Dopamine D1 and D2 antagonists attenuate amphetamine-produced enhancement of responding for conditioned reward in rats

Antagonism of D2 receptors via raclopride ameliorates amphetamine-induced associative learning deficits in male mice

Dopamine levels in the dorsomedial striatum (DMS) are highly dynamic and are thought to underly the encoding of action-outcome associations. Although it is known that amphetamine disrupts the learning that is required for goal-directed action, the role of D1 and D2 receptors in this process has not been established. In this study, we examined the role of D1 and D2 receptor antagonists on learning in response to amphetamine. We used the outcome-specific devaluation task to examine goal-directed action in male C57BL6/J mice treated systemically with either a D1 antagonist (SCH-23990; 0.01 mg/kg) or a D2 antagonist (raclopride; 0.5 mg/kg) and then administered amphetamine (1 mg/kg). The mice were injected repeatedly throughout the instrumental training phase of the task to assess the impact on the learning of action-outcomes, and the subsequent choice test assessing performance of goal-directed action was conducted drug free. Effects of chronic drug administration on locomotor behaviour was assessed before and after the choice test. Treatment during learning with either amphetamine, or the D1 or D2 antagonists, impaired the subsequent performance of goal-directed action. The amphetamine-induced impairment in goal-directed action was reversed in mice treated with raclopride, but not when treated with SCH-23990. By contrast, amphetamine-induced hyperactivity was reversed in mice treated with SCH-23990, but not in mice treated with raclopride. Taken together, these data support the role of a balance of dopamine receptor signalling after amphetamine treatment. While overall D1 receptor availability is necessary to promote learning, in a state of elevated dopamine, modifying D2 receptor function can ameliorate learning deficits.

Enhancement of memory consolidation by an avoidance conditioned stimulus: Modulation by the D3 receptor

Conditioned stimuli (CS) paired with foot-shock can enhance memory consolidation. Because the dopamine D3R has been implicated in mediating various responses to CSs, the current study explored its potential role in modulation of memory consolidation by an avoidance CS. Male Sprague-Dawley rats trained to avoid foot-shocks in a two-way signalled active avoidance task (8 sessions, 30 trials per session, 0.8 mA foot-shock) were pre-treated with the D3R antagonist NGB-2904 (Vehicle, 0.1 or 5 mg/kg) and exposed to the CS immediately after the sample phase of an object recognition memory task. Discrimination ratios were assessed 72 h later. Immediate, but not delayed (6 h), post-sample exposure to the CS enhanced object recognition memory and this effect was dose-dependently blocked by NGB-2904. Control experiments with the beta-noradrenergic receptor antagonist propranolol (10 or 20 mg/kg) and D2R antagonist pimozide (0.2 or 0.6 mg/kg) indicated that NGB-2904 targeted post-training memory consolidation. Exploring the pharmacological selectivity of the D3R effect, it was found that: 1) 5 mg/kg NGB-2904 blocked conditioned memory modulation produced by post-sample exposure to a "weak" CS (one day of avoidance training) and concurrent stimulation of catecholamine activity by 10 mg/kg bupropion; 2) post-sample exposure to a "weak" CS and concurrent administration of the D3R agonist 7-OH-DPAT (1 mg/kg) enhanced consolidation of object memory. Finally, because 5 mg/kg NGB-2904 had no effect on modulation by avoidance training in the presence of foot-shocks, the findings herein support the hypothesis that the D3R plays an important role in modulation of memory consolidation by CSs.

Influence of Selective Dopamine Agonist Ropinirole on Conditioned Place Preference and Somatic Signs of Morphine Withdrawal in Rats

The underlying mechanism of dependence and rewarding effects of morphine is imperative to understand. The primary aim of this study was to investigate whether ropinirole D2/3 agonist affects the rewarding and reinforcing properties of morphine-induced conditioned place preference (CPP) and withdrawal syndromes in rats. On day one, the animals were randomly divided to conduct the pre-test. The morphine (10 mg/kg, i.p.) and/or saline was administered on alternate days in an 8-day CPP session. On day 10, 15 min prior to the post-conditioning test (expression), a single dose of ropinirole (1, 2, and 5 mg/kg, i.p.) was given to rats. In extinction session, ropinirole was injected daily, and CPP was extinguished by repeated testing, with intervals of 3 days. Finally, reinstatement was assessed by administering ropinirole (1, 2, and 5 mg/kg) 15 min before the morphine injection. Morphine dependence was developed by administering increasing doses of morphine (10–50 mg/kg, i.p.). To assess withdrawal symptoms, ropinirole (1, 2, and 5 mg/kg) was injected 15 min before naloxone (2 mg/kg, s.c.) administration. The present study confirms that ropinirole attenuates expression and reinstatement of CPP, while it precipitates the extinction of morphine-induced CPP. Naloxone-precipitated morphine withdrawal symptoms, including wet dog shakes and weight loss, were attenuated although jumping was increased by a single ropinirole injection. Thus, ropinirole was influential in attenuating expression, reducing drug seeking and weakening reinstatement via the dopaminergic system. These findings show that ropinirole might affect neuro-adaptive changes related to dependence.

