Paradoxical reinforcing properties of apomorphine: effects of nucleus accumbens and area postrema lesions

Morphine-element interactions - The influence of selected chemical elements on neural pathways associated with addiction

Addiction is a pressing social problem worldwide and opioid dependence can be considered the strongest and most difficult addiction to treat. Mesolimbic and mesocortical dopaminergic pathways play an important role in modulation of cognitive processes and decision making and, therefore, changes in dopamine metabolism are considered the central basis for the development of dependence. Disturbances caused by excesses or deficiency of certain elements have a significant impact on the functioning of the central nervous system (CNS) both in physiological conditions and in pathology and can affect the cerebral reward system and therefore, may modulate processes associated with the development of addiction. In this paper we review the mechanisms of interactions between morphine and zinc, manganese, chromium, cadmium, lead, fluoride, their impact on neural pathways associated with addiction, and on antinociception and morphine tolerance and dependence.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Chromium picolinate reduces morphine-dependence in rats, while increasing brain serotonin levels

Chromium is an essential trace element with anti-diabetic and anti-depressant effect; the latter is considered related to chromium properties of increasing brain serotonin. Cr³⁺ salts were shown to improve some forced swimming-parameters and to induce rewarding effects, which are additive to those of morphine, but Cr effect on addictive processes has not been tested.

AIM

The present study aimed to assess chromium picolinate (CrPi) influence on morphine-dependence in rats.

MATHERIAL AND METHODS

We used five groups of 10 rats. Groups 1 and 2 (controls) received saline, respectively CrPi, 0.01 mg/kg/day, for 10 days. In groups 3, 4 and 5 dependence was induced with progressively-increased morphine doses (from 5 - day 1-90 mg/kg/day - day 10, s.c.). Group 3 received only morphine, while groups 4 and 5 received CrPi, i.p., 10 and respectively 5 μg/kg/day, during the 10 days of dependence induction. On day 11, groups 3, 4, and 5 were administered 90 mg/kg morphine, and, 2 h later, all rats received naloxone, 2 mg/kg s.c., to precipitate withdrawal. We compared withdrawal intensity in group 3 vs. groups 4 and 5, assessing both individual symptoms and Gellert-Holtzman global withdrawal score. Upon rats sacrifice at the end of the experiments, brain serotonin (5HT) in certain areas and serum Cr were assessed.

RESULTS

Some withdrawal signs were unequally influenced by CrPi: compulsive mastication was reduced by both CrPi doses (p < 0.05), while teeth chattering and grooming were significantly reduced only by the higher dose (p < 0.05). Withdrawal score was reduced by both CrPi doses: from 132.4 ± 9.87 - group 3 to 122.2 ± 6.47 - group 4 (p < 0.01 vs. group 3) and 124.1 ± 8.41 - group 5 (p < 0.05 vs. group 3). CrPi reduction of withdrawal is accompanied by increased brain 5 H T, mainly in the prefrontal cortex (646.3 ± 8.51 - group 3 vs. 661.5 ± 14.63 - group 4, p < 0.01 and 660.7 ± 14.01 pg/mg tissue - group 5, p < 0.05 vs. group 3). CrPi also increases brain 5 H T in non-dependent rats (prefrontal cortex: 541.6 ± 31.80, group 1 and 565.5 ± 16. 46 pg/mg tissue, group 2, p < 0.05). Administration of CrPi determined a dose-dependent increase of serum Cr.

CONCLUSIONS

Our study evidenced a slight, but significant reduction of morphine dependence in rats induced by chromium picolinate, accompanied by increased brain serotonin. This might be considered a supplementary evidence for chromium anti-depressant effect and its serotonin-mediated mechanisms.

Memantine increases NMDA receptor level in the prefrontal cortex but fails to reverse apomorphine-induced conditioned place preference in rats

Studies have shown that inflammation and neurodegeneration may accompany the development of addiction to apomorphine and that the glutamate NMDA receptor antagonist, memantine, may be neuroprotective. The similarity between apomorphine and dopamine with regard to their chemical, pharmacological and toxicological properties provided a basis for investigating the mechanism of action of the former agent. In this study, we investigated whether memantine would suppress apomorphine-seeking behavior in rats subjected to apomorphine-induced place preference conditioning, through modulation of NMDA receptors in the prefrontal cortex. Repeated apomorphine (1 mg/kg) treatment induced conditioned place preference (CPP) and had no significant effect on NMDA receptor levels in the prefrontal cortex. Prior treatment with memantine (5 mg/kg or 10 mg/kg) increased the levels of NMDA receptors in the prefrontal cortex but did not suppress CPP induced by apomorphine. These data give further support to the addictive effect of apomorphine and demonstrate that blockade of NMDA receptors by memantine is unable to suppress apomorphine-seeking behavior.

Pramipexole induced place preference after L-dopa therapy and nigral dopaminergic loss: linking behavior to transcriptional modifications

RATIONALE

Impulsive-compulsive disorders (ICD) in patients with Parkinson's disease (PD) have been described as behavioral or substance addictions including hypersexuality, gambling, or compulsive medication use of the dopamine replacement therapy (DRT).

OBJECTIVES

A remaining challenge is to understand the neuroadaptations leading to reward bias in PD patients under DRT.

METHODS

To this end, the appetitive effect of the D2/D3 agonist pramipexole was assessed after chronic exposure to L-dopa in an alpha-synuclein PD rat model.

RESULTS

Association of progressive nigral loss and chronic L-dopa was required to observe a pramipexole-induced place preference. This behavioral outcome was inhibited by metabotropic glutamate receptor 5 (mGluR5) antagonism while transcriptional profiling highlighted regulations potentially related to the context of psychostimulant addiction.

CONCLUSION

This study provides evidences strongly suggesting that PD-like lesion and L-dopa therapy were concomitant factors involved in striatal remodeling underlying the pramipexole-induced place preference. Molecular and pharmacological data suggest a key involvement of the glutamatergic pathway in this behavioral outcome.

