Transient overexpression of striatal D2 receptors impairs operant motivation and interval timing

Complementary opposing D2-MSNs and D1-MSNs dynamics during interval timing

The role of striatal pathways in cognitive processing is unclear. We studied dorsomedial striatal cognitive processing during interval timing, an elementary cognitive task that requires mice to estimate intervals of several seconds and involves working memory for temporal rules as well as attention to the passage of time. We harnessed optogenetic tagging to record from striatal D2-dopamine receptor-expressing medium spiny neurons (D2-MSNs) in the indirect pathway and from D1-dopamine receptor-expressing MSNs (D1-MSNs) in the direct pathway. We found that D2-MSNs and D1-MSNs exhibited distinct dynamics over temporal intervals as quantified by principal component analyses and trial-by-trial generalized linear models. MSN recordings helped construct and constrain a four-parameter drift-diffusion computational model. This model predicted that disrupting either D2-MSNs or D1-MSNs would increase interval timing response times and alter MSN firing. In line with this prediction, we found that optogenetic inhibition or pharmacological disruption of either D2-MSNs or D1-MSNs increased interval timing response times. Pharmacologically disrupting D2-MSNs or D1-MSNs also changed MSN dynamics and degraded trial-by-trial temporal decoding. Together, our findings demonstrate that D2-MSNs and D1-MSNs make complementary contributions to interval timing despite opposing dynamics, implying that striatal direct and indirect pathways work together to shape temporal control of action. These data provide novel insight into basal ganglia cognitive operations beyond movement and have implications for human striatal diseases and therapies targeting striatal pathways.

Amphetamine increases motivation of humans and mice as measured by breakpoint, but does not affect an Electroencephalographic biomarker

Translation of drug targets from preclinical studies to clinical trials has been aided by cross-species behavioral tasks, but evidence for brain-based engagement during task performance is still required. Cross-species progressive ratio breakpoint tasks (PRBTs) measure motivation-related behavior and are pharmacologically and clinically sensitive. We recently advanced elevated parietal alpha power as a cross-species electroencephalographic (EEG) biomarker of PRBT engagement. Given that amphetamine increases breakpoint in mice, we tested its effects on breakpoint and parietal alpha power in both humans and mice. Twenty-three healthy participants performed the PRBT with EEG after amphetamine or placebo in a double-blind design. C57BL/6J mice were trained on PRBT with EEG (n = 24) and were treated with amphetamine or vehicle. A second cohort of mice was trained on PRBT without EEG (n = 40) and was treated with amphetamine or vehicle. In humans, amphetamine increased breakpoint. In mice, during concomitant EEG, 1 mg/kg of amphetamine significantly decreased breakpoint. In cohort 2, however, 0.3 mg/kg of amphetamine increased breakpoint consistent with human findings. Increased alpha power was observed in both species as they reached breakpoint, replicating previous findings. Amphetamine did not affect alpha power in either species. Amphetamine increased effort in humans and mice. Consistent with previous reports, elevated parietal alpha power was observed in humans and mice as they performed the PRBT. Amphetamine did not affect this EEG biomarker of effort. Hence, these findings support the pharmacological predictive validity of the PRBT to measure effort in humans and mice and suggest that this EEG biomarker is not directly reflective of amphetamine-induced changes in effort.

Core clock gene, Bmal1, is required for optimal second-level interval production

Perception and production of second-level temporal intervals are critical in several behavioral and cognitive processes, including adaptive anticipation, motor control, and social communication. These processes are impaired in several neurological and psychological disorders, such as Parkinson's disease and attention-deficit hyperactivity disorder. Although evidence indicates that second-level interval timing exhibit circadian patterns, it remains unclear whether the core clock machinery controls the circadian pattern of interval timing. To investigate the role of core clock molecules in interval timing capacity, we devised a behavioral assay called the interval timing task to examine prospective motor interval timing ability. In this task, the mouse produces two separate nose pokes in a pretrained second-level interval to obtain a sucrose solution as a reward. We discovered that interval perception in wild-type mice displayed a circadian pattern, with the best performance observed during the late active phase. To investigate whether the core molecular clock is involved in the circadian control of interval timing, we employed Bmal1 knockout mice (BKO) in the interval timing task. The interval production of BKO did not display any difference between early and late active phase, without reaching the optimal interval production level observed in wild-type. In summary, we report that the core clock gene Bmal1 is required for the optimal performance of prospective motor timing typically observed during the late part of the active period.

Chronic jet lag reduces motivation and affects other mood-related behaviors in male mice

Introduction: The circadian system regulates various physiological processes such as sleep-wake cycles, hormone secretion, metabolism, and the reaction to both natural and drug-based rewards. Chronic disruption of the circadian system caused by unsteady synchronization with light-dark (LD) schedules, such as advancing chronic jet lag (CJL), leads to adverse physiological effects and pathologies, and is linked with changes in mood and depressive behaviors in humans and rodent models. Methods: C57BL/6J male mice were subjected to circadian disruption through phase advances of 6 h every 2 days (CJL +6/2). Mice under 12:12-h LD cycle were used as controls. After 8 weeks under these conditions, a battery of behavioral tests was performed to assess if mood-related behaviors were affected. Results: Compared to controls under 24 h LD cycles, mice under CJL presented desynchronization of activity-rest rhythms that led to several behavioral impairments, including a decrease in motivation for food reward, and an increase in anxiety, anhedonia, and depressive-like behavior. Conclusion: Chronic circadian disruption, caused by an experimental CJL protocol, affects mood-related and reward-related behaviors in mice. Understanding the importance of the circadian system and its potential role for disruption due to CJL is important for maintaining good health and well-being.

Dopamine D2 receptors in nucleus accumbens cholinergic interneurons increase impulsive choice

Impulsive choice, often characterized by excessive preference for small, short-term rewards over larger, long-term rewards, is a prominent feature of substance use and other neuropsychiatric disorders. The neural mechanisms underlying impulsive choice are not well understood, but growing evidence implicates nucleus accumbens (NAc) dopamine and its actions on dopamine D2 receptors (D2Rs). Because several NAc cell types and afferents express D2Rs, it has been difficult to determine the specific neural mechanisms linking NAc D2Rs to impulsive choice. Of these cell types, cholinergic interneurons (CINs) of the NAc, which express D2Rs, have emerged as key regulators of striatal output and local dopamine release. Despite these relevant functions, whether D2Rs expressed specifically in these neurons contribute to impulsive choice behavior is unknown. Here, we show that D2R upregulation in CINs of the mouse NAc increases impulsive choice as measured in a delay discounting task without affecting reward magnitude sensitivity or interval timing. Conversely, mice lacking D2Rs in CINs showed decreased delay discounting. Furthermore, CIN D2R manipulations did not affect probabilistic discounting, which measures a different form of impulsive choice. Together, these findings suggest that CIN D2Rs regulate impulsive decision-making involving delay costs, providing new insight into the mechanisms by which NAc dopamine influences impulsive behavior.

