Selective suppression of schedule-induced ethanol drinking by antialcoholic drugs in rats

Effects of disulfiram and calcium cyanamide, antialcoholic drugs, on schedule-induced ethanol drinking as well as on schedule-controlled response (lever-pressing) under a fixed interval 1 min schedule of food reinforcement were investigated in Wistar strain rats. When ethanol solution was available, the schedule-induced ethanol drinking decreased depending on the ethanol concentration (2-8%). However, the dose of ethanol intake during the 1 hr experimental session was at maximum (2.8 g/kg) when 4% ethanol solution was available. Thereafter, 4% ethanol solution was used in the experiment for studying the effects of disulfiram and calcium cyanamide on the schedule-induced ethanol drinking. Disulfiram (100-200 mg/kg, p.o.), pretreated at 1 hr before the start of the experiment, tended to suppress schedule-induced water drinking. However, the same treatment of calcium cyanamide (5-10 mg/kg, p.o.) did not produce a marked change in it. In contrast, disulfiram (100 and 200 mg/kg) and calcium cyanamide (5 and 10 mg/kg) markedly suppressed schedule-induced ethanol drinking without eliciting a marked change in schedule-controlled response. The present results suggest that both disulfiram and calcium cyanamide selectively suppress ethanol drinking in rats.     

Effects of isolation-rearing on voluntary consumption of ethanol, sucrose and saccharin solutions in Fawn Hooded and Wistar rats

These experiments examined the hypothesis that isolation-rearing and strain influence hedonic mechanisms. In experiment 1, voluntary consumption of ethanol and water was monitored in the home cage of Fawn Hooded (FH) and Wistar rats. FH rats were found to consume more ethanol at low concentrations than Wistar rats, independent of rearing condition, and isolation-reared rats were found to consume more of high ethanol concentrations, independent of strain. In experiment 2, isolation-reared rats were found to consume more sucrose, independent of concentration, than socially reared rats. In experiment 3, Fawn Hooded rats were found to be more sensitive to low concentration solutions of saccharin, and to consume less of the high concentration solutions, while isolation-rearing was found to enhance consumption of high concentrations. Thus, hedonic processes are independently modulated by strain and rearing conditions, although the effects of isolation-rearing appear to be exacerbated in Fawn Hooded rats. Received: 19 August 1997 / Final version: 26 November 1997     

Naloxone attenuates incubated sucrose craving in rats

Rationale Cue-induced craving precedes drug relapse and contributes to eating disorders. Opiate antagonists have been demonstrated to be effective at reducing cravings for drugs and food. Craving, as defined as responding for a stimulus previously associated with a reward, increases, or incubates, over forced abstinence in an animal model of relapse. Objectives This paper aims to determine anticraving effects of the opiate antagonist, naloxone, on the incubation of sucrose craving. Methods 106 male Long-Evans rats lever pressed for 10% sucrose solution 2 h/day for 10 days. On either day 1 or 30 of forced abstinence, rats responded in extinction for 6 h and then were injected (ip) with either saline or naloxone (0.001, 0.01, 0.1, 1, or 10 mg/kg). The rats then responded for 1 h for presentation of a tone + light cue previously presented with every sucrose delivery during self-administration training. Results The rats responded more in extinction and following saline on day 30 vs day 1 (an incubation of craving). Except for a trend for a decrease in responding following 10 mg/kg on day 1, naloxone was primarily effective on day 30. On day 30, naloxone significantly reduced responding at all doses except for 0.1 mg/kg. Conclusions The time-dependent increase in sensitivity to an opiate antagonist is consistent with time-dependent changes in the opiate system following forced abstinence from sucrose. These changes may partly underlie the incubation of sucrose craving. In addition, these findings could be used to support the use of naloxone as an anticraving medication in protracted abstinence.     

Naloxone depresses osmoregulatory drinking in rats

The effect of an opiate antagonist, naloxone, on hypertonic NaCl-induced drinking was studied in rats in a within-subject design. Naloxone reduced drinking at all dosage levels used (0.3–10.0 mg/kg) when compared to a control condition. These results extend previous findings of naloxone mediated reduction in fluid intake in water deprived and osmotically challenged animals. Naloxone's effect on fluid intake seems to be independent of procedure employed, and thus quite general. Possible mechanisms were considered.     

