Apomorphine-induced context-specific behavioural sensitization is prevented by the D1 antagonist SCH-23390 but potentiated and uncoupled from contextual cues by the D2 antagonist sulpiride

Behaviorally conditioned effects of psychoactive drugs in experimental animals: What we have learned from nearly a century of research and what remains to be learned

Continuous treatment with drugs is a crucial requirement for managing various clinical conditions, including chronic pain and neuropsychiatric disorders such as depression or schizophrenia. Associative learning processes, i.e. Pavlovian conditioning, can play an important role for the effects of drugs and could open new avenues for optimizing patient treatment. In this narrative literature review, we summarize available data in experimental animals regarding the behaviorally conditioned effects of psychostimulants such as d-amphetamine and cocaine, the dopamine receptor agonist apomorphine, the dopamine receptor antagonist haloperidol, morphine and antidepressant drugs. In each section, the drug under discussion is briefly introduced, followed by a detailed examination of conditioning features, including doses and dosing regimens, characteristics of the conditioning process such as test environments or specific conditioned stimuli, testing and conditioned response characteristics, possible extinction or reconditioning or reversal training, neural mechanisms, and finally, the potential clinical relevance of the research area related to the drug. We focus on key outcomes, delve into methodical issues, identify gaps in current knowledge, and suggest future research directions.

Sex similarities and dopaminergic differences in interval timing

Rodent behavioral studies have largely focused on male animals, which has limited the generalizability and conclusions of neuroscience research. Working with humans and rodents, we studied sex effects during interval timing that requires participants to estimate an interval of several seconds by making motor responses. Interval timing requires attention to the passage of time and working memory for temporal rules. We found no differences between human females and males in interval timing response times (timing accuracy) or the coefficient of variance of response times (timing precision). Consistent with prior work, we also found no differences between female and male rodents in timing accuracy or precision. In female rodents, there was no difference in interval timing between estrus and diestrus cycle stages. Because dopamine powerfully affects interval timing, we also examined sex differences with drugs targeting dopaminergic receptors. In both female and male rodents, interval timing was delayed after administration of sulpiride (D2-receptor antagonist), quinpirole (D2-receptor agonist), and SCH-23390 (D1-receptor antagonist). By contrast, after administration of SKF-81297 (D1-receptor agonist), interval timing shifted earlier only in male rodents. These data illuminate sex similarities and differences in interval timing. Our results have relevance for rodent models of both cognitive function and brain disease by increasing represenation in behavioral neuroscience.

Imaging Kappa Opioid Receptors in the Living Brain with Positron Emission Tomography

Kappa opioid receptor (KOR) neuroimaging using positron emission tomography (PET) has been immensely successful in all phases of discovery and validation in relation to radiotracer development from preclinical imaging to human imaging. There are now several KOR-specific PET radiotracers that can be utilized for neuroimaging, including agonist and antagonist ligands, as well as C-11 and F-18 variants. These technologies will increase KOR PET utilization by imaging centers around the world and have provided a foundation for future studies. In this chapter, I review the advances in KOR radiotracer discovery, focusing on ligands that have been translated into human imaging, and highlight key attributes unique to each KOR PET radiotracer. The utilization of these radiotracers in KOR PET neuroimaging can be subdivided into three major investigational classes: the first, measurement of KOR density; the second, measurement of KOR drug occupancy; the third, detecting changes in endogenous dynorphin following activation or deactivation. Given the involvement of the KOR/dynorphin system in a number of brain disorders including, but not limited to, pain, itch, mood disorders and addiction, measuring KOR density in the living brain will offer insight into the chronic effects of these disorders on KOR tone in humans. Notably, KOR PET has been successful at measuring drug occupancy in the human brain to guide dose selection for maximal therapeutic efficacy while avoiding harmful side effects. Lastly, we discuss the potential of KOR PET to detect changes in endogenous dynorphin in the human brain, to elucidate neural mechanisms and offer critical insight into disease-modifying therapeutics. We conclude with comments on other translational neuroimaging modalities such as MRI that could be used to study KOR-dynorphin tone in the living human brain.

