Distinguishing whether dopamine regulates liking, wanting, and/or learning about rewards

Adaptor protein-3 produces synaptic vesicles that release phasic dopamine

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high-frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a distinct population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

The association between lifelong personality and clinical phenotype in the FTD-ALS spectrum

Introduction

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are two phenotypes of the same neurodegenerative disease, the FTD-ALS spectrum. What determines the development of one rather than the other phenotype is still unknown. Based on the clinical observation that patients' personality seems to differ between the two phenotypes, i.e., ALS patients tend to display kind, prosocial behaviors whereas FTD patients tend to present anti-social behaviors, and that these traits are often reported as pre-existing the disease onset by caregivers, we set up to study experimentally patients' personality in their premorbid life.

Methods

We first tested for differences between groups, then tested the association between premorbid personality and current functional organization of the brain. Premorbid personality of a cohort of forty patients, 27 FTD and 13 ALS, was explored through the NEO Personality Inventory 3 (NEO-PI-3), which analyses the five main personality factors, completed by the caregiver with reference to patient's personality 20 years before symptoms onset (premorbid). A subgroup of patients underwent a brain MRI including structural and resting-state functional MRI (rsfMRI).

Results

A significant difference between FTD and ALS in premorbid personality emerged in the Openness (133.92 FTD vs. 149.84 ALS, p = 0.01) and Extraversion (136.55 FTD vs. 150.53 ALS, p = 0.04) factors. This suggests that ALS patients had been, in their premorbid life, more open to new experiences, more sociable and optimistic than FTD patients. They also showed greater functional connectivity than both FTD and a control group in the Salience resting state network, over and above differences in gray matter atrophy. Finally, there was a positive correlation between premorbid Openness and functional connectivity in the Salience network across all patients, suggesting a possible association between premorbid personality and current functional organization of the brain, irrespective of the degree of atrophy.

Discussion

Our proof-of-concept results suggest that premorbid personality may eventually predispose to the development of one, rather than the other, phenotype in the FTD-ALS spectrum.

Impact of manganese accumulation on Na,K-ATPase expression and function in the cerebellum and striatum of C57Bl/6 mice

Overexposure to Mn causes a neurological disorder-manganism-with motor symptoms that overlap closely with disorders associated with haploinsufficiency in the gene encoding for α3 isoform of Na+,K+-ATPase (NKA). The present study was designed to test the hypothesis that behavioral changes in the mouse model of manganism may be associated with changes in the expression and activity of α3 NKA in the cerebellum (CB) and striatum (STR)-the key brain structures responsible for motor control in adult mice. C57Bl/6 mice were exposed to MnCl2 at 0.5 g/L (in drinking water) for up to eight weeks. After four weeks of Mn consumption, Mn levels were increased in the CB only. Behavioral tests demonstrated decreased performance of Mn-treated mice in the shuttle box test (third through sixth weeks), and the inclined grid walking test (first through sixth weeks), suggesting the development of learning impairment, decreased locomotion, and motor discoordination. The activity of NKA significantly decreased, and the expression of α1-α3 isoforms of NKA increased in the second week in the CB only. Thus, signs of learning and motor disturbances developing in this model of manganism are unlikely to be directly linked to disturbances in the expression or activity of NKA in the CB or STR. Whether these early changes may contribute to the pathogenesis of later behavioral deficits remains to be determined.

Dopamine-mediated striatal activity and function is enhanced in GlyRα2 knockout animals

The glycine receptor alpha 2 (GlyRα2) is a ligand-gated ion channel which upon activation induces a chloride conductance. Here, we investigated the role of GlyRα2 in dopamine-stimulated striatal cell activity and behavior. We show that depletion of GlyRα2 enhances dopamine-induced increases in the activity of putative dopamine D1 receptor-expressing striatal projection neurons, but does not alter midbrain dopamine neuron activity. We next show that the locomotor response to d-amphetamine is enhanced in GlyRα2 knockout animals, and that this increase correlates with c-fos expression in the dorsal striatum. 3-D modeling revealed an increase in the neuronal ensemble size in the striatum in response to D-amphetamine in GlyRα2 KO mice. Finally, we show enhanced appetitive conditioning in GlyRα2 KO animals that is likely due to increased motivation, but not changes in associative learning or hedonic response. Taken together, we show that GlyRα2 is an important regulator of dopamine-stimulated striatal activity and function.

Dopamine Release Plateau and Outcome Signals in Dorsal Striatum Contrast with Classic Reinforcement Learning Formulations

We recorded dopamine release signals in medial and lateral sectors of the striatum as mice learned consecutive visual cue-outcome conditioning tasks including cue association, cue discrimination, reversal, and probabilistic discrimination task versions. Dopamine release responses in medial and lateral sites exhibited learning-related changes within and across phases of acquisition. These were different for the medial and lateral sites. In neither sector could these be accounted for by classic reinforcement learning as applied to dopamine-containing neuron activity. Cue responses ranged from initial sharp peaks to modulated plateau responses. In the medial sector, outcome (reward) responses during cue conditioning were minimal or, initially, negative. By contrast, in lateral sites, strong, transient dopamine release responses occurred at both cue and outcome. Prolonged, plateau release responses to cues emerged in both regions when discriminative behavioral responses became required. In most sites, we found no evidence for a transition from outcome to cue signaling, a hallmark of temporal difference reinforcement learning as applied to midbrain dopamine activity. These findings delineate reshaping of dopamine release activity during learning and suggest that current views of reward prediction error encoding need review to accommodate distinct learning-related spatial and temporal patterns of striatal dopamine release in the dorsal striatum.

Adaptor Protein-3 Produces Synaptic Vesicles that Release Phasic Dopamine

The burst firing of midbrain dopamine neurons releases a phasic dopamine signal that mediates reinforcement learning. At many synapses, however, high firing rates deplete synaptic vesicles (SVs), resulting in synaptic depression that limits release. What accounts for the increased release of dopamine by stimulation at high frequency? We find that adaptor protein-3 (AP-3) and its coat protein VPS41 promote axonal dopamine release by targeting vesicular monoamine transporter VMAT2 to the axon rather than dendrites. AP-3 and VPS41 also produce SVs that respond preferentially to high frequency stimulation, independent of their role in axonal polarity. In addition, conditional inactivation of VPS41 in dopamine neurons impairs reinforcement learning, and this involves a defect in the frequency dependence of release rather than the amount of dopamine released. Thus, AP-3 and VPS41 promote the axonal polarity of dopamine release but enable learning by producing a novel population of SVs tuned specifically to high firing frequency that confers the phasic release of dopamine.

