Neuroleptics block the positive reinforcing effects of amphetamine but not of morphine as measured by place conditioning

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?

What roles do mesolimbic and neostriatal dopamine systems play in reward? Do they mediate the hedonic impact of rewarding stimuli? Do they mediate hedonic reward learning and associative prediction? Our review of the literature, together with results of a new study of residual reward capacity after dopamine depletion, indicates the answer to both questions is 'no'. Rather, dopamine systems may mediate the incentive salience of rewards, modulating their motivational value in a manner separable from hedonia and reward learning. In a study of the consequences of dopamine loss, rats were depleted of dopamine in the nucleus accumbens and neostriatum by up to 99% using 6-hydroxydopamine. In a series of experiments, we applied the 'taste reactivity' measure of affective reactions (gapes, etc.) to assess the capacity of dopamine-depleted rats for: 1) normal affect (hedonic and aversive reactions), 2) modulation of hedonic affect by associative learning (taste aversion conditioning), and 3) hedonic enhancement of affect by non-dopaminergic pharmacological manipulation of palatability (benzodiazepine administration). We found normal hedonic reaction patterns to sucrose vs. quinine, normal learning of new hedonic stimulus values (a change in palatability based on predictive relations), and normal pharmacological hedonic enhancement of palatability. We discuss these results in the context of hypotheses and data concerning the role of dopamine in reward. We review neurochemical, electrophysiological, and other behavioral evidence. We conclude that dopamine systems are not needed either to mediate the hedonic pleasure of reinforcers or to mediate predictive associations involved in hedonic reward learning. We conclude instead that dopamine may be more important to incentive salience attributions to the neural representations of reward-related stimuli. Incentive salience, we suggest, is a distinct component of motivation and reward. In other words, dopamine systems are necessary for 'wanting' incentives, but not for 'liking' them or for learning new 'likes' and 'dislikes'.

Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

GABAergic neurons in the ventral tegmental area (VTA) play a primary role in local inhibition of mesocorticolimbic dopamine (DA) neurons but are not physiologically or anatomically well characterized. We used in vivo extracellular and intracellular recordings in the rat VTA to identify a homogeneous population of neurons that were distinguished from DA neurons by their rapid-firing, nonbursting activity (19.1 +/- 1.4 Hz), short-duration action potentials (310 +/- 10 microseconds), EPSP-dependent spontaneous spikes, and lack of spike accommodation to depolarizing current pulses. These non-DA neurons were activated both antidromically and orthodromically by stimulation of the internal capsule (IC; conduction velocity, 2.4 +/- 0.2 m/sec; refractory period, 0.6 +/- 0.1 msec) and were inhibited by stimulation of the nucleus accumbens septi (NAcc). Their firing rate was moderately reduced, and their IC-driven activity was suppressed by microelectrophoretic application or systemic administration of NMDA receptor antagonists. VTA non-DA neurons were recorded intracellularly and showed relatively depolarized resting membrane potentials (-61.9 +/- 1.8 mV) and small action potentials (68.3 +/- 2.1 mV). They were injected with neurobiotin and shown by light microscopic immunocytochemistry to be multipolar cells and by electron microscopy to contain GABA but not the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Neurobiotin-filled dendrites containing GABA received asymmetric excitatory-type synapses from unlabeled terminals and symmetric synapses from terminals that also contained GABA. These findings indicate that VTA non-DA neurons are GABAergic, project to the cortex, and are controlled, in part, by a physiologically relevant NMDA receptor-mediated input from cortical structures and by GABAergic inhibition.

Place conditioning with the dopamine D1-like receptor agonist SKF 82958 but not SKF 81297 or SKF 77434

While self-administration and place conditioning studies have shown that dopamine D2-like receptor agonists produce reward-related learning, the effects of dopamine D1-like receptor agonists remain equivocal. The present study tested three dopamine D1-like receptor agonists for their ability to induce a place preference. Like control rats treated with amphetamine (2.0 mg/kg i.p.), rats treated with SKF 82958 (+/- -6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1-phenyl-1H- 3-benzazepine hydrobromide; 0.05 but not 0.01, 0.025, 0.075, or 0.10 mg/kg s.c. and/or i.p.) during conditioning showed a significant increase in the amount of time spent on the drug-paired side during the drug free test. Neither SKF 81297 (+/- -6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; 0.25, 0.50, 1.0, 2.0, and 4.0 mg/kg i.p.) nor SKF 77434 (+/- -7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride; 0.20, 1.0, 5.0, and 10.0 mg/kg i.p.) produced place conditioning. Significant increases in locomotion were seen at some doses of all drugs. Results show for the first time that systemic administration of a dopamine D1-like receptor agonist produces a place preference and are consistent with previous findings showing that dopamine D1-like receptor activation produces reward-related learning.

Dopamine D3 receptor mutant mice exhibit increased behavioral sensitivity to concurrent stimulation of D1 and D2 receptors

The dopamine D3 receptor is expressed primarily in regions of the brain that are thought to influence motivation and motor functions. To specify in vivo D3 receptor function, we generated mutant mice lacking this receptor. Our analysis indicates that in a novel environment, D3 mutant mice are transiently more active than wild-type mice, an effect not associated with anxiety state. Moreover, D3 mutant mice exhibit enhanced behavioral sensitivity to combined injections of D1 and D2 class receptor agonists, cocaine and amphetamine. However, the combined electrophysiological effects of the same D1 and D2 agonists on single neurons within the nucleus accumbens were not altered by the D3 receptor mutation. We conclude that one function of the D3 receptor is to modulate behaviors by inhibiting the cooperative effects of postsynaptic D1 and other D2 class receptors at systems level.

Absence of opiate rewarding effects in mice lacking dopamine D2 receptors

Dopamine receptors have been implicated in the behavioural response to drugs of abuse. These responses are mediated particularly by the mesolimbic dopaminergic pathway arising in the ventral tegmental area and projecting to the limbic system. The rewarding properties of opiates and the somatic expression of morphine abstinence have been related to changes in mesolimbic dopaminergic activity that could constitute the neural substrate for opioid addiction. These adaptive responses to repeated morphine administration have been investigated in mice with a genetic disruption of the dopaminergic D2 receptors. Although the behavioural expression of morphine withdrawal was unchanged in these mice, a total suppression of morphine rewarding properties was observed in a place-preference test. This effect is specific to the drug, as mice lacking D2 receptors behaved the same as wild-type mice when food is used as reward. We conclude that the D2 receptor plays a crucial role in the motivational component of drug addiction.

