Enhancing excitability of dopamine neurons promotes motivational behaviour through increased action initiation

Adolescent THC impacts on mPFC dopamine-mediated cognitive processes in male and female rats

Rationale

Adolescent cannabis use is linked to later-life changes in cognition, learning, and memory. Rodent experimental studies suggest Δ 9 -tetrahydrocannabinol (THC) influences development of circuits underlying these processes, especially in the prefrontal cortex, which matures during adolescence.

Objective

We determined how 14 daily THC injections (5mg/kg) during adolescence persistently impacts medial prefrontal cortex (mPFC) dopamine-dependent cognition.

Methods

In adult Long Evans rats treated as adolescents with THC (AdoTHC), we quantify performance on two mPFC dopamine-dependent reward-based tasks-strategy set shifting and probabilistic discounting. We also determined how acute dopamine augmentation with amphetamine (0, 0.25, 0.5 mg/kg), or specific chemogenetic stimulation of ventral tegmental area (VTA) dopamine neurons and their projections to mPFC impacts probabilistic discounting.

Results

AdoTHC sex-dependently impacts acquisition of cue-guided instrumental reward seeking, but has minimal effects on set-shifting or probabilistic discounting in either sex. When we challenged dopamine circuits acutely with amphetamine during probabilistic discounting, we found reduced discounting of improbable reward options, with AdoTHC rats being more sensitive to these effects than controls. In contrast, neither acute chemogenetic stimulation of VTA dopamine neurons nor pathway-specific chemogenetic stimulation of their projection to mPFC impacted probabilistic discounting in control rats, although stimulation of this cortical dopamine projection slightly disrupted choices in AdoTHC rats.

Conclusions

These studies confirm a marked specificity in the cognitive processes impacted by AdoTHC exposure. They also suggest that some persistent AdoTHC effects may alter amphetamine-induced cognitive changes in a manner independent of VTA dopamine projections to mPFC, or via alterations of non-VTA dopamine neurons.

Zona incerta dopamine neurons encode motivational vigor in food seeking

Energy deprivation triggers food seeking to ensure homeostatic consumption, but the neural coding of motivational vigor in food seeking during physical hunger remains unknown. Here, we report that ablation of dopamine (DA) neurons in zona incerta (ZI) but not ventral tegmental area potently impaired food seeking after fasting. ZI DA neurons and their projections to paraventricular thalamus (PVT) were quickly activated for food approach but inhibited during food consumption. Chemogenetic manipulation of ZI DA neurons bidirectionally regulated feeding motivation to control meal frequency but not meal size for food intake. Activation of ZI DA neurons promoted, but silencing of these neurons blocked, contextual memory associate with food reward. In addition, selective activation of ZI DA projections to PVT promoted food seeking for food consumption and transited positive-valence signals. Together, these findings reveal that ZI DA neurons encode motivational vigor in food seeking for food consumption through their projections to PVT.

Zona incerta dopamine neurons encode motivational vigor in food seeking

Energy deprivation triggers food seeking to ensure homeostatic consumption, but the neural coding of motivational vigor in food seeking during physical hunger remains unknown. Here, we report that ablation of dopamine (DA) neurons in zona incerta (ZI) but not ventral tegmental area potently impaired food seeking after fasting. ZI DA neurons were quickly activated for food approach but inhibited during food consumption. Chemogenetic manipulation of ZI DA neurons bidirectionally regulated feeding motivation to control meal frequency but not meal size for food intake. In addition, activation of ZI DA neurons and their projections to paraventricular thalamus transited positive-valence signals to promote acquisition and expression of contextual food memory. Together, these findings reveal that ZI DA neurons encode motivational vigor in food seeking for homeostatic eating.

One Sentence Summary

Activation of ZI DA neurons vigorously drives and maintains food-seeking behaviors to ensure food consumption triggered by energy deprivation through inhibitory DA ZI-PVT transmissions that transit positive-valence signals associated with contextual food memory.

Nucleus accumbens in the pathogenesis of major depressive disorder: A brief review

Major depressive disorder (MDD) is the most prevalent mental disorder characterized by anhedonia, loss of motivation, avolition, behavioral despair and cognitive abnormalities. Despite substantial advancements in the pathophysiology of MDD in recent years, the pathogenesis of this disorder is not fully understood. Meanwhile,the treatment of MDD with currently available antidepressants is inadequate, highlighting the urgent need for clarifying the pathophysiology of MDD and developing novel therapeutics. Extensive studies have demonstrated the involvement of nuclei such as the prefrontal cortex (PFC), hippocampus (HIP), nucleus accumbens (NAc), hypothalamus, etc., in MDD. NAc,a region critical for reward and motivation,dysregulation of its activity seems to be a hallmark of this mood disorder. In this paper, we present a review of NAc related circuits, cellular and molecular mechanisms underlying MDD and share an analysis of the gaps in current research and possible future research directions.

Astrocytes mediate long-lasting synaptic regulation of ventral tegmental area dopamine neurons

The plasticity of glutamatergic transmission in the ventral tegmental area (VTA) represents a fundamental mechanism in the modulation of dopamine neuron burst firing and phasic dopamine release at target regions. These processes encode basic behavioral responses, including locomotor activity, learning and motivated behaviors. Here we describe a hitherto unidentified mechanism of long-term synaptic plasticity in mouse VTA. We found that the burst firing in individual dopamine neurons induces a long-lasting potentiation of excitatory synapses on adjacent dopamine neurons that crucially depends on Ca²⁺ elevations in astrocytes, mediated by endocannabinoid CB1 and dopamine D2 receptors co-localized at the same astrocytic process, and activation of pre-synaptic metabotropic glutamate receptors. Consistent with these findings, selective in vivo activation of astrocytes increases the burst firing of dopamine neurons in the VTA and induces locomotor hyperactivity. Astrocytes play, therefore, a key role in the modulation of VTA dopamine neuron functional activity.

