Dissociation in effects of lesions of the nucleus accumbens core and shell on appetitive pavlovian approach behavior and the potentiation of conditioned reinforcement and locomotor activity by D-amphetamine

A novel perspective on the role of nucleus accumbens neurons in encoding associative learning

The nucleus accumbens (NAc) has been considered a key brain region for encoding reward/aversion and cue-outcome associations. These processes are encoded by medium spiny neurons that express either dopamine receptor D1 (D1-MSNs) or D2 (D2-MSNs). Despite the well-established role of NAc neurons in encoding reward/aversion, the underlying processing by D1-/D2-MSNs remains largely unknown. Recent electrophysiological, optogenetic and calcium imaging studies provided insight on the complex role of D1- and D2-MSNs in these behaviours and helped to clarify their involvement in associative learning. Here, we critically discuss findings supporting an intricate and complementary role of NAc D1- and D2-MSNs in associative learning, emphasizing the need for additional studies in order to fully understand the role of these neurons in behaviour.

Immp2l knockdown in male mice increases stimulus-driven instrumental behaviour but does not alter goal-directed learning or neuron density in cortico-striatal circuits in a model of Tourette syndrome and autism spectrum disorder

Cortico-striatal neurocircuits mediate goal-directed and habitual actions which are necessary for adaptive behaviour. It has recently been proposed that some of the core symptoms of autism spectrum disorder (ASD) and Gilles de la Tourette syndrome (GTS), such as tics and other repetitive behaviours, may emerge because of imbalances in these neurocircuits. We have recently developed a model of ASD and GTS by knocking down Immp2l, a mitochondrial gene frequently associated with these disorders. The current study sought to determine whether Immp2l knockdown (KD) in male mice alters flexible, goal- or cue- driven behaviour using procedures specifically designed to examine response-outcome and stimulus-response associations, which underlie goal-directed and habitual behaviour, respectively. Whether Immp2l KD alters neuron density in cortico-striatal neurocircuits known to regulate these behaviours was also examined. Immp2l KD mice and wild type-like mice (WT) were trained on Pavlovian and instrumental learning procedures where auditory cues predicted food delivery and lever-press responses earned a food outcome. It was demonstrated that goal-directed learning was not changed for Immp2l KD mice compared to WT mice, as lever-press responses were sensitive to changes in the value of the food outcome, and to contingency reversal and degradation. There was also no difference in the capacity of KD mice to form habitual behaviours compared to WT mice following extending training of the instrumental action. However, Immp2l KD mice were more responsive to auditory stimuli paired with food as indicated by a non-specific increase in lever response rates during Pavlovian-to-instrumental transfer. Finally, there were no alterations to neuron density in striatum or any prefrontal cortex or limbic brain structures examined. Thus, the current study suggests that Immp2l is not necessary for learned maladaptive goal or stimulus driven behaviours in ASD or GTS, but that it may contribute to increased capacity for external stimuli to drive behaviour. Alterations to stimulus-driven behaviour could potentially influence the expression of tics and repetitive behaviours, suggesting that genetic alterations to Immp2l may contribute to these core symptoms in ASD and GTS. Given that this is the first application of this battery of instrumental learning procedures to a mouse model of ASD or GTS, it is an important initial step in determining the contribution of known risk-genes to goal-directed versus habitual behaviours, which should be more broadly applied to other rodent models of ASD and GTS in the future.

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.

Nucleus Accumbens Shell Neurons Encode the Kinematics of Reward Approach Locomotion

The nucleus accumbens (NAc) is considered an interface between motivation and action, with NAc neurons playing an important role in promoting reward approach. However, the encoding by NAc neurons that contributes to this role remains unknown. We recorded 62 NAc neurons in male Wistar rats (n = 5) running towards rewarded locations in an 8-arm radial maze. Variables related to locomotor approach kinematics were the best predictors of the firing rate for most NAc neurons. Nearly 18% of the recorded neurons were inhibited during the entire approach run (locomotion-off cells), suggesting that reduction in firing of these neurons promotes initiation of locomotor approach. 27% of the neurons presented a peak of activity during acceleration followed by a valley during deceleration (acceleration-on cells). Together, these neurons accounted for most of the speed and acceleration encoding identified in our analysis. In contrast, a further 16% of neurons presented a valley during acceleration followed by a peak just prior to or after reaching reward (deceleration-on cells). These findings suggest that these three classes of NAc neurons influence the time course of speed changes during locomotor approach to reward.

Intravenous Injection of GluR2-3Y Inhibits Repeated Morphine-Primed Reinstatement of Drug Seeking in Rats

Studies have demonstrated that the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor is essential to drug addiction. In this study, we explored the influence of GluR2-3Y, an interfering peptide to prevent the endocytosis of AMPA receptors containing the GluR2 subunit, on morphine-seeking behavior in the rat self-administration model. After self-administration was established, the rats received intravenous injections of GluR2-3Y during the extinction sessions. There were no significant differences in both active and inactive pokes compared to the control group of rats that received GluR2-3S, indicating that GluR2-3Y has no significant influences on the extinction of morphine self-administration. The other two groups of rats were trained, extinguished, and reinstated by repeated morphine priming (respectively, called Prime 1, Prime 2, and Prime 3). Only one intravenous injection of GluR2-3Y was performed before Prime 1. Compared to the control group, GluR2-3Y did not affect Prime 1, but significantly attenuated the morphine-seeking behavior during repeated morphine-primed reinstatement, indicating an inhibitory after effect of GluR2-3Y on morphine-seeking behavior in rats. The long-term depression (LTD) in the nucleus accumbens (NAc) shell was also assessed. Pretreatment with GluR2-3Y altered the ability of LTD induction to the level of that in the naive group, while pretreatment with GluR2-3S had no effects on LTD. Our results demonstrated that the intravenous injection of GluR2-3Y, to block the endocytosis of AMPA receptors, inhibited the reinstatement of morphine-seeking behavior, which may be induced by modulating the neuronal plasticity in the NAc shell of rats.

Medial orbitofrontal cortical regulation of different aspects of Pavlovian and instrumental reward seeking

RATIONALE

The medial subregion of the orbitofrontal cortex (mOFC) is thought to play an important role representing the expected outcome of a given course of action, as lesioning or inactivating this cortical region results in the adoption of choice strategies based more on observable (rather than previously learned) information. Despite this, its role in mediating basic associative learning remains to be fully clarified.

OBJECTIVE

The present series of experiments examined the role of the mOFC in (1) Pavlovian conditioned approach, (2) conditioned reinforcement, (3) extinction, and (4) cue-induced reinstatement of food-seeking behavior.

METHODS

Separate cohorts of rats went through Pavlovian or instrumental training. Intra-mOFC infusions of either saline or GABA agonists (to temporarily inactivate neural activity) were given prior to Pavlovian approach, conditioned reinforcement, first or second day of instrumental extinction training, or cue-induced reinstatement test days.

RESULTS

mOFC inactivation increased lever-CS contacts in Pavlovian conditioned approach and (2) had no effect on conditioned reinforcement. These manipulations (3) accelerated within-session instrumental extinction during the initial extinction session, but impaired subsequent extinction learning on drug-free days. (4) mOFC inactivation induced differential effects on reinstatement that depended on baseline performance. mOFC inactivation abolished reinstatement in "Reinstater" rats (who displayed robust responding under control conditions) and robustly increased reinstatement in "Non-Reinstater" rats (who showed little reinstatement under control conditions) suggesting that individual differences in reinstatement may be supported by differences in mOFC mediated representations of expected outcomes.

