Extinction of ethanol-induced conditioned place preference and conditioned place aversion: effects of naloxone

 Four experiments examined the effect of naloxone pretreatment on the expression and extinction of ethanol-induced conditioned place preference (experiments 1, 2, 4) or conditioned place aversion (experiments 1, 3). DBA/2 J mice received four pairings of a distinctive tactile (floor) stimulus (CS) with injection of ethanol (2 g/kg) given either immediately before or after 5-min exposure to the CS. A different stimulus was paired with injection of saline. Pre-CS injection of ethanol produced conditioned place preference, whereas post-CS injection of ethanol produced conditioned place aversion. Both behaviors extinguished partially during repeated choice testing after vehicle injection. Naloxone (10 mg/kg) had little effect on the initial expression of conditioned place preference, but facilitated its extinction. Moreover, repeated naloxone testing resulted in the expression of a weak conditioned place aversion to the CS that initially elicited a place preference. In contrast, naloxone (1.5 or 10 mg/kg) enhanced expression of conditioned place aversion, thereby increasing its resistance to extinction. A control experiment (experiment 4) indicated that repeated testing with a different aversive drug, lithium chloride, did not affect rate of extinction or produce an aversion to the CS previously paired with ethanol. These findings do not support the suggestion that naloxone facilitates the general processes that underlie extinction of associative learning. Also, these data are not readily explained by the conditioning of place aversion at the time of testing. Rather, naloxone’s effects appear to reflect a selective influence on maintenance of ethanol’s conditioned rewarding effect, an effect that may be mediated by release of endogenous opioids. Overall, these findings encourage further consideration of the use of opiate antagonists in the treatment of alcoholism. Received: 4 December 1997 / Final version: 16 February 1998     

Handling blocks expression of conditioned place aversion but not conditioned place preference produced by ethanol in mice

Previous findings implicate opioid receptors in the expression of the conditioned rewarding and aversive properties of ethanol. We have recently reported that the conditioned rewarding effect of ethanol is mediated by opioid receptors in the ventral tegmental area (VTA). We attempted to determine whether VTA opioid receptors also mediate the expression of the conditioned aversive properties of ethanol. However, the magnitude of conditioned place aversion (CPA) was not consistent with our previous findings and prevented us from making definitive conclusions. We hypothesized that the handling required to make intracranial infusions in mice alters the expression of CPA, but not conditioned place preference (CPP). Therefore, non-operated animals underwent a Pavlovian conditioning procedure for either ethanol CPA or CPP. Just before testing, half of the animals were held by the scruff of the neck to mimic intracranial infusion handling. Animals conditioned for CPA did not express CPA if they were handled. However, animals conditioned for CPP exhibited robust CPP, regardless of handling. These findings provide additional evidence that the conditioned rewarding and aversive effects of ethanol are mediated by different neural mechanisms.     

GABAA receptors modulate ethanol-induced conditioned place preference and taste aversion in mice