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.

Oxytocin Receptor-Expressing Neurons in the Paraventricular Thalamus Regulate Feeding Motivation through Excitatory Projections to the Nucleus Accumbens Core

Oxytocin receptors (OTR) have been found in the paraventricular thalamus (PVT) for the regulation of feeding and maternal behaviors. However, the functional projections of OTR-expressing PVT neurons remain largely unknown. Here, we used chemogenetic and optogenetic tools to test the role of OTR-expressing PVT neurons and their projections in the regulation of food intake in both male and female OTR-Cre mice. We found chemogenetic activation of OTR-expressing PVT neurons promoted food seeking under trials with a progressive ratio schedule of reinforcement. Using Feeding Experimentation Devices for real-time meal measurements, we found chemogenetic activation of OTR-expressing PVT neurons increased meal frequency but not cumulative food intake because of a compensatory decrease in meal sizes. In combination with anterograde neural tracing and slice patch-clamp recordings, we found optogenetic stimulation of PVT OTR terminals excited neurons in the posterior basolateral amygdala (pBLA) and nucleus accumbens core (NAcC) as well as local PVT neurons through monosynaptic glutamatergic transmissions. Photostimulation of OTR-expressing PVT-NAcC projections promoted food-seeking, whereas selective activation of PVT-pBLA projections produced little effect on feeding. In contrast to selective activation of OTR terminals, photostimulation of a broader population of glutamatergic PVT terminals exerted direct excitation followed by indirect lateral inhibition on neurons in both NAcC and anterior basolateral amygdala. Together, these results suggest that OTR-expressing PVT neurons are a distinct population of PVT glutamate neurons that regulate feeding motivation through projections to NAcC.SIGNIFICANCE STATEMENT The paraventricular thalamus plays an important role in the regulation of feeding motivation. However, because of the diversity of paraventricular thalamic neurons, the specific neuron types promoting food motivation remain elusive. In this study, we provide evidence that oxytocin receptor-expressing neurons are a specific group of glutamate neurons that primarily project to the nucleus accumbens core and posterior amygdala. We found that activation of these neurons promotes the motivation for food reward and increases meal frequency through projections to the nucleus accumbens core but not the posterior amygdala. As a result, we postulate that oxytocin receptor-expressing neurons in the paraventricular thalamus and their projections to the nucleus accumbens core mainly regulate feeding motivation but not food consumption.

Intermittent social stress produces different short- and long-term effects on effort-based reward-seeking behavior

Previous studies show that intermittent social defeat (ISD) stress increases self-administration of psychostimulants, which suggests that ISD promotes reward-seeking behavior and, ultimately, increases vulnerability to develop drug abuse. The present study investigates whether ISD alters cost/benefit evaluations to promote reward-seeking behavior and whether these alterations are time-dependent. Male rats performed two different tasks that assessed their motivation to seek and consume food rewards. An effort-discounting task in which rats chose between less and more effortful options (i.e., 1 lever-press versus 2, 5, 10 or 20 lever-presses) associated with low- and high-reward (i.e., 1 sugar pellet versus 3 sugar pellets), respectively; and a progressive ratio task in which rats had to increase their effort (more lever presses) to obtain a sugar pellet. ISD consisted of exposing animals to social defeat once every three days for ten days (4 stress episodes). Rats were tested 24-48 h after stress episodes, and 1 week and 6 weeks after the last stress episode. In the effort-discounting task, stressed animals showed a decrease in their preference for high rewards associated with more effort (i.e., 10 and 20 lever-presses). These effects were transient and not maintained one week after stress. In the progressive ratio task, stressed animals showed an increase in the number of lever presses to obtain rewards that emerged six weeks after the last stress episode. These results suggest different short- and long-term effects on the motivation for rewards after ISD and indicate temporal dynamic adaptations in the function of the brain reward system.

Increased elasticity of sucrose demand during hyperdopaminergic states in rats

RATIONALE

Deficits in cost-benefit decision-making are a core feature of several psychiatric disorders, including substance addiction, eating disorders and bipolar disorder. Mesocorticolimbic dopamine signalling has been implicated in various processes related to cognition and reward, but its precise role in reward valuation and cost-benefit trade-off decisions remains incompletely understood.

OBJECTIVES

We assessed the role of mesocorticolimbic dopamine signalling in the relationship between price and consumption of sucrose, to better understand its role in cost-benefit decisions.

METHODS

Dopamine neurons in the ventral tegmental area (VTA) were chemogenetically activated in rats, and a behavioural economics approach was used to quantify the relationship between price and consumption of sucrose. Motivation for sucrose was also assessed under a progressive ratio (PR) schedule of reinforcement. To further gauge the role of dopamine in cost-benefit trade-offs for sucrose, the effects of treatment with D-amphetamine and the dopamine receptor antagonist alpha-flupentixol were assessed.