Neurobiology of reward-related learning

A major goal in psychology is to understand how environmental stimuli associated with primary rewards come to function as conditioned stimuli, acquiring the capacity to elicit similar responses to those elicited by primary rewards. Our neurobiological model is predicated on the Hebbian idea that concurrent synaptic activity on the primary reward neural substrate-proposed to be ventral tegmental area (VTA) dopamine (DA) neurons-strengthens the synapses involved. We propose that VTA DA neurons receive both a strong unconditioned stimulus signal (acetylcholine stimulation of DA cells) from the primary reward capable of unconditionally activating DA cells and a weak stimulus signal (glutamate stimulation of DA cells) from the neutral stimulus. Through joint stimulation the weak signal is potentiated and capable of activating the VTA DA cells, eliciting a conditioned response. The learning occurs when this joint stimulation initiates intracellular second-messenger cascades resulting in enhanced glutamate-DA synapses. In this review we present evidence that led us to propose this model and the most recent evidence supporting it.

Preadolescent dopamine receptor antagonism increases postadolescent reward-related operant behaviors that may depend on dopamine receptor hypersensitivity

The dopaminergic system has a long history of being associated with reward-related activities but the developmental consequences of blocking dopamine receptor function on reward-based associative learning has been less studied. To this end, male, Long Evans rats were systemically (i.p.) treated with the dopamine receptor (DAr) antagonist, flupenthixol (0.25 mg/kg), or saline, from postnatal day (P)18 - 24 (preadolescence) then trained on an operant conditioning task from P41 - P45 (postadolescent) without drug treatment. The preadolescent flupenthixol group showed elevated active lever responses and locomotor activity during the drug-free test. Another group of rats was given flupenthixol prior to each acquisition session from P41 - 45 which significantly suppressed both active lever presses and locomotor activity. Separate groups of rats were treated with flupenthixol or saline from P18 - 24 then treated with apomorphine or saline on P41 followed by assessment of c-Fos labeling in the nucleus accumbens. Early flupenthixol treatment was associated with more apomorphine-induced c-Fos labeling in the nucleus accumbens shell than the early saline-apomorphine group, indicating a sensitized response. These findings suggest that preadolescent dopamine receptor blockade may lead to a sensitized postadolescent dopaminergic response that underlies enhanced behavioral responses in the presence of rewarding stimuli.

Effects of 5-HT1A, 5-HT2A and 5-HT2C receptor agonists and antagonists on responding for a conditioned reinforcer and its enhancement by methylphenidate

OBJECTIVES

These experiments examined the effects of selective 5-HT_1A, 5-HT_2A and 5-HT_2C receptor ligands on responding for a conditioned reinforcer (CRf). Effects of these ligands were measured under basal conditions and following elevated dopamine (DA) activity produced by the DA reuptake inhibitor methylphenidate.

METHODS

Water-restricted rats learned to associate a conditioned stimulus (CS) with water in operant chambers. Subsequently, two response levers were made available; responding on one lever delivered the CS (now a CRf), while responding on the second lever had no consequences. The effects of agonist and antagonists of 5-HT_1A (8-hydroxy-2(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT) and N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY100635)), 5-HT_2A (DOI and M100907) and 5-HT_2C (Ro60-0175 and SB242084) receptors on responding were examined alone, as well as in the presence of methylphenidate.

RESULTS

Responding for a CRf was reduced by the agonists 8-OH-DPAT, DOI and Ro60-0175. 8-OH-DPAT also reduced responding for water and seemed to impair responding in a non-specific fashion. None of the receptor antagonists affected responding. Methylphenidate dose-dependently enhanced responding for a CRf, and this was attenuated by DOI and Ro60-0175. Conversely, the 5-HT_2C receptor antagonist SB242084 potentiated the effect of methylphenidate.

CONCLUSIONS

No evidence was found for a behaviourally selective effect of 5-HT_1A receptor ligands on responding for a CRf. Activation of 5-HT_2A receptors selectively inhibits responding for a CRf. 5-HT_2C receptor ligands exerted bidirectional modulation of responding for a CRf, especially when DA activity was increased. This indicates that 5-HT_2C receptor activity is an important modulator of DA-dependent reward-related behaviours.

Safety out of control: dopamine and defence

We enjoy a sophisticated understanding of how animals learn to predict appetitive outcomes and direct their behaviour accordingly. This encompasses well-defined learning algorithms and details of how these might be implemented in the brain. Dopamine has played an important part in this unfolding story, appearing to embody a learning signal for predicting rewards and stamping in useful actions, while also being a modulator of behavioural vigour. By contrast, although choosing correct actions and executing them vigorously in the face of adversity is at least as important, our understanding of learning and behaviour in aversive settings is less well developed. We examine aversive processing through the medium of the role of dopamine and targets such as D2 receptors in the striatum. We consider critical factors such as the degree of control that an animal believes it exerts over key aspects of its environment, the distinction between 'better' and 'good' actual or predicted future states, and the potential requirement for a particular form of opponent to dopamine to ensure proper calibration of state values.

The effect of a dopamine antagonist on conditioning of sexual arousal in women

RATIONALE

Dopamine (DA) plays a key role in reward-seeking behaviours. Accumulating evidence from animal and human studies suggests that human sexual reward learning may also depend on DA transmission. However, research on the role of DA in human sexual reward learning is completely lacking.