Neurobiological constraints on behavioral models of motivation

The application of neurobiological tools to behavioral questions has produced a number of working models of the mechanisms mediating the rewarding and aversive properties of stimuli. The authors review and compare three models that differ in the nature and number of the processes identified. The dopamine hypothesis, a single system model, posits that the neurotransmitter dopamine plays a fundamental role in mediating the rewarding properties of all classes of stimuli. In contrast, both nondeprived/deprived and saliency attribution models claim that separate systems make independent contributions to reward. The former identifies the psychological boundary defined by the two systems as being between states of nondeprivation (e.g. food sated) and deprivation (e.g. hunger). The latter identifies a boundary between liking and wanting systems. Neurobiological dissociations provide tests of and explanatory power for behavioral theories of goal-directed behavior.

The effect of ginseng extract on locomotor sensitization and conditioned place preference induced by methamphetamine and cocaine in mice

Repeated i.p. injections of 2 mg/kg methamphetamine (MA) or 20 mg/kg cocaine at 48-h intervals induced reverse tolerance to their ambulation-enhancing effects (behavioral sensitization). Furthermore, the reappearance of the sensitized state was observed at the time of readministration of MA or cocaine even after a 30-day discontinuation of drug administration. A concomitant injection of ginseng extract (GE), 200 mg/kg, i.p., suppressed the development of reverse tolerance and the reappearance of sensitization to MA and cocaine. Conditioned place preference to MA (1, 2, and 4 mg/kg, i.p.) and cocaine (1, 4, 10, and 20 mg/kg, i.p.), was completely blocked by GE, 200 mg/kg, i.p. combined treatment with MA of cocaine. Meanwhile, spontaneous motor activity and place preference were not affected by GE alone. These results provide evidence that GE may be useful clinically for the prevention of adverse actions of MA and cocaine.

Morphine- and cocaine-induced conditioned place preference: effects of quinpirole and preclamol

The role of dopamine in opioid reward is unresolved. Furthermore, the issue is somewhat unclear regarding cocaine and the place preference paradigm. In the present study we investigated whether the drugs activating dopamine autoreceptors affect cocaine- and morphine-induced place preference in rats. Neither the dopamine D2/D3 receptor agonist, quinpirole (0.05 mg/kg, SC), nor the partial dopamine autoreceptor agonist, preclamol (2 or 8 mg/kg, SC), induced place conditioning by itself. Quinpirole had no significant influence on the place preference induced either by morphine (3 mg/kg, SC) or cocaine (5 mg/kg, IP). Preclamol, when given at the dose of 8 mg/kg SC, significantly attenuated the effect of cocaine but failed to modify the effect of morphine. Our results suggest that the rewarding properties of morphine involve DA-independent mechanisms whereas in the cocaine-induced reward the role of brain DA is critical. Furthermore, as regards place conditioning, we propose that the activation of DA autoreceptors is not sufficient to reliably modify the rewarding effect of cocaine.

Chronic morphine increases hippocampal acetylcholine release: possible relevance in drug dependence

Previous studies have shown that cocaine and amphetamine stimulate acetylcholine release in the hippocampus via an action of endogenously released dopamine on dopamine D1 and D2 receptors. The present study was aimed at clarifying if the property of stimulating hippocampal acetylcholine release was shared by morphine. The acute administration of morphine (10 mg/kg i.p.) failed to modify acetylcholine release in the hippocampus. However, after repeated administration (10 mg/kg i.p. twice daily) morphine acquired the ability to stimulate hippocampal acetylcholine release. Thus, at days 5 and 7 of chronic morphine treatment, a challenge dose of morphine (10 mg/kg i.p.) increased acetylcholine release by 50 and 100%, respectively. Concomitantly with the development of the stimulant property on acetylcholine release, morphine also acquired that of producing behavioural stimulation and lost that of producing sedation and catalepsy. The morphine-induced increase in acetylcholine output was suppressed by the blockade of dopamine D1 receptors with SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine) (0.1 mg/kg s.c.), which also suppressed the morphine-induced motor stimulation. Moreover, repeated morphine administration markedly potentiated the stimulant effect of the dopamine D1/D2 receptor agonist apomorphine (R(-)-10, 11-dihydroxyaporphine) (0.1 or 0.5 mg/kg s.c.) both on hippocampal acetylcholine release and on behaviour. These results may suggest that the enhancement of hippocampal acetylcholine release as well as the development of behavioural sensitisation after chronic morphine could be related to the development of dopamine receptor supersensitivity. Moreover, increased acetylcholine transmission in the hippocampus may play a role in the 'memory' of the rewarding effects of drugs of abuse.

Effects of haloperidol in a response-reinstatement model of heroin relapse

The present study employed an animal model of drug relapse in which previously extinguished heroin self-administration behavior was reinstated following a single reinforced trial. Male albino rats were trained to traverse a straight-alley for a reinforcer consisting of a single IV injection of 0.06 mg/kg diacetylmorphine (heroin). Once the alley-running had been established, the heroin reinforcer was removed and the operant behavior permitted to extinguish over trials. On treatment day, animals were injected 45 min prior to testing with 0.0, 0.075, 0.10, 0.15 or 0.3 mg/kg of the dopamine receptor antagonist, haloperidol. A single trial was then conducted during which some animals continued to experience extinction conditions while others were injected with the heroin reinforcer upon entry into the goal box. The effects of these manipulations were determined during an additional single test trial conducted 24 h later when the subjects were no longer drugged. While heroin produced a reliable reinstatement in operant responding, this effect was dose-dependently prevented by pretreatment with haloperidol. These data suggest that dopamine receptor antagonism alters the reinforcing consequences of heroin administration as measured by heroin's ability to reinstate operant behavior following a prolonged period of nonreinforced responding.

