Narcotic antagonists attenuate drinking induced by water deprivation in a primate

Economic choice between remifentanil and food in squirrel monkeys

Traditional approaches for evaluating if compounds are reinforcing, and thus a risk for abuse, include preclinical self-administration procedures conducted in the absence of alternative reinforcers. While the track record of this approach for determining abuse potential is good, that for predicting efficacy of addiction treatments is not. An alternate approach would be economic choice between drug and nondrug rewards, with parametrically varied options from trial to trial. This would promote goal-directed decisions between reward modalities and should provide metrics that reflect changes in internal state that influence desirability of a given option. We report herein a high throughput economic choice procedure in which squirrel monkeys choose between a short-lived opiate, remifentanil, and a palatable food reward. Stimuli on touchscreens indicate the amount of each reward type offered by varying the number of reward-specific elements. The rapid clearance of remifentanil avoids accumulation of confounding levels of drug, and permits a large number of trials with a wide range of offers of each reward modality. The use of a single metric encompassing multiple values of each reward type within a session enables estimation of indifference values using logistic regression. This indifference value is sensitive to reward devaluation within each reward domain, and is therefore a useful metric for determining shifts in reward preference, as shown with satiation and pharmacological treatment approaches.

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.

Diprenorphine as a stimulus in drug discrimination learning

Using the conditioned taste aversion baseline of drug discrimination learning, animals were trained to discriminate diprenorphine from distilled water. In subsequent generalization tests, the opiate antagonists naltrexone and naloxone and the mixed opiate agonist/antagonist nalorphine substituted for the diprenorphine stimulus in a dose-dependent manner, while the opiate agonist morphine and the nonopiate pentobarbital failed to substitute even at the highest doses tested. That a range of opiate antagonists substituted for the diprenorphine stimulus (and an opiate agonist and a nonopiate failed to substitute) suggest that diprenorphine's antagonist properties may mediate the discrimination, presumably by blocking endogenous opiate activity. The ability of these drugs to substitute for the diprenorphine stimulus may also be a function of this receptor activity. The differences in the specific generalization patterns reported in the present assessment and those of earlier reports were discussed.

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.

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.

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.

Differential effects of CGS 8216 and naltrexone on ingestional behaviour

Effects of the pyrazoloquinoline CGS 8216 (a partial benzodiazepine receptor inverse agonist) and the opiate antagonist, naltrexone, were compared in several tests of ingestion in non-deprived and deprived male rats. Both naltrexone (0.1-10.0 mg/kg, SC) and CGS 8216 (1.25-10.0 mg/kg, IP) significantly reduced the consumption of a highly palatable saccharin-glucose solution by non-deprived rats. Both compounds were also effective in reducing, dose-dependently, the intake of palatable sweet or oily mash by non-deprived animals. Hence, naltrexone and CGS 8216 attenuated palatability-induced ingestional responses, and sweet taste was not necessary for this effect to occur. The two drugs also reduced the intake of the saccharin-glucose solution in food-deprived rats, but their effects diverged in water-deprived animals. CGS 8216 had relatively little effect in the thirsty animals, whereas the effect of naltrexone was enhanced. This difference was underscored in a final test of deprivation-induced consumption of water. Naltrexone reduced the drinking, but CGS 8216 had no effect. Taken together, these data indicate that CGS 8216 was more selective in its effects on ingestion.