Impact of Calorie-Restricted Cafeteria Diet and Treadmill Exercise on Sweet Taste in Diet-Induced Obese Female and Male Rats

The goal of the present study was to evaluate the sweet taste function in obese rats fed with a 30% calorie-restricted cafeteria diet (CAFR) and/or subjected to moderate treadmill exercise (12-17 m/min, 35 min, 5 days per week) for 9 weeks. A two-bottle preference test, a taste reactivity test, and a brief-access licking test were carried out when animals were aged 21 weeks; biometric and metabolic parameters were also measured along the interventions. Two separate experiments for females and males were performed. Behaviorally, CAF diet decreased sucrose intake and preference, as well as perceived palatability, in both sexes and decreased hedonic responses in males. Compared to the CAF diet, CAFR exerted a corrective effect on sweet taste variables in females by increasing sucrose intake in the preference test and licking responses, while exercise decreased sucrose intake in both sexes and licking responses in females. As expected, CAF diet increased body weight and Lee index and worsened the metabolic profile in both sexes, whereas CAFR diet ameliorated these effects mainly in females. Exercise had no noticeable effects on these parameters. We conclude that CAF diet might diminish appetitive behavior toward sucrose in both sexes, and that this effect could be partially reverted by CAFR diet in females only, while exercise might exert protective effects against overconsumption of sucrose in both sexes.

The Arousal-motor Hypothesis of Dopamine Function: Evidence that Dopamine Facilitates Reward Seeking in Part by Maintaining Arousal

Dopamine facilitates approach to reward via its actions on dopamine receptors in the nucleus accumbens. For example, blocking either D1 or D2 dopamine receptors in the accumbens reduces the proportion of reward-predictive cues to which rats respond with cued approach. Recent evidence indicates that accumbens dopamine also promotes wakefulness and arousal, but the relationship between dopamine's roles in arousal and reward seeking remains unexplored. Here, we show that the ability of systemic or intra-accumbens injections of the D1 antagonist SCH23390 to reduce cued approach to reward depends on the animal's state of arousal. Handling the animal, a manipulation known to increase arousal, was sufficient to reverse the behavioral effects of the antagonist. In addition, SCH23390 reduced spontaneous locomotion and increased time spent in sleep postures, both consistent with reduced arousal, but also increased time spent immobile in postures inconsistent with sleep. In contrast, the ability of the D2 antagonist haloperidol to reduce cued approach was not reversible by handling. Haloperidol reduced spontaneous locomotion but did not increase sleep postures, instead increasing immobility in non-sleep postures. We place these results in the context of the extensive literature on dopamine's contributions to behavior, and propose the arousal-motor hypothesis. This novel synthesis, which proposes that two main functions of dopamine are to promote arousal and facilitate motor behavior, accounts both for our findings and many previous behavioral observations that have led to disparate and conflicting conclusions.

Food wanting is mediated by transient activation of dopaminergic signaling in the honey bee brain

The biological bases of wanting have been characterized in mammals, but whether an equivalent wanting system exists in insects remains unknown. In this study, we focused on honey bees, which perform intensive foraging activities to satisfy colony needs, and sought to determine whether foragers leave the hive driven by specific expectations about reward and whether they recollect these expectations during their waggle dances. We monitored foraging and dance behavior and simultaneously quantified and interfered with biogenic amine signaling in the bee brain. We show that a dopamine-dependent wanting system is activated transiently in the bee brain by increased appetite and individual recollection of profitable food sources, both en route to the goal and during waggle dances. Our results show that insects share with mammals common neural mechanisms for encoding wanting of stimuli with positive hedonic value.

The effect of unexpected rewards on decision making in cuttlefish

Despite numerous studies demonstrating the cognitive ability of cephalopods, there is currently no study showing an emotion-like behavior in this group of animals. To examine whether cuttlefish have different internal states, we developed a behavioral paradigm to assess if prior surprised events are able to alter the choice made by cuttlefish. By presenting unexpected food rewards to cuttlefish before the test, we investigated whether the reaction time of choosing between two shrimps, an intuitive response toward the prey without previous learning, at three different levels of discriminative tests (easy, difficult, and ambiguous), are different compared to the one without an unexpected reward. This behavioral paradigm serves to demonstrate whether cuttlefish are aware of ambiguous situations, and their choice outcome and reaction time are dependent of their internal states. The results show that the response latency was significantly shortened in the difficult and ambiguous tests when choosing from two shrimps that are either moderately different in size or similar sizes, respectively, when cuttlefish have received unexpected rewards before the test. These results were compared with tests during which the cuttlefish did not receive any reward in advance. Furthermore, this shortening of latency did not result in a difference in choice outcome during the difficult and ambiguous tests. Interestingly, even when cuttlefish have obtained the expected food rewards or simply made tentacular strike without prey capture each time before test, these prior experiences were sufficient to shorten the response latency in the difficult and ambiguous tests. However, different from the result of unexpected rewards, food consumption alone or prey capture failure did affect the choice outcome during the simple and difficult tests. Taken together, our findings suggest that pre-test treatments of unexpected and expected rewards or simply unsuccessful visual attack seem to induce cuttlefish to adopt different foraging behaviors. This context dependent decision making suggests that cuttlefish’s foraging strategies are influenced by the previously surprised event and their internal states. It also shows a speed-accuracy tradeoff in difficult and ambiguous situations when foraging for prey. This observation may lead to a future investigation of the presence of emotional state in cephalopods.

Ketamine as a Treatment for Anorexia Nervosa: A Narrative Review

Anorexia nervosa (AN) is a highly complex disorder to treat, especially in severe and enduring cases. Whilst the precise aetiology of the disorder is uncertain, malnutrition and weight loss can contribute to reductions in grey and white matter of the brain, impairments in neuroplasticity and neurogenesis and difficulties with cognitive flexibility, memory and learning. Depression is highly comorbid in AN and may be a barrier to recovery. However, traditional antidepressants are often ineffective in alleviating depressive symptoms in underweight patients with AN. There is an urgent need for new treatment approaches for AN. This review gives a conceptual overview for the treatment of AN with ketamine. Ketamine has rapid antidepressant effects, which are hypothesised to occur via increases in glutamate, with sequelae including increased neuroplasticity, neurogenesis and synaptogenesis. This article provides an overview of the use of ketamine for common psychiatric comorbidities of AN and discusses particular safety concerns and side effects. Potential avenues for future research and specific methodological considerations are explored. Overall, there appears to be ample theoretical background, via several potential mechanisms, that warrant the exploration of ketamine as a treatment for adults with AN.

