Mapping projections of molecularly defined dopamine neuron subtypes using intersectional genetic approaches

Molecular and spatial transcriptomic classification of midbrain dopamine neurons and their alterations in a LRRK2G2019S model of Parkinson's disease

Several studies have revealed that midbrain dopamine (DA) neurons, even within a single neuroanatomical area, display heterogeneous properties. In parallel, studies using single cell profiling techniques have begun to cluster DA neurons into subtypes based on their molecular signatures. Recent work has shown that molecularly defined DA subtypes within the substantia nigra (SNc) display distinctive anatomic and functional properties, and differential vulnerability in Parkinson's disease (PD). Based on these provocative results, a granular understanding of these putative subtypes and their alterations in PD models, is imperative. We developed an optimized pipeline for single-nuclear RNA sequencing (snRNA-seq) and generated a high-resolution hierarchically organized map revealing 20 molecularly distinct DA neuron subtypes belonging to three main families. We integrated this data with spatial MERFISH technology to map, with high definition, the location of these subtypes in the mouse midbrain, revealing heterogeneity even within neuroanatomical sub-structures. Finally, we demonstrate that in the preclinical LRRK2G2019S knock-in mouse model of PD, subtype organization and proportions are preserved. Transcriptional alterations occur in many subtypes including those localized to the ventral tier SNc, where differential expression is observed in synaptic pathways, which might account for previously described DA release deficits in this model. Our work provides an advancement of current taxonomic schemes of the mouse midbrain DA neuron subtypes, a high-resolution view of their spatial locations, and their alterations in a prodromal mouse model of PD.

Generation and validation of a D1 dopamine receptor Flpo knock-in mouse

BACKGROUND

Dopamine is a powerful neuromodulator of diverse brain functions, including movement, motivation, reward, and cognition. D1-type dopamine receptors (D1DRs) are the most prevalently expressed dopamine receptors in the brain. Neurons expressing D1DRs are heterogeneous and involve several subpopulations. Although these neurons can be studied with BAC-transgenic rodents, these models have some limitations especially when considering their integration with conditional or intersectional genetic tools.

NEW METHOD

We developed a novel Drd1-P2A-Flpo (Drd1-Flpo) mouse line in which the Flpo gene was knocked in immediately after the Drd1 gene using CRISPR-Cas9. We validated the Drd1-Flpo line by confirming Flp expression and functionality specific to D1DR+ neurons with immunohistochemistry and in situ hybridization.

COMPARISON WITH EXISTING METHODS

The Drd1-Flpo line is a useful resource for studying subpopulations of D1DR+ neurons with intersectional genetic tools.

CONCLUSIONS

We demonstrated brain-wide GFP expression driven by Drd1-Flpo, suggesting that this mouse line may be useful for comprehensive anatomical and functional studies in many brain regions. The Drd1-Flpo model will advance the study of dopaminergic signaling by providing a new tool for investigating the diverse roles of D1DR+ neurons and their subpopulations in brain disease.

The ventral tegmental area dopamine to basolateral amygdala projection supports acquisition of cocaine self-administration

Dopamine signaling in the amygdala is known to play a role in associative learning and memory, including the process of learning to associate environmental cues with the reinforcing properties of drugs like cocaine. Evidence suggests that the ventral tegmental area (VTA) dopamine (DA) projection specifically to the basolateral amygdala (BLA) participates in establishing cocaine-cue associations that can promote later craving- and relapse-like responses to the cue alone. In order to further investigate the specific role of VTA-BLA projections in cocaine-reinforced learning, we used chemogenetics to manipulate VTA DA inputs to the BLA during cocaine self-administration, cue- and cocaine-primed reinstatement, and conditioned place preference. We found inhibiting DA input to the BLA during cocaine self-administration inhibited acquisition and weakened the ability of the previously cocaine-paired cue to elicit cocaine-seeking, while acutely inhibiting the pathway on the day of cue-induced reinstatement testing had no effect. Conversely, exciting the projection during self-administration boosted the salience of the cocaine-paired cue as indicated by enhanced responding during cue-induced reinstatement. Importantly, interfering with DA input to the BLA had no impact on the ability of cocaine to elicit a place preference or induce reinstatement in response to a priming cocaine injection. Overall, we show that manipulation of projections underlying DA signaling in the BLA may be useful for developing therapeutic interventions for substance use disorders.

Fiber photometry in neuroscience research: principles, applications, and future directions

In recent years, fluorescent sensors are enjoying a surge of popularity in the field of neuroscience. Through the development of novel genetically encoded sensors as well as improved methods of detection and analysis, fluorescent sensing has risen as a new major technique in neuroscience alongside molecular, electrophysiological, and imaging methods, opening up new avenues for research. Combined with multiphoton microscopy and fiber photometry, these sensors offer unique advantages in terms of cellular specificity, access to multiple targets - from calcium dynamics to neurotransmitter release to intracellular processes - as well as high capability for in vivo interrogation of neurobiological mechanisms underpinning behavior. Here, we provide a brief overview of the method, present examples of its integration with other tools in recent studies ranging from cellular to systems neuroscience, and discuss some of its principles and limitations, with the aim of introducing new potential users to this rapidly developing and potent technique.

