Suppression of deprivation-induced water intake in the rat by opioid antagonists: central sites of action

Role of Naltrexone in Improving Compulsive Drinking in Psychogenic Polydipsia

Psychogenic polydipsia or self-induced water intoxication is a potentially lethal condition seen in many chronic psychiatric patients. This is a literature review based on therapeutic significance of Naltrexone in improving compulsive water drinking behavior in chronic psychiatrically ill patients with psychogenic polydipsia. Naltrexone is an opioid antagonist approved by FDA for alcohol dependence. Extensive literature search provides a line of evidence that suggests correlation of opioid receptor with compulsive water ingestion in animals. However, there is limited data regarding clinical utility of naltrexone in improving psychogenic polydipsia in human species. This review highlights the necessity for further research and trials to elucidate the role of naltrexone in human psychogenic drinking behavior.

Intrinsic properties of the sodium sensor neurons in the rat median preoptic nucleus

The essential role of the median preoptic nucleus (MnPO) in the integration of chemosensory information associated with the hydromineral state of the rat relies on the presence of a unique population of sodium (Na+) sensor neurons. Little is known about the intrinsic properties of these neurons; therefore, we used whole cell recordings in acute brain slices to determine the electrical fingerprints of this specific neural population of rat MnPO. The data collected from a large sample of neurons (115) indicated that the Na+ sensor neurons represent a majority of the MnPO neurons in situ (83%). These neurons displayed great diversity in both firing patterns induced by transient depolarizing current steps and rectifying properties activated by hyperpolarizing current steps. This diversity of electrical properties was also present in non-Na+ sensor neurons. Subpopulations of Na+ sensor neurons could be distinguished, however, from the non-Na+ sensor neurons. The firing frequency was higher in Na+ sensor neurons, showing irregular spike discharges, and the amplitude of the time-dependent rectification was weaker in the Na+ sensor neurons than in non-Na+ sensor neurons. The diversity among the electrical properties of the MnPO neurons contrasts with the relative function homogeneity (Na+ sensing). However, this diversity might be correlated with a variety of direct synaptic connections linking the MnPO to different brain areas involved in various aspects of the restoration and conservation of the body fluid homeostasis.

Acute HPA axis response to naltrexone differs in female vs. male smokers

BACKGROUND

Both opioid antagonist administration and cigarette smoking acutely increase hypothalamic-pituitary-adrenal (HPA) axis activity as measured by adrenocorticotropic hormone (ACTH) and cortisol levels. However, male and female smokers may differ in their response to the opioid antagonist naltrexone, which may be partially mediated by sex differences in HPA axis function. Smokers, as a group, have frequently been shown to have HPA axis dysfunction, which may have relevance to the course and maintenance of nicotine dependence. The purpose of this study was to examine possible sex differences in HPA axis function by comparing stress-hormone response to naltrexone within healthy male and female smokers. Additionally, exploratory analyses compared the combined effects of naltrexone and cigarette smoking on hormonal responsivity between the sexes.

METHOD

Thirty-eight healthy smokers (22 men) were tested in two separate morning sessions after 12h of smoking abstinence. For women, self-reports of menstrual cycle information were obtained prior to each session (date of last menstruation, cycle length, reproductive phase, etc.). Each participant received 50mg naltrexone or placebo capsule (in random order) and plasma levels of ACTH and cortisol were assessed at regular intervals for several hours. A subgroup of 12 participants underwent a similar, additional session in which they smoked a single cigarette three hours after naltrexone administration.

RESULTS

Naltrexone significantly increased ACTH and cortisol levels in women, but not men (DrugxSexxTime, p<0.05). A post hoc analysis suggested that women at an estimated 'high estrogen' phase had a greater cortisol response (DrugxEstrogen level, p<0.05) than those at an estimated 'low estrogen' phase. Exploratory analyses showed that smoking a single cigarette potentiated naltrexone-induced increases in ACTH (p<0.05) and cortisol (p<0.01) in all participants.

CONCLUSION

The findings support the hypothesis that women are more sensitive to opioid antagonism at the level of the HPA axis. Although further studies are needed to examine mechanisms underlying these responses, both results may have clinical implications for the use of naltrexone as a treatment for nicotine dependence.

