Bidirectional effects of dopamine D2 receptor antagonists on responding for a conditioned reinforcer

α4-Containing GABAA Receptors on DRD2 Neurons of the Nucleus Accumbens Mediate Instrumental Responding for Conditioned Reinforcers and Its Potentiation by Cocaine

Extrasynaptic GABAA receptors (GABAARs) composed of α4, β, and δ subunits mediate GABAergic tonic inhibition and are potential molecular targets in the modulation of behavioral responses to natural and drug rewards. These GABAARs are highly expressed within the nucleus accumbens (NAc), where they influence the excitability of the medium spiny neurons. Here, we explore their role in modulating behavioral responses to food-conditioned cues and the behavior-potentiating effects of cocaine. α4-Subunit constitutive knock-out mice (α4-/-) showed higher rates of instrumental responding for reward-paired stimuli in a test of conditioned reinforcement (CRf). A similar effect was seen following viral knockdown of GABAAR α4 subunits within the NAc. Local infusion of the α4βδ-GABAAR-preferring agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol; Gaboxadol) into the NAc had no effect on responding when given alone but reduced cocaine potentiation of responding for conditioned reinforcers in wild-type, but not α4-/- mice. Finally, specific deletion of α4-subunits from dopamine D2, but not D1, receptor-expressing neurons (DRD2 and DRD1 neurons), mimicked the phenotype of the constitutive knockout, potentiating CRf responding, and blocking intra-accumbal THIP attenuation of cocaine-potentiated CRf responding. These data demonstrate that α4-GABAAR-mediated inhibition of DRD2 neurons reduces instrumental responding for a conditioned reinforcer and its potentiation by cocaine and emphasize the importance of GABAergic signaling within the NAc in mediating the effects of cocaine.

Dopamine D1 and D2 Receptors Are Important for Learning About Neutral-Valence Relationships in Sensory Preconditioning

Dopamine neurotransmission has been ascribed multiple functions with respect to both motivational and associative processes in reward-based learning, though these have proven difficult to tease apart. In order to better describe the role of dopamine in associative learning, this series of experiments examined the potential of dopamine D1- and D2-receptor antagonism (or combined antagonism) to influence the ability of rats to learn neutral valence stimulus-stimulus associations. Using a sensory preconditioning task, rats were first exposed to pairings of two neutral stimuli (S2-S1). Subsequently, S1 was paired with a mild foot-shock and resulting fear to both S1 (directly conditioned) and S2 (preconditioned) was examined. Initial experiments demonstrated the validity of the procedure in that measures of sensory preconditioning were shown to be contingent on pairings of the two sensory stimuli. Subsequent experiments indicated that systemic administration of dopamine D1- or D2-receptor antagonists attenuated learning when administered prior to S2-S1 pairings. However, the administration of a more generic D1R/D2R antagonist was without effect. These effects remained constant regardless of the affective valence of the conditioning environment and did not differ between male and female rats. The results are discussed in the context of recent suggestions that dopaminergic systems encode more than a simple reward prediction error, and provide potential avenues for future investigation.

Cell type-specific activation of mitogen-activated protein kinase in D1 receptor-expressing neurons of the nucleus accumbens potentiates stimulus-reward learning in mice

Medium spiny neurons (MSN) in the nucleus accumbens (NAc) are a fundamental component of various aspects of motivated behavior. Although mitogen-activated protein kinase (MAPK) signaling plays a crucial role in several types of learning, the cell type-specific role of MAPK pathway in stimulus-reward learning and motivation remains unclear. We herein investigated the role of MAPK in accumbal MSNs in reward-associated learning and memory. During the acquisition of Pavlovian conditioning, the number of phosphorylated MAPK1/3-positive cells was increased significantly and exclusively in the NAc core by 7-days of extensive training. MAPK signaling in the respective D1R- and D2R-MSNs was manipulated by transfecting an adeno-associated virus (AAV) plasmid into the NAc of Drd1a-Cre and Drd2-Cre transgenic mice. Potentiation of MAPK signaling shifted the learning curve of Pavlovian conditioning to the left only in Drd1a-Cre mice, whereas such manipulation in D2R-MSNs had negligible effects. In contrast, MAPK manipulation in D2R-MSNs of the NAc core significantly increased motivation for food rewards as found in Drd1a-Cre mice. These results suggest that MAPK signaling in the D1R-MSNs of NAc core plays an important role in stimulus-reward learning, while MAPK signaling in both D1R- and D2R-MSNs is involved in motivation for natural rewards.

