Naloxone Suppression Research Articles

Naloxone's suppression of spontaneous and food-conditioned locomotor activity is diminished in mice lacking either the dopamine D 2 receptor or enkephalin

Mice lacking the D 2 dopamine receptor (D 2 −/−) and congenic to the C57BL/6J background were tested for opioid-mediated locomotor activity to examine the involvement of the D 2 dopamine receptor in opioid pharmacology. Morphine-stimulated locomotor activity did not significantly differ between the two genotypes. The opioid antagonist naloxone dose-dependently decreased spontaneous motor activity in wild-type mice but was without significant effect in D 2 −/− mice. The magnitude of food-conditioned increases in locomotor activity in wild-type mice and D 2 −/− mice was similar but naloxone did not decrease conditioned motor activity in D 2 −/− mice. Spontaneous locomotor activity of mice lacking the endogenous opioids β-endorphin and/or enkephalin was also tested and we found that naloxone did not reduce activity in mice specifically lacking enkephalin. We suggest that the D 2 dopamine receptor is necessary for modulation of spontaneous locomotor activity stimulated by the endogenous opioid enkephalin.

Naloxone suppression and morphine enhancement of voluntary wheel-running activity in rats

This study examines the effects of the opioid receptor antagonist, naloxone, and the agonist, morphine, on voluntary wheel-running activity (WR) in rats. Male Sprague–Dawley rats were given 1-h access to a running wheel under non-deprived conditions. Naloxone injections (1.0, 0.5, or 0.25 mg/kg, ip), administered immediately before access to running wheels, dose-dependently suppressed WR. In another experiment, subjects were given 6-h access to running wheels under nondeprived conditions for 5 consecutive days. Morphine injections (2.0 mg/kg, sc) were found to increase WR after an initial suppression. These data demonstrate that naloxone inhibits WR, while morphine both suppresses and enhances WR depending on time and dose. These are in agreement with data on other behaviors that indicate that endogenous opioid systems play a major role in the mediation of motivational behaviors.

Inhibition by Naloxone of Promoter Activity of the Neurofilament Gene in SK-N-SH Cells

Chronic administration of morphine is known to decrease the levels of neurofilaments (NFs) in the ventral tegmental area. We ligated a promoter region of the mouse 68-KDa neurofilament (NF-68) gene to the pGL3-enhancer vector containing a luciferase gene, transfected it into SK-N-SH cells and then analyzed transcriptional activity in the cells treated with agonists or antagonists of opiate receptors. The activity of the NF-68 promoter was suppressed by naloxone about 55% at 10(-5) M and 30% at 10(-7) M at 48 h, but suppressed not by morphine. Naltrexone at 10(-5) M suppressed the promoter activity about 20%, but levallorphan, DAMGO, DPDPE and U50488 did not. The inhibition by naloxone was dose-dependent and not reversed by morphine. The inhibitory effect of naloxone was not observed in N18TG-2 cells and PC12 cells. Experiments with various deletion mutants revealed that a region responsible for naloxone suppression spans from -328 to -101 in the gene. These results suggest that naloxone has the ability to suppress transcriptional activity in some neurons.

Naloxone's effects on operant responding depend upon level of deprivation

Naloxone's effects on initiation, maintenance, and maximal response effort to acquire food were examined in rats maintained under different levels of food deprivation. In Experiment 1, naloxone was administered SC to rats responding under an FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule. Naloxone did not increase time to acquire the first pellet. Naloxone's suppression of subsequent intake and lowest effective dose were inversely related to level of deprivation. In Experiment 2, rats responded for food under a Progressive Ratio 2 reinforcement schedule. Breakpoint was lowered only when rats were maintained with free access to food. Decreases in response and running rate were inversely related to deprivation level. Results are consistent with the hypothesis that opioids are involved in the maintenance but not the initiation of feeding.

The effects of food schedule adaptation on the ability of naloxone to suppress the aquisition of schedule-induced polydipsia

Naloxone suppressed the acquisition of schedule-induced polydipsia (SIP) in rats given no previous exposure to the feeding schedule. Adaptation to the feeding schedule prior to SIP acquisition attenuated this suppression. Specifically, water consumption, bout probability, licks/bout and maximum lick rates during the interpellet interval (IPI) were significantly increased by adaptation. Although adaptation attenuated the suppressive effects of naloxone on SIP, this attenuation was not complete. Adapted, naloxone-treated subjects displayed both decreased water consumption and bout probability as compared to distilled water-treated controls. Unlike the effects of adaptation on naloxone's suppression of SIP, adaptation completely eliminated naloxone's suppression of feeding. That adapted subjects ate at control levels while still displaying a lower level of SIP suggests that the suppressive effect of naloxone on the acquisition of SIP is not an indirect effect of naloxone on feeding, but rather a direct effect of naloxone on developing SIP. Given that naloxone has a general suppressive effect on drinking (including SIP), what remains to be determined is why naloxone has no effect on established SIP. Possible explanations for this are discussed.