OBJECTIVES

To investigate whether DA antagonism attenuates classical conditioning of sexual response in humans.

METHODS

Healthy women were randomly allocated to one of two treatment conditions: haloperidol (n = 29) or placebo (n = 29). A differential conditioning paradigm was applied with genital vibrostimulation as unconditional stimulus (US) and neutral pictures as conditional stimuli (CSs). Genital arousal was assessed, and ratings of affective value and subjective sexual arousal were obtained.

RESULTS

Haloperidol administration affected unconditional genital responding. However, no significant effects of medication were found for conditioned responding.

CONCLUSIONS

No firm conclusions can be drawn about whether female sexual reward learning implicates DA transmission since the results do not lend themselves to unambiguous interpretation.

Cocaine self-administration disrupts mesolimbic dopamine circuit function and attenuates dopaminergic responsiveness to cocaine

Dopaminergic projections from the ventral midbrain to the nucleus accumbens (NAc) have long been implicated in encoding associations between reward availability and environmental stimuli. As such, this circuit is instrumental in guiding behaviors towards obtaining maximal rewards based on previous experience. Cocaine acts on the dopamine system to exert its reinforcing effects and it is thought that cocaine-induced dysregulation of dopamine neurotransmission contributes to the difficulty that cocaine addicts exhibit in selecting environmentally appropriate behaviors. Here we used cocaine self-administration combined with in vivo fast scan cyclic voltammetry in anesthetised rats to examine the function of the ventral tegmental area to NAc projection neurons. Over 5 days of cocaine self-administration (fixed-ratio 1; 1.5 mg/kg/injection; 40 injections/day), animals increased their rate of intake. Following cocaine self-administration, there was a marked reduction in ventral tegmental area-stimulated NAc dopamine release. Additionally, there was a decreased augmentation of stimulated dopamine overflow in response to a cocaine challenge. These findings demonstrate that cocaine induces a hypodopaminergic state, which may contribute to the inflexible drug-taking and drug-seeking behaviors observed in cocaine abusers. Additionally, tolerance to the ability of cocaine to elevate dopamine may lead to increased cocaine intake in order to overcome decreased effects, another hallmark of cocaine abuse.

Amphetamine modulates brain signal variability and working memory in younger and older adults

Better-performing younger adults typically express greater brain signal variability relative to older, poorer performers. Mechanisms for age and performance-graded differences in brain dynamics have, however, not yet been uncovered. Given the age-related decline of the dopamine (DA) system in normal cognitive aging, DA neuromodulation is one plausible mechanism. Hence, agents that boost systemic DA [such as d-amphetamine (AMPH)] may help to restore deficient signal variability levels. Furthermore, despite the standard practice of counterbalancing drug session order (AMPH first vs. placebo first), it remains understudied how AMPH may interact with practice effects, possibly influencing whether DA up-regulation is functional. We examined the effects of AMPH on functional-MRI-based blood oxygen level-dependent (BOLD) signal variability (SD(BOLD)) in younger and older adults during a working memory task (letter n-back). Older adults expressed lower brain signal variability at placebo, but met or exceeded young adult SD(BOLD) levels in the presence of AMPH. Drug session order greatly moderated change-change relations between AMPH-driven SD(BOLD) and reaction time means (RT(mean)) and SDs (RT(SD)). Older adults who received AMPH in the first session tended to improve in RT(mean) and RT(SD) when SD(BOLD) was boosted on AMPH, whereas younger and older adults who received AMPH in the second session showed either a performance improvement when SD(BOLD) decreased (for RT(mean)) or no effect at all (for RT(SD)). The present findings support the hypothesis that age differences in brain signal variability reflect aging-induced changes in dopaminergic neuromodulation. The observed interactions among AMPH, age, and session order highlight the state- and practice-dependent neurochemical basis of human brain dynamics.

Responding for a conditioned reinforcer, and its enhancement by nicotine, is blocked by dopamine receptor antagonists and a 5-HT(2C) receptor agonist but not by a 5-HT(2A) receptor antagonist

An aspect of nicotine reinforcement that may contribute to tobacco addiction is the effect of nicotine to enhance the motivational properties of reward-associated cues, or conditioned stimuli (CSs). Several studies have now shown that nicotine enhances responding for a stimulus that has been paired with a natural reinforcer. This effect of nicotine to enhance responding for a conditioned reinforcer is likely due to nicotine-induced enhancements in mesolimbic dopaminergic activity, but this has not been directly assessed. In this study, we assessed roles for dopamine (DA) D1 or D2 receptors, and two serotonin (5-HT) receptor subtypes known to modulate DA activity, the 5-HT2C or 5-HT2A subtypes, on nicotine-enhanced responding for a conditioned reinforcer. Water-restricted rats were exposed to Pavlovian conditioning sessions, where a CS was paired with water delivery. Then, in a second phase, animals were required to perform a novel, lever-pressing response for presentations of the CS as a conditioned reinforcer. Nicotine (0.4 mg/kg) enhanced responding for the conditioned reinforcer. To examine potential roles for dopamine (DA) and serotonin (5-HT) receptors in this effect, separate groups of animals were used to assess the impact of administering the D1 receptor antagonist SCH 23390, D2 receptor antagonist eticlopride, 5-HT2C receptor agonist Ro 60-0175, or 5-HT2A receptor antagonist M100907 on nicotine-enhanced responding for conditioned reinforcement. SCH 23390, eticlopride, and Ro 60-0175 all reduced responding for conditioned reinforcement, and the ability of nicotine to enhance this effect. M100907 did not alter this behavior. Together, these studies indicate that DA D1 and D2 receptors, but not 5-HT2A receptors, contribute to the effect of nicotine to enhance responding for a conditioned reinforcer. This effect can also be modulated by 5-HT2C receptor activation.