CONCLUSIONS

These findings have important implications for understanding how the mOFC uses stimulus-outcome and action-outcome expectancies to guide behavior, and how dysfunction within this region may contribute to pathological patterns of reward seeking.

D2/3 Agonist during Learning Potentiates Cued Risky Choice

Impulse control and/or gambling disorders can be triggered by dopamine agonist therapies used to treat Parkinson's disease, but the cognitive and neurobiological mechanisms underlying these adverse effects are unknown. Recent data show that adding win-paired sound and light cues to the rat gambling task (rGT) potentiates risky decision-making and impulsivity via the dopamine system, and that changing dopaminergic tone has a greater influence on behavior while subjects are learning task contingencies. Dopamine agonist therapy may therefore be potentiating risk-taking by amplifying the behavioral impact of gambling-related cues on novel behavior. Here, we show that ropinirole treatment in male rats transiently increased motor impulsivity but robustly and progressively increased choice of the high-risk/high-reward options when administered during acquisition of the cued but not uncued rGT. Early in training, ropinirole increased win-stay behavior after large unlikely wins on the cued rGT, indicative of enhanced model-free learning, which mediated the drug's effect on later risk preference. Ex vivo cFos imaging showed that both chronic ropinirole and the addition of win-paired cues suppressed the activity of dopaminergic midbrain neurons. The ratio of midbrain:prefrontal cFos+ neurons was lower in animals with suboptimal choice patterns and tended to predict risk preference across all rats. Network analyses further suggested that ropinirole induced decoupling of the dopaminergic cells of the VTA and nucleus accumbens but only when win-paired cues were present. Frontostriatal activity uninformed by the endogenous dopaminergic teaching signal therefore appeared to perpetuate risky choice, and ropinirole exaggerated this disconnect in synergy with reward-paired cues.SIGNIFICANCE STATEMENT D2/3 receptor agonists, used to treat Parkinson's disease, can cause gambling disorder through an unknown mechanism. Ropinirole increased risky decision-making in rats, but only when wins were paired with casino-inspired sounds and lights. This was mediated by increased win-stay behavior after large unlikely wins early in learning, indicating enhanced model-free learning. cFos imaging showed that ropinirole suppressed activity of midbrain dopamine neurons, an effect that was mimicked by the addition of win-paired cues. The degree of risky choice rats exhibited was uniquely predicted by the ratio of midbrain dopamine:PFC activity. Depriving the PFC of the endogenous dopaminergic teaching signal may therefore drive risky decision-making on-task, and ropinirole acts synergistically with win-paired cues to amplify this.

Mu opioid receptor stimulation in the medial preoptic area or nucleus accumbens facilitates song and reward in flocking European starlings

It has been proposed that social cohesion in gregarious animals is reinforced both by a positive affective state induced by social interactions and by the prevention of a negative state that would be caused by social separation. Opioids that bind to mu opioid receptors (MORs) act in numerous brain regions to induce positive and to reduce negative affective states. Here we explored a potential role for MORs in affective states that may impact flocking behavior in mixed-sex flocks of nonbreeding European starlings, Sturnus vulgaris. Singing behavior, which is considered central to flock cohesion, and other social behaviors were quantified after infusions of the MOR agonist D-Ala2, N-Me-Phe4, glycinol5-ENK (DAMGO) into either the medial preoptic area (POM) or the nucleus accumbens (NAC), regions previously implicated in affective state and flock cohesion. We focused on beak wiping, a potential sign of stress or redirected aggression in this species, to provide insight into a presumed negative state. We also used conditioned place preference (CPP) tests to provide insight into the extent to which infusions of DAMGO into POM or NAC that stimulated song might be rewarding. We found that MOR stimulation in either POM or NAC dose-dependently promoted singing behavior, reduced beak wiping, and induced a CPP. Subtle differences in responses to MOR stimulation between NAC and POM also suggest potential functional differences in the roles of these two regions. Finally, because the location of NAC has only recently been identified in songbirds, we additionally performed a tract tracing study that confirmed the presence of dopaminergic projections from the ventral tegmental area to NAC, suggesting homology with mammalian NAC. These findings support the possibility that MORs in POM and NAC play a dual role in reinforcing social cohesion in flocks by facilitating positive and reducing negative affective states.

Transcriptomics in the nucleus accumbens shell reveal sex- and reinforcer-specific signatures associated with morphine and sucrose craving

Incubation of craving is a well-documented phenomenon referring to the intensification of drug craving over extended abstinence. The neural adaptations that occur during forced abstinence following chronic drug taking have been a topic of intense study. However, little is known about the transcriptomic changes occurring throughout this window of time. To define gene expression changes associated with morphine consumption and extended abstinence, male and female rats underwent 10 days of morphine self-administration. Separate drug-naive rats self-administered sucrose in order to compare opioid-induced changes from those associated with natural, non-drug rewards. After one or 30 days of forced abstinence, rats were tested for craving, or nucleus accumbens shell tissue was dissected for RNA sequencing. Morphine consumption was predictive of drug seeking after extended (30 days) but not brief (1 day) abstinence in both sexes. Extended abstinence was also associated with robust sex- and reinforcer-specific changes in gene expression, suggesting sex differences underlying incubation of morphine and sucrose seeking respectively. Importantly, these changes in gene expression occurred without re-exposure to drug-paired cues, indicating that chronic morphine causes long-lasting changes in gene expression that prime the system for increased craving. These findings lay the groundwork for identifying specific therapeutic targets for curbing opioid craving without impacting the natural reward system in males and females.

Medial orbitofrontal cortex and nucleus accumbens mediation in risk assessment behaviors in adolescents and adults

Risk assessment behaviors are necessary for gathering risk information and guiding decision-making. Risky decision-making heightens during adolescence, possibly as a result of low risk awareness and an increase in sensitivity to reward-associated cues and experiences. Higher adolescent engagement in high-risk behaviors may be, in part, due to developing circuits that contribute to risk assessment behaviors. Nucleus accumbens (NAc) activity is linked to risky decision-making and receives inputs carrying sensory and emotional information. Namely, the medial orbitofrontal cortex (MO) contributes to behavior guided by reward probability and sends direct projections to the NAc (MO→NAc), which may permit risk assessment in a mature circuit. Here, we evaluated risk assessment behaviors in adult and adolescent rats during elevated plus maze (EPM) exploration, including stretch and attend postures, head dips, and rears. We found that adolescents exhibited fewer EPM risk assessment behaviors than adults. We also quantified MO→NAc projections using a fluorescent anterograde tracer, Fluoro-Ruby, in both age groups. Labeled MO→NAc pathways exhibited greater total fluorescence in adults than in adolescents, indicating MO→NAc fibers increase over development. Using a disconnection approach to measure the contribution of the MO-NAc pathway in adults, we found that ipsilateral inactivation of the MO-NAc did not alter risk assessment behavior; however, MO-NAc disconnection reduced the number of stretch-and-attend postures. Together, this work suggests that the development of MO-NAc pathways can contribute to age-dependent differences in risk assessment.