Towards Novel Potential Molecular Targets for Antidepressant and Antipsychotic Pharmacotherapies

Depression and schizophrenia are two highly prevalent and severely debilitating neuropsychiatric disorders. Both conventional antidepressant and antipsychotic pharmacotherapies are often inefficient clinically, causing multiple side effects and serious patient compliance problems. Collectively, this calls for the development of novel drug targets for treating depressed and schizophrenic patients. Here, we discuss recent translational advances, research tools and approaches, aiming to facilitate innovative drug discovery in this field. Providing a comprehensive overview of current antidepressants and antipsychotic drugs, we also outline potential novel molecular targets for treating depression and schizophrenia. We also critically evaluate multiple translational challenges and summarize various open questions, in order to foster further integrative cross-discipline research into antidepressant and antipsychotic drug development.

Dopamine D2 receptors in nucleus accumbens cholinergic interneurons increase impulsive choice

Impulsive choice, often characterized by excessive preference for small, short-term rewards over larger, long-term rewards, is a prominent feature of substance use and other neuropsychiatric disorders. The neural mechanisms underlying impulsive choice are not well understood, but growing evidence implicates nucleus accumbens (NAc) dopamine and its actions on dopamine D2 receptors (D2Rs). Because several NAc cell types and afferents express D2Rs, it has been difficult to determine the specific neural mechanisms linking NAc D2Rs to impulsive choice. Of these cell types, cholinergic interneurons (CINs) of the NAc, which express D2Rs, have emerged as key regulators of striatal output and local dopamine release. Despite these relevant functions, whether D2Rs expressed specifically in these neurons contribute to impulsive choice behavior is unknown. Here, we show that D2R upregulation in CINs of the mouse NAc increases impulsive choice as measured in a delay discounting task without affecting reward magnitude sensitivity or interval timing. Conversely, mice lacking D2Rs in CINs showed decreased delay discounting. Furthermore, CIN D2R manipulations did not affect probabilistic discounting, which measures a different form of impulsive choice. Together, these findings suggest that CIN D2Rs regulate impulsive decision-making involving delay costs, providing new insight into the mechanisms by which NAc dopamine influences impulsive behavior.

Appetitive Motivation and Associated Neurobiology Change Differentially across the Life Course of Mouse Offspring Exposed to Peri- and Postnatal High Fat Feeding

Alterations in neural pathways that regulate appetitive motivation may contribute to increased obesity risk in offspring born to mothers fed a high fat (HF) diet. However, current findings on the impact of maternal obesity on motivation in offspring are inconclusive, and there is no information about the long-lasting effects in aged animals. This study examined the longitudinal effect of perinatal and chronic postnatal HF intake on appetitive motivation in young and aged offspring. Female C57Bl/6 were fed either a control (C) or HF diet before mating through to lactation. At weaning, offspring were maintained on the C or HF diet, generating the following four diet groups: C/C, C/HF, HF/C, and HF/HF based on the pre/post weaning diet. At 6 months, motivation was higher in HF/C females, but lower in male and female C/HF and HF/HF mice. By 12 months, this difference was lost, as C-fed animals became less motivated, while motivation increased in HF-fed mice. The mRNA levels of dopamine receptor 1 and 2 increased with age, while cannabinoid receptor 1 and μ-opioid receptor expression remained stable or decreased in mesolimbic and mesocortical dopaminergic pathways. Results from this study suggest that perinatal and chronic postnatal HF feeding produced opposite effects on appetitive motivation in young adult offspring mice, which was also reflected in the shift in motivation over time. These results have significant implications for patterns of hedonic eating across the life course and the relative risk of obesity at different time points.

Dysbindin-1A modulation of astrocytic dopamine and basal ganglia dependent behaviors relevant to schizophrenia

The mechanisms underlying the dichotomic cortical/basal ganglia dopaminergic abnormalities in schizophrenia are unclear. Astrocytes are important non-neuronal modulators of brain circuits, but their role in dopaminergic system remains poorly explored. Microarray analyses, immunohistochemistry, and two-photon laser scanning microscopy revealed that Dys1 hypofunction increases the reactivity of astrocytes, which express only the Dys1A isoform. Notably, behavioral and electrochemical assessments in mice selectively lacking the Dys1A isoform unraveled a more prominent impact of Dys1A in behavioral and dopaminergic/D2 alterations related to basal ganglia, but not cortical functioning. Ex vivo electron microscopy and protein expression analyses indicated that selective Dys1A disruption might alter intracellular trafficking in astrocytes, but not in neurons. In agreement, Dys1A disruption only in astrocytes resulted in decreased motivation and sensorimotor gating deficits, increased astrocytic dopamine D2 receptors and decreased dopaminergic tone within basal ganglia. These processes might have clinical relevance because the caudate, but not the cortex, of patients with schizophrenia shows a reduction of the Dys1A isoform. Therefore, we started to show a hitherto unknown role for the Dys1A isoform in astrocytic-related modulation of basal ganglia behavioral and dopaminergic phenotypes, with relevance to schizophrenia.

Dysregulation of dopamine neurotransmission in the nucleus accumbens in immobilization-induced hypersensitivity

Cast immobilization causes sensory hypersensitivity, which is also a symptom of neuropathic pain and chronic pain. However, the mechanisms underlying immobilization-induced hypersensitivity remain unclear. The present study investigated the role of dopamine neurotransmission in the nucleus accumbens shell (NAcSh) of rats with cast immobilization-induced mechanical hypersensitivity using in vivo microdialysis. Cast immobilization of the hind limb decreased the paw withdrawal threshold (PWT). Mechanical stimulation of the cast-immobilized hind limb induced a decrease in dopamine in the NAcSh, and this decrease was associated with the upregulation of presynaptic D2-like receptors. A D2-like receptor antagonist infused into the NAcSh reversed the decrease in PWT in rats with cast immobilization, whereas a D2-like receptor agonist infused into the NAcSh induced a decrease in PWT in control rats. In addition, the expression of the D2 receptor (Drd2) mRNA in the NAcSh was increased by cast immobilization. Importantly, systemic administration of the D2-like receptor antagonist reversed the decrease in PWT in rats with cast immobilization. As dopamine levels regulated by presynaptic D2-like receptors did not correlate with the PWT, it is presumed that the D2-like receptor antagonist or agonist acts on postsynaptic D2-like receptors. These results suggest that immobilization-induced mechanical hypersensitivity is attributable to the upregulation of postsynaptic D2-like receptors in the NAc. Blockade of D2-like receptors in the NAcSh is a potential therapeutic strategy for immobilization-induced hypersensitivity.

Motivational disturbances in rodent models of neuropsychiatric disorders

Motivated behavior is integral to the survival of individuals, continuously directing actions toward rewards or away from punishments. The orchestration of motivated behavior depends on interactions among different brain circuits, primarily within the dopaminergic system, that subserve the analysis of factors such as the effort necessary for obtaining the reward and the desirability of the reward. Impairments in motivated behavior accompany a wide range of neuropsychiatric disorders, decreasing the patients’ quality of life. Despite its importance, motivation is often overlooked as a parameter in neuropsychiatric disorders. Here, we review motivational impairments in rodent models of schizophrenia, depression, and Parkinson’s disease, focusing on studies investigating effort-related behavior in operant conditioning tasks and on pharmacological interventions targeting the dopaminergic system. Similar motivational disturbances accompany these conditions, suggesting that treatments aimed at ameliorating motivation levels may be beneficial for various neuropsychiatric disorders.