A precision drinking device for rats tested with water, etonitazene, and ethanol

An inexpensive system for detecting tongue licking and for delivering liquids to rats is described. Liquids were dispensed under a heterogenous chain schedule of reinforcement in which depressions of a lever were required before tongue licking produced liquid delivery. Three rats drank water dispensed by the system at lowest, highest, and midmost valve settings. The volume of water delivered increased logarithmically from 0.00270 ml to 0.01180 ml across valve settings. In additional tests, drug-experienced rats were tested with either 5.0 microgram/ml etonitazene HCl, water, or 8% w/v ethanol available with the valve at its widest setting. More of the drug solutions were consumed than water suggesting that the drugs served as reinforcers. Intakes of both drugs were compared with intakes previously determined in dipper delivery systems using the same rats. This liquid delivery system has several advantageous features in that it is inexpensive to construct, allows quick and easy control of the volume delivered, minimizes evaporation, and, along with the heterogenous chain schedule of reinforcement, insures that liquid is delivered only when oral contact is made with it.     

Time-related facilitation and suppression of drinking by muscarinic anticholinergic drugs in water non-deprived rats

Effects of tertiary anticholinergic drugs, atropine (1.3, 2.5, 5 and 10 mg/kg, s.c.) and scopolamine (0.13, 0.25, 0.5 and 1 mg/kg, s.c.), and a quaternary anticholinergic drug, methylatropine (1.3, 2.5, 5 and 10 mg/kg, s.c.), on the drinking behavior were investigated in water non-deprived rats that were housed in a 12-hr light-dark situation (light period: 6:00-18:00) with a free access to food. Atropine, 5 mg/kg, and scopolamine, 0.13 and 0.25 mg/kg, administered at 12:00, significantly increased the drinking during the 12:00-18:00 period. Furthermore, lower to medium doses of atropine increased the drinking during the 18:00-6:00 period. In contrast, the drinking did not change during the 12:00-18:00 period, but decreased during the 18:00-6:00 period in a dose-dependent manner after administration of methylatropine at 12:00, whereas the drinking during the 18:00-24:00 period decreased in a dose-dependent manner when both the tertiary and quaternary drugs were administered at 18:00. However, the drinking during the 24:00-6:00 period increased in the rats that were administered atropine at the dose of 5 or 10 mg/kg at 18:00, while the drinking still decreased after methylatropine at the same time. The present results suggest that in water non-deprived rats, central muscarinic cholinergic blockade is effective for both increasing and decreasing drinking behavior, depending on the doses, when the drug is administered, and time span between the drug administration and the behavior observation. It is also suggested that peripheral cholinergic blockade monotonously suppresses the drinking behavior.     

Naloxone, but not Tyr-MIF-1, reduces volitional ethanol drinking in rats: correlation with degree of spontaneous preference

The possible relationship between the actions of ethanol and opiates led us to examine the effect of opiate antagonists on ethanol intake in rats with a free choice of water. Naloxone (NAL) significantly reduced intake of ethanol. This effect was much greater in "high-preferring" (ethanol/total fluid intake greater than 60%) than in "low-preferring" (ethanol/total fluid intake less than 30%) rats. Furthermore, a correlation was found between the degree of spontaneous preference (ethanol/total fluid intake ratio) and the reduction of ethanol drinking by NAL. Sensitivity to NAL increased with increased preference for ethanol. Neither Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) nor MIF-1 (Pro-Leu-Gly-NH2) caused a significant modification of ethanol intake. This study shows that NAL can reduce volitional ethanol intake in rats and provides further evidence that Tyr-MIF-1 does not always act like NAL.     

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.     

