Conditioned activation induced by ethanol: role in sensitization and conditioned place preference.

Previous studies of ethanol-induced activation and place preference conditioning have shown that repeated exposure to ethanol produces sensitization to ethanol's locomotor activating effect in mice. This experiment was designed to determine whether the behavioral sensitization to ethanol that occurs during place preference conditioning is due to development of a Pavlovian conditioned activity response. Mice (DBA/2J) in the experimental group (BEFORE) received four pairings of a distinctive floor stimulus with ethanol (2 g/kg, IP); a different floor stimulus was paired with saline (counterbalanced). Mice in two control groups were exposed equally to each floor stimulus and were handled and injected as often as experimental mice. One control group (AFTER) always received ethanol in the home cage 1 h after exposure to the floor stimulus, while the other control group (NO-DRUG) never received ethanol during conditioning. BEFORE group mice showed a significant conditioned place preference, whereas control mice did not. Activity tests after saline or ethanol indicated higher activity levels in BEFORE mice compared to control mice, regardless of floor stimulus. Moreover, BEFORE mice were more active on their CS+ floor than on their CS- floor during saline tests; activity was equally elevated on both floors during ethanol tests. These results support the hypothesis that sensitization to ethanol's activating effect is mediated by Pavlovian conditioning. Further, they suggest that place conditioning established-associative control by two kinds of stimuli; the specific tactile cues serving as CS+ and CS- and the general environmental cues common to both CS+ and CS- trials.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of test activity in ethanol-induced disruption of place preference expression in mice

Rationale Reduced expression of a drug-induced conditioned place preference (CPP) may reflect a decrease in the drug’s conditioned rewarding effects. However, CPP is also open to disruption by processes unrelated to the underlying motivation. In unpublished studies, we previously observed that ethanol pretreatment before testing disrupted expression of ethanol-induced CPP in DBA/2J mice. We hypothesized that this interference effect was due to large ethanol-induced increases in activity. Objective The present studies were designed to examine the relationship between test activity and expression of ethanol-induced CPP both in the presence and absence of ethanol. To assess the generality of this relationship, we examined these effects both in DBA/2J (which are highly activated by ethanol) and in NZB/B1NJ mice (which show similar CPP, but less ethanol-induced activation). Materials and methods In separate experiments, inbred mice from each strain underwent ethanol (2 g/kg) place conditioning. Saline or ethanol was then administered immediately before the test. Results Ethanol, given immediately before the test, blocked the expression of ethanol CPP in DBA/2J, but not in NZB/B1NJ mice. Moreover, ethanol significantly increased test activity levels in DBA/2J and to a much lesser degree in NZB/B1NJ mice. Correlation analyses showed an inverse phenotypic relationship between preference and test activity, reflecting stronger preferences in less active mice. Conclusions Disruption of ethanol-CPP observed in DBA/2J mice may be a consequence of high ethanol-induced activity levels. More generally, these studies suggest that competing behaviors can affect expression of a drug-induced CPP independent of affecting the conditioned rewarding effects of the drug.     

Conditioned reinforcement in rats established with self-administered nicotine and enhanced by noncontingent nicotine

Rationale Nicotine is widely assumed to convey reinforcing properties upon tobacco-related stimuli through associative learning. We have proposed that the reinforcement derived from these conditional stimuli can be inflated by a nonassociative “reinforcement-enhancing” effect of nicotine. Objectives Experiment 1 investigated whether nicotine could establish a stimulus as a conditioned reinforcer. Using the same subjects, Experiment 2 examined whether responding for a nicotine-associated stimulus was enhanced by response-independent administration of nicotine. Materials and methods Self-administered nicotine (Paired group, 0.03 mg kg¹ infusion⁻¹) or saline (conditional stimulus or CS-Only group) was paired with a stimulus light (CS). An Unpaired group, yoked to the Paired group, received equal exposure to nicotine and the CS, but each event was temporally separated. To test for conditioning, the CS was then made contingent upon a novel lever-pressing response. In Experiment 2, a subset of the paired rats (self-administering) continued to lever press while receiving contingent nicotine and the CS. To determine whether nicotine enhanced responding for the CS, two remaining subsets of the Paired group responded for the CS while receiving nicotine (YNIC) or saline (YSAL) yoked to the self-administering rats. All remaining control groups received response-contingent CS presentations, together with yoked nicotine or saline. Results Pairing self-administered nicotine with the CS promoted the acquisition of a novel response for the CS. In Experiment 2, the Paired YNIC group responded at higher rates than control groups receiving YNIC or YSAL. Conclusions Nicotine can establish stimuli as conditioned reinforcers for which noncontingent nicotine can enhance responding.     