RESULTS

Chemogenetic activation of VTA dopamine neurons increased demand elasticity, while responding for sucrose under a PR schedule of reinforcement was augmented upon stimulation of VTA dopamine neurons. Treatment with amphetamine partially replicated the effects of chemogenetic dopamine neuron activation, whereas treatment with alpha-flupentixol reduced free consumption of sucrose and had mixed effects on demand elasticity.

CONCLUSIONS

Stimulation of mesocorticolimbic dopaminergic neurotransmission altered cost-benefit trade-offs in a complex manner. It reduced the essential value of palatable food, increased incentive motivation and left free consumption unaltered. Together, these findings imply that mesocorticolimbic dopamine signalling differentially influences distinct components of cost expenditure processes aimed at obtaining rewards.

Identification of Novel Neurocircuitry Through Which Leptin Targets Multiple Inputs to the Dopamine System to Reduce Food Reward Seeking

BACKGROUND

Leptin reduces the motivation to obtain food by modulating activity of the mesolimbic dopamine (DA) system upon presentation of cues that predict a food reward. Although leptin directly reduces the activity of ventral tegmental area (VTA) DA neurons, the majority of leptin receptor (LepR)-expressing DA neurons do not project to the nucleus accumbens, the projection implicated in driving food reward seeking. Therefore, the precise locus of leptin action to modulate motivation for a food reward is unresolved.

METHODS

We used transgenic mice expressing Cre recombinase under the control of the LepR promoter, anatomical tracing, optogenetics-assisted patch-clamp electrophysiology, in vivo optogenetics with fiber photometric calcium measurements, and chemogenetics to unravel how leptin-targeted neurocircuitry inhibits food reward seeking.

RESULTS

A large number of DA neurons projecting to the nucleus accumbens are innervated by local VTA LepR-expressing GABA (gamma-aminobutyric acid) neurons. Leptin enhances the activity of these GABA neurons and thereby inhibits nucleus accumbens-projecting DA neurons. In addition, we find that lateral hypothalamic LepR-expressing neurons projecting to the VTA are inhibited by leptin and that these neurons modulate DA neurons indirectly via inhibition of VTA GABA neurons. In accordance with such a disinhibitory function, optogenetically stimulating lateral hypothalamic LepR projections to the VTA potently activates DA neurons in vivo. Moreover, we found that chemogenetic activation of lateral hypothalamic LepR neurons increases the motivation to obtain a food reward only when mice are in a positive energy balance.

CONCLUSIONS

We identify neurocircuitry through which leptin targets multiple inputs to the DA system to reduce food reward seeking.

Pain induces adaptations in ventral tegmental area dopamine neurons to drive anhedonia-like behavior

The persistence of negative affect in pain leads to co-morbid symptoms such as anhedonia and depression-major health issues in the United States. The neuronal circuitry and contribution of specific cellular populations underlying these behavioral adaptations remains unknown. A common characteristic of negative affect is a decrease in motivation to initiate and complete goal-directed behavior, known as anhedonia. We report that in rodents, inflammatory pain decreased the activity of ventral tegmental area (VTA) dopamine (DA) neurons, which are critical mediators of motivational states. Pain increased rostromedial tegmental nucleus inhibitory tone onto VTA DA neurons, making them less excitable. Furthermore, the decreased activity of DA neurons was associated with reduced motivation for natural rewards, consistent with anhedonia-like behavior. Selective activation of VTA DA neurons was sufficient to restore baseline motivation and hedonic responses to natural rewards. These findings reveal pain-induced adaptations within VTA DA neurons that underlie anhedonia-like behavior.

Activation of ventral tegmental area neurotensin Receptor-1 neurons promotes weight loss

Dopamine (DA) neurons in the ventral tegmental area (VTA) modulate physical activity and feeding behaviors that are disrupted in obesity. Yet, the heterogeneity of VTA DA neurons has hindered determination of which ones might be leveraged to support weight loss. We hypothesized that increased activity in the subset of VTA DA neurons expressing neurotensin receptor-1 (NtsR1) might promote weight loss behaviors. To test this, we used Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) to activate VTA NtsR1 neurons in normal weight and diet-induced obese mice. Acute activation of VTA NtsR1 neurons (24hr) significantly decreased body weight in normal weight and obese mice by reducing food intake and increasing physical activity. Moreover, daily activation of VTA NtsR1 neurons in obese mice sustained weight loss over 7 days. Activating VTA NtsR1 neurons also suppressed how much mice worked to obtain sucrose rewards, even when there was high motivation to consume. However, VTA NtsR1 neural activation was not reinforcing, nor did it invoke liabilities associated with whole-body NtsR1 agonism such as anxiety, vasodepressor response or hypothermia. Activating VTA NtsR1 neurons therefore promotes dual behaviors that support weight loss without causing adverse effects, and is worth further exploration for managing obesity.

Oxytocin activation of paraventricular thalamic neurons promotes feeding motivation to attenuate stress-induced hypophagia

The neuropeptide oxytocin (OT) regulates important brain functions including feeding through activating OT receptors in multiple brain areas. Both OT fibers and OT receptors have been reported in the paraventricular thalamus (PVT), an area that was revealed to be important for the control of emotion, motivation, and food intake. However, the function and modulation of PVT OT signaling remain unknown. Here, we used a progressive ratio (PR) schedule of reinforcement to examine the role of PVT OT signaling in regulating the motivation for food and patch-clamp electrophysiology to study the modulation of OT on PVT neurons in brain slices. We demonstrate that PVT OT administration increases active lever presses to earn food rewards in both male and female mice under PR trials and OT receptor antagonist atosiban inhibits OT-induced increase in motivated lever presses. However, intra-PVT OT infusion does not affect food intake in normal conditions but attenuates hypophagia induced by stress and anxiety. Using patch-clamp recordings, we find OT induces long-lasting excitatory effects on neurons in all PVT regions, especially the middle to posterior PVT. OT not only evokes tonic inward currents but also increases the frequency of spontaneous excitatory postsynaptic currents on PVT neurons. The excitatory effect of OT on PVT neurons is mimicked by the specific OT receptor agonist [Thr4, Gly7]-oxytocin (TGOT) and blocked by OT receptor antagonist atosiban. Together, our study reveals a critical role of PVT OT signaling in promoting feeding motivation to attenuate stress-induced hypophagia through exciting PVT neurons.