The ventral striatum contributes to the activity of the motor cortex and motor outputs in monkeys

The ventral striatum (VSt) is thought to be involved in the vigor of motivated behavior and is suggested to be a limbic-motor interface between limbic areas involved in motivational processes and neural circuits regulating behavioral outputs. However, there is little direct evidence demonstrating the involvement of the VSt in motor control for motivated behaviors. To clarify the functional role of the VSt in motor control, we investigated the effect of reversible pharmacological inactivation of the VSt on the oscillatory activity of the sensorimotor cortices and motor outputs in two macaque monkeys. VSt inactivation reduced movement-related activities of the primary motor cortex and premotor area at 15–120 Hz and increased those at 5–7 Hz. These changes were accompanied by reduced torque outputs but had no effect on the correct performance rate. The present study provides direct evidence that the VSt regulates activities of the motor cortices and motor output.

Striatal dopamine dissociates methylphenidate effects on value-based versus surprise-based reversal learning

Psychostimulants such as methylphenidate are widely used for their cognitive enhancing effects, but there is large variability in the direction and extent of these effects. We tested the hypothesis that methylphenidate enhances or impairs reward/punishment-based reversal learning depending on baseline striatal dopamine levels and corticostriatal gating of reward/punishment-related representations in stimulus-specific sensory cortex. Young healthy adults (N = 100) were scanned with functional magnetic resonance imaging during a reward/punishment reversal learning task, after intake of methylphenidate or the selective D_2/3-receptor antagonist sulpiride. Striatal dopamine synthesis capacity was indexed with [¹⁸F]DOPA positron emission tomography. Methylphenidate improved and sulpiride decreased overall accuracy and response speed. Both drugs boosted reward versus punishment learning signals to a greater degree in participants with higher dopamine synthesis capacity. By contrast, striatal and stimulus-specific sensory surprise signals were boosted in participants with lower dopamine synthesis. These results unravel the mechanisms by which methylphenidate gates both attention and reward learning.

The mechanisms underpinning the variability in methylphenidate’s effects on cognition remain unclear. Here, the authors show that such effects reflect changes in striatal dopamine-related output gating of task-relevant cortical signals, and that these changes depend on baseline dopamine synthesis capacity.

Oscillation/Coincidence-Detection Models of Reward-Related Timing in Corticostriatal Circuits

The major tenets of beat-frequency/coincidence-detection models of reward-related timing are reviewed in light of recent behavioral and neurobiological findings. This includes the emphasis on a core timing network embedded in the motor system that is comprised of a corticothalamic-basal ganglia circuit. Therein, a central hub provides timing pulses (i.e., predictive signals) to the entire brain, including a set of distributed satellite regions in the cerebellum, cortex, amygdala, and hippocampus that are selectively engaged in timing in a manner that is more dependent upon the specific sensory, behavioral, and contextual requirements of the task. Oscillation/coincidence-detection models also emphasize the importance of a tuned ‘perception’ learning and memory system whereby target durations are detected by striatal networks of medium spiny neurons (MSNs) through the coincidental activation of different neural populations, typically utilizing patterns of oscillatory input from the cortex and thalamus or derivations thereof (e.g., population coding) as a time base. The measure of success of beat-frequency/coincidence-detection accounts, such as the Striatal Beat-Frequency model of reward-related timing (SBF), is their ability to accommodate new experimental findings while maintaining their original framework, thereby making testable experimental predictions concerning diagnosis and treatment of issues related to a variety of dopamine-dependent basal ganglia disorders, including Huntington’s and Parkinson’s disease.

Choice-selective sequences dominate in cortical relative to thalamic inputs to NAc to support reinforcement learning

How are actions linked with subsequent outcomes to guide choices? The nucleus accumbens, which is implicated in this process, receives glutamatergic inputs from the prelimbic cortex and midline regions of the thalamus. However, little is known about whether and how representations differ across these input pathways. By comparing these inputs during a reinforcement learning task in mice, we discovered that prelimbic cortical inputs preferentially represent actions and choices, whereas midline thalamic inputs preferentially represent cues. Choice-selective activity in the prelimbic cortical inputs is organized in sequences that persist beyond the outcome. Through computational modeling, we demonstrate that these sequences can support the neural implementation of reinforcement-learning algorithms, in both a circuit model based on synaptic plasticity and one based on neural dynamics. Finally, we test and confirm a prediction of our circuit models by direct manipulation of nucleus accumbens input neurons.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Cortico-Striatal Control over Adaptive Goal-Directed Responding Elicited by Cues Signaling Sucrose Reward or Punishment

The medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) have been associated with the expression of adaptive and maladaptive behavior elicited by fear-related and drug-associated cues. However, reported effects of mPFC manipulations on cue-elicited natural reward-seeking and inhibition thereof have been varied, with few studies examining cortico-striatal contributions in tasks that require adaptive responding to cues signaling reward and punishment within the same session. The current study aimed to better elucidate the role of mPFC and NAc subdivisions, and their functional connectivity in cue-elicited adaptive responding using a novel discriminative cue responding task. Male Long-Evans rats learned to lever-press on a VR5 schedule for a discriminative cue signaling reward, and to avoid pressing the same lever in the presence of another cue signaling punishment. Postacquisition, prelimbic (PL) and infralimbic (IL) areas of the mPFC, NAc core, shell, PL-core, or IL-shell circuits were pharmacologically or chemogenetically inhibited while animals performed under (1) nonreinforced (extinction) conditions, where the appetitive and aversive cues were presented in alternating trials alone or as a compound stimulus; and (2) reinforced conditions, whereby cued responding was accompanied by associated outcomes. PL and IL inactivation attenuated nonreinforced and reinforced goal-directed cue responding, whereas NAc core and shell inactivation impaired nonreinforced responding for the appetitive, but not aversive cue. Furthermore, PL-core and IL-shell inhibition disinhibited nonreinforced but not reinforced cue responding. Our findings implicate the mPFC as a site of confluence of motivationally significant cues and outcomes, and in the regulation of nonreinforced cue responding via downstream NAc targets.SIGNIFICANCE STATEMENT The ability to discriminate and respond appropriately to environmental cues that signal availability of reward or punishment is essential for survival. The medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) have been implicated in adaptive and maladaptive behavior elicited by fear-related and drug-associated cues. However, less is known about the role they play in orchestrating adaptive responses to natural reward and punishment cues within the same behavioral task. Here, using a novel discriminative cue responding task combined with pharmacological or chemogenetic inhibition of mPFC, NAc and mPFC-NAc circuits, we report that mPFC is critically involved in responding to changing cued response-outcomes, both when the responses are reinforced, and nonreinforced. Furthermore, the mPFC coordinates nonreinforced discriminative cue responding by suppressing inappropriate responding via downstream NAc targets.

Deconstructing and reconstructing behaviour relevant to mental health disorders: The benefits of a psychological approach, with a focus on addiction

RUTHERFORD, L.G. and Milton, A.L. Deconstructing and reconstructing behaviour relevant to mental health disorders: what can psychology offer? NEUROSCI BIOBEHAV REV XX(X)XXX-XXX, 2021. - Current treatments for mental health disorders are successful only for some patients, and there is an unmet clinical need for new treatment development. One challenge for treatment development has been how best to model complex human conditions in animals, where mechanism can be more readily studied with a range of neuroscientific techniques. We suggest that an approach to modelling based on associative animal learning theory provides a good framework for deconstructing complex mental health disorders such that they can be studied in animals. These individual simple models can subsequently be used in combination to 'reconstruct' a more complex model of the mental health disorder of interest. Using examples primarily from the field of drug addiction, we explore the 'psychological approach' and suggest that in addition to facilitating translation and backtranslation of tasks between animal models and patients, it is also highly concordant with the concept of triangulation.