FOS expression induced by an ethanol-paired conditioned stimulus

To identify brain areas involved in ethanol-induced Pavlovian conditioning, brains of male DBA/2J mice were immunohistochemically analyzed for FOS expression after exposure to a conditioned stimulus (CS) previously paired with ethanol (2 g/kg) in two experiments. Mice were trained with a procedure that normally produces place preference (Before: ethanol before the CS) or one that normally produces place aversion (After: ethanol after the CS). Control groups received unpaired ethanol injections in the home cage (Delay) or saline only (Na?ve). On the test day, mice were exposed to the 5-min CS 90 min before sacrifice. Before groups showed a conditioned increase in activity, whereas the After group showed a conditioned decrease in activity. FOS expression after a drug-free CS exposure was significantly higher in Before-group mice than in control mice in the bed nucleus of the stria terminalis (Experiment 1) and anterior ventral tegmental area (Experiments 1-2). Conditioned FOS responses were also seen in areas of the extended amygdala and hippocampus (Experiment 2). However, no conditioned FOS changes were seen in any brain area examined in After-group mice. Overall, these data suggest an important role for the mesolimbic dopamine pathway, extended amygdala and hippocampus in ethanol-induced conditioning.     

Localization of genes influencing ethanol-induced conditioned place preference and locomotor activity in BXD recombinant inbred mice

Genetic differences in ethanol's ability to induce conditioned place preference were studied in 20 BXD Recombinant Inbred (RI) mouse strains and in the C57BL/6J and DBA/2J progenitor strains. Male mice from each strain were exposed to a Pavlovian conditioning procedure in which a distinctive floor stimulus (CS+) was paired four times with ethanol (2 g/kg). A different floor stimulus (CS-) was paired with saline. Control mice were injected only with saline. Floor preference testing without ethanol revealed significant genetic differences in conditioned place preference, with some strains spending nearly 80% time on the ethanolpaired floor while others spent only 50% (i.e., no preference). Control mice showed genetic differences in unconditioned preference for the floor cues, but unconditioned preference was not genetically correlated with conditioned preference. There were also substantial genetic differences in ethanol-stimulated activity, but contrary to psychomotor stimulant theory, ethanol-induced activity on conditioning trials was not positively correlated with strength of conditioned place preference. However, there was a significant negative genetic correlation (r=–0.42) between test session activity and preference. Quantitative trait loci (QTL) analyses showed strong associations (P<0.01) between conditioned place preference and marker loci on chromosomes 4, 8, 9, 18 and 19. Weaker associations (0.01<P<0.05) were identified on several other chromosomes. Analysis also yielded several significant QTL for unconditioned preference, ethanol-stimulated activity, and sensitization. Overall, these data support the conclusion that genotype influences ethanol-induced conditioned place preference, presumably via genetic differences in sensitivity to ethanol's rewarding effects. Moreover, several chromosomal regions containing candidate genes of potential relevance to ethanol-induced conditioned place preference have been identified.     

Effects of removing food on maintenance of drinking initiated by pairings of sipper and food