Dopamine D1 receptor involvement in latent inhibition and overshadowing

Latent inhibition (LI) manifests as poorer conditioning to a stimulus that has previously been experienced without consequence. There is good evidence of dopaminergic modulation of LI, as the effect is reliably disrupted by the indirect dopamine (DA) agonist amphetamine. The disruptive effects of amphetamine on LI are reversed by both typical and atypical antipsychotics, which on their own are able to facilitate LI. However, the contribution of different DA receptors to these effects is poorly understood. Amphetamine effects on another stimulus selection procedure, overshadowing, have been suggested to be D1-mediated. Thus, in the current experiments, we systematically investigated the role of D1 receptors in LI. First, we tested the ability of the full D1 agonist SKF 81297 to abolish LI and compared the effects of this drug on LI and overshadowing. Subsequently, we examined whether the D1 antagonist SCH 23390 can lead to the emergence of LI under conditions that do not produce the effect in normal animals (weak pre-exposure). Finally, we tested the ability of SCH 23390 to block amphetamine-induced disruption of LI. We found little evidence that direct stimulation of D1 receptors abolishes LI (although there was some attenuation of LI at 0.4 mg/kg SKF 81297). Similarly, SCH 23390 failed to enhance LI. However, SCH 23390 did block amphetamine-induced disruption of LI. These data indicate that, while LI may be unaffected by selective manipulation of activity at D1 receptors, the effects of amphetamine on LI are to some extent dependent on actions at D1 receptors.

Enhanced incentive motivation for sucrose-paired cues in adolescent rats: possible roles for dopamine and opioid systems

Vulnerability to the effects of drugs of abuse during adolescence may be related to altered incentive motivation, a process believed to be important in addiction. Incentive motivation can be seen when a neutral stimulus acquires motivational properties through repeated association with a primary reinforcer. We compared adolescent (postnatal day (PND) 24-50) and adult (>PND 70) rats on a measure of incentive motivation: responding for a conditioned reinforcer (CR). Rats learned to associate the delivery of 0.1 ml of 10% sucrose with a conditioned stimulus (CS; light and tone); 30 pairings per day were given over 14 days. Then, we measured responding on a lever delivering the CS (now a CR) after injections of amphetamine (0, 0.25 or 0.5 mg/kg). We also examined responding for CR when the CS and sucrose were paired or unpaired during conditioning, and responding for the primary reinforcer (10% sucrose) in control experiments. Finally, we examined the effects of D(1) and D(2) dopamine receptor antagonists (SCH 39166 and eticlopride, respectively) and an opioid receptor antagonist (naltrexone) on responding for a CR in adolescent rats. Adolescents but not adults acquired responding for a CR, but adolescents responded less than adults for the primary reinforcer. Responding for a CR depended upon the pairing of the CS and sucrose during conditioning. Both dopamine and opioid receptor antagonists reduced responding for the CR. Therefore, incentive motivation may be enhanced in adolescents compared with adults, and incentive motivation may be mediated in part by both dopamine and opioid systems.

Amphetamine-induced enhancement of responding for conditioned reward in rats: interactions with repeated testing

RATIONALE

The mesolimbic dopamine system underlies the ability of reward-related stimuli to control operant behavior. Previous work has shown that amphetamine potentiates operant responding for conditioned rewards (CRs).

OBJECTIVES

Here, we asked whether the profile of this amphetamine-produced potentiation changes with repeated CR presentation, i.e., as the CR is being extinguished.

METHODS

Amphetamine (0-1.0 mg/kg, i.p.), administered over four daily sessions using a Latin square design, dose-dependently increased lever pressing for a 'lights-off' stimulus previously paired with food in rats.

RESULTS

The amphetamine-produced enhancement of responding for CR was significantly modulated with repeated CR exposure: it was strongest on day 1 and became less pronounced in subsequent sessions whereas the CR effect persisted. In further experiments, rats receiving LiCl devaluation of the primary reward failed to show a significant reduction in the amphetamine-produced enhancement of responding for CR.

CONCLUSIONS

The nature of the dissociable effects of amphetamine on responding for CR versus the CR effect itself remains to be elucidated.

The effects of adolescent methylphenidate self-administration on responding for a conditioned reward, amphetamine-induced locomotor activity, and neuronal activation

BACKGROUND

Abuse of methylphenidate (Ritalin) is rising, particularly during adolescence and early adulthood, but the long-term effects of its abuse during adolescence are unclear.

