Central and peripheral contributions of endogenous opioid systems to nutrient selection in rats

The contribution of central and peripheral sites to opioid mediation of energy intake and dietary self-selection of the three macronutrients, protein, fat, and carbohydrate, was examined in male rats. Animals given access to either Purina Chow or a self-selection regime were injected with either the opioid antagonist, naltrexone (0.0, 0.1, 1.0, and 5.0 mg/kg, IP), or quarternary naltrexone (0.0, 0.1, 1.0, and 5.0 mg/kg, IP), an opioid antagonist that does not readily enter the central nervous system. Animals received injections at the beginning of an 8-h feeding period, and nutrient intakes were measured at 1, 2, 4, and 8 h postinjection. Naltrexone and its quarternary analogue differed in their effects both on total energy intake and macronutrient selection. Naltrexone led to significant decreases in total energy intake in animals on both dietary regimes, whereas quarternary naltrexone did not modify energy intake of animals given access to either diet. Naltrexone produced a sustained reduction in fat intake and initial decreases in carbohydrate and protein intakes. Quarternary naltrexone did not modify overall energy intake but did lead to modifications in nutrient choice. In contrast to naltrexone, quarternary naltrexone resulted in increased fat intake, decreased carbohydrate intake, and a small reduction in protein intake. These data suggest that both peripheral and central sites contribute to opioid effects on patterns of nutrient choice.     

Differential effects of amphetamine and fenfluramine on dietary self-selection in rats

Daily caloric intakes and dietary self-selection of the three macronutrients, protein, fat and carbohydrate were examined in female rats following administration of d-amphetamine sulfate (0.0, 0.5, 1.0 and 2.0 mg/kg, IP) or fenfluramine hydrochloride (0.0, 1.5, 3.0 and 6.0 mg/kg, IP). Animals were maintained on ground Purina Chow or one of two self-selection regimes, one with a high-caloric fat ration (7.85 kcal/g) and the other with a fat ration isocaloric to the carbohydrate and protein rations (3.76 kcal/g). Animals received drug injections at the beginning of a daily 8-hour feeding period with nutrient intakes measured at 2, 4 and 8 hrs following injections. While both amphetamine and fenfluramine led to dose-related decreases in total caloric intakes, the two drugs resulted in different temporal patterns of feeding. Amphetamine produced its greatest effect on caloric intake during the first 2 hours of the feeding period, whereas fenfluramine suppressed caloric intake equivalently across the 8-hour feeding period. The two anorectic drugs also led to different patterns of nutrient choice. When animals were given the high-caloric fat ration, amphetamine selectively decreased fat intake while fenfluramine produced decreases in both protein and fat intakes, sparing carbohydrate intake. In contrast, when animals were given the isocaloric fat ration, amphetamine resulted in a general suppression of nutrient intakes while fenfluramine led to a sustained decrease in fat intake with a relative sparing of protein and carbohydrate consumption.     

Increased fat consumption induced by morphine administration in rats

Patterns of caloric intakes and dietary self-selection of the three macronutrients, protein, fat and carbohydrate were examined in male rats following the administration of morphine sulfate (0.0, 1.0, 10.0 and 20.0 mg/kg, IP). Animals were given access to either ground Purina Chow or one of two dietary self-selection regimes, one with a high-fat ration (7.8 kcal/g) and the other with a fat ration isocaloric to the carbohydrate and protein rations (3.8 kcal/g). Animals received morphine injections at the beginning of a six-hour feeding period and nutrient intakes were measured at 1,2,4 and 6 hours postinjection. Similar patterns of macronutrient choice were observed for both animals maintained on the high-fat regime and animals with access to the isocaloric components following morphine injections. As a function of morphine injections, animals on both self-selection regimes increased fat intake while suppressing carbohydrate intake and exhibiting little modifications in protein intake.     

Morphine selectively influences macronutrient intake in the rat

Dietary self-selection of the three macronutrients, protein, carbohydrate and fat, was examined in male rats following the administration of three doses of morphine sulphate: 10 mg, 15 mg, and 30 mg/kg body weight. Intakes of all three macronutrients were suppressed in a dose-dependent manner for a two-hour period following morphine administration. Both protein and carbohydrate intakes remained suppressed for a six-hour feeding period after morphine injections. In contrast, animals increased fat intake during the final four hours of the six-hour feeding period resulting in an overall increase in fat intake.     