Heroin-induced suppression of saccharin intake in water-deprived and water-replete rats

Rats suppress intake of a saccharin conditioned stimulus (CS) when paired with an aversive unconditioned stimulus such as lithium chloride. This phenomenon is referred to as a conditioned taste aversion (CTA). Rats also suppress intake of a saccharin CS when paired with a rewarding sucrose solution and when paired with a drug of abuse. Although the suppressive effects of drugs of abuse have long been interpreted as CTAs, evidence suggests that rats may suppress intake of the saccharin CS following taste-drug pairings because they are anticipating the rewarding rather than the aversive properties of the drug. Oddly, however, while all other drugs of abuse tested suppress intake of a gustatory CS, the highly reinforcing drug, heroin, is reportedly ineffective. The present study reexamined this issue in both water-deprived and water-replete rats using procedures that sustain both morphine- and cocaine-induced suppression of CS intake. The results showed that heroin greatly reduced CS intake following saccharin-heroin pairings and that this effect was less variable when assessed in water-replete subjects. When taken with other reports, these data suggest that rats suppress intake of a saccharin CS in anticipation of the availability of all drugs of abuse tested.     

Modeling binge-like ethanol drinking by peri-adolescent and adult P rats

Alcohol binge-drinking, especially among adolescents and young adults, is a serious public health concern. The present study examined ethanol binge-like drinking by peri-adolescent [postnatal days (PNDs 30-72)] and adult (PNDs 90-132) alcohol-preferring (P) rats with a drinking-in-the-dark-multiple-scheduled-access (DID-MSA) procedure used by our laboratory. Male and female P rats were provided concurrent access to 15% and 30% ethanol for three 1-h sessions across the dark cycle 5 days/week. For the 1st week, adolescent and adult female P rats consumed 3.4 and 1.6 g/kg of ethanol, respectively, during the 1st hour of access, whereas for male rats the values were 3.5 and 1.1 g/kg of ethanol, respectively. Adult intakes increased to ~ 2.0 g/kg/h and adolescent intakes decreased to ~ 2.5 g/kg/h across the 6 weeks of ethanol access. The daily ethanol intake of adult DID-MSA rats approximated or modestly exceeded that seen in continuous access (CA) rats or the selection criterion for P rats (≥ 5 g/kg/day). However, in general, the daily ethanol intake of DID-MSA peri-adolescent rats significantly exceeded that of their CA counterparts. BELs were assessed at 15-min intervals across the 3rd hour of access during the 4th week. Ethanol intake was 1.7 g/kg vs. 2.7 g/kg and BELs were 57 mg% vs. 100 mg% at 15- and 60-min, respectively. Intoxication induced by DID-MSA in female P rats was assessed during the 1st vs. 4th week of ethanol access. Level of impairment did not differ between the 2 weeks (106 vs. 97 s latency to fall, 120 s criterion) and was significant (vs. naïve controls) only during the 4th week. Overall, these findings support the use of the DID-MSA procedure in rats, and underscore the presence of age- and sex-dependent effects mediating ethanol binge-like drinking in P rats.     

Opiate dependence produced by ad libitum drinking of morphine in water,saline, and sucrose vehicles

Rats were given ad libitum morphine water, saline morphine, or sucrose morphine as their only source of liquid. Measures of liquid, food, caloric, and morphine intake along with body weight were taken daily, thereby monitoring the effects of morphine ingestion on these indices, and observing the course of dependence over time. To assess the degree of dependence the animals were given a two-bottle choice test between the drug solution and the vehicle, following a suitable ingestion period. The results indicated that the rats neglected morphine water and saline morphine in favor of the drug-free alternative. Concomitant morphine withdrawal signs (body weight loss, anorexia, adipsia, rhinorrhea, diarrhea, and irritability) were observed, thus indicating the animals were drug dependent. When morphine was given in sucrose solutions, the rats showed a marked preference for the drug solution over the vehicle.This work was supported by NSF research grants B023365 and P2B0349, and funds from the Graduate School of the University of Wisconsin-Milwaukee to K. A. Khavari.     