Vigor, Effort-Related Aspects of Motivation and Anhedonia

In this chapter we provide an overview of the pharmacological and circuit mechanisms that determine the willingness to expend effort in pursuit of rewards. A particular focus will be on the role of the mesolimbic dopamine system, as well the contributing roles of limbic and cortical brains areas involved in the evaluation, selection, and invigoration of goal-directed actions. We begin with a review of preclinical studies, which have provided key insights into the brain systems that are necessary and sufficient for effort-based decision-making and have characterized novel compounds that enhance selection of high-effort activities. Next, we summarize translational studies identifying and expanding this circuitry in humans. Finally, we discuss the relevance of this work for understanding common motivational impairments as part of the broader anhedonia symptom domain associated with mental illness, and the identification of new treatment targets within this circuitry to improve motivation and effort-expenditure.

Ramping activity in the striatum

Control of the timing of behavior is thought to require the basal ganglia (BG) and BG pathologies impair performance in timing tasks. Temporal interval discrimination depends on the ramping activity of medium spiny neurons (MSN) in the main BG input structure, the striatum, but the underlying mechanisms driving this activity are unclear. Here, we combine an MSN dynamical network model with an action selection system applied to an interval discrimination task. We find that when network parameters are appropriate for the striatum so that slowly fluctuating marginally stable dynamics are intrinsically generated, up and down ramping populations naturally emerge which enable significantly above chance task performance. We show that emergent population activity is in very good agreement with empirical studies and discuss how MSN network dysfunction in disease may alter temporal perception.

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.

Not All Mice Are Created Equal: Interval Timing Accuracy and Scalar Timing in 129, Swiss-Webster, and C57BL/6 Mice

Many species, including humans, show both accurate timing − appropriate time estimation in the seconds to minutes range − and scalar timing − time estimation error varies linearly with estimated duration. Behavioral paradigms aimed at investigating interval timing are expected to evaluate these dissociable characteristics of timing. However, when evaluating interval timing in models of neuropsychiatric disease, researchers are confronted with a lack of adequate studies about the parent (background) strains, since accuracy and scalar timing have only been demonstrated for the C57BL/6 strain of mice (Buhusi, Aziz, Winslow, Carter, Swearingen, & Buhusi (2009) Behav. Neurosci., 123, 1102–1113). We used a peak-interval (PI) procedure with three intervals − a protocol in which other species, including humans, demonstrate accurate, scalar timing − to evaluate timing accuracy and scalar timing in three strains of mice frequently used in genetic and behavioral studies: 129, Swiss-Webster (SW), and C57BL/6. C57BL/6 mice showed accurate, scalar timing, while 129 and SW mice showed departures from accuracy and/or scalar timing. Results suggest that the genetic background/strain of the mouse is a critical variable for studies investigating interval timing in genetically engineered mice. Our study validates the PI procedure with multiple intervals as a proper technique, and the C57BL/6 strain as the most suitable genetic background to date for behavioral investigations of interval timing in genetically engineered mice modeling human disorders. In contrast, studies using mice in 129, SW, or mixed-background strains should be interpreted with caution, and thorough investigations of accuracy and scalar timing should be conducted before a less studied strain of mouse is considered for use in timing studies.

Secondary auditory cortex mediates a sensorimotor mechanism for action timing

The ability to accurately determine when to perform an action is a fundamental brain function and vital to adaptive behavior. The behavioral mechanism and neural circuit for action timing, however, remain largely unknown. Using a new, self-paced action timing task in mice, we found that deprivation of auditory, but not somatosensory or visual input, disrupts learned action timing. The hearing effect was dependent on the auditory feedback derived from the animal's own actions, rather than passive environmental cues. Neuronal activity in the secondary auditory cortex was found to be both correlated with and necessary for the proper execution of learned action timing. Closed-loop, action-dependent optogenetic stimulation of the specific task-related neuronal population within the secondary auditory cortex rescued the key features of learned action timing under auditory deprivation. These results unveil a previously underappreciated sensorimotor mechanism in which the secondary auditory cortex transduces self-generated audiomotor feedback to control action timing.

Impulsivity and risk-seeking as Bayesian inference under dopaminergic control

Bayesian models successfully account for several of dopamine (DA)'s effects on contextual calibration in interval timing and reward estimation. In these models, tonic levels of DA control the precision of stimulus encoding, which is weighed against contextual information when making decisions. When DA levels are high, the animal relies more heavily on the (highly precise) stimulus encoding, whereas when DA levels are low, the context affects decisions more strongly. Here, we extend this idea to intertemporal choice and probability discounting tasks. In intertemporal choice tasks, agents must choose between a small reward delivered soon and a large reward delivered later, whereas in probability discounting tasks, agents must choose between a small reward that is always delivered and a large reward that may be omitted with some probability. Beginning with the principle that animals will seek to maximize their reward rates, we show that the Bayesian model predicts a number of curious empirical findings in both tasks. First, the model predicts that higher DA levels should normally promote selection of the larger/later option, which is often taken to imply that DA decreases 'impulsivity,' and promote selection of the large/risky option, often taken to imply that DA increases 'risk-seeking.' However, if the temporal precision is sufficiently decreased, higher DA levels should have the opposite effect-promoting selection of the smaller/sooner option (higher impulsivity) and the small/safe option (lower risk-seeking). Second, high enough levels of DA can result in preference reversals. Third, selectively decreasing the temporal precision, without manipulating DA, should promote selection of the larger/later and large/risky options. Fourth, when a different post-reward delay is associated with each option, animals will not learn the option-delay contingencies, but this learning can be salvaged when the post-reward delays are made more salient. Finally, the Bayesian model predicts correlations among behavioral phenotypes: Animals that are better timers will also appear less impulsive.

How changes in dopamine D2 receptor levels alter striatal circuit function and motivation

It was first posited, more than five decades ago, that the etiology of schizophrenia involves overstimulation of dopamine receptors. Since then, advanced clinical research methods, including brain imaging, have refined our understanding of the relationship between striatal dopamine and clinical phenotypes as well as disease trajectory. These studies point to striatal dopamine D2 receptors, the main target for all current antipsychotic medications, as being involved in both positive and negative symptoms. Simultaneously, animal models have been central to investigating causal relationships between striatal dopamine D2 receptors and behavioral phenotypes relevant to schizophrenia. We begin this article by reviewing the circuit, cell-type and subcellular locations of dopamine D2 receptors and their downstream signaling pathways. We then summarize results from several mouse models in which D2 receptor levels were altered in various brain regions, cell-types and developmental periods. Behavioral, electrophysiological and anatomical consequences of these D2 receptor perturbations are reviewed with a selective focus on striatal circuit function and alterations in motivated behavior, a core negative symptom of schizophrenia. These studies show that D2 receptors serve distinct physiological roles in different cell types and at different developmental time points, regulating motivated behaviors in sometimes opposing ways. We conclude by considering the clinical implications of this complex regulation of striatal circuit function by D2 receptors.