Postsynaptic mu-opioid receptor response in the median preoptic nucleus is altered by a systemic sodium challenge in rats

The median preoptic nucleus (MnPO) is an integrator site for the chemosensory and neural signals induced by a perturbation in the hydromineral balance, and it is highly involved in controlling fluid and electrolyte ingestion. Here, we hypothesize that opioid peptides, previously recognized to control ingestive behaviors, may regulate the excitability of MnPO neurons and that this regulatory action may depend on the natriuric (Na(+)) status of body fluid compartments. Our results show that activation of mu-, but not delta-, opioid receptors (OR) triggered a membrane hyperpolarization by recruiting a G-protein-regulated inward-rectifier K(+) (GIRK) conductance in 41% of the neurons tested. Interestingly, 24 h Na(+) depletion strengthened this opioid-mediated control of neuronal excitability. In Na(+)-depleted animals, the neuronal population displaying the mu-OR-induced hyperpolarization expanded to 60% (Z-test, P = 0.012), whereas Na(+) repletion restored this population to the control level (39%; Z-test, P = 0.037). Among the neurons displaying mu-OR-induced hyperpolarization, Na(+) depletion specifically increased the neuronal population responsive to variation in ambient Na(+) (from 27% to 43%; Z-test, P = 0.029). In contrast, Na(+) repletion dramatically reduced the population that was unresponsive to Na(+) (from 17% to 3%; Z-test, P = 0.031). Neither the basic properties of the neurons nor the characteristics of the mu-OR-induced response were altered by the body Na(+) challenge. Our results indicate that an episode of Na(+) depletion/Na(+) repletion modifies the organization of the opioid-sensitive network of the MnPO. Such network plasticity might be related to the avid salt ingestion triggered by repeated Na(+) depletion.

Enhanced sensitivity to naltrexone-induced drinking suppression of fluid intake and sucrose consumption in maternally separated rats

Early-life stress has been identified as a risk factor in the development of a host of disorders, including substance abuse; however the link between early postnatal stress and changes in measures of reward has not been thoroughly researched. The current study had two main objectives: 1) to determine the impact of maternal separation (an animal model of early-life stress) on the consumption of 10% and 2.5% sucrose solutions by Long-Evans rat dams and male and female offspring, and 2) to determine the effect of the opioid antagonist naltrexone (0.1-3.0 mg/kg) on drinking by each of those groups. Dam-pup separations occurred for varying lengths of time during the first two postnatal weeks. In Experiment 1, a two-bottle choice test (sucrose solution vs. water) was administered across five days to both nonhandled (NH) and maternally-separated (MS) offspring as adults and to dams 2-4 weeks post-weaning. In Experiment 2, naltrexone was administered prior to two-bottle choice tests. MS males and the dams of MS litters exhibited increased intake of total fluid and sucrose solutions, whereas results from females were less consistent. Naltrexone elicited a greater decrease in fluid intake and sucrose intake in male MS offspring compared to male NH offspring. These results indicate that early postnatal stress alters both sucrose consumption, a non-drug measure of reward, and apparently the brain opioid systems that mediate naltrexone-induced drinking suppression.

Reinforcing properties of ethanol in neonatal rats: involvement of the opioid system

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanol's gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanol's reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

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.

Evidence for a mu-opioid-opioid connection between the paraventricular nucleus and ventral tegmental area in the rat

The paraventricular nucleus (PVN) and the ventral tegmental area (VTA) have been shown to be involved in opioid mediated feeding behavior. The present study examined whether mu-opioid signalling between the PVN and VTA affected feeding behavior. Male Sprague-Dawley rats were cannulated with one cannula placed in the PVN and two cannulae placed in the VTA, which allowed for co-administration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Bilateral administration of DAMGO (1.2, 2.4 and 4.9 nmol) into the VTA stimulated feeding dose dependently at 2.4 and 4.9 nmol (P<0.05). The DAMGO (2.4 nmol)-induced increase of food intake following injection into the PVN was blocked by bilateral injection of NTX (6.6, 13.2 and 26.5 nmol) into the VTA at 2 and 4 h (P<0.05). In the reverse situation, the DAMGO (2.4 nmol)-induced increase of food intake following injection into the VTA was blocked by injection of NTX (13.2 and 26.5 nmol) into the PVN at 2 and 4 h (P<0.05). The present study suggests that a bidirectional mu-opioid-opioid signalling pathway exists between the PVN and the VTA which influences feeding.

