaluated whether administration of opioids shortly before LiCl in rats influences 1) development of conditioned taste aversion (CTA) and 2) activation of NTS neurons and OT/VP cells. Neuronal activation was assessed by applying c-Fos immunohistochemical staining. Three opioids were used: morphine (MOR), a mu-agonist, butorphanol tartrate (BT), a mixed mu/kappa-agonist, and nociceptin/orphanin FQ (N/OFQ), which binds to an ORL1 receptor. BT and N/OFQ completely blocked acquisition of CTA. MOR alleviated but did not eliminate the aversive effects. Each of the opioids decreased LiCl-induced activation of NTS neurons as well as OT and VP cells in the PVN and SON. We conclude that opioids antagonize aversive properties of LiCl, presumably by suppressing activation of pathways that encompass OT and VP cells and NTS neurons.

Prenatal heroin exposure. Effects on development, acoustic startle response, and locomotion in weanling rats

The purpose of this study was to investigate the effects of prenatal heroin exposure on the offspring in postnatal behavioral development. Pregnant Sprague-Dawley rats were injected daily (s.c.) with 10mg/kg of heroin from gestational day 8 to 20. The control dam received saline injections and the pair-fed dam received saline and was yoked to a weight-matched heroin-treated dam. Litters were culled to eight to ten pups and weighed at postnatal day (PND) 1, 8, 15, and 22. Acoustic prepulse inhibition and habituation were parameters used for evaluating the sensorimotor gating and simple form of learning respectively. Locomotor activity and rearing were assessed using the photobeam activity system. All behavioral tests were performed on the offspring at PND 21 to 23. Results showed that heroin treatment significantly reduced maternal food intake, water consumption, and weight gain. Both heroin-exposed and pair-fed groups showed a marked reduction in birth weight in both male and female pups when compared with controls; however the postnatal weight gain in heroin-exposed pups was significantly lower than the pair-fed group by 3 weeks postnatally, particularly in the female pups. These female pups also showed a significant increase in ambulation and rearing when compared to the pair-fed pups. The habituation rate in both types of behavioral tests was also decreased in these female pups as compared to control and pair-fed groups. The present study indicated that prenatal heroin exposure could result in a marked retardation of postnatal development and learning. These effects are sex related.

Diet Restriction Increases Enkephalin- and Dynorphin-like Immunoreactivity in Rat Brain and Attenuates Long-term Retention of Passive Avoidance

The present study examines effects of diet restriction (DR) on behavior and on the opioid peptides enkephalin (Enk) and dynorphin (Dyn). Female rats were assigned to ad libitum food intake (AL), DR 60% (DR60) or 40% (DR40) of AL. After 4 weeks, DR reduced fearful behavior in the elevated plus maze. DR rats displayed good retention of passive avoidance at 24 h, but DR40 rats had reduced retention, at 5 and 11 days post training. Changes in Enk- and Dyn-like immunoreactivity (LI) in the hippocampal mossy fibers (MF), hypothalamus, septum, central nucleus of amygdala (CeAm) and thalamus depended on the severity of DR. In DR60, Enk-LI and Dyn-LI were not changed except for reduction in CeAm. In DR40, Dyn-LI increased significantly above AL levels in MF, CeAm and hypothalamus, whereas Enk-LI increased significantly above AL levels in the CA3 subregion of the MF system and in thalamus. Serum glucose was tightly correlated with Enk-LI reaching highest values in the MF (r= -0.82). Increased opioid-LI in CeAm and MF was associated with reduced fearfulness in the elevated plus maze. Thus, hippocampal and amygdala opioid subsystems are uniquely sensitive to DR and may be relevant to psychophysiological problems in human starvation including anorexia.

STZ-induced diabetes decreases and insulin normalizes POMC mRNA in arcuate nucleus and pituitary in rats

Effects of streptozotocin (STZ)-induced diabetes and insulin on opioid peptide gene expression were examined in rats. In experiment 1, three groups were administered STZ (75 mg/kg ip single injection). Two groups were killed at either 2 or 4 wk. In the third group, insulin treatment (7.0 IU/kg x 1 day for 3 wk) was initiated 1 wk after STZ injection. STZ induced hyperphagia and reduced weight gain. Insulin decreased food intake and increased body weight relative to diabetes. Proopiomelanocortin (POMC) mRNA in arcuate nucleus (Arc) and pituitary decreased in diabetes and normalized after insulin treatment. Prodynorphin (proDyn) mRNA increased in diabetes and normalized in the pituitary after insulin but not in the Arc. Diabetes did not alter proenkephalin (proEnk) expression in the Arc or pituitary, nor dynorphin A1-17 or beta-endorphin in paraventricular nucleus (PVN). alpha-Melanocyte-stimulating hormone (alpha-MSH) peptide levels were decreased in the PVN and normalized following insulin treatment. Diabetes increased Arc neuropeptide Y mRNA, and insulin suppressed this increase. In experiment 2, insulin (2.5 IU/kg sc) daily for 1 wk in normal rats increased Arc POMC mRNA, but not proDyn and proEnk mRNA. These results suggest that Arc POMC expression and PVN alpha-MSH peptide levels decrease in diabetes. Also, insulin may influence Arc and pituitary POMC activity in neurons that regulate energy metabolism.

Acute and chronic effects of morphine under a progressive-ratio 25 schedule of food delivery

The present study examined the effects of morphine in pigeons responding under a progressive-ratio 25 schedule of food delivery. Morphine initially reduced response rates and breaking points. With chronic exposure, tolerance developed to these effects. The magnitude of the observed tolerance was not obviously different from that previously reported under a PR 5 schedule of food delivery. In addition, when drug effects were compared under the fixed-ratio 25 and fixed-ratio 100 components comprised by the progressive-ratio schedule, comparable tolerance was observed. Although prior studies using other procedures have shown that ratio size modulates the development of tolerance to morphine and other drugs, the present data suggest that this relation is constrained, and is not easily observed under progressive-ratio schedules.

Association between the amygdala and nucleus of the solitary tract in mu-opioid induced feeding in the rat

The central nucleus of the amygdala (CNA) and the nucleus of the solitary tract (NTS) are important in the regulation of ingestive behavior. We evaluated whether opioid-opioid signaling between the CNA and rostral NTS (rNTS) affect feeding behavior. To test this, rats were doubly cannulated with one cannula placed in the rNTS and one cannula in the CNA, allowing for co-administration of an opioid agonist into one site and an opioid antagonist into the other. Tyr-D-Ala-Gly-(me) Phe-Gly-ol (DAMGO) (2 nmol) injected into the CNA (CNA DAMGO) increased feeding more than two-fold compared to the vehicle-injected rats. This increase in food intake was blocked when doses of 26.5 and 79 nmol of naltrexone (NTX) were injected into the rNTs. In the reverse situation, rNTS DAMGO increased food intake above control levels, and CNA NTX blocked DAMGO-induced feeding when administrated in doses of 26.5 and 79 nmol. This suggests that a bi-directional opioid-opioid signaling pathway exists between the CNA and the rNTS which influences feeding via mu-opioid receptors.