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Choose your path: Divergent basolateral amygdala efferents differentially mediate incentive motivation, flexibility and decision-making

To survive in a complex environment, individuals form associations between environmental stimuli and rewards to organize and optimize reward seeking behaviors. The basolateral amygdala (BLA) uses these learned associations to inform decision-making processes. In this review, we describe functional projections between BLA and its cortical and striatal targets that promote learning and motivational processes central to decision-making. Specifically, we compare and contrast divergent projections from the BLA to the orbitofrontal (OFC) and to the nucleus accumbens (NAc) and examine the roles of these pathways in associative learning, value-guided decision-making, choice behaviors, as well as cue and context-driven drug seeking. Finally, we consider how these projections are involved in disorders of motivation, with a focus on Substance Use Disorder.

Integrating the Roles of Midbrain Dopamine Circuits in Behavior and Neuropsychiatric Disease

Dopamine (DA) is a behaviorally and clinically diverse neuromodulator that controls CNS function. DA plays major roles in many behaviors including locomotion, learning, habit formation, perception, and memory processing. Reflecting this, DA dysregulation produces a wide variety of cognitive symptoms seen in neuropsychiatric diseases such as Parkinson’s, Schizophrenia, addiction, and Alzheimer’s disease. Here, we review recent advances in the DA systems neuroscience field and explore the advancing hypothesis that DA’s behavioral function is linked to disease deficits in a neural circuit-dependent manner. We survey different brain areas including the basal ganglia’s dorsomedial/dorsolateral striatum, the ventral striatum, the auditory striatum, and the hippocampus in rodent models. Each of these regions have different reported functions and, correspondingly, DA’s reflecting role in each of these regions also has support for being different. We then focus on DA dysregulation states in Parkinson’s disease, addiction, and Alzheimer’s Disease, emphasizing how these afflictions are linked to different DA pathways. We draw upon ideas such as selective vulnerability and region-dependent physiology. These bodies of work suggest that different channels of DA may be dysregulated in different sets of disease. While these are great advances, the fine and definitive segregation of such pathways in behavior and disease remains to be seen. Future studies will be required to define DA’s necessity and contribution to the functional plasticity of different striatal regions.

Incentive motivation: 'wanting' roles of central amygdala circuitry

The central nucleus of amygdala (CeA) mediates positively-valenced reward motivation as well as negatively-valenced fear. Optogenetic or neurochemical stimulation of CeA circuitry can generate intense incentive motivation to pursue and consume a paired natural food, sex, or addictive drug reward, and even create maladaptive 'wanting what hurts' such as attraction to a shock rod. Evidence indicates CeA stimulations selectively amplify incentive motivation ('wanting') but not hedonic impact ('liking') of the same reward. Further, valence flips can occur for CeA contributions to motivational salience. That is, CeA stimulation can promote either incentive motivation or fearful motivation, even in the same individual, depending on situation. These findings may carry implications for understanding CeA roles in neuropsychiatric disorders involving aberrant motivational salience, ranging from addiction to paranoia and anxiety disorders.

Involvement of the dopaminergic system in the reward-related behavior of pregabalin

There has been an increase in cases of drug addiction and prescription drug abuse worldwide. Recently, pregabalin abuse has been a focus for many healthcare agencies, as highlighted by epidemiological studies. We previously evaluated the possibility of pregabalin abuse using the conditioned place preference (CPP) paradigm. We observed that a 60 mg/kg dose could induce CPP in mice and that pregabalin-rewarding properties were mediated through glutamate neurotransmission. Notably, the dopaminergic reward circuitry is also known to play a crucial role in medication-seeking behavior. Therefore, this study aimed to explore the possible involvement of dopaminergic receptor-1 in pregabalin-induced CPP. Mice were randomly allocated to receive saline or the dopamine-1 receptor antagonist SKF-83566 (0.03 mg/kg, intraperitoneal). After 30 min, the mice received either saline or pregabalin (60 mg/kg) during the conditioning phase. Among the control groups that received saline or SKF-83566, the time spent in the two conditioning chambers was not significantly altered. However, among the pregabalin-treated group, there was a marked increase in the time spent in the drug-paired chamber compared to the time spent in the vehicle-paired chamber. Notably, blocking dopamine-1 receptors with SKF-83566 completely prevented pregabalin-induced place preference, thus demonstrating the engagement of the dopaminergic system in pregabalin-induced reward-related behavior.

Computational phenotyping of brain-behavior dynamics underlying approach-avoidance conflict in major depressive disorder

Adaptive behavior requires balancing approach and avoidance based on the rewarding and aversive consequences of actions. Imbalances in this evaluation are thought to characterize mood disorders such as major depressive disorder (MDD). We present a novel application of the drift diffusion model (DDM) suited to quantify how offers of reward and aversiveness, and neural correlates thereof, are dynamically integrated to form decisions, and how such processes are altered in MDD. Hierarchical parameter estimation from the DDM demonstrated that the MDD group differed in three distinct reward-related parameters driving approach-based decision making. First, MDD was associated with reduced reward sensitivity, measured as the impact of offered reward on evidence accumulation. Notably, this effect was replicated in a follow-up study. Second, the MDD group showed lower starting point bias towards approaching offers. Third, this starting point was influenced in opposite directions by Pavlovian effects and by nucleus accumbens activity across the groups: greater accumbens activity was related to approach bias in controls but avoid bias in MDD. Cross-validation revealed that the combination of these computational biomarkers were diagnostic of patient status, with accumbens influences being particularly diagnostic. Finally, within the MDD group, reward sensitivity and nucleus accumbens parameters were differentially related to symptoms of perceived stress and depression. Collectively, these findings establish the promise of computational psychiatry approaches to dissecting approach-avoidance decision dynamics relevant for affective disorders.

Dissociating brain systems that respond to contingency and valence during monetary loss avoidance in adolescence

Negative reinforcement processes allow individuals to avoid negative and/or harmful outcomes. They depend on the brain's ability to differentiate; (i) contingency from non-contingency, separately from (ii) judgements about positive and negative valence. Thirty-three males (8-18 years) performed a cued reaction-time task during fMRI scanning to differentiate the brain's responses to contingency and valence during loss avoidance. In two conditions, cues indicated no -contingency between participants' responses and monetary loss - (1) CERTAIN LOSS (negative valence) of €0.20, €1 or €5 or (2) CERTAIN LOSS AVOIDANCE (positive valence). In a third condition, cues indicated a contingency between short reaction times and avoidance of monetary loss. As expected participants had shorter reaction times in this latter condition where CONDITIONAL LOSS AVOIDANCE cues activated salience and motor-response-preparation brain networks - independent of the relative valence of the contrast (CERTAIN LOSS or CERTAIN LOSS AVOIDANCE). Effects of valence were seen toward the session's end where CERTAIN LOSS AVOIDANCE cues activated ventral striatum, medial-orbitofrontal cortex and medial-temporal areas more than CERTAIN LOSS. CONDITIONAL LOSS AVOIDANCE trials with feedback indicating "success" activated ventral striatum more than "failure feedback". The findings support the hypothesis that brain networks controlling contingency and valence processes during negative reinforcement are dissociable.