Differential effects of ventral striatal lesions on the conditioned place preference induced by morphine or amphetamine

The present experiment examined the role of the ventral striatum in the rewarding effect of morphine and amphetamine by testing whether lesions of cell bodies within this region disrupt the development of a conditioned place preference to either drug. Bilateral, N-methyl-D-aspartate- or kainic acid-induced lesions of the ventral striatum block a conditioned place preference to amphetamine (1.5 mg/kg x 3 pairings) but not to morphine (2 mg/kg x 3 pairings). Because both lesions spared anterior portions of the ventral striatum, we examined the effect of larger or more selective ventral striatal lesions on a conditioned place preference induced by morphine. Destruction of the entire ventral striatum reduced, but did not eliminate, a conditioned place preference to morphine, whereas selective lesions of the anterior ventral striatum were ineffective. These results indicate that the ventral striatum is not critically involved in morphine's rewarding effect and support the suggestion that the rewarding effects of opiates and stimulants do not involve identical neural substrates.

Effect of nucleus accumbens dopamine depletion on motivational aspects involved in initiation of cocaine and heroin self-administration in rats

The involvement of mesolimbic dopamine (DA) systems in motivational aspects of drug-taking behavior during initiation of drug self-administration was investigated using a recently developed behavioral paradigm. In separate experiments animals were allowed to self-administer cocaine or heroin (0.16 and 0.32 mg . kg-1 per inf) during 5 consecutive daily 3-h sessions. During a 15-min period preceding the last four self-administration sessions lever-press behavior was measured in absence of the drug as an index of the motivational aspects involved in drug-taking behavior. The effect of 6-hydroxydopamine (6-OHDA) lesion of the nucleus accumbens (NAC) on lever-press behavior before and during self-administration was measured. Destruction of DA terminals in the NAC did not affect initiation of heroin self-administration nor the lever-press behavior during the period preceding the self-administration sessions. In cocaine animals 6-OHDA lesion of the NAC decreased the total intake of cocaine during the self-administration sessions and impaired discriminative lever-responding for the drug, both during cocaine self-administration, and during preceding periods when no cocaine was available. It is concluded that DAergic systems in the NAC might be involved in the reinforcement and/or motivational processes underlying cocaine self-administration. The present findings, however, do not support the notion of a critical role of NAC DA in the motivational aspects of drug-taking behavior in general.

Individual differences in reactivity to the rewarding/aversive properties of drugs: assessment by taste and place conditioning

The ability of individual differences in the strength of conditioned taste avoidance (CTA) to predict strength of place conditioning produced by the same drug was assessed. In Phase 1, rats were assigned to High CTA and Low CTA groups on the basis of their intake of saccharin solution previously paired with morphine, amphetamine, lithium, or fenfluramine. In Phase 2, the rats received place conditioning training with the same drug used during Phase 1. The rats that displayed the strongest amphetamine-induced CTA also displayed the strongest amphetamine-induced place preference, suggesting that a common mechanism mediates both effects. On the other hand, the strength of the CTA was unrelated to the strength of the place preference or place aversion produced by morphine, lithium, or fenfluramine.

Discriminative stimulus properties of CGS 10746B: similarity to dopamine D1 receptor antagonists

CGS 10746B is an imidazole-derivative related to the atypical antipsychotic clozapine which produces a decrease in dopamine release without altering dopamine metabolism or occupying D2 receptors. Rats were trained on an appetitively-motivated, two-choice, operant task to discriminate 20.0 mg/kg CGS 10746B from its vehicle. CGS 10746B was highly discriminable, producing rapid acquisition of the discrimination, and its effects were dose-responsive allowing generation of an ED50 value of 6.16 mg/kg. Substitution tests were conducted with other typical and atypical antipsychotic compounds: haloperidol, chlorpromazine, clozapine and SCH 23390. Additional tests examined generalization from the CGS 10746B stimulus properties to the calcium channel blocker isradipine, as well as to the anticholinergics atropine, scopolamine and methylscopolamine, as well as to the serotonergic agonist DOI. Clozapine and SCH 23390 were the only substances to substitute for the CGS 10746B stimulus cue. Results are discussed in terms of potential D1 receptor selectivity of CGS 10746B.

DNQX in the nucleus accumbens inhibits cocaine-induced conditioned place preference

Several lines of evidence suggest that activation of both AMPA/kainate receptors and dopaminergic receptors in the nucleus accumbens may be required for psychostimulant drug induced reward. However, it has been reported that dopaminergic antagonists fail to block acquisition of conditioned place preference to cocaine. The goal of these experiments was to determine whether AMPA receptor antagonist injected into the nucleus accumbens could block conditioned place preference elicited by cocaine under conditions where dopaminergic antagonists do not inhibit acquisition of place preference. DNQX (1 microgram/0.5 microliter), injected into the nucleus accumbens just before systemic injections of cocaine (20 mg/kg i.p.) during the training sessions, attenuated the acquisition of place preference. This suggests that AMPA receptors are involved in acquisition of place preference to cocaine. By contrast, fluphenazine (2.5 micrograms/0.5 microliter), injected into the nucleus accumbens during training, did not alter cocaine-induced place preference. Analysis of locomotor activity showed that the ability of flyphenazine to inhibit cocaine-induced hyperactivity progressively decreased with each training session. These observations suggest that the failure of dopaminergic antagonists to block cocaine-induced place preference may be related to adaptations occurring following repeated exposure to these drugs. Both DNQX and fluphenazine blocked the expression of conditioned place preference in rats that had been previously trained with cocaine alone. This result suggests that both AMPA and dopaminergic receptors are involved in the expression of a conditioned place preference to cocaine.

Tegmental pedunculopontine nucleus lesions do not block cocaine reward

Previous studies have implicated the tegmental pedunculopontine (TPP) nucleus in mediating the rewarding effects of opiates, food, and amphetamine, provided that animals are not in aversive motivational states induced by food--or drug--withdrawal. We wondered if bilateral TPP lesions could block the reinforcing effects of systemic cocaine in a place conditioning paradigm. Both lesioned and sham animals acquired cocaine place preferences. TPP-lesioned animals subsequently failed to acquire place preferences when conditioned with morphine, replicating previous data with TPP lesions. It is possible that our cocaine place conditioning protocol induced aversions during drug withdrawal, thus explaining the inability of TPP lesions to block conditioning. We looked for place aversions by conditioning animals at various times postinjection of cocaine. At no time point following drug withdrawal from cocaine were significant conditioned aversions observed. Cocaine's systemic motivational effects are mediated by a substrate separate from the TPP substrate underlying the rewarding effects of opiates, food, and amphetamine.