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

The effects of naltrexone methobromide, a quaternary derivative of the opioid antagonist naltrexone, were investigated on deprivation (24 h)-induced water intake in the unilaterally cannulated rats. Naltrexone methobromide reduced post-deprivational water intake with an ED50 of 7.3 micrograms when tested at 30 min (peak effect) after intracerebroventricular administration. It also dose-dependently (0.3-10 micrograms) depressed water intake, with peak effects at 15 min, after microinjection into the paraventricular hypothalamic nucleus and into the supraoptic hypothalamic nucleus. The drug did not produce any other effects on behaviors. The ED50S were 1.4 micrograms when given into the paraventricular nucleus, and 3.3 micrograms when given into the supraoptic nucleus, respectively. Although injections of higher doses (1.0, 3.0 and/or 10 micrograms) of the drug into the preoptic area, zona incerta, and corpus callosum significantly suppressed water intake, other behavioral manifestations, such as rotational behaviors, convulsions, body shakes, head swaying, and/or backward locomotion were manifested simultaneously with the reduction in drinking. When injected into the lateral hypothalamic area, water intake was not significantly affected by the drug. These findings suggest that the paraventricular and supraoptic hypothalamic nuclei are important sites of action in the naltrexone-induced suppression of water intake.

Adrenalectomy and the anorectic effects of benzodiazepine inverse agonists and opiate antagonists in rats fed a palatable diet

The benzodiazepine (BZ) receptor inverse agonists, FG 7142 (1.25-10.0 mg/kg, IP) and CGS 8216 (2.5-20.0 mg/kg, IP), significantly attenuated the consumption of a palatable sweetened diet by non-deprived male rats in a 30 min test. Adrenalectomy failed to affect the reduction in food intake produced by these two drugs. Similarly, the anorectic effects of the opiate antagonists, naltrexone (0.3-3.0 mg/kg, SC) and diprenorphine (0.3-3.0 mg/kg, SC) in the same feeding paradigm were unaffected by adrenalectomy. So far as palatability-induced feeding in concerned, anorectic effects of BZ inverse agonists and opiate-antagonists appear to be adrenal-independent in the rat. The benzodiazepines, clonazepam (0.3 mg/kg, IP) and diazepam (1.0 mg/kg, IP), stimulated food consumption in both adrenalectomized and sham-operated animals.

Naloxone and fluid consumption in rats: dose-response relationships for 15 days

Rats were given daily intraperitoneal injections of 10.0, 1.0, 0.1, 0.01, 0.001 or 0.0 mg/kg naloxone for 15 days. Each day after the injections, animals were allowed access to a 20% sucrose solution for two hours and to tap water for the subsequent 10 hours. Consumption of the sucrose solution by the group that received 1.0 mg/kg was reliably decreased on Day 1 and 2, reflecting the suppressive effect of naloxone at that dose. By Day 3 until the end of the experiment, however, the suppression was no longer significant, suggesting that tolerance had developed. A similar effect was seen with the group given the highest dose, 10.0 mg/kg; although drinking was significantly less than the control in each of the 15 sessions, this group showed a trend to increase intake over the days of the experiment, thus also indicating possible tolerance to the effect of naloxone. Drinking patterns of the other groups did not differ statistically from the control. Thus, the low doses had no ability to suppress consumption, and the lowest dose that did lower it soon lost that ability; the highest dose continued to suppress drinking throughout the study but with decreasing efficacy. High performance liquid chromatography (HPLC) demonstrated that the naloxone remained intact over the 15 days of the experiment, supporting the suggestion that tolerance to naloxone might have developed.

Suppression of food and water intake after intracerebroventricular infusion of morphine and naloxone in rabbits

The effect of intracerebroventricular infusion of morphine and naloxone on food and water intake was investigated in rabbits. Morphine hydrochloride at a dose of 120, 10 and 5 micrograms produced statistically significant suppression of 24-h food and water intake. The same effect ensued after infusion of naloxone at dose of 120 and 10 micrograms. Postmorphine aphagia was accompanied by a rise in blood free fatty acids and normal glucose levels. No changes were seen after naloxone.