Intracerebral administration of naloxone and drinking in water-deprived rats

In 24-hr water-deprived rats, naloxone, at various doses (0, 12.5, 25, 50 μg/rat), was administered prior to a 15-min drinking period. Infusions were made bilaterally into each lateral ventricle, frontal cortex, lateral preoptic area, lateral hypothalamus, and caudate nucleus. Naloxone reliably reduced water consumption at 50 μg/rat when infused into the lateral ventricles and lateral hypothalamic areas. When comparable doses of naloxone were given by peripheral injection, no effect on drinking was observed. There appeared to be a trend developing for greater sensitivity to naloxone when infusions were made into a particular part of the hypothalamus. These data support the idea that naloxone reduces drinking by acting at central opiate receptors.     

Delayed effects of naloxone on responsiveness to environmental novelty in rats

Two experiments were conducted to examine further the hypothesized involvement of endorphins in responsiveness to environmental novelty. In Experiment 1, rats were treated with naloxone hydrochloride (0.5–5.0 mg/kg, SC) before initial exposure to a novel arena (Day 1) and then retested in the arena 24 h later (Day 2). Only naloxone (5 mg/kg) significantly affected Day 1 performance, producing a selective reduction in locomotor activity. However, compared to saline controls, all groups that had previously received naloxone showed marked reductions in both locomotor activity and rearing upon Day 2 retest. In Experiment 2, naloxone (0.5–5.0 mg/kg) was without significant effect on performance in naive animals which had been injected on Day 1 but not exposed to the arena until Day 2. These data suggest that the delayed effects of naloxone relate specifically to the initial experience of environmental novelty rather than receptor changes or metabolite influences resulting from acute antagonist treatment. Results are discussed in relation to a possible action of naloxone upon mechanisms of attention and/or memory.     

Influence of naloxone and methysergide on the analgesic effect of clomipramine in rats

The effects of the tricyclic antidepressant clomipramine were studied in two analgesic tests in rats: (1) vocalization threshold response; and (2) scored behavioral response to electric shock to the tail. Clomipramine (20-50 mg/kg i.p.) produced analgesia, decreasing behavioral response scores and increasing vocalization threshold. Morphine also reduced the response scores in the second test. Naloxone (0.8 mg/kg i.p) or methysergide (20 mg/kg i.p.) (no effect when given alone) abolished the analgesic effect of clomipramine as evaluated by vocalization threshold response. Naloxone alone (0.6 or 2 mg/kg i.p.) increased the behavioral response at 20 and 30 V but did not modify the score at 40 V. Naloxone reduced the analgesic effect of clomipramine or morphine in the behavioral test. These results suggest that the analgesic effect of clomipramine could involve both serotonergic and endorphin central systems.     

Behavioral alterations produced by chronic naloxone injections

Repeated blockade of the endorphin receptors eventually induces symptoms resembling an opiate abstinence syndrome, despite the complete absence of opiate narcotics. Rats were injected with 0.6 mg/kg naloxone or with injection vehicle alone twice a day for six days. They were observed twice a day for four subsequent days. Body shakes, head shakes, scratches and total symptoms were significantly elevated in the naloxone treated group over controls. Symptoms were completely reversed by a small dose of morphine but not by naloxone. In a second experiment, rats were injected for ten days with the same dosage of naloxone. The abstinence-like syndrome began after six days of naloxone and continued for several days after cessation of injections. Total symptoms, body shakes, scratches and aggression were significantly elevated over controls and were reversed by morphine but not by naloxone.     