Insulin and endocannabinoids in the mesolimbic system

Easy access to palatable food and an abundance of food-related cues exacerbate non-homeostatic feeding. The metabolic and economical sequelae of non-homeostatic feeding outweigh those of homeostatic feeding and contribute significantly to the global obesity pandemic. The mesolimbic dopamine system is the primary central circuit that governs the motivation to consume food. Insulin and endocannabinoids (eCBs) are two major, presumably opposing, players in regulating homeostatic and non-homeostatic feeding centrally and peripherally. Insulin is generally regarded as a postprandial satiety signal, whereas eCBs mainly function as pre-prandial orexinergic signals. In this review, we discuss the effects of insulin and eCB-mediated actions within the mesolimbic pathways. We propose that insulin and eCBs have regional- and time course-dependent roles. We discuss their mechanisms of actions in the ventral tegmental area and nucleus accumbens, as well as how their mechanisms converge to finely tune dopaminergic activity and food intake.

Different baseline physical activity predicts susceptibility and resilience to chronic social defeat stress in mice: Involvement of dopamine neurons

Physical inactivity, the fourth leading mortality risk factor worldwide, is associated with chronic mental illness. Identifying the mechanisms underlying different levels of baseline physical activity and the effects of these levels on the susceptibility to stress is very important. However, whether different levels of baseline physical activity influence the susceptibility and resilience to chronic social defeat stress (CSDS), and the underlying mechanisms in the brain remain unclear. The present study segregated wild-type mice into low baseline physical activity (LBPA) and high baseline physical activity (HBPA) groups based on short term voluntary wheel running (VWR). LBPA mice showed obvious susceptibility to CSDS, while HBPA mice were resilient to CSDS. In addition, the expression of tyrosine hydroxylase (TH) in the ventral tegmental area (VTA) was lower in LBPA mice than in HBPA mice. Furthermore, activation of TH neurons in the VTA of LBPA mice by chemogenetic methods increased the levels of VWR and resilience to CSDS. In contrast, inhibiting TH neurons in the VTA of HBPA mice lowered the levels of VWR and increased their susceptibility to CSDS. Thus, this study suggests that different baseline physical activities might be mediated by the dopamine system. This system also affects the susceptibility and resilience to CSDS, possibly via alteration of the baseline physical activity. This perspective on the neural control and impacts on VWR may aid the development of strategies to motivate and sustain voluntary physical activity. Furthermore, this can maximize the impacts of regular physical activity toward stress-reduction and health promotion.

Zona incerta neurons projecting to the ventral tegmental area promote action initiation towards feeding

KEY POINTS

The zona incerta (ZI) and ventral tegmental area (VTA) are brain areas that are both implicated in feeding behaviour. The ZI projects to the VTA, although it has not yet been investigated whether this projection regulates feeding. We experimentally (in)activated the ZI to VTA projection by using dual viral vector technology, and studied the effects on feeding microstructure, the willingness to work for food, general activity and body temperature. Activity of the ZI to VTA projection promotes feeding by facilitating action initiation towards food, as reflected in meal frequency and the willingness to work for food reward, without affecting general activity or directly modulating body temperature. We show for the first time that activity of the ZI to VTA projection promotes feeding, which improves the understanding of the neurobiology of feeding behaviour and body weight regulation.

ABSTRACT

Both the zona incerta (ZI) and the ventral tegmental area (VTA) have been implicated in feeding behaviour. The ZI provides prominent input to the VTA, although it has not yet been investigated whether this projection regulates feeding. Therefore, we investigated the role of ZI to VTA projection neurons in the regulation of several aspects of feeding behaviour. We determined the effects of (in)activation of ZI to VTA projection neurons on feeding microstructure, food-motivated behaviour under a progressive ratio schedule of reinforcement, locomotor activity and core body temperature. To activate or inactivate ZI neurons projecting to the VTA, we used a combination of canine adenovirus-2 in the VTA, as well as Cre-dependent designer receptors exclusively activated by designer drugs (DREADD) or tetanus toxin (TetTox) light chain in the ZI. TetTox-mediated inactivation of ZI to VTA projection neurons reduced food-motivated behaviour and feeding by reducing meal frequency. Conversely, DREADD-mediated chemogenetic activation of ZI to VTA projection neurons promoted food-motivated behaviour and feeding. (In)activation of ZI to VTA projection neurons did not affect locomotor activity or directly regulate core body temperature. Taken together, ZI neurons projecting to the VTA exert bidirectional control overfeeding behaviour. More specifically, activity of ZI to VTA projection neurons facilitate action initiation towards feeding, as reflected in both food-motivated behaviour and meal initiation, without affecting general activity.