  Rationale: GABA_A receptor antagonists have been shown to reduce ethanol self-administration and ethanol-induced conditioned taste aversion (CTA) in rats, suggesting a role for the GABA_A receptor in modulating ethanol’s motivational effects. Objectives: The present experiments examined the effects of the GABA_A receptor antagonists, bicuculline and picrotoxin, on the acquisition of ethanol-induced conditioned place preference (CPP) and CTA in male DBA/2J mice. Methods: Mice in the CPP experiments received four pairings of ethanol (2 g/kg) with a distinctive floor stimulus for a 5-min conditioning session (CS+ sessions). During CS+ sessions, mice also received bicuculline (0, 1.0, 3.0, or 5.0 mg/kg) or picrotoxin (2.0 mg/kg) before an injection of ethanol. On intervening days (CS– sessions), the pretreatment injection was always vehicle followed by saline injections that were paired with a different floor type. For the preference test, all mice received saline injections and were placed on a half grid and half hole floor for a 60-min session. For the CTA experiments, mice were adapted to a 2-h per day water restriction regimen followed by five conditioning trials every 48 h. During conditioning trials, subjects received an injection of vehicle, bicuculline (0.5 and 2.0 mg/kg), or picrotoxin (0.75 and 2.5 mg/kg) before injection of 2 g/kg ethanol or saline following 1-h access to a saccharin solution. Results: Both picrotoxin and the lowest dose of bicuculline (1.0 mg/kg) significantly increased the magnitude of CPP relative to vehicle-treated controls. Picrotoxin alone did not produce place conditioning. Ethanol-stimulated locomotor activity was significantly reduced during conditioning trials with picrotoxin and the higher doses of bicuculline (3.0 and 5.0 mg/kg). Bicuculline did not alter ethanol-induced CTA; however, picrotoxin dose-dependently increased the magnitude of ethanol-induced CTA. Bicuculline and picrotoxin did not produce CTA when administered alone. Conclusions: Overall, these results suggest that blockade of GABA_A receptors with bicuculline and picrotoxin enhances ethanol’s motivational effects in the CPP paradigm; however, only picrotoxin enhances ethanol’s motivational effects in the CTA paradigm. Received: 12 September 1998 / Final version: 21 December 1998     

Nicotine place preference in a biased conditioned place preference design

Conditioned place preference (CPP) is often more effectively produced with nicotine using a biased procedure. Interpretation of results can be problematic, however, given that doses that produce CPP in rats have acute anxiolytic and residual anxiogenic effects. We tested three groups of male rats in a biased, 2-chambered apparatus. Over eight conditioning days, one group (paired group) received four alternating injections of nicotine paired with the non-preferred (white) chamber and of saline in the preferred (black) chamber. A second group (counterbalanced group) received two nicotine injections each paired with the black and white chambers, with saline pairings on alternate days. A third group (saline control) received saline injections paired with both chambers. Following conditioning, the paired group spent significantly more time in the initially non-preferred chamber relative to saline-treated controls, suggesting CPP. The counterbalanced group did not show a significant preference shift, providing evidence that the observed preference shift in the paired group was not due to a drug-induced unconditioned reduction in aversion. Although this finding is consistent with the notion that nicotine produced CPP through its rewarding effects, we cannot discount the possibility of a conditioned reduction in aversion to the non-preferred chamber. For the paired group, a negative correlation was found between time spent in the white chamber before conditioning and preference shift following conditioning, suggesting that animals showing greater initial aversion to a non-preferred context are more likely to form CPP.     

Ethanol-induced conditioned place aversion in mice

Previous studies have shown that ethanol produces conditioned place preference (CPP) in mice when injections are given immediately before exposure to the conditioned stimulus (CS). Paradoxically, however, injection of ethanol immediately after the CS produces conditioned place aversion (CPA). Four experiments were conducted to characterize the parametric boundaries of CPA produced by post-CS ethanol exposure. Experiment 1 showed that CPA is positively related to ethanol dose, with significant CPA at 2 and 4 g / kg, but not at 1 g / kg. Experiment 2 revealed an inverse relationship between CPA and trial duration, i.e. significant CPA occurred when the trial duration was 5, 15 or 30 min, but not when it was 60 or 90 min. Experiment 3 indicated that ethanol pre-exposure (eight daily injections) significantly reduced subsequent development of CPA. Finally, experiment 4 showed that repeated exposure to the CS alone (six 30 min exposures to each CS) after CS-ethanol pairings produced complete extinction of CPA. The same extinction procedure also completely eliminated CPP induced by pre-CS injections of ethanol. Overall, these studies demonstrate that CPA induced by post-CS ethanol injection is influenced by many of the same variables that affect CPP produced by pre-CS ethanol injection in mice. However, these findings do not resolve the issue of whether the 'before-versus-after' effect in ethanol place conditioning is better explained by assuming ethanol produces only rewarding effects or by assuming that ethanol produces both rewarding and aversive effects.     