Area postrema lesions attenuate LiCl-induced c-Fos expression correlated with conditioned taste aversion learning

Lesions of the area postrema (AP) block many of the behavioral and physiological effects of lithium chloride (LiCl) in rats, including formation of conditioned taste aversions (CTAs). Systemic administration of LiCl induces c-Fos immunoreactivity in several brain regions, including the AP, nucleus of the solitary tract (NTS), lateral parabrachial nucleus (latPBN), supraoptic nucleus (SON), paraventricular nucleus (PVN), and central nucleus of the amygdala (CeA). To determine which of these brain regions may be activated in parallel with the acquisition of LiCl-induced CTAs, we disrupted CTA learning in rats by ablating the AP and then quantified c-Fos-positive cells in these brain regions in sham- and AP-lesioned rats 1 h following LiCl or saline injection. Significant c-Fos induction after LiCl was observed in the CeA and SON of AP-lesioned rats, demonstrating activation independent of an intact AP. LiCl-induced c-Fos was significantly attenuated in the NTS, latPBN, PVN and CeA of AP-lesioned rats, suggesting that these regions are dependent on AP activation. Almost all of the lesioned rats showed some damage to the subpostremal NTS, and some rats also had damage to the dorsal motor nucleus of the vagus; this collateral damage in the brainstem may have contributed to the deficits in c-Fos response. Because c-Fos induction in several regions was correlated with magnitude of CTA acquisition, these regions are implicated in the central mediation of lithium effects during CTA learning.

Insights into pathophysiology of punding reveal possible treatment strategies

Punding is a stereotyped behavior characterized by an intense fascination with a complex, excessive, nongoal oriented, repetitive activity. Men tend to repetitively tinker with technical equipment such as radio sets, clocks, watches and car engines, the parts of which may be analyzed, arranged, sorted and cataloged but rarely put back together. Women, in contrast, incessantly sort through their handbags, tidy continuously, brush their hair or polish their nails. Punders are normally aware of the inapposite and obtuse nature of the behavior; however, despite the consequent self-injury, they do not stop such behavior. The most common causes of punding are dopaminergic replacement therapy in patients affected by Parkinson's disease (PD) and cocaine and amphetamine use in addicts. The vast majority of information about punding comes from PD cases. A critical review of these cases shows that almost all afflicted patients (90%) were on treatment with drugs acting mainly on dopamine receptors D1 and D2, whereas only three cases were reported in association with selective D2 and D3 agonists. Epidemiological considerations and available data from animal models suggest that punding, drug-induced stereotypies, addiction and dyskinesias all share a common pathophysiological process. Punding may be related to plastic changes in the ventral and dorsal striatal structures, including the nucleus accumbens, and linked to psychomotor stimulation and reward mechanisms. Possible management guidelines are proposed.

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

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

Yoked delivery of cocaine is aversive and protects against the motivation for drug in rats

In Experiment 1, water-deprived rats had 5-min access to saccharin followed by active or yoked intravenous delivery of saline or cocaine (0.33 mg/infusion). Both cocaine groups avoided intake of the saccharin cue following saccharin-cocaine pairings; however, the rats in the yoked condition exhibited greater avoidance of the taste cue than did the actively administering rats. Experiment 2 evaluated subsequent self-administration behavior on fixed- and progressive-ratio schedules of reinforcement. The results showed that prior yoked exposure to cocaine reduced subsequent drug-taking behavior on a progressive-ratio but not on a fixed-ratio schedule. Finally, Experiment 3 used a choice test to determine the impact of yoked drug delivery on the relative preference for cocaine versus water. The results showed that rats with a history of self-administering cocaine preferred to perform operant behaviors on the side of the chamber previously paired with cocaine, whereas the rats with a history of yoked delivery of cocaine avoided this side. These data show that, in most rats, the unpredictable, uncontrollable delivery of cocaine protects against the subsequent motivation for cocaine through an aversive mechanism.

Behavioral and electrophysiological indices of negative affect predict cocaine self-administration

The motivation to seek cocaine comes in part from a dysregulation of reward processing manifested in dysphoria, or affective withdrawal. Learning is a critical aspect of drug abuse; however, it remains unclear whether drug-associated cues can elicit the emotional withdrawal symptoms that promote cocaine use. Here we report that a cocaine-associated taste cue elicited a conditioned aversive state that was behaviorally and neurophysiologically quantifiable and predicted subsequent cocaine self-administration behavior. Specifically, brief intraoral infusions of a cocaine-predictive flavored saccharin solution elicited aversive orofacial responses that predicted early-session cocaine taking in rats. The expression of aversive taste reactivity also was associated with a shift in the predominant pattern of electrophysiological activity of nucleus accumbens (NAc) neurons from inhibitory to excitatory. The dynamic nature of this conditioned switch in affect and the neural code reveals a mechanism by which cues may exert control over drug self-administration.

Compulsive drug use linked to sensitized ventral striatal dopamine transmission

OBJECTIVE

A small group of Parkinson's disease (PD) patients compulsively use dopaminergic drugs despite causing harmful social, psychological, and physical effects and fulfil core Diagnostic and Statistical Manual (of Mental Disorders) Fourth Edition criteria for substance dependence (dopamine dysregulation syndrome [DDS]). We aimed to evaluate levodopa-induced dopamine neurotransmission in the striatum of patients with DDS compared with PD control patients.

METHODS

We used a two-scan positron emission tomography protocol to calculate the percentage change in (11)C-raclopride binding potential from a baseline withdrawal (off drug) state to the binding potential after an oral dose of levodopa. We related the subjective effects of levodopa to the effects on endogenous dopamine release of a pharmacological challenge with levodopa in eight control PD patients and eight patients with DDS.

RESULTS

PD patients with DDS exhibited enhanced levodopa-induced ventral striatal dopamine release compared with levodopa-treated patients with PD not compulsively taking dopaminergic drugs. The sensitized ventral striatal dopamine neurotransmission produced by levodopa in these individuals correlated with self-reported compulsive drug "wanting" but not "liking" and was related to heightened psychomotor activation (punding).

INTERPRETATION

This provides evidence that links sensitization of ventral striatal circuitry in humans to compulsive drug use.

