Distinct reward processing by subregions of the nucleus accumbens

Altered dynamic functional connectivity of nucleus accumbens subregions in major depressive disorder: the interactive effect of childhood trauma and diagnosis

Major depressive disorder (MDD) with childhood trauma represents a heterogeneous clinical subtype of depression. Previous research has observed alterations in the reward circuitry centered around the nucleus accumbens (NAc) in MDD patients. However, limited investigations have focused on aberrant functional connectivity (FC) within NAc subregions among MDD with childhood trauma. Thus, this study adopts analyses of both static FC (sFC) and dynamic FC (dFC) to examine neurobiological changes in MDD with childhood trauma. The bilateral nucleus accumbens shell (NAc-shell) and nucleus accumbens core (NAc-core) were selected as the seeds. Four participant groups were included: MDD with childhood trauma (n = 48), MDD without childhood trauma (n = 30), healthy controls (HCs) with childhood trauma (n = 57), and HCs without childhood trauma (n = 46). Our findings revealed both abnormal sFC and dFC between NAc-shell and NAc-core and regions including the middle occipital gyrus (MOG), anterior cingulate cortex, and inferior frontal gyrus in MDD with childhood trauma. Furthermore, a significant correlation was identified between the dFC of the left NAc-shell and the right MOG in relation to childhood trauma. Additionally, abnormal dFC moderated the link between childhood abuse and depression severity. These outcomes shed light on the neurobiological underpinnings of MDD with childhood trauma.

Comparison of dopamine release and uptake parameters across sex, species and striatal subregions

For over four decades, fast-scan cyclic voltammetry (FSCV) has been used to selectively measure neurotransmitters such as dopamine (DA) with high spatial and temporal resolution, providing detailed information about the regulation of DA in the extracellular space. FSCV is an optimal method for determining concentrations of stimulus-evoked DA in brain tissue. When modelling diseases involving disturbances in DA transmission, preclinical rodent models are especially useful because of the availability of specialized tools and techniques that serve as a foundation for translational research. There is known heterogeneity in DA dynamics between and within DA-innervated brain structures and between males and females. However, systematic evaluations of sex- and species-differences across multiple areas are lacking. Therefore, using FSCV, we captured a broad range of DA dynamics across five sub-regions of the dorsal and ventral striatum of males and females of both rats and mice that reflect the functional heterogeneity of DA kinetics and dynamics within these structures. While numerous differences were found, in particular, we documented a strong, consistent pattern of increased DA transporter activity in females in all of the regions surveyed. The data herein are intended to be used as a resource for further investigation of DA terminal function.

The nucleus accumbens in reward and aversion processing: insights and implications

The nucleus accumbens (NAc), a central component of the brain's reward circuitry, has been implicated in a wide range of behaviors and emotional states. Emerging evidence, primarily drawing from recent rodent studies, suggests that the function of the NAc in reward and aversion processing is multifaceted. Prolonged stress or drug use induces maladaptive neuronal function in the NAc circuitry, which results in pathological conditions. This review aims to provide comprehensive and up-to-date insights on the role of the NAc in motivated behavior regulation and highlights areas that demand further in-depth analysis. It synthesizes the latest findings on how distinct NAc neuronal populations and pathways contribute to the processing of opposite valences. The review examines how a range of neuromodulators, especially monoamines, influence the NAc's control over various motivational states. Furthermore, it delves into the complex underlying mechanisms of psychiatric disorders such as addiction and depression and evaluates prospective interventions to restore NAc functionality.

Tetherless Optical Neuromodulation: Wavelength from Orange-red to Mid-infrared

Optogenetics, a technique that employs light for neuromodulation, has revolutionized the study of neural mechanisms and the treatment of neurological disorders due to its high spatiotemporal resolution and cell-type specificity. However, visible light, particularly blue and green light, commonly used in conventional optogenetics, has limited penetration in biological tissue. This limitation necessitates the implantation of optical fibers for light delivery, especially in deep brain regions, leading to tissue damage and experimental constraints. To overcome these challenges, the use of orange-red and infrared light with greater tissue penetration has emerged as a promising approach for tetherless optical neuromodulation. In this review, we provide an overview of the development and applications of tetherless optical neuromodulation methods with long wavelengths. We first discuss the exploration of orange-red wavelength-responsive rhodopsins and their performance in tetherless optical neuromodulation. Then, we summarize two novel tetherless neuromodulation methods using near-infrared light: upconversion nanoparticle-mediated optogenetics and photothermal neuromodulation. In addition, we discuss recent advances in mid-infrared optical neuromodulation.