Alpha-Glycerylphosphorylcholine Increases Motivation in Healthy Volunteers: A Single-Blind, Randomized, Placebo-Controlled Human Study

Alpha-glycerylphosphorylcholine (αGPC) is a precursor of acetylcholine and can increase acetylcholine concentration in the brain. In addition, αGPC has a role in cholinergic function as well as monoaminergic transmission, including dopaminergic and serotonergic systems. These monoaminergic systems are related to feelings and emotions, including motivation, reward processing, anxiety, and depression. However, the precise effects of αGPC on human feelings and emotions remain to be elucidated. In this study, we investigated changes in the subjective feelings of healthy volunteers using the KOKORO scale before and after administering αGPC. Thirty-nine volunteers participated in a single-blind, placebo-controlled design. Participants completed a KOKORO scale test to quantify self-reported emotional states, three times each day for two weeks preceding treatment and then for a further two weeks while self-administering treatment. αGPC treatment show a tendency to increase motivation during the intervention period. Furthermore, motivation at night was significantly higher in the αGPC group than in the placebo group (p < 0.05). However, αGPC did not show any effects on anxiety. These data suggest that αGPC can be used to increase motivation in healthy individuals.

Inhibitory Effect of Apomorphine on Focal and Nonfocal Plasticity in the Human Motor Cortex

Dopamine is crucial for neuroplasticity, which is considered to be the neurophysiological foundation of learning and memory. The specific effect of dopamine on plasticity such as long-term potentiation (LTP) and long-term depression (LTD) is determined by receptor subtype specificity, concentration level, and the kind of plasticity induction technique. In healthy human subjects, the dopamine precursor levodopa (L-DOPA) exerts a dosage-dependent non-linear effect on motor cortex plasticity. Low and high dosage L-DOPA impaired or abolished plasticity, while medium-dose preserved and reversed plasticity in previous studies. Similar dosage-dependent effects were also observed for selective D1-like and D2-like receptor activation that favor excitatory and inhibitory plasticity, respectively. However, such a dosage-dependent effect has not been explored for a nonselective dopamine agonist such as apomorphine in humans. To this aim, nonfocal and focal motor cortex plasticity induction using paired associative stimulation (PAS) and transcranial direct current stimulation (tDCS) were performed respectively in healthy participants under 0.1, 0.2, 0.3 mg apomorphine or placebo drug. Transcranial magnetic stimulation-elicited motor-evoked potentials were used to monitor motor cortical excitability alterations. We hypothesized that, similar to L-DOPA, apomorphine will affect motor cortex plasticity. The results showed that apomorphine with the applied dosages has an inhibitory effect for focal and nonfocal LTP-like and LTD-like plasticity, which was either abolished, diminished or reversed. The detrimental effect on plasticity induction under all dosages of apomorphine suggests a predominantly presynaptic mechanism of action of these dosages.

Cannabis and exercise: Effects of Δ9-tetrahydrocannabinol on preference and motivation for wheel-running in mice

Recent surveys have revealed close links between cannabis and exercise. Specifically, cannabis usage before and/or after exercise is an increasingly common habit primarily aimed at boosting exercise pleasure, motivation, and performance whilst facilitating post-exercise recovery. However, whether these beliefs reflect the true impact of cannabis on these aspects of exercise is unknown. This study has thus examined the effects of cannabis' main psychoactive ingredient, namely Δ⁹-tetrahydrocannabinol (THC), on (i) mouse wheel-running preference and performance and (ii) running motivation and seeking behaviour. Wheel-running preference and performance were investigated using a T-maze with free and locked wheels located at the extremity of either arm. Running motivation and seeking were assessed by a cued-running operant task wherein wheel-running was conditioned by nose poking. Moreover, because THC targets cannabinoid type 1 (CB₁) receptors, i.e. receptors previously documented to control running motivation, this study also assessed the role of these receptors in running preference, performance, and craving-like behaviour. Whilst acute blockade or genetic deletion of CB₁ receptors decreased running preference and performance in the T-maze, THC proved ineffective on either variable. The failure of THC to affect running variables in the T-maze extended to running motivation, as assessed by cued-running under a progressive ratio (PR) reinforcement schedule. This ineffectiveness of THC was not related to the treatment protocol because it successfully increased motivation for palatable food. Although craving-like behaviour, as indexed by a cue-induced reinstatement of running seeking, was found to depend on CB₁ receptors, THC again proved ineffective. Neither running motivation nor running seeking were affected when CB₁ receptors were further stimulated by increasing the levels of the endocannabinoid 2-arachidonoylglycerol. These results, which suggest that the drive for running is insensitive to the acute stimulation of CB₁ receptors, raise the hypothesis that cannabis is devoid of effect on exercise motivation. Future investigation using chronic administration of THC, with and without other cannabis ingredients (e.g. cannabidiol), is however required before conclusions can be drawn.

The impact of childhood experience of starvations on the health of older adults: Evidence from China

This paper used pooled cross-sectional data from the Chinese Longitudinal Healthy Longevity Survey to comprehensively investigate how childhood experiences of starvation affect the health of older Chinese adults. The study found that the childhood experience of starvation was negatively correlated with self-rated health, functional health and cognitive health among older adults. After using the model and variable substitution methods to address the endogeneity problems caused by omitted variables, the negative effects of childhood experiences of starvation on the health of older adults were still present. The Karlson-Holm-Breen decomposition method was used to test the mediation effects, and it was found that childhood experiences of starvation had adverse effects on the health of older adults through endowment insurance, household income, education and nutrition. Consequently, the government should strengthen nutrition or other related health interventions for children and make longer-term plans for improving the health of older adults.

What is reinforcement sensitivity? Neuroscience paradigms for approach‐avoidance process theories of personality

Reinforcement sensitivity is a concept proposed by Gray (1973) to describe the biological antecedents of personality, and has become the common mechanism among a family of personality theories concerning approach and avoidance processes. These theories suggest that 2–3 biobehavioural systems mediate the effects of reward and punishment on emotion and motivation, and that individual differences in the functioning of these systems manifest as personality. Identifying paradigms for operationalising reinforcement sensitivity is therefore critical for testing and developing these theories, and evaluating their footprint in personality space. In this paper I suggest that, while traditional self‐report paradigms in personality psychology may be less‐than‐ideal for this purpose, neuroscience paradigms may offer operations of reinforcement sensitivity at multiple levels of approach and avoidance processes. After brief reflection on the use of such methods in animal models—which first spawned the concept of reinforcement sensitivity—recent developments in four domains of neuroscience are reviewed. These are psychogenomics, psychopharmacology, neuroimaging and category‐learning. By exploring these paradigms as potential operations of reinforcement sensitivity we may enrich our understanding of the putative biobehavioural bases of personality. Copyright © 2008 John Wiley & Sons, Ltd.