Participation of Dopamine D1 and D2 Receptors in the Rapid-Onset Behavioral Sensitization to Modafinil

Studies on the abuse potential of modafinil, a psychostimulant-like drug used to treat narcolepsy, are still controversial. While some studies claim no potential for abuse, increasing evidence suggests that modafinil induces abuse-related effects, including rapid-onset behavioral sensitization (i.e., a type of sensitization that develops within hours from the drug priming administration). The rapid-onset sensitization paradigm is a valuable tool to study the neuroplastic changes that occur quickly after drug administration, and shares neuroadaptations with drug abuse in humans. However, the mechanisms involved in the rapid-onset behavioral sensitization induced by modafinil are uncertain. Our aim was to investigate the possible involvement of dopamine D1 and D2 receptors on acute modafinil-induced hyperlocomotion and on the induction and expression of rapid-onset behavioral sensitization induced by modafinil in male Swiss mice. Treatment with the D1 receptor antagonist SCH 23390 or the D2 receptor antagonist sulpiride attenuated the acute modafinil-induced hyperlocomotion in a dose-dependent manner. Pretreatment with either antagonist before the priming injection of modafinil prevented the development of sensitization in response to a modafinil challenge 4 h later. However, only SCH 23390 decreased the expression of modafinil-induced rapid-onset behavioral sensitization. Taken together, the present findings provide evidence of the participation of D1 and D2 receptors on the development of rapid-onset behavioral sensitization to modafinil, and point to a prominent role of D1 receptors on the expression of this phenomenon.

Conditioned increase of locomotor activity induced by haloperidol

Dopamine antagonist drugs have profound effects on locomotor activity. In particular, the administration of the D2 antagonist haloperidol produces a state that is similar to catalepsy. In order to confirm whether the modulation of the dopaminergic activity produced by haloperidol can act as an unconditioned stimulus, we carried out two experiments in which the administration of haloperidol was repeatedly paired with the presence of distinctive contextual cues that served as a Conditioned Stimulus. Paradoxically, the results revealed a dose-dependent increase in locomotor activity following conditioning with dopamine antagonist (Experiments 1) that was susceptible of extinction when the conditioned stimulus was presented repeatedly by itself after conditioning (Experiment 2). These data are interpreted from an associative perspective, considering them as a result of a classical conditioning process.

The acquisition, extinction and spontaneous recovery of Pavlovian drug conditioning induced by post-trial dopaminergic stimulation/inhibition

In contextual drug conditioning, the onset of the drug treatment is contiguous with the contextual cues. Evidence suggests that drug conditioning also can occur if there is a discontinuity between the onset of the drug effect and offset of the contextual cues. Here we examine whether post-trial contextual drug conditioning conforms to several Pavlovian conditioning tenets namely: acquisition, extinction and spontaneous recovery. Six groups of rats received apomorphine (0.05 or 2.0mg/kg) and vehicle immediately or after a 15min delay following a 5min non-drug exposure to an open-field during three successive days (conditioning phase). The extinction phase occurred on days 4-8, in which all post-trial treatments were vehicle injections. After 2days of non-testing, the final test was performed. The results showed that on the first test day, the activity levels of the 6 groups were statistically equivalent. On test day 2, there were marked differences in activity levels selectively between the two immediate post-trial apomorphine treatment groups. The immediate low dose apomorphine group displayed a reduction in activity and the immediate high dose group an increase in activity relative to their day 1 levels. The activity levels of both vehicle groups and both apomorphine delay groups remained equivalent to their day 1 activity levels. On test day 3, the differences in activity levels between the two immediate post-trial apomorphine groups increased but the activity levels of the vehicle groups and the 15min delay post-trial apomorphine groups remained unchanged. In the extinction phase, the conditioned activity differences between the two immediate post-trial apomorphine groups were gradually eliminated. During the final test, the activity differences between the immediate post-trial apomorphine groups were partially restored, indicative of spontaneous recovery. These findings are consistent with several basic elements of Pavlovian conditioning and are supportive of drug induced trace conditioning.

Inhibition of apomorphine-induced behavioral sensitization in rats pretreated with fluoxetine

Despite a number of clinically useful effects, there is growing evidence that psychosis and impulse control disorders develop in patients on apomorphine therapy. Evidence suggests a critical role of serotonin-1A receptors in psychosis, drug abuse, and in the mechanism of action of the prototypical selective serotonin reuptake inhibitor fluoxetine. We investigated whether fluoxetine can prevent apomorphine-induced behavioral sensitization in a rat model of psychosis. Animals treated with fluoxetine (5 and 10 mg/kg) for 2 weeks were subsequently cotreated with apomorphine (1.0 mg/kg) for 7 days. A single injection of apomorphine increased motor activity, whereas repeated daily injections produced a progressive sensitization of motor behavior. The sensitization effects of apomorphine did not occur in fluoxetine-pretreated and subsequently cotreated animals. To further elucidate the mechanism involved in the inhibition of apomorphine sensitization in fluoxetine-treated animals, we found that apomorphine-induced motor behavior was much greater in repeated apomorphine-treated than repeated saline-treated animals. It was also greater in apomorphine and fluoxetine-cotreated animals, but not in animals pretreated and cotreated with fluoxetine. The mechanism involved in the inhibition of apomorphine sensitization in fluoxetine-pretreated animals is discussed. The findings introduce an innovative approach for extending the therapeutic use of apomorphine and classical psychostimulant drugs.