Opioid stimulation in the ventral tegmental area facilitates the onset of maternal behavior in rats

This research investigated the effect of an increase or decrease in opioid activity in the ventral tegmental area (VTA) on the onset of maternal behavior in rats. In Experiment 1, the latency to show maternal behavior toward foster rat pups (sensitization latency) was determined in maternally naive female rats given either nothing or a unilateral intra-VTA injection of morphine sulfate (MS) (0.0, 0.01, 0.03, 0.1 or 0.3 microgram), on the first three days of a 10-day period of constant exposure to pups. Rats treated with 0.03 microgram MS had significantly shorter sensitization latencies than did rats treated with 0.0 microgram MS, 0.01 microgram MS, or receiving no treatment (higher doses of morphine produced intermediate results). The facilitating effect of intra-VTA MS on the onset of maternal behavior was blocked by pretreatment with naltrexone hydrochloride and was found to have a specific site of action in the VTA (MS injections dorsal to the VTA were ineffective). In Experiment 2, sensitization latencies were determined in periparturitional rats given a bilateral intra-VTA injection of either the opioid antagonist naltrexone methobromide (quaternary naltrexone), its vehicle, a sham injection, or left untreated 40 min after delivery of the last pup. The mothers' own pups were removed at delivery; mothers were nonmaternal at the time of testing. Quaternary naltrexone treatment produced significantly slower sensitization to foster pups than did control conditions. Total activity and pup-directed activity did not differ significantly with treatment. The results demonstrate that increased opioid activity in the VTA facilitates the onset of maternal behavior in inexperienced nonpregnant female rats, and decreased opioid activity in the VTA disrupts the rapid onset of maternal behavior at parturition.

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.

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.

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

Opioid receptor subtype antagonists differentially alter different types of water intake such that mu2 receptors modulate deprivation-induced water intake, kappa receptors modulate hypertonic saline-induced water intake, and mu2, delta1 and kappa receptors modulate water intake following Angiotensin II (ANG II). Water intake stimulated by peripheral administration of the beta-adrenergic agonist, isoproterenol is attenuated by naloxone and is thought to be mediated by release of renin and production of ANG II. The present study examined whether systemic and i.c.v. administration of general opioid antagonists and central administration of specific opioid receptor subtype antagonists would selectively alter water intake following isoproterenol in rats. Both systemic (1 mg/kg s.c.) and central (1-20 micrograms) naltrexone reduced water intake induced by isoproterenol (25 micrograms/kg s.c.) over a 2-h period. The mu receptor antagonist, beta-funaltrexamine (B-FNA: 1-20 micrograms), but not the mu1 antagonist, naloxonazine (50 micrograms), dose-dependently reduced isoproterenol drinking. Both the kappa antagonist, nor-binaltorphamine (Nor-BNI, 5-20 micrograms) and the delta1 antagonist, [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 1-40 micrograms) also dose-dependently reduced isoproterenol drinking. These data implicate mu2, kappa and delta1 sites in the opioid modulation of isoproterenol drinking.

Differential modulation of angiotensin II and hypertonic saline-induced drinking by opioid receptor subtype antagonists in rats