Naloxone reduces diuretic responses induced by water, alcohol or congenital lack of vasopressin in rats

The effects of naloxone (2 and 10 mg kg-1 s.c.) were compared in several kinds of experimental polyuria: alcohol- or water-loaded rats and Brattleboro rats (i.e. animals with congenital lack of vasopressin). In normal rats, both water and alcohol increased urine flow and decreased urinary osmolality. Alcohol induced a more marked diuretic response than water. In normally hydrated rats, naloxone (2 and 10 mg kg-1 s.c.) failed to modify urine flow, urinary osmolality, Na+ and K+ urinary excretion, and urine creatinine concentration. The two doses of naloxone decreased urine flow and increased osmolality in both water- and alcohol-loaded rats. In Brattleboro rats, naloxone (10 mg kg-1 s.c.) reduced urine flow and urinary creatine whereas the low dose (2 mg kg-1 s.c.) was without effect. Since it is well known that the mechanism of water- or alcohol-induced diuresis is an inhibition of vasopressin release, the present results suggest that naloxone could prevent this inhibition. They indicate that endogenous opioid peptides may exert an inhibitory control on vasopressin release.

Naloxone suppresses fluid consumption in tests of choice between sodium chloride solutions and water in male and female water-deprived rats

The effects of naloxone on fluid consumption by water-deprived rats trained to choose between a saline solution and water in a 15-min drinking test were examined. Rats of each sex were allocated to three groups and given access to 0.125% NaCl, 0.6% NaCl, and 1.7% NaCl, respectively, as the alternative to water. Under control conditions they drank substantially more of the hypotonic salt solutions than water, but drank slightly more water than hypertonic salt solution. Naloxone generally reduced fluid consumption, dose-dependently (0.01-10 mg/kg). In the cases of the two hypotonic solutions, the suppressant effect of naloxone was limited to saline solution. The usually low levels of water consumption were unaffected. In the case of the hypertonic solution, naloxone suppressed salt and water intakes by equivalent amounts. The effects of naloxone in the tests with the two higher salt concentrations depended upon sex. There was on example of a significant naloxone-induced reduction in saline preference (females; 0.125% NaCl v H2O). In other instances, saline preferences were not significantly modified. The results are briefly discussed in relation to current suggestions that naloxone may affect fluid consumption in ways which are taste-dependent (e.g., taste sensitivity, palatability, reward). An alternative view is also considered, that the effects of naloxone may be taste-independent, at least in the particular case of drinking in a two-choice test with saline and water.

Patterns of drinking in the rat following the administration of opiate antagonists

Durations of drinking were recorded for water-deprived rats as they drank to satiety, following SC injections of naloxone (0.1-10.0 mg/kg), naltrexone (0.1-10.0 mg/kg) or saline vehicle. The results provided evidence for the effects of opiate antagonists on the temporal pattern of drinking exhibited by water-deprived animals. A separate, time-sampling procedure was used to supplement the drinking duration data, and showed that the opiate antagonists may suppress water consumption during a period 2.5-7.5 min after the start of the initial drinking bout. A second experiment confirmed that the pattern of drinking displayed during schedule-induced polydipsia in the rat is resistant to any suppressant effect of a moderate dose of an opiate antagonist. The similarity between opiate receptor blockade and water preloading in their effect on drinking in response to water deprivation, and lack of effect on schedule-induced polydipsia is discussed. Opiate antagonists may affect drinking principally by imposing a thirst satiety signal.

Naloxone blocks exercise-stimulated water intake in the rat

To examine whether opiate receptors modulate exercise-induced water intake, we measured water intake during four consecutive hours after a one-hour swim stress in male, Sprague-Dawley rats. Increased cumulative water intake was found four hours following exercise and this response was naloxone-reversible (P = 0.06). Suppression of water intake in the naloxone-treated, exercised group was most marked in the first two hours after exercise (P less than 0.05). Non-exercised rats consumed water at a constant, linear rate (P less than 0.05) whether treated with naloxone or saline. These results indicate an endogenous opioid role in regulating exercise-induced water intake in the rat, but do not delineate whether this role reflects a non-specific stress behavior or specific physiological processes related to thirst.