Where Do We Stand in the Domestic Dog ( Canis familiaris ) Positive-Emotion Assessment: A State-of-the-Art Review and Future Directions

Although there have been a growing number of studies focusing on dog welfare, the research field concerning dog positive-emotion assessment remains mostly unexplored. This paper aims to provide a state-of-the-art review and summary of the scattered and disperse research on dog positive-emotion assessment. The review notably details the current advancement in dog positive-emotion research, what approaches, measures, methods, and techniques have been implemented so far in emotion perception, processing, and response assessment. Moreover, we propose possible future research directions for short-term emotion as well as longer-term emotional states assessment in dogs. The review ends by identifying and addressing some methodological limitations and by pointing out further methodological research needs.

Hippocampal Input to the Nucleus Accumbens Shell Enhances Food Palatability

BACKGROUND

Insight into the neural basis of hedonic processing has come from studies of food palatability in rodents. Pharmacological manipulations of the nucleus accumbens shell (NAcSh) have repeatedly been demonstrated to increase hedonic taste reactivity, yet the contribution of specific NAcSh circuit components is unknown.

METHODS

Bidirectional optogenetic manipulations were targeted to the principal NAcSh projection neurons and afferent pathways in mice during free feeding assays. Number of licks per bout of consumption was used as a measure of food palatability as it was confirmed to track sucrose concentration and subjective flavor preferences.

RESULTS

Photoinhibition of NAcSh neurons, whether general or cell-type specific, was found to alter consumption without affecting its hedonic impact. Among the principal excitatory afferent pathways, we showed that ventral hippocampal (vHipp) input alone enhances palatability upon low-frequency photostimulation time-locked to consumption. This enhancement in palatability was independent of opioid signaling and not recapitulated by NAcSh or dopamine neuron photostimulation. We further demonstrated that vHipp input photostimulation is sufficient to condition a flavor preference, while its inhibition impedes sucrose-driven flavor preference conditioning.

CONCLUSIONS

These results demonstrate a novel contribution of vHipp-NAcSh pathway activity to palatability that may relate to its innervation of a particular region or neuronal ensemble in the NAcSh. These findings are consistent with the evidence that vHipp-NAcSh activity is relevant to the pathophysiology of anhedonia and depression as well as the increasing appreciation of hippocampal involvement in people's food pleasantness ratings, hunger, and weight.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

Reward Processing under Chronic Pain from the Perspective of "Liking" and "Wanting": A Narrative Review

The therapeutic goals of patients with chronic pain are not only to relieve pain but also to improve the quality of life. Chronic pain negatively affects various aspects of daily life, such as by decreasing the motivation to work and reward sensitivity, which may lead to difficulties in daily life or even unemployment. Human and animal studies have shown that chronic pain damages reward processing; the exploration of associated internal mechanisms may aid the development of treatments to repair this damage. Incentive salience theory, used widely to describe reward processing, divides this processing into “liking” (reward-induced hedonic sensory impact) and “wanting” (reward-induced motivation) components. It has been employed to explain pathological changes in reward processing induced by psychiatric disorders. In this review, we summarize the findings of studies of reward processing under chronic pain and examine the effects of chronic pain on “liking” and “wanting.” Evidence indicates that chronic pain compromises the “wanting” component of reward processing; we also discuss the neural mechanisms that may mediate this effect. We hope that this review aids the development of therapies to improve the quality of life of patients with chronic pain.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Individual behavioural predictors of amphetamine-induced emission of 50 kHz vocalization in rats

Measurement of ultrasonic vocalizations (USVs) produced by adult rats represents a highly useful index of emotional arousal. The associations found between 50 kHz USV production and a variety of behavioural and pharmacological protocols increasingly suggests they serve as a marker of positive motivational states. This study used a powerful within-subjects design to investigate the relationships among individual differences in approach to a sweet-food reward, predisposition to emit 50 kHz USVs spontaneously, and 50 kHz USVs emission following acute systemic administration of amphetamine. Both approach motivation and predisposition to call were found to not correlate with each other but did predict 50 kHz USV response to acute amphetamine. These two behavioural phenotypes appear to represent dissociable predictors of acute amphetamine-induced emission of 50 kHz USVs in a non-sensitization paradigm. In contrast to that, a measure of sucrose preference was not found to predict 50 kHz USV emission following amphetamine. Acute amphetamine was also found to increase average sound frequency of emitted USVs and selectively increase the proportion of Trill subtype 50 kHz USVs. Together, these data demonstrate that acute amphetamine-induced 50 kHz USVs in the adult rat represent more than just a univariate motivational state and may represent the product of dissociable subsystems of emotional behavior.

Medium spiny neurons of the anterior dorsomedial striatum mediate reversal learning in a cell-type-dependent manner

The striatum has been implicated in the regulation of cognitive flexibility. Abnormalities in the anterior dorsomedial striatum (aDMS) are revealed in many mental disorders in which cognitive inflexibility is commonly observed. However, it remains poorly understood whether the aDMS plays a special role in flexible cognitive control and what the regulation pattern is in different neuronal populations. Based on the reversal learning task in mice, we showed that optogenetic activation in dopamine receptor 1-expressing medium spiny neurons (D1R-MSNs) of the aDMS impaired flexibility; meanwhile, suppressing these neurons facilitated behavioral performance. Conversely, D2R-MSN activation accelerated reversal learning, but it induced no change through neuronal suppression. The acquisition and retention of discrimination learning were unaffected by the manipulation of any type of MSN. Through bi-direct optogenetic modulation in D1R-MSNs of the same subject in a serial reversal learning task, we further revealed the function of D1R-MSNs during different stages of reversal learning, where inhibiting and exciting the same group of neurons reduced perseverative errors and increased regressive errors. Following D1R- and D2R-MSN activation in the aDMS, neuronal activity of the mediodorsal thalamus decreased and increased, respectively, in parallel with behavioral impairment and facilitation, but not as a direct result of the activation of the striatal MSNs. We propose that D1R- and D2R-MSN sub-populations in the aDMS exert opposing functions in cognitive flexibility regulation, with more important and complex roles of D1R-MSNs involved. Mental disorders with cognitive flexibility problems may feature an underlying functional imbalance in the aDMS' two types of neurons.