Schedule-induced ethanol polydipsia: enhancement by saccharin

The effect of adding sodium saccharin to a 5% ethanol solution on intake was examined. One set of animals were maintained at 80% of their free-feeding weight, in their home-cages with either 5% ethanol, or 5% ethanol-0.25% sodium saccharin as the only fluid available for three months (home-cage condition). A second set of animals were maintained in cages with automatic food dispensers that provided a 24 hr feeding regimen known to produce ethanol overdrinking (schedule-induced condition). These animals had 5% ethanol as their only available fluid for one month, followed by the 5% ethanol-0.25% sodium saccharin mixture for two months. No significant differences in ethanol intake were found between the 2 home-cage conditions (5% ethanol = 11.6 g ethanol/kg/day; 5% ethanol in 0.25% sodium saccharin = 11.7 g ethanol/kg/day. However, the addition of saccharin in the schedule-induced condition produced a marked increase in ethanol intake (5% ethanol = 13.1 g ethanol/kg/day; 5% ethanol in 0.25% sodium saccharin = 15.1 g ethanol/kg/day). The home-cage animals showed no sign of an abstinence syndrome upon substitution of water for ethanol. In the schedule-induced group, severe tonic-clonic seizures occured as a result of ethanol withdrawal.     

Naloxone suppresses feeding and drinking but not wheel running in rats

The effects of naloxone hydrochloride on food and water intake and number of wheel revolutions were measured in male rats. The administration of 10 mg/kg naloxone but not 1 mg/kg suppressed the 3-hr food and water intake in nondeprived rats. Naloxone injections (1 mg/kg or 10 mg/kg) were ineffective in altering the number of wheel revolutions in nondeprived or food deprived rats. These results support the interpretation that the suppressive effects of naloxone previously reported with deprived rats are evident in nondeprived rats and are specific to feeding and drinking.     

Naloxone attenuates voluntary ethanol intake in rats selectively bred for high ethanol preference

The effect of naloxone on voluntary ethanol intake was examined in rats which were selectively bred for oral ethanol preference (High Alcohol Drinking or HAD line). Rats of the HAD line were treated with naloxone in doses of 0.05-18.0 mg/kg b.wt. before access to water alone or to a free-choice between a 10% (v/v) ethanol solution and water. Naloxone suppressed water intake when water was presented as the sole source of fluid. In contrast, naloxone produced a dose-dependent decrease in ethanol consumption, without altering water intake, when rats were given a free-choice between the ethanol solution and water. Selective suppression of ethanol consumption by naloxone was not attributable to changes in blood ethanol concentrations or ethanol elimination rates following naloxone treatment. It appears that although naloxone may attenuate the positively reinforcing properties of both ethanol and water, ethanol drinking is a subset of consummatory behaviors that is particularly sensitive to opioid receptor blockade. The results suggest that activation of the endogenous opioid system may be an important mechanism which serves to maintain continued ethanol drinking.     

Total avoidance of saccharin consumption by rats after repeatedly paired injections of ethanol or LiCl

Rats injected with ethanol or LiCl following consumption of novel saccharin solution drank less saccharin than non-poisoned controls on a subsequent exposure with degree of aversion positively related to dose of ethanol (2–5 g/kg). While a single pairing of saccharin with ethanol or LiCl resulted in partial avoidance of saccharin,solution, repeated conditioning trials led to total avoidance of saccharin consumption by animals injected with the higher doses of ethanol or with LiCl. These results, characteristic of emetic-induced aversions, support the explanation of the limited consumption of ethanol by rats under ad lib, free-choice conditions as a result of acquired aversion to the oronasal sensory stimuli of ethanol after association with pharmacologically aversive aftereffects of consumed ethanol.     

Induction of dependence on ethanol by free-choice drinking in alcohol-preferring rats

Studies were performed to examine whether chronic, voluntary consumption of ethanol by the selectively-bred, alcohol-preferring P-rats produces physical dependence. Body weight reduction, food restriction and flavoring the 10% ethanol solution increased ethanol consumption from 7 to 14 g ethanol/kg body weight/day when water was freely available. Under similar conditions, consumption by selectively-bred, alcohol-nonpreferring NP-rats increased from 1 to 12 g/kg/day. Removal of ethanol after eight weeks induced physical signs of withdrawal in both lines of animals. In two subsequent studies, P-rats were given food, water and unflavored 10% ethanol ad lib for 15 and 20 weeks; ethanol consumption was 7.2 and 5.6 g/kg/day, respectively. Upon removal of ethanol, manifestations of withdrawal, scored blind in one experiment, developed in 85% of the animals and persisted for 72 hours. Importantly, none in the control groups of P and NP rats given water only exhibited these signs. The ethanol withdrawn groups were hyperactive in both the open-field and the head-poke apparatus. These results indicate that sufficient ethanol was voluntarily consumed by the selectively-bred alcohol-preferring P-rats under free-feeding conditions to produce physical dependence.     