METHODS

In experiment 1, we examined the effect of adolescent methylphenidate self-administration (0.0625 mg/infusion), as compared with cocaine self-administration (0.125 mg/infusion), under a fixed ratio 1 schedule of reinforcement in male Sprague-Dawley rats during adolescence (postnatal day (PND) 32-47) on adult dopamine-mediated behaviors (PND >70). These included responding for a conditioned reward (CR), a measure of incentive motivation, and amphetamine-induced locomotor activity. In experiment 2, we aimed to replicate and enhance the effects observed in experiment 1, and we also examined the effects of methylphenidate self-administration during adolescence on adult amphetamine-induced zif268 messenger ribonucleic acid (mRNA) expression.

RESULTS

Adolescent rats self-administered both cocaine and methylphenidate. There was no effect of adolescent drug self-administration on adult baseline or amphetamine-induced responding for a CR. However, both adolescent methylphenidate and cocaine self-administration increased amphetamine-induced locomotion. Adolescent methylphenidate self-administration also enhanced amphetamine-induced zif268 mRNA expression in the nucleus accumbens.

CONCLUSIONS

Our findings suggest that repeated, behaviorally contingent exposure to methylphenidate during adolescence enhances responsivity to the locomotor-stimulating and neuronal activating effects of amphetamine but not incentive motivation.

Bupropion and nicotine enhance responding for nondrug reinforcers via dissociable pharmacological mechanisms in rats

RATIONALE

Nicotine serves as a primary reinforcer but also potently enhances responding for nonnicotine stimuli with reinforcing properties. One of the most successful pharmacotherapies for smoking cessation, bupropion, also increases responding for nondrug reinforcers such as food and brain stimulation rewards.

OBJECTIVE

The present studies investigated whether treatment with bupropion and nicotine had similar effects on responding for a reinforcing visual stimulus (VS). They also investigated whether the effects of bupropion and nicotine depended on common pharmacological substrates.

RESULTS

Nicotine (0.4 mg/kg base) enhanced responding for the VS, and this enhancing effect increased across testing sessions, replicating our previous findings. Bupropion (3, 10, and 30 mg/kg salt) dose-dependently increased responding for the VS. Treatment with 10 and 30 mg/kg bupropion resulted in a profile similar to nicotine; operant responding increased over repeated drug treatments. The reinforcement enhancing effect of nicotine, but not bupropion, was blocked by pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine. In contrast, the reinforcement enhancing effect of bupropion, but not nicotine, was blocked by pretreatment with the alpha noradrenergic antagonist prazosin.

CONCLUSION

The reinforcement enhancing effects of nicotine and bupropion increased over time and repeated treatments suggesting a shared mechanism of action. However, the reinforcement enhancing effects of nicotine are mediated by nicotinic acetylcholine receptors, whereas the reinforcement enhancing effects of bupropion were mediated by alpha noradrenergic receptors.

Reduction of impulsivity with amphetamine in an appetitive fixed consecutive number schedule with cue for optimal performance in rats

RATIONALE

Impulsivity is a key feature of many psychopathologies such as mania, personality disorders or attention deficit-hyperactivity disorder (ADHD). Most experimental paradigms assessing impulsive behaviour also require non-specific capacities such as time estimation. This may interact with the measures and mask the beneficial effects of psychostimulants-the most commonly used treatment for ADHD-on impulsivity, given that these drugs speed up the internal clock.

OBJECTIVES

The present experiment investigated the effects of suppressing behaviours non-specific to impulsivity in a fixed consecutive number (FCN) schedule and examined whether amphetamine, previously shown to increase impulsive responses in this task, could have beneficial effects when impulsive responses are promoted.

MATERIALS AND METHODS

Food-deprived rats were trained to press one lever of a two-lever operant chamber eight times before pressing the other lever to obtain food. Premature ending of responses resulted in absence of food delivery and reset the counter. A cue light indicating the required number of presses was present (FCN8(cue)) and removed after training (FCN8). Rats were then trained under an FCN16(cue) schedule to be challenged with d-amphetamine (0.125, 0.25 and 0.5 mg/kg).

RESULTS

The cue improved performances, and similar scores were obtained under FCN16(cue) compared to FCN8. Premature responses under these two conditions were unrelated. Amphetamine reduced impulsive responses in FCN16(cue) at the lower dose.

CONCLUSIONS

Suppression of capacities non-specific to impulsivity in the FCN schedule, associated with conditions that permit the expression of inhibitory deficits, allows the beneficial effects of psychostimulants observed clinically to be evidenced experimentally.

Rostral-caudal differences in the effects of intra-VTA muscimol on cocaine self-administration