Motor thalamus supports striatum-driven reinforcement

Reinforcement has long been thought to require striatal synaptic plasticity. Indeed, direct striatal manipulations such as self-stimulation of direct-pathway projection neurons (dMSNs) are sufficient to induce reinforcement within minutes. However, it’s unclear what role, if any, is played by downstream circuitry. Here, we used dMSN self-stimulation in mice as a model for striatum-driven reinforcement and mapped the underlying circuitry across multiple basal ganglia nuclei and output targets. We found that mimicking the effects of dMSN activation on downstream circuitry, through optogenetic suppression of basal ganglia output nucleus substantia nigra reticulata (SNr) or activation of SNr targets in the brainstem or thalamus, was also sufficient to drive rapid reinforcement. Remarkably, silencing motor thalamus—but not other selected targets of SNr—was the only manipulation that reduced dMSN-driven reinforcement. Together, these results point to an unexpected role for basal ganglia output to motor thalamus in striatum-driven reinforcement.

Genetic inhibition of neurotransmission reveals role of glutamatergic input to dopamine neurons in high-effort behavior

Midbrain dopamine neurons are crucial for many behavioral and cognitive functions. As the major excitatory input, glutamatergic afferents are important for control of the activity and plasticity of dopamine neurons. However, the role of glutamatergic input as a whole onto dopamine neurons remains unclear. Here we developed a mouse line in which glutamatergic inputs onto dopamine neurons are specifically impaired, and utilized this genetic model to directly test the role of glutamatergic inputs in dopamine-related functions. We found that while motor coordination and reward learning were largely unchanged, these animals showed prominent deficits in effort-related behavioral tasks. These results provide genetic evidence that glutamatergic transmission onto dopaminergic neurons underlies incentive motivation, a willingness to exert high levels of effort to obtain reinforcers, and have important implications for understanding the normal function of the midbrain dopamine system.

Sexually dimorphic structural abnormalities in major connections of the medial forebrain bundle in alcoholism

Background

The mesocorticolimbic system is particularly susceptible to the effects of chronic alcoholism. Disruption of this system has been linked to drug seeking and the development of Reward Deficiency Syndrome, a neurobiological framework for describing the development and relapsing patterns of addictions. In this study, we evaluated the association of alcoholism and sex with major connections of the medial forebrain bundle (MFB), a prominent mesocorticolimbic fiber pathway connecting the ventral tegmental area with the basal forebrain. Given sex differences in clinical consequences of alcohol consumption, we hypothesized that alcoholic men and women would differ in structural abnormalities of the MFB.

Methods

Diffusion magnetic resonance imaging (dMRI) data were acquired from 30 abstinent long-term alcoholic individuals (ALC; 9 men) and 25 non-alcoholic controls (NC; 8 men). Major connections of the MFB were extracted using multi-tensor tractography. We compared groups on MFB volume, fractional anisotropy (FA), radial diffusivity (RD), and axial diffusivity (AD), with hemisphere and sex as independent variables. We also evaluated associations between abnormal structural measures and drinking measures.

Results

Analyses revealed significant group-by-sex interactions for FA and RD: while ALC men had lower FA and higher RD compared to NC men, ALC women had higher FA and lower RD compared to NC women. We also detected a significant negative association between FA and number of daily drinks in ALC women.

Conclusion

Alcoholism is associated with sexually dimorphic structural abnormalities in the MFB. The results expand upon other findings of differences in brain reward circuitry of alcoholic men and women.

Pleasure: The missing link in the regulation of sleep

Although largely unrecognized by sleep scholars, sleeping is a pleasure. This report aims first, to fill the gap: sleep, like food, water and sex, is a primary reinforcer. The levels of extracellular mesolimbic dopamine show circadian oscillations and mark the "wanting" for pro-homeostatic stimuli. Further, the dopamine levels decrease during waking and are replenished during sleep, in opposition to sleep propensity. The wanting of sleep, therefore, may explain the homeostatic and circadian regulation of sleep. Accordingly, sleep onset occurs when the displeasure of excessive waking is maximal, coinciding with the minimal levels of mesolimbic dopamine. Reciprocally, sleep ends after having replenished the limbic dopamine levels. Given the direct relation between waking and mesolimbic dopamine, sleep must serve primarily to gain an efficient waking. Pleasant sleep (i.e. emotional sleep), can only exist in animals capable of feeling emotions. Therefore, although sleep-like states have been described in invertebrates and primitive vertebrates, the association sleep-pleasure clearly marks a difference between the sleep of homeothermic vertebrates and cool blooded animals.

Opioid and nondopamine reward circuitry and state-dependent mechanisms

A common notion is that essentially all addictive drugs, including opioids, activate dopaminergic pathways in the brain reward system, and the inappropriate use of such drugs induces drug dependence. However, an opioid reward response is reportedly still observed in several models of dopamine depletion, including in animals that are treated with dopamine blockers, animals that are subjected to dopaminergic neuron lesions, and dopamine-deficient mice. The intracranial self-stimulation response is enhanced by stimulants but reduced by morphine. These findings suggest that dopaminergic neurotransmission may not always be required for opioid reward responses. Previous findings also indicate the possibility that dopamine-independent opioid reward may be observed in opioid-naive states but not in opioid-dependent states. Therefore, a history of opioid use should be considered when evaluating the dopamine dependency of opioid reward.

Smoking addiction: the shift from head to hands: Approach bias towards smoking-related cues in low-dependent versus dependent smokers

The dual process theory is central to several models of addiction, implying both an increase of stimulus salience and deficits in inhibitory control. Our major aim is to provide behavioral evidence for an approach bias tendency in smokers and more specifically during smoking cue exposure. The second aim is to examine whether this bias differs in low-dependent versus dependent smokers. Thirty-two smokers (17 low dependent and 15 dependent; cut-off FTND of 4) and 28 non-smokers performed a modified Go/NoGo task using tobacco-related words and neutral words as stimuli. Smokers generally made more mistakes and tended to be faster for smoking-related cues specifically. Low dependents acknowledged more their dependency in declarative questionnaires while making more errors and being slower specifically on smoking cues; dependent smokers were less prone to indicate their addiction, but were faster and accurate when it came to picking the smoking cues. These results suggest that a shift has operated from a mental preoccupation with smoking in the low-dependent group, to smoking as a motor habit in our dependent group. This finding invites experts to rethink smoking addiction in the light of this crucial moment, namely, the shift "from head to hands".