Opioid modulation of ingestion includes general opioid antagonism of different forms of water intake, mu 2 receptor modulation of deprivation-induced water intake and delta 2 receptor modulation of saccharin intake. Water intake is stimulated by both central administration of angiotensin II (ANG II) and peripheral administration of a hypertonic saline solution; both responses are reduced by general opioid antagonists. The present study examined whether specific opioid receptor subtype antagonists would selectively alter each form of water intake in rats. Whereas systemic naltrexone (0.1-2.5 mg/kg, s.c.) reduced water intake induced by either peripheral ANGII (500 micrograms/kg, s.c.) or hypertonic saline (3 ml/kg, 10%), intracerebroventricular (i.c.v.) naltrexone (1-50 micrograms) only inhibited central ANGII (20 ng)-induced hyperdipsia. Both forms of drinking were significantly and dose-dependently inhibited by the selective kappa antagonist, nor-binaltorphamine (Nor-BNI, 1-20 micrograms). Whereas both forms of drinking were transiently reduced by the mu-selective antagonist, beta-funaltrexamine (beta-FNA, 1-20 micrograms), the mu 1 antagonist, naloxonazine (40 micrograms) stimulated drinking following hypertonic saline. The delta 1 antagonist, [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 1-40 micrograms) significantly reduced drinking following ANGII, but not following hypertonic saline; the delta antagonist, naltrindole failed to exert significant effects. These data indicate that whereas kappa opioid binding sites modulate hyperdipsia following hypertonic saline, mu 2, delta 1, and kappa opioid binding sites modulate hyperdipsia following ANGII. The mu 1 opioid binding site may normally act to inhibit drinking following saline.

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

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

Central opioid receptor subtype antagonists differentially reduce intake of saccharin and maltose dextrin solutions in rats

Opioid modulation of ingestion includes general opioid antagonism of deprivation-induced water intake and intake of sucrose and saccharin solutions. Previous studies using selective subtype antagonists indicated that opioid effects upon deprivation-induced water intake occurred through the mu2 receptor and that opioid effects upon sucrose intake occurred through kappa and mu2 receptors. The present study compared the effects of intracerebroventricular administration of opioid receptor subtype antagonists upon intakes of a saccharin solution and a maltose dextrin (MD) solution to determine which receptor subtypes were involved in modulation of ingestion of different preferred tastants. Significant reductions in saccharin intake (1 h) occurred following naltrexone (20-50 micrograms: 66%) and naltrindole (delta, 20 micrograms: 75%), whereas [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1, 40 micrograms: 45%) had transient (5 min) effects. Neither beta-funaltrexamine (B-FNA, mu), naloxonazine (mu1), nor nor-binaltorphamine (Nor-BNI, kappa) significantly altered saccharin intake. Significant reductions in MD intake (1 h) occurred following naltrexone (5-50 micrograms: 69%) and B-FNA (1-20 micrograms: 38%). MD intake was not reduced by naltrindole, DALCE, naloxonazine and Nor-BNI. Peak antagonist effects were delayed (20-25 min) to reflect interference with the maintenance, rather than the initiation of saccharin or MD intake. Comparisons of opioid antagonist effects across intake situations revealed that naltrexone had consistently low ID40 values for saccharin (29 nmol), MD (25 nmol), sucrose (6 nmol) and deprivation (38 nmol) intake. Despite its significant effects relative to naloxonazine, B-FNA had significantly higher ID40 values for saccharin (800 nmol), MD (763 nmol) and sucrose (508 nmol) relative to deprivation (99 nmol) intake, suggesting that mu2 receptors may be mediating maintenance of intake rather than taste effects. Nor-BNI had low ID40 values for intake of sucrose (4 nmol), but not for saccharin (168 nmol), MD (153 nmol) and deprivation (176 nmol), suggesting that kappa receptors may mediate ingestion of sweet-tasting stimuli. That delta (naltrindole: ID40 = 60 nmol), but not delta 1 (DALCE: ID40 = 288 nmol) antagonists consistently reduce saccharin intake suggests a role for the delta 2 receptor subtype in the modulation of hedonic orosensory signals.

Effects of the opiates on the paraventricular nucleus in genetically polydipsic mice