Endomorphins have orexigenic and anxiolytic activities in mice

This study was designed to investigate the effects of endomorphin 1 and 2, recently identified mu-opioid receptor selective peptides, on food intake and anxiety in non-food-deprived mice. The intracerebroventricular (i.c.v.) injection of either endomorphin 1 or 2 (0.03-30 nmol) increased food intake in a dose-related manner. A significant increase was observed 20 min after i.c.v. injection of endomorphin 1 or 2 and continued for 4 h. In the elevated plus maze test, the i.c.v. injection of endomorphin 1 (30 nmol) significantly decreased the normal preference for the closed arms. These results suggest that endomorphin produces orexigenic and anxiolytic effects, and that the mu-opioid receptor contributes to the regulation of feeding and anxiety in mice.

Effects of the opioid antagonist naltrexone on feeding induced by DAMGO in the central nucleus of the amygdala and in the paraventricular nucleus in the rat

The paraventricular nucleus of the hypothalamus (PVN) and the central nucleus of the amygdala (CNA) are two forebrain structures which are important in regulation of ingestive behavior. DAMGO is one of the most reliable and potent mu-selective opioid ligands that increases feeding in both of these brain nuclei. Administration of naloxone, an opioid antagonist, into the CNA prior to DAMGO blocks DAMGO-induced increases in food intake. The effect of this drug combination on food intake has not been evaluated in the PVN. However, intra-PVN injection of naloxone decreases deprivation and NPY-induced feeding. It has been suggested that CNA may modulate activity of midbrain and caudal brainstem centers via the hypothalamus. Based on these data, we evaluated whether an opioid-opioid interaction is present between the CNA and PVN which might affect feeding behavior. To test this, rats were doubly cannulated with 1 cannula placed in the PVN and 1 cannula in the CNA, allowing for co-administration of the opioid agonist into the PVN and the opioid antagonist into the CNA, and vice versa. CNA DAMGO increased feeding more than two-fold as compared to the vehicle-injected rats. When doses of 10, 12.5 and 25 micrograms of naltrexone (NTX) were injected into the PVN, CNA DAMGO no longer increased food intake above control levels. In the reverse situation, PVN DAMGO also increased food intake above control levels. However, when NTX was administrated unilaterally into the CNA at a relatively high dose (25 micrograms) or bilaterally (12.5 micrograms), PVN DAMGO-induced feeding was not altered. This suggests that an opioid-opioid signaling pathway exists from the CNA to the PVN which influences feeding via mu opioid receptors, whereas such a pathway from the PVN to the CNA does not seem to exist.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Food intake and food choice: the role of the endogenous opioid peptides in the marsupial Sminthopsis crassicaudata

Endogenous opioid peptides activate food seeking behaviour and influence macronutrient choice in a number of animal species and previous studies have suggested that the palatability of food is strongly modulated by the opioid feeding system. The effect of opioid peptides on appetite and food choice in marsupials has not been evaluated. The aim of these studies was to determine the effect of mu, delta and K opioid receptors on food intake and food choice in the marsupial Sminthopsis crassicaudata. When offered a choice of mealworms or laboratory diet after 24 h food deprivation, S. crassicaudata ate predominantly mealworms. After a 24 h fast, adult male S. crassicaudata were injected peripherally with opioid receptor antagonists or saline. Animals were re-fed with either their laboratory diet alone, or a choice of laboratory diet and mealworms. In animals re-fed with laboratory diet alone, naloxone at doses of 15 and 10 mg/kg produced a 31% (P < 0.05) and 38% (P < 0.05) respectively reduction in food intake in the first 30 min after laboratory diet was re-introduced, but lower doses had no effect. The selective delta antagonist naltrindole at 20 mg/kg resulted in a 65% (P < 0.01) reduction in food intake compared to controls between 30 and 60 min. The selective kappa opioid antagonist nor-binaltorphimine had no effect on the intake of laboratory diet. In animals offered a choice of laboratory diet and mealworms, naloxone doses of 1, 5, 10, 15 and 20 mg/kg significantly decreased intake in the first 0.5 h after re-feeding, due to a preferential suppression of the intake of mealworms. Naltrindole and nor-binaltorphimine had no effect on food choice. These studies demonstrate that endogenous opioid peptides influence both food intake and choice in S. crassicaudata and that the role of the opioid feeding system is in part modulated by food palatability. In S. crassicaudata these effects appear to occur predominantly by a mu opioid receptor mechanism.

Opioid-receptor subtype agonist-induced enhancements of sucrose intake are dependent upon sucrose concentration

Selective mu ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO)), delta1 ([D-Pen2, D-Pen5]-enkephalin (DPDPE)), delta2 ([D-Ala2, Glu4]-Deltorphin (Delt II)), kappa1 (U50488H) and kappa3 (naloxone benzoylhydrazone (NalBzOH)) opioid agonists each stimulate food intake in rats. Whereas studies with selective opioid antagonists implicate mu and kappa1 receptors in the mediation of sucrose intake, studies with selective opioid agonists implicate mu and delta receptors in the mediation of saccharin intake. The present study determined if specific delta1, delta2, kappa1, kappa3 and mu opioid-receptor subtype agonists produced similar alterations in sucrose intake as a function of sucrose concentration (0.5%, 2.5%, 10%) across a 1-h time-course. Each of these agonists significantly increased sucrose intake with variations in pattern, magnitude, and consistency as a function of sucrose concentration. Whereas the mu opioid agonist, DAMGO, and the delta1 opioid agonist, DPDPE, each enhanced sucrose intake at higher (2.5%, 10%), but not lower (0.5%), concentrations, the delta2 opioid agonist, Delt II, increased sucrose intake at lower (0.5%, 2.5%), but not higher (10%), concentrations. Kappa opioid agonists produced less consistent effects. The kappa1 opioid agonist, U50488H, increased sucrose intake at high (10%) concentrations and decreased sucrose intake at low (0.5%) concentrations, and the kappa3 opioid agonist, NalBzOH, inconsistently increased sucrose intake at the 0.5% (20 microg) and 10% (1 microg) concentrations. Thus, these data further implicate mu, delta1, and delta2 opioid mediation of palatable intake, particularly of its orosensory characteristics.