Role of the Medial Orbitofrontal Cortex and Ventral Tegmental Area in Effort-Related Responding

The posterior subdivision of the medial orbitofrontal cortex (mOFC-p) mediates the willingness to expend effort to reach a selected goal. However, the neural circuitry through which the mOFC-p modulates effort-related function is as yet unknown. The mOFC-p projects prominently to the posterior ventral tegmental area (pVTA). Therefore, we analyzed the role of the mOFC-p and interactions with the pVTA in effort-related responding using a combination of behavioral, pharmacological, and neural circuit analysis methods in rats. Pharmacological inhibition of the mOFC-p was found to increase lever pressing for food under a progressive ratio (PR) schedule of reinforcement. These findings provide further support for a modulation of effort-related function by the mOFC-p. Then, we investigated effects of disconnecting the mOFC-p and pVTA on PR responding using unilateral pharmacological inhibition of both areas. This asymmetric intervention was also found to increase PR responding suggesting that the mOFC-p controls effort-related function through interactions with the pVTA. Possibly, a reduced excitatory mOFC-p drive on pVTA gamma-aminobutyric acid (GABA)ergic relays disinhibits VTA dopamine neurons which are known to support PR responding. Collectively, our findings suggest that the mOFC-p and pVTA are key components of a neural circuit mediating the willingness to expend effort to reach a goal.

Effects of brief inhibition of the ventral tegmental area dopamine neurons on the cocaine seeking during abstinence

Preclinical studies strongly suggest that cocaine seeking depends on the neuronal activity of the ventral tegmental area (VTA) and phasic dopaminergic (DA) signaling. Notably, VTA pharmacological inactivation or dopamine receptor blockade in the forebrain may induce behavioral inhibition in general and acute aversive states in particular, thus reducing cocaine seeking indirectly. Such artifacts hinder successful translation of these findings in clinical studies and practice. Here, we aimed to evaluate if dynamic VTA manipulations effectively reduce cocaine seeking. We used male tyrosine hydroxylase (TH) IRES-Cre⁺ rats along with optogenetic tools to inhibit directly and briefly VTA DA neurons during conditioned stimulus (CS)-induced cocaine seeking under extinction conditions. The behavioral effects of optogenetic inhibition were also assessed in the real-time dynamic place aversion, conditioned place aversion, and CS-induced food-seeking tests. We found that brief and nondysphoric/nonsedative pulses of VTA photo-inhibition (1 s every 9 s, ie, for 10% of time) attenuated CS-induced cocaine seeking under extinction conditions in rats expressing archaerhodopsin selectively on the TH⁺ neurons. Furthermore, direct inhibition of the VTA DA activity reduced CS-induced cocaine seeking 24 hours after photo-modulation. Importantly, such effect appears to be selective for cocaine seeking as similar inhibition of the VTA DA activity had no effect on CS-induced food seeking. Thus, briefly inhibiting VTA DA activity during CS-induced cocaine seeking drastically and selectively reduces seeking without behavioral artifacts such as sedation or dysphoria. Our results point to the therapeutic possibilities of coupling nonpharmacologic treatments with extinction training in reducing cocaine addiction.

The Nucleus Accumbens: A Common Target in the Comorbidity of Depression and Addiction

The comorbidity of depression and addiction has become a serious public health issue, and the relationship between these two disorders and their potential mechanisms has attracted extensive attention. Numerous studies have suggested that depression and addiction share common mechanisms and anatomical pathways. The nucleus accumbens (NAc) has long been considered a key brain region for regulating many behaviors, especially those related to depression and addiction. In this review article, we focus on the association between addiction and depression, highlighting the potential mediating role of the NAc in this comorbidity via the regulation of changes in the neural circuits and molecular signaling. To clarify the mechanisms underlying this association, we summarize evidence from overlapping reward neurocircuitry, the resemblance of cellular and molecular mechanisms, and common treatments. Understanding the interplay between these disorders should help guide clinical comorbidity prevention and the search for a new target for comorbidity treatment.

Lateral hypothalamic area neuropeptides modulate ventral tegmental area dopamine neurons and feeding

Understanding how the brain coordinates energy status with the motivation to eat is crucial to identify strategies to improve disordered body weight. The ventral tegmental area (VTA), known as the core of the mesolimbic system, is of particular interest in this regard because it controls the motivation to consume palatable, calorie-dense foods and to engage in volitional activity. The VTA is largely composed of dopamine (DA) neurons, but modulating these DA neurons has been alternately linked with promoting and suppressing feeding, suggesting heterogeneity in their function. Subsets of VTA DA neurons have recently been described based on their anatomical distribution, electrophysiological features, connectivity and molecular expression, but to date there are no signatures to categorize how DA neurons control feeding. Assessing the neuropeptide receptors expressed by VTA DA neurons may be useful in this regard, as many neuropeptides mediate anorexic or orexigenic responses. In particular, the lateral hypothalamic area (LHA) releases a wide variety of feeding-modulating neuropeptides to the VTA. Since VTA neurons intercept LHA neuropeptides known to either evoke or suppress feeding, expression of the cognate neuropeptide receptors within the VTA may point to VTA DA neuronal mechanisms to promote or suppress feeding, respectively. Here we review the role of the VTA in energy balance and the LHA neuropeptide signaling systems that act in the VTA, whose receptors might be used to classify how VTA DA neurons contribute to energy balance.