Two experiments evaluated the effects of removing food presentations on the maintenance of drinking induced by experience with sipper — food pairings. In Exp 1, ethanol drinking was induced in non-deprived Long-Evans rats by Pavlovian conditioning procedures employing an ethanol sipper as conditioned stimulus (CS) and food pellet as unconditioned stimulus (US). The Paired/Ethanol group received presentations of the ethanol sipper CS followed immediately by the response-independent presentation of the food pellet US. The Random/Ethanol group received the ethanol sipper CS and food US randomly with respect to one another. For both groups, the concentration of ethanol in the sipper CS [(3%, 4%, 6%, 8% (vol./vol.)] was increased across sessions, and, as in previous studies employing low concentrations of ethanol in non-deprived rats (i.e., maintained with free access to food in their home cages), the two procedures induced comparable levels of sipper CS-directed ethanol drinking. Removing food US presentations had no effect on sipper CS-directed ethanol drinking in either group. In Exp 2, groups of non-deprived Long-Evans rats were trained either with water or ethanol in the sipper CS paired with food US. Removing food US presentations had no effect on ethanol drinking in the Paired/Ethanol group, but water drinking in the Paired/Water group declined systematically across sessions. Results indicate that food US presentations contribute to the maintenance of water drinking but not to the maintenance of ethanol drinking. Implications for accounts of ethanol drinking based on Pavlovian sign-tracking, behavioral economics and intermittent sipper procedures are considered.     

β2-Subunit-containing nicotinic acetylcholine receptors are involved in nicotine-induced increases in conditioned reinforcement but not progressive ratio responding for food in C57BL/6 mice

Rationale Nicotine administration potentiates conditioned reinforcement in rats, an effect that persists for weeks after chronic exposure. Little is known regarding the nicotinic receptor subtypes that may mediate this effect. Objective The purpose of this study was to determine whether β2-subunit-containing nicotinic acetylcholine receptors (β2*nAChRs) are necessary for lasting effects of nicotine on conditioned and primary reinforcement in mice. Methods β2 knockout (β2KO) and wild-type (WT) mice received 14 days of nicotine exposure (NIC, 200 μg/ml in 2% saccharin) or saccharin alone (SAC) in their drinking water. Five days later, mice received paired presentations of a conditioned stimulus (CS) with water unconditioned stimulus (US) or explicitly unpaired presentations of the CS and US during Pavlovian discriminative approach training. Training was followed by two conditioned reinforcement tests. Mice were subsequently tested for food-reinforced responding in the absence of explicit cues followed by a progressive ratio test. Results During conditioned reinforcement testing, only mice in the paired condition showed increased responding in the CS-reinforced aperture over inactive apertures. WT-NIC mice showed enhanced conditioned reinforcement compared to WT-SAC animals. β2KO-SAC mice showed elevated conditioned reinforcement compared to WT-SAC subjects, but β2KO-NIC and β2KO-SAC mice did not differ in responding with conditioned reinforcement. Prior nicotine exposure did not alter food-reinforced responding but resulted in elevated break points for food in both genotypes. Conclusion These data show that nicotine exposure enhances conditioned reinforcement in mice and indicate that β2*nAChRs are necessary for nicotine-dependent enhancement of incentive aspects of motivation but not motivation for primary reinforcement measured by progressive ratio responding.     

Effects of intra-amygdala R(+) 7-OH-DPAT on intra-accumbens d-amphetamine-associated learning I. Pavlovian conditioning

We have previously obtained evidence that the mesoamygdaloid dopamine projection modulates the acquisition of a conditioned response (CR) elicited by presentation of a conditioned stimulus (CS) predicting the availability of a natural (sucrose) reward. This property was found to be dependent upon D₃, but not D₁ or D₂, dopamine receptor activation. The aim of the present study was to determine whether the mesoamygdaloid dopamine projection is similarly involved in the acquisition of a drug-associated CR. Thus, two groups of rats with guide cannulae aimed at the nucleus accumbens and amygdala were trained using a Pavlovian conditioning procedure in which an initially neutral CS was paired with a computer-controlled, bilateral intra-accumbens infusion of d-amphetamine (the unconditioned stimulus; US). Conditioning sessions were conducted in standard operant chambers, with each session consisting of a single CS-US trial. For one group of rats, CS presentation was positively correlated with the drug US (Paired group), while for the second group CS and US presentations were negatively correlated (Unpaired group). During training, locomotor activity was recorded and was utilised as the measure both of the unconditioned (UR) and conditioned response (CR). A within-subjects design was utilised to investigate the effect of post-session bilateral intra-amygdala administration of R(+) 7-OH-DPAT on the development of the drug-associated CR. Hence, both Paired and Unpaired groups were exposed to two different CSs which were presented on alternate sessions. Post-session bilateral intra-amygdala administration of R(+) 7-OH-DPAT (10 nmol) followed sessions in which one CS was presented, while intra-amygdala vehicle followed sessions in which the alternate CS was presented. The development of a CR occurred only in the presence of a CS that had been positively correlated with presentation of the drug US. Post-session, intra-amygdala administration of R(+) 7-OH-DPAT enhanced the acquisition of this CR. However, R(+) 7-OH-DPAT was without effect upon the unconditioned response to intra-accumbens d-amphetamine. Our previous data indicate a comparable effect of R(+) 7-OH-DPAT on conditioning to a CS associated with a non-drug, natural reward. Therefore, taken together, these findings suggest that D₃ dopamine receptors within the amygdala modulate specifically the acquisition of Pavlovian conditioned responses, regardless of whether drug or natural rewards constitute the US. Received: 28 November 1997/Final version: 9 April 1998     