Cortical alpha-synuclein preformed fibrils do not affect interval timing in mice

One hallmark feature of Parkinson's disease (PD) is Lewy body pathology associated with misfolded alpha-synuclein. Previous studies have shown that striatal injection of alpha-synuclein preformed fibrils (PFF) can induce misfolding and aggregation of native alpha-synuclein in a prion-like manner, leading to cell death and motor dysfunction in mouse models. Here, we tested whether alpha-synuclein PFFs injected into the medial prefrontal cortex results in deficits in interval timing, a cognitive task which is disrupted in human PD patients and in rodent models of PD. We injected PFF or monomers of human alpha-synuclein into the medial prefrontal cortex of mice pre-injected with adeno-associated virus (AAV) coding for overexpression of human alpha-synuclein or control protein. Despite notable medial prefrontal cortical synucleinopathy, we did not observe consistent deficits in fixed-interval timing. These results suggest that cortical alpha-synuclein does not reliably disrupt fixed-interval timing.

Dopamine and the interdependency of time perception and reward

Time is a fundamental dimension of our perception of the world and is therefore of critical importance to the organization of human behavior. A corpus of work - including recent optogenetic evidence - implicates striatal dopamine as a crucial factor influencing the perception of time. Another stream of literature implicates dopamine in reward and motivation processes. However, these two domains of research have remained largely separated, despite neurobiological overlap and the apothegmatic notion that "time flies when you're having fun". This article constitutes a review of the literature linking time perception and reward, including neurobiological and behavioral studies. Together, these provide compelling support for the idea that time perception and reward processing interact via a common dopaminergic mechanism.

Disruptions in effort-based decision-making following acute optogenetic stimulation of ventral tegmental area dopamine cells

The dopamine system has been implicated in decision-making particularly when associated with effortful behavior. We examined acute optogenetic stimulation of dopamine cells in the ventral tegmental area (VTA) as mice engaged in an effort-based decision-making task. Tyrosine hydroxylase-Cre mice were injected with Cre-dependent ChR2 or eYFP control virus in the VTA. While eYFP control mice showed effortful discounting, stimulation of dopamine cells in ChR2 mice disrupted effort-based decision-making by reducing choice toward the lever associated with a preferred outcome and greater effort. Surprisingly, disruptions in effortful discounting were observed in subsequent test sessions conducted in the absence of optogenetic stimulation, however during these sessions ChR2 mice displayed enhanced high choice responding across trial blocks. These findings suggest increases in VTA dopamine cell activity can disrupt effort-based decision-making in distinct ways dependent on the timing of optogenetic stimulation.

Rational inattention and tonic dopamine

Slow-timescale (tonic) changes in dopamine (DA) contribute to a wide variety of processes in reinforcement learning, interval timing, and other domains. Furthermore, changes in tonic DA exert distinct effects depending on when they occur (e.g., during learning vs. performance) and what task the subject is performing (e.g., operant vs. classical conditioning). Two influential theories of tonic DA-the average reward theory and the Bayesian theory in which DA controls precision-have each been successful at explaining a subset of empirical findings. But how the same DA signal performs two seemingly distinct functions without creating crosstalk is not well understood. Here we reconcile the two theories under the unifying framework of 'rational inattention,' which (1) conceptually links average reward and precision, (2) outlines how DA manipulations affect this relationship, and in so doing, (3) captures new empirical phenomena. In brief, rational inattention asserts that agents can increase their precision in a task (and thus improve their performance) by paying a cognitive cost. Crucially, whether this cost is worth paying depends on average reward availability, reported by DA. The monotonic relationship between average reward and precision means that the DA signal contains the information necessary to retrieve the precision. When this information is needed after the task is performed, as presumed by Bayesian inference, acute manipulations of DA will bias behavior in predictable ways. We show how this framework reconciles a remarkably large collection of experimental findings. In reinforcement learning, the rational inattention framework predicts that learning from positive and negative feedback should be enhanced in high and low DA states, respectively, and that DA should tip the exploration-exploitation balance toward exploitation. In interval timing, this framework predicts that DA should increase the speed of the internal clock and decrease the extent of interference by other temporal stimuli during temporal reproduction (the central tendency effect). Finally, rational inattention makes the new predictions that these effects should be critically dependent on the controllability of rewards, that post-reward delays in intertemporal choice tasks should be underestimated, and that average reward manipulations should affect the speed of the clock-thus capturing empirical findings that are unexplained by either theory alone. Our results suggest that a common computational repertoire may underlie the seemingly heterogeneous roles of DA.

Endocannabinoid modulation of dopamine release during reward seeking, interval timing, and avoidance

Endocannabinoids (eCBs) are neuromodulators that influence a wide range of neural systems and behaviors. In the current review, we describe our recent research showing how eCBs, particularly 2-arachidonoylglycerol (2-AG), concurrently shape mesolimbic dopamine (DA) release and associated behavior. We will restrict our discussion by emphasizing three distinct behaviors: reward seeking, interval timing, and active avoidance. During reward seeking we find that 2-AG is necessary to observe cue-evoked DA release events that are thought to represent the value of a rewarding outcome. We then describe data showing that 2-AG modulates unique patterns of DA release and behavior observed under conditions of periodic reinforcement. These data are discussed within the context of interval timing and adjunctive behavior. eCB modulation of DA release is also implicated in defensive behavior, including the avoidance of harm. As in reward seeking, our data suggest that the concentration of DA that is evoked by a warning signal can represent the value of an avoidance outcome. And, disrupting eCB signaling concomitantly reduces the concentration of the avoidance value signal and active avoidance. Disruptions in reward seeking, interval timing, and defensive behavior are commonly observed in a variety of movement disorders (e.g., Parkinson's and Huntington's disease) and disorders of motivation (e.g., addiction). We believe our data on eCB-DA interactions have implications for the development of novel pharmacotherapies to treat these disorders. Thus, we conclude by discussing how eCB pharmacology might be harnessed to treat disorders of movement and motivation.

Timing behavior in genetic murine models of neurological and psychiatric diseases

How timing behavior is altered in different neurodevelopmental and neurodegenerative disorders is a contemporary research question. Genetic murine models (GMM) that offer high construct validity also serve as useful tools to investigate this question. But the literature on timing behavior of different GMMs largely remains to be consolidated. The current paper addresses this gap by reviewing studies that have been conducted with GMMs of neurodevelopmental (e.g. ADHD, schizophrenia, autism spectrum disorder), neurodegenerative disorders (e.g., Alzheimer's disease, Huntington's disease) as well as circadian and other mutant lines. The review focuses on those studies that specifically utilized the peak interval procedure to improve the comparability of findings both within and between different disease models. The reviewed studies revealed timing deficits that are characteristic of different disorders. Specifically, Huntington's disease models had weaker temporal control over the termination of their anticipatory responses, Alzheimer's disease models had earlier timed responses, schizophrenia models had weaker temporal control, circadian mutants had shifted timed responses consistent with shifts in the circadian periods. The differences in timing behavior were less consistent for other conditions such as attention deficit and hyperactivity disorder and mutations related to intellectual disability. We discuss the implications of these findings for the neural basis of an internal stopwatch. Finally, we make methodological recommendations for future research for improving the comparability of the timing behavior across different murine models.