Reinstatement of nicotine-conditioned place preference by drug priming: effects of calcium channel antagonists

Reinstatement of drug-seeking behaviour in animals is relevant to drug relapse in humans. In the present study, we used the conditioned place preference paradigm to investigate the establishment, extinction, reinstatement and cross-reinstatement of nicotine-induced place conditioning in rats. Nicotine produced a place preference to the initially less-preferred compartment paired with its injections during conditioning (0.5 mg/kg, i.p., three drug sessions). Once established, nicotine place preference was extinguished by repeated training. Following this extinction phase, the reinstatement of place conditioning was investigated. For this purpose, nicotine-experienced rats were challenged with nicotine (0.5 mg/kg, i.p.) or morphine (10 mg/kg, i.p.). These priming injections of both drugs renewed a marked preference for the compartment previously paired with nicotine. In the second step, we examined the influence of the calcium channel antagonists, nimodipine (10 and 20 mg/kg, i.p.) and flunarizine (5 and 10 mg/kg, i.p.), on the reinstatement of nicotine-conditioned place preference induced by priming doses of nicotine and morphine. It was shown that the calcium channel blockers dose dependently attenuated the reinstatement of nicotine place preference induced by both drugs. These findings support the hypothesis that similar neural calcium-dependent mechanisms are involved in nicotine- and morphine-induced reinstatement. Finally, the conditioned place preference paradigm appears to be a useful tool for studies of the relapse of drug-seeking behaviour in laboratory animals.     

Pentylenetetrazole can induce a conditioned place preference

Thirty-two male Sprague-Dawley rats were randomly selected into 4 groups (n=8/group) and conditioned in a standard place preference task. The groups differed in the dose of pentylenetetrazole (PTZ) administered prior to conditioning trials. With respect to the three treatment groups, placement into the, initially, nonpreferred side of the CPP apparatus was preceded by injections of 5.6, 10, or 17.8 mg/kg PTZ. The control group was injected with hypertonic saline (1.8% w/v) on the rats', initially, nonpreferred sides and isotonic saline (0.9% w/v) on their preferred sides, to control for any irrivative effects of PTZ injections in the treatment groups. Six pairs of drug-saline conditioning trials were conducted with each subject. PTZ produced a dose-dependent increase in the amount of time spent in the drug-associated environment. Saline control subjects' preference scores did not change over the course of the study. These data suggest that PTZ is not aversive in the place learning task; more importantly, the data suggest that a dose-dependent shift in the hedonic valences associated with environmental stimuli can occur when these stimuli are repeatedly paired with PTZ administration. The data are discussed in terms of the stimulus properties of PTZ and the hypothetical “anxiety” state the drug may produce.     

Injection timing determines whether intragastric ethanol produces conditioned place preference or aversion in mice

Previous studies have shown that mice develop conditioned place preference (CPP) when ethanol is administered by intraperitoneal (ip) or intravenous (iv) injection. The present studies examined CPP in mice using the intragastric (ig) route of administration. Inbred mice were surgically implanted with chronic intragastric cannulae and exposed to an unbiased place conditioning procedure in which infusion of ethanol (2 or 4 g/kg) was paired with a conditioned stimulus (CS+). A different CS was paired with water. In Experiments 1-2, ethanol was infused just before exposure to CS+. Contrary to previous studies involving intraperitoneal injection, infusion of 4 g/kg ig ethanol produced a significant conditioned place aversion (CPA). However, when a 5-min delay was inserted between infusion and CS exposure (Experiments 3-4), the same dose produced CPP. These outcomes are not consistent with expectations derived from a recent study in selectively bred rats, suggesting that sensitivity to ethanol reward is enhanced by intragastric administration. However, the finding that intragastric ethanol can produce either CPP or CPA depending on dose and injection timing is consistent with previous intraperitoneal ethanol studies in mice. Although the parameters differ for each route of administration, it appears that the same underlying processes can be invoked to explain how manipulation of injection timing affects the direction of ethanol-induced place conditioning. More specifically, in both cases, CPA can be attributed to an initial, short-lived aversive effect, whereas CPP can be attributed to a delayed rewarding effect of ethanol.     

