Social influences on morphine-conditioned place preference in adolescent BALB/cJ and C57BL/6J mice

Rationale  

Among human adolescents, drug use is substantially influenced by the attitudes and behaviors of peers. Social factors also affect the drug-seeking behaviors of laboratory animals. Conditioned place preference (CPP) experiments indicate that social context can influence the degree to which rodents derive a rewarding experience from drugs of abuse. However, the precise manner by which social factors alter drug reward in adolescent rodents remains unknown.     

The discriminative stimulus properties of methylphenidate in C57BL/6J mice

Methylphenidate (MPH) therapy for attention-deficit/hyperactivity disorder is common in children and adults. Concerns regarding abuse of MPH prompted studies to better understand its pharmacology. We used an established drug discrimination task to determine whether MPH could be discriminated by C57BL/6J (B6) mice. B6 mice learned to discriminate cues produced by racemic MPH (dl-MPH 5.0 mg/kg) or half the dose of pure d-isomer (2.5 mg/kg), and dose-response tests established appropriate reductions in discrimination with declining dose. Importantly, the two drug forms generalized to each other completely in substitution tests; consistent with reports that the l-isomer is pharmacodynamically inactive. An additional experiment indicated that lower doses (1 and 2 mg/kg) of dl-MPH did not support acquisition of MPH discrimination despite extensive training. Mice acquired discrimination of dl-MPH only when the dose was increased to 4 mg/kg. Thus, although these lower doses increased drug lever responding in mice trained on the higher dose, their stimuli were not sufficient to support acquisition of the discrimination task. These findings correspond to earlier studies conducted in our laboratory on threshold doses needed to produce stimulatory effects of motor activity in B6 mice. These preclinical findings provide insight into the relative potency, and by extension, efficacy of dl-MPH versus d-MPH doses.     

DBA/2J mice develop stronger lithium chloride-induced conditioned taste and place aversions than C57BL/6J mice

Genetic differences in lithium-induced conditioned aversion were examined using both place- and taste-conditioning procedures. In the place-conditioning procedure, adult male C57BL/6J (B6) and DBA/2J (D2) mice were exposed to a differential conditioning procedure in which each mouse received four 30-min pairings of a distinctive floor cue immediately after IP injections of either 0.75, 1.5, or 3. 0 mEq/kg LiCl. A different floor cue was paired with saline injections. A separate group of control mice received saline injections paired with both floor types. Subsequent floor preference testing revealed greater conditioned aversion in D2 mice compared to B6 mice in groups receiving 3.0 mEq/kg LiCl. Lower LiCl doses did not produce conditioning in either strain. In a conditioned taste-aversion procedure, fluid-restricted mice received four trials in which access to 0.2 M NaCl solution was followed by IP injection of either 0.75, 1.5, 3.0, or 6.0 mEq/kg LiCl. D2 mice showed stronger conditioned taste aversion than B6 mice at all doses, suggesting that taste conditioning may be a more sensitive index of aversive drug sensitivity than place conditioning. These findings are not well explained by strain differences in general learning ability or by strain differences in stimulus salience or innate preference. Rather, these data appear more consistent with previous studies showing strain differences in lithium pharmacokinetics and in general sensitivity to aversive events.     

Effects of withdrawal from chronic escalating-dose binge cocaine on conditioned place preference to cocaine and striatal preproenkephalin mRNA in C57BL/6J mice

Relapse is a serious problem for the effective treatment of cocaine addiction.RationaleExamining cocaine re-exposure-induced behavioral and neurobiological alterations following chronic escalating-dose binge cocaine administration and withdrawal may provide insight into the neurobiological basis of cocaine relapse.ObjectivesOur goal was to determine how exposure to chronic escalating-dose cocaine affects development of subsequent cocaine-induced conditioned place preference (CPP) and changes in endogenous opioid systems.MethodsMice were injected with either escalating-dose binge cocaine (15–30 mg/kg/injection × 3/day) or saline for 14-days and conditioned with 15 mg/kg of cocaine or saline (once per day for 10-days), starting either 1 or 14-days after the last day of binge injections.ResultsMice exposed to chronic escalating cocaine did not develop CPP to cocaine when conditioning commenced on the first day of withdrawal (CPP test on day 10 of withdrawal). By contrast, mice did develop CPP to cocaine when conditioning started on the 14th day of withdrawal (CPP test on day 24 of withdrawal). Furthermore, preproenkephalin (Penk) mRNA levels in caudate putamen were significantly higher in mice that received 14-day withdrawal from escalating-dose binge cocaine before the CPP procedure (tested 24 days post-binge) than those that received 1-day withdrawal (tested 10 days post-binge).ConclusionsThe rewarding effect of cocaine was blunted in early withdrawal from chronic escalating exposure, but recovered in more prolonged withdrawal. Time-dependent elevations in Penk mRNA levels may be part of the underlying mechanisms of this effect.     