A role for reward valuation in the serotonergic modulation of impulsivity

RATIONALE

Impulsive behavior is a deleterious component of a number of mental health disorders but has few targeted pharmacotherapies. One contributing factor to the difficulty in understanding the neural substrates of disordered impulsivity is the diverse presentations of impulsive behavior. Defining the behavioral and cognitive processes which contribute to different subtypes of impulsivity is important for understanding the neural underpinnings of dysregulated impulsive behavior.

METHODS

Using a mouse model for disordered impulsivity, our goal was to identify behavioral and cognitive processes that are associated with increased impulsivity. Specifically, we were interested in the facets of impulsivity modulated by serotonin signaling. We used mice lacking the serotonin 1B receptor (5-HT_1BR) and measured different types of impulsivity as well as goal-directed responding, extinction, habitual-like behavior, cue reactivity, and reward reactivity.

RESULTS

Mice lacking expression of 5-HT_1BR had increased levels of impulsive action, goal-directed responding, and motivation, with no differences seen in rate of extinction, development of habitual behavior, delay discounting, or effort-based discounting. Interestingly, mice lacking 5-HT_1BR expression also showed an overall increase in the choice of higher value rewards, increased hedonic responses to sweet rewards, and responded more for cues that predict reward. We developed a novel paradigm to demonstrate that increasing anticipated reward value could directly increase impulsive action. Furthermore, we found that 5-HT_1BR KO-induced impulsivity could be ameliorated by decreasing the reward value relative to controls, suggesting that the increased 5-HT_1BR-associated impulsive action may be a result of increased reward valuation.

CONCLUSIONS

Taken together, these data show that the effects of serotonin on impulsive action are mediated through the modulation of hedonic value, which may alter the reward representations that motivate action. Overall, this data supports a role for reward value as an important substrate in impulsive action which may drive clinically relevant increases in impulsivity.

Dissociating the effects of dopamine D2 receptors on effort-based versus value-based decision making using a novel behavioral approach

Cost-benefit decision making is essential for organisms to adapt to their ever-changing environment. Most studies of cost-benefit decision making involve choice conditions in which effort and value are varied simultaneously. This prevents identification of the aspects of cost-benefit decision making that are affected by experimental manipulations. We developed operant assays to isolate the individual impacts of effort and value manipulations on cost-benefit decision making. In the concurrent effort choice (CEC) task, mice choose between exerting two distinct types of effort: the number of responses and the duration of a response, to earn the same reward. By parametrically varying response cost, psychometric functions are obtained that reflect how the two types of effort scale against one another. Direct manipulations of effort shift the functions. Because reward value is held constant in this task, differences in scaling of the two response types must be related to the effort manipulations. In the concurrent value choice (CVC) task, mice make the same type of response to earn rewards of different value (e.g., pellets vs. sucrose solutions). Here the effort required to earn one reward type is parametrically varied to obtain the psychometric function that scales the value of the two rewards into the number of responses subjects will pay to earn one reward over the other. Direct value manipulations shift these functions. We tested the effect of the dopamine D2 receptor antagonist, haloperidol, on performance in the CEC and CVC assays and found that D2R signaling is important for effort-based, but not value-based decision making. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Circadian modulation of motivation in mice

Most living organisms have a circadian timing system adapted to optimize the daily rhythm of exposure to the environment. This circadian system modulates several behavioral and physiological processes, including the response to natural and drug rewards. Food is the most potent natural reward across species. Food-seeking is known to be mediated by dopaminergic and serotonergic transmission in cortico-limbic pathways. In the present work, we show evidence of a circadian modulation of motivation for food reward in young (4-months old) and aged (over 1.5 years old) C57BL/6 mice. Motivation was assayed through the progressive ratio (PR) schedule. Mice under a 12:12 light/dark (LD) cycle exhibited a diurnal rhythm in motivation, becoming more motivated during the night, coincident with their active phase. This rhythm was also evident under constant dark conditions, indicating the endogenous nature of this modulation. However, circadian arrhythmicity induced by chronic exposure to constant light conditions impaired the performance in the task causing low motivation levels. Furthermore, the day/night difference in motivation was also evident even without caloric restriction when using a palatable reward. All these results were found to be unaffected by aging. Taken together, our results indicate that motivation for food reward is regulated in a circadian manner, independent of the nutritional status and the nature of the reward, and that this rhythmic modulation is not affected by aging. These results may contribute to improve treatment related to psychiatric disorders or drugs of abuse, taking into account potential mechanisms of circadian modulation of motivational states.

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.

A Touchscreen Motivation Assessment Evaluated in Huntington's Disease Patients and R6/1 Model Mice

Apathy is pervasive across many neuropsychiatric disorders but is poorly characterized mechanistically, so targeted therapeutic interventions remain elusive. A key impediment has been the lack of validated assessment tools to facilitate translation of promising findings between preclinical disease models and patients. Apathy is a common symptom in Huntington's disease. Due to its established genetic basis and the availability of defined animal models, this disease offers a robust translational framework for linking motivated behavior with underlying neurobiology and an ideal context in which to evaluate a quantitative, translational apathy assessment method. In this study we therefore aimed to demonstrate the validity of using touchscreen-delivered progressive ratio tasks to mirror apathy assessment in Huntington's disease patients and a representative mouse model. To do this we evaluated Huntington's disease patients (n = 23) and age-matched healthy controls (n = 20), and male R6/1 mice (n = 23) and wildtype controls (n = 29) for apathy-like behavior using touchscreen-delivered progressive ratio tasks. The primary outcome measure of the assessment was breakpoint, defined as the highest number of touchscreen responses emitted before task engagement ceased. Patients and R6/1 mice were both found to exhibit significantly reduced breakpoints relative to their respective control groups, consistent with apathy-like behavior. This performance was also not associated with motoric differences in either species. These data demonstrate the utility of touchscreen-delivered progressive ratio tasks in detecting clinically relevant motivational deficits in Huntington's disease. This approach may offer a platform from which clinically relevant mechanistic insights concerning motivation symptoms can be derived and provide an effective route for translation of promising preclinical findings into viable therapeutic interventions.

The Neural Basis of Timing: Distributed Mechanisms for Diverse Functions

Timing is critical to most forms of learning, behavior, and sensory-motor processing. Converging evidence supports the notion that, precisely because of its importance across a wide range of brain functions, timing relies on intrinsic and general properties of neurons and neural circuits; that is, the brain uses its natural cellular and network dynamics to solve a diversity of temporal computations. Many circuits have been shown to encode elapsed time in dynamically changing patterns of neural activity-so-called population clocks. But temporal processing encompasses a wide range of different computations, and just as there are different circuits and mechanisms underlying computations about space, there are a multitude of circuits and mechanisms underlying the ability to tell time and generate temporal patterns.