Dopaminergic modulation of cognitive function-implications for L-DOPA treatment in Parkinson's disease

It is well recognised that patients with Parkinson's disease exhibit cognitive deficits, even in the earliest disease stages. Whereas, L-DOPA therapy in early Parkinson's disease is accepted to improve the motor symptoms, the effects on cognitive performance are more complex: both positive and negative effects have been observed. The purpose of the present article is to review the effects of L-DOPA medication in Parkinson's disease on cognitive functions in the broad domains of cognitive flexibility and working memory. The review places the effects in Parkinson's disease within a framework of evidence from studies with healthy human volunteers, rodents and non-human primates as well as computational modeling work. It is suggested that beneficial or detrimental effects of L-DOPA are observed depending on task demands and basal dopamine levels in distinct parts of the striatum. The study of the beneficial and detrimental cognitive effects of L-DOPA in Parkinson's disease has substantial implications for the understanding and treatment development of cognitive abnormalities in Parkinson's disease as well as normal health.

Role of the lateral parabrachial nucleus in apomorphine-induced conditioned consumption reduction: cooling lesions and relationship of c-Fos-like immunoreactivity to strength of conditioning

The following experiments were designed to determine whether the lateral parabrachial nucleus (lPBN) mediates acquisition of conditioned consumption reduction induced by apomorphine, an agent that also has reinforcing properties. Temporary cooling lesions of the PBN blocked acquisition of apomorphine-induced conditioned consumption reduction. In addition, both apomorphine and LiCl activated c-Fos-like immunoreactivity (c-FLI) in the central, external, and crescent lPBN, and there was a strong correspondence between amount of c-FLI expression and strength of conditioned consumption reduction in these subnuclei. Taken together, these results support the hypothesis that the lPBN mediates apomorphine-induced conditioned consumption reduction, as is true for LiCl. Furthermore, they raise the possibility that the specific part of the lPBN mediating this conditioning effect of apomorphine and LiCl is 1 of the 3 subnuclei.

Compulsive use of dopamine replacement therapy in Parkinson's disease: reward systems gone awry?

Dopamine replacement therapy (DRT) is the most effective treatment for Parkinson's disease (PD); it provides substantial benefit for most patients, extends independence, and increases survival. A few patients with PD, however, take increasing quantities of medication far beyond those required to treat their motor disabilities. These patients demand rapid drug escalation and continue to request more DRT despite the emergence of increasingly severe drug-induced motor complications and harmful behavioural consequences. In this article we detail the features of compulsive DRT-seeking and intake in PD, in relation to theories of compulsive drug use.

Effects of conditioned food aversions on nutritional behavior in humans

Conditioned food aversion (CFA) and taste aversion (CTA) are widely occurring phenomena mediating rejection of solids or liquids, the ingestion of which has induced the onset of post-ingestional malaise. It is a powerful and durable imprint learning that may influence food choice and intake in all animals, including humans. For ethical reasons, CTA has been extensively investigated in a wide variety of laboratory animal's species but only incidentally in humans. Nevertheless, convincing evidence has been provided that CFA and CTA learning are possible in a wide range of human subjects. The results in humans may have some limitations in accuracy since data are sparse, sometimes indirect, and poorly controlled. There is only limited information on the extent of CFA in the elderly since most studies have employed questionnaire and/or interview methods on young people (i.e. college students). The present review evaluates the literature derived both from laboratory animals and humans. In the first instance, the salient features of food and taste aversion learning and the neural mechanisms involved in this learning behavior will be examined. Then, the problems encountered when trying to assess the role of learned food and taste aversions in the nutritional status of healthy as well as sick young or elderly people will be considered. In particular, the importance of CFA on the nutritional status of cancer patients and treatment of alcoholism will be examined. It is concluded that the data are compelling enough to warrant further research and, some indications and recommendations are suggested.

The role of nucleus accumbens dopamine in motivated behavior: a unifying interpretation with special reference to reward-seeking

Studies addressing behavioral functions of dopamine (DA) in the nucleus accumbens septi (NAS) are reviewed. A role of NAS DA in reward has long been suggested. However, some investigators have questioned the role of NAS DA in rewarding effects because of its role in aversive contexts. As findings supporting the role of NAS DA in mediating aversively motivated behaviors accumulate, it is necessary to accommodate such data for understanding the role of NAS DA in behavior. The aim of the present paper is to provide a unifying interpretation that can account for the functions of NAS DA in a variety of behavioral contexts: (1) its role in appetitive behavioral arousal, (2) its role as a facilitator as well as an inducer of reward processes, and (3) its presently undefined role in aversive contexts. The present analysis suggests that NAS DA plays an important role in sensorimotor integrations that facilitate flexible approach responses. Flexible approach responses are contrasted with fixed instrumental approach responses (habits), which may involve the nigro-striatal DA system more than the meso-accumbens DA system. Functional properties of NAS DA transmission are considered in two stages: unconditioned behavioral invigoration effects and incentive learning effects. (1) When organisms are presented with salient stimuli (e.g., novel stimuli and incentive stimuli), NAS DA is released and invigorates flexible approach responses (invigoration effects). (2) When proximal exteroceptive receptors are stimulated by unconditioned stimuli, NAS DA is released and enables stimulus representations to acquire incentive properties within specific environmental context. It is important to make a distinction that NAS DA is a critical component for the conditional formation of incentive representations but not the retrieval of incentive stimuli or behavioral expressions based on over-learned incentive responses (i.e., habits). Nor is NAS DA essential for the cognitive perception of environmental stimuli. Therefore, even without normal NAS DA transmission, the habit response system still allows animals to perform instrumental responses given that the tasks take place in fixed environment. Such a role of NAS DA as an incentive-property constructor is not limited to appetitive contexts but also aversive contexts. This dual action of NAS DA in invigoration and incentive learning may explain the rewarding effects of NAS DA as well as other effects of NAS DA in a variety of contexts including avoidance and unconditioned/conditioned increases in open-field locomotor activity. Particularly, the present hypothesis offers the following interpretation for the finding that both conditioned and unconditioned aversive stimuli stimulate DA release in the NAS: NAS DA invigorates approach responses toward 'safety'. Moreover, NAS DA modulates incentive properties of the environment so that organisms emit approach responses toward 'safety' (i.e., avoidance responses) when animals later encounter similar environmental contexts. There may be no obligatory relationship between NAS DA release and positive subjective effects, even though these systems probably interact with other brain systems which can mediate such effects. The present conceptual framework may be valuable in understanding the dynamic interplay of NAS DA neurochemistry and behavior, both normal and pathophysiological.