Time-dependent changes in cognitive flexibility performance during intermittent social stress: Relevance for motivation and reward-seeking behavior

Repeated exposure to stress produces cognitive impairments that can lead to psychiatric disorders. How and when these cognitive impairments occur during repeated exposure to stress is not well understood. The present study investigates the time course effects of the exposure to intermittent episodes of social stress on cognitive flexibility performance. Male rats were trained to perform a cognitive flexibility (set-shifting) task. Then they were submitted to intermittent social defeat stress, which consisted of exposing animals to social defeat once every three days for ten days (four stress episodes). Set-shifting performance was evaluated before and in between social stress episodes, and up to ten days after the end of the stress protocol. Plasma levels of corticosterone after tail-pinch were also measured after the last set-shifting session. Intermittent exposure to social stress did not impair cognitive flexibility but produced short-and long-term changes in set-shifting performance. After the third social defeat episode, stressed animals required significantly more time to respond to cues (instrumental action) and to poke in the food-trough when no food pellet was delivered. These effects were reversed ten days after stress and suggest a decrease in motivation to pursue rewards. In contrast, stressed animals responded more accurately to light cues that predicted rewards. This effect, which emerged ten days after stress, suggests an increase in the reactivity to salient cues in the long-term. These time-dependent behavioral changes might help to understand the transition from social stress to stress-related disorders including drug abuse.

Optogenetic and chemogenetic approaches to manipulate attention, impulsivity and behavioural flexibility in rodents

Studies manipulating neural activity acutely with optogenetic or chemogenetic intervention in behaving rodents have increased considerably in recent years. More often, these circuit-level neural manipulations are tested within an existing framework of behavioural testing that strives to model complex executive functions or symptomologies relevant to multidimensional psychiatric disorders in humans, such as attentional control deficits, impulsivity or behavioural (in)flexibility. This methods perspective argues in favour of carefully implementing these acute circuit-based approaches to better understand and model cognitive symptomologies or their similar isomorphic animal behaviours, which often arise and persist in overlapping brain circuitries. First, we offer some practical considerations for combining long-term, behavioural paradigms with optogenetic or chemogenetic interventions. Next, we examine how cell-type or projection-specific manipulations to the ascending neuromodulatory systems, local brain region or descending cortical glutamatergic projections influence aspects of cognitive control. For this, we primarily focus on the influence exerted on attentional and motor impulsivity performance in the (3-choice or) 5-choice serial reaction time task, and impulsive, risky or inflexible choice biases during alternative preference, reward discounting or reversal learning tasks.

Shared Behavioral and Neurocircuitry Disruptions in Drug Addiction, Obesity, and Binge Eating Disorder: Focus on Group I mGluRs in the Mesolimbic Dopamine Pathway

Accumulated data from clinical and preclinical studies suggest that, in drug addiction and states of overeating, such as obesity and binge eating disorder (BED), there is an imbalance in circuits that are critical for motivation, reward saliency, executive function, and self-control. Central to these pathologies and the extensive topic of this Review are the aberrations in dopamine (DA) and glutamate (Glu) within the mesolimbic pathway. Group I metabotropic glutamate receptors (mGlus) are highly expressed in the mesolimbic pathway and are poised in key positions to modulate disruptions in synaptic plasticity and neurotransmitter release observed in drug addiction, obesity, and BED. The use of allosteric modulators of group I mGlus has been studied in drug addiction, as they offer several advantages over traditional orthosteric agents. However, they have yet to be studied in obesity or BED. With the substantial overlap between the neurocircuitry involved in drug addiction and eating disorders, group I mGlus may also provide novel targets for obesity and BED.

Blockade of muscarinic acetylcholine receptors facilitates motivated behaviour and rescues a model of antipsychotic-induced amotivation

Disruptions to motivated behaviour are a highly prevalent and severe symptom in a number of neuropsychiatric and neurodegenerative disorders. Current treatment options for these disorders have little or no effect upon motivational impairments. We assessed the contribution of muscarinic acetylcholine receptors to motivated behaviour in mice, as a novel pharmacological target for motivational impairments. Touchscreen progressive ratio (PR) performance was facilitated by the nonselective muscarinic receptor antagonist scopolamine as well as the more subtype-selective antagonists biperiden (M1) and tropicamide (M4). However, scopolamine and tropicamide also produced increases in non-specific activity levels, whereas biperiden did not. A series of control tests suggests the effects of the mAChR antagonists were sensitive to changes in reward value and not driven by changes in satiety, motor fatigue, appetite or perseveration. Subsequently, a sub-effective dose of biperiden was able to facilitate the effects of amphetamine upon PR performance, suggesting an ability to enhance dopaminergic function. Both biperiden and scopolamine were also able to reverse a haloperidol-induced deficit in PR performance, however only biperiden was able to rescue the deficit in effort-related choice (ERC) performance. Taken together, these data suggest that the M1 mAChR may be a novel target for the pharmacological enhancement of effort exertion and consequent rescue of motivational impairments. Conversely, M4 receptors may inadvertently modulate effort exertion through regulation of general locomotor activity levels.