Conditioned place preference after single doses or "binge" cocaine in C57BL/6J and 129/J mice

The rewarding effect of cocaine as reflected by the development of conditioned place preference was examined in C57BL/6J and 129/J mice. Cocaine was administered in a single daily dose (2.5, 5, 10 and 20 mg/kg ip) or in a "binge" pattern (15 mg/kg ip x3, hourly). Mice remained in the conditioning compartment for 30 min immediately after each injection. Single injections of cocaine from 5 to 20 mg/kg induced conditioned place preference in each strain of mice. Only C57BL/6J mice developed conditioned place preference after "binge" cocaine administration. Both strains showed significantly greater locomotion in the conditioning compartment across the range of single doses of cocaine and after "binge" cocaine administration, but only 129/J mice showed sensitization. When mice that had received the single 10 mg/kg dose were retested 4 weeks later, the amount of time spent in the preferred side was significantly reduced compared to the initial test in the 129/J, but not in C57BL/6J mice. Thus, the persistence of conditioned place preference is strain dependent. The fact that 129/J mice did not develop conditioned place preference after "binge" cocaine administration, but did after single doses, suggests that the rewarding effects of cocaine are influenced by pattern of administration, a factor that may be relevant to the development of human cocaine addiction.     

Susceptibility to conditioned place preference induced by addictive drugs in mice of the C57BL/6 and DBA/2 inbred strains

Rationale In previous studies, we have demonstrated that mice of the inbred strain C57BL/6J (C57) are more susceptible to amphetamine-induced conditioned place preference (CPP) than DBA/2J (DBA) mice. Moreover, we also observed parallel strain differences for the locomotor-stimulant effects of the drug. However, other studies have reported either no difference or opposite strain differences for cocaine- and morphine-induced CPP as well as for the locomotor effects of these drugs, suggesting that amphetamine-related behavioral phenotypes might depend on a specific pharmacological action of the psychostimulant. Objectives This study was aimed at testing strain differences for cocaine- and morphine-related behavioral phenotypes in the same experimental protocol and conditions previously used for amphetamine. Methods C57 and DBA mice were tested for CPP induced by cocaine (0, 5, 10, and 20 mg/kg) and morphine (0, 5, 7.5, and 10 mg/kg). Locomotor activity data were simultaneously obtained by measuring distance moved during all different CPP phases and unconditioned locomotor activity, behavioral sensitization and conditioned hyperactivity were measured together with CPP. Results (a) Either cocaine or morphine promoted significant CPP at lower doses in C57 than in DBA mice; (b) only drug-trained C57 mice showed a significant CPP compared with the control group; and (c) only C57 mice showed dose-dependent effects of cocaine on CPP. Moreover, there was no relationship between drug-induced CPP and locomotion. Conclusions The results demonstrate that C57 and DBA mice differ in their sensitivity to cocaine- and morphine-induced CPP and suggest that the two strains differ in sensitivity to the positive incentive properties of drugs of abuse.     