Conditioned stimulus control of morphine hyperthermia

Classical conditioning of morphine hyperthermia was examined using an explicit conditioned stimulus (CS) paired with intravenous (IV) morphine administration. Rats were implanted with a jugular vein cannula and a biotelemetry device for monitoring body temperature. The animals were housed 24 h/day in the chambers in which all testing occurred. The CS was a 15-min light/noise stimulus. The unconditioned stimulus (US) was an infusion of morphine (5 mg/kg). Rats were assigned to either the Paired group, which received morphine with the CS, or the Unpaired group, which received explicitly unpaired presentations of the CS and US. The CS-morphine pairings resulted in development of a conditioned hyperthermic response in the Paired group evoked by the CS in the absence of morphine. The development of morphine hyperthermia was more rapid in the Paired group in the presence of the CS than in its absence in the same group and more rapid in the Paired group than in the Unpaired group during the CS. These results clearly show that learning affects the response to morphine administered repeatedly. In contrast to previous studies, conditioned hyperthermia was elicited within 15 min by a discrete CS in a situation where the response was not confounded by handling or the stress of injection.     

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     

Spinal cord alpha-2 noradrenergic receptors mediate conditioned analgesia

The present experiment investigated the effects of direct spinal administration of the monoaminergic receptor blockers yohimbine, phentolamine and methysergide on the expression of conditioned analgesia. Animals in the Paired group received classical conditioning trials in which one context was paired with footshock administration (1 mA shock for 15 s). Animals in the Unpaired control group were administered shock in a second, different, context. On the test day animals within each condition were administered saline (20 μl), yohimbine (30 μg), phentolamine (30 μg), or methysergide (30 μg) prior to receiving a hot plate test (50° C) in the context previously used to shock the Paired group. These ligands were administered into the spinal fluid through a chronic, indwelling spinal catheter. Animals in the Paired group which received saline displayed longer paw lick latencies than saline-treated animals in the Unpaired group. Yohimbine, but not phentolamine or methysergide, attenuated this conditioned analgesia. These results suggest that spinal cord noradrenergic substrates mediate conditioned analgesia, and that this mediation occurs specifically through the alpha-2 noradrenergic receptor.     

Effects of ethanol on Pavlovian autoshaping in rats

 Approach responses, consummatory behaviors, and directed motor responses maintained by food reward resemble autoshaping CRs and are increased by lower doses of ethanol. This study evaluated the effects of presession IP injections of ethanol doses (0.00, 0.25, 0.50, 0.70, or 1.00 g/kg) on the acquisition of lever-press autoshaping CR performance in groups of male Long-Evans hooded rats. Paired groups received 15 daily sessions of Pavlovian autoshaping procedures, wherein the insertion of a retractable lever for 5 s (CS) was followed by the response-independent presentation of food (US). Ethanol facilitated lever-press autoshaping CR acquisition, as revealed by dose-related increases in the number of trials on which CRs were performed. The form of the dose-effect curve was inverted U-shaped with maximal responding induced during sessions 1–5 by the 0.70 g/kg ethanol dose. A similar dose-effect curve was observed during sessions 11–15, revealing that the effects of ethanol on autoshaping CR performance were relatively stable. A pseudoconditioning control group injected presession with 0.50 g/kg ethanol received training wherein the food US was presented randomly with respect to the lever CS. Few lever-presses were performed by the Random 0.50 group, indicating that ethanol’s effects on autoshaping CR acquisition and maintenance observed in the Paired 0.50 group were not due to its psychomotor activating effects. A non-injection control group performed more autoshaping CRs than did the control group injected presession with saline, indicating that daily presession IP injections per se suppress autoshaping CR performance. Results reveal that low doses of ethanol enhance Pavlovian conditioning of directed motor and consummatory-like responding maintained by food reward. Implications for autoshaping accounts of impulsivity and drug abuse are considered. Received: 15 December 1997 / Final version: 27 March 1998     