Serotonin and conditioning: focus on Pavlovian psychostimulant drug conditioning

Serotonin containing neurons are located in nuclei deep in the brainstem and send axons throughout the central nervous system from the spinal cord to the cerebral cortex. The vast scope of these connections and interactions enable serotonin and serotonin analogs to have profound effects upon sensory/motor processes. In that conditioning represents a neuroplastic process that leads to new sensory/motor connections, it is apparent that the serotonin system has the potential for a critical role in conditioning. In this article we review the basics of conditioning as well as the serotonergic system and point up the number of non-associative ways in which manipulations of serotonin neurotransmission have an impact upon conditioning. We focus upon psychostimulant drug conditioning and review the contribution of drug stimuli in the use of serotonin drugs to investigate drug conditioning and the important impact drug stimuli can have on conditioning by introducing new sensory stimuli that can create or mask a CS. We also review the ways in which experimental manipulations of serotonin can disrupt conditioned behavioral effects but not the associative processes in conditioning. In addition, we propose the use of the recently developed memory re-consolidation model of conditioning as an approach to assess the possible role of serotonin in associative processes without the complexities of performance effects related to serotonin treatment induced alterations in sensory/motor systems.

Activation of postsynaptic D2 dopamine receptors in the rat dorsolateral striatum prevents the amnestic effect of systemically administered neuroleptics

Systemically administered antipsychotics bind to dopamine (DA) D2 receptors expressed in both pre- and postsynaptic neurons of different striatal sites and present an amnestic effect on learning and memory of conditioned avoidance responses (CAR). The aim of this study was to test whether blockade of the pre- or post-synaptic D2 receptors of the dorsolateral striatum of rats is the mechanism by which systemically administered antipsychotics present this amnestic effect. CAR learning and memory was evaluated in rats that received i.p. administrations of pre- or postsynaptic doses of the antipsychotic sulpiride combined with intra-DLS infusion of the D2 agonist quinpirole. Intra-DLS quinpirole itself was not amnestic and this effect was prevented by co-administration of presynaptic dose of sulpiride. However, sulpiride was amnestic when administered systemically in a post- but not presynaptic dose. This amnestic effect of sulpiride was prevented by the co-administration of quinpirole into the DLS. These results show that a blockade of postsynaptic D2 receptors in the DLS is necessary and sufficient to produce the amnestic effect of neuroleptics on CARs.

Role of the D1 receptor for the dopamine agonist-induced one-trial behavioral sensitization of preweanling rats

RATIONALE

The neural mechanisms mediating the ontogeny of behavioral sensitization are poorly understood.

OBJECTIVE

The purpose of the present study was to determine the role of the D1 receptor for the induction of dopamine agonist-induced behavioral sensitization during the preweanling period.

METHODS

In the first experiment, the early ontogeny of R-propylnorapomorphine (NPA)-induced behavioral sensitization was examined by pretreating male and female rats with saline or NPA (0.5, 1, or 2 mg/kg, intraperitoneally (IP)) before placement in activity chambers on postnatal day (PD) 12, 16, 20, or 24. One day later, rats were tested with lower doses of NPA and the occurrence of locomotor sensitization was determined. In subsequent experiments, rats were injected with saline or the D1 receptor antagonist SCH23390 (0.1, 0.5, 1, or 5 mg/kg, IP) 0, 15, 30, or 60 min before cocaine, methamphetamine (METH), or NPA pretreatment. The next day, rats were tested with the same dopamine agonist again and sensitized responding was assessed.

RESULTS

NPA produced one-trial behavioral sensitization at all ages tested. In preweanling rats, SCH23390, regardless of dose, was ineffective at preventing the induction of cocaine-, METH-, or NPA-induced one-trial behavioral sensitization.

CONCLUSIONS

The present results are in partial contrast to adult rodent studies, in which SCH23390 blocks the induction of METH- and apomorphine-induced behavioral sensitization, but not cocaine sensitization. When these findings are considered together, it appears that D1 receptor stimulation is not necessary for the induction of behavioral sensitization during the preweanling period, although D1 receptors may play a more important role in adulthood.