Abundant collateralization of temporal lobe projections to the accumbens, bed nucleus of stria terminalis, central amygdala and lateral septum

Behavioral flexibility is subserved in part by outputs from the cerebral cortex to telencephalic subcortical structures. In our earlier evaluation of the organization of the cortical-subcortical output system (Reynolds and Zahm, J Neurosci 25:11757-11767, 2005), retrograde double-labeling was evaluated in the prefrontal cortex following tracer injections into pairs of the following subcortical telencephalic structures: caudate-putamen, core and shell of the accumbens (Acb), bed nucleus of stria terminalis (BST) and central nucleus of the amygdala (CeA). The present study was done to assess patterns of retrograde labeling in the temporal lobe after similar paired tracer injections into most of the same telencephalic structures plus the lateral septum (LS). In contrast to the modest double-labeling observed in the prefrontal cortex in the previous study, up to 60-80 % of neurons in the basal and accessory basal amygdaloid nuclei and amygdalopiriform transition area exhibited double-labeling in the present study. The most abundant double-labeling was generated by paired injections into structures affiliated with the extended amygdala, including the CeA, BST and Acb shell. Injections pairing the Acb core with the BST or CeA produced significantly fewer double-labeled neurons. The ventral subiculum exhibited modest amounts of double-labeling associated with paired injections into the Acb, BST, CeA and LS. The results raise the issue of how an extraordinarily collateralized output from the temporal lobe may contribute to behavioral flexibility.

Weight gain after STN-DBS: The role of reward sensitivity and impulsivity

Weight gain has been reported after deep brain stimulation of the subthalamic nucleus (STN-DBS), a widely used treatment for Parkinson's disease (PD). This nucleus has been repeatedly found to be linked both to reward and to inhibitory control, two key aspects in the control of food intake. In this study, we assessed whether weight gain experienced by patients with PD after STN-DBS, might be due to an alteration of reward and inhibitory functions. Eighteen patients with PD were compared to eighteen healthy controls and tested three times: before surgery, in ON medication and after surgery, respectively five days after the implantation in ON medication/OFF stimulation and at least three months after surgery in ON medication/ON stimulation. All participants were assessed for depression (Beck Depression Inventory), anhedonia (Snaith-Hamilton Pleasure Scale) and impulsiveness (Barratt Impulsiveness Scale). They performed a battery of tests assessing food reward sensitivity (Liking, Wanting and Preference) and a food go/no-go task. Results showed that body weight significantly increased after STN-DBS. A few days after surgery, patients were slower and more impulsive in the go/no-go task, showed a higher preference for high calorie (HC) foods and rated foods as less tasty. Months after subthalamic stimulation, the performance on the go/no-go task improved while no differences were observed in reward sensitivity. Interestingly, weight gain resulted greater in patients with higher levels of attentional impulsiveness pre-surgery, higher wanting for low calorie (LC) foods and impulsivity in the go/no-go task in ON medication/ON stimulation. However, only wanting and attentional impulsivity significantly predicted weight change. Furthermore, weight gain resulted associated with the reduction of l-Dopa after surgery and disease's duration. In conclusion, our findings are consistent with the view that weight gain in PD after STN-DBS has a multifactorial nature, which reflects the complex functional organization of the STN.

Diurnal rhythms in psychological reward functioning in healthy young men: 'Wanting', liking, and learning

A range of evidence suggests that human reward functioning is partly driven by the endogenous circadian system, generating 24-hour rhythms in behavioural measures of reward activation. Reward functioning is multifaceted but literature to date is largely limited to measures of self-reported positive mood states. The aim of this study was to advance the field by testing for hypothesised diurnal variation in previously unexplored components of psychological reward: 'wanting', liking, and learning using subjective and behavioural measures. Risky decision making (automatic Balloon Analogue Risk Task), affective responsivity to positive images (International Affective Pictures System), uncued self-reported discrete emotions, and learning-contingent reward (Iowa Gambling Task) were measured at 10.00 hours, 14.00 hours, and 19.00 hours in a counterbalanced repeated measures design with 50 healthy male participants (aged 18-30). As hypothesised, risky decision making (unconscious 'wanting') and ratings of arousal towards positive images (conscious wanting) exhibited a diurnal waveform with indices highest at 14.00 hours. No diurnal rhythm was observed for liking (pleasure ratings to positive images, discrete uncued positive emotions) or in a learning-contingent reward task. Findings reaffirm that diurnal variation in human reward functioning is most pronounced in the motivational 'wanting' components of reward.

Models of neuromodulation for computational psychiatry

Psychiatry faces fundamental challenges: based on a syndrome-based nosology, it presently lacks clinical tests to infer on disease processes that cause symptoms of individual patients and must resort to trial-and-error treatment strategies. These challenges have fueled the recent emergence of a novel field-computational psychiatry-that strives for mathematical models of disease processes at physiological and computational (information processing) levels. This review is motivated by one particular goal of computational psychiatry: the development of 'computational assays' that can be applied to behavioral or neuroimaging data from individual patients and support differential diagnosis and guiding patient-specific treatment. Because the majority of available pharmacotherapeutic approaches in psychiatry target neuromodulatory transmitters, models that infer (patho)physiological and (patho)computational actions of different neuromodulatory transmitters are of central interest for computational psychiatry. This article reviews the (many) outstanding questions on the computational roles of neuromodulators (dopamine, acetylcholine, serotonin, and noradrenaline), outlines available evidence, and discusses promises and pitfalls in translating these findings to clinical applications. WIREs Cogn Sci 2017, 8:e1420. doi: 10.1002/wcs.1420 For further resources related to this article, please visit the WIREs website.