Naloxone Decreases Responding for Conditioned Reinforcement in Rats

That opioids can mediate unconditioned reinforcement is well established, but there is little evidence indicating whether they modify conditioned reinforcement. Here, a tone which initially served as a discriminative stimulus for the availability of water reinforcement was established as a conditioned stimulus. When later given a choice between pressing a lever producing the tone (but not water) or one which produced no effect, rats chose the tone-producing lever 66% of the time. Naloxone (3.0 mg/kg) reduced overall responding and completely eliminated the preference for the tone-producing lever. Results are briefly discussed in terms of the importance of understanding mechanisms serving conditioned reinforcement.

Accelerated habit formation following amphetamine exposure is reversed by D1, but enhanced by D2, receptor antagonists

Repeated exposure to the psychostimulant amphetamine has been shown to disrupt goal-directed instrumental actions and promote the early and abnormal development of goal-insensitive habitual responding (Nelson and Killcross, 2006). To investigate the neuropharmacological specificity of this effect as well as restore goal-directed responding in animals with pre-training amphetamine exposure, animals were treated with the non-selective dopamine antagonist α-flupenthixol, the selective D₁ antagonist SCH 23390 or the selective D₂ antagonist eticlopride, prior to instrumental training (three sessions). Subsequently, the reinforcer was paired with LiCL-induced gastric-malaise and animals were given a test of goal-sensitivity both in extinction and reacquisition. The effect of these dopaminergic antagonists on the sensitivity of lever press performance to outcome devaluation was assessed in animals with pre-training exposure to amphetamine (Experiments 1A–C) or in non-sensitized animals (Experiment 2). Both α-flupenthixol and SCH23390 reversed accelerated habit formation following amphetamine sensitization. However, eticlopride appeared to enhance this effect and render instrumental performance compulsive as these animals were unable to inhibit responding both in extinction and reacquisition, even though a consumption test confirmed they had acquired an aversion to the reinforcer. These findings demonstrate that amphetamine induced-disruption of goal-directed behavior is mediated by activity at distinct dopamine receptor subtypes and may represent a putative model of the neurochemical processes involved in the loss of voluntary control over behavior.

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.

D1 receptor antagonist-induced long-term depression in the medial prefrontal cortex of rat, in vivo: an animal model of psychiatric hypofrontality

The objective of the following experiment was to induce a pathogenic hypofrontal condition by administering a dopamine-1 receptor (D(1)R) antagonist to rats. The pathophysiological effect of this manipulation upon glutamate-based long-term potentiation (LTP) in the medial prefrontal cortex (mPFC) was examined in vivo. Subjects were surgically implanted with stimulating electrodes into the corpus callosum and recording electrodes into the mPFC. High-frequency stimulation (HFS) was combined with the administration of the selective D(1)R family agonist A68930 hydrochloride (0.4 mg/kg/mL) and the selective D(1)R family antagonist SKF 83566 (0.15 mg/kg/mL). The administration of SKF 83566 hydrobromide prevented mPFC LTP, and resulted in HFS-induced long-term depression. This indicates that D(1)R activation is necessary for the induction of mPFC glutamate-based LTP. This is supported by our finding that the administration of A68930 hydrochloride combined with HFS induced LTP comparable with saline control levels, suggesting that D(1)R activation is necessary for the induction of baseline levels of mPFC LTP. Given that the mPFC governs executive behaviours that are subserved by LTP, such as working memory, these findings are relevant for the study of psychopathological conditions in which hypodopaminergic conditions exist in the mPFC and are correlated with psychiatric symptomotology, such as drug addiction and schizophrenia.

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.

A mechanistic account of striatal dopamine function in human cognition: Psychopharmacological studies with cabergoline and haloperidol

The authors test a neurocomputational model of dopamine function in cognition by administering to healthy participants low doses of D2 agents cabergoline and haloperidol. The model suggests that DA dynamically modulates the balance of Go and No-Go basal ganglia pathways during cognitive learning and performance. Cabergoline impaired, while haloperidol enhanced, Go learning from positive reinforcement, consistent with presynaptic drug effects. Cabergoline also caused an overall bias toward Go responding, consistent with postsynaptic action. These same effects extended to working memory and attentional domains, supporting the idea that the basal ganglia/dopamine system modulates the updating of prefrontal representations. Drug effects interacted with baseline working memory span in all tasks. Taken together, the results support a unified account of the role of dopamine in modulating cognitive processes that depend on the basal ganglia.