Opposite effects of typical and atypical anti-psychotic drugs on sensitized dopamine receptors: sub-chronic low dose Olanzapine exposure reverses sensitization but a similar regimen of low dose haloperidol potentiates sensitization effects

RATIONALE

Both typical and atypical antipsychotic drugs are D2 receptor antagonists but yet appear to have markedly different effects upon the induction of dopamine sensitization.

OBJECTIVE

This study aims to compare the effects of subchronic regimens of low-dose olanzapine and haloperidol administered to rats previously sensitized to apomorphine.

METHODS

Initially, rats received apomorphine (2.0 mg/kg) or vehicle treatments for five consecutive days followed by a conditioning test and an apomorphine challenge test. Next, there was an antipsychotic exposure phase in which three vehicle groups and three apomorphine groups received 10 daily injections of either vehicle, haloperidol (0.03 mg/kg) or olanzapine (0.01 mg/kg). In the final phase, all groups were given a second conditioning test and apomorphine challenge test.

RESULTS

Apomorphine induced sensitization and conditioning effects. Following haloperidol exposure, apomorphine conditioned and sensitization effects were potentiated but, in contrast, olanzapine exposure eliminated apomorphine sensitization effects. In addition, the sensitization induced by apomorphine transformed the low-dose haloperidol treatment into a potent locomotor stimulant treatment. In the vehicle groups, haloperidol and olanzapine exposure effects were equivalent and not different from vehicle treatment.

CONCLUSIONS

The profound differences observed between typical and atypical antipsychotic exposure in animals with an upregulated dopamine system are consistent with clinical evidence for lower risk of psychomotor disturbances with chronic treatment with atypical antipsychotic. Importantly, the finding that a very low dose of olanzapine reversed sensitization effects suggests that low-dose olanzapine may have clinical utility in a variety of disorders linked to sensitization of the dopamine system.

Extending therapeutic use of psychostimulants: focus on serotonin-1A receptor

INTRODUCTION

Despite a number of medicinally important pharmacological effects, the therapeutic use of psychostimulants is limited because of abuse potential and psychosis following long term use. Development of pharmacological agents for improving and extending therapeutic use of psychostimulants in narcolepsy, attention deficit hyperactivity disorder, Parkinson's disease, obesity and as cognitive enhancer is an important research imperative. In this regard, one potential target system is the 5-hydroxytryptamine (5-HT; serotonin) neurotransmitter system. The focus of the present article is to evaluate a potential role of 5-HT-1A receptor in the alleviation of abuse potential and psychosis-induced by prescription psychostimulants amphetamines and apomorphine.

METHOD

Synaptic contacts between dopamine systems and 5-HT-1A receptors are traced. Studies on serotonin-1A influences on the modulation of dopamine neurotransmission and psychostimulant-induced behavioral sensitization are accumulated.

RESULTS

Inhibition of amphetamine and apomorphine-induced behavioral sensitization by co administration of 5-HT-1A agonists cannot be explained in terms of direct activation of 5-HT-1A receptors, because activation of pre- as well as postsynaptic 5-HT-1A receptors tends to increase dopamine neurotransmission.

CONCLUSION

Long term use of amphetamine and apomorphine produces adaptive changes in 5-HT-1A receptor mediated functions, which are prevented by the co-use of 5-HT-1A agonists. In view of extending medicinal use of psychostimulants, it is important to evaluate the effects of co-use of 5-HT-1A agonists on potential therapeutic profile of amphetamine and apomorphine in preclinical research. It is also important to evaluate the functional significance of 5-HT-1A receptors on psychostimulant-induced behaviors in other addiction models such as drug self-administration and reinstatement of drug seeking behavior.

Residual dopamine receptor desensitization following either high- or low-dose sub-chronic prior exposure to the atypical anti-psychotic drug olanzapine

RATIONALE

Anti-psychotic drugs are antagonists of dopamine D2 receptors and repeated administration may lead to the development of dopamine receptor supersensitivity.

OBJECTIVES

The objective of this study is to investigate the effects of sub-chronic olanzapine treatments upon the induction of dopamine receptor supersensitivity.

METHODS

Rats were administered ten daily low or high doses of the atypical anti-psychotic drug olanzapine (0.01 or 1.0 mg/kg). After 5 days of withdrawal, all groups received 2.0 mg/kg apomorphine on five successive days. Five days after the apomorphine sensitization protocol, in separate experiments, either a conditioning test or an apomorphine sensitization test was conducted.