Dopamine antagonism does not impair learning of Pavlovian conditioned approach to manipulable or non-manipulable cues but biases responding towards goal tracking

Dopamine's (DA) role in reward-processing is currently discussed as either providing a teaching signal to guide learning or mediating the transfer of incentive salience (i.e. motivational aspects) from unconditioned stimuli (US) to conditioned stimuli (CS). We used a Pavlovian conditioned approach (PCA) procedure to further investigate DAs contribution to these processes. Experiment 1 assessed the acquisition of PCA to a manipulable lever cue for 7days under DA-blockade with Flupenthixol (FLU; 225μg/kg) or Saline (SAL) treatment, followed by 6-days off-drug testing. FLU decreased the number of conditioned responses (CR) during the treatment phase, but cessation of treatment resulted in an immediate increase in CR to levels comparable to SAL controls; notably, CR in FLU-treated rats were restricted to goal tracking behaviour. During continued off-drug testing, rats from the FLU group developed sign tracking with a similar temporal pattern as controls. In experiment 2, acquisition of PCA to a non-manipulable auditory cue was investigated. FLU reduced the number of CR during treatment, and removing DA antagonism resulted in a similar rapid increase of CR as seen in experiment 1. These data complement other reports by demonstrating that, independently from the physical properties of the CS, DA is not required for learning predictive aspects of a CS-US relationship but for the development of behaviour (namely sign tracking) which is based on the motivational aspects of a CS-US relationship.

Architectural Representation of Valence in the Limbic System

In order to thrive, animals must be able to recognize aversive and appetitive stimuli within the environment and subsequently initiate appropriate behavioral responses. This assignment of positive or negative valence to a stimulus is a key feature of emotional processing, the neural substrates of which have been a topic of study for several decades. Until recently, the result of this work has been the identification of specific brain regions, such as the basolateral amygdala (BLA) and nucleus accumbens (NAc), as important to valence encoding. The advent of modern tools in neuroscience has allowed further dissection of these regions to identify specific populations of neurons signaling the valence of environmental stimuli. In this review, we focus upon recent work examining the mechanisms of valence encoding, and provide a model for the systematic investigation of valence within anatomically-, genetically-, and functionally defined populations of neurons.

Cocaine can generate a stronger conditioned reinforcer than food despite being a weaker primary reinforcer

The present study aimed to test the hypothesis that cues associated with drug-taking behavior become extra strong motivators of behavior compared with cues paired with non-drug reinforcers. In experiment 1, rats were trained to lever press for intravenous cocaine infusions and grain pellets. Each reinforcer was paired with a distinct audiovisual cue. When allowed to choose between these alternatives, rats chose grain on ~70-80 percent of trials. However, after extinguishing lever pressing, reintroduction of press-contingent cues during a test for cue-induced reinstatement generated more cocaine seeking than grain seeking (also observed on 3- and 8-week follow-up tests). To examine whether the same pattern of results would occur with two non-drug reinforcers, experiment 2 replicated experiment 1 using grain and sucrose as reinforcement alternatives. Rats chose sucrose over grain on ~70-80 percent of choice trials and also responded more for the sucrose cue than for the grain cue on the reinstatement test. The disconnect between primary and conditioned reinforcements in experiment 1 but not in experiment 2 suggests that drug cues may become exceptionally strong motivators of drug seeking. These results are consistent with cue-focused theories of addiction and may offer insight into the persistent cue-driven drug-seeking behavior observed in addiction.

Explanatory pluralism: An unrewarding prediction error for free energy theorists

Courtesy of its free energy formulation, the hierarchical predictive processing theory of the brain (PTB) is often claimed to be a grand unifying theory. To test this claim, we examine a central case: activity of mesocorticolimbic dopaminergic (DA) systems. After reviewing the three most prominent hypotheses of DA activity-the anhedonia, incentive salience, and reward prediction error hypotheses-we conclude that the evidence currently vindicates explanatory pluralism. This vindication implies that the grand unifying claims of advocates of PTB are unwarranted. More generally, we suggest that the form of scientific progress in the cognitive sciences is unlikely to be a single overarching grand unifying theory.

Roles of "Wanting" and "Liking" in Motivating Behavior: Gambling, Food, and Drug Addictions

The motivation to seek out and consume rewards has evolutionarily been driven by the urge to fulfill physiological needs. However in a modern society dominated more by plenty than scarcity, we tend to think of motivation as fueled by the search for pleasure. Here, we argue that two separate but interconnected subcortical and unconscious processes direct motivation: "wanting" and "liking." These two psychological and neuronal processes and their related brain structures typically work together, but can become dissociated, particularly in cases of addiction. In drug addiction, for example, repeated consumption of addictive drugs sensitizes the mesolimbic dopamine system, the primary component of the "wanting" system, resulting in excessive "wanting" for drugs and their cues. This sensitizing process is long-lasting and occurs independently of the "liking" system, which typically remains unchanged or may develop a blunted pleasure response to the drug. The result is excessive drug-taking despite minimal pleasure and intense cue-triggered craving that may promote relapse long after detoxification. Here, we describe the roles of "liking" and "wanting" in general motivation and review recent evidence for a dissociation of "liking" and "wanting" in drug addiction, known as the incentive sensitization theory (Robinson and Berridge 1993). We also make the case that sensitization of the "wanting" system and the resulting dissociation of "liking" and "wanting" occurs in both gambling disorder and food addiction.

Dopamine, reward learning, and active inference

Temporal difference learning models propose phasic dopamine signaling encodes reward prediction errors that drive learning. This is supported by studies where optogenetic stimulation of dopamine neurons can stand in lieu of actual reward. Nevertheless, a large body of data also shows that dopamine is not necessary for learning, and that dopamine depletion primarily affects task performance. We offer a resolution to this paradox based on an hypothesis that dopamine encodes the precision of beliefs about alternative actions, and thus controls the outcome-sensitivity of behavior. We extend an active inference scheme for solving Markov decision processes to include learning, and show that simulated dopamine dynamics strongly resemble those actually observed during instrumental conditioning. Furthermore, simulated dopamine depletion impairs performance but spares learning, while simulated excitation of dopamine neurons drives reward learning, through aberrant inference about outcome states. Our formal approach provides a novel and parsimonious reconciliation of apparently divergent experimental findings.