The nucleus accumbens shell: a neural hub at the interface of homeostatic and hedonic feeding

Feeding behavior is a complex physiological process regulated by the interplay between homeostatic and hedonic feeding circuits. Among the neural structures involved, the nucleus accumbens (NAc) has emerged as a pivotal region at the interface of these two circuits. The NAc comprises distinct subregions and in this review, we focus mainly on the NAc shell (NAcSh). Homeostatic feeding circuits, primarily found in the hypothalamus, ensure the organism's balance in energy and nutrient requirements. These circuits monitor peripheral signals, such as insulin, leptin, and ghrelin, and modulate satiety and hunger states. The NAcSh receives input from these homeostatic circuits, integrating information regarding the organism's metabolic needs. Conversely, so-called hedonic feeding circuits involve all other non-hunger and -satiety processes, i.e., the sensory information, associative learning, reward, motivation and pleasure associated with food consumption. The NAcSh is interconnected with hedonics-related structures like the ventral tegmental area and prefrontal cortex and plays a key role in encoding hedonic information related to palatable food seeking or consumption. In sum, the NAcSh acts as a crucial hub in feeding behavior, integrating signals from both homeostatic and hedonic circuits, to facilitate behavioral output via its downstream projections. Moreover, the NAcSh's involvement extends beyond simple integration, as it directly impacts actions related to food consumption. In this review, we first focus on delineating the inputs targeting the NAcSh; we then present NAcSh output projections to downstream structures. Finally we discuss how the NAcSh regulates feeding behavior and can be seen as a neural hub integrating homeostatic and hedonic feeding signals, via a functionally diverse set of projection neuron subpopulations.

Distinct Action Signals by Subregions in the Nucleus Accumbens during STOP-Change Performance

The nucleus accumbens (NAc) is thought to contribute to motivated behavior by signaling the value of reward-predicting cues and the delivery of anticipated reward. The NAc is subdivided into core and shell, with each region containing different populations of neurons that increase or decrease firing to rewarding events. While there are numerous theories of functions pertaining to these subregions and cell types, most are in the context of reward processing, with fewer considering that the NAc might serve functions related to action selection more generally. We recorded from single neurons in the NAc as rats of both sexes performed a STOP-change task that is commonly used to study motor control and impulsivity. In this task, rats respond quickly to a spatial cue on 80% of trials (GO) and must stop and redirect planned movement on 20% of trials (STOP). We found that the activity of reward-excited neurons signaled accurate response direction on GO, but not STOP, trials and that these neurons exhibited higher precue firing after correct trials. In contrast, reward-inhibited neurons significantly represented response direction on STOP trials at the time of the instrumental response. Finally, the proportion of reward-excited to reward-inhibited neurons and the strength of precue firing decreased as the electrode traversed the NAc. We conclude that reward-excited cells (more common in core) promote proactive action selection, while reward-inhibited cells (more common in shell) contribute to accurate responding on STOP trials that require reactive suppression and redirection of behavior.

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Molecular and circuit determinants in the globus pallidus mediating control of cocaine-induced behavioral plasticity

The globus pallidus externus (GPe) is a central component of the basal ganglia circuit, receiving strong input from the indirect pathway and regulating a variety of functions, including locomotor output and habit formation. We recently showed that it also acts as a gatekeeper of cocaine-induced behavioral plasticity, as inhibition of parvalbumin-positive cells in the GPe (GPe PV ) prevents the development of cocaine-induced reward and sensitization. However, the molecular and circuit mechanisms underlying this function are unknown. Here we show that GPe PV cells control cocaine reward and sensitization by inhibiting GABAergic neurons in the substantia nigra pars reticulata (SNr GABA ), and ultimately, selectively modulating the activity of ventral tegmental area dopamine (VTA DA ) cells projecting to the lateral shell of the nucleus accumbens (NAcLat). A major input to GPe PV cells is the indirect pathway of the dorsomedial striatum (DMS D 2 ), which receives DAergic innervation from collaterals of VTA DA →NAcLat cells, making this a closed-loop circuit. Cocaine likely facilitates reward and sensitization not directly through actions in the GPe, but rather in the upstream DMS, where the cocaine-induced elevation of DA triggers a depression in DMS D 2 cell activity. This cocaine-induced elevation in DA levels can be blocked by inhibition of GPe PV cells, closing the loop. Interestingly, the level of GPe PV cell activity prior to cocaine administration is correlated with the extent of reward and sensitization that animals experience in response to future administration of cocaine, indicating that GPe PV cell activity is a key predictor of future behavioral responses to cocaine. Single nucleus RNA-sequencing of GPe cells indicated that genes encoding voltage-gated potassium channels KCNQ3 and KCNQ5 that control intrinsic cellular excitability are downregulated in GPe PV cells following a single cocaine exposure, contributing to the elevation in GPe PV cell excitability. Acutely activating channels containing KCNQ3 and/or KCNQ5 using the small molecule carnosic acid, a key psychoactive component of Salvia rosmarinus (rosemary) extract, reduced GPe PV cell excitability and also impaired cocaine reward, sensitization, and volitional cocaine intake, indicating its potential as a therapeutic to counteract psychostimulant use disorder. Our findings illuminate the molecular and circuit mechanisms by which the GPe orchestrates brain-wide changes in response to cocaine that are required for reward, sensitization, and self-administration behaviors.