We have found that dopamine (DA) in the ventral tegmental area (VTA) plays an important role in cocaine self-administration. DA in the VTA acts at D1-type receptors on the terminals of GABA afferents causing release of this neurotransmitter. Thus, the neurochemical pathways whereby VTA DA might be involved in cocaine self-administration may include GABA neurotransmission. In the present study, we investigated this possibility. Rats were prepared with intravenous catheters and bilateral guide cannulae positioned to allow microinjections directly into the VTA or a site 1 mm dorsal to it. The rats were then trained to self-administer cocaine (1.0 mg/kg/injection) under a fixed-ratio 1 schedule of reinforcement and tested with microinjections of muscimol (0, 0.05 and 0.1 microg/0.25 microl) or picrotoxin (0, 0.025 and 0.05 microg/0.25 microl) or trained under a progressive ratio (PR) schedule and tested with vehicle and 0.05 microg/0.25 microl muscimol. Muscimol in the VTA, but not immediately dorsal to it, significantly reduced cocaine intake under the FR1 schedule. Furthermore, when analyzed by rostral/caudal site of injection, it was found that rostral injections of muscimol significantly reduced cocaine self-administration whereas caudal injections produced non-significant decreases in self-administration. Inspection of individual records revealed no signs of non-specific behavioral effects of the muscimol treatments. Muscimol in the rostral VTA also significantly increased break points in responding under the PR schedule. Intra-VTA picrotoxin did not significantly affect cocaine self-administration. These data suggest that stimulation of GABA-A receptors in the VTA is involved in cocaine self-administration and reward and that this involvement is more pronounced in the rostral than in the caudal VTA.

The serial reaction time task in the rat: effects of D1 and D2 dopamine-receptor antagonists

Sequential behaviour, probably reflecting procedural learning, has intensively been investigated in humans and monkeys using so-called serial reaction time tasks (SRTT), where serial stimuli are either presented in a random or sequential fashion. Learning of sequences is typically inferred from faster reaction times to such sequences as compared to random blocks of stimuli. Work with such tasks has shown that sequential behaviour seems to be mediated by specific brain systems, including the basal ganglia and the neurotransmitter dopamine. We have recently developed a rat version of the human serial reaction time task, in which rats have to respond to visual stimuli in one of four spatial locations by nose-poking in order to obtain food reward under a fixed ratio schedule (FR13). Here, we used a test version where random and sequential condition phases (10 min each) were alternated within-sessions. In support of our previous work, we found that well-trained (i.e. skilled) rats display superior performance under sequential than random conditions, namely, faster reaction times and higher response accuracies. Furthermore, we investigated the effects of selective dopamine-receptor blockade, by systemically administering SKF 83566, a D1 antagonist (.05-.15 mg/kg), or raclopride, a D2 antagonist (.05-.20 mg/kg), in two separate experiments. Both antagonists impaired responding to the conditioned visual stimuli in a dose-related way, i.e. they decreased, or even blocked, nose-poke rates. In those rats, which kept responding, the speeding of reaction times during sequential conditions was no longer observed with the D1 antagonist, whereas the enhancements in accuracy were preserved, or even enhanced as compared to vehicle. The D2 antagonist also impaired instrumental behaviour, but did not alter sequence effects on accuracy or reaction times. In contrast to responses to the conditioned stimuli, reaction times to the unconditioned stimuli (food pellets) were not substantially affected by either drug. These results are discussed with respect to methodological factors, and the possible role of dopamine for instrumental behaviour, in general, and sequential behaviour, in specific.

D1-receptor drugs and cocaine-seeking behavior: investigation of receptor mediation and behavioral disruption in rats

RATIONALE

Dopamine D1-receptor antagonists and agonists both attenuate cocaine-seeking behavior (i.e., operant responding in the absence of cocaine reinforcement) elicited by a cocaine prime or cocaine-paired stimuli. It remains unclear whether these effects are D1-receptor mediated.

OBJECTIVES

The present study tested whether a D1 antagonist (SCH-23390) would reverse the attenuating effects of a D1 agonist (SKF-81297) on cocaine-seeking behavior and whether behavioral disruption is involved in these effects.

METHODS

Rats trained to press a lever for cocaine reinforcement with light and tone cues paired with each infusion underwent daily extinction sessions during which responding had no scheduled consequences (i.e., neither cocaine nor the cocaine-paired stimulus complex was available). After responding diminished, the effects of the D1 antagonist on the dose-response functions of the D1 agonist for reinstatement of cocaine-seeking behavior by response-contingent cue presentations or cocaine priming were examined. A separate experiment assessed the effects of the agonist on the dose-response function of the antagonist for cue reinstatement. Stereotyped behavior and activity were also measured during each test session.

RESULTS

The attenuating effects of SKF-81297 on cocaine-seeking behavior during cocaine-primed reinstatement were reversed by co-administration of SCH-23390. However, no evidence for reversal of the attenuation during cue reinstatement was found even though agonist-induced stereotypy and antagonist-induced hypoactivity were reversed by co-administration of the two drugs during the same test session.

CONCLUSIONS

The findings suggest that the attenuating effects of D1-receptor drugs on cocaine-seeking behavior during cocaine reinstatement are mediated by dopamine D1 receptors; however, it remains unclear whether the effects of these drugs on cocaine-seeking behavior during cue reinstatement are D1-receptor mediated. Nevertheless, it is evident that the attenuation of cocaine-seeking behavior by these drugs is not simply due to behavioral disruption.