The inbred mice, STR/N, are known to possess extreme polydipsia with no known abnormality in vasopressin system and the kidney function. Our previous studies indicate that the opiate antagonists given intracerebroventricularly strongly attenuated spontaneous drinking. To determine the site(s) of action, the present study was undertaken. Microinjections of naltrexone methobromide and a selective kappa-receptor antagonist, nor-binaltorphimine (nor-BNI), into the paraventricular nucleus of the hypothalamus (PVN) greatly attenuated drinking of the STR/N for 0.5 to 16 h after injections, while in the two control groups, non-polydipsic STR/1N and Swiss/Webster strains, drinking was not affected by the injections. Food intake was not much altered in all groups. Studies of PVN neurons in vitro (n = > 160 for each group) showed that basal firing rates and patterns were similar in the STR/N and the control groups. Morphine added to the medium inhibited some but excited none in all strains tested. The threshold for the inhibitory action was higher in the polydipsic STR/N mice (10(-8) M), compared to that in the control, S/W mice (10(-9) M). Further, a proportion of neurons inhibited by morphine in the PVN was significantly smaller (P < 0.01) in the STR/N (41.7%), compared to that in the control (64.9%). Dynorphin had very similar effect to that of morphine, but the proportion of cells inhibited was 25.4% in the STR/N, and 70.4% in the S/W. Prior applications of naloxone to the medium prevented the action of both morphine and dynorphin. Under the synaptic blockade (in a low Ca2+ and high Mg2+ medium) the inhibitory effect of the opiates persisted. We concluded that the PVN is at least one of the possible sites where the opiates are acting to cause the polydipsia in the STR/N mice.

Opioid modulation of thermal dehydration-induced thirst in rats

Male Sprague-Dawley rats were utilized to study the effects of the opioid receptor antagonists, naloxone and naltrexone, on thirst induced by thermal dehydration. In an initial experiment, the depressant effect of naloxone (1.0 mg/kg, IP) on the water intake of rats deprived of water for 24 h was confirmed. In subsequent experiments, rats were thermally dehydrated by exposing them without water to a 40 degrees C environment for 1-4 h. Following heat exposure, rats were injected with either naloxone or naltrexone either IP or ICV. Fifteen minutes later, rats were provided with water and water intake was measured for 2 h. Both naloxone and naltrexone had dose (0.1-5.0 mg/kg, IP)-dependent effects of reducing water intake of rats thermally dehydrated for 3 h. Water intake of rats thermally dehydrated for 2 or 4 h was also attenuated by pretreatment with naloxone. Rats thermally dehydrated for 3 h exhibited decreases in water intake following ICV injection of either naloxone or naltrexone at a dose of 50 micrograms. Neither naloxone nor naltrexone had an effect on urine output in any experiment. The water intake data support the hypothesis that thirst induced by thermal dehydration in rats is modulated by an opioid mechanism.

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.

Intracerebroventricular naltrexone treatment attenuates acquisition of intravenous cocaine self-administration in rats

The influence of centrally administered naltrexone, an opiate antagonist, on acquisition of intravenous cocaine self-administration behaviour in rats was examined. On five consecutive days, three hours per day, they could self-administer a cocaine solution (30 micrograms per infusion) through an indwelling cannula. Treatment consisted of daily injections of naltrexone (2 or 5 micrograms) or placebo into the lateral ventricle 30 minutes before testing. Naltrexone treatment dose dependently attenuated the rate of cocaine self-infusion. Both self-infusion rate and rate of responding on the reinforcement lever in the group treated with 5 micrograms naltrexone differed from placebo, whereas rate of responding on a dummy lever did not. These findings a) support the notion that opioid systems play a role in cocaine reinforcement, and b) suggest that naltrexone exerts its effect on cocaine reinforcement through action in the central nervous system.

Kappa-opioid antagonist strongly attenuates drinking of genetically polydipsic mice

Effects of opioid antagonists on the genetic polydipsia of the STR/N strain of mice were investigated. Naltrexone (0.5-5.0 mg/kg) injected subcutaneously before dark period attenuated spontaneous drinking for the first 3 h after injection only in the inbred polydipsic mice (STR/N), whose water intake was 5 times that of controls (non-polydipsic mutant, STR/1N, and Swiss/Webster mice). The highest dose (5 mg/kg) of naltrexone administration reduced drinking also during the next 3-6 h period and overnight feeding. Cerebroventricular (i.c.v.) injection of naltrexone, 1.0 and 2.5 micrograms (per mouse), suppressed drinking only in the polydipsic mice, while the higher dose (5.0 micrograms) attenuated drinking and feeding of both the polydipsic mice and their controls. However, i.c.v. injection of specific kappa-receptor antagonist, nor-binaltorphimine (nor-BNI, 0.5-2.5 micrograms), suppressed drinking only in the polydipsic strain of mice at one-half dose of that needed for naltrexone. Furthermore, even a higher dose of nor-BNI administration was without effect on food intake in all strains. These findings suggest that the central opioid system plays an important role in causing the polydipsia in the STR/N mice, probably through the kappa-opioid receptor.