Conditioned taste aversion and place preference with buspirone and gepirone

The effects of the nonbenzodiazepine anxiolytics buspirone and gepirone were compared to diazepam at 1, 3, and 10 mg/kg using the conditioned taste aversion (CTA) paradigm. Buspirone and gepirone produced stronger CTA than diazepam (3 and 10 mg/kg) across repeated conditioning trials, indicating that these nonbenzodiazepine anxiolytics may have stronger aversive properties than diazepam. The effects of buspirone and gepirone (1 and 3 mg/kg) were also assessed using the conditioned place preference (CPP) paradigm. Both buspirone (1 and 3 mg/kg) and gepirone (3 mg/kg only) produced CPP, indicating that these drugs may have rewarding properties, and that buspirone is more potent than gepirone in producing CPP. These findings demonstrate that buspirone and gepirone have affective properties similar to abused drugs, and may therefore have abuse potential. It was also demonstrated that buspirone (3 mg/kg), but not gepirone (3 mg/kg), increased dopamine (DA) synthesis in the nucleus accumbens, a mesolimbic brain area thought to be involved in drug reward.     

Paradoxical simultaneous occurrence of amphetamine-induced conditioned taste aversion and conditioned place preference with the same single drug injection: A new “pre- and post-association” experimental paradigm

The paradoxical phenomenon of co-existing physically aversive and psychologically rewarding effects of drugs is a crucial issue for drug addiction. The present study employed a new experimental paradigm to test whether the rewarding and aversive properties of amphetamine (AMPH) can exist simultaneously. Rats were given a 15 min period of exposure to saccharin injected with 0.15 M NaCl or 1.5 mg/kg AMPH and then were confined to one compartment of a test box for 30 min. After three paired and unpaired cycles, the aversive and rewarding effects were assessed. A reduction in consumption of the paired flavored solution provided evidence of avoidance while preference for the AMPH injection context provided evidence of rewarding effects. The present findings demonstrate that the development of AMPH-induced rewarding and aversive effects depends on the particular behavioral conditions and support both the task-dependent drug effects hypothesis and the reward comparison hypothesis. The formation of associations with stimuli that comes before (pre) vs. after (post) the unconditioned stimulus and the role of the dopaminergic system in such associations are discussed.     

Baclofen alters ethanol-stimulated activity but not conditioned place preference or taste aversion in mice.

The present experiments examined the effects of the GABA(B) receptor agonist, baclofen, on the acquisition of ethanol-induced conditioned place preference (CPP) and conditioned taste aversion (CTA) in male DBA/2J mice. Mice in the CPP experiment received four pairings of ethanol (2g/kg) with a distinctive floor stimulus for a 5-min conditioning session (CS+ sessions). On intervening days (CS- sessions), mice received saline injections paired with a different floor type. On CS+ days, mice also received one of four doses of baclofen (0.0. 2.5, 5.0, or 7.5 mg/kg) 15 min before an injection of ethanol. For the preference test, all mice received saline injections, and were placed on a half-grid and half-hole floor for a 60-min session. Baclofen dose dependently reduced ethanol-stimulated activity, but did not alter the magnitude of ethanol-induced CPP at any dose. For the CTA experiment, mice were adapted to a 2-h per day water restriction regimen followed by five conditioning trials every 48 h. During conditioning trials, subjects received an injection of saline or baclofen (2.0 and 6.0 mg/kg) 15 min before injection of 2 g/kg ethanol or saline following 1-h access to a saccharin solution. Baclofen did not alter the magnitude of ethanol-induced CTA at any dose. In addition, baclofen alone did not produce a CTA. Overall, these studies show that activation of GABA(B) receptors with baclofen reduces ethanol-induced locomotor activation, but does not alter ethanol's rewarding or aversive effects in the CPP and CTA paradigms in DBA/2J mice.     