Effects of acute withdrawal on ethanol-induced conditioned place preference in DBA/2J mice

Rationale

Reexposure to ethanol during acute withdrawal might facilitate the transition to alcoholism by enhancing the rewarding effect of ethanol.

Objective

The conditioned place preference (CPP) procedure was used to test whether ethanol reward is enhanced during acute withdrawal.

Methods

DBA/2J mice were exposed to an unbiased one-compartment CPP procedure. Ethanol (0.75, 1.0, or 1.5 g/kg IP) was paired with a distinctive floor cue (CS+), whereas saline was paired with a different floor cue (CS?). The withdrawal (W) group received CS+ trials during acute withdrawal produced by a large dose of ethanol (4 g/kg) given 8 h before each trial. The no-withdrawal (NW) group did not experience acute withdrawal during conditioning trials but was matched for acute withdrawal experience. Floor preference was tested in the absence of ethanol or acute withdrawal.

Results

All groups eventually showed a dose-dependent preference for the ethanol-paired cue, but development of CPP was generally more rapid and stable in the W groups than in the NW groups. Acute withdrawal suppressed the normal activating effect of ethanol during CS+ trials, but there were no group differences in test activity.

Conclusions

Acute withdrawal enhanced ethanol’s rewarding effect as indexed by CPP. Since this effect depended on ethanol exposure during acute withdrawal, the enhancement of ethanol reward was likely mediated by the alleviation of acute withdrawal, i.e., negative reinforcement. Enhancement of ethanol reward during acute withdrawal may be a key component in the shift from episodic to chronic ethanol consumption that characterizes alcoholism.     

Conditioned reinforcing properties of stimuli paired with self-administered cocaine, heroin or sucrose: implications for the persistence of addictive behaviour

Conditioned environmental stimuli are known to be important determinants of drug seeking. Traditional models of drug seeking under the control of conditioned stimuli have focused on the ability of conditioned reinforcers either to reinstate extinguished responding or to maintain prolonged chains of drug seeking under second-order schedules. These models have consistently suggested that it is the conditioned reinforcing, rather than other, effects of Pavlovian drug stimuli that most profoundly influence drug seeking. However, the impact of drug-associated conditioned reinforcers has not been studied directly and in isolation, not least because the instrumental seeking response is invariably the same as that which was previously reinforced with the drug itself. The purpose of the present study was, therefore, to investigate the conditioned reinforcing properties of drug-paired CSs using an acquisition of a new response procedure in which an animal learns to make a new instrumental response reinforced solely by the CS. It was found that CSs paired with either cocaine, heroin or sucrose supported the rapid acquisition of lever pressing for the CS that persisted over months of repeated, intermittent testing. Furthermore, rats did not acquire the lever press response when the CS was not paired with drug, suggesting that for this stimulus to acquire conditioned reinforcing properties, it must be predictively associated with the drug's effect. Moreover, lever pressing for the CS could not be explained as coincidental to an over-riding Pavlovian approach response to the location of the lever, since animals also acquired discriminated lever pressing when the CS was above the opposite, inactive lever. Extinction decreased responding with conditioned reinforcement, but only when the CS-US association was devalued prior to, and not after, acquisition of the lever press response, providing evidence for the establishment of habitual CS-maintained responding that may explain the persistence of drug-seeking responses in animal models of addiction and relapse.     