Emerging roles of striatal dopamine D2 receptors in motivated behaviour: Implications for psychiatric disorders

Impaired motivation has been a long recognized negative symptom of schizophrenia, as well as a common feature of non-psychotic psychiatric disorders, responsible for a significant share of functional burden, and with limited treatment options. The striatum and dopamine signalling system play a central role in extracting motivationally relevant information from the environment, selecting which behavioural direction the animal should follow, and the vigour with which to engage it. Much of this function relies on striatal projection neurons, known as medium spiny neurons (MSNs) expressing dopamine D2 receptors (D2Rs), or D2-MSNs. However, determining the precise nature of D2-MSNs in regulating motivated behaviour in both healthy individuals and experimental manipulations of D2-MSN function has at times yielded a somewhat confusing picture since their activity has been linked to either enhancement or dampening of motivation in animal models. In this MiniReview, we describe the latest data from rodent studies that investigated how D2Rs exert their modulatory effect on motivated behaviour by regulating striatal indirect pathway neuronal activity. We will include a discussion about how functional selectivity of D2Rs towards G protein-dependent or β-arrestin-dependent signalling differentially affects motivated behaviour. Lastly, we will describe a recent preclinical attempt to improve motivation by exploiting serotonin receptor-mediated modulation of dopamine transmission in the striatum.

Adapting the flow of time with dopamine

The modulation of interval timing by dopamine (DA) has been well established over decades of research. The nature of this modulation, however, has remained controversial: Although the pharmacological evidence has largely suggested that time intervals are overestimated with higher DA levels, more recent optogenetic work has shown the opposite effect. In addition, a large body of work has asserted DA's role as a "reward prediction error" (RPE), or a teaching signal that allows the basal ganglia to learn to predict future rewards in reinforcement learning tasks. Whether these two seemingly disparate accounts of DA may be related has remained an open question. By taking a reinforcement learning-based approach to interval timing, we show here that the RPE interpretation of DA naturally extends to its role as a modulator of timekeeping and furthermore that this view reconciles the seemingly conflicting observations. We derive a biologically plausible, DA-dependent plasticity rule that can modulate the rate of timekeeping in either direction and whose effect depends on the timing of the DA signal itself. This bidirectional update rule can account for the results from pharmacology and optogenetics as well as the behavioral effects of reward rate on interval timing and the temporal selectivity of striatal neurons. Hence, by adopting a single RPE interpretation of DA, our results take a step toward unifying computational theories of reinforcement learning and interval timing. NEW & NOTEWORTHY How does dopamine (DA) influence interval timing? A large body of pharmacological evidence has suggested that DA accelerates timekeeping mechanisms. However, recent optogenetic work has shown exactly the opposite effect. In this article, we relate DA's role in timekeeping to its most established role, as a critical component of reinforcement learning. This allows us to derive a neurobiologically plausible framework that reconciles a large body of DA's temporal effects, including pharmacological, behavioral, electrophysiological, and optogenetic.

Dorsal striatum dopamine oscillations: Setting the pace of food anticipatory activity

Predicting the uncertainties of the ever-changing environment provides a competitive advantage for animals. The need to anticipate food sources has provided a strong evolutionary drive for synchronizing behavioural and internal processes with daily circadian cycles. When food is restricted to a few hours per day, rodents exhibit increased wakefulness and foraging behaviour preceding the arrival of food. Interestingly, while the master clock located in the suprachiasmatic nucleus entrains daily rhythms to the light cycle, it is not necessary for this food anticipatory activity. This suggests the existence of a food-entrained oscillator located elsewhere. Based on the role of nigrostriatal dopamine in reward processing, motor function, working memory and internal timekeeping, we propose a working model by which food-entrained dopamine oscillations in the dorsal striatum can enable animals maintained on a restricted feeding schedule to anticipate food arrival. Finally, we summarize how metabolic signals in the gut are conveyed to the nigrostriatal pathway to suggest possible insight into potential input mechanisms for food anticipatory activity.

Overexpression of striatal D2 receptors reduces motivation thereby decreasing food anticipatory activity

Dopamine has been implicated in circadian timing underlying the food entrainable oscillator (FEO) circuitry and overexpression of the dopamine D2 receptor (D2R) in the striatum has been reported to reduce motivation to obtain food rewards in operant tasks. In the present study, we explored both of these mechanisms by examining food anticipatory activity (FAA) in dopamine D2 receptor-overexpressing (D2R-OE) mice under various durations of food availability. First, we noted that at baseline, there were no differences between D2R-OE mice and their littermates in activity level, food intake, and body weight or in circadian activity. Under conditions of very restricted food availability (4 or 6 hr), both genotypes displayed FAA. In contrast, under 8-hr food availability, control mice showed FAA, but D2R-OE mice did not. Normalization of D2R by administration of doxycycline, a tetracycline analogue, rescued FAA under 8-hr restricted food. We next tested for circadian regulation of FAA. When given ad libitum access to food, neither D2R-OE nor controls were active during the daytime. However, after an interval of food restriction, all mice showed elevated locomotor activity at the time of previous food availability in the day, indicating circadian timing of anticipatory activity. In summary, motivation is reduced in D2R-OE mice but circadian timing behavior is not affected. We conclude that an increase in striatal D2R reduces FAA by modulating motivation and not by acting on a clock mechanism.

Striatal dopamine D2 receptors regulate effort but not value-based decision making and alter the dopaminergic encoding of cost

Deficits in goal-directed motivation represent a debilitating symptom for many patients with schizophrenia. Impairments in motivation can arise from deficits in processing information about effort and or value, disrupting effective cost-benefit decision making. We have previously shown that upregulated dopamine D2 receptor expression within the striatum (D2R-OE mice) decreases goal-directed motivation. Here, we determine the behavioral and neurochemical mechanisms behind this deficit. Female D2R-OE mice were tested in several behavioral paradigms including recently developed tasks that independently assess the impact of Value or Effort manipulations on cost-benefit decision making. In vivo microdialysis was used to measure extracellular dopamine in the striatum during behavior. In a value-based choice task, D2R-OE mice show normal sensitivity to changes in reward value and used reward value to guide their actions. In an effort-based choice task, D2R-OE mice evaluate the cost of increasing the number of responses greater relative to the effort cost of longer duration responses compared to controls. This shift away from choosing to repeatedly execute a response is accompanied by a dampening of extracellular dopamine in the striatum during goal-directed behavior. In the ventral striatum, extracellular dopamine level negatively correlates with response cost in controls, but this relationship is lost in D2R-OE mice. These results show that D2R signaling in the striatum, as observed in some patients with schizophrenia, alters the relationship between effort expenditure and extracellular dopamine. This dysregulation produces motivation deficits that are specific to effort but not value-based decision making, paralleling the effort-based motivational deficits observed in schizophrenia.

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.