Delta and kappa opioid receptor subtypes and ingestion: antagonist and glucoprivic effects

Delta and kappa opioid receptors have been differentiated into further subtypes based upon both biochemical and pharmacological analgesic assays. Whereas hyperphagia elicited by the kappa1 receptor agonist, U50488H is blocked by general and kappa1 opioid antagonists, hyperphagia elicited by the kappa3 receptor agonist, naloxone benzoylhydrazone (NalBzOH) is blocked by general, but not kappa1 opioid antagonists. The first study assessed the opioid antagonist profile of hyperphagia elicited by centrally administered delta1 ([D-Pen2, D-Pen5]-enkephalin, DPDPE: 5-50 microg) and delta2 ([D-Ala2, Glu4]-Deltorphin, Delt II: 5-50 microg) agonists following central pretreatment with general (naltrexone), delta1 ([D-Ala2, Leu5, Cys6]-enkephalin, DALCE) and delta2 (naltrindole isothiocyanate, NTII) opioid antagonists. It is also important to determine whether selective opioid receptor subtype agonists are capable of altering intake in ingestive situations other than spontaneous feeding. The second study examined whether centrally administered delta1, delta2, kappa1 or kappa3 agonists altered the pattern and magnitude of hyperphagia elicited by 2-deoxy-D-glucose (2DG: 50-400 mg/kg, IP). DPDPE-induced hyperphagia was significantly reduced by naltrexone and NTII, but not DALCE. Delt II-induced hyperphagia was significantly reduced by DALCE and NTII, but not naltrexone. Pairing Delt II (5 microg) with low (100-200 mg/kg) 2DG doses significantly enhanced intake, producing a leftward (3-fold) shift in 2DG's hyperphagic dose-response curve. In contrast, DPDPE failed to alter 2DG-induced hyperphagia, and kappa1 and kappa3 opioid agonists each produced small, but significant increases in 2DG-induced hyperphagia. The antagonist data suggest the possibility of physiological and pharmacological interactions between delta receptor subtypes in mediating food intake, and it would appear that delta2 opioid receptors exert facilitatory effects upon glucoprivic hyperphagia.

Interactions between angiotensin II and delta opioid receptor subtype agonists upon water intake in rats

Delta opioid receptor agonists, like those of mu and kappa receptors, stimulate water intake. To assess the relative contributions of delta1 and delta2 receptors in the modulation of water intake stimulated by Angiotensin II (AII), the present study examined the respective actions of [D-Pen2, D-Pen5]-enkephalin (DPDPE: 5-20 ug, i.c.v.) and [D-Ala2, Glu4]-Deltorphin (Delt II: 5-20 ug, i.c.v.) upon water intake per se, and upon AII (0.02-20 ng, i.c.v.)-induced hyperdipsia in rats. Both DPDPE and Delt II dose-dependently stimulated spontaneous water intake. An ineffective (5 ug) dose of DPDPE differentially altered water intake when paired with AII, significantly increasing intake at the 0.02 ng dose, not changing intake at the 0.2 ng dose and significantly decreasing intake at the 2 ng dose. In contrast, pairing ineffective doses of Delt II (5 ug) and AII (0.2 ng) significantly increased water intake that persisted when either the Delt II dose (1 ug) or the AII dose (0.02 ng) was lowered. However, neither delta1 nor delta2 opioid agonists significantly altered the ED50 for AII-induced drinking. Thus, delta2 opioid agonism appeared more consistent and reliable than delta1 opioid agonism in stimulating water intake when paired with AII, and these data indicate interactions between delta1 and delta2 agonists and AII in mediating water intake.

Acute and chronic effects of morphine in pigeons responding under a progressive-ratio schedule of food delivery

Although progressive-ratio schedules have often been used by behavioral pharmacologists to index the relative reinforcing effects of drugs of abuse, they have been ignored in the study of tolerance to opioids. The present study examined tolerance to morphine in pigeons responding under a progressive-ratio 5 schedule of food delivery. Acute administrations of morphine produced general dose-dependent reductions in response rates and breaking points. Dose-response curves for both measures shifted rightward substantially (roughly fivefold) following chronic (daily) exposure to morphine, indicating that tolerance developed to the drug's effects.

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.

Differential effects of intrahypothalamic administration of opioids on food intake in naive and tolerant rats

We investigated the effects of intrahypothalamic administrations of the opioid agonists morphine (MOR) and ketocyclazocine (KCZ) and antagonists naltrexone (NALTX) and Mr2266 on food intake (FI) during light and dark phases of the diurnal cycle, after acute or chronic administration in rats. Acute intralateral hypothalamic (LH) administration of MOR or KCZ (1 microgram/rat) enhanced FI during dark and light phases, respectively, whereas intraventromedial hypothalamic (VMH) injections resulted in moderate hyperphagia during dark phases by both mu and kappa agonists. The receptor specificity was evident from blockade of the responses to MOR or KCZ by the respective antagonists NALTX and Mr2266. After repeated administrations of MOR and KCZ, FI responses to the test dose of these agonists injected in LH were modulated in opposite directions. However, the adaptative changes in FI after intra-VMH injection of KCZ were similar to those seen with MOR. These results are discussed in light of a differential opioid receptor involvement and their possible functional interactions within the hypothalamus during food intake.

Alterations in deprivation, glucoprivic and sucrose intake following general, mu and kappa opioid antagonists in the hypothalamic paraventricular nucleus of rats

While opioid agonists administered into the hypothalamic paraventricular nucleus increase food intake in rats, naloxone reduces deprivation-induced intake. Ventricular administration of either mu (beta-funaltrexamine) or kappa (nor-binaltorphamine) opioid antagonists reduces spontaneous, deprivation, glucoprivic and palatable intake. The present study assessed whether microinjections of either general, mu or kappa opioid antagonists into the paraventricular nucleus altered either deprivation (24 h) intake, 2-deoxy-D-glucose hyperphagia or sucrose intake in rats. Deprivation intake was significantly reduced by nor-binaltorphamine (5 micrograms, 68 nmol, 30-33%), beta-funaltrexamine (5 micrograms, 100 nmol, 26-29%) or naltrexone (10 micrograms, 260 nmol, 26%) in the paraventricular nucleus. 2-Deoxy-D-glucose hyperphagia was significantly reduced only after 2 h by naltrexone (10 micrograms, 260 nmol, 69%), norbinaltorphamine (20 micrograms, 272 nmol, 69%) or beta-funaltrexamine (20 micrograms, 400 nmol, 83%) in the paraventricular nucleus. Sucrose intake was significantly reduced by nor-binaltorphamine (5 micrograms, 68 nmol, 27-36%), naltrexone (5-10 micrograms, 130-260 nmol, 18-31%) and beta-funaltrexamine (5 micrograms, 100 nmol, 20%) in the paraventricular nucleus. These data indicate that general, mu and kappa opioid antagonists administered into the hypothalamic paraventricular nucleus produce similar patterns of effects upon different forms of food intake as did ventricular administration, implicating this nucleus as part of the circuitry underlying opioid mediation of ingestion.

Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

The mu, delta, and kappa opioid receptors are the three main types of opioid receptors found in the central nervous system (CNS) and periphery. These receptors and the peptides with which they interact are important in a number of physiological functions, including analgesia, respiration, and hormonal regulation. This study examines the expression of mu, delta, and kappa receptor mRNAs in the rat brain and spinal cord using in situ hybridization techniques. Tissue sections were hybridized with 35S-labeled cRNA probes to the rat mu (744-1,064 b), delta (304-1,287 b), and kappa (1,351-2,124 b) receptors. Each mRNA demonstrates a distinct anatomical distribution that corresponds well to known receptor binding distributions. Cells expressing mu receptor mRNA are localized in such regions as the olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, and dorsal root ganglia. Cellular localization of delta receptor mRNA varied from mu or kappa, with expression in such regions as the olfactory bulb, allo- and neocortex, caudate-putamen, nucleus accumbens, olfactory tubercle, ventromedial hypothalamus, hippocampus, amygdala, red nucleus, pontine nuclei, reticulotegmental nucleus, motor and spinal trigeminal, linear nucleus of the medulla, lateral reticular nucleus, spinal cord, and dorsal root ganglia. Cells expressing kappa receptor mRNA demonstrate a third pattern of expression, with cells localized in regions such as the claustrum, endopiriform nucleus, nucleus accumbens, olfactory tubercle, medial preoptic area, bed nucleus of the stria terminalis, amygdala, most hypothalamic nuclei, median eminence, infundibulum, substantia nigra, ventral tegmental area, raphe nuclei, paratrigeminal and spinal trigeminal, nucleus of the solitary tract, spinal cord, and dorsal root ganglia. These findings are discussed in relation to the physiological functions associated with the opioid receptors.

Butorphanol increases food-reinforced operant responding in satiated rats

In the present series of studies we examined the effect of butorphanol tartrate on food-reinforced operant responding in satiated rats. In the first experiment, 8.0 mg/kg butorphanol was administered subcutaneously, once per day for 4 days, to satiated rats responding under an fixed ratio 10 (FR 10) reinforcement schedule. In the second experiment, butorphanol (0, 0.3, 1.0, 3.0, 10.0 mg/kg) was administered to satiated rats responding under an FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule for 4 consecutive days. Repeated butorphanol administration increased total amount of food consumed over sessions in both experiments. Under the FR 80 schedule component, butorphanol initially increased latency to acquire the first pellet, an effect attenuated by repeated administration. Whereas vehicle administration was associated with consumption of relatively large quantities of food within the first 10 min of receiving the first pellet, butorphanol was associated with continued feeding as the session progressed. These data suggest that butorphanol-induced food intake is associated with maintenance rather than initiation of feeding.

Food deprivation history and cocaine self-administration: an animal model of binge eating

In this two-part study, an animal model of binge eating was first produced, then the rate of acquisition of cocaine self-administration was assessed. Initially, 16 female weanling rats were food deprived (DEPR) at 25, 95, and 143 days of age. Another group of 16 age-matched controls was allowed ad lib access to food. Each time the DEPR group was food deprived, they were allowed to recover to normal weight. They were then injected with butorphanol tartrate (BUTR), an opioid that stimulates feeding, and food intake was measured for 4 h. All rats given BUTR consumed significantly more food than those given saline. Animals with DEPR history consumed food over a longer period of time, and at h 4 after BUTR injection, they consumed significantly more food than controls. In the second part of the experiment, an autoshaping procedure was used to quantitatively evaluate the rate of acquisition of cocaine self-administration. By day 30, 86% of the DEPR and 69% of the control groups had acquired cocaine self-administration.

The role of multiple opioid receptors in the maintenance of stimulation-induced feeding

Feeding induced by lateral hypothalamic electrical stimulation is sensitive to opioid antagonism and has previously been blocked by naloxone and antibodies to dynorphin A fragments. In the present study, high affinity receptor-selective antagonists were used to determine the particular opioid receptor type(s) that mediates stimulation-induced feeding (SIF). Separate groups of rats were used to conduct i.c.v. dose-response studies with TCTAP (mu), naltrindole (delta) and norbinaltorphimine (kappa). TCTAP, at the highest dose tested (i.e. 5.0 nmol) and norbinaltorphimine, at doses of 10.0 and 50.0 nmol, increased the brain stimulation frequency threshold for eliciting SIF. Naltrindole, at doses up to 50.0 nmol, had no effect. Results of another study, recently conducted in this laboratory, indicate that the present doses of TCTAP and norbinaltorphimine have no effect on thresholds for lateral hypothalamic self-stimulation. This suggests that mu and kappa opioid activity are associated with feeding, rather than the eliciting brain stimulation, and excludes non-specific performance deficits as an explanation of elevated SIF thresholds. In the SIF test, where 5 determinations of threshold are obtained in serial order, naloxone characteristically increases thresholds toward the end of a test while conventional appetite suppressants increase thresholds uniformly throughout a test. TCTAP and norbinaltorphimine produced a 'naloxone-like' pattern of threshold elevation, suggesting that mu and kappa receptors are involved in the process whereby endogenous opioid activity sustains feeding once initiated.

The effect of selective opioid antagonists on butorphanol-induced feeding

Butorphanol tartrate (BT) potently stimulates food intake in satiated rats. The opioid receptor profile of BT is complex and is dependent upon the assay and animal species studied. In the present study we utilized three selective opioid antagonists; namely beta-funaltrexamine (beta-FNA), naltrindole (NTI) and norbinaltorphimine (nor-BNI), to probe the opioid receptor profile of BT as an orexigenic agent. Intracerebroventricular administration of nor-BNI (kappa) antagonized the feeding effects of BT (8 mg/kg, s.c.) at doses of 1, 10 and 100 nmol at the 1-2 h time point and decreased feeding at all time points for the 10 nmol dose. After 1 h, the 100 nmol dose of nor-BNI decreased BT-induced feeding by about 72%. In contrast, intraventricular injection of only the highest dose of the selective mu opioid antagonist, beta-FNA (50 nmol), decreased BT-induced feeding. Intraventricular administration of the delta opioid agonist, NTI, failed to alter BT-induced feeding at doses as high as 50 nmol. These data suggest that BT is dependent upon the kappa and perhaps the mu opioid receptors to increase food intake in satiated rats.