Naloxone suppression of self-stimulation is independent of response difficulty

The action of the opiate antagonist naloxone on relatively easy (nose-poke) and relatively difficult (lever-press) self-stimulation behaviors was compared, in order to determine if opiate antagonists suppress self-stimulation by interfering with the ability of the animal to respond, or by reducing the reinforcement value of the stimulation. Naloxone (0.2, 2.0 and 20 mg/kg) significantly suppressed both nose-poking and lever-pressing self-stimulation rates, and the degree of suppression was virtually identical for both tasks at all doses examined. If naloxone had interfered with the ability of the animal to respond, then lever-pressing-which requires more motor output than nose-poking-should have been more suppressed than nose-poking. The results suggest that opiate antagonists do not interfere with the ability of the animal to respond, and are therefore consistent with the hypothesis that these drugs reduce the reinforcement value of the stimulation.

Lesions in ventromedial hypothalamus or amygdala do not affect naloxone's suppression of water consumption

Rats received lesions in the ventromedial hypothalamus (VMH) or amygdala, or were sham operated, and then were tested for two-hour intake of water after injections of naloxone (2 mg/kg), MIF-1 (2, 4, or 8 mg/kg), or diluent. There was a significant effect of test compound, with naloxone reducing consumption relative to the diluent control and the largest dose of MIF-1. Although MIF-1 tended to suppress drinking, the effect did not reach significance. There was no main effect for lesions, indicating that the amygdala and the VMH do not play a critical role in the effect of naloxone or MIF-1 on water consumption. A significant lesion by time interaction occurred, however, with amygdala-lesioned rats drinking the most in the first 30 min but much less after that. The VMH rats drank the most in the 30–60 min interval, but there were no differences in groups after 60 min. Thus, it appears that the intact VMH and basolateral amygdala are not necessary for naloxone's suppression of water consumption in the rat.

Food intake: Opioid/purine interactions

The exogenous opioids butorphanol tartrate (BT) and ethylketocyclazocine (EKC) have been reported to stimulate feeding in rats. In this study we evaluated the effects of purines (known to suppress feeding) and the adenosine antagonist, caffeine, on opioid induced feeding. Adenosine and inosine significantly suppressed BT and EKC induced feeding at various doses and time points. Caffeine enhanced food consumption was suppressed by various doses of naloxone, but was not suppressed by adenosine or inosine. Although caffeine itself induced further feeding, it did not enhance BT induced food consumption. Adenosine and inosine failed to suppress BT induced feeding when 12.5 mg/kg of caffeine was administered to the rats suggesting blockade of the adenosine receptor by caffeine. In contrast to 12.5 mg/kg caffeine, high dose caffeine (50 mg/kg) suppressed BT induced feeding over a 4 hour time period. Adenosine (50 mg/kg) and inosine (50 mg/kg) injected one hour after injection of BT and caffeine (50 mg/kg) reversed the suppressive effect of high dose caffeine in BT induced feeding. These studies indicate that opioid induced feeding can be suppressed by adenosine and inosine. Also, caffeine can reverse the suppressive effect of adenosine and inosine on feeding and vice versa. Naloxone's suppression of caffeine enhanced food consumption indicate that at least part of caffeine's effect on food intake may be mediated through an opioid mechanism.

Naloxone suppression of food and water intake and cholecystokinin reduction of feeding is attentuated in weanling rats with dorsomedial hypothalamic lesions

In Experiment 1, sham operated (SCON) and dorsomedial hypothalamic nuclei (DMN) lesioned (L) rats were given saline or naloxone (0.1, 1.0 or 2.0 mg/kg) just prior to the onset of the dark cycle, lights out. Compared to saline injections, naloxone at all doses suppressed the cumulative food intake of the SCON during the second and third hr of measurement. Naloxone was without significant effect on the food intake of DMNL rats. Similar results were obtained in Experiment 2, except that naloxone at 2.0 mg/kg significantly suppressed the DMNL rats' food intake by the fourth hr of measurement. Cumulative water intake of both groups was significantly suppressed by naloxone in both experiments but its effects appeared to be attenuated in the DMNL group. In a preliminary trial cholecystokinin octapeptide (3.0 and 6.0 μg/kg) given at the onset of the dark cycle significantly suppressed the food intake of the SCON group but had on signficant effect on the DMNL rats. The possibility exists that the reduced food intake and lower body weight of DMNL rats may partially result from damage to an opioid system. The data also tentatively suggest the DMN may play a role in cholecystokinin-induced satiety.

Naloxone reduces fluid consumption: Relationship of this effect to conditioned taste aversion and morphine dependence

Naloxone reduces fluid consumption in rats. It also produces a conditioned taste aversion (CTA). No reliable correlation was found between these two measures, suggesting that the reductions in drinking are unrelated to the illness-producing effects indexed by the CTA test. Measures of CTA using naloxone were also uncorrelated with measures of voluntary oral consumption of sweetened morphine solutions. In addition, naloxone suppression of drinking was unaffected by prior morphine administration with the administration of morphine given in a variety of ways, including dependence-producing regimens of injections.

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 endorphin receptors.