Striatal dopamine 2 receptor upregulation during development predisposes to diet-induced obesity by reducing energy output in mice

Dopaminergic signaling in the striatum, particularly at dopamine 2 receptors (D2R), has been a topic of active investigation in obesity research in the past decades. However, it still remains unclear whether variations in striatal D2Rs modulate the risk for obesity and if so in which direction. Human studies have yielded contradictory findings that likely reflect a complex nonlinear relationship, possibly involving a combination of causal effects and compensatory changes. Animal work indicates that although chronic obesogenic diets reduce striatal D2R function, striatal D2R down-regulation does not lead to obesity. In this study, we evaluated the consequences of striatal D2R up-regulation on body-weight gain susceptibility and energy balance in mice. We used a mouse model of D2R overexpression (D2R-OE) in which D2Rs were selectively up-regulated in striatal medium spiny neurons. We uncover a pathological mechanism by which striatal D2R-OE leads to reduced brown adipose tissue thermogenesis, reduced energy expenditure, and accelerated obesity despite reduced eating. We also show that D2R-OE restricted to development is sufficient to promote obesity and to induce energy-balance deficits. Together, our findings indicate that striatal D2R-OE during development persistently increases the propensity for obesity by reducing energy output in mice. This suggests that early alterations in the striatal dopamine system could represent a key predisposition factor toward obesity.

Striatal dopamine and the temporal control of behavior

Striatal dopamine strongly regulates how individuals use time to guide behavior. Dopamine acts on D1- and D2- dopamine receptors in the striatum. However, the relative role of these receptors in the temporal control of behavior is unclear. To assess this, we trained rats on a task in which they decided to start and stop a series of responses based on the passage of time and evaluated how blocking D1 or D2-dopamine receptors in the dorsomedial or dorsolateral striatum impacted performance. D2 blockade delayed the decision to start and stop responding in both regions, and this effect was larger in the dorsomedial striatum. By contrast, dorsomedial D1 blockade delayed stop times, without significantly delaying start times, whereas dorsolateral D1 blockade produced no detectable effects. These findings suggest that striatal dopamine may tune decision thresholds during timing tasks. Furthermore, our data indicate that the dorsomedial striatum plays a key role in temporal control, which may be useful for localizing neural circuits that mediate the temporal control of action.

Genetic influence alters the brain synchronism in perception and timing

Background

Studies at the molecular level aim to integrate genetic and neurobiological data to provide an increasingly detailed understanding of phenotypes related to the ability in time perception.

Main Text

This study suggests that the polymorphisms genetic SLC6A4 5-HTTLPR, 5HTR2A T102C, DRD2/ANKK1-Taq1A, SLC6A3 3’-UTR VNTR, COMT Val158Met, CLOCK genes and GABRB2 A/C as modification factor at neurochemical levels associated with several neurofunctional aspects, modifying the circadian rhythm and built-in cognitive functions in the timing. We conducted a literature review with 102 studies that met inclusion criteria to synthesize findings on genetic polymorphisms and their influence on the timing.

Conclusion

The findings suggest an association of genetic polymorphisms on behavioral aspects related in timing. However, order to confirm the paradigm of association in the timing as a function of the molecular level, still need to be addressed future research.

Impaired recruitment of dopamine neurons during working memory in mice with striatal D2 receptor overexpression

The dopamine (DA) system plays a major role in cognitive functions through its interactions with several brain regions including the prefrontal cortex (PFC). Conversely, disturbances in the DA system contribute to cognitive deficits in psychiatric diseases, yet exactly how they do so remains poorly understood. Here we show, using mice with disease-relevant alterations in DA signaling (D2R-OE mice), that deficits in working memory (WM) are associated with impairments in the WM-dependent firing patterns of DA neurons in the ventral tegmental area (VTA). The WM-dependent phase-locking of DA neurons to 4 Hz VTA-PFC oscillations is absent in D2R-OE mice and VTA-PFC synchrony deficits scale with their WM impairments. We also find reduced 4 Hz synchrony between VTA DA neurons and selective impairments in their representation of WM demand. These results identify how altered DA neuron activity—at the level of long-range network activity and task-related firing patterns—may underlie cognitive impairments.

Disrupted dopamine neuron firing is thought to contribute to cognitive dysfunction in psychiatric disorders. Here the authors show that mice overexpressing D2R in the striatum, commonly seen in schizophrenia, are also impaired in recruitment of dopamine neurons during working memory performance.

Timing Deficits in ADHD: Insights From the Neuroscience of Musical Rhythm

Everyday human behavior relies upon extraordinary feats of coordination within the brain. In this perspective paper, we argue that the rich temporal structure of music provides an informative context in which to investigate how the brain coordinates its complex activities in time, and how that coordination can be disrupted. We bring insights from the neuroscience of musical rhythm to considerations of timing deficits in Attention Deficit/Hyperactivity Disorder (ADHD), highlighting the significant overlap between neural systems involved in processing musical rhythm and those implicated in ADHD. We suggest that timing deficits warrant closer investigation since they could lead to the identification of potentially informative phenotypes, tied to neurobiological and genetic factors. Our novel interdisciplinary approach builds upon recent trends in both fields of research: in the neuroscience of rhythm, an increasingly nuanced understanding of the specific contributions of neural systems to rhythm processing, and in ADHD, an increasing focus on differentiating phenotypes and identifying distinct etiological pathways associated with the disorder. Finally, we consider the impact of musical experience on rhythm processing and the potential value of musical rhythm in therapeutic interventions.

Schizophrenia-Related Microdeletion Impairs Emotional Memory through MicroRNA-Dependent Disruption of Thalamic Inputs to the Amygdala

Individuals with 22q11.2 deletion syndrome (22q11DS) are at high risk of developing psychiatric diseases such as schizophrenia. Individuals with 22q11DS and schizophrenia are impaired in emotional memory, anticipating, recalling, and assigning a correct context to emotions. The neuronal circuits responsible for these emotional memory deficits are unknown. Here, we show that 22q11DS mouse models have disrupted synaptic transmission at thalamic inputs to the lateral amygdala (thalamo-LA projections). This synaptic deficit is caused by haploinsufficiency of the 22q11DS gene Dgcr8, which is involved in microRNA processing, and is mediated by the increased dopamine receptor Drd2 levels in the thalamus and by reduced probability of glutamate release from thalamic inputs. This deficit in thalamo-LA synaptic transmission is sufficient to cause fear memory deficits. Our results suggest that dysregulation of the Dgcr8–Drd2 mechanism at thalamic inputs to the amygdala underlies emotional memory deficits in 22q11DS.

•

Thalamic inputs to the lateral amygdala (LA) are impaired in 22q11DS mice

•

Thalamo-LA disruption is sufficient to cause associative fear memory deficits

•

Deficiency in microRNA-processing Dgcr8 causes thalamo-LA and fear memory deficits

•

Fear memory deficits in 22q11DS mice are rescued by thalamic Drd2 inhibition

Scalar timing in memory: A temporal map in the hippocampus

Many essential tasks, such as decision making, rate calculation and planning, require accurate timing in the second to minute range. This process, known as interval timing, involves many cortical areas such as the prefrontal cortex, the striatum, and the hippocampus. Although the neurobiological origin and the mechanisms of interval timing are largely unknown, we have developed increasingly accurate mathematical and computational models that can mimic some properties of time perception. The accepted paradigm of temporal durations storage is that the objective elapsed time from the short-term memory is transferred to the reference memory using a multiplicative "memory translation constant" K*. It is believed that K* has a Gaussian distribution due to trial-related variabilities. To understand K* genesis, we hypothesized that the storage of temporal memories follows a topological map in the hippocampus, with longer durations stored towards dorsal hippocampus and shorter durations stored toward ventral hippocampus. We found that selective removal of memory cells in this topological map model shifts the peak-response time in a manner consistent with the current experimental data on the effect of hippocampal lesions on time perception. This opens new avenues for experimental testing of our topological map hypothesis. We found numerically that the relative shift is determined both by the lesion size and its location and we suggested a theoretical estimate for the memory translation constant K*.