Influence of the dose and the number of drug-context pairings on the magnitude and the long-lasting retention of cocaine-induced conditioned place preference in C57BL/6J mice

Rationale The place conditioning procedure is increasingly used to study relapse in drug seeking in mice. However, the retention course of drug-induced place preference has not been systematically characterized.Methods The effects of cocaine doses and number of conditioning trials on both the magnitude and the persistence of cocaine-induced conditioned place preference (CPP) were investigated in C57BL/6J mice. Twelve groups of animals were injected with saline, 4, 8 or 12 mg/kg cocaine (i.p.) and submitted to an unbiased counterbalanced place conditioning protocol including one, two or four drug-pairing sessions. Subsequently, the animals were tested at various time intervals after the last conditioning session.Results One cocaine-pairing session was insufficient to induce a CPP. Two and four pairing sessions resulted in significant place preferences of similar magnitude for all tested doses of cocaine, the place preference induced by the greatest number of pairing sessions being the strongest. In the two-pairing groups, place preference lasted less than 14 days for any tested dose of cocaine. In contrast, all four-pairing groups still showed significant place preference 28 days after the last conditioning session. However, the magnitude of cocaine place preference slowly declined at a rate that was dependent upon cocaine dose. On the 35-day post-conditioning interval, only the 12-mg/kg cocaine group still displayed a significant place preference, whereas place preference was undetectable at 42 and 56 days post-conditioning for all groups.Conclusions The number of cocaine-pairing sessions, but not cocaine dose, affected the magnitude of cocaine place preference in mice when tested 1 day after the last conditioning session. In contrast, both cocaine doses and the number of pairing sessions affected the persistence of cocaine place preference. Overall, these results demonstrate that cocaine-induced place preference is a long lasting phenomenon that is strongly affected by the number of drug-pairing trials.     

Intravenous cocaine-induced activity in A/J and C57BL/6J mice: behavioral sensitization and conditioned activity

The stimulant properties of cocaine have been extensively investigated in the mouse using either intraperitoneal (i.p.) or subcutaneous (s.c.) administration of drug. However, cocaine use in humans often involves intravenous (i.v.) administration of drug. The purpose of this study was to develop a methodology for studying i.v. cocaine-induced activity in the mouse, which allows within-session determination of the dose-response function, and assessment of the development of behavioral sensitization and conditioned activity. The stimulant effects of i.v. cocaine (3-25 mg/kg) were investigated in C57BL/6J and A/J mice both acutely and following repeated treatments (four treatments at 48 hour intervals), in addition to the conditioned activating properties of the cocaine-paired context. Cocaine produced a dose-dependent increase in measures of motor activity in both strains of mice. Repeated cocaine treatments resulted in the development of behavioral sensitization to the stimulant properties of the drug at all doses tested, and exposure to the cocaine-paired context in the absence of drug revealed the development of conditioned activity. While both C57BL/6J and A/J strains displayed these phenomena, differences were observed between ambulation and total beam breaks, highlighting differences between multiple behavioral end-points. Both strains of mice displayed conditioned activity of a higher magnitude than their response to novelty, in addition to a positive relationship between the number of drug-environment pairings and the magnitude of the conditioned response. In summary, these data extend to the i.v. route of administration previous observations on cocaine-induced activity and conditioned activity.     

PDE7A expression on alcohol consumption and alcohol withdrawal in striatum of C57BL/6J mice

OBJECTIVE To observe PDE7A expression on alcohol consumption and alcohol withdrawal in the striatum of C57BL/6J mice. METHODS Forty adult female C57BL/6J mice were randomly divided into 3 groups: water intake control, alcohol(10% v/v) drinking(18 h · d~(-1) for 7 d), and alcohol withdrawal(for 24 h).PDE7A expression was analyzed using Western blotting in striatal tissues from mice. RESULTS Compared to the control, alcohol drinking mice displayed increases in the expression of PDE7A in the striatum, which decreased to normal 24 h after withdrawal. CONCLUSION The results suggest that PDE7, in particular PDE7A, may play a unique role in the regulation of ethanol depdendence.     

Naltrexone effects on ethanol consumption and response to ethanol conditioned cues in C57BL/6 mice