RESULTS

During the anti-psychotic treatment the high dose of olanzapine induced profound locomotion suppression, whereas the low dose had no effect upon locomotion. The apomorphine treatments given to the vehicle control group generated locomotor sensitization. This sensitization effect was attenuated by the same degree for both the low or high dose prior olanzapine treatments. Also, the low and high-dose olanzapine pre-treatments diminished subsequent apomorphine-conditioned and apomorphine-sensitized locomotor responses.

CONCLUSIONS

The equivalent attenuation of the apomorphine sensitization produced by both olanzapine doses indicates that this effect was unrelated to the direct effects of olanzapine upon locomotion. Furthermore, the persistence of the desensitization effects well after the termination of the olanzapine treatments is indicative of a residual desensitization of the dopamine system. These findings are of importance when considering the use of atypical anti-psychotic drugs in the treatment of psychoses and other disorders in which overactivity of the dopamine system is considered a contributory factor.

Dopamine D1 receptor signaling system regulates ryanodine receptor expression in ethanol physical dependence

BACKGROUND

Ryanodine receptors (RyRs) amplifying activity-dependent calcium influx via calcium-induced calcium release play an important role in central nervous system functions including learning, memory, and drug abuse. In this study, we investigated the role and the regulatory mechanisms of RyR expression under continuous exposure of mice to ethanol (EtOH) vapor for 9 days.

METHODS

The model of EtOH physical dependence was prepared as follows: 8-week-old male ddY mice were exposed to EtOH vapor for 9 days. Protein and mRNA of RyR-1, RyR-2, and RyR-3 in the frontal cortex and limbic forebrain were determined by Western blot and real-time RT-PCR analysis, respectively.

RESULTS

Exposure of mice to EtOH vapor for 9 days induced significant withdrawal signs when estimated with withdrawal score, which was dose-dependently suppressed by intracerebroventricular administration of dantrolene, an RyR antagonist. Protein levels of RyR-1 and RyR-2 in the frontal cortex and limbic forebrain significantly increased during EtOH vapor exposure for 9 days with increased expression of their mRNA, whereas that of RyR-3 in these 2 brain regions showed no changes. Increased proteins and mRNA of RyR-1 and RyR-2 were completely abolished by SCH23390, a selective antagonist of dopamine D1 receptors (D1DRs), but not by sulpiride, a selective antagonist of D2DRs.

CONCLUSIONS

RyRs play a critical role in the development of EtOH physical dependence and that the up-regulation of RyRs in the brain of mouse, showing EtOH physical dependence is regulated by D1DRs.

Memory re-consolidation and drug conditioning: an apomorphine conditioned locomotor stimulant response can be enhanced or reversed by a single high versus low apomorphine post-trial treatment

RATIONALE

Psychostimulant sensitization can have transformative effects upon contextual stimuli such as acquired conditioned stimuli and conditioned incentive motivational properties.

OBJECTIVE

The aim of this study is to induce apomorphine sensitization and conduct non-drug exposures to the contextual cues followed by post-trial treatments designed to associate increases/decreases in dopamine activity with re-consolidation of the contextual cue conditioned stimulus.

METHODS

Separate groups received five daily apomorphine (2.0 mg/kg) treatments, paired or unpaired to the test environment. Two days later, a 30-min non-drug conditioning test was performed. Subsequently, there were three brief (5 min) conditioning tests on successive days. After removal from the test environment on the three test days, all groups received post-trial treatment with vehicle, 0.05, and 2.0 mg/kg apomorphine. One day later, a second 30-min conditioning test was conducted.

RESULTS

There was a sensitized and a conditioned locomotor stimulant response in the paired groups. After the first and second post-trial treatments with 0.05 mg/kg apomorphine, the conditioned stimulant response in the paired group was transformed into a conditioned inhibitory response. In contrast, the conditioned stimulant response of the paired group administered with apomorphine 2.0 mg/kg post-trial was amplified. The apomorphine post-trial treatments administered to the unpaired groups or 2 h post-trial to paired groups were without effect.

CONCLUSIONS

These findings suggest that sensitization substantially enhances the associative sensitivity of contextual stimuli and imply that brief exposure to cues linked to drugs of addiction followed by treatments that inhibit neurotransmitter systems may provide a new direction in drug abuse treatment.