Neurobiology of Obsessive-Compulsive Disorder from Genes to Circuits: Insights from Animal Models

Obsessive-compulsive disorder (OCD) is a chronic, severe psychiatric disorder that has been ranked by the World Health Organization as one of the leading causes of illness-related disability, and first-line interventions are limited in efficacy and have side-effect issues. However, the exact pathophysiology underlying this complex, heterogeneous disorder remains unknown. This scenario is now rapidly changing due to the advancement of powerful technologies that can be used to verify the function of the specific gene and dissect the neural circuits underlying the neurobiology of OCD in rodents. Genetic and circuit-specific manipulation in rodents has provided important insights into the neurobiology of OCD by identifying the molecular, cellular, and circuit events that induce OCD-like behaviors. This review will highlight recent progress specifically toward classic genetic animal models and advanced neural circuit findings, which provide theoretical evidence for targeted intervention on specific molecular, cellular, and neural circuit events.

A review article on the development of dopaminergic neurons and establishment of dopaminergic neuron-based in vitro models by using immortal cell lines or stem cells to study and treat Parkinson's disease

The primary pathological hallmark of Parkinson's disease (PD) is the degeneration of dopaminergic (DA) neurons in the substantia nigra pars compacta, a critical midbrain region. In vitro models based on DA neurons provide a powerful platform for investigating the cellular and molecular mechanisms of PD and testing novel therapeutic strategies. A deep understanding of DA neuron development, including the signalling pathways and transcription factors involved, is essential for advancing PD research. This article first explores the differentiation and maturation processes of DA neurons in the midbrain, detailing the relevant signalling pathways. It then compares various in vitro models, including primary cells, immortalized cell lines, and stem cell-based models, focusing on the advantages and limitations of each. Special attention is given to the role of immortalized and stem cell models in PD research. This review aims to guide researchers in selecting the most appropriate model for their specific research goals. Ethical considerations and clinical implications of using stem cells in PD research are also discussed.

Dopaminergic system and neurons: Role in multiple neurological diseases

The dopaminergic system is a complex and powerful neurotransmitter system in the brain. It plays an important regulatory role in motivation, reward, cognition, and motor control. In recent decades, research in the field of the dopaminergic system and neurons has increased exponentially and is gradually becoming a point of intervention in the study and understanding of a wide range of neurological diseases related to human health. Studies have shown that the dopaminergic system and neurons are involved in the development of many neurological diseases (including, but not limited to Parkinson's disease, schizophrenia, depression, attention deficit hyperactivity disorder, etc.) and that dopaminergic neurons either have too much stress or too weak function in the dopaminergic system can lead to disease. Therefore, targeting dopaminergic neurons is considered key to treating these diseases. This article provides a comprehensive review of the dopaminergic system and neurons in terms of brain region distribution, physiological function and subtypes of dopaminergic neurons, as well as the role of the dopaminergic system and neurons in a variety of diseases.

Implementation and validation of single-cell genomics experiments in neuroscience

Single-cell or single-nucleus transcriptomics is a powerful tool for identifying cell types and cell states. However, hypotheses derived from these assays, including gene expression information, require validation, and their functional relevance needs to be established. The choice of validation depends on numerous factors. Here, we present types of orthogonal and functional validation experiment to strengthen preliminary findings obtained using single-cell and single-nucleus transcriptomics as well as the challenges and limitations of these approaches.

TARGET-seq: Linking single-cell transcriptomics of human dopaminergic neurons with their target specificity

Dopaminergic (DA) neurons exhibit significant diversity characterized by differences in morphology, anatomical location, axonal projection pattern, and selective vulnerability to disease. More recently, scRNAseq has been used to map DA neuron diversity at the level of gene expression. These studies have revealed a higher than expected molecular diversity in both mouse and human DA neurons. However, whether different molecular expression profiles correlate with specific functions of different DA neurons or with their classical division into mesolimbic (A10) and nigrostriatal (A9) neurons, remains to be determined. To address this, we have developed an approach termed TARGET-seq (Tagging projections by AAV-mediated RetroGrade Enrichment of Transcriptomes) that links the transcriptional profile of the DA neurons with their innervation of specific target structures in the forebrain. Leveraging this technology, we identify molecularly distinct subclusters of human DA neurons with a clear link between transcriptome and axonal target-specificity, offering the possibility to infer neuroanatomical-based classification to molecular identity and target-specific connectivity. We subsequently used this dataset to identify candidate transcription factors along DA developmental trajectories that may control subtype identity, thus providing broad avenues that can be further explored in the design of next-generation A9 and A10 enriched DA-neurons for drug screening or A9 enriched DA cells for clinical stem cell-based therapies.