Suppression of deprivation-induced food and water intake in rats and mice by naloxone

Naloxone, an opiate antagonist, was administered to male and female rats and male mice after periods of food or water deprivation ranging from 12 to 48 hr. Naloxone (0.01-10 mg/kg) reduced postdeprivational water intake in most groups of rats and mice in a dose-related manner. Naloxone suppression of water consumption appeared to be independent of sexual differences in rats, and phase of the diurnal cycle, and length of the deprivation interval in both rats and mice. Postdeprivational food intake in male rats and mice was also reduced by naloxone in a dose-dependent fashion. This naloxone effect was less pronounced than actions observed with water intake, and tended to diminish with lengthening food deprivation periods. In general, mice appeared to be less sensitive than rats to naloxone suppression of food and water intake. Naloxone appears to markedly reduce appetitive behavior, particularly water intake, following deprivation in both rats and mice. The fact that low doses of naloxone can elicit these effects suggests that the drug is acting at specific tissue sites, possibly endorphine recpetors.     

Comparison of the suppression by naloxone of water intake induced in rats by hyperosmolarity,hypovolemia, and angiotensin

The effect of naloxone upon water consumption by rats was assessed using two intensities each of IV NaCl (hyperosmolarity), SC polyethylene glycol (hypovolemia), and IV angiotensin II. In each case naloxone produced a dose-related reduction in the amount drunk. Angiotensin-induced drinking was most easily inhibited, and was abolished by only 1 mg/kg naloxone. In contrast, 1 mg/kg naloxone produced only a 50% reduction in NaCl-induced drinking, and hypovolemia-induced drinking was not completely reversed by 5 mg/kg. Naloxone was without effect upon the natriuresis after NaCl, or the hypertension during AII administrations. Parallels are drawn between the effects of naloxone on these types of thirst, and of other perturbations including brain damage and taste adulteration.     

Interaction of acute and chronic stress with respiration: Modification by naloxone

Rats exposed to inescapable foot shock displayed an increase in respiratory rate, tidal volume and minute volume. Naloxone HCl (5 mg/kg, SC) potentiated the foot shock-induced increase in ventilation. Inhalation of high (5% and 10%) concentrations of carbon dioxide enhanced the stimulation of ventilation observed in both the acute stressed animals and the acute stress-naloxone treated group. Chronic daily foot shock sessions (11 days) attenuated the respiratory stimulation produced by acute foot shock and the potentiation induced by naloxone. The appearance of foot shock-induced stimulation of respiration paralleled the production of acute foot shock-induced analgesia. On the other hand, chronic foot shock attenuated both stress-related analgesia and respiratory stimulation. These results strongly suggests stress can influence respiratory function through activation or release of the endogenous opioids. It is postulated that the endorphinergic system functions as a compensatory system which prevents excessive stimulation of respiration by stress.     

Increased carbohydrate consumption by rats as a function of 2-deoxy-D-glucose administration

Dietary self-selection was examined following the administration of the glucoprivic agent, 2-deoxy-D-glucose (2-DG), in adult male rats given access to separate sources of the three macronutrients, protein, fat and carbohydrate. All animals received injections (IP) of saline, 250, 500 and 750 mg/kg 2-DG with nutrient intakes measured at 2, 4, 6 and 24 hrs following injections. Animals consumed significantly more carbohydrate at 4, 6 and 24 hrs after injections of 500 and 750 mg/kg 2-DG than after saline injections. In contrast, fat intake was significantly suppressed by all three doses of 2-DG at 2 hr, by 250 and 750 mg/kg 2-DG at 4 and 6 hrs, and by 750 mg/kg 2-DG at 24 hr after injections. Protein intake was significantly decreased by all three doses of 2-DG at 2 hr after injections. As a result of the increase in carbohydrate intake and complimentary decrease in fat intake following 2-DG injections, total caloric intake of animals given the self-selection regime was not modified as a function of drug administration. In comparison, rats given a single nutritionally complete diet (ground Purina Laboratory Chow) consumed significantly more calories following 2-DG administration than following saline injections. The ability of animals to make appropriate modifications in nutrient selection following regulatory challenges is discussed.     