The role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Conditioned place preference induced by delta 9-tetrahydrocannabinol: comparison with cocaine, morphine, and food reward

The rewarding property of delta 9-tetrahydrocannabinol (THC), the psychoactive constituent of marijuana and hashish, was studied using the conditioned place preference paradigm, and compared to that of cocaine, morphine, and food reward. The results of Experiment 1 demonstrated that 2.0 and 4.0 mg/kg doses produced a reliable shift in preference for the THC-paired compartment. The THC place preference observed at 2.0 and 4.0 mg/kg was nearly equivalent to that produced by low doses of cocaine (5.0 mg/kg), morphine (4.0 mg/kg), and food in non food-deprived animals. The second experiment used a different conditioning procedure that included a washout period for THC. The results of Experiment 2 demonstrated that a THC place preference could be obtained using a lower dose of THC (1.0 mg/kg), and that this THC place preference was equivalent to that produced by 10 mg/kg cocaine. At higher doses (2.0 and 4.0 mg/kg), THC produced a dose-dependent place aversion. These results suggest that THC's action on brain reward substrates, previously demonstrated by electrical brain stimulation reward, in vivo brain microdialysis, and in vivo brain electrochemistry studies, reflects itself behaviorally in increased appetitive motivational value for environmental stimuli associated with ingestion of marijuana and hashish.

Conditioned place preference using opiate and stimulant drugs: a meta-analysis

A meta-analysis was conducted on the data obtained from published articles that have used the conditioned place preference (CPP) paradigm to assess the rewarding effects of morphine, heroin, amphetamine and cocaine in rats. Using a histogram analysis of the data, significant dose-effect curves were evident with all of the drugs examined, except for cocaine. Analysis of the data also revealed that several methodological variables moderated the effect size for CPP, at least with some of the drugs examined. In particular, the following methodological variables significantly moderated CPP effect size: strain of rat used; housing condition (single or group cages); type of apparatus (2 or 3 compartments); preconditioning test (present or absent); route of drug administration; intervening saline trials (present or absent); conditioning trial duration; and drug compartment (nonpreferred, counterbalanced or white). No significant effect size differences were evident using sex, number of drug trials, or test duration as moderator variables in the analyses. These meta-analytic results may be useful to investigators for maximizing the effect size of drug-induced CPP.

The dopamine receptor agonist 7-OH-DPAT modulates the acquisition and expression of morphine-induced place preference

The present study investigated the effects of systemic administration of the putative dopamine D3 receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) on the acquisition and expression of morphine-induced place preference in male Wistar rats. Using a a 3-day schedule of conditioning it was found that 7-OH-DPAT in a broad dose range (0.01, 0.25 and 5.0 mg/kg) did not produce significant place preference. However, the administration of either 0.25 or 5.0 mg/kg of 7-OH-DPAT 15 min prior to the exposure to morphine (1 mg/kg) prevented the acquisition of a morphine place preference, whereas the 0.01 mg/kg dose of the dopamine receptor agonist was uneffective. In addition, when 7-OH-DPAT was acutely administered 15 min prior to the testing session of an already established morphine place preference, the 0.01 mg/kg dose prevented the expression of this conditioned response. This effect was not observed with either 0.25 and 5.0 mg/kg doses of this dopamine D3 receptor agonist. It was suggested that the different dose related effects of 7-OH-DPAT on the acquisition and expression of morphine place preference might be related to the intrinsic ability of this agonist for interacting with pre- and postsynaptic dopamine D3 receptors located in limbic projecting areas of the mesencephalic dopamine system, although involvement of dopamine D2 receptors cannot be excluded. The pattern of effects seen with 7-OH-DPAT suggests that it may be useful for treating opiate dependence and craving.

Reinforcing properties of fencamfamine: involvement of dopamine and opioid receptors

Fencamfamine (FCF) is a psychostimulant classified as an indirect dopamine agonist. The conditioning place preference (CPP) paradigm was used to investigate the reinforcing properties of FCF. After initial preferences had been determined, animals were conditioned with FCF (1.75, 3.5, or 7.0 mg/kg; IP). Only at the dose of 3.5 mg/kg FCF produced a significant place preference. Pretreatment with SCH23390 (0.05 mg/kg; SC) or naloxone (1.0 mg/kg; SC) 10 min before FCF (3.5 mg/kg, IP) blocked both FCF-induced hyperactivity and CPP. Pretreatment with metoclopramide (10.0 mg/kg; IP) or pimozide (1.0 mg/kg, IP), respectively, 30 min or 4 h before FCF (3.5 mg/kg; IP), which blocked the FCF-induced locomotor activity, failed to influence place conditioning produced by FCF. In conclusion, the present study suggests that dopamine D1 and opioid receptors are related to FCF reinforcing effect, while dopamine D2 subtype receptor was ineffective in modifying FCF-induced CPP.

Conditioned place aversion produced by dopamine release inhibition

CGS 10746B, a dopamine release inhibitor with properties similar to the atypical antipsychotic clozapine, was assessed as to its behavioral properties using spontaneous locomotor activity and the conditioned place preference test. Rats conditioned with interperitoneally administered doses of 1.25, 2.5, 5.0, 10.0, 20.0 or 30.0 mg/kg CGS 10746B showed a conditioned place aversion, whereas only doses of 5.0 mg/kg or greater suppressed locomotor activity. Results are discussed in terms of dopaminergic mediation of conditioned place preference and spontaneous locomotor activity and methodological concerns involved in employing the conditioned place preference test with drugs that produce opposing affective cues.