Reduction of food intake and morphine analgesia by central glybenclamide

Previous research has indicated the presence of a reciprocal relationship between food intake and opioid-mediated analgesia. We believe the cellular candidate most likely acting as a common mediator of both ingestive and nociceptive behaviors is the ATP-sensitive K+ channel (K+ATP). This ion channel appears to be opened by mu and delta 1 opioid receptor agonists in the service of analgesia, and closed as cellular ATP availability rises. To further examine the role of the K+ATP in the relationship between feeding and opioid function, we administered 80 nmol of glybenclamide (a K+ATP antagonist) to male SD rats via the lateral ventricle. Chow consumption in the treated animals was significantly reduced over the following 48 h (F = 2.62, p < 0.013), with the peak effect (78% of control) occurring at 6 h. In the tail-flick test, 4 mg/kg morphine sulfate provided analgesia of 42.38 +/- 8.4% and 18.89 +/- 7.67% in vehicle and treated animals, respectively (p < 0.05, n = 8/group, one-tailed t-test). These results support the hypothesis that food intake and analgesia are reciprocally modulated through activity at the K+ATP.

Involvement of mu-opioid receptors in alcohol drinking by alcohol-preferring AA rats

The present study examined the role of mu- and delta-opioid receptors in alcohol drinking using antagonists selective for these receptor types. Food- and water-sated male and female AA (Alko, alcohol) rats consistently drank 10% alcohol during daily 30-min access periods in their home cages in the middle of the 12-h light phase. On 3 consecutive days, the animals received the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP, 1 micrograms ICv), the delta-receptor antagonist N,N-diallyl-Try-Aib-Aib-Phe-Leu-OH (ICI 174,864, 3 micrograms ICV), or saline 15 min before the alcohol access period. Relative to saline, the mu-antagonist CTOP decreased alcohol drinking both by males and females progressively over the 3 treatment days, with a continued suppression on the first days after the termination of the administration. Treatment with the delta-antagonist ICI 174,864 had no effect on alcohol drinking in males, and produced transient hind limb dysfunction and barrel rolling in over half of the females. These results suggest that selective blockage of mu-opioid receptors is sufficient to suppress alcohol drinking in AA rats.

Opioid agonist properties of two oxime derivatives of naltrexone, NPC 831 and NPC 836

Two novel oxime derivatives of naltrexone, 6-[2-phenylethyl]-oximino naltrexone (NPC 831) and 6-[3-phenylpropyl]-oximino naltrexone (NPC 836) were potent agonists at opioid receptors. Both compounds inhibited binding to all three opioid receptor subtypes with nanomolar affinities. In vivo, NPC 831 and NPC 836 were equipotent to morphine and more potent than the kappa-selective agonist U-50,488H to produce analgesia. ED50 values of 4.02 mg/kg for NPC 831 and 2.24 mg/kg for NPC 836 were generated for inhibition of the tail-flick response in the rat, and ED50 values of 0.05 mg/kg for NPC 831 and 0.02 mg/kg for NPC 836 were calculated for inhibition of the writhing response in the mouse. Bombesin-induced scratching was used to evaluate NPC 831 and NPC 836 for kappa-agonist properties, and the A50, defined as the percent antagonism of the bombesin-induced response, was 1.86 mg/kg for NPC 831 and 0.08 mg/kg for NPC 836, compared to an A50 of 1.54 mg/kg for U-50,488H. These data suggest that NPC 831 and NPC 836 possess potent mu- and kappa-agonist properties in vivo, with NPC 836 being approximately twice as potent as NPC 831 to produce analgesia and 20 times as potent as NPC 831 to inhibit the scratching response produced by bombesin.

Free-feeding and free-drinking patterns of male rats following treatment with opiate kappa agonists

Three experiments investigated the effects of PD117302 and U50,488H on the patterns of food and water intake by male rats. Experiment 1 demonstrated early dose-related suppression of food and water intake after PD117302 (0, 1.25, 2.5, 5 mg/kg). The initial suppression of drinking was followed by a sustained increase 4-12 h after drug administration. Experiment 2 demonstrated that 2.5 mg/kg PD117302 failed to increase food intake whether given at the beginning of the night (high baseline food intake) or the beginning of the day (low baseline food intake). Experiment 3 showed that 0.5 mg/kg U50,488H significantly enhanced meal size but, at doses of 0.5, 1.0, and 2.0 mg/kg, had no effect on overall food intake. U50,488H also produced delayed, dose-related increases in water intake. The results suggest kappa receptors may have limited importance in modulating ad lib food intake and demonstrate the behavioural characteristics of increased drinking after excessive urine output.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Central opioid receptor subtype antagonists differentially alter sucrose and deprivation-induced water intake in rats

The present study compared the effectiveness of centrally-administered opioid receptor subtype antagonists to inhibit intake of either a 10% sucrose solution under ad libitum conditions, or water following 24 h of water deprivation. Full dose-response functions were evaluated over a 1 h period for the following antagonists: naltrexone (general: 1-50 micrograms), nor-binaltorphamine (Nor-BNI, kappa: 1-20 micrograms), beta-funaltrexamine (beta-FNA, mu: 1-20 micrograms), naltrindole (delta 2: 1-20 micrograms), [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1: 10-40 micrograms) and naloxonazine (mu 1: 10-50 micrograms). Naltrexone significantly and dose-dependently inhibited both sucrose intake (64-67%) and deprivation-induced water intake (53-67%). Nor-BNI significantly and dose-dependently inhibited sucrose intake (53-55%), but failed to significantly affect (28%) deprivation-induced water intake. beta-FNA significantly and dose-dependently inhibited both sucrose intake (31-34%) and deprivation-induced water intake (36-50%). Naltrindole failed to significantly alter either sucrose intake (24%) or deprivation-induced water intake (16%). Whereas DALCE significantly, but transiently (15-20 min) inhibited sucrose intake (28%), it failed to significantly alter deprivation-induced water intake (14%). Naloxonazine significantly, but transiently (5-10 min) stimulated sucrose intake at low doses (26%), but non-significantly reduced sucrose intake at higher doses (20%). Naloxonazine failed to significantly alter deprivation-induced water intake (16% reduction). These data indicate that whereas the kappa and mu 2 binding sites participate in the opioid modulation of sucrose intake, the mu 2 binding site participates in the opioid modulation of deprivation-induced water intake.