Central angiotensin converting enzyme blockade and thirst

The role of endogenous brain angiotensin II (AII) in various thirst states was examined in the rat using the angiotensin converting enzyme inhibitor, captopril. Intracerebroventricular (ICV) captopril (7 micrograms) significantly attenuated the dipsogenic response to centrally administered angiotensin I (AI) (200 ng) for up to 2 hours. The same dose of captopril significantly potentiated the dipsogenic response to ICV AII (100 ng) but failed to alter the dipsogenic response to ICV carbachol (200 pmoles). Central pretreatment with captopril (7 micrograms), for 30 minutes, failed to alter markedly the cumulative water intake of 24 hour water deprived rats. However, a small, significant 8% decrease in water intake was noted in ICV captopril treated rats 60 minutes following the return of water. The same dose of captopril, administered intraperitoneally, significantly potentiated the cumulative water intake of 24 hour water deprived rats. Central pretreatment with captopril (7 micrograms), for 30 minutes, failed to alter the cumulative water intake of rats treated intraperitoneally with hypertonic saline (0.75 M given at a dose of 1% of the body weight). From these studies it would appear that central angiotensin converting enzyme plays only a minor role in thirst induced by water deprivation.

Minireview. Benzodiazepine-opiate antagonist interactions in relation to feeding and drinking behavior

Benzodiazepines reliably produce overconsumption of food and fluids. Opiate antagonists, naloxone and naltrexone, block the benzodiazepine-induced hyperphagia and hyperdipsia at low doses. Hence, activation of endogenous opioid mechanisms may be closely involved in the benzodiazepine facilitatory effects on ingestional behavior. Evidence is reviewed that opiate antagonists diminish feeding and drinking responses, and may enhance satiety processes in feeding and drinking, in addition to selectively diminishing the palatability of attractive foods and fluids. It is proposed that a single mechanism of action of the opiate antagonists would be sufficient to account for both effects on feeding and drinking. Biochemical data confirm that acute benzodiazepine treatment in vivo is associated with a naloxone-reversible release of striatal enkephalin. It is possible therefore that there is a close association between the behavioral and biochemical data, which both show that acute benzodiazepine effects are reversed by opiate antagonists. The implied relationship between benzodiazepine and endogenous opioid mechanisms may be relevant to the question of concurrent opiate-benzodiazepine abuse.

The effects of naloxone, diprenorphine, and diazepam on responding suppressed by pre-shock and pre-food stimuli

The lever-pressing of rats was reinforced with food according to a variable-interval 1-min schedule. In one group, occasional illumination of cue lights for 30-sec periods was followed by a brief electric shock; responding was suppressed during these periods. Naloxone (0.01-10 mg/kg) did not change the degree to which responding was suppressed during the pre-shock stimulus. Diprenorphine (0.1-10 mg/kg) slightly attenuated suppression, and diazepam (1.0-3.0 mg/kg) increased responding during the stimulus to normal levels. These results confirm that opiate antagonists do not always enhance the effects of shock on behavior. In a second group, occasional illumination of the cue lights for 20-sec periods was followed by delivery of free food pellets. Responding was also suppressed during the pre-food stimulus. Neither naloxone nor diprenorphine had any effect on response rate during this stimulus. In contrast to the results of earlier studies using benzodiazepines, diazepam (1.0-3.0 mg/kg) produced a marked attenuation of response suppression during the pre-food stimulus.

Effects of opiate antagonists and putative mu- and kappa-agonists on milk intake in rat and squirrel monkey

The effects of a number of relatively pure opiate antagonists (naloxone, naltrexone, diprenorphine), and putative mu- (morphine, etorphine) and kappa- (ketocyclazocine, ethylketocyclazocine) receptor agonists on sweetened condensed milk intake were examined over a broad range of doses in non-deprived rats and squirrel monkeys. The antagonists consistently decreased milk intake in both the rat and squirrel monkey. There were, however, species differences: diprenorphine was 30 times more potent than either naloxone or naltrexone in the squirrel monkey, but was of similar potency in the rat. The effects of the opiate agonists were more variable than those of the antagonists. In both species, all agonists decreased milk intake at high doses that also produced behavioral depression. Significant increases in drinking were produced only by low doses of ketocyclazocine and ethylketocyclazocine in the rat. The suppression of milk intake by the antagonists supports a modulatory role of opiate receptors in the control of drinking behavior, however, the effects of the agonists on drinking are less easily interpreted within this conceptual framework.