Opposite effects of low versus high dose haloperidol treatments on spontaneous and apomorphine induced motor behavior: evidence that at a very low dose haloperidol acts as an indirect dopamine agonist

Anti-psychotic drugs are antagonists at the dopamine D2 receptors and repeated administration can lead to the development of dopamine receptor supersensitivity. In two experiments, separate groups of rats were administered 10 daily low or high doses of the typical anti-psychotic drug haloperidol (0.03 or 1.0 mg/kg). The high dose decreased locomotion whereas, the low dose increased locomotion. After 5 days of withdrawal, all groups received 2.0 mg/kg apomorphine on 5 successive days. The apomorphine treatments given to the vehicle group generated a progressive locomotion sensitization effect and this effect was potentiated by pre-exposure to 0.03 mg/kg haloperidol. Initially, the prior high dose of haloperidol exaggerated the apomorphine locomotor stimulant effect but with repeated apomorphine treatments desensitization developed. Following a 5-day withdrawal period an apomorphine challenge test was conducted and apomorphine sensitization was absent in the haloperidol high dose pre-exposure group but potentiated in the low dose pre-exposure group. In the second replication experiment a conditioning test instead of a sensitization challenge test was conducted 5 days after completion of the 5-day apomorphine treatment protocol. The repeated apomorphine treatments induced conditioned hyper- locomotion and this conditioned effect was prevented by the prior high dose haloperidol pre-exposure but enhanced by the prior low dose haloperidol pre-exposure. Two new key findings are (a) that a low dose haloperidol regimen can function as a dopamine agonist and these effects persist after withdrawal and (b) that repeated apomorphine treatments can desensitize D2 receptors previously sensitized by a high dose haloperidol treatment regimen.

Reversal of apomorphine locomotor sensitization by a single post-conditioning trial treatment with a low autoreceptor dose of apomorphine: a memory re-consolidation approach

Sensitization is a common feature of psychostimulants and sensitization effects are generally considered to be linked to the addictive properties of these drugs. We used a conventional paired/unpaired Pavlovian protocol to induce a context specific sensitization to the locomotor stimulant effect of a high dose of apomorphine (2.0mg/kg). Two days following a 5 session sensitization induction phase, a brief 5min non-drug test for conditioning was conducted. Only the paired groups exhibited locomotor stimulant conditioned response effects. Immediately following this brief test for conditioning, the paired and the unpaired groups received injections of 0.05mg/kg apomorphine, 2.0mg/kg apomorphine or vehicle designed to differentially impact memory re-consolidation of the conditioning. Two days later, all groups received a sensitization challenge test with 2.0mg/kg apomorphine. The 2.0mg/kg apomorphine post-trial treatment potentiated sensitization while the 0.05mg/kg eliminated sensitization. These effects were only observed in the paired groups. The activation of dopaminergic systems by the high dose of apomorphine strengthened the drug/environment association whereas the inhibition of dopamine activity by the low auto-receptor dose eliminated this association. The results point to the importance of conditioning to context specific sensitization and targeting memory re-consolidation of conditioning as a paradigm to modify sensitization.

Context-driven changes in L-DOPA-induced behaviours in the 6-OHDA lesioned rat

Both contralateral rotational behaviour and dyskinetic abnormal involuntary movements (AIMs) are induced by the administration of l-DOPA in the unilateral 6-OHDA lesioned rat model of Parkinson's disease. Since rotational responses can be conditioned to environmental cues we have investigated the extent to which drug-induced AIMS may also be conditioned by exteroceptive cues and experience. In Experiment I, 6-OHDA lesioned rats received repeated daily injections of l-DOPA either in their home cage (control) or in association with a brief (20 mins) exposure to the rotometers (paired). To assess conditioning, all animals then received two tests in the rotometer bowls. Following injection of saline the paired group both rotated more contralaterally and displayed manifest AIMs, neither of which were exhibited by the control rats. Moreover, following injection of l-DOPA, the paired group showed a trend for increased AIMs compared to controls. Two further studies provided longer exposure to the conditioning environments in counterbalanced designs. Although, using these parameters, re-exposure in the presence of saline did not induce context-dependent AIMs, a strong context-specific component of the sensitised response to l-DOPA was seen; chronic administration of drug produced a significantly stronger behavioural response in animals paired with a particular environment for drug administration than controls. This data suggests that part of the sensitisation of behavioural responding to l-DOPA administration is not solely a pharmacological phenomenon, but is also conditioned to the environmental context in which the drug is administered. This has clear implications for the clinical observation and experimental measurement of drug-induced dyskinesia in Parkinson's disease patients and animal models.