Effect of Modulation of the Astrocytic Glutamate Transporters' Expression on Cocaine-Induced Reinstatement in Male P Rats Exposed to Ethanol

AIM

Reinforcing properties of ethanol and cocaine are mediated in part through the glutamatergic system. Extracellular glutamate concentration is strictly maintained through several glutamate transporters, such as glutamate transporter 1 (GLT-1), cystine/glutamate transporter (xCT) and glutamate aspartate transporter (GLAST). Previous findings revealed that cocaine and ethanol exposure downregulated GLT-1 and xCT, and that β-lactam antibiotics restored their expression.

METHODS

In this study, we investigated the effect of ampicillin/sulbactam (AMP/SUL) (200 mg/kg, i.p.), a β-lactam antibiotic, on cocaine-induced reinstatement and locomotor activity in male alcohol preferring (P) rats using free choice ethanol (15 and 30%, v/v) and water. We also investigated the effect of co-exposure to ethanol and cocaine (20 mg/kg, i.p.) on GLT-1, xCT and GLAST expression in the nucleus accumbens (NAc) core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

RESULTS

Cocaine exposure decreased ethanol intake and preference. Cocaine and ethanol co-exposure acquired place preference and increased locomotor activity compared to ethanol-exposed rats. GLT-1 and xCT expression were downregulated after cocaine and ethanol co-exposure in the NAc core and shell, but not in dmPFC. AMP/SUL attenuated reinstatement to cocaine as well attenuated the decrease in locomotor activity and ethanol intake and preference. These effects were associated with upregulation of GLT-1 and xCT expression in the NAc core/shell and dmPFC. GLAST expression was not affected after ethanol and cocaine co-exposure or AMP/SUL treatment.

CONCLUSION

Our findings demonstrate that astrocytic glutamate transporters within the mesocorticolimbic area are critical targets in modulating cocaine-seeking behavior while being consuming ethanol.

The effect of the mGlu8 receptor agonist, (S)-3,4-DCPG on acquisition and expression of morphine-induced conditioned place preference in male rats

Background

The nucleus accumbens (NAc) plays a principal role in drug reward. It has been reported that metabotropic glutamate receptors (mGlu receptors) play a key role in the rewarding pathway(s). Previous studies have shown the vast allocation of the different types of mGlu receptors, including mGlu8 receptors, in regions that are associated with opioid rewards, such as the NAc. The aim of the present study was to evaluate the role of mGlu8 receptors within the NAc in the acquisition and expression phases of morphine induced conditioned place preference (CPP). Adult male Wistar rats were bilaterally implanted by two cannulas' in the NAc and were evaluated in a CPP paradigm. Selective mGlu8 receptor allosteric agonist (S-3,4-DCPG) was administered at doses of 0.03, 0.3, and 3 μg/0.5 μL saline per side into the NAc on both sides during the 3 days of morphine (5 mg/kg) conditioning (acquisition) phase, or before place preference test, or post-conditioning (expression) phase of morphine-induced CPP.

Results

The results revealed that intra-accumbal administration of S-3,4-DCPG (0.3 and 3 μg) markedly decreased the acquisition in a dose-dependent manner but had no effect on expression of morphine-induced CPP.

Conclusions

The findings suggest that activation of mGlu8 receptors in the NAc dose-dependently blocks the establishment of morphine-induced CPP and reduces the rewarding properties of morphine which may be related to the glutamate activity into the NAc and in reward pathway(s). These data suggest that mGlu8 receptor may be involved in conditioned morphine reward.

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.

Effects of Cocaine Exposure on Astrocytic Glutamate Transporters and Relapse-Like Ethanol-Drinking Behavior in Male Alcohol-Preferring Rats

AIM

Glutamate has been considered as neurotransmitter that is critical in triggering relapse to drugs of abuse, including ethanol and cocaine. Extracellular glutamate concentrations are tightly regulated by several mechanisms, including reuptake through glutamate transporters. Glutamate transporter type 1 (GLT-1) is responsible for clearing the majority of extracellular glutamate. The astrocytic cystine/glutamate antiporter (xCT) regulates also glutamate homeostasis. In this study, we investigated the effects of cocaine exposure and ampicillin/sulbactam (AMP/SUL), a β-lactam antibiotic known to upregulate GLT-1 and xCT, on relapse-like ethanol intake and the expression of astrocytic glutamate transporters in mesocorticolimbic brain regions.

METHODS

Male alcohol-preferring (P) rats had free access to ethanol for 5 weeks. On Week 6, rats were exposed to either cocaine (20 mg/kg, i.p.) or saline for 12 consecutive days. Ethanol bottles were then removed for 7 days; during the last 5 days, either AMP/SUL (100 or 200 mg/kg, i.p.) or saline was administered to the P rats. Ethanol bottles were reintroduced, and ethanol intake was measured for 4 days.

RESULTS

Cocaine exposure induced an alcohol deprivation effect (ADE), which was associated in part by a decrease in the expression of GLT-1 and xCT in the nucleus accumbens (NAc) core. AMP/SUL (100 mg/kg, i.p.) attenuated the ADE, while AMP/SUL (200 mg/kg, i.p.) reduced ethanol intake during 4 days of ethanol re-exposure and upregulated GLT-1 and xCT expression in the NAc core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

CONCLUSION

This study suggests that these astrocytic glutamate transporters might be considered as potential targets for the treatment of polysubstance abuse.

Opioid-induced structural and functional plasticity of medium-spiny neurons in the nucleus accumbens

Opioid Use Disorder (OUD) is a chronic relapsing clinical condition with tremendous morbidity and mortality that frequently persists, despite treatment, due to an individual's underlying psychological, neurobiological, and genetic vulnerabilities. Evidence suggests that these vulnerabilities may have neurochemical, cellular, and molecular bases. Key neuroplastic events within the mesocorticolimbic system that emerge through chronic exposure to opioids may have a determinative influence on behavioral symptoms associated with OUD. In particular, structural and functional alterations in the dendritic spines of medium spiny neurons (MSNs) within the nucleus accumbens (NAc) and its dopaminergic projections from the ventral tegmental area (VTA) are believed to facilitate these behavioral sequelae. Additionally, glutamatergic neurons from the prefrontal cortex, the basolateral amygdala, the hippocampus, and the thalamus project to these same MSNs, providing an enriched target for synaptic plasticity. Here, we review literature related to neuroadaptations in NAc MSNs from dopaminergic and glutamatergic pathways in OUD. We also describe new findings related to transcriptional, epigenetic, and molecular mechanisms in MSN plasticity in the different stages of OUD.

Effects of methylphenidate on reinforcement learning depend on working memory capacity

RATIONALE

Brain catecholamines have long been implicated in reinforcement learning, exemplified by catecholamine drug and genetic effects on probabilistic reversal learning. However, the mechanisms underlying such effects are unclear.

OBJECTIVES AND METHODS

Here we investigated effects of an acute catecholamine challenge with methylphenidate (20 mg, oral) on a novel probabilistic reversal learning paradigm in a within-subject, double-blind randomised design. The paradigm was designed to disentangle effects on punishment avoidance from effects on reward perseveration. Given the known large individual variability in methylphenidate's effects, we stratified our effects by working memory capacity and trait impulsivity, putatively modulating the effects of methylphenidate, in a large sample (n = 102) of healthy volunteers.