RATIONALE: The conditions under which naltrexone reduces ethanol consumption and its effect on behavior controlled by ethanol conditioned stimuli remain unclear. OBJECTIVES: The objectives were to determine the effects of naltrexone on ethanol consumption by C57BL/6 (B6) mice when injected subcutaneously (expt 1) or delivered by osmotic minipump (expt 2), and on ethanol conditioned cues (expt 3). METHODS: Naltrexone effects on ethanol consumption and preference were measured in a continuous access two-bottle choice paradigm in groups of mice implanted with osmotic minipumps delivering 0-3.0 mg/kg per day or injected subcutaneously with 0-6.0 mg/kg doses. Naltrexone's (0-3.0 mg/kg) effect on ethanol-conditioned cues was indexed by its effect on the expression of ethanol place conditioning (expt 3). RESULTS: Naltrexone produced a transient reduction in ethanol consumption and a consistent reduction in preference when injected; however, it had no effect on ethanol consumption or preference when delivered continuously by osmotic minipump. Naltrexone attenuated the expression of ethanol place conditioning in a U-shaped dose-response function. CONCLUSIONS: The transient reduction in ethanol consumption produced by injected naltrexone and the absence of an effect when continuously delivered confirms a report that maintaining naltrexone at steady state levels may antagonize its attenuation of ethanol consumption. The reduced expression of ethanol place conditioning in naltrexone-injected mice suggests that the drug can attenuate the reinforcing effects of ethanol conditioned stimuli as was recently reported for lever responding maintained by ethanol conditioned stimuli in rats. These effects were observed at naltrexone doses with no readily apparent adverse side-effects, supporting its usefulness for treating alcoholism.     

Effects of acute alcohol administration on object recognition learning in C57BL/6J mice.

The goal of the present study was to investigate effects of alcohol intoxication on the object recognition learning task. Male C57BL/6J mice habituated to saline injections and exploratory arena received different doses of ethanol (0, 1.6 or 2.4 g/kg) before or after a 10-min training session. During training, animals were exposed to a small object (a marble or a die). On the next day, during a 10-min testing session, animals were exposed to two objects: the familiar object from the previous day and a novel object. Analysis of behavior during testing showed that mice injected with 0 and 1.6 g/kg of ethanol before training spent more time exploring a novel than a familiar object during testing. In contrast, mice injected with 2.4 g/kg ethanol spent equal amounts of time exploring the novel and the familiar object. Mice injected with this dose of ethanol after training did not show a decreased ratio of object exploration during testing. Analysis of behavior during training showed that mice injected with this dose of ethanol spent less time exploring the object, although their locomotor activity was not decreased. Our results show that in C57BL/6J mice, ethanol intoxication interferes with exploratory activity during object exploration, but not with consolidation of memory.     

Responding for conditioned reinforcement in C57BL/6 and CD-1 mice,and Sprague-Dawley rats: Effects of methylphenidate and amphetamine

Rationale

Characterization of responding for conditioned reinforcement in mice is important to implement genetic tools in examining the neurobiological mechanisms underlying reward-related learning and incentive motivation.

Methods

Inbred C57BL/6 mice, outbred CD-1 mice, and outbred Sprague–Dawley rats underwent Pavlovian conditioning in which a conditioned stimulus (CS) was paired with saccharin. Subsequently, subjects were allowed to respond for that CS in tests of responding for conditioned reinforcement. Experiments measured the effects of methylphenidate (MPH) and amphetamine (AMPH) on lever pressing for conditioned reinforcement in mice and rats. We further examined the stability of responding for conditioned reinforcement in mice after repeated testing and the extinction of this behaviour following omission of the reinforcer. We also determined whether the CS exhibited reinforcing properties if it was not paired with saccharin.

Results

C57BL/6 and CD-1 mice learned to respond for a conditioned reinforcer similarly to rats, and the behaviour was stable over time. MPH increased responding in CD-1 mice and rats, but not in C57BL/6 mice. AMPH only increased responding in rats. Responding was reduced following omission of the conditioned reinforcer, and responding was only established when the CS was paired with saccharin.

Conclusions

These experiments characterize a conditioned reinforcement test which produces stable responding in two different mouse backgrounds. These findings also show that dopaminergic psychomotor stimulants can differently affect rats and mice in tests of responding for conditioned reinforcement.     

Stimulatory effects of ethanol in C57BL/6 mice

Although ethanol stimulation is well documented in several species including humans, there is some controversy about whether the stimulation occurs in the highly inbred mouse strain, C57BL/6. Since inbred mouse strains are frequently used to elucidate mechanisms for individual differences in reaction to alcohol, the present study was undertaken to more completely characterize the behavioral effects of ethanol and to help resolve some of the controversy regarding the drug's stimulatory effect on C57 mice. Activity of female C57BL/6cr mice was assessed in either a lighted or dark environment for 20 min after injections of water or ethanol at doses of 0.5, 1.0, 2.0, 4.0 g/kg. Elevated activity (stimulation) was observed in mice injected with relatively low ethanol doses and tested in the light. The 2.0 g/kg dose produced a transient elevation in activity which declined rapidly across time. Animals tested under the dark condition were not stimulated by the drug but had activity reductions to high doses of ethanol. The detection of ethanol-induced stimulation appears to be related to the performance of control mice rather than a light-related difference in ethanol sensitivity.     