Repeated testing attenuates conditioned place preference with cocaine

Cocaine-treated rats acquired a preference for cocaine-associated contextual stimuli (CS) relative to saline-injected control rats. However, when animals were given repeated tests for conditioned place preference intermittent between conditioning trials, they displayed an attenuation in strength of conditioning. This attenuation was not due to pharmacologic tolerance (Experiment 1), but rather reflected a disruption in learning due to exposure to the CS alone (Experiment 2). Like other examples of classical conditioning, the strength of the conditioned response (CR) as assessed by the conditioned place preference model may be influenced by partial reinforcement.     

Clonidine produces a conditioned place preference in rats

The possibility that the -adrenergic agonist clonidine can act as a reinforcing agent was investigated using the conditioned place preference paradigm. Using two different variants of this method we were able to demonstrate reinforcing properties of clonidine at doses of 200 and 400 g/kg. These results are consistent with those obtained by other investigators using the self-administration technique, and support the view that adrenergic mechanisms may be involved in reinforcement.     

Dimenhydrinate produces a conditioned place preference in rats

Dimenhydrinate (DMH; trade names Gravol and Dramamine) is a compound of diphenhydramine (DP) and 8-chlorotheophylline in equimolar ratios. DMH has been reported to be abused by humans for its euphoric and hallucinogenic properties but few studies have evaluated its reinforcing effects in animals. To evaluate the hypothesis that DMH and its constituents DP and 8-chlorotheophylline are rewarding in animals, rats were tested for conditioned place preference (CPP). The paradigm consisted of pre-exposure (three 15-min sessions of access to both sides of the chamber), conditioning [eight 30-min pairings of one side with drug (four sessions) and, on alternate days, the other side with vehicle (four sessions)] and test phases (three 15-min sessions of access to both sides of the chamber). Significant preferences for the drug-paired location were found on test session one after conditioning with 60.0, but not 25.0, 40.0 or 50.0 mg/kg of DMH, and after conditioning with 37.8 but not 27.0 or 32.4 mg/kg of DP. No preference was found after conditioning with 23.0, 27.6 or 32.2 mg/kg of 8-chlorotheophylline. All three drugs stimulated locomotor activity during conditioning sessions and DMH and DP showed sensitization over conditioning sessions. DMH doses that showed sensitization (25.0 and 40.0 mg/kg) were lower than the dose (60.0 mg/kg) that produced a CPP revealing a dissociation of locomotor stimulating versus rewarding effects. Results reveal that DMH and DP have rewarding properties, although the molar equivalent dose-response curve for DP appeared to be further to the right than that for DMH. Future investigations into the neurotransmitter systems modulating this effect are awaited.     

Magnesium-induced conditioned place preference in mice

A conditioned place preference procedure was used in mice to test the hypothesis that magnesium possesses reinforcing properties. Mice were conditioned to the nonpreferred end of a three-compartment straight shuttle box with MgCl2 injections alternating with saline injections on the preferred end. Dose of MgCl2 was varied (0, 15, 30, 125 mg/kg) as well as number of conditioning trials (8 or 16). On the day after the first postconditioning test, animals were given acute injections of 5 mg/kg cocaine, or other test drug, to determine if the conditioned effect on behavior would be potentiated, maintained or blocked by these test drugs. Results demonstrated that 15 mg/kg MgCl2 induced the greatest amount of conditioning and that increasing the number of MgCl2/place pairings did not enhance the amount of conditioning, but rather, it decreased it. Amphetamine potentiated MgCl2-induced place preference; cocaine and pentobarbital maintained it; and haloperidol blocked it. These data indicate that MgCl2 has some primary reinforcing properties in mice and that MgCl2 shares stimulus properties with other stimulants and reinforcing substances.     