RESULTS

Contrary to our prediction, methylphenidate did not alter performance in the reversal phase of the task. Our key finding is that methylphenidate altered learning of choice-outcome contingencies in a manner that depended on individual variability in working memory span. Specifically, methylphenidate improved performance by adaptively reducing the effective learning rate in participants with higher working memory capacity.

CONCLUSIONS

This finding emphasises the important role of working memory in reinforcement learning, as reported in influential recent computational modelling and behavioural work, and highlights the dependence of this interplay on catecholaminergic function.

The Projection From Ventral CA1, Not Prefrontal Cortex, to Nucleus Accumbens Core Mediates Recent Memory Retrieval of Cocaine-Conditioned Place Preference

Drug-paired cues inducing memory retrieval by expressing drug-seeking behaviors present a major challenge to drug abstinence. How neural circuits coordinate for drug memory retrieval remains unclear. Here, we report that exposure of the training chamber where cocaine-conditioned place preference (CPP) was performed increased neuronal activity in the core of nucleus accumbens (AcbC), ventral CA1 (vCA1), and medial prefrontal cortex (mPFC), as shown by elevated pERK and c-Fos levels. Chemogenetic inhibition of neuronal activity in the vCA1 and AcbC, but not mPFC, reduced the time spent in the cocaine-paired compartment, suggesting that the vCA1 and AcbC are required for the retrieval of cocaine-CPP memory and are key nodes recruited for cocaine memory storage. Furthermore, chemogenetic inhibition of the AcbC-projecting vCA1 neurons, but not the AcbC-projecting mPFC neurons, decreased the expression of cocaine-CPP. Optogenetic inhibition of the vCA1–AcbC projection, but not the mPFC–AcbC projection, also reduced the preference for the cocaine-paired compartment. Taken together, the cue-induced natural recall of cocaine memory depends on vCA1–AcbC circuits. The connectivity from the vCA1 to the AcbC may store the information of the cue–cocaine reward association critically required for memory retrieval. These data thus provide insights into the neural circuit basis of retrieval of drug-related memory.

Potential beneficial effects of caffeine administration in the neonatal period of an animal model of schizophrenia

Obstetric complications, like maternal hypertension and neonatal hypoxia, disrupt brain development, leading to psychiatry disorders later in life, like schizophrenia. The exact mechanisms behind this risk are not yet well known. Spontaneously hypertensive rats (SHR) are a well-established model to study neurodevelopment of schizophrenia since they exhibit behavioral alterations mimicking schizophrenia that can be improved with antipsychotic drugs. SHR mothers are hypertensive, and the SHR offspring develop in preeclampsia-like conditions. Hypoxic conditions increase levels of adenosine, which play an important role in brain development. The enhanced levels of adenosine at birth could be related to the future development of schizophrenia. To investigate this hypothesis adenosine levels of brain neonatal Wistar rats and SHR were quantified. After that, caffeine, an antagonist of adenosinergic system, was administrated on PND (postnatal day) 7 (neurodevelopmental age similar to a human at delivery) and rats were observed at adolescent and adult ages. We also investigated the acute effects of caffeine at adolescent and adult ages. SHR control adolescent and adult groups presented behavioral deficits like hyperlocomotion, deficit in social interaction (SI), and contextual fear conditioning (CFC). In SHR, neonatal caffeine treatment on PND 7 normalized hyperlocomotion, improved SI, and CFC observed at adolescent period and adult ages, showing a beneficial effect on schizophrenia-like behaviors. Wistar rats neonatally treated with caffeine exhibited hyperlocomotion, deficit in SI and CFC when observed at adolescent and adult ages. Acutely caffeine treatment administrated at adolescent and adult ages increased locomotion and decreased SI time of Wistar rats and impair CFC in adult Wistars. No effects were observed in SHR. In conclusion, caffeine can be suggested as a useful drug to prevent behavioral deficits observed in this animal model of prenatal hypoxia-induced schizophrenia profile when specifically administered on PND 7.

The transition to compulsion in addiction

Compulsion is a cardinal symptom of drug addiction (severe substance use disorder). However, compulsion is observed in only a small proportion of individuals who repeatedly seek and use addictive substances. Here, we integrate accounts of the neuropharmacological mechanisms that underlie the transition to compulsion with overarching learning theories, to outline how compulsion develops in addiction. Importantly, we emphasize the conceptual distinctions between compulsive drug-seeking behaviour and compulsive drug-taking behaviour (that is, use). In the latter, an individual cannot stop using a drug despite major negative consequences, possibly reflecting an imbalance in frontostriatal circuits that encode reward and aversion. By contrast, an individual may compulsively seek drugs (that is, persist in seeking drugs despite the negative consequences of doing so) when the neural systems that underlie habitual behaviour dominate goal-directed behavioural systems, and when executive control over this maladaptive behaviour is diminished. This distinction between different aspects of addiction may help to identify its neural substrates and new treatment strategies.

Amphetamine disrupts haemodynamic correlates of prediction errors in nucleus accumbens and orbitofrontal cortex

In an uncertain world, the ability to predict and update the relationships between environmental cues and outcomes is a fundamental element of adaptive behaviour. This type of learning is typically thought to depend on prediction error, the difference between expected and experienced events and in the reward domain that has been closely linked to mesolimbic dopamine. There is also increasing behavioural and neuroimaging evidence that disruption to this process may be a cross-diagnostic feature of several neuropsychiatric and neurological disorders in which dopamine is dysregulated. However, the precise relationship between haemodynamic measures, dopamine and reward-guided learning remains unclear. To help address this issue, we used a translational technique, oxygen amperometry, to record haemodynamic signals in the nucleus accumbens (NAc) and orbitofrontal cortex (OFC), while freely moving rats performed a probabilistic Pavlovian learning task. Using a model-based analysis approach to account for individual variations in learning, we found that the oxygen signal in the NAc correlated with a reward prediction error, whereas in the OFC it correlated with an unsigned prediction error or salience signal. Furthermore, an acute dose of amphetamine, creating a hyperdopaminergic state, disrupted rats' ability to discriminate between cues associated with either a high or a low probability of reward and concomitantly corrupted prediction error signalling. These results demonstrate parallel but distinct prediction error signals in NAc and OFC during learning, both of which are affected by psychostimulant administration. Furthermore, they establish the viability of tracking and manipulating haemodynamic signatures of reward-guided learning observed in human fMRI studies by using a proxy signal for BOLD in a freely behaving rodent.

Dissociable dopaminergic and pavlovian influences in goal-trackers and sign-trackers on a model of compulsive checking in OCD

RATIONALE

Checking is a functional behaviour that provides information to guide behaviour. However, in obsessive-compulsive disorder (OCD), checking may escalate to dysfunctional levels. The processes underpinning the transition from functional to dysfunctional checking are unclear but may be associated with individual differences that support the development of maladaptive behaviour. We examined one such predisposition, sign-tracking to a pavlovian conditioned stimulus, which we previously found associated with dysfunctional checking. How sign-tracking interacts with another treatment with emerging translational validity for OCD-like checking, chronic administration of the dopamine D₂ receptor agonist quinpirole, is unknown.

OBJECTIVES

We tested how functional and dysfunctional checking in the rat observing response task (ORT) was affected by chronic quinpirole administration in non-autoshaped controls and autoshaped animals classified as sign-trackers or goal-trackers.