Failure to discriminate conspecifics in amygdaloid-lesioned mice

An experiment was conducted to investigate the role of the amygdala in the discriminative behavior of mice using the conditioned individual preference (CIP) method, a modified conditioned place preference (CPP) paradigm. CIP training of the subject mice involved 6 consecutive days of alternate IP injections of morphine (3 mg/kg) or saline followed by associations with a stimulus mouse in one compartment of the preference box. After the CIP training, the subject mice were given a choice between the morphine-associated and the saline-associated stimulus mice. Normal and sham-operated mice showed preference for the morphine-associated stimulus mouse. On the other hand, mice receiving bilateral amygdala lesions before or after CIP training did not show any preference for either stimulus mouse. These results support the view that the amygdala influences expression and acquisition of conditioned discriminant behaviors of mice by possibly interacting centrally, through its opiate receptors, with the peripherally injected morphine.

Lack of evidence for an involvement of nucleus accumbens dopamine D1 receptors in the initiation of heroin self-administration in the rat

The involvement of dopamine D1 receptor systems in the reinforcing properties of opiate reward was studied by examining the effect of the dopamine D1 antagonist SCH23390 on the initiation of heroin self-administration in rats. The D1 antagonist was administered daily systemically or locally in the nucleus accumbens (NAC), after which the animals were allowed to self-administer heroin (IV) in a 3-h session for 5 consecutive days. Systemic treatment with SCH23390 (0.17 and 0.5 mg.kg-1) significantly decreased heroin intake during initiation of heroin self-administration, while a dose of 0.06 mg.kg-1 was not effective. Local administration of SCH23390 (0.5 and 2.5 micrograms/site) in the NAC did not affect heroin intake. Both systemic and intra-accumbal administration of SCH23390 dose dependently decreased motor behavior measured in a small open field. The attenuation of heroin intake during initiation of heroin self-administration by blockade of dopamine D1 receptor systems may be due to a decrease in the reinforcing effects of heroin or more likely to a reduction in non-reinforcement-related behavior. The dopamine D1 receptors present in the NAC are probably not involved in opiate reward.

Assessment of the relative contribution of peripheral and central components in cocaine place conditioning

A balanced place conditioning paradigm was used to assess the contribution of peripheral and central factors mediating place conditioning induced by cocaine HCl. The first experiment was conducted to examine changes in locomotor activity and extracellular dopamine (DA) concentrations in the nucleus accumbens (NACC) following intraperitoneal (IP) injections of cocaine HCl (15 mg/kg) or cocaine methiodide (19.6 mg/kg). IP cocaine HCl significantly increased locomotor activity and extracellular NACC DA, whereas IP cocaine methiodide failed to increase either locomotor activity or extracellular DA in the NACC. In the second experiment, IP cocaine HCl (15 mg/kg) induced a significant conditioned place preference; however, neither IP procaine HCl (25 or 50 mg/kg) nor IP cocaine methiodide (4.9, 9.8, or 19.6 mg/kg) induced preferences for the drug-paired compartment. In the third experiment, intracerebroventricular (ICV) infusions of cocaine HCl (25 micrograms/2 microliters) or cocaine methiodide (1 or 5 micrograms/2 microliters) induced significant place conditioning for the drug-paired compartment. These results suggest place conditioning induced by cocaine HCl is mediated centrally and that the local anaesthetic properties alone do not contribute to this effect to any significant degree.

Voluntary oral morphine self-administration in rats: effect of haloperidol or ondansetron

Rats were exposed to increasing concentrations of morphine hydrochloride (up to 0.4 mg/ml) in 5% w/v sucrose solution as their sole source of drinking water. Physical dependence was established as determined by the precipitation of withdrawal behaviour following administration of 1 mg/kg IP naloxone hydrochloride on day 23. The choice between either a 5% w/v sucrose solution or a 5% w/v sucrose solution containing 0.4 mg/ml morphine hydrochloride 4 days following withdrawal resulted in rats being categorized into two groups based on their respective consumption of the morphine-containing solution. The amount of morphine solution voluntarily consumed by approximately half the rats were sufficiently high as to lead to a relapse into physical dependence to morphine. The high preference for morphine shown by these rats could not be attributed to the taste of the morphine solution. Naive rats or rats exposed to a 5% w/v sucrose solution for 23 days failed to consume significant quantities of the morphine-containing solution when provided with a choice. The administration of either an IM slow-release formulation of 70.5 mg/kg haloperidol decanoate (= 50 mg/kg haloperidol) or 10 micrograms/kg IP ondansetron hydrochloride daily did not alter morphine ingestion in the high morphine-preferring rats.

The effect of 5-HT3 receptor antagonists on the morphine-induced excitation of A10 dopamine cells: electrophysiological studies

In extracellular recordings from chloral hydrate anesthetized rats the 5-HT3 antagonist, BRL 46470A, failed to prevent or reverse the increase in dopamine cell firing rate produced by systemic or iontophoretically applied morphine. A second 5-HT3 antagonist, tropesitron, was similarly found to be ineffective in antagonizing the effects of systemic morphine. These results suggest that previous microdialysis reports that 5-HT3 antagonists can prevent the increase in extracellular dopamine levels in the nucleus accumbens produced by morphine are not due to an action of these compounds in suppressing the excitatory effects of morphine on A10 dopamine cell firing rate.

Neuropeptide Y: intraaccumbens injections produce a place preference that is blocked by cis-flupenthixol

Neuropeptide Y (NPY) has been localized in the nucleus accumbens (NAcc), where it may influence dopamine (DA) neurotransmission. Extensive data implicate NAcc DA in reward-related learning, raising the possibility that NPY microinjected into the NAcc may induce rewarding effects mediated by DA. This hypothesis was tested using the conditioned place preference (CPP) paradigm. Each experiment consisted of three distinct phase: preconditioning (three 15-min exposures to an apparatus with two compartments connected by a tunnel); conditioning (four 30-min pairing of one compartment with drug and four similar pairings of the other compartment with vehicle); and test (three 15-min exposures to the apparatus). A significant increase in the time spent in the drug-paired compartment from preconditioning to test was taken as evidence of a CPP. Two experiments showed that systemic (2.0 mg/kg, IP) or intraaccumbens amphetamine (10.0 micrograms in 0.5 microliters on each side) produced a CPP. The third experiment showed that intraaccumbens NPY (0.1 micrograms in 0.5 microliter on each side) produced a CPP. This CPP was blocked by pretreatment with a dose of the DA receptor blocker cis-flupenthixol (20.0 micrograms in 0.5 microliter on each side in the NAcc) that, alone, produced no CPP effect. These results strongly suggest that NPY applied to the NAcc is rewarding. In addition, these rewarding properties of NPY may be mediated by DA neurotransmission.