Failure to establish a conditioned place preference with ethanol in rats

Previous studies have demonstrated that many drugs of abuse are able to produce a conditioned place preference in rats. We sought to determine if ethanol, injected in a wide range of doses, could also produce a conditioned place preference. Statistical analysis of our results indicated that the IP administration of the drug (50, 100, 150, 300, 600, 800, or 1000 mg/kg) failed to produce either a conditioned place preference or aversion compared to vehicle injected control rats. Under similar testing conditions a conditioned place preference was obtained with amphetamine (2 mg/kg) and this preference was not secondary to conditioned hyperactivity. In another experiment, rats were injected with ethanol through indwelling jugular cannulae at doses similar to those reported [24,26] to support (1, 2 mg/kg) or not to support (8 mg/kg) self-administration by rats. We also failed to obtain a conditioned place preference using these doses. Blood and brain ethanol levels, determined 1, 2 or 5 minutes after the administration of 2 mg/kg (IV) indicated very low ethanol levels. These results may suggest that rats do not self-administer ethanol for its intoxicating properties, and that the affective state produced by ethanol administration per se is not readily conditionable to environmental cues.     

Morphine and ethanol pretreatment effects on expression and extinction of ethanol-induced conditioned place preference and aversion in mice

Psychopharmacology - Opioid receptor antagonists reliably alter the expression or extinction of ethanol’s conditioned motivational effects as indexed by the place conditioning procedure,...     

Mianserin enhancement of ethanol-induced conditioned place preference

This experiment examined the influence of mianserin, a 5-HT(2) receptor antagonist, on the rewarding effect of ethanol in a place conditioning paradigm. Swiss-Webster mice received four pairings of a tactile stimulus with drug treatment consisting of two i.p. injections separated by a 30min interval. Drug treatment groups were as follows: saline (10mg/kg) followed by ethanol (2mg/kg); mianserin (10mg/kg) followed by ethanol; mianserin followed by saline. A different stimulus was paired with two saline injections. During conditioning, ethanol produced increases in locomotor activity that were reduced by mianserin. Mianserin alone reduced activity levels. As expected, group saline-ethanol showed a nonsignificant trend towards conditioned place preference. However, mianserin enhanced the acquisition of ethanol preference, whereas mianserin alone did not produce either place preference or aversion. The results are consistent with the notion that serotonergic systems are important in the response to ethanol, and further suggest that 5-HT(2) receptor blockade increases the rewarding effects of ethanol.     

Toluene inhalation produces a conditioned place preference in rats

Toluene is a widely abused solvent with demonstrated addictive potential in humans. Here we explore if conditioned place preference can be used to study the abuse-related effects of inhaled toluene in rats. Animals were confined to a distinctive compartment of a three-compartment chamber while exposed to toluene vapor and later tested for preference for that compartment compared to appropriate control subjects. In this study, a flame ionization detector was used for on-line monitoring of toluene vapor concentrations inside the conditioning apparatus coupled with computerized recording of the time spent by the animals on the test day in each of the chambers. Sprague-Dawley rats were exposed to 810, 1895 or 4950 ppm of toluene vapors in either the black or white compartment during 30-min pairing sessions given every other day alternating with air exposure for the total of six pairings for each treatment. Rats that received air in both sides (control group) did not show any preference for either side with approximately equal time spent in each compartment on the test day (241 +/- 33 and 234 +/- 34 s, for white and black box, respectively). However, the 1895- and 4950-ppm test groups, but not the 810-ppm group, demonstrated a significant preference for the side paired with toluene exposure. When a subsequent test session was performed during toluene exposures, no conditioned place preference was observed. Thus, toluene produced a clear conditioned place preference that appears to be most evident when animals are not intoxicated. This procedure should be useful for further studies of the abuse-related effects of abused inhalants.