Accumbal Adenosine A2A Receptors Enhance Cognitive Flexibility by Facilitating Strategy Shifting

The deficits of cognitive flexibility (including attentional set-shifting and reversal learning) concomitant with dysfunction of the striatum are observed in several neuropsychiatric disorders. Rodent and human studies have identified the striatum [particularly the dorsomedial striatum (DMS) and nucleus accumbens (NAc)] as the critical locus for control of cognitive flexibility, but the effective neuromodulator and pharmacological control of cognitive flexibility remains to be determined. The adenosine A_2A receptors (A_2ARs) are highly enriched in the striatopallidal neurons where they integrate dopamine and glutamate signals to modulate several cognitive behaviors, but their contribution to cognitive flexibility control is unclear. In this study, by coupling an automated operant cognitive flexibility task with striatal subregional knockdown (KD) of the A_2AR via the Cre-loxP strategy, we demonstrated that NAc A_2AR KD improved cognitive flexibility with enhanced attentional set-shifting and reversal learning by decreasing regressive and perseverative errors, respectively. This facilitation was not attributed to mnemonic process or motor activity as NAc A_2AR KD did not affect the visual discrimination, lever-pressing acquisition, and locomotor activity, but was associated with increased attention and motivation as evident by the progressive ratio test (PRT). In contrast to NAc A_2ARs, DMS A_2ARs KD neither affected visual discrimination nor improved set-shifting nor reversal learning, but promoted the effort-related motivation. Thus, NAc and DMS A_2ARs exert dissociable controls of cognitive flexibility with NAc A_2ARs KD selectively enhancing cognitive flexibility by facilitating strategy shifting with increased motivation/attention.

Latency to Reward Predicts Social Dominance in Rats: A Causal Role for the Dopaminergic Mesolimbic System

Reward signals encoded in the mesolimbic dopaminergic system guide approach/seeking behaviors to all varieties of life-supporting stimuli (rewards). Differences in dopamine (DA) levels have been found between dominant and submissive animals. However, it is still unclear whether these differences arise as a consequence of the rewarding nature of the acquisition of a dominant rank, or whether they preexist and favor dominance by promoting reward-seeking behavior. Given that acquisition of a social rank determines animals' priority access to resources, we hypothesized that differences in reward-seeking behavior might affect hierarchy establishment and that modulation of the dopaminergic system could affect the outcome of a social competition. We characterized reward-seeking behaviors based on rats' latency to get a palatable-reward when given temporary access to it. Subsequently, rats exhibiting short (SL) and long (LL) latency to get the rewards cohabitated for more than 2 weeks, in order to establish a stable hierarchy. We found that SL animals exhibited dominant behavior consistently in social competition tests [for palatable-rewards and two water competition tests (WCTs)] after hierarchy was established, indicating that individual latency to rewards predicted dominance. Moreover, because SL animals showed higher mesolimbic levels of DA than LL rats, we tested whether stimulation of mesolimbic DA neurons could affect the outcome of a social competition. Indeed, a combination of optical stimulation of mesolimbic DA neurons during individual training and during a social competition test for palatable rewards resulted in improved performance on this test.

Modulating Dopamine Signaling and Behavior with Chemogenetics: Concepts, Progress, and Challenges

For more than 60 years, dopamine (DA) has been known as a critical modulatory neurotransmitter regulating locomotion, reward-based motivation, and endocrine functions. Disturbances in DA signaling have been linked to an array of different neurologic and psychiatric disorders, including Parkinson's disease, schizophrenia, and addiction, but the underlying pathologic mechanisms have never been fully elucidated. One major obstacle limiting interpretation of standard pharmacological and transgenic interventions is the complexity of the DA system, which only appears to widen as research progresses. Nonetheless, development of new genetic tools, such as chemogenetics, has led to an entirely new era for functional studies of neuronal signaling. By exploiting receptors that are engineered to respond selectively to an otherwise inert ligand, so-called Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), chemogenetics enables pharmacological remote control of neuronal activity. Here we review the recent, extensive application of this technique to the DA field and how its use has advanced the study of the DA system and contributed to our general understanding of DA signaling and related behaviors. Moreover, we discuss the challenges and pitfalls associated with the chemogenetic technology, such as the metabolism of the DREADD ligand clozapine N-oxide (CNO) to the D2 receptor antagonist clozapine. We conclude that despite the recent concerns regarding CNO, the chemogenetic toolbox provides an exceptional approach to study neuronal function. The huge potential should promote continued investigations and additional refinements to further expound key mechanisms of DA signaling and circuitries in normal as well as maladaptive behaviors.

Differential effects of chemogenetic inhibition of dopamine and norepinephrine neurons in the mouse 5-choice serial reaction time task

Attention-deficit/hyperactivity disorder (ADHD) is a psychiatric disorder characterized by inattention, aberrant impulsivity, and hyperactivity. Although the underlying pathophysiology of ADHD remains unclear, dopamine and norepinephrine signaling originating from the ventral tegmental area (VTA) and locus coeruleus (LC) is thought to be critically involved. In this study, we employ Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) together with the mouse 5-Choice Serial Reaction Time Task (5-CSRTT) to investigate the necessary roles of these catecholamines in ADHD-related behaviors, including attention, impulsivity, and motivation. By selective inhibition of tyrosine hydroxylase (TH)-positive VTA dopamine neurons expressing the Gi-coupled DREADD (hM4Di), we observed a marked impairment of effort-based motivation and subsequently speed and overall vigor of responding. At the highest clozapine N-oxide (CNO) dose tested (i.e. 2 mg/kg) to activate hM4Di, we detected a reduction in locomotor activity. DREADD-mediated inhibition of LC norepinephrine neurons reduced attentional performance in a variable stimulus duration test designed to increase task difficulty, specifically by increasing trials omissions, reducing mean score, and visual processing speed. These findings show that VTA dopamine and LC norepinephrine neurons differentially affect attention, impulsive and motivational control. In addition, this study highlights how molecular genetic probing of selective catecholamine circuits can provide valuable insights into the mechanisms underlying ADHD-relevant behaviors.