Chronic cadmium exposure attenuates conditioned place preference produced by cocaine and other drugs

Adult male rats were exposed ad lib for 40 days to 100 ppm dietary cadmium chloride (group cadmium) or an identical diet with no added cadmium (group control). Conditioned place preference (CPP) was conducted in a two-chamber apparatus in which all drugs were paired with the least-preferred side as determined by a pretest. In Experiment 1, animals received 0, 2.5, or 5 mg/kg cocaine HCl (IP) for 4 days and vehicle only for 4 days. Control animals showed a place preference for the drug side at 2.5 and 5 mg/kg, while the cadmium-exposed animals showed a preference at 5 mg/kg only. In Experiment 2, animals received 0, 5, or 10 mg/kg of the D1/D2 dopamine receptor agonist apomorphine HCl (SC) for 4 days and vehicle only for 4 days. Control animals showed a place preference at 5 and 10 mg/kg, while metal-exposed animals showed a preference at 10 mg/kg only. To determine the possible effects of alterations of learning mechanisms by cadmium, a conditioned place aversion (CPA) procedure was employed for Experiment 3. Animals received 0, 10, or 40 mg/kg lithium chloride (IP) for 4 days or vehicle only for 4 days. Control animals showed a significant place aversion at 40 mg/kg, while cadmium-exposed animals did not. These findings are discussed within a framework of possible metal-induced disturbance of neurochemical function and/or associative processing.

Taste aversion learning: a contemporary perspective

Food aversion learning has attracted widespread interest because it is a highly adaptive, powerful type of learning with both practical and theoretical ramifications. It has features that make it unusual and robust when compared with other learning paradigms. It has relevance to human problems in that it is likely to contribute to food choice and appetite problems in certain clinical situations. And the robustness of this learning makes it a promising model for neurobiologists interested in understanding neural mechanisms of plasticity. This review provides a broad overview of these aspects of taste aversion learning and points to areas where questions remain and additional research is needed.

GR205171 blocks apomorphine and amphetamine-induced conditioned taste aversions

The tachykinin NK1 receptor antagonist, GR205171 ([2-methoxy-5-(5-trifluoromethyl-tetrazol-1-yl)-benzyl]-(2S-phenyl -piperidin-3S-yl)-amine), is a potent inhibitor of emesis induced by a wide variety of emetogens. This is in contrast to 5-HT3 (5-hydroxytryptamine3) receptor antagonists, such as ondansetron, which have a more restricted antiemetic profile. The present study evaluated the efficacy of GR205171, in comparison with ondansetron to block the acquisition of a conditioned taste aversion induced by either apomorphine (0.25 mg kg(-1) s.c.) or by amphetamine (0.5 mg kg(-1) s.c.) in rats. Pretreatment with GR205171 (0.1-1.0 mg kg(-1) s.c.) and ondansetron (0.001-0.1 mg kg(-1) s.c.) produced a dose-dependent blockade of conditioned taste aversions evoked by apomorphine. In contrast, the acquisition of conditioned taste aversions induced by amphetamine was inhibited by GR205171 (0.3-0.5 mg kg(-1) s.c.), but only attenuated by ondansetron (0.001-0.1 mg kg(-1) s.c.). These results suggest that tachykinin NK1 receptor antagonists may have potential in the treatment of drug-induced conditioned aversive behaviour and nausea.

The effects of cooling the area postrema of male rats on conditioned taste aversions induced by LiC1 and apomorphine

Although permanent lesion studies have demonstrated that the area postrema (AP), a chemoreceptor trigger zone, is part of the neural mechanism for conditioned taste aversions (CTAs), its exact role remains questionable. It has been suggested that the attenuated acquisition of a CTA after permanent lesions of the AP is the result of an inability to recognize the conditioned taste as novel. The present series of experiments was designed to test the hypothesis that lesions of the AP interfered with LiCl processing and not recognition of taste novelty. This was accomplished by using the reversible lesioning procedure, cooling, only during administration of the illness-inducing agent. In Expt. 1, measurement of thermal lines around the tip of the cold probe in the AP indicated that our cooling procedures allowed the majority of the AP to be cooled to temperatures that suppress neuronal activity and transsynaptic transmission, but not axonal transmission. In Expts. 2 and 3, rats were injected with either LiCl or apomorphine after consumption of a 10% sucrose solution. Cooling of the AP was initiated 5 min before administration of one of the illness-inducing agents and was continued for 55 min after injection. The rats were tested later for acquisition while the neural function of the AP was preserved. Our experimental results demonstrated that cooling the AP could attenuate the CTA induced by LiCl, but had no effect on the CTA induced by apomorphine. Since the AP was functional when the rats encountered the novel sucrose solution both before and after conditioning, but not functional when LiCl was given, these results do not support the recognition of taste novelty hypothesis.

Borna disease virus and the brain

Viruses with the ability to establish persistent infection in the central nervous system (CNS) can induce progressive neurologic disorders associated with diverse pathological manifestations. Clinical, epidemiological, and virological evidence supports the hypothesis that viruses contribute to human mental diseases whose etiology remains elusive. Therefore, the investigation of the mechanisms whereby viruses persist in the CNS and disturb normal brain function represents an area of research relevant to clinical and basic neurosciences. Borna disease virus (BDV) causes CNS disease in several vertebrate species characterized by behavioral abnormalities. Based on its unique features, BDV represents the prototype of a new virus family. BDV provides an important model for the investigation of the mechanisms and consequences of viral persistence in the CNS. The BDV paradigm is amenable to study virus-cell interactions in the CNS that can lead to neurodevelopmental abnormalities, immune-mediated damage, as well as alterations in cell differentiated functions that affect brain homeostasis. Moreover, seroepidemiological data and recent molecular studies indicate that BDV is associated with certain neuropsychiatric diseases. The potential role of BDV and of other yet to be uncovered BDV-related viruses in human mental health provides additional impetus for the investigation of this novel neurotropic infectious agent.