METHODS

Sign-trackers or goal-trackers were trained on the ORT before the effects of chronic quinpirole administration on checking were assessed. Subsequently, the effects on checking of different behavioural challenges, including reward omission and the use of unpredictable reinforcement schedules, were tested.

RESULTS

Prior autoshaping increased checking. Sign-trackers and goal-trackers responded differently to quinpirole sensitization, reward omission and reinforcement uncertainty. Sign-trackers showed greater elevations in dysfunctional checking, particularly during uncertainty. By contrast, goal-trackers predominantly increased functional checking responses, possibly in response to reduced discrimination accuracy in the absence of cues signalling which lever was currently active.

CONCLUSIONS

The results are discussed in terms of how pavlovian associations influence behaviour that becomes compulsive in OCD and how this may be dependent on striatal dopamine D₂ receptors.

Striatopallidal neurons control avoidance behavior in exploratory tasks

The dorsal striatum has been linked to decision-making under conflict, but the mechanism by which striatal neurons contribute to approach-avoidance conflicts remains unclear. We hypothesized that striatopallidal dopamine D2 receptor (D2R)-expressing neurons promote avoidance, and tested this hypothesis in two exploratory approach-avoidance conflict paradigms in mice: the elevated zero maze and open field. Genetic elimination of D2Rs on striatopallidal neurons (iMSNs), but not other neural populations, increased avoidance of the open areas in both tasks, in a manner that was dissociable from global changes in movement. Population calcium activity of dorsomedial iMSNs was disrupted in mice lacking D2Rs on iMSNs, suggesting that disrupted output of iMSNs contributes to heightened avoidance behavior. Consistently, artificial disruption of iMSN output with optogenetic stimulation heightened avoidance of open areas of these tasks, while inhibition of iMSN output reduced avoidance. We conclude that dorsomedial striatal iMSNs control approach-avoidance conflicts in exploratory tasks, and highlight this neural population as a potential target for reducing avoidance in anxiety disorders.

Effects of inactivation of the ventral tegmental area on prefronto-accumbens activity and sexual motivation in male rats

Sexual motivation requires the processing of sexual stimuli. The prefrontal cortex (PFC) and nucleus accumbens (NAcc) receive dopaminergic innervation from the ventral tegmental area (VTA). Both structures participate in processing stimuli, and their adequate functioning is modulated by dopamine and other neurotransmitters. This study was designed to determine the effect of inactivation of the VTA on sexual motivation, relative power (RP) and electroencephalographic (EEG) correlation of the PFC and NAcc in male rats. A total of 20 rats implanted with electrodes in the left medial prefrontal cortex (mPFC) and NAcc, and with bilateral cannulae in the VTA, were divided into two groups of 10 rats each, one injected with tetrodotoxin (TTX), the other with a vehicle solution (VEH). EEGs from the mPFC and NAcc were recorded during the awake-quiet state in the presence of either a receptive or non-receptive female. The TTX group showed a lower preference for the receptive female accompanied by a lower RP of the 8-13 and 14-30 Hz bands in the mPFC. Also, in the presence of the receptive female, the TTX group had a lower RP of the 8-13 Hz band in the NAcc, but a higher prefronto-accumbens correlation in the same band. These results provide evidence that VTA activity is necessary for the adequate functioning of the mPFC and NAcc and, therefore, also for the adequate processing of sexually-relevant stimuli that allows the induction and maintenance of sexual motivation in male rats.

A novel role for acid-sensing ion channels in Pavlovian reward conditioning

Pavlovian fear conditioning has been shown to depend on acid-sensing ion channel-1A (ASIC1A); however, it is unknown whether conditioning to rewarding stimuli also depends on ASIC1A. Here, we tested the hypothesis that ASIC1A contributes to Pavlovian conditioning to a non-drug reward. We found effects of ASIC1A disruption depended on the relationship between the conditional stimulus (CS) and the unconditional stimulus (US), which was varied between five experiments. In experiment 1, when the CS preceded the US signaling an upcoming reward, Asic1a-/- mice exhibited a deficit in conditioning compared to Asic1a+/+ mice. Alternatively, in experiment 2, when the CS coinitiated with the US and signaled immediate reward availability, the Asic1a-/- mice exhibited an increase in conditioned responses compared to Asic1a+/+ mice, which contrasted with the deficits in the first experiment. Furthermore, in experiments 3 and 4, when the CS partially overlapped in time with the US, or the CS was shortened and coinitiated with the US, the Asic1a-/- mice did not differ from control mice. The contrasting outcomes were likely because of differences in conditioning because in experiment 5 neither the Asic1a-/- nor Asic1a+/+ mice acquired conditioned responses when the CS and US were explicitly unpaired. Taken together, these results suggest that the effects of ASIC1A disruption on reward conditioning depend on the temporal relationship between the CS and US. Furthermore, these results suggest that ASIC1A plays a critical, yet nuanced role in Pavlovian conditioning. More research will be needed to deconstruct the roles of ASIC1A in these fundamental forms of learning and memory.

Dopamine signals related to appetitive and aversive events in paradigms that manipulate reward and avoidability

Using environmental cues to acquire good and avoid harmful things is critical for survival. Rewarding and aversive outcomes both drive behavior through reinforcement learning and sometimes occur together in the environment, but it remains unclear how these signals are encoded within the brain and if signals for positive and negative reinforcement are encoded similarly. Recent studies demonstrate that the dopaminergic system and interconnected brain regions process both positive and negative reinforcement necessary for approach and avoidance behaviors, respectively. Here, we review these data with a special focus on behavioral paradigms that manipulate both expected reward and the avoidability of aversive events to reveal neural correlates related to value, prediction error encoding, motivation, and salience.

Behavioral and neural mechanisms underlying habitual and compulsive drug seeking

Addiction is characterized by compulsive drug use despite negative consequences. Here we review studies that indicate that compulsive drug use, and in particular punishment resistance in animal models of addiction, is related to impaired cortical control over habitual behavior. In humans and animals, instrumental behavior is supported by goal-directed and habitual systems that rely on distinct corticostriatal networks. Chronic exposure to addictive drugs or stress has been shown to bias instrumental response strategies toward habit learning, and impair prefrontal cortical (PFC) control over responding. Moreover, recent work has implicated prelimbic PFC hypofunction in the punishment resistance that has been observed in a subset of animals with an extended history of cocaine self-administration. This may be related to a broader role for prelimbic PFC in mediating adaptive responding and behavioral flexibility, including exerting goal-directed control over behavior. We hypothesize that impaired cortical control and reduced flexibility between habitual and goal-directed systems may be critically involved in the development of maladaptive, compulsive drug use.

Dopamine: Functions, Signaling, and Association with Neurological Diseases

The dopaminergic system plays important roles in neuromodulation, such as motor control, motivation, reward, cognitive function, maternal, and reproductive behaviors. Dopamine is a neurotransmitter, synthesized in both central nervous system and the periphery, that exerts its actions upon binding to G protein-coupled receptors. Dopamine receptors are widely expressed in the body and function in both the peripheral and the central nervous systems. Dopaminergic signaling pathways are crucial to the maintenance of physiological processes and an unbalanced activity may lead to dysfunctions that are related to neurodegenerative diseases. Unveiling the neurobiology and the molecular mechanisms that underlie these illnesses may contribute to the development of new therapies that could promote a better quality of life for patients worldwide. In this review, we summarize the aspects of dopamine as a catecholaminergic neurotransmitter and discuss dopamine signaling pathways elicited through dopamine receptor activation in normal brain function. Furthermore, we describe the potential involvement of these signaling pathways in evoking the onset and progression of some diseases in the nervous system, such as Parkinson's, Schizophrenia, Huntington's, Attention Deficit and Hyperactivity Disorder, and Addiction. A brief description of new dopaminergic drugs recently approved and under development treatments for these ailments is also provided.