Effects of naloxone and its quaternary form on fluid consumption in rats

Three studies were performed on albino rats to determine the effects of naloxone and its quaternary derivative, naloxone methylbromide, on fluid consumption. The doses of the quaternary naloxone were equated with naloxone by molarity and effectiveness in order to facilitate direct comparisons. All rats were deprived of food and water for 12 hr and exposed to a 20% sucrose solution for a 2 hr period. In Experiment 1, a low (0.01 mg/kg) dose of naloxone or an equated dose of quaternary naloxone was given ICV and immediate access allowed to the fluid on four consecutive days. Animals receiving naloxone were not significantly different from controls, and rats receiving quaternary naloxone exhibited seizures, resulting in decreased consumption. In Experiment 2, the low dose of naloxone or the equated dose of quaternary naloxone was given IP for four consecutive days and neither was significantly different from controls. In Experiment 3, animals were given an IP dose of either 1 mg/kg naloxone, a 1 mg/kg or 50 mg/kg dose of quaternary naloxone, or saline and tested for a single 2 hr period. The doses of 1 mg/kg naloxone and 50 mg/kg quaternary naloxone produced significantly less drinking than controls. In all studies, the initial 30 min period produced the most drinking. Suppression of drinking by a dose of 50 mg/kg quaternary naloxone suggested, in contrast to other studies, that it may cross the blood-brain barrier at high doses.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Effects of opiate antagonists and of morphine on chlordiazepoxide-induced hyperdipsia in the water-deprived rat

The effects of naloxone hydrochloride (0.01-10 mg X kg-1), naltrexone hydrochloride (0.01-10 mg X kg-1), and morphine sulphate (0.01-10 mg X kg-1) on the increased water consumption provoked by administration of chlordiazepoxide (10 mg X kg-1) were investigated in male rats which had been adapted to a 22 hr water-deprivation schedule. As previously reported, both naloxone and naltrexone dose-relatedly reduced water ingestion. Naloxone at 1 mg X kg-1 and naltrexone at 0.1 mg X kg-1 completely blocked the chlordiazepoxide-induced hyperdipsia. Since both opiate antagonists removed the chlordiazepoxide-induced effects in small doses, their effect can plausibly be attributed to opiate receptor blockade. Hence, chlordiazepoxide-induced hyperdipsia may depend upon the activation of endogenous opioid mechanisms. Morphine had little effect on drinking, unless a large dose (10 mg X kg-1) was used, when the thirst-induced and chlordiazepoxide-induced drinking were attenuated. The data provided no evidence that morphine, a mu opiate agonist, enhanced chlordiazepoxide-induced water consumption. The results are considered in relation to other relevant behavioural and biochemical findings.

Exaggerated salt appetite of spontaneously hypertensive rats is decreased by central angiotensin-converting enzyme blockade

Injection of angiotensin II (AII) into the cerebral ventricles at doses as low as 1 pmol h-1 results in a marked stimulation of salt and water ingestion in the rat. Evidence that AII is produced in the central nervous system independently of the circulating renin-angiotensin system (RAS) raises the possibility that endogenous brain AII is involved in the physiological regulation of thirst. The role of brain AII in salt appetite is still unclear. Here we confirm that the spontaneously hypertensive rat (SHR), believed to have elevated levels of brain AII, possesses an exaggerated salt appetite compared with its normotensive controls. We also show that this exaggerated salt appetite is reduced by chronic central treatment with the angiotensin-converting enzyme inhibitor, captopril, while that of the normotensive controls is unaffected. Our study suggests that a central neuropeptide, probably AII, is involved in the maintenance of the exaggerated salt appetite in this model of hypertension.