Centrally administered opioid antagonists, nor-binaltorphimine, 16-methyl cyprenorphine and MR2266, suppress intake of a sweet solution

Three opioid antagonists (MR2266, 16-methyl cyprenorphine and nor-binaltorphimine) were tested independently for their ability to suppress the intake of a highly palatable saccharin and glucose (S/G) solution after central administration. MR2266 is an equally potent antagonist at kappa (kappa) and mu (mu) opioid receptors. Nor-binaltorphimine (N-BNI) and 16-methyl cyprenorphine (M80) are two recently developed opioid antagonists that were chosen based upon their ability to act more selectively than naloxone at kappa and delta (delta) opioid receptor types, respectively. Prior research has demonstrated that when dissolved in acid and administered centrally, MR2266 (20 micrograms) fails to suppress S/G intake. Because all three antagonists are rather insoluble in water, they were dissolved in dimethyl sulfoxide (DMSO). Rats with chronic ventricular cannula were allowed to consume S/G for a 0.5 hr bout. They received a single intracerebroventricular (ICV) injection of antagonist (MR2266: 0, 10, 20 and 40 micrograms; M80: 0, 5, 10, 20 and 40 micrograms or N-BNI: 0, 1, 3, and 10 micrograms) 10 min prior to the start of the drinking bout. Administration of DMSO alone failed to alter drinking relative to saline, whereas each antagonist significantly attenuated S/G intake. We conclude that, when dissolved in DMSO, these antagonists suppress drinking by blockade of opioid receptors.

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.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

The opioid antagonist, MR2266, specifically decreases saline intake in the mouse

The effects of the opioid antagonist, Mr2266 [(-)-(1R,5R,9R)-5,9-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomo rph an] on the intake of water and saline (0.9%) were investigated in the mouse, deprived of water for 24 hr. In an attempt to evaluate motor functions, the behavior after treatment with Mr2266 was also examined by using multi-dimensional behavioral analyses. Although smaller doses (1.0, 3.0 and 10.0 mg/kg) of Mr2266 failed to affect significantly the intake of water, a larger dose (30.0 mg/kg) elicited a significant attenuation in the intake of water. During a 30 min observation, Mr2266 (30.0 mg/kg) depressed markedly linear locomotion, while other behavioral responses, such as rearing and grooming, remained unchanged. In contrast, 1.0-30.0 mg/kg doses of the drug produced a significant reduction in the intake of saline. The drug Mr2266 had no significant effects on the latency to start drinking at any doses tested. These results suggest that Mr2266 specifically blocks the intake of saline of the mouse through the mediation of opioid systems.

Effects of intrahypothalamic administration of opioid peptides selective for mu-, kappa, and delta-receptors on different schedules of water intake in the rat

The effects of opioid peptides selective for mu, kappa, and delta-opioid receptors were investigated on 3 different schedules of water intake in the rat: spontaneous, deprivational (12 h), and hypertonic saline-induced drinking. Peptides were injected into the paraventricular and supraoptic hypothalamic nuclei, D-Ala2-NMePhe4-Gly(ol)-enkephalin, a mu-selective opioid agonist, tended to increase water intake in non-deprived rats, but 0.01 and 0.1 microgram significantly decreased water intake for 45 min in deprived rats, and for up to 60 min in hypertonic saline-injected rats when injected into the paraventricular hypothalamic nucleus. The kappa-selective agonist, dynorphin A1-13 (0.1, 0.3, 1.0 and 3.0 micrograms)and the delta-selective agonist, [D-Pen2,L-Pen5]enkephalin (0.3 and 3.0 micrograms) did not affect spontaneous, deprivational or hypertonic saline-induced water intakes when injected into either the paraventricular or supraoptic hypothalamic nuclei. Thus, a mu-selective opioid peptide produced dose- and time-dependent effects on drinking that were pharmacologically and anatomically specific, and dependent upon the schedule of water intake.