Nicotine attenuates place aversion induced by naloxone in single-dose,morphine-treated rats

Rationale Acute physical dependence refers to the withdrawal syndrome precipitated by an opioid antagonist administered several hours after either a single dose or a short-term infusion of an opioid agonist.Objectives We examined the mechanism of nicotine-induced attenuation of naloxone-precipitated withdrawal syndrome when used to produce an aversive motivational state in a place-conditioning paradigm.Methods The effect of nicotine was investigated through place aversion induced by naloxone in morphine-pretreated rats. Additionally, the mechanism of nicotine action in this model was explored specifically in relation to the dopaminergic system through the use of dopamine receptor antagonist and agonist.Results Place avoidance behavior was potently elicited by naloxone (0.5 mg/kg s.c.) 24 h after a single exposure to morphine (10 mg/kg s.c.). Avoidance behavior was attenuated by pretreatment with a 0.2-mg/kg dose of nicotine 15 min prior to naloxone administration. The effect of nicotine was completely blocked by mecamylamine, but not hexamethonium. The dopamine receptor antagonists haloperidol (0.05, 0.1 mg/kg, s.c.), SCH23390 (0.1 mg/kg, s.c.), raclopride (1.0 mg/kg, s.c.) and eticlopride (0.1 mg/kg, s.c.) showed effects similar to mecamylamine. Additionally, the dopamine receptor agonist apomorphine (0.03, 0.1, 0.3 mg/kg, s.c.) inhibited naloxone-induced place aversion in morphine-treated rats.Conclusion The inhibitory effect of nicotine on place aversion induced by naloxone-precipitated morphine withdrawal may involve a dopaminergic portion of the central nervous system.     

The effect of opiates and naloxone on food intake in virgin and lactating rats

Lactation provides an excellent model of non-obese hyperphagia. There is accumulating evidence that endogenous opioids play a role in the modulation of the hormonal changes that occur during lactation. Because endogenous opioids appear also to play a role in the regulation of feeding, we studied the effects of the opiate agonist, butorphanol tartrate, and an opiate antagonist, naloxone, on food intake in virgin female rats and in rats during early, mid and late lactation and during post-weaning. It has been reported that female rats are less sensitive to the suppressant effects of nalmefene, an opioid antagonist, than male rats. Therefore, we also examined the effect of naloxone, an opioid antagonist, on spontaneous nocturnal feeding and 24 hour food deprivation-induced food intake in virgin female rats. We found that female rats were relatively insensitive to the food suppressant effects of naloxone following 24 hour food deprivation, while male rats tested under similar conditions had a decreased intake in response to naloxone. Despite the marked hyperphagia that occurred during lactation, there were minimal alterations in the response to opiate agonists and antagonists during this time period. Our data suggest that endogenous opioids may not play a pivotal role in the hyperphagia of lactation.     

Suppression of drinking by naloxone in rats homo- and heterozygous for diabetes insipidus

The effects of the opiate antagonist, naloxone, alone and in combination with morphine, were examined on drinking induced by water deprivation in homo- and heterozygous Brattleboro rats manifesting an inherited diabetes insipidus. Both naloxone and a structurally-related congener, naltrexone (0.01–10 mg/kg), attenuated water consumption in adose-related fashion of 1 hr water-deprived homozygotes, which exhibit a complete absence of vasopressin. Drinking was also reduced by the two drugs in 24 hr water-deprived heterozygotes, which have detectable levels of vasopressin. Morphine pretreatment enhanced the antidipsogenic effects of naloxone in a dose- and time-dependent manner. The administration of 10 mg/kg of morphine 3 hr before testing, which itself did not affect drinking, maximally potentiated the suppressant effects of naloxone on drinking in homozygotes. This potentiating effect of morphine persisted for at least 48 hr. These results indicate that vasopressin is not essential for the antidipsogenic effects of the narcotic antagonists. The polydipsic Brattleboro rat may provide a convenient animal model for studies of the effects of opiate agonists and antagonists on drinking behavior.     

Continuous infusion of naloxone: effects on behavior and oxygen consumption

Twenty-eight hours of endorphin receptor blockade by subcutaneous naloxone infusion produced behavioral and respiratory symptoms resembling opiate abstinence syndrome. Rats were implanted subcutaneously with two Alzet osmotic minipumps delivering 0.7 mg/kg per hour naloxone or with two control minipumps containing distilled water only. They were observed for 10 minutes under blind conditions at 16 and 28 hours post-implantation. The naloxone-infused rats showed significantly more wet dog shakes, abdominal writhes and overall abstinence-like symptoms than did the control rats. These symptoms decreased after 28 hours despite continued naloxone infusion. Acute administration of naloxone failed to produce abstinence-like symptoms, even when combined with the trauma of carrying two implanted water-filled minipumps for 28 hours. In another experiment, naloxone-infused rats showed a highly significant 53.4% elevation of O2 consumption over water-infused control rats in a pure O2 atmosphere at 28 hours after implantation. This difference disappeared at 48 hours post-implantation. In contrast to the effect of naloxone infusion, acute administration of three different doses of naloxone failed to significantly increase O2 consumption.     