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.

Inhibition by MK-801 of morphine-induced conditioned place preference and postsynaptic dopamine receptor supersensitivity in mice

Intraperitoncel injection of morphine (5 mg/kg) in mice every other day for 8 days produced conditioned place preference (CPP). CPP effects were evaluated by assessing the difference in time spent in the drug-paired compartment and the saline-paired compartment of the place conditioning apparatus. The injection of a noncompetitive NMDA antagonist, MK-801 (0.05 and 0.1 mg/kg. IP), prior to and during morphine treatment in mice inhibited morphine-induced CPP. The development of postsynaptic dopamine (DA) receptor supersensitivity in mice displaying a morphine-induced CPP was evidenced by the enhanced response in ambulatory activity to the DA agonist, apomorphine (2 mg/kg). MK-801 inhibited that development of postsynaptic DA receptor supersensitivity. MK-801 also inhibited apomorphine-induced climbing behavior, suggesting that MK-801 inhibits dopaminergic activation mediated via the NMDA receptor. These results suggest that the development of morphine-induced CPP may be associated with the development of postsynaptic DA receptor supersensitivity. The development of morphine-induced CPP and DA receptor supersensitivity may be closely related to NMDA receptor-mediated dopaminergic activity, because morphine-induced changes in sensitivity to apomorphine, as well as apomorphine-induced climbing behavior in morphine treated mice, were both blocked by MK-801.

Characterization of the conditioned taste aversion produced by 7-OH-DPAT in rats

Using the conditioned taste aversion preparation, we described the dose-effect curve for the reputed dopamine D3 agonist (+/-)7-OH-DPAT (0, 0.001, 0.01, 0.1, 1.0, or 10 mg/kg). Rats received a 0.1% saccharin taste paired on repeated occasions with one of the 7-OH-DPAT doses. The 0.1, 1.0, and 10.0 mg/kg doses of 7-OH-DPAT produced a significant conditioned saccharin aversion. This aversion was evident regardless of whether saccharin intake of 7-OH-DPAT-treated rats was compared to their own water consumption or to saccharin intake by saline-treated rats.

Toxin-induced conditioned changes in taste reactivity and the role of the chemosensitive area postrema

Conditioned taste avoidances (CTAs) are an important component of behavioral regulation of ingestion. In the laboratory CTAs can be produced by pairing a novel taste stimulus with the physiological feedback produced by a toxin, such as lithium. Such toxins putatively activate a chemosensitive brainstem structure, the area postrema, which ultimately results in the production of a CTA. The present review describes a series of studies which examined conditioned changes in taste reactivity responses (TRRs) when a novel intraoral sucrose taste was paired with the effects of an intraperitoneal (IP) injection of LiCl, and the role of the area postrema in the formation of conditioned palatability shifts. It was first of all necessary to examine the effects of area postrema ablations on TRRs to a range of intraoral sucrose and quinine stimulus intensities. In the first study area postrema lesioned rats exhibited concentration dependent changes in TRRs to these taste stimuli that were very similar to those exhibited by sham lesioned rats. The second study demonstrated that 30 s intraoral infusions of sucrose (0.3 M), presented at 5 or 10 min intervals following an IP injection of LiCl (3.0 meq), resulted in conditioned changes in TRRs. These were characterized by orderly, gradual reductions in ingestive responses and increases in aversive responses. Finally, when area postrema lesioned rats (Study 3) were subjected to this conditioning procedure (brief sucrose presentations paired with the effects of LiCl) no evidence for conditioned or unconditioned changes in TRRs to sucrose were obtained. Lesioned rats injected with LiCl behaved similarly to sham lesioned rats injected with NaCl. These series of studies provide evidence indicating that the chemosensitive area postrema mediates the formation of conditioned palatability shifts induced by treatment with a toxin such as lithium.

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.

Subsensitivity to rewarding and locomotor stimulant effects of a dopamine agonist following chronic mild stress

Chronic exposure to very mild unpredictable stress has previously been found to reduce or abolish the acquisition of place preference conditioning. In the present study, chronic mild stress was found to abolish the acquisition of preferences for a distinctive environment paired with systemic administration of amphetamine (0.5 mg/kg) or quinpirole (100-400 micrograms/kg) or with quinpirole (0.75 micrograms) administered bilaterally within the nucleus accumbens. The locomotor stimulant effects of quinpirole (100-400 micrograms/kg) were also attenuated in stressed animals. The result suggest that decreased sensitivity to reward following chronic mild stress results from a decreased sensitivity of dopamine D2 receptors within the nucleus accumbens.

Lesions of the lateral parabrachial nuclei disrupt aversion learning induced by electrical stimulation of the area postrema

The research about the neural basis of taste aversion learning (TAL) has pointed out the area postrema (AP) as a fundamental structure implied in the processing of certain toxic stimuli. Likewise, recent studies demonstrated that electric stimulation of the AP is an efficient substitute of the aversive stimulus. The lateral parabrachial nucleus (PBN1), one of the subnuclei of the parabrachial complex, is the main anatomic rostral connection of the AP. In the experiment presented here, we demonstrate that TAL induced by electric stimulation of the AP is interrupted when the PBN1 is lesioned, thus giving support to the functional role of this anatomic system (AP-PBN1) in the codification of aversive stimuli processed by the AP.

Remoxipride, a specific D2 dopamine antagonist: An examination of its self-administration liability and its effects on d-amphetamine self-administration

The self-administration liability of remoxipride, a specific dopamine D2 antagonist, by laboratory rats was evaluated using an intravenous self-administration paradigm. It was observed that remoxipride failed to support self-administration behavior across the three doses tested. In addition, remoxipride pretreatment attenuated d-amphetamine self-administration. The findings of the present study provide support for the notion that remoxipride appears to have functional similarity in self-administration paradigms as other D2 antagonists.