Effects of beta-funaltrexamine on ingestive behaviors in the rat

The effects of beta-funaltrexamine (beta-FNA), an irreversible mu-opioid receptor antagonist, were determined on water and food intake of non-deprived rats. Intracerebroventricular administration of 1.25 or 2.5 micrograms of beta-FNA did not affect drinking or eating. However, 5.0 micrograms first transiently increased food intake and then reduced both water and food intake for at least 72 h. Locomotor activity was unaffected by 5.0 micrograms of beta-FNA; thus, changes in ingestive behavior were not a secondary consequence of drug-induced behavioral stimulation or depression. The early increase in food intake may be due to the short lived and reversible kappa-agonist activity of beta-FNA. On the other hand, selective blockade of mu-opioid receptors appears sufficient to reduce the intake of water and food.

Apomorphine markedly potentiates naltrexone-induced hypodipsia in the rat

This article describes the effects of apomorphine on naltrexone-induced decreases in water intake of the rat deprived of water for 24 h. Apomorphine alone at reasonable doses (0.03, 0.1, 0.3 and 1.0 mg/kg) failed to affect water intake of the rat, but a higher dose (3.0 mg/kg) abolished water intake completely, accompanied by marked stereotypy. Naltrexone (0.1, 1.0 and 10.0 mg/kg) produced a dose-dependent reduction in water intake. A 0.3-mg/kg dose of apomorphine which is considered to activate preferentially presynaptic dopamine autoreceptors enhanced markedly naltrexone (1.0 and 10.0 mg/kg)-induced decreases in water intake. Only apomorphine at 1.0 mg/kg caused a significant prolongation of the latency to start drinking. Apomorphine (0.3 mg/kg), naltrexone (0.1, 1.0 and 10.0 mg/kg) or their combinations did not produce a marked effect on locomotor activity in the rat. These results suggest that apomorphine is capable of potentiating naltrexone-induced decreases in water intake through the mediation of presynaptic dopamine autoreceptors without causing motor dysfunction.

Antagonist treatment in nucleus accumbens or periaqueductal grey affects heroin self-administration

The role of opiate receptors in the periaqueductal grey and nucleus accumbens in maintenance of intravenous heroin self-administration was examined by means of intracranial microinjections of the quaternary opiate antagonist methyl naltrexone. Over a dose range of 0-3.0 micrograms, pre-session infusions of methyl naltrexone in either brain site produced dose-related increases in responding for heroin (0.06 mg/kg/infusion) on a CRF schedule, without causing significant changes in responding on a second activity control lever. Involvement of the periaqueductal grey was also examined in animals administering a lower heroin dose (0.03 mg/kg/infusion) in shorter sessions in order to minimize drug exposure prior to treatment. In this experiment, infusion of methyl naltrexone produced selective increases in responding for heroin, whereas treatment with the identical dose of methyl naltrexone had no effect on cocaine self-administration (1.0 mg/kg/infusion) in the same animals. With respect to the nucleus accumbens, these data confirm its involvement in opiate self-administration. Data for the periaqueductal grey provide the first evidence that opiate receptors in the vicinity of this brain region may play a role in intravenous opiate self-administration.

Opiate antagonists and rewarding brain stimulation

This review examines the literature on the effects of opiate antagonists on brain stimulation (ICSS) reward. Antagonists should have predictable effects if endogenous opioids modulate ICSS. Naloxone is the antagonist most often used, and it has produced inconsistent results in some ICSS paradigms. When schedules of intermittent reinforcement are used, however, naloxone reliably reduces the rate of responding. It reverses the effects of opiate agonists on ICSS behavior, and it also attenuates the effects of psychomotor stimulants, such as amphetamine. The results produced by naloxone are consistent with a modulatory effect of endogenous opioid systems on reward, and suggest that the opiate and dopamine systems together exert significant control over ICSS. Further research is needed to characterize better the actions of the antagonists on ICSS behavior, and productive research directions are proposed. Data obtained in future studies might suggest how the endogenous opioid systems modulate both natural and brain stimulation reward.