Interactive effects of ethanol and nicotine on learning in C57BL/6J mice depend on both dose and duration of treatment

Objective and rationale Alcohol and nicotine are commonly co-abused; one possible explanation for co-abuse is that each drug ameliorates the aversive effects of the other. Both drugs have dose-dependent effects on learning and memory. Thus, this study examined the interactive effects of acute ethanol and acute, chronic, or withdrawal from chronic nicotine on fear conditioning in C57BL/6J mice. Materials and methods Conditioning consisted of auditory conditioned stimulus-foot-shock unconditioned stimulus pairings. For acute studies, saline or ethanol, then saline or nicotine was administered before training, and saline or nicotine was also administered before testing. For chronic and withdrawal studies, saline or nicotine was administered chronically, and ethanol or saline was administered before training. Results Acute nicotine (0.09 mg/kg) reversed ethanol-induced deficits (1.0 and 1.5 g/kg) in contextual and cued fear conditioning, whereas a low dose of ethanol (0.25 g/kg) reversed nicotine (6.3 mg kg⁻¹ day⁻¹) withdrawal-induced deficits in contextual conditioning. Tolerance developed for the effects of nicotine on ethanol-induced deficits in conditioning and cross-tolerance between chronic nicotine and acute ethanol was seen for the enhancing effects of ethanol on conditioning. Conclusions The complex and sometimes polar actions of ethanol and nicotine on behavior may contribute to co-abuse of these drugs. Specifically, smoking may initially reduce the aversive effects of ethanol, but tolerance develops for this effect. In addition, low doses of alcohol may lessen nicotine withdrawal symptoms.     

Diet-induced obesity and cardiovascular regulation in C57BL/6J mice

1. In the present study, we determined the effect of diet-induced obesity on cardiovascular and metabolic regulation in mice at standard laboratory temperatures (ambient temperature (Ta) = 22 degrees C) and during exposure to thermoneutrality (Ta = 30 degrees C). 2. Male C57BL/6J (B6) mice fed a high-fat diet (HFF; n = 17) or chow (CHW; n = 14) for 15 weeks were surgically instrumented with telemetry devices, housed in metabolic chambers and assigned to either control or atenolol treatment (25 mg/kg per day in drinking water) to determine the effects of obesity on baseline cardiovascular function and on the responses to thermoneutrality and 24 h fasting. Mean arterial pressure (MAP), heart rate (HR), arterial pressure and HR variability (time and frequency domain), oxygen consumption (VO2) and locomotor activity were determined. 3. The HFF mice exhibited increased bodyweight (+10.6 +/- 4.1 g), moderate light period hypertension (+8.6 +/- 2.6 mmHg), no difference in HR and increased HR variability at standard laboratory temperature compared with CHW controls. Atenolol produced less of a decrease in HR in HFF mice (-42 +/- 10 b.p.m.) compared with CHW controls (-73 +/- 15 b.p.m.). Acute exposure to thermoneutrality (Ta = 30 degrees C) reduced HR similarly in both HFF and CHW mice (approximately 175 b.p.m.), but reduced MAP less in HFF than in CHW mice (-7.3 +/- 2.5 and -15.2 +/- 1.0 mmHg), respectively. Atenolol treatment had only minor effects on the HR response to thermonuetrality (-114 +/- 13 and -129 +/- 8 b.p.m. in HFF and CHW mice, respectively). The HFF mice displayed greater fasting-induced reductions in light period MAP than did CHW mice (-10.0 +/- 1.1 vs-3.1 +/- 3.5 mmHg, respectively), whereas HR was decreased equally in both groups. Fasting-induced increases in HR variability were attenuated in HFF mice. 4. We conclude that diet-induced obesity produced generally minor changes in cardiovascular regulation in B6 mice at baseline, some of which are distinct from the effects of diet-induced obesity in larger animal models. In contrast, acute variations in Ta or caloric availability produce pronounced alterations in cardiovascular function in either lean or obese mice, which are generally evident after atenolol and, thus, presumably not due exclusively to variation in cardiac sympathetic activity. Interestingly, the degree of obesity induced hypertension was augmented when mice were studied at thermonuetrality. The results suggest an important unrecognized role for vagal tone in the regulation of cardiovascular function in mice and support the need for considerable caution when using mouse models of obesity to examine regulation of cardiovascular function. We argue that mouse physiology studies should be performed in thermoneutral conditions.     