Place conditioning with dopamine D1 and D2 agonists injected peripherally or into nucleus accumbens

The conditioned place preference technique was used to assess the affective properties of the direct dopamine D1 agonist, SKF38393, and the direct D2 agonist, LY171555 (quinpirole). A three compartment apparatus was used: the animals' pre-experimental preference for the two choice compartments was equal and, within each experimental group, half the rats received drug pairings in each choice compartment. Intraperitoneal injections of SKF38393 produced conditioned place aversions at all doses tested (1.0-4.0 mg/kg); LY171555 produced weak conditioned place preferences at 1.0 and 2.0 mg/kg, but no reliable effect at 4.0 mg/kg. Bilateral intra-accumbens microinjections of SKF38393 produced strong preferences at all doses tested (0.5-2.0 micrograms/side); LY171555 produced strong preferences at two doses (0.5 and 1.0 micrograms/side) and no effect at a third dose (2.0 micrograms/side). These results suggest that activation of either D1 or D2 receptors in the nucleus accumbens can produce reward, and that D1 receptors (and possibly also D2 receptors) located elsewhere in the brain or in the periphery may mediate aversive effects.

Effect of learned behavior upon conditioned place preference to cathinone

The purpose of this study was to examine whether first training rats to discriminate the stimulus cues produced by an indirect dopamine agonist, cathinone, would influence a subsequent test of preference. The conditioned place preference (CPP) paradigm was used to evaluate the reinforcing effects of l-cathinone in four differently treated groups of rats. Half of the animals were trained to discriminate the interoceptive cues produced by 0.8 mg/kg cathinone in a two-lever, food-motivated operant task. The other animals were equally divided between two groups, one receiving saline and noncontingent reinforcements on the same schedule as those trained to discriminate cathinone; the other group, the "yoked-control" rats, received the same cathinone and saline regimen of administration as the discrimination-trained animals. Results of CPP testing indicate that cathinone produced a statistically significant conditioned place preference only in the group trained to discriminate cathinone and not in the saline or yoked control groups. Furthermore, when half of the cathinone discrimination-trained rats were pretreated with the dopamine release inhibitor CGS 10746B, the conditioned place preference to cathinone was attenuated. The results would indicate that pairing cathinone with a nonpreferred environment tended to make the rat spend more time in that environment and the amount of time spent in the cathinone-associated environment can be increased by prior discrimination training and decreased by diminished dopamine function in the brain.

Nicotine-induced conditioned taste aversions are enhanced in rats with lesions of the area postrema

Lesions, which destroy the area postrema and damage the adjacent nucleus of the solitary tract, attenuate or abolish conditioned taste aversions (CTA) induced by a variety of pharmacological agents. In the present experiment 2 groups of male rats received lesions of the area postrema and 2 groups were given sham lesions. One lesioned group and one sham-lesioned group were twice conditioned with 30-min access to a novel 0.15% saccharin solution followed by injection of nicotine (1 mg/kg, IP). The other 2 groups were similarly conditioned with saccharin followed by saline injections. In subsequent two-bottle choice tests (saccharin vs. water), the saline-injected rats exhibited strong preferences for saccharin, the sham-lesioned rats injected with nicotine showed a weak but significant (p less than 0.05) aversion to saccharin, and the area postrema-lesioned rats injected with nicotine displayed a significantly (p less than 0.05) stronger CTA than the drug-injected sham-lesioned animals. In Phase 2 all rats were given novel chocolate metrecal (30 min) followed by injection of scopolamine HCl (1 mg/kg, IP). The area postrema-lesioned rats showed significant (p less than 0.01) preference for the chocolate taste relative to the aversions shown by the sham-lesioned animals. Thus, area postrema lesions attenuated a scopolamine-induced CTA, but enhanced a nicotine-induced aversion. These results suggest that nicotine and scopolamine act at different neural sites in producing CTAs.

The functional relevance of the area postrema in drug-induced aversion learning

Research into the neural mechanisms involved in the acquisition of learned aversions induced by drug points toward the area postrema (AP) as one of the structures implicated in the detection of drug aversive consequences. The evidence suggest that although the AP is indeed involved in drug-induced learned aversions, its functional integrity is not always a necessary requisite for learning to take place. The aim in this study was to determine whether the AP is essentially or selectively involved in all learned aversions induced by scopolamine methyl nitrate (SMN) using different number of trials with the aversive stimulus. In Experiment 1, AP-lesioned rats were injected with SMN fifteen minutes after consuming a flavoured solution during three consecutive trials. A single-stimulus test failed to detect learned aversions, which were, however, evident in two subsequent choice-tests. In one-trial paradigms, however, choice-tests as well as single-stimulus tests failed to detect learned aversions in AP-lesioned rats, both when SMN was injected immediately after stimulus intake (Experiment 2) and when a fifteen-minute delay was introduced (Experiment 3). The results suggested that the AP is not essential for the acquisition of SMN-induced aversion learning with three consecutive trials if learning is detected with a choice-test, although effective single-trial learning does apparently require a functional AP.

Apomorphine-induced flavor-drug associations: a dose-response analysis by the taste reactivity test and the conditioned taste avoidance test

Apomorphine is a positively reinforcing drug at low to moderate doses, but appears to lose its reinforcing properties at higher doses. In Experiment 1, across a range of doses (0.5-15.0 mg/kg, intraperitoneally), apomorphine produced a CTA over 5 conditioning/testing trials which was not dose-dependent by a single bottle test. The rejection taste reactivity responses of chin rubbing and gaping, however, only occurred at the highest dose of apomorphine (15 mg/kg). In Experiment 2, a CTA test which was designed to more effectively discriminate among the different drug dose conditions indicated that the doses of 2.5, 7.5 and 15 mg/kg of apomorphine produce CTAs of equivalent strength. Our results support the contention that CTAs produced by positively reinforcing drugs are not accompanied by a palatability shift.