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.     

Nonselective suppression of operant ethanol and sucrose self-administration by the mGluR7 positive allosteric modulator AMN082

Emerging evidence indicates that specific metabotropic glutamate receptors (mGluRs) modulate ethanol self-administration. In general, inhibition of glutamate transmission through blockade of postsynaptic mGluRs, or activation of presynaptic mGluRs, inhibits ethanol self-administration. The goal of this preclinical study was to further characterize mGluR regulation of ethanol self-administration by examining effects of AMN082, an allosteric positive modulator of presynaptic mGluR7 activity. Separate groups of C57BL/6J male mice were trained to self-administer ethanol or sucrose on a fixed-ratio 4 schedule of reinforcement during 1 h sessions. On test days, mice were pretreated with AMN082 (0, 1.0, 3.0, 5.6, or 10 mg/kg) 30 min prior to self-administration sessions. Functional specificity and activity was examined by testing the effects of AMN082 (0-10 mg/kg) on open-field locomotor activity and HPA axis function as measured by plasma corticosterone levels. AMN082 (10 mg/kg) produced a significant reduction in ethanol and sucrose reinforced responding, and inhibited locomotor activity. Plasma corticosterone levels were significantly increased following AMN082 (5.6 and 10 mg/kg) suggesting a dose-dependent dissociation between the behavioral and hormonal effects of the compound. These data suggest that activation of mGluR7 by AMNO82 produces nonspecific reductions in motivated behavior that are associated with negative effects on motor activity.     

Naloxone administration following operant training of sucrose/water discrimination in the rat

 The suppression of food intake observed following naloxone administration has often been ascribed to palatability or taste. Unfortunately, many confounds become apparent when attempts are made to isolate such factors in the investigation of ingestive behaviors. In the present study, rats (two groups) were trained to discriminate either a 10% or 5% sucrose solution from water (0.1 ml). These mildly food deprived subjects (95% of free-feeding weight) were trained to press the appropriate lever in a two-lever operant chamber following sampling of sucrose or water; successful responding was reinforced by delivery of a 45 mg grain food pellet. Following random exposure to reduced sucrose concentrations tested under extinction, a sucrose concentration gradient (1.0, 0.5, 0.1, 0.05, 0.01 and 0.005% sucrose solution) was established for both training groups under IP saline administration. Data collected under IP saline were then compared to those collected following random IP naloxone administration (3.0, 1.0, 0.3 and 0.1 mg/kg). No significant differences were observed between the sucrose concentration gradients obtained under saline and those obtained under naloxone, suggesting that the anorectic effect of naloxone is not primarily determined by discrimination of sweet taste. Received: 4 September 1996 / Final version: 16 October 1996     

An inter-gender effect on ethanol drinking in rats: proximal females increase ethanol drinking in males

Three groups of male Long-Evans hooded rats were assessed for effects of social opportunity on drinking of ethanol or water. The ethanol/female group received intermittent presentations of a sipper containing ethanol that was followed by 15 s of social interaction opportunity with a female rat. The ethanol/male group received similar training except the social interaction opportunity was with a male rat. The water/female group received training similar to the ethanol/female group except that the sipper contained water. For the ethanol groups, the concentration of ethanol [3%, 4%, 6%, 8% and 10% (vol/vol)] in the sipper was increased across sessions. With 10% ethanol in the sipper, social opportunity with females induced more drinking and ethanol intake than did social opportunity with males. Social opportunity with females induced more intake of ethanol than water. Post-session plasma samples revealed social opportunity with females induced higher corticosterone and testosterone levels than did social opportunity with males, irrespective of the sipper fluid. This study documents, for the first time, an inter-gender effect on ethanol drinking in rats.