An Interaction between Serotonin Receptor Signaling and Dopamine Enhances Goal-Directed Vigor and Persistence in Mice

The functionally selective 5-HT2C receptor ligand SB242084 can increase motivation and have rapid onset anti-depressant-like effects. We sought to identify the specific behavioral effects of SB242084 treatment and elucidate the mechanism in female and male mice. Using a quantitative behavioral approach, we determined that SB242084 increases the vigor and persistence of goal-directed activity across different types of physical work, particularly when work requirements are demanding. We found this influence of SB242084 on effort, rather than reward to be reflected in striatal DA measured during behavior. Using in vivo fast scan cyclic voltammetry, we found that SB242084 has no effect on reward-related phasic DA release in the NAc. Using in vivo microdialysis to measure tonic changes in extracellular DA, we also found no changes in the NAc. In contrast, SB242084 treatment increases extracellular DA in the dorsomedial striatum, an area that plays a key role in response vigor. These findings have several implications. At the behavioral level, this work shows that the capacity to work in demanding situations can be increased, without a generalized increase in motor activity or reward value. At the circuit level, we identified a pathway restricted potentiation of DA release and showed that this was the reason for the increased response vigor. At the cellular level, we show that a specific serotonin receptor cross talks to the DA system. Together, this information provides promise for the development of treatments for apathy, a serious clinical condition that can afflict patients with psychiatric and neurological disorders.SIGNIFICANCE STATEMENT Motivated behaviors are modulated by reward value, effort demands, and cost-benefit computations. This information drives the decision to act, which action to select, and the intensity with which the selected action is performed. Because these behavioral processes are all regulated by DA signaling, it is very difficult to influence selected aspects of motivated behavior without affecting others. Here we identify a pharmacological treatment that increases the vigor and persistence of responding in mice, without increasing generalized activity or changing reactions to rewards. We show that the 5-HT2C-selective ligand boosts motivation by potentiating activity-dependent DA release in the dorsomedial striatum. These results reveal a novel strategy for treating patients with motivational deficits, avolition, or apathy.

Phencyclidine increased while isolation rearing did not affect progressive ratio responding in rats: Investigating potential models of amotivation in schizophrenia

BACKGROUND

Schizophrenia is a debilitating neurodevelopmental disorder affecting 1% of the global population with heterogeneous symptoms including positive, negative, and cognitive. While treatment for positive symptoms exists, none have been developed to treat negative symptoms. Animal models of schizophrenia are required to test targeted treatments and since patients exhibit reduced effort (breakpoints) for reward in a progressive ratio (PR) task, we examined the PR breakpoints of rats treated with the NMDA receptor antagonist phencyclidine or those reared in isolation - two common manipulations used to induce schizophrenia-relevant behaviors in rodents.

METHODS

In two cohorts, the PR breakpoint for a palatable food reward was examined in Long Evans rats after: 1) a repeated phencyclidine regimen; 2) A subchronic phencyclidine regimen followed by drug washout; and 3) post-weaning social isolation.

RESULTS

Rats treated with repeated phencyclidine and those following washout from phencyclidine exhibited higher PR breakpoints than vehicle-treated rats. The breakpoint of isolation reared rats did not differ from those socially reared, despite abnormalities of these rats in other schizophrenia-relevant behaviors.

CONCLUSION

Despite their common use for modeling other schizophrenia-relevant behaviors neither phencyclidine treatment nor isolation rearing recreated the motivational deficits observed in patients with schizophrenia, as measured by PR breakpoint. Other manipulations, and negative symptom-relevant behaviors, require investigation prior to testing putative therapeutics.

Deficits in temporal processing correlate with clinical progression in Huntington's disease

OBJECTIVES

Precise temporal performance is crucial for several complex tasks. Time estimation in the second-to-minutes range-known as interval timing-involves the interaction of the basal ganglia and the prefrontal cortex via dopaminergic-glutamatergic pathways. Patients with Huntington's disease (HD) present deficits in cognitive and motor functions that require fine control of temporal processing. The objective of the present work was to assess temporal cognition through a peak-interval time (PI) production task in patients with HD and its potential correlation with the Unified Huntington's Disease Rating Scale (UHDRS).

MATERIALS AND METHODS

Patients with molecular diagnosis of HD and controls matched by age, sex and educational level (n=18/group) were tested for interval timing in short- (3 seconds), medium- (6 seconds) and long (12 seconds)-duration stimuli.

RESULTS

Significant differences were observed in the PI task, with worse performance in HD compared to controls. Patients underestimated real time (left-shifted Peak location) for 6- and 12-second intervals (P<.05) and presented decreased temporal precision for all the intervals evaluated (P<.01). Importantly, a significant correlation was found between time performance and the UHDRS (P<.01). Patients' responses also deviated from the scalar property.

CONCLUSIONS

Our results contribute to support that timing functions are impaired in HD in correlation with clinical deterioration. Recordings of cognitive performance related to timing could be a potential useful tool to measure the neurodegenerative progression of movement disorder-related pathologies.

Functional coding variation in the presynaptic dopamine transporter associated with neuropsychiatric disorders drives enhanced motivation and context-dependent impulsivity in mice

Recent genetic analyses have provided evidence that clinical commonalities associated with different psychiatric diagnoses often have shared mechanistic underpinnings. The development of animal models expressing functional genetic variation attributed to multiple disorders offers a salient opportunity to capture molecular, circuit and behavioral alterations underlying this hypothesis. In keeping with studies suggesting dopaminergic contributions to attention-deficit hyperactivity disorder (ADHD), bipolar disorder (BPD) and autism spectrum disorder (ASD), subjects with these diagnoses have been found to express a rare, functional coding substitution in the dopamine (DA) transporter (DAT), Ala559Val. We developed DAT Val559 knock-in mice as a construct valid model of dopaminergic alterations that drive multiple clinical phenotypes, and here evaluate the impact of lifelong expression of the variant on impulsivity and motivation utilizing the 5- choice serial reaction time task (5-CSRTT) and Go/NoGo as well as tests of time estimation (peak interval analysis), reward salience (sucrose preference), and motivation (progressive ratio test). Our findings indicate that the DAT Val559 variant induces impulsivity behaviors that are dependent upon the reward context, with increased impulsive action observed when mice are required to delay responding for a reward, whereas mice are able to withhold responding if there is a probability of reward for a correct rejection. Utilizing peak interval and progressive ratio tests, we provide evidence that impulsivity is likely driven by an enhanced motivational phenotype that also may drive faster task acquisition in operant tasks. These data provide critical validation that DAT, and more generally, DA signaling perturbations can drive impulsivity that can manifest in specific contexts and not others, and may rely on motivational alterations, which may also drive increased maladaptive reward seeking.