The Psychopharmacology of Effort-Related Decision Making: Dopamine, Adenosine, and Insights into the Neurochemistry of Motivation

Effort-based decision making is studied using tasks that offer choices between high-effort options leading to more highly valued reinforcers versus low-effort/low-reward options. These tasks have been used to study the involvement of neural systems, including mesolimbic dopamine and related circuits, in effort-related aspects of motivation. Moreover, such tasks are useful as animal models of some of the motivational symptoms that are seen in people with depression, schizophrenia, Parkinson's disease, and other disorders. The present review will discuss the pharmacology of effort-related decision making and will focus on the use of these tasks for the development of drug treatments for motivational dysfunction. Research has identified pharmacological conditions that can alter effort-based choice and serve as models for depression-related symptoms (e.g., the vesicular monoamine transport-2 inhibitor tetrabenazine and proinflammatory cytokines). Furthermore, tests of effort-based choice have identified compounds that are particularly useful for stimulating high-effort work output and reversing the deficits induced by tetrabenazine and cytokines. These studies indicate that drugs that act by facilitating dopamine transmission, as well as adenosine A_2A antagonists, are relatively effective at reversing effort-related impairments. Studies of effort-based choice may lead to the identification of drug targets that could be useful for treating motivational treatments that are resistant to commonly used antidepressants such as serotonin transport inhibitors.

Differences in the dopaminergic reward system in rats that passively and actively behave in the Porsolt test

The aim of the study was to assess appetitive responses and central dopaminergic neurotransmission in passive and active rats divided according to their immobility time in the Porsolt swim test and exposed to restraint stress. Passive rats had more episodes of appetitive 50-kHz ultrasonic vocalization (USV) during rat encounter after social isolation and spent significantly more time in the amphetamine-associated context in conditioned place preference test, compared to active rats. Restraint stress decreased sucrose preference, but increased appetitive vocalization and reinforced the conditioned place preference only in passive animals that was associated with increased dopamine concentration in the amygdala. Restraint stress increased also the level of Cocaine- and Amphetamine Regulated Transcript (CART) peptide, a neuromodulator linked to dopamine neurotransmission, in the central nucleus of amygdala, while decreasing it the nucleus accumbens shell in passive rats. In the parvocellular region of paraventricular nucleus of the hypothalamus passive animals had a higher expression of CART compared to passive restraint rats and active control rats. The obtained results show that active and passive rats in the Porsolt test differ significantly in response to appetitive stimuli, which can be additionally changed under stress conditions. The underlying mechanisms are probably associated with differences in dopaminergic activity and CART signaling in reward system.

Antidepressant and pro-motivational effects of repeated lamotrigine treatment in a rat model of depressive symptoms

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Oxygen responses within the nucleus accumbens are associated with individual differences in effort exertion in rats

Goal‐directed motivated behaviour is crucial for everyday life. Such behaviour is often measured, in rodents, under a progressive ratio (PR) schedule of reinforcement. Previous studies have identified a few brain structures critical for supporting PR performance. However, the association between neural activity within these regions and individual differences in effort‐related behaviour is not known. Presently, we used constant potential in vivo oxygen amperometry, a surrogate for functional resonance imaging in rodents, to assess changes in tissue oxygen levels within the nucleus accumbens (NAc) and orbitofrontal cortex (OFC) in male Wistar rats performing a PR task. Within both regions, oxygen responses to rewards increased as the effort exerted to obtain the rewards was larger. Furthermore, higher individual breakpoints were associated with greater magnitude NAc oxygen responses. This association could not be explained by temporal confounds and remained significant when controlling for the different number of completed trials. Animals with higher breakpoints also showed greater magnitude NAc oxygen responses to rewards delivered independently of any behaviour. In contrast, OFC oxygen responses were not associated with individual differences in behavioural performance. The present results suggest that greater NAc oxygen responses following rewards, through a process of incentive motivation, may allow organisms to remain on task for longer and to overcome greater effort costs.

Dopamine, Effort-Based Choice, and Behavioral Economics: Basic and Translational Research

Operant behavior is not only regulated by factors related to the quality or quantity of reinforcement, but also by the work requirements inherent in performing instrumental actions. Moreover, organisms often make effort-related decisions involving economic choices such as cost/benefit analyses. Effort-based decision making is studied using behavioral procedures that offer choices between high-effort options leading to relatively preferred reinforcers vs. low effort/low reward choices. Several neural systems, including the mesolimbic dopamine (DA) system and other brain circuits, are involved in regulating effort-related aspects of motivation. Considerable evidence indicates that mesolimbic DA transmission exerts a bi-directional control over exertion of effort on instrumental behavior tasks. Interference with DA transmission produces a low-effort bias in animals tested on effort-based choice tasks, while increasing DA transmission with drugs such as DA transport blockers tends to enhance selection of high-effort options. The results from these pharmacology studies are corroborated by the findings from recent articles using optogenetic, chemogenetic and physiological techniques. In addition to providing important information about the neural regulation of motivated behavior, effort-based choice tasks are useful for developing animal models of some of the motivational symptoms that are seen in people with various psychiatric and neurological disorders (e.g., depression, schizophrenia, Parkinson’s disease). Studies of effort-based decision making may ultimately contribute to the development of novel drug treatments for motivational dysfunction.