Endogenous opioids may be involved in idazoxan-induced food intake

In this study it has been shown that the unexpected increase in food consumption, produced by the alpha 2-adrenoceptor antagonist idazoxan (10 mg/kg, i.p.) in rats, was significantly attenuated by small doses of the opioid antagonist (-)-naloxone (0.1, 1 mg/kg, i.p.) and totally inhibited by a small dose of naltrexone (1 mg/kg, i.p.). On the other hand, idazoxan-induced feeding was not affected by (+)-naloxone (0.1, 1 mg/kg, i.p.), which is inactive at opioid receptors. In addition, idazoxan-induced food consumption was not blocked by the delta-opioid antagonist, naltrindole (0.1, 1 mg/kg, i.p.) nor by the mu/delta-antagonist, RX8008M (16-methyl cyprenorphine; 0.1, 1 mg/kg, i.p.), which clearly discriminates between mu/delta- and kappa-opioid receptor function in vivo. These findings suggest that idazoxan may lead to the release of endogenous opioid peptides, which subsequently stimulate feeding by activation of kappa-, as opposed to mu- or delta-opioid receptors. This response is unlikely to be due to alpha 2-adrenoceptor blockade, since other highly selective alpha 2-adrenoceptor antagonists do not increase food intake and, instead may reflect the high affinity of idazoxan for non-adrenoceptor idazoxan binding sites.

Naloxone benzoylhydrazone, a kappa 3 opioid agonist, stimulates food intake in rats

Naloxone benzoylhydrazone (NalBzoH) is a selective, short-acting agonist at the kappa 3 opioid receptor and a slowly dissociating potent antagonist at the mu opioid receptor. Given the important role of kappa receptors in the opioid control of food intake, the present study examined the central and peripheral effects of NalBzoH upon food intake. Central administration of NalBzoH (1-20 micrograms, i.c.v.) significantly increased food intake for up to 12 h, but failed to alter intake or body weight after 24 or 48 h. The 12 h duration of NalBzoH-mediated effects may be due to either persistent kappa 3 receptor occupancy, and/or activation of an ingestive system which maintains its activity. Peripheral administration of NalBzoH (20 mg/kg, s.c.) significantly increased food intake for up to 1 h. To distinguish kappa 1 (U50,488H) and kappa 3 (NalBzoH) hyperphagic effects, these agonist effects were compared following pretreatment with either naltrexone or the kappa 1 antagonist, nor-binaltorphamine (Nor-BNI). Whereas naltrexone significantly reduced both U50,488H and NalBzoH hyperphagia, Nor-BNI blocked U50,448H, but not NalBzoH hyperphagia. These data indicate a distinct role for the kappa 3 receptor in ingestive behavior separable from that of kappa 1 effects.

Beta-funaltrexamine (beta-FNA) decreases deprivation and opioid-induced feeding

We studied the effect of the mu antagonist, beta-funaltrexamine (beta-FNA) on deprivation and opioid-induced feeding. Intracerebroventricular pre-treatment of 20 h deprived rats with 0.1, 1, 10 and 20 nmol of beta-FNA decreased feeding by 24%, 50%, 50% and 38% during the first hour. Central administration of beta-FNA (0.1, 1 and 10 nmol) also decreased feeding induced by the mu opioid agonist, DAMGO by 57%, 60% and 71%. Feeding induced by the delta agonist, DSLET, was decreased by pre-treatment with beta-FNA; but only during the 1-2 h time points, a time when relatively little food was ingested. Intraventricular injection of beta-FNA failed to alter feeding stimulated by the kappa opioid agonist, U-50,488H. These data further substantiate a role for the opioid receptor in deprivation and opioid-induced feeding.

Effects of acute and chronic morphine on food intake in rats: modulation by oxytocin and vasopressin

The effects of acute and chronic morphine administration and the interaction with oxytocin and vasopressin on food intake response were investigated at various intervals during a 24-h schedule in rats. Acute morphine (5 mg/kg, IP) produced a generalized hyperphagic effect in both light (0-6 h) and dark (6-24 h) phases, the most marked effects being at 0-1 h, 1-3 h and 6-24 h. Chronic morphine (7 days) in an escalating dose schedule (5-35 mg/kg/day) produced (a) an enhancement of the hyperphagic effect in the light phase and (b) an attenuation of the food intake response during the dark phase. Neither oxytocin nor vasopressin had any significant influence on food intake, per se, after either acute or chronic administrations. However, both OXY and AVP reduced the hyperphagic response to acute morphine throughout the 24-h observation period. Further, on chronic administration, both neurohypophyseal peptides blocked the enhancements of morphine-induced hyperphagia (reverse tolerance) during light phase, whereas only vasopressin was effective in attenuating the reduction of hyperphagia (tolerance) during dark phase. These results are discussed in light of complex opiate-oxytocin/vasopressin interactions in the regulation of food intake.

Ingestive behavior following central [D-Ala2, Leu5, Cys6]-enkephalin (DALCE), a short-acting agonist and long-acting antagonist at the delta opioid receptor

DALCE (1-40 micrograms, ICV), a short-acting agonist and long-acting antagonist at the delta opioid receptor, was examined for its effects upon food intake in rats under spontaneous, deprivation, glucoprivic and palatable conditions. DALCE (10 micrograms) significantly stimulated free feeding for up to 10 h but only minimally decreased (40 micrograms) food intake and body weight after 24-72 h. DALCE, administered prior to food deprivation (24 h), failed to affect subsequent 24-h intake and sporadically decreased intake and body weight change after 48-72 h. 2-Deoxy-D-glucose (650 mg/kg, IP) hyperphagia was transiently (2 h) decreased by long-term DALCE (10 micrograms) pretreatment. Hyperphagia following exposure to a high-fat diet was significantly potentiated by long-term DALCE (1 microgram) pretreatment. DALCE (10 micrograms) hyperphagia (2-10 h) was eliminated by central pretreatment with either naltrexone (20 micrograms) or the kappa antagonist, nor-binaltorphamine (20 micrograms) but was minimally affected by central pretreatment with the mu antagonist, beta-funaltrexamine (20 micrograms) or long-term DALCE (40 micrograms). The general inability of the antagonist actions of DALCE to alter these forms of feeding argues against a role for the delta opioid receptor in these responses.

Nor-binaltorphimine decreases deprivation and opioid-induced feeding

We evaluated the effect of the kappa antagonist, nor-binaltorphimine (nor-BNI) on deprivation and opioid-induced feeding in rats. Intracerebroventricular administration of nor-BNI (100 nmol) decreased deprivation-induced feeding for as long as 24 h, albeit in a fairly weak manner (maximum decrease of approximately 28%). Nor-BNI (1, 10 and 100 nmol) decreased feeding induced by the kappa ligand U-50,488H by as much as 85% during the first hour of the study. This kappa antagonist also decreased feeding induced by the delta agonist DSLET and the mu agonist DAMGO. Based on previous studies indicating that nor-BNI is a selective kappa antagonist, we conclude that not only U-50,488H (kappa), but also DSLET (delta) and DAMGO (mu)-induced feeding are dependent upon an active kappa receptor.