Naloxone reduces fluid intake in rats with open gastric fistulas

Water-deprived rats, fixed with chronically indwelling gastric fistulas, drank with the fistulas open (sham drinking) for 20 min. The subjects were given three doses of naloxone (0.0, 1.0, and 10 mg/kg, SC) 15 min before the opportunity to sham drink. The mean water intakes on days of 0.0, 1.0 and 10 mg/kg of naloxone were 43.3, 36.3, and 23.1 g, respectively. Naloxone clearly reduced fluid intake in rats engaged in sham drinking. This finding, that naloxone reduces fluid intake when post-ingestional absorption is prevented, lends support to the idea that naloxone modifies central neural regulatory processes. The results of these experiments do not support the hypothesis that the endorphins are involved exclusively with drive-reduction.     

Alcohol induced discoordination is not reversed by naloxone

It has previously been reported that prior administration of naloxone may prevent the decrement in performance produced by alcohol. To be of clinical value, however, naloxone must be shown to reverse rather than prevent this decrement. This study examined the effect of naloxone given after consumption of alcohol. A double blind balanced crossover protocol was used to examine the effect of either 1.2 mg or 10 mg naloxone on the sensory-motor impairment produced by blood alcohol concentrations maintained between 75 and 85 mg/100 ml. This alcohol concentration significantly impaired two measures of sensory-motor performance, but there was no evidence that either dose of naloxone could reverse this decrement. We tested our subjects for a chlorpropamide alcohol flush but none gave a positive response. These results indicate that naloxone (1.2 mg or 10 mg) does not reverse the sensory-motor impairment produced by alcohol intoxication in subjects who do not exhibit a chlorpropamide alcohol flush. Nearly all the subjects exhibited somnolence after receiving alcohol and naloxone (1.2 mg or 10 mg) but not after receiving alcohol and saline.     

Social crowding enhances aversiveness of naloxone in rats

If endorphins mediate various behavioural states including reactions to stress and social affect, then experimental manipulation of such states should alter the stimulus properties of opioid antagonists. The results of experiments in which the aversiveness of naloxone was increased by chronic environmental stress produced by rearing and maintaining male rats under conditions of severe social crowding, support this theoretical proposition. All rats acquired aversions to flavoured solutions that had been presented for 15 min immediately prior to a systemic injection of naloxone. The degree of the conditioned aversion was greater in rats maintained in crowded conditions. Transferring rats from crowded conditions to individual accommodation 3 weeks before the start of aversive conditioning did not attenuate the increased aversiveness of the antagonist. It is suggested that social interaction under conditions of chronic crowding induces a prolonged increase in the level of tonic activity in endorphinergic systems.     

The anorectic action of naloxone is attenuated by adaptation to a food-deprivation schedule

Recent studies have shown that naloxone and other opiate antagonists can reduce the amounts of food and water consumed by laboratory animals, a finding consistent with a role for endogenous opioids in the control of appetite. Because there have also been some failures to observe an anorectic action of naloxone, a study was carried out in which the effects of the drug on food intake were investigated using two different experimental procedures. In naive rats deprived of food for 24 h, both naloxone (0.1, 1.0 and 10.0 mg/kg) and fenfluramine (1.0, 3.0 and 10.0 mg/kg) produced dose-related decreases in food and water intake. In rats which has been adapted to receiving food for only 6 h each day, fenfluramine produced a similar effect whereas naloxone had no effect on food intake and reduced water consumption only at the highest dose. A second experiment showed that the different actions of a 1.0 mg/kg dose of naloxone in the two procedures were not due to differences in the duration of the immediately preceding period of food deprivation or in the time during which the rats were handled. These results show that the anorectic action of naloxone can be attenuated by adaptation to a schedule of repeated food deprivation.