Effects of the AMPA/kainate receptor antagonist DNQX in the nucleus accumbens on drug-induced conditioned place preference

Activation of AMPA/kainate glutamatergic receptors in the nucleus accumbens may be a component of the mechanism of drug induced reward. To test this hypothesis, 6,7-dinitroquinoxaline-2,3-dione (DNQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate glutamatergic receptor anatagonist, was injected into the nucleus accumbens before the administration of amphetamine or morphine during the training phase (acquisition) of a conditioned place preference paradigm. Rats were then tested for place preference in the absence of drugs. In other experiments, DNQX was given before testing for place preference (expression) but not during the training phase. Bilateral injection of DNQX (1 microgram/0.5 microliters/side) inhibited acquisition of place preference to amphetamine (1 mg/kg) but not morphine (10 mg/kg). During acquisition, DNQX marginally attenuated the locomotor stimulation elicited by amphetamine during the first but not subsequent training sessions, while the combination of morphine plus DNQX produced marked akinesia during each training session. When given prior to testing, DNQX inhibited the expression of place preference induced by amphetamine and morphine but did not affect locomotor activity. The results suggest that activation of AMPA/kainate receptors is involved in the primary reward stimulation (acquisition of place preference) of amphetamine but not morphine and in behaviors elicited by memory of primary reward stimulation (expression of place preference) for both drugs. Furthermore, locomotor activity during conditioning is not necessary for acquisition of place preference.

Trends in place preference conditioning with a cross-indexed bibliography; 1957-1991

The purpose of this work is to present a perspective of the conditioned place preference (CPP) test by offering an overview of the empirical research from 1957-1991. The intent is not to extensively analyze the controversies inherent to any behavioral technique but rather to present a survey of research using a descriptive statistics approach to explore topical issues. The objectives of this work are three-fold: (a) to provide an exhaustive bibliography of the CPP literature including articles, journal abstracts, book chapters and critical reviews; (b) to provide a cross-index of identified key words/drugs tested; and (c) to give an overview of selected procedural issues underlying CPP testing.

Conditioned place preference produced by the psychostimulant cathinone

Previous work has indicated that the psychostimulant cathinone produces a location preference in the conditioned place preference task. The present study expanded upon this earlier work by examining the dose-response nature of cathinone-induced conditioned place preference, as well as testing its effect upon spontaneous locomotor activity. At doses ranging from 0.2 to 1.6 mg/kg, cathinone produced a conditioned place preference at all but the lowest dose, and the highest dose but not the lowest dose increased locomotor activity. Results are discussed in terms of dopaminergic mediation of conditioned place preference and the relationship between conditioned place preference and locomotion being subserved by the same neuronal system.

Potentiation by low doses of selected neuroleptics of food-induced conditioned place preference in rats

Numerous data support the hypothesis that dopamine (DA) plays a crucial role in reward-related processes and in incentive learning in animals and man. The possibility that various neuroleptics exhibiting a high affinity for the dopaminergic D2 (and D3) receptors could reinforce DA transmission was studied using the conditioned place preference paradigm (CPP) in rats. This was done by examining the ability of these compounds to potentiate the reinforcing properties of food in hungry rats subjected to a version of the CPP paradigm which consisted of repeated pairings of food with a single environmental cue, the floor texture of an open field. During the test session when food was no longer available in the open field, the increase in the time spent by drug-free rats on the food-paired texture was assumed to indicate the perceived rewarding value of the food. This time was significantly lengthened when the specific D2 (D3)-receptor antagonists sulpiride (4 mg/kg), amisulpride (0.5, 1 mg/kg) or pimozide (0.03, 0.06 mg/kg) were administered before the food conditioning sessions. Larger doses of these compounds as well as haloperidol, metoclopramide and the non-specific D1-D2 antagonist, chlorpromazine, regardless of the doses tested, did not exhibit this effect, but rather reduced the food-induced CPP, an action usually associated with neuroleptics. The positive effects of amisulpride was reversed by a D1 receptor antagonist, SCH 23390 (0.01 mg/kg). These results suggest that, as with amphetamine (0.5 mg/kg), some D2-specific neuroleptics enhance the incentive value of food in a narrow range of low doses, an effect proposed to reflect a "prohedonic" property.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the CS-UCS interval and extinction on place conditioning and analgesic tolerance with morphine

In experiment 1, a CS-UCS interval study of place conditioning and analgesic tolerance with morphine was conducted. Morphine (10 mg/kg i.p.) was administered to separate groups of rats either 2 h prior to, 1 h prior to, immediately prior to, immediately after or 2 h after 30-min confinement in one end compartment of a place conditioning apparatus. A total of three choice tests was given, one after every six morphine injections. A preference for the end compartment contingent upon morphine injection was shown in groups that received morphine prior to end compartment placement. Groups that received morphine after end compartment placement were not different in their preference behavior from groups that received only saline during place conditioning training. A hot-plate test for tolerance to the analgesic effect of morphine was given at the end of all choice testing. All groups that had received morphine during place conditioning training were equally tolerant. These results indicate a dissociation between the analgesic effect of morphine and the effect that produces place preference, since the former was not affected by temporal parameters that did affect the latter. In the second experiment, the effect of extinction on a morphine-induced place preference was studied using extinction procedures that, in contrast to previous studies, equated exposure to both end compartments. Whereas the morphine-induced place preference was undiminished by a 10-day retention period in which animals received saline injections in the home cage, extinction trials during the same period eliminated the place preference. These results provide evidence that morphine-induced place preferences involve associative processes.