Quasi-asymptotic development of conditioned hyperactivity induced by intermittent injections of cocaine in C57BL/6J mice

The emergence of a conditioned cocaine-induced hyperlocomotion was examined in C57BL/6J mice using a procedure that has not been used previously. Two days after a session of preexposure to the test chambers under saline, a first group of mice (cocaine-cued) received five once-daily injections of 10-mg/kg s.c. cocaine every other day (on the odd days of the chronic treatment period) and a saline injection on the 5 days following each cocaine injection day (on the even days of the treatment period), in all cases before being placed in the test chamber. Another group of mice (saline-cued) received 10 injections of saline on both the even and the odd days in the same context, and a third group of mice (cocaine-uncued) received five injections of saline on the even days in the test context and five injections of cocaine on the odd days in an alternative context. On the odd days sessions, the cocaine-cued group showed significant repeated increases in locomotion without behavioural sensitisation being induced, whereas the saline-cued levels of locomotion remained on baseline levels. On the first even session, the three groups did not differ from each other and showed lower levels of locomotion than on the preexposure session. During the two following even sessions, the cocaine-cued group showed an increase in locomotion that levelled off on the two remaining sessions, whereas the saline-cued and the cocaine-uncued groups (which presented comparable values) exhibited significantly lower levels of locomotion. That pattern of successive placebo responses resembles the typical S-shaped development of a Pavlovian conditioned response, albeit the increase described here was quite rapid. The protocol used here may provide an additional method for the experimental analysis of stimulant-induced conditioned placebo activity.     

Nicotine-produced relearning deficit in C57BL/6J and DBA/2J mice

Acute nicotine administration has been shown to influence the acquisition and retention of learning tasks. In order to investigate the many possible behavioral and pharmacological effects of nicotine, a modified 2×2 statedependent learning design was used to assess nicotine's effects on active avoidance learning. Male and female mice of the C57BL/6J (C57) and DBA/2J (DBA) inbred strains were injected with a control solution or with 0.5, 1.0, or 2.0 mg/kg nicotine 5 min before the start of training and, following a 24-h period, 5 min before retraining. Nicotine had no effect on the acquisition of the learning task but, depending on strain and sex, did have an effect on relearning. Relearning in the C57 males was unaffected by nicotine injection, whereas the most prominent effect of nicotine in the C57 females and the DBA males and females was a retrieval deficit. The prevalence of a nicotine-induced retrieval deficit in the present experiment suggests that those mechanisms underlying the retrieval of previously learned information are, in part, mediated or modulated by perturbations within nicotine-sensitive areas of the central nervous system.     

Low alcohol preference among the "high alcohol preference" C57/BL10 mice; factors affecting such preference

The effects of age, ethanol concentration and minor stress on the variation in alcohol preference of C57 strain mice were determined. In two bottle choice tests, an older population of mice contained slightly more low-preference mice than a younger population. A wide range of ethanol preference was consistently seen in young mice for 8% and 6% ethanol, but the previously reported biphasic pattern of distribution was revealed only with 8% ethanol. Very few animals showed high preference for concentrations of 10% or 12% ethanol. Moving low alcohol preference mice to a new location (but not repeated cage changing or ultrasonic noise) significantly increased the alcohol preference. Exploratory locomotor activity did not correlate with the subsequent alcohol consumption. Blood and brain alcohol concentrations showed that the differences in alcohol preference were not due to differences in metabolism of ethanol. The C57 strain mice with low preference for alcohol provides a valuable model for the study of the effects of minor stress on alcohol consumption.