Morphine as a conditioned stimulus in a conditioned emotional response paradigm

A Pavlovian conditioning experiment was conducted to determine whether morphine (6 mg/kg, IP) could act as a conditioned stimulus (CS) when paired with an electric shock unconditioned stimulus (US), and later produce a conditioned suppression of drinking (CR) in water deprived rats. Seven groups were tested for conditioning after exposure to one of the following conditioning procedures: (1) morphine paired with shock; (2) morphine alone with no shock; (3) shock but no morphine; (4) no shock and no morphine; (5) morphine paired with vocalizations of shocked rats; (6) saline paired with shock; (7) saline alone with no shock. Groups 1 and 2 tested whether morphine could act as a CS. Groups 3 and 4 tested for sensitization. Group 5 tested whether exposure to the vocalizations of other rats could act as a US when paired with a morphine CS. Groups 6 and 7 tested whether cues associated with the injection procedure could act as a CS. Only subjects in group 1 showed conditioned suppression of drinking, when compared to control groups. Overall, the results indicate that morphine could act as a conditioned stimulus and that several of the more obvious possible sources of artifact did not significantly contribute to the CR that it produced.     

Associational and nonassociational mechanisms in locomotor sensitization to the dopamine agonist quinpirole

A pairing paradigm was employed to explore the contribution of associational mechanisms to the expression of sensitization to the dopamine agonist quinpirole. Rats received ten quinpirole injections in the test environment (Group Paired) or in the home cage (Group Unpaired), and saline in the alternate environment. A third group received saline injections in both environments (Group Acute). Subjects received quinpirole on the 11th injection as a test for locomotor sensitization, and saline on the next injection as a test for conditioned activity. The range of discriminative stimuli predicting a drug versus a non-drug injection was increased across three independent experiments in an effort to detect a possible associational effect. Regardless of the strength of discriminative stimuli, both Paired and Unpaired groups showed locomotor sensitization to 0.5 mg/kg quinpirole compared with the Acute group. However, the Paired group showed more locomotion than the Unpaired group in the last minutes of the sensitization test. With a lower sensitizing dose of quinpirole (0.1 mg/kg) used in one experiment, only the Paired group showed locomotor sensitization. For both doses, the Paired, but not the Unpaired groups showed conditioned locomotion. It is suggested that with moderate doses of quinpirole, expression of locomotor sensitization does not require drug-signalling cues though such signals may have a modulatory influence. With lower quinpirole doses, however, quinpirole sensitization is context-dependent.     

Distal and proximal pre-exposure to ethanol in the place conditioning task: tolerance to aversive effect,sensitization to activating effect,but no change in rewarding effect

RATIONALE: The literature offers many examples of tolerance to ethanol's inhibitory/depressant effects and sensitization to its activating effects. There are also many examples of tolerance to ethanol's aversive effects as measured in the conditioned taste aversion and conditioned place aversion (CPA) procedures. However, there are very few demonstrations of either tolerance or sensitization to ethanol's rewarding or reinforcing effects. OBJECTIVE: The present studies were designed to examine effects of two forms of ethanol pre-exposure (distal or proximal) on ethanol's rewarding and aversive effects as indexed by the place conditioning procedure. METHOD: Male inbred (DBA/2J) mice were exposed to ethanol (2 g/kg IP) in an unbiased place conditioning procedure that normally produces either conditioned place preference (CPP) (ethanol injection before CS exposure) or CPA (ethanol injection after CS exposure). In the distal pre-exposure studies (experiments 1 and 2), mice initially received a series of four ethanol injections (0, 2, or 4 g/kg) in the home cage at 48-h intervals during the week before place conditioning. In the proximal pre-exposure studies (experiments 3-4), mice were injected with ethanol 65 min before (experimental groups) or 65 min after (control groups) each paired ethanol injection on CS+ trials. RESULTS: Distal pre-exposure produced a robust sensitization to ethanol's activating effect, whereas proximal pre-exposure generally reduced the activation normally produced by the paired ethanol injection. Both forms of pre-exposure interfered with CPA, but had no effect on CPP. CONCLUSIONS: These studies suggest that both forms of pre-exposure reduced ethanol's aversive effect, but had no impact on ethanol's rewarding effect. In general, the detrimental effects of pre-exposure on CPA are explained best in terms of a reduction in ethanol's efficacy as an aversive unconditioned stimulus (i.e. tolerance), although explanations based on other types of associative interference are also possible. The failure to affect CPP with pre-exposure treatments that reduced or eliminated CPA suggests that these behaviors are mediated by independent, motivationally opposite effects of ethanol. Moreover, these results indicate dissociation between sensitization to ethanol's locomotor activating effect and changes in its rewarding effect. To the extent that motivational processes measured by CPP and CPA normally contribute to ethanol drinking, the present findings suggest that increases in ethanol intake seen after chronic ethanol exposure are more likely caused by tolerance to ethanol's aversive effect rather than sensitization to its rewarding or reinforcing effect.     