Dopamine in the Brain: Hypothesizing Surfeit or Deficit Links to Reward and Addiction

Recently there has been debate concerning the role of brain dopamine in reward and addiction. David Nutt and associates eloquently proposed that dopamine (DA) may be central to psycho stimulant dependence and some what important for alcohol, but not important for opiates, nicotine or even cannabis. Others have also argued that surfeit theories can explain for example cocaine seeking behavior as well as non-substance-related addictive behaviors. It seems prudent to distinguish between what constitutes “surfeit” compared to” deficit” in terms of short-term (acute) and long-term (chronic) brain reward circuitry responsivity. In an attempt to resolve controversy regarding the contributions of mesolimbic DA systems to reward, we review the three main competing explanatory categories: “liking”, “learning”, and “wanting”. They are (a) the hedonic impact -liking reward, (b) the ability to predict rewarding effects-learning and (c) the incentive salience of reward-related stimuli -wanting. In terms of acute effects, most of the evidence seems to favor the “surfeit theory”. Due to preferential dopamine release at mesolimbic-VTA-caudate-accumbens loci most drugs of abuse and Reward Deficiency Syndrome (RDS) behaviors have been linked to heightened feelings of well-being and hyperdopaminergic states.The “dopamine hypotheses” originally thought to be simple, is now believed to be quite complex and involves encoding the set point of hedonic tone, encoding attention, reward expectancy, and incentive motivation. Importantly, Willuhn et al. shows that in a self-administration paradigm, (chronic) excessive use of cocaine is caused by decreased phasic dopamine signaling in the striatum. In terms of chronic addictions, others have shown a blunted responsivity at brain reward sites with food, nicotine, and even gambling behavior. Finally, we are cognizant of the differences in dopaminergic function as addiction progresses and argue that relapse may be tied to dopamine deficiency. Vulnerability to addiction and relapse may be the result of the cumulative effects of dopaminergic and other neurotransmitter genetic variants and elevated stress levels. We therefore propose that dopamine homeostasis may be a preferred goal to combat relapse.

Phasic dopamine release induced by positive feedback predicts individual differences in reversal learning

Striatal dopamine (DA) is central to reward-based learning. Less is known about the contribution of DA to the ability to adapt previously learned behavior in response to changes in the environment, such as a reversal of response-reward contingencies. We hypothesized that DA is involved in the rapid updating of response-reward information essential for successful reversal learning. We trained rats to discriminate between two levers, where lever availability was signaled by a non-discriminative cue. Pressing one lever was always rewarded, whereas the other lever was never rewarded. After reaching stable discrimination performance, a reversal was presented, so that the previously non-rewarded lever was now rewarded and vice versa. We used fast-scan cyclic voltammetry to monitor DA release in the ventromedial striatum. During discrimination performance (pre-reversal), cue presentation induced phasic DA release, whereas reward delivery did not. The opposite pattern was observed post-reversal: Striatal DA release emerged after reward delivery, while cue-induced release diminished. Trial-by-trial analysis showed rapid reinstatement of cue-induced DA release on trials immediately following initial correct responses. This effect of positive feedback was observed in animals that learned the reversal, but not in 'non-learners'. In contrast, neither pre-reversal responding and DA signaling, nor post-reversal DA signaling in response to negative feedback differed between learners and non-learners. Together, we show that phasic DA dynamics in the ventromedial striatum encoding reward-predicting cues are associated with positive feedback during reversal learning. Furthermore, these signals predict individual differences in learning that are not present prior to reversal, suggesting a distinct role for dopamine in the adaptation of previously learned behavior.

Injection of a dopamine type 2 receptor antagonist into the dorsal striatum disrupts choices driven by previous outcomes, but not perceptual inference

Decisions are often driven by a combination of immediate perception and previous experience. In this study, we investigated how these two sources of information are integrated and the neural systems that mediate this process. Specifically, we injected a dopamine type 1 antagonist (D1A; SCH23390) or a dopamine type 2 antagonist (D2A; eticlopride) into the dorsal striatum while macaques performed a task in which their choices were driven by perceptual inference and/or reinforcement of past choices. We found that the D2A affected choices based on previous outcomes. However, there were no effects of the D2A on choices driven by perceptual inference. We found that the D1A did not affect perceptual inference or reinforcement learning. Finally, a Bayesian model applied to the results suggested that the D2A may be increasing noise in the striatal representation of value, perhaps by disrupting the striatal population that normally represents value.

Compromised NMDA/Glutamate Receptor Expression in Dopaminergic Neurons Impairs Instrumental Learning, But Not Pavlovian Goal Tracking or Sign Tracking

Behavior is shaped to a dramatic degree by the occurrence of rewards, through both pavlovian and instrumental conditioning processes; these mechanisms give rise to both normal and abnormal behavior. It is crucial to understand the neural mechanisms that give rise to normal actions and how they lead to pathological behaviors, such as overeating and drug addictions.

Two theories regarding the role for dopamine neurons in learning include the concepts that their activity serves as a (1) mechanism that confers incentive salience onto rewards and associated cues and/or (2) contingency teaching signal reflecting reward prediction error. While both theories are provocative, the causal role for dopamine cell activity in either mechanism remains controversial. In this study mice that either fully or partially lacked NMDARs in dopamine neurons exclusively, as well as appropriate controls, were evaluated for reward-related learning; this experimental design allowed for a test of the premise that NMDA/glutamate receptor (NMDAR)-mediated mechanisms in dopamine neurons, including NMDA-dependent regulation of phasic discharge activity of these cells, modulate either the instrumental learning processes or the likelihood of pavlovian cues to become highly motivating incentive stimuli that directly attract behavior. Loss of NMDARs in dopamine neurons did not significantly affect baseline dopamine utilization in the striatum, novelty evoked locomotor behavior, or consumption of a freely available, palatable food solution. On the other hand, animals lacking NMDARs in dopamine cells exhibited a selective reduction in reinforced lever responses that emerged over the course of instrumental learning. Loss of receptor expression did not, however, influence the likelihood of an animal acquiring a pavlovian conditional response associated with attribution of incentive salience to reward-paired cues (sign tracking). These data support the view that reductions in NMDAR signaling in dopamine neurons affect instrumental reward-related learning but do not lend support to hypotheses that suggest that the behavioral significance of this signaling includes incentive salience attribution.

Pleasure systems in the brain

Pleasure is mediated by well-developed mesocorticolimbic circuitry and serves adaptive functions. In affective disorders, anhedonia (lack of pleasure) or dysphoria (negative affect) can result from breakdowns of that hedonic system. Human neuroimaging studies indicate that surprisingly similar circuitry is activated by quite diverse pleasures, suggesting a common neural currency shared by all. Wanting for reward is generated by a large and distributed brain system. Liking, or pleasure itself, is generated by a smaller set of hedonic hot spots within limbic circuitry. Those hot spots also can be embedded in broader anatomical patterns of valence organization, such as in a keyboard pattern of nucleus accumbens generators for desire versus dread. In contrast, some of the best known textbook candidates for pleasure generators, including classic pleasure electrodes and the mesolimbic dopamine system, may not generate pleasure after all. These emerging insights into brain pleasure mechanisms may eventually facilitate better treatments for affective disorders.