Pharmacologic profile of NPC 168 (naltrexone phenyl oxime), a novel compound with activity at opioid receptors

NPC 168 (naltrexone phenyl oxime) was synthesized as a novel opioid antagonist and evaluated in several in vitro and in vivo assays. NPC 168 inhibited binding to the mu, delta and kappa subtypes of the opioid receptor with nanomolar potencies. The potency of NPC 168 to antagonize morphine-induced analgesia was slightly less than that of naltrexone and nalmefene following either intraperitoneal (ED50 = 0.07 mg/kg) or oral (ED50 = 0.82 mg/kg) administration. The duration of action of NPC 168 was approximately 8 hr following subcutaneous administration, compared to 4 hr for nalmefene, to antagonize oxymorphonazine-induced analgesia. The long duration of action of NPC 168 was substantiated by pharmacokinetic data that demonstrated rapid uptake and slow clearance of NPC 168 from brain. NPC 168 (5, 10 and 20 mg/kg) also inhibited cumulative 6-hr food intake in rats that were deprived of food for 24 hr, but chronic administration of this compound to rats over a three-week period resulted in a marginal reduction in cumulative body weight gain. NPC 168 at doses of up to 10 mg/kg did not produce a conditioned taste aversion. However, NPC 168 was slightly more toxic than either naltrexone or nalmefene when administered parenterally, and as toxic as nalmefene when administered by the oral route. These data demonstrate that NPC 168 is a novel opioid antagonist with a longer duration of action than either naltrexone or nalmefene.

The effect of transcutaneous electrical nerve stimulation (TENS) on catecholamine metabolism during pacing-induced angina pectoris and the influence of naloxone

Two invasive studies (invasive study I and invasive study II) showed positive effects of transcutaneous electrical nerve stimulation (TENS) in pacing-induced angina pectoris in terms of increased tolerance to pacing, improved lactate metabolism and less anginal pain. Invasive study I demonstrated a decrease in left ventricular afterload by TENS treatment as reflected by a fall in systolic blood pressure, and this fact was thought to be explained by reduced sympathetic activity since arterial levels of epinephrine and norepinephrine dropped during TENS in TENS responders. In invasive study II, the influence of naloxone on the effects of TENS in pacing-induced angina pectoris was studied in 11 patients with severe coronary artery disease. The patients were catheterized and treated with TENS on 2 occasions; one with a single intravenous (i.v.) dose of saline as placebo and one with a single i.v. dose of 50 mg naloxone, double-blind, in random order. Treatment with TENS increased tolerance to pacing (P less than 0.01 with placebo and P less than 0.01 with naloxone, respectively) and improved lactate metabolism (P less than 0.05 with placebo and P less than 0.01 with naloxone, respectively). The positive effects of TENS were thus reproducible and not reversed by single i.v. doses of naloxone. The results of this study indicate that the effects of TENS on the heart are not mediated by beta-endorphin but do not exclude activation of more short-acting opioids like delta or kappa receptor agonists (met-enkephalin and/or dynorphin) since naloxone has a low affinity for these receptors. It is also possible that non-opioid mechanisms are of importance.(ABSTRACT TRUNCATED AT 250 WORDS)

Opposite dopaminergic activity in lateral and median hypothalamic nuclei in relation to the feeding effect of D-Ser2-Leu-Enk-Thr6 (DSLET)

The Leu-enkephalin analogue D-Ser2-Leu-Enk-Thr6 (DSLET) had been shown to enhance feeding in rats, increase dopaminergic activity in the striatum like other opiate agonists, and particularly to decrease dopaminergic activity in the hypothalamus. In this study, the latter effect was found to be localized in the hypothalamic nuclei involved in the regulation of feeding such as the paraventricular (PVN), ventromedian (VMH), dorsomedian (DMH) nuclei and the lateral hypothalamus (LH). DSLET produced the same decrease in dopaminergic activity in the LH as in the whole hypothalamus. In the median nuclei (PVN and VMH and to a lesser extent in the DMH), an opposite effect was observed, resembling that in the striatum. The relevance of these opposite variations with regard to the feeding effect of DSLET is discussed. The decreased dopaminergic activity in the LH would appear to be the most specifically related to the behavioural effect given the known role of dopamine in this region. These data reconcile apparently contradictory aspects of the role of dopamine and the functional opposition between the lateral and median hypothalamus in food intake control.

Selective opioid receptor antagonist effects upon intake of a high-fat diet in rats

Short-term (2 h) intake of a high-fat diet in rats was significantly inhibited by intravenous (0.1-10 mg/kg: 39-67%) and central (1-5 micrograms, i.c.v.: 51%) naloxone. The irreversible mu opioid antagonist, beta-funaltrexamine (10 micrograms, i.c.v.: 37%), but not the irreversible mu 1 antagonist, naloxonazine (10 mg/kg, i.v.) inhibited intake, suggesting mu 2 receptor mediation. The delta antagonist, ICI 174864 (1-10 micrograms, i.c.v.: 41%) inhibited high-fat diet intake only at doses that also produced motor dysfunction.

Effects of a selective mu opioid receptor agonist and naloxone on the intake of sodium chloride solutions

Endogenous opioid peptides are thought to play a role in mediating the palatability or rewarding aspects of sweet tastes. There is also evidence, however, which suggests that opioids may influence the preference for the taste of salt as well. In the present studies, we measured the effects of central administration of naloxone and the mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on the ingestion of salt solutions. In non-deprived rats given a choice of water and 0.6% saline, ICV injections of DAGO (1 and 3 nmol) significantly increased the intake of 0.6% saline; baseline water intake was minimal and was unaffected by DAGO. When rats were given a choice between water and 1.7% saline, DAGO stimulated both water and saline intake. Because 1.7% saline is a hypertonic solution, the increase in water intake may have been secondary to saline intake. In rats on a deprivation schedule in which water and 0.6% saline were available for only 2-3 h/day, there was a tendency for DAGO to increase 0.6% saline intake and decrease water intake, though these effects were not significant. In rats given water and 1.7% saline, DAGO increased saline intake and had no effect on water intake. Naloxone was also tested in water-deprived rats. Naloxone (20 and 50 micrograms) significantly decreased 0.6% saline intake; baseline water intake was low (3-5 ml) and was unaffected by naloxone. When rats were given a choice between water and 1.7% saline, naloxone (50 micrograms) significantly reduced water intake, while intake of 1.7% saline was slightly increased. These results suggest a role for central mu opioid receptors in mediating the preference for salt solutions.

Effects of the kappa opioid receptor antagonist MR-2266-BS on the acquisition of ethanol preference

Using a paradigm by which rats forced to drink a weak ethanol solution (2.5% w/v) (conditioning session) develop ethanol preference in consecutive retention testing days, the effects of the administration of the kappa opioid antagonist MR-2266-BS, prior to or after the forced ethanol session, were studied. Pre-conditioning subcutaneous (s.c.) administration of 1 mg/kg of MR-2266-BS induced a decrease in subsequent ethanol consumption without significantly modifying the acquisition of ethanol preference. Post-conditioning administration of MR-2266-BS (0.1, 1, 5 or 10 mg/kg) induced both a dose-dependent reduction in ethanol consumption and in preference throughout the three following days. The results of the present study provide further support of the involvement of kappa-type opioids on drinking behavior, and suggest that kappa receptors may be involved in the consumption and development of preference to ethanol.