Qualitative and quantitative differences in the operant runway behavior of rats working for cocaine and heroin reinforcement

Animals were trained to traverse a straight alley for drug reinforcement consisting of five IV injections of either 0.75 mg/kg/injection cocaine (n = 6) or 0.06 mg/kg/injection heroin (n = 6). Testing involved single daily trials during which the latency to leave the start box and the time required to traverse the alley were recorded for each animal. In addition, input from 12 pairs of infrared photocell detector/emittors placed along the length of the alley provided information on the precise location of the animal at 0.1-s intervals throughout the course of each trial. This information was recorded by computer and provided the basis for construction of graphic representations of each trial in the form of spatiotemporal records that revealed the precise route the subject took in getting to the goal box. The experiment revealed substantial differences in the runway behavior of heroin and cocaine animals. While the heroin group exhibited typical patterns of operant performance in that both start latency and goal times decreased gradually over the course of the experiment, cocaine animals were reliably slower than heroin subjects to leave the start box and exhibited a progressive increase in goal times over trials. The latter effect appeared to be a consequence of a "stop and retreat" behavior that was observed in all six cocaine subjects and increased in frequency as the experiment progressed. Because the runway behaviors exhibited here were emitted prior to delivery of the drug reinforcer, they suggest that the motivational state underlying drug-seeking behavior is qualitatively different for heroin- and cocaine-reinforced animals.

Pimozide prevents the development of conditioned place preferences induced by rewarding locus coeruleus stimulation

Electrical stimulation in the vicinity of the cell bodies of the locus coeruleus (LC) has been shown to support self-stimulation behaviors in rats. In the present study, a Conditioned Place Test, sensitive to both rewarding and aversive qualities of brain stimulation, was employed to determine (a) whether rewarding locus coeruleus stimulation would result in place preferences and (b) if so, whether dopamine receptor antagonism would affect the development of such place preferences. Animals were pretreated with pimozide (0.0, 0.5 or 1.0 mg/kg) prior to exposure to two distinctive environments only one of which was paired with locus coeruleus stimulation. Rats that received vehicle injections prior to stimulation/place pairings developed strong preferences for the stimulation-paired environment while those animals pretreated with 0.5 mg/kg pimozide showed no reliable shift in preference from baseline performance. Additionally, animals injected with the 1.0 mg/kg dose of pimozide exhibited mild place aversions to the stimulation-paired environment. It is hypothesized that dopamine neurotransmission is important for the rewarding effects of locus coeruleus stimulation without which such stimulation appears to be aversive.

Orally self-administered cocaine: reinforcing efficacy by the place preference method

In three separate place preference conditioning (PPC) experiments, groups of rats were exposed to different modes of receiving cocaine: IP cocaine doses (7.5 mg/kg), PO cocaine self-administered bolus doses (15 mg/kg), and 1-h schedule-induced cocaine-solution drinking sessions (19.1 mg/kg). Oral cocaine self-administration of PO bolus and schedule induction took place in situations that preceded transfer into an apparatus for PPC sessions. Thus, the reinforcing efficacies of the pharmacological consequences of both oral cocaine self-administration methods were evaluated by a procedure separate from the self-administration behavior itself. The IP cocaine dose imposition and the two oral cocaine self-administration arrangements all resulted in dose-exposure conditions sufficient for the production of PPC. The serum and brain cocaine pharmacokinetics sufficient for the production of reinforcing efficacy were measured and related to previous data.

Reward and abuse of opiates

The dependence creating properties of drugs are mediated by structures in the brain. The mesolimbic system seems to play a crucial role in the behaviourally reinforcing effects of opiates and other drugs of abuse. The significance of dopamine in opiate reinforcement is still a matter of debate, in spite of the large number of studies on this subject. Dopamine appears to be involved in conditioning processes and in drug self-administration behaviour only once it has been established. Neuropeptides, centrally active fragments of hormones, may play a role in the individual vulnerability for the development of drug dependence. Administration of a number of wellknown neuropeptides attenuates the acquisition of drug self-administration behaviour. The virtues and flaws of some widely used animal models for drug dependence are discussed.

Infusions of 6-hydroxydopamine into the nucleus accumbens abolish the analgesic effect of amphetamine but not of morphine in the formalin test

The effects of bilateral infusions of 6-hydroxydopamine into the nucleus accumbens on analgesia produced by D-amphetamine and morphine were examined, in separate experiments, in the formalin test in rats. The lesions in the two experiments were not significantly different and dopamine in the nucleus accumbens, olfactory tubercle and striatum was depleted to 21.1%, 40.3% and 65.0% of control values, respectively. D-Amphetamine (0.75 and 2.0 mg/kg) and morphine (3.0 and 6.0 mg/kg) attenuated the response to formalin in unlesioned control rats. The analgesic effect of amphetamine was severely reduced by 6-hydroxydopamine lesions, and the residual analgesia was correlated with the amount of dopamine in the nucleus accumbens, but not with dopamine levels in the olfactory tubercle or striatum. Lesions also attenuated the locomotor stimulant effect of amphetamine. The analgesic effect of morphine was not altered by 6-hydroxydopamine infusions, nor was there any correlation between the analgesic effect of morphine and dopamine concentration in the nucleus accumbens, olfactory tubercle or striatum. The results indicate that the dopamine innervation of the nucleus accumbens is not critical for the analgesic effect of morphine but plays a major role in the analgesic effect of amphetamine.

Opposite effects of prefrontal cortex and nucleus accumbens infusions of flupenthixol on stimulant-induced locomotion and brain stimulation reward

Ventral tegmental area (VTA) stimulation produced conditioned place preferences for stimulation-paired environments the magnitudes of which were dose-dependently reduced by systemic application of the dopamine antagonist, haloperidol (0.0, 0.15, 0.3 mg/kg). Bilateral microinjections of cis-flupenthixol (FLU) into the nucleus accumbens (0.0, 1.0, 5.0 or 10.0 micrograms) also resulted in reductions in the size of stimulation-induced place preferences as well as reductions in the magnitude of the hyperlocomotor response to 1.5 mg/kg (s.c.) D-amphetamine. Comparable microinjections of FLU into the medial prefrontal cortex (PFC) produced diametrically opposite effects: the size of VTA stimulation-induced place preferences was either unaffected (1.0 and 5.0 microgram groups) or slightly increased (10 micrograms group) and amphetamine-stimulated hyperlocomotion was dose-dependently potentiated. These behavioral findings suggest a dopamine-mediated modulatory role for the PFC over reward relevant elements within the nucleus accumbens.