Differential behavioral responses to dopaminergic stimulation of nucleus accumbens subregions in the rat

The following experiments investigated the behavioral response to local microinfusion of dopamine (DA) and selective DA agonists into the core and shell subregions of the nucleus accumbens. Rats were implanted with chronic indwelling cannulae aimed at these subregions. Two experiments were conducted. In experiment 1, the response to DA (0, 2, 5, 10 microg/0.5 microl/side), the D-1 agonist SKF-82598 (0, 0.1, 1.0 microg), the D-2/3 agonist quinpirole (0, 1, 5, 15 microg) and the D-3 preferring agonist pramipexole (0.1, 1.0, 10.0 microg) was examined in photocell activity cages. Locomotor (horizontal) and rearing (vertical) activities were measured. DA and SKF-82958 induced relatively greater increases in activity following stimulation of the shell as compared with the core. Quinpirole induced a dose-dependent suppression of activity after infusion into both sites, although the core was more sensitive to the suppressive effect than the shell. Pramipexole induced time-dependent, biphasic effects that were small in magnitude and did not differentiate between site. In experiment 2, an observation procedure was used to record behaviors (locomotion, rearing, feeding, drinking). Dopamine (0, 2, 10 microg) elicited greater increases in rearing and feeding behavior in the shell than in the core. SKF-82958 (0, 0.75 microg) enhanced locomotion and rearing to a similar extent in both subregions in this test, whereas a mixture of a low dose (0.25 microg) of the D-1 and D-2 agonists selectively induced behavioral activation in the shell. In contrast to the results in the activity cage test, quinpirole (0, 1, 5 microg) increased motor activity at the lower dose when infused into the shell but not into the core. No alterations in feeding were observed following infusion of selective agonists, and no changes in drinking were found with any of the treatments. In summary, the shell appears to be relatively more sensitive to the motor activating effects of DA agonists than the core. Moreover, circuits associated with shell may be preferentially involved in feeding.

Intranucleus accumbens amphetamine infusions enhance responding maintained by a stimulus complex paired with oral ethanol self-administration

Six male Long-Evans rats were trained to self-administer 10% ethanol (v/v) during 30 min operant sessions. A licking response on an empty drinking tube resulted in the presentation of reinforcement from an automatic dipper. During the initiation of ethanol self-administration, a tone-light stimulus complex was paired with all ethanol presentations. When 10% ethanol maintained responding, guide cannulae aimed at the nucleus accumbens (NAcc) were implanted into the brain. The ability of the paired stimulus complex to reinforce a new operant response (i.e., a lever press) was then examined. To test for the development of the new response, responding on one lever resulted in presentation of only the paired tone-light stimulus complex (contingency-associated lever) while responding on an alternate lever had no programmed consequences (no contingency-associated lever). Prior to some new response sessions, amphetamine (5-20 microg/microl) was infused into the NAcc to examine the influence of dopamine on responding maintained by the stimulus complex. Ethanol intake during the sessions prior to new response testing averaged 0.49 +/- 0.07 g/g. During new response sessions no significant differences in lever pressure during no-drug conditions (control, sham, injection or vehicle injection) were observed between the contingency-associated and no contingency-associated levers. Intra-NAcc infusion of amphetamine (5-20 microg/microl) resulted in significant increases in lever pressing only on the contingency-associated lever. These data suggest that increasing NAcc dopamine levels with amphetamine enhanced the ability of the stimulus complex to function as a reinforcer. Further studies examining the ability of potentially more salient stimuli (i.e., taste of ethanol) to function as conditioned reinforcers associated with ethanol self-administration are warranted due to the apparent inability of the paired tone-light stimulus complex to function as a reinforcer without amphetamine-induced activation of the NAcc.

Enhanced reward-related responding following cholera toxin infusion into the nucleus accumbens

In recent years, considerable focus has been directed to understanding how drugs of abuse affect neuronal function at the molecular level. For example, repeated administration of stimulants or opiates can induce long-lasting alterations in gene expression, transcription factors, and signal transduction pathways. Our laboratory previously showed that intraaccumbens infusion of cholera toxin (CTX), which alters the Gs protein such that production of cyclic Adenosine Monophosphate (AMP) is upregulated, causes pronounced, long-lasting motor activation and sensitization to stimulants. In the present experiments, the effect of intraaccumbens infusion of cholera toxin on reward-related responding was investigated. The conditioned reinforcement (CR) paradigm was employed, which measures an animal's instrumental response to obtain presentation of a stimulus previously paired with a primary reward. When this stimulus supports acquisition of a new operant response (lever-pressing), it is termed a conditioned reinforcer (CR). In the first experiment, the effects of bilateral intraaccumbens infusion of CTX (100 ng/1 microliter) were examined on previously-established responding. CTX treatment resulted in enhanced responding for the CR. This enhancement developed over several days and reached its peak 3 days following infusion. In the second experiment, the influence of CTX was examined on acquisition of responding for the CR. The group treated with CTX (100 ng) discriminated between the CR and control (NCR) lever earlier than the vehicle-infused group, and showed greater levels of responding on the CR lever. In the third experiment, it was determined that infusion of CTX (300 ng bilaterally) into the anterior dorsal striatum did not affect levels of responding, although a later test with cocaine in these animals (25 mg/kg, intraperitoneally) (i.p.) indicated that they were capable of potentiated responding. These data are interpreted as evidence that the G(S) protein-cyclic AMP second messenger system within the nucleus accumbens is directly involved in reward-related behavior.

Neurobehavioural mechanisms of reward and motivation

The analysis of the behavioural and neural mechanisms of reinforcement and motivation has benefited from the recent application of learning theory and better anatomical knowledge of the connectivity of certain key neural structures, such as the nucleus accumbens. This progress has enabled the dissection of motivational processes into components that can begin to be related to the functioning of specific limbic cortical structures that project to different compartments of the ventral striatum.