Effects of opiate agonists and an antagonist on food intake and brain neurotransmitters in normophagic and obese "cafeteria" rats

The relationship between the effects of opiates on food intake and on central monoamines in various brain areas was investigated in normophagic and obese "cafeteria" rats. Three agonists, beta-endorphin, dynorphin, and D-Ser2-Leu-Enk-Thr6 (DSLET) and an antagonist, naltrexone, were used. The three agonists enhanced feeling in normophagic rats but had different dopaminergic effects. Serotonergic metabolism increased concomitantly with the enhancement of feeding by the agonists, whereas it decreased following treatment with the antagonist naltrexone. In the cafeteria rats, although the feeding effects of dynorphin and DSLET occurred earlier, there was a complete lack of monoaminergic effects. beta-Endorphin was completely devoid of effects in this model. There would, thus, appear to be a positive correlation between the behavioural effects of these opiates and serotonergic metabolism in normophagic rats, while stimulated feeding situations ("cafeteria" rats) the disruption of a monoaminergic modulation does not prohibit a direct effect on feeding.

Endogenous opioid peptides in the control of food intake in cats

In this report, we investigated the role of exogenous and endogenous enkephalins on food intake in the cat, using, respectively, exogenous [D-Ala2-Met5]-enkephalin (DAME) and acetorphan (Ac) in order to inhibit the degradation of endogenous enkephalins. In addition, the selective peripheral antagonist naltrexone methylbromide (NTxMB) and the nonselective antagonist naloxone (Nx) were used in an attempt to discriminate central and peripheral opioid receptors. In 18-hours food-deprived animals, Ac (5 mg/kg IV) increased milk intake during sham feeding (+18%, p less than 0.05), but did not modify it in feeding conditions. Nx (1 mg/kg SC) reduced milk intake in sham-feeding experiments (-67%, p less than 0.01) more than in milk-feeding conditions (-30%, p less than 0.01). NTxMB (1 mg/kg SC) did not modify milk intake in sham-feeding but decreased it in feeding experiments. In nonfasted animals, Ac did not modify food intake. IV infusion of DAME (50 micrograms/kg) resulted in a reduction of daily food intake (-32%, p less than 0.01). Nx (1 mg/kg SC) decreased the earlier 30 min intake followed by reduction of daily intake (-30%, p less than 0.01). NTxMB (1 and 4 mg/kg SC) increased the 30-min intake dose dependently, without significant change in daily intake. In conclusion, Ac increases food intake in sham-feeding conditions, suggesting that endogenous enkephalins are likely to be involved in the stimulation of food intake. The effects of Nx and NTxMB furthermore suggest both a central activation, and a peripheral inhibition of food intake by opiates when food is allowed to proceed normally through the digestive tract.

Centrally administered opioid peptides stimulate saccharin intake in nondeprived rats

Endogenous opioid peptides are thought to play a role in mediating the pleasurable or rewarding aspects of the ingestion of certain foods and liquids. We therefore measured the effects of central administration of selective opioid agonists and naloxone on the intake of two concentrations of saccharin solution. All tests were performed on nondeprived rats, such that the taste of the solutions provided the primary incentive to consume. Intracerebroventricular (ICV) administration of the selective mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and the selective delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (3 nmol) increased intake of a 0.15% saccharin solution by approximately 10 ml over 3 hr. Water was available simultaneously, but intake was minimal. The selective kappa agonist U-50,488H did not increase intake of the saccharin solution. Naloxone (30 and 100 micrograms, ICV) caused a 44% reduction in saccharin solution intake in the first hour; two- and three-hour cumulative intakes were not different from control. DAGO and DTLET were also tested when rats were given a weaker saccharin solution (0.006%) along with water. Both agonists caused small increases in saccharin and water intake, but the increases above baseline were much smaller than those observed with the more palatable 0.15% saccharin solution. These results are consistent with reports by others which suggest that endogenous opioids influence taste preferences or palatability. Further, they indicate a role for central mu and delta opioid receptors in the mediation of this influence.

Influence of naloxone on the effects of high frequency transcutaneous electrical nerve stimulation in angina pectoris induced by atrial pacing

The influence of naloxone on the effects of high frequency transcutaneous electrical nerve stimulation in angina pectoris induced by atrial pacing was studied in 11 patients with severe coronary artery disease. The patients were catheterised and treated with transcutaneous electrical nerve stimulation on two occasions, double blind and in random order, with a single intravenous dose of saline or with a single intravenous dose of 50 mg naloxone. Treatment with transcutaneous electrical nerve stimulation increased tolerance to pacing and significantly improved lactate metabolism with placebo and with naloxone. The positive effects of transcutaneous electrical nerve stimulation were thus reproducible and not reversed by single intravenous doses of naloxone. The results indicate that the effects of transcutaneous electrical nerve stimulation on the heart are not mediated by beta endorphin but they do not exclude activation of more short-acting opioids such as delta or kappa receptor agonists (met-enkephalin or dynorphin or both) because naloxone has a low affinity for these receptors. Non-opioid mechanisms may also be important.

Inhibition of drinking by naltrexone in the rat: interaction with the dopamine D-1 antagonist SCH 23390 and the D-2 antagonist sulpiride

The involvement of dopamine receptors in water intake was investigated in the rat deprived of water for 24 hr. A 0.03 mg/kg dose of SCH 23390 markedly enhanced naltrexone (0.1 and 10.0 mg/kg)-induced hypodipsia, whilst the drug alone significantly decreased water intake at doses of 0.01 to 3.0 mg/kg, accompanied by marked motor dysfunction. Sulpiride (20.0 and 40.0 mg/kg) did not markedly affect water intake and naltrexone-induced hypodipsia. Consistent with previous results, apomorphine (0.3 mg/kg) alone was without marked effects, while it produced a marked potentiation of naltrexone (1.0 and 10.0 mg/kg)-induced hypodipsia. SCH 23390 (0.003 mg/kg) and sulpiride (40.0 mg/kg) completely antagonized the enhancing effects of apomorphine on naltrexone-induced hypodipsia. Similar effects were also seen in the latency to begin drinking. In contrast to the effects on naltrexone-induced hypodipsia, it appears that both dopamine D-1 and D-2 receptors play a key role in the effects of apomorphine on naltrexone-induced hypodipsia in the rat.