The use of place conditioning in studying the neuropharmacology of drug reinforcement

The place conditioning paradigm has proven successful in identifying the neural mechanisms of drug reinforcement. Two classes of drugs, opiates and psychomotor stimulants, have received the most study, and in each case an important role for DA neurons of the mesolimbic system has been established. Moreover, both receptor subtypes, D1 and D2, appear to be involved. Despite this progress, the substrates of drug reward are not completely understood. First, a role for DA has not been established for all stimulants: DA receptor blockade failed to affect conditioned place preferences produced by the stimulants methylphenidate, nomifensine, or bupropion. Second, preliminary evidence suggests that intact serotonergic transmission is important in morphine place conditioning, but a similar consistent finding has not been observed with amphetamine place conditioning. Further study may reveal an interesting dissociation of serotonin's role in the rewarding effects of psychomotor stimulants and opiates. Finally, the role of the opiate receptor subtype kappa is not known; also, the significance of the several anatomical sites that support opiate place conditioning remains unclear.

Gamma radiation-induced conditioned taste aversions in rats: a comparison of the protective effects of area postrema lesions with differing doses of radiation

Lesions which destroy the area postrema (AP) and damage the adjacent nucleus of the solitary tract (NTS) attenuate or abolish conditioned taste aversions (CTA) induced by a variety of pharmacological agents as well as exposure to radiation. In the present experiment, 4 groups of male rats received lesions of AP and 4 groups were given sham lesions. One sham-lesioned and one AP-lesioned group were given a single pairing of 1-hr access to a novel 0.10% sodium saccharin solution followed immediately with exposure to 0, 100, 200, or 400 rad of gamma radiation, respectively. Four days later all groups were given daily two-bottle preference tests (saccharin vs. water) on 4 consecutive days. The sham-lesioned groups exposed to the radiation (100, 200, or 400 rad) developed profound aversions to the saccharin on all test days (p less than 0.001). In contrast, all of the AP-lesioned groups as well as the sham-irradiated (0 rad) sham-lesioned group exhibited strong, comparable (p greater than 0.30) preferences for saccharin. Thus, lesion of AP abolished the radiation-induced CTA at all dose levels of radiation. These results raise the possibility of pharmacological intervention at the level of AP to prevent radiation-induced CTA in cancer patients undergoing radiation therapy.

Sensitization to and conditioning with apomorphine in pigeons

Pigeons that repeatedly experienced the effect of apomorphine in the same environment showed an augmented behavioural response to the same drug dose as compared with controls that experienced the effect of the drug dose in differing environments. Sensitization, an increase in the behavioural response that is observed in pigeons when the same dose of apomorphine is repeatedly administered, may thus be mainly due to a conditioning of the drug response to incidental environmental cues. Apomorphine injections also induced place preferences. Pigeons that had experienced a particular environment under the influence of apomorphine subsequently favoured that environment to one they had experienced while under saline. This suggests that apomorphine administration has reinforcing properties for birds, much as it has for mammals.

Norepinephrine stimulates behavioral activation in rats following depletion of nucleus accumbens dopamine

Intraventricular (ICV) infusion of norepinephrine (NE) produces locomotor activation in rats that is greatly potentiated by prior depletion of whole brain catecholamines by ICV injection of 6-hydroxydopamine (6OHDA). In a series of experiments, the neural substrates of this potentiated locomotor response were examined. One group of animals received ICV infusion of 6OHDA to deplete whole brain catecholamines. Other rats were pretreated with desmethylimipramine (DMI) and then received 6OHDA infusions into the nucleus accumbens (NAC) to selectively deplete dopamine (DA) from this region. One week later, all animals were tested for their locomotor response to ICV infusion of NE. Both groups of rats exhibited a greatly potentiated locomotor response to ICV NE compared to corresponding sham-lesioned animals. Both ICV and NAC 6OHDA-injected animals also exhibited a supersensitive locomotor response to the DA receptor agonist apomorphine. These results suggest that NE-induced locomotor activation in ICV 6OHDA-treated rats results from the actions of NE on supersensitive NAC DA receptors.

Microinjections of D-Ala2-Met5-enkephalinamide placed into nucleus accumbens suppress hypothalamically elicited hissing in the cat

The effects of D-Ala2-Met5-enkephalinamide (DAME) upon the hissing component of hypothalamically elicited affective defense behavior in the cat were examined in this study. Microinjections of DAME placed into the nucleus accumbens significantly suppressed this response in a dose and time dependent manner. This dose dependent suppression of affective defense decreased toward baseline levels at 60 and 90 min following delivery of 1 and 10 micrograms/0.5 microliters of DAME, respectively. Similar injections placed into the caudate nucleus had no effects upon this response. Neither vehicle control nor naloxone placed into nucleus accumbens was found to significantly alter latencies for hissing. Naloxone injected into nucleus accumbens prior to administration of either a 1-microgram or a 10-micrograms dose of DAME blocked the suppressive effects of DAME that were observed when this drug was administered alone. These findings suggest that opioid receptors in the nucleus accumbens play an important role in the regulation of the hissing component of hypothalamically elicited affective defense behavior in the cat.

Studies on D1 and D2 dopamine receptor involvement in conditioned taste aversions

This series of studies investigated the ability of compounds selective for either the D1 or D2 dopamine receptor to induce a conditioned taste aversion (CTA) in thirsty rats. Neither the D1 antagonist SCH23390 (0.12-0.60 mg/kg) nor the D2 antagonist haloperidol (0.125-0.375 mg/kg) were able to induce CTAs to a saccharin solution. In contrast, the D1 agonist SKF38393 produced a dose-dependent taste aversion which was stereoselective to the (R-) enantiomer. The aversion to (R,S)-SKF38393 was not blocked by pretreatment with either SCH23390 or haloperidol, suggesting that the aversion is not mediated through stimulation of either dopamine receptor subtype. The D2 dopamine receptor agonist quinpirole was also found to produce a dose-dependent CTA. This aversion was blocked by injections of haloperidol and was attenuated following injections of domperidone, suggesting involvement of peripheral dopamine receptors in the aversion. Pretreatment with SCH23390 failed to affect the quinpirole-induced CTA, providing additional evidence that the D1 and D2 dopamine receptor subtypes can function independently of one another in the control of behavior. Finally, it does not appear that the area postrema is importantly involved in these taste aversions since lesions of this brain region did not affect the CTAs induced by either SKF38393 or quinpirole.