Discrete cues paired with naloxone-precipitated withdrawal from acute morphine dependence elicit conditioned withdrawal responses

Acute bolus doses of morphine induce a state of acute opioid dependence as measured by naloxone-precipitated withdrawal. Repeated morphine and precipitated withdrawal experience further enhances naloxone-induced withdrawal severity, partly because of direct neuroadaptation to repeated morphine, and partly because of conditioned associations of context and withdrawal experience. To determine whether a discrete tone/light conditioned stimulus could elicit conditioned withdrawal responses in acute dependence, rats trained on a fixed-ratio-15 operant schedule for food reward received morphine (5.6 mg/kg) 4x at daily or weekly intervals, with each morphine injection followed at 4 h by naloxone (1.0 mg/kg) and an operant session. The conditioned stimulus was presented to a Paired group after each naloxone injection. Separate control groups experienced the conditioned stimulus either at a different time of the day or on a different day of the week than naloxone (Unpaired), received naloxone without any conditioned stimulus exposure [Paired-no conditioned stimulus (Paired-NO CS)] or received vehicle instead of naloxone before conditioned stimulus presentation (NaI-Naive). On the test day, all rats received vehicle before conditioned stimulus exposure. The conditioned stimulus alone reliably suppressed responding in Paired groups relative to control conditions with either daily or weekly intervals between conditioning sessions. The administration of morphine 4 h before conditioned stimulus exposure on the test day was not necessary to observe conditioned withdrawal. Thus, conditioned withdrawal is reliably established to discrete cues associated with naloxone-precipitated withdrawal from acute, infrequent (weekly) opioid exposure.     

Genetic differences in the rewarding and activating effects of morphine and ethanol

The influence of genotype on the rewarding and locomotor activating effects of morphine and ethanol was examined in the place conditioning paradigm. Two inbred mouse strains (C57BL/6J and DBA/2J) were exposed to a differential conditioning procedure in which each mouse received four pairings of a distinctive floor stimulus with IP injection of morphine (0, 2.5, 5 or 10 mg/kg) or ethanol (0, 1, 2, 3 or 4 g/kg). A different floor stimulus was paired with saline. Conditioning trials lasted 30 min and each experiment concluded with a floor preference test in the absence of drug. In accord with previous studies, morphine evoked a dose-dependent increase in activity during conditioning that was greater in C57BL/6J mice than in DBA/2J mice. In contrast, ethanol produced a dose-dependent increase in activity that was greater in DBA/2J than in C57BL/6J mice. Both strains showed conditioned place preference with morphine, but only the DBA/2J strain showed conditioned place preference with ethanol. No conditioned place aversion was seen. With both drugs, stronger place preference conditioning was obtained in DBA/2J mice, supporting the general conclusion that sensitivity to drug reward is influenced by genotype. The fact that the same genotype is more sensitive to the rewarding effects of two different drugs supports theories postulating commonality in the biological mechanisms of drug reward. Although the outcome of the ethanol study supports predictions of the psychomotor stimulant theory of addiction concerning the relationship between drug-induced activation and reward, the outcome of the morphine study does not. The direction of the strain difference in conditioned place preference is opposite to what might be predicted on the basis of strain differences previously reported in drug consumption and preference studies, suggesting that genetic differences in drug consumption may not accurately reflect postabsorptive motivational effects of drug.     

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.