Transcriptional characterization of cocaine withdrawal versus extinction within nucleus accumbens

Substance use disorder is characterized by a maladaptive imbalance wherein drug seeking persists despite negative consequences or drug unavailability. This imbalance correlates with neurobiological alterations some of which are amplified during forced abstinence, thereby compromising the capacity of extinction-based approaches to prevent relapse. Cocaine use disorder (CUD) exemplifies this phenomenon in which neurobiological modifications hijack brain reward regions such as the nucleus accumbens (NAc) to manifest craving and withdrawal-like symptoms. While increasing evidence links transcriptional changes in the NAc to specific phases of addiction, genome-wide changes in gene expression during withdrawal vs. extinction (WD/Ext) have not been examined in a context- and NAc-subregion-specific manner. Here, we used cocaine self-administration (SA) in rats combined with RNA-sequencing (RNA-seq) of NAc subregions (core and shell) to transcriptionally profile the impact of experiencing withdrawal in the home cage or in the previous drug context or experiencing extinction training. As expected, home-cage withdrawal maintained drug seeking in the previous drug context, whereas extinction training reduced it. By contrast, withdrawal involving repetitive exposure to the previous drug context increased drug-seeking behavior. Bioinformatic analyses of RNA-seq data revealed gene expression patterns, networks, motifs, and biological functions specific to these behavioral conditions and NAc subregions. Comparing transcriptomic analysis of the NAc of patients with CUD highlighted conserved gene signatures, especially with rats that were repetitively exposed to the previous drug context. Collectively, these behavioral and transcriptional correlates of several withdrawal-extinction settings reveal fundamental and translational information about potential molecular mechanisms to attenuate drug-associated memories.

Nucleus accumbens neurons dynamically respond to appetitive and aversive associative learning

To survive, individuals must learn to associate cues in the environment with emotionally relevant outcomes. This association is partially mediated by the nucleus accumbens (NAc), a key brain region of the reward circuit that is mainly composed by GABAergic medium spiny neurons (MSNs), that express either dopamine receptor D1 or D2. Recent studies showed that both populations can drive reward and aversion, however, the activity of these neurons during appetitive and aversive Pavlovian conditioning remains to be determined. Here, we investigated the relevance of D1- and D2-neurons in associative learning, by measuring calcium transients with fiber photometry during appetitive and aversive Pavlovian tasks in mice. Sucrose was used as a positive valence unconditioned stimulus (US) and foot shock was used as a negative valence US. We show that during appetitive Pavlovian conditioning, D1- and D2-neurons exhibit a general increase in activity in response to the conditioned stimuli (CS). Interestingly, D1- and D2-neurons present distinct changes in activity after sucrose consumption that dynamically evolve throughout learning. During the aversive Pavlovian conditioning, D1- and D2-neurons present an increase in the activity in response to the CS and to the US (shock). Our data support a model in which D1- and D2-neurons are concurrently activated during appetitive and aversive conditioning.

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

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

A putative loop connection between VTA dopamine neurons and nucleus accumbens encodes positive valence to compensate for hunger

Dopamine (DA) signal play pivotal roles in regulating motivated behaviors, including feeding behavior, but the role of midbrain DA neurons in modulating food intake and neural circuitry mechanisms remain largely unknown. Here, we found that activating but not inhibiting ventral tegmental area (VTA) DA neurons reduces mouse food intake. Furthermore, DA neurons in ventral VTA, especially neurons projecting to the medial nucleus accumbens (NAc), are activated by refeeding in the 24 h fasted mice. Combing neural circuitry tracing, optogenetic, chemogenetic, and pharmacological manipulations, we established that the VTA→medial NAc→VTA loop circuit is critical for the VTA DA neurons activation-induced food intake reduction. Moreover, activating either VTA DA neurons or dopaminergic axons in medial NAc elevates positive valence, which will compensate for the hungry-induced food intake. Thus, our study identifies a subset of positive valence-encoded VTA DA neurons forming possible loop connections with medial NAc that are anorexigenic.

Application of the wholebrain calculation interactive framework to map whole-brain neural connectivity networks

The aim of this work was to develop a simple and feasible method of mapping the neural network topology of the mouse brain. Wild-type C57BL/6 J mice (n = 10) aged 8-10 weeks were injected with the cholera toxin subunit B (CTB) tracer in the anterior (NAcCA) and posterior (NAcCP) parts of the nucleus accumbens (NAc) core and in the medial (NAcSM) and lateral (NAcSL) parts of the NAc shell. The labeled neurons were reconstructed using the WholeBrain Calculation Interactive Framework. The NAcCA receives neuronal projections from the olfactory areas (OLF) and isocortex; the thalamus and isocortex project more fibers to the NAcSL, and the hypothalamus send more fiber projections to the NAcSM. Cell resolution can be automatically annotated, analyzed, and visualized using the WholeBrain Calculation Interactive Framework, making large-scale mapping of mouse brains at cellular and subcellular resolutions easier and more accurate.