Morphine place conditioning is differentially affected by CCKA and CCKB receptor antagonists

In the present study we have examined the interaction between the selective cholecystokinin (CCK)A and CCKB receptor antagonists, devazepide and L365-260 on morphine conditioned place preference (CPP). Using an unbiased procedure, morphine (1.5 mg/kg) produced a reliable CPP which was observed irrespective of the conditioning compartment type. Pretreatment with devazepide (0.001-0.01 mg/kg s.c.) produced a dose related attenuation of this response. At higher doses (0.1-1 mg/kg) this antagonism became variable and dependent on the training compartment with blockade only observed when conditioning was to the white/rough textured environment. This profile has also been reported for the serotonin (5-HT)3 receptor antagonist ondansetron. The CCKB antagonist L365-260 (0.000001-0.01 mg/kg) failed to antagonize the morphine CPP, if anything a mild potentiation was observed. To study this further we examined the interaction between L365-260 (0.01 mg/kg) and a subthreshold dose of morphine (0.3 mg/kg). At these doses neither drug elicited CPP, however when co-administered a significant CPP was recorded. Finally, L365-260 at 1 mg/kg induced a mild but significant CPP when administered alone. These results suggest a differential role of CCK receptor subtypes on reward-related behaviour and complement previous studies suggesting bimodal effects of CCK systems on mesolimbic dopamine function.

The amphetamine conditioned place preference: differential involvement of dopamine receptor subtypes and two dopaminergic terminal areas

We investigated involvement of dopamine receptor subtypes and two dopaminergic terminal areas in the acquisition and the expression of the amphetamine conditioned place preference (CPP). When injected systemically before conditioning, both D1 and D2 dopamine antagonists blocked acquisition in a dose-dependent manner. When injected systemically before testing, the effects of the same D1 and D2 antagonists differed. The selective D1 antagonist SCH23390 dose-dependently blocked expression of the previously established conditioned behavior within the dose range that also blocked acquisition. In contrast, D2 antagonists failed to block expression of the amphetamine CPP at doses which blocked acquisition. Expression was, however, blocked by higher doses of D2 antagonists, which may have lost their selectivity for the D2 dopamine receptor. The expression of the CPP was also blocked by microinjections of SCH23390 or sulpiride into nucleus accumbens, but not into striatum. In a control experiment, sodium pentobarbital, which significantly reduced spontaneous locomotor activity in a manner similar to the higher doses of the dopamine antagonists, had no effect on the expression of the amphetamine CPP when given before testing. Finally, electrolytic lesions of the dorsal striatum potentiated the amphetamine CPP. These findings indicate that the dopamine released by amphetamine interacts with both D1 and D2 dopamine receptors to establish a CPP, but that the expression of the CPP may involve activation of the D1 dopamine receptor in the nucleus accumbens.

Haloperidol attenuates conditioned place preferences produced by electrical stimulation of the medial prefrontal cortex

A Conditioned Place Preference test procedure [Ettenberg and Duvauchelle (13)] was used to investigate the effects of dopamine antagonist challenge on the rewarding properties of medial prefrontal cortex (MPFC) electrical stimulation. Rats exhibited strong preferences for the side of a two-compartment test apparatus in which they experienced sessions of experimenter-administered 0.5-s trains of MPFC sine-wave 60-Hz stimulation. Pretreatment with the neuroleptic dopamine antagonist drug, haloperidol (0.0, 0.15, or 0.3 mg/kg IP), resulted in a dose-dependent reduction in the magnitude of observed place preferences. Preference tests were conducted 24 hours after drug-conditioning trials and, hence, were not subject to motoric or other nonspecific actions of the neuroleptic treatments. In a control experiment, haloperidol did not block the place aversions produced by dorsomedial tegmental stimulation. Animals can, therefore, recall place-associations formed in the presence of haloperidol, a result which challenges "state-dependent learning" explanations of the drug's actions. Together, these results are consistent with the view that dopamine neurotransmission is involved in the rewarding consequences of electrical stimulation in the medial prefrontal cortex.

Naloxone blockade of amphetamine place preference conditioning

Amphetamine and naloxone were examined in place conditioning, in order to study possible interactions between endogenous opioids and catecholamines in reinforcement. After initial preferences were determined, animals were conditioned with amphetamine alone (1.0 mg/kg SC), naloxone alone (0.02, 0.2 or 2.0 mg/kg SC) or combinations of amphetamine plus naloxone. A reliable, long-lasting preference for the compartment associated with amphetamine was observed, reflecting the reinforcing properties of this drug. No preference or aversion was observed in animals that received saline in both compartments. Naloxone (0.02, 0.2 and 2.0 mg/kg) produced a dose-dependent place aversion; while the lowest dose had effects similar to saline, the higher doses produced significant place aversions. Naloxone, at all three doses examined, prevented the ability of amphetamine to produce a place preference. Thus, the lowest dose of naloxone, having no effects alone in place conditioning was still able to block the reinforcing effects of amphetamine. These results suggest that the reinforcing effects of amphetamine are dependent on activation of opiate receptors, and provide further evidence that interactions between endogenous opioids and catecholamines may be important in reinforcement.

Place avoidance conditioning with D-amphetamine: the effect of screening and the use of preferred and non-preferred sides

The effect of screening animals to identify unconditioned preferences for stimuli used in subsequent place conditioning, and the effect of pairing drug injections with originally most or least preferred stimuli were investigated in place conditioning with 0.5 and 2.0 mg/kg D-amphetamine. A choice test to identify conditioned approach and avoidance responses was given after every two end compartment drug trials until a total of 10 choice tests had been given. During drug trials, the drug was injected 15 min after the animal was placed in the end compartment and the animal remained in the end compartment for 30 min following drug injection. In confirmation of previous research from our laboratory, during choice tests, animals avoided the end compartment that had been paired with D-amphetamine and this avoidance occurred regardless of whether the animals had been initially screened or not, and regardless of whether the originally most or least preferred stimuli were used as the drug-associated end compartment.