The functional output of the mesolimbic dopamine system

In summary, the nucleus accumbens, located at the interface of the limbic projections from the amygdala, hippocampus, and cingulate cortex, and receiving extrapyramidal fibers from midbrain DA-containing nuclei, is well situated to form neural circuitry that mediates the behaviorally activating properties of several stimulants. Efferent GABAergic fibers projecting from the nucleus accumbens to the ventral pallidum translate integrated limbic and extrapyramidal information to lower motor circuitry; some of this information appears to be carried by ventral pallidal efferent fibers projecting to the dorsomedial nucleus of the thalamus. It seems very possible that activation of this circuitry by positive reinforcing environmental stimuli, through the release of endogenous DA or opiate compounds, might contribute to motivated behavior. Indeed, environmentally generated locomotor activity can be blocked by disruption of this circuitry following destruction of N. Acc. DA terminals. It is also tempting to speculate that pathological changes in activity within this system might disrupt normal reinforcement contingencies, and contribute to the affective components of both psychiatric and neurologic disease states.

Area postrema and alcohol: effects of area postrema lesions on ethanol self-administration, pharmacokinetics, and ethanol-induced conditioned taste aversion

An investigation was carried out to test the hypothesis that the area postrema (AP) may detect ethanol as a blood-borne toxin and thereby mediate aversive postingestinal effects of the drug. These aversive effects in turn may impose an upper limit on the amount of drug that can be consumed. In Experiment 1 rats had continuous access to water and a 4% ethanol solution. Animals with AP lesions drank more ethanol than sham-operated controls. No differences in drug disposition or metabolism were found when these rats were injected with ethanol and blood ethanol levels were measured. If the AP lesions resulted in increased ethanol drinking because of a reduction in the aversive effects of the drug, then the lesions might also be expected to attenuate a conditioned taste aversion induced by ethanol. In Experiment 2, groups of lesioned and sham-operated rats drank a novel tasting fluid and then were given i.p. injections of ethanol (0.9, 1.2, or 1.6 g/kg) or vehicle. Similar degrees of aversion to the taste of the fluid developed in both the lesioned and the sham-control groups. Since the AP lesion did not result in the attenuation of the ethanol-induced conditioned taste aversion, it was suggested that the AP may not mediate the aversive consequences of ethanol and that the increased ethanol self-administration observed in Experiment 1 may be due to other effects of the lesion.

Selective D1 and D2 dopamine agonists produce opposing effects in place conditioning but not in conditioned taste aversion learning

The neurotransmitter, dopamine (DA), has been implicated in place conditioning but the role of D1 and D2 receptors has not been investigated. In Experiment 1, the effects of SKF 38393 (0, 0.01, 0.1, 1.0, 10.0 mg/kg) and quinpirole (0, 0.01, 0.1, 1.0, 2.0, 4.0 mg/kg), preferential D1 and D2 receptor agonists, respectively, were evaluated and compared to (+)-amphetamine (0, 0.01, 0.1, 1.0, 2.0, 4.0 mg/kg). The experiment consisted of three phases. During the preexposure phase, rats explored two distinctive end compartments adjoined by a small tunnel. The time spent in each compartment was recorded. During the 8-day conditioning phase, rats were treated with drug and confined to one compartment for 30 min. On alternate days, rats received saline and were placed in the opposite compartment. Test days occurred over the remaining three days during which drug-free animals explored both compartments. Rats conditioned with (+)-amphetamine demonstrated a dose-dependent increase in time spent in the drug-paired environment from preexposure to test indicating the establishment of a conditioned place preference. Treatment with quinpirole also resulted in a conditioned place preference, however, only an intermediate dose was effective. In contrast, SKF 38393 produced a dose-dependent decrease in time spent on the drug-paired side suggesting the establishment of a place aversion. The idea that D1 receptors may be exclusively involved in mediating the aversive properties of psychomotor stimulants was tested in Experiment 2 employing a conditioned taste aversion paradigm. The results did not support this notion; it was found that both quinpirole and SKF 38393 produced a conditioned taste aversion.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids attenuate taste aversions produced by toxins in rats

Conditioned taste aversions (CTAs) develop if toxicosis is induced after an animal eats or drinks. Usually, if a second drug is administered after consumption and prior to the toxin, it either adds to the CTA produced by the toxin or else has no noticeable effect. However, glucocorticoids (dexamethasone, cortisol, methylprednisolone, and prednisolone) attenuate CTAs produced by cyclophosphamide. Dexamethasone was tested most extensively and is also effective against CTAs produced by carmustine, cisplatin, copper sulfate, cytarabine, dactinomycin, doxorubicin, lithium chloride, and mechlorethamine. Delta-9 THC, domperidone, haloperidol, metoclopramide, and scopolamine were ineffective against CTAs produced by cyclophosphamide, although they are used medically for palliative purposes. Prochlorperazine attenuated CTAs but to a much smaller extent than the glucocorticoids. These results are interpreted as cross-validation of recent reports that glucocorticoids alleviate clinically observed distress produced by cancer chemotherapies.

The dopamine D2 receptor agonists, quinpirole and bromocriptine produce conditioned place preferences

1. The conditioned place preference paradigm was used to examine the role of the D2 receptor in mediating the reinforcing effects of dopamine (DA) agonists. 2. During the 3-day pre-exposure phase, rats explored two distinctive end compartments which were adjoined by a small tunnel. During the 8-day conditioning phase, groups of rats were treated with the selective D2 receptor agonists, quinpirole (0.01, 0.025, 0.05, 0.10, 0.25, 1.0 and 5.0 mg/kg IP) or bromocriptine (0, 0.01, 0.1, 0.5, 1.0, 5.0 and 10.0 mg/kg IP) and confined to one compartment for 30 min. On alternate days, rats received vehicle injections and were placed in the opposite compartment. Test days occurred over the remaining 3 days during which untreated animals explored both compartments. 3. Rats conditioned with quinpirole or bromocriptine showed significant increases in time spent in the drug-paired environment from pre-exposure to test indicating the establishment of conditioned place preferences. 4. This suggests a functional role for the D2 receptor in mediating the rewarding effects of DA agonists.