Investigating dopamine and glucocorticoid systems as underlying mechanisms of anhedonia

RATIONALE

Anhedonia, a deficit in reward processing, is an endophenotype of several neuropsychiatric conditions. Despite its prevalence and debilitating effects, treatments for anhedonia are lacking, primarily because its underlying mechanisms are poorly understood. Dopamine (DA) has been implicated in anhedonia through its role in reward-related learning; glucocorticoid systems may also be involved in that anhedonia is often preceded by chronic stress.

OBJECTIVE

This study investigated DA and glucocorticoid systems in anhedonia using a rat version of the probabilistic reward task (PRT).

METHODS

Adult male Wistar rats were trained on the PRT and then tested following: (1) activation or inhibition of DA activity induced by amphetamine (AMPH) or pramipexole (PRAMI) injections, (2) chronic mild stress (CMS), or (3) glucocorticoid system activation (dexamethasone (DEX)) or inhibition (mifepristone (MIFE)).

RESULTS

AMPH increased and PRAMI decreased response bias, pointing to enhanced and diminished reward responsiveness with DA agonism and antagonism, respectively. CMS reduced response bias but only in a subpopulation of rats. DEX also decreased response bias, suggesting that glucocorticoid processes contribute to anhedonia, although glucocorticoid inhibition (MIFE) had no effect. None of the manipulations altered the ability to detect and respond to reward-paired stimuli.

CONCLUSIONS

These results confirm a role of DA in anhedonia and elucidate the contribution of the glucocorticoid system to this effect. In addition, chronic stress may interfere with normal DA functioning, leading to impaired reward-related learning in some animals. These findings may direct future treatment of anhedonia by targeting DA and glucocorticoid systems, as well as a possible interaction between the two.

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.

Region-specific changes in markers of neuroplasticity revealed in HIV-1 transgenic rats by low-dose methamphetamine

Methamphetamine abuse co-occurring with HIV infection presents neuropathology in brain regions that mediate reward and motivation. A neuronal signaling cascade altered acutely by meth and some HIV-1 proteins is the mitogen-activated protein kinase (MAPK) pathway. It remains unknown if chronic co-exposure to meth and HIV-1 proteins converge on MAPK in vivo. To make this determination, we studied young adult Fischer 344 HIV-1 transgenic (Tg) and non-Tg rats that self-administered meth (0.02-0.04 mg/kg/0.05 ml iv infusion, 2 h/day for 21 days) and their saline-yoked controls. One day following the operant task, rats were killed. Brain regions involved in reward-motivation [i.e., nucleus accumbens (NA) and ventral pallidum (VP)], were assayed for a MAPK cascade protein, extracellular signal-regulated kinase (ERK), and a downstream transcription factor, ΔFosB. In the NA, activated (phosphorylated; p) ERK-to-ERK ratio (pERK/ERK) was increased in meth-exposed Tg rats versus saline Tg controls, and versus meth non-Tg rats. ΔFosB was increased in meth Tg rats versus saline and meth non-Tg rats. Assessment of two targets of ΔFosB-regulated transcription revealed (1) increased dopamine D1 receptor (D1R) immunoreactivity in the NA shell of Tg-meth rats versus saline Tg controls, but (2) no changes in the AMPA receptor subunit, GluA2. No changes related to genotype or meth occurred for ERK, ΔFosB or D1R protein in the VP. Results reveal a region-specific activation of ERK, and increases in ΔFosB and D1R expression induced by HIV-1 proteins and meth. Such effects may contribute to the neuronal and behavioral pathology associated with meth/HIV comorbidity.

Expression and localization of CB1R, NAPE-PLD, and FAAH in the vervet monkey nucleus accumbens

Extensive rodent literature suggests that the endocannabinoid (eCB) system present in the nucleus accumbens (NAc) modulates dopamine (DA) release in this area. However, expression patterns of the cannabinoid receptor type 1 (CB1R), the synthesizing enzyme N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD), and the degradation enzyme fatty acid amide hydrolase (FAAH) in the NAc have not yet been described in non-human primates. The goal of this study is therefore to characterize the expression and localization of the eCB system within the NAc of vervet monkeys (Chlorocebus sabaeus) using Western blots and immunohistochemistry. Results show that CB1R, NAPE-PLD, and FAAH are expressed across the NAc rostrocaudal axis, both in the core and shell. CB1R, NAPE-PLD, and FAAH are localized in medium spiny neurons (MSNs) and fast-spiking GABAergic interneurons (FSIs). Dopaminergic projections and astrocytes did not express CB1R, NAPE-PLD, or FAAH. These data show that the eCB system is present in the vervet monkey NAc and supports its role in the primate brain reward circuit.

Drug Cues, Conditioned Reinforcement, and Drug Seeking: The Sequelae of a Collaborative Venture With Athina Markou

Athina Markou spent a research period in my laboratory, then in the Department of Anatomy in Cambridge University, in 1991 to help us establish a cocaine-seeking procedure. Thus we embarked on developing a second-order schedule of intravenous cocaine reinforcement to investigate the neural basis of the pronounced effects of cocaine-associated conditioned stimuli on cocaine seeking. This brief review summarizes the fundamental aspects of cocaine seeking measured using this approach and the importance of the methodology in enabling us to define the neural mechanisms and circuitry underlying conditioned reinforcement and cocaine, heroin, and alcohol seeking. The shift over time and experience of control over drug seeking from a limbic cortical-ventral striatal circuit underlying goal-directed drug seeking to a dorsal striatal system mediating habitual drug seeking are also summarized. The theoretical implications of these data are discussed, thereby revealing the ways in which the outcomes of a collaboration can endure.

Toward a Unifying Account of Impulsivity and the Development of Self-Control

Impulsivity has traditionally been thought to involve various behavioral traits that can be measured using different laboratory protocols. Whereas some authors regard different measures of impulsivity as reflecting fundamentally distinct and unrelated behavioral tendencies (fragmentation approach), others regard those different indexes as analogue forms of the same behavioral tendency, only superficially different (unification approach). Unifying accounts range from mere intuitions to more sophisticated theoretical systems. Some of the more complete attempts at unifying are intriguing but have validity weaknesses. We propose a new unifying attempt based on theoretical points posed by other authors and supplemented by theory and research on associative learning. We then apply these assumptions to characterize the paradigms used to study impulsivity in laboratory settings and evaluate their scope as an attempt at unification. We argue that our approach possesses a good balance of parsimony and empirical and theoretical grounding, as well as a more encompassing scope, and is more suitable for experimental testing than previous theoretical frameworks. In addition, the proposed approach is capable of generating a new definition of impulsivity and outlines a hypothesis of how self-control can be developed. Finally, we examine the fragmentation approach from a different perspective, emphasizing the importance of finding similarities among seemingly different phenomena.