Effects of naloxone and diprenorphine on spontaneous activity in rats and mice

The narcotic antagonist naloxone has been reported to decrease locomotor activity in the rat, presumably blocking endogenous opiate systems. Naloxone has a greater affinity for receptors which preferentially bind morphine and other opiate alkaloids as compared to receptors that bind endogenous opioid peptides. Diprenorphine, another pure opiate antagonist, binds with equal affinity to both receptor subtypes. Therefore, the effects of the narcotic antagonists naloxone and diprenorphine on spontaneous activity were compared in rats and mice, tested individually and in pairs. Only naloxone (10 mg/kg) affected spontaneous activity in rats tested individually, decreasing both gross and fine activity. In rats tested in pairs, naloxone (1.0 and 10 mg/kg) decreased both fine and gross activity, while diprenorphine (10 mg/kg) produced significant decreases only in fine activity. In mice tested individually, naloxone produced modest (nonsignificant) decreases in activity while diprenorphine (10 mg/kg) significantly enhanced activity. Neither opiate antagonist produced consistent effects on activity in paired mice. These results illustrate the species and situation dependence of the effects of opiate antagonists and point out the need for testing more than one narcotic antagonist in research designed to provide inferential information concerning possible physiological functions of endogenous opioid peptides.

Novel phenylpiperidine opioid antagonists and partial agonists: effects on fluid consumption

The effects of five opioid antagonists, a racemate partial agonist and its agonist and antagonist optical isomers were studied on deprivation-induced drinking. All compounds had a phenylpiperidine nucleus. The antagonists produced dose-related decreases in drinking, and the potencies for decreasing drinking correlated with morphine-antagonist doses. The racemic partial agonist and its agonist isomer decreased drinking at doses higher than those which produced marked analgesia. Within the class of phenylpiperidine drugs studied, some had less specificity than naloxone for the mu-receptors as compared to the delta-receptor, but the suppression of drinking was not related to changes in mu-to-delta ratios.

Drinking, but not feeding, is opiate-sensitive in hamsters

The long-lasting opiate antagonist, naltrexone (NTX), was examined for its effects on various types of consummatory behavior in male golden hamsters and rats. Rat, but not hamster, 24 hr food and water intakes were significantly decreased by four daily NTX (10.0 mg/kg) injections. Hamsters displayed a minimal night to day feeding ratio compared to rats. Hamsters increased food intake following insulin (50 U/kg) administration, but not after 24 hr food deprivation (FD) or 2-deoxy-D-glucose (2-DG; 800 mg/kg) injections. NTX (1.0 and 10 mg/kg) had no effect on feeding, but markedly attenuated hamster drinking induced by 48 hr water deprivation or hypertonic saline injection. Dexamethasone (DEX), a glucocorticoid which depletes pituitary beta-endorphin and produces anorexia in rats, had no effect on daily hamster intake. Since the normal feeding profile of the hamster is similar to that of naloxone and DEX-treated rats, hamsters appear to lack an opiate-sensitive feeding system. In contrast, stimulated drinking behavior of hamsters operates through an opiate-sensitive mechanism. Thus, there are marked species differences concerning the involvement of endogenous opioids in consummatory behavior.

A temporal analysis of naloxone's suppressant effect on drinking

Licking activity was monitored in water deprived rats following various doses of naloxone. It was found that naloxone, at doses ranging from 0.01 to 10.0 mg/kg, had very little effect on initial drinking. However, naloxone did slow sustained drinking after 2 to 6 min into the bout, dependent upon dose. We takes these results to mean that naloxone is interfering with those processes which serve to sustain continued drinking throughout a drinking bout.