The D1 agonist SKF 38393 attenuates amphetamine-produced enhancement of responding for conditioned reward in rats

The present study investigated the hypothesis that the D1 subtype of DA receptors is critically involved in reward-related learning. The effects of SKF 38393, a D1-specific agonist, on amphetamine-produced enhancement of responding for conditioned reward were tested. We exposed 69 male Wistar rats to an experimental design consisting of three phases. The preexposure phase consisted of five sessions during which the rats were exposed to an operant chamber containing two levers. One lever produced a lights-off stimulus (3 s) and the other a tone stimulus (3 s). This was followed by four conditioning sessions during which the levers were removed and the rats were exposed to pairings of the lights-off stimulus with food. This phase was followed by two test sessions during which the levers were present and the number of responses made on each lever was calculated as a ratio of the number of responses made during the preexposure phase. A group receiving saline during the test sessions showed a higher ratio of responding for the lights off stimulus than the tone stimulus, demonstrating that the lights-off stimulus had become a conditioned reward. Amphetamine [2.0 mg/kg, intraperitoneally (i.p.), 5 min before the test] enhanced responding specifically on the lever producing the conditioned reward. Groups receiving SKF 38393 (5.0, 10.0, and 20.0 mg/kg, i.p., 5 min before the test) failed to show significantly greater responding for the lights-off stimulus than the tone, indicating a reduction or elimination of the conditioned reward effect. Moreover, SKF 38393 dose dependently reduced the amphetamine-produced enhancement of responding for conditioned reward.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of systemic and intracerebral injections of D1 and D2 agonists on brain stimulation reward

That dopamine (DA) plays a role in reward-related learning is well documented but the mechanisms through which it acts are not well understood. The present set of experiments investigated the role of DA receptor subtypes within DA-innervated forebrain regions in brain stimulation reward (BSR). Thirty-two rats were implanted with electrodes in the ventral tegmental area (VTA) and cannulae aimed at the caudal nucleus accumbens (NAcc), the caudate-putamen (CP) or cortex. Rate-frequency functions were determined by logarithmically decreasing the number of cathodal pulses in a stimulation train from a value that sustained maximal responding to one that did not sustain responding (thresholds). After BSR thresholds stabilized rats received treatments with DA agonists and their effects on thresholds were analyzed. Systemic treatments consisted of injections of (+)-amphetamine (1.0 mg/kg, i.p., 10 min before testing), the D2 agonist quinpirole (1.0 mg/kg, i.p., 10 min before testing), the novel D1 agonist A-77636 (3.0 mg/kg, s.c., 90 min before testing) or their vehicle (distilled H(2)0). Central treatments consisted of microinjections of quinpirole (0.3-10.0 micrograms/0.5 microliter) directly into the caudal NAcc, CP or cortex or A-77636 (30 micrograms/0.5 microliter) into the caudal NAcc or CP. Results showed that all three agonists, when injected systemically, significantly reduced the threshold frequency required for VTA BSR, indicating a potentiative effect on reward. Central injections of quinpirole in the caudal NAcc, CP or cortex produced significant increases in BSR thresholds indicative of reduced rewarding efficacy of stimulation. Central injections of A-77636 into the caudal NAcc, but not the CP, were associated with a reduction in VTA BSR thresholds, suggesting an increase in reward. These results suggest that stimulation of D1 or D2 receptors enhances the rewarding effect of brain stimulation. In the case of the systemic quinpirole enhancement of reward, the present results suggest that this may not occur in the caudal NAcc, CP or cortex. Finally, the present results suggest that D1 receptor stimulation in the caudal NAcc can facilitate reward-related learning.

Rostral-caudal differences in effects of nucleus accumbens amphetamine on VTA ICSS

The effects of amphetamine along the rostrocaudal axis of the nucleus accumbens (NAcc) on ventral tegmental area (VTA) intracranial self-stimulation (ICSS) were studied. Eighteen rats were trained to lever press for ICSS in the VTA. Rate-frequency functions were determined by logarithmically decreasing the frequency of cathodal pulses in a stimulation train from a value that induced maximal responding to one that induced no responding (thresholds). After ICSS thresholds stabilized, (+)-amphetamine (20.0 micrograms/0.5 microliter) or its vehicle, distilled H2O (0.5 microliter), were injected directly into the rostral NAcc (n = 6) or the caudal NAcc (n = 8) or the caudate-putamen (CP) (n = 5) just dorsal to the caudal NAcc. Results showed that amphetamine in the caudal NAcc significantly decreased ICSS thresholds without affecting asymptomatic rates of responding, indicating a potentiation of the rewarding efficacy of VTA stimulation. Amphetamine in the rostral NAcc or CP produced smaller, non-significant, decreases in ICSS thresholds. Further analyses revealed a significant positive correlation (r13 = 0.51, P < 0.05) between the site of injection along the rostrocaudal axis of the NAcc and the size of the amphetamine-produced potentiation of VTA stimulation reward. Others have reported topographical differences, including dopamine terminal density and D1 receptor density, in the NAcc. The present results indicate that these anatomical and neurochemical differences appear to be correlated with behavioural differences.