Manipulation of operant responding for an ethanol-paired conditioned stimulus in the rat by pharmacological alteration of the serotonergic system

It is becoming increasingly clear that environmental stimuli play a critical role in the maintenance of drug taking behaviour. This has led to investigations into the neural mechanisms by which environmental stimuli can come to control behaviour using paradigms such as conditioned reinforcement. The majority of this work has involved the use of food-paired conditioned stimulus rodent paradigms. Relatively few studies have attempted to investigate the neuropharmacology of behaviour maintained by presentation of a stimulus paired with ethanol drinking. Several lines of research support an important role for brain serotonin (5-HT) neurotransmitter systems in the control of alcohol drinking behaviour. The aim of the present study was, initially, to establish a procedure in which rats respond for an ethanol-paired conditioned stimulus, and second, to study the effects of a range of serotonergic compounds previously shown to be effective in reducing oral ethanol self-administration, on responding for this conditioned stimulus. Results showed that the 5-HT releaser d-fenfluramine, the selective serotonin reuptake inhibitor fluoxetine, the 5-HT1A receptor agonist 8-hydroxy-2[di-n-propylamino]tetralin, the partial 5-HT1A receptor agonist buspirone, and the 5-HT1B/5-HT2C receptor agonist 1-(3-trifluoromethylphenyl)piperazine, but not the 5-HT2A/5-HT2C receptor agonist 1-(2,5-dimethoxy-4-iodophenylaminopropane)-2, selectively reduced responding on a lever leading to presentation of an ethanol paired conditioned stimulus. In addition the non-specific D1/D2 dopamine receptor antagonist haloperidol was active in this paradigm. Results are consistent with involvement of the dopaminergic and 5-HT systems, in particular activation of 5-HT1A and 5-HT1B receptor subtypes, in mediation of the conditioned or secondary reinforcing properties of ethanol.

Nicotine as an Addictive Substance: A Critical Examination of the Basic Concepts and Empirical Evidence

The present review is a critical analysis of the concepts behind and the empirical data supporting the view that tobacco use represents an addiction to nicotine. It deals with general aspects of the notion of addiction, while concentrating on specific problems associated with incorporating nicotine into current frameworks. The notion of addiction suffers from unprecedented definitional difficulties. The definitions offered by various authorities are very different, even contradictory. Definitions that reasonably include nicotine are so broad and vague that they allow many trivial things, such as salt, sugar, and watching television, to be considered addictive. Definitions that exclude the trivia also exclude nicotine. The addiction hypothesis, in general, is strongly shaped by views that certain drugs bring about a molecular level subversion of rationality. The main human evidence for this is verbal reports of smokers who say that they can't quit. On the other hand, the existence of many millions of successful quitters suggests that most people can quit. Some smokers don't quit, but whether they can't is another matter. The addiction hypothesis would be greatly strengthened by the demonstration that any drug of abuse produces special changes in the brain. It has yet to be shown that any drug produces changes in the brain different from those produced by many innocuous substances and events. The effects of nicotine on the brain are similar to those of sugar, salt, exercise, and other harmless substances and events. Apart from numerous conceptual and definitional inadequacies with the addiction concept in general, the notion that nicotine is addictive lacks reasonable empirical support. Nicotine does not have the properties of reference drugs of abuse. There are so many findings that conflict so starkly with the view that nicotine is addictive that it increasingly appears that adhering to the nicotine addiction thesis is only defensible on extra-scientific grounds.

D2 dopamine receptor blockade immediately post-training enhances retention in hidden and visible platform versions of the water maze

Considerable evidence shows that post-training administration of dopamine agonists can enhance memory through actions on consolidation processes, but relatively little is known regarding the effects of dopamine antagonists on consolidation. These experiments investigated the effects of post-training systemic administration of the D2 receptor antagonist sulpiride on consolidation of memory for two versions of the Morris water maze task. Rats trained in either the hidden (spatial) or visible (cued) platform version received a subcutaneous injection of sulpiride or vehicle immediately following training. Retention testing 48 hr later revealed that relative to vehicle controls, sulpiride reduced platform latencies in both task versions, suggesting that like dopamine agonists, sulpiride can also have memory-enhancing effects.