Corticonigral projections recruit substantia nigra pars lateralis dopaminergic neurons for auditory threat memories

Dopaminergic neurons (DANs) in the lateral substantia nigra project to the tail of striatum (TS), which is involved in threat conditioning. Auditory cortex also contributes to threatening behaviors, but whether it directly interacts with midbrain DANs and how these interactions might influence threat conditioning remain unclear. Here, functional mapping revealed robust excitatory input from auditory and temporal association cortexes to substantia nigra pars lateralis (SNL) DANs, but not to pars compacta (SNc) DANs. SNL DANs exhibited unique firing patterns, with irregular pacemaking and higher maximal firing, reflecting different channel complements than SNc DANs. Behaviorally, inhibiting cortex to SNL projections impaired memory retrieval during auditory threat conditioning. Thus, we demonstrate robust corticonigral projections to SNL DANs, contrasting with previous observations of sparse cortical input to substantia nigra DANs. These findings distinguish SNL DANs from other nigral populations, highlighting their role in threatening behaviors and expanding knowledge of cortex to midbrain interactions.

Proportion and distribution of neurotransmitter-defined cell types in the ventral tegmental area and substantia nigra pars compacta

Most studies on the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) have focused on dopamine neurons and their role in processes such as motivation, learning, movement, and associated disorders such as addiction and Parkinson's disease. However there has been increasing attention on other VTA and SNc cell types that release GABA, glutamate, or a combination of neurotransmitters. Yet the relative distributions and proportions of neurotransmitter-defined cell types across VTA and SNc has remained unclear. Here, we used fluorescent in situ hybridization in male and female mice to label VTA and SNc neurons that expressed mRNA encoding the canonical vesicular transporters for dopamine, GABA, or glutamate: vesicular monoamine transporter (VMAT2), vesicular GABA transporter (VGAT), and vesicular glutamate transporter (VGLUT2). Within VTA, we found that no one type was particularly more abundant, instead we observed similar numbers of VMAT2+ (44%), VGAT+ (37%) and VGLUT2+ (41%) neurons. In SNc we found that a slight majority of neurons expressed VMAT2 (54%), fewer were VGAT+ (42%), and VGLUT2+ neurons were least abundant (16%). Moreover, 20% of VTA neurons and 10% of SNc neurons expressed more than one vesicular transporter, including 45% of VGLUT2+ neurons. We also assessed within VTA and SNc subregions and found remarkable heterogeneity in cell-type composition. And by quantifying density across both anterior-posterior and medial-lateral axes we generated heatmaps to visualize the distribution of each cell type. Our data complement recent single-cell RNAseq studies and support a more diverse landscape of neurotransmitter-defined cell types in VTA and SNc than is typically appreciated.

Transient nicotine exposure in early adolescent male mice freezes their dopamine circuits in an immature state

How nicotine acts on developing neurocircuitry in adolescence to promote later addiction vulnerability remains largely unknown, but may hold the key for informing more effective intervention efforts. We found transient nicotine exposure in early adolescent (PND 21-28) male mice was sufficient to produce a marked vulnerability to nicotine in adulthood (PND 60 + ), associated with disrupted functional connectivity in dopaminergic circuits. These mice showed persistent adolescent-like behavioral and physiological responses to nicotine, suggesting that nicotine exposure in adolescence prolongs an immature, imbalanced state in the function of these circuits. Chemogenetically resetting the balance between the underlying dopamine circuits unmasked the mature behavioral response to acute nicotine in adolescent-exposed mice. Together, our results suggest that the perseverance of a developmental imbalance between dopamine pathways may alter vulnerability profiles for later dopamine-dependent psychopathologies.

On the genesis and unique functions of zinc neuromodulation

In addition to the essential structural and catalytic functions of zinc, evolution has adopted synaptic zinc as a neuromodulator. In the brain, synaptic zinc is released primarily from glutamatergic neurons, notably in the neocortex, hippocampus, amygdala, and auditory brainstem. In these brain areas, synaptic zinc is essential for neuronal and sensory processing fine-tuning. But what niche does zinc fill in neural signaling that other neuromodulators do not? Here, we discuss the evolutionary history of zinc as a signaling agent and its eventual adoption as an essential neuromodulator in the mammalian brain. We then attempt to describe the unique roles that zinc has carved out of the vast and diverse landscape of neuromodulators.

Distinct dynamics and intrinsic properties in ventral tegmental area populations mediate reward association and motivation

Ventral tegmental area (VTA) dopamine neurons regulate reward-related associative learning and reward-driven motivated behaviors, but how these processes are coordinated by distinct VTA neuronal subpopulations remains unresolved. Here, we compare the contribution of two primarily dopaminergic and largely non-overlapping VTA subpopulations, all VTA dopamine neurons and VTA GABAergic neurons of the mouse midbrain, to these processes. We find that the dopamine subpopulation that projects to the nucleus accumbens (NAc) core preferentially encodes reward-predictive cues and prediction errors. In contrast, the subpopulation that projects to the NAc shell preferentially encodes goal-directed actions and relative reward anticipation. VTA GABA neuron activity strongly contrasts VTA dopamine population activity and preferentially encodes reward outcome and retrieval. Electrophysiology, targeted optogenetics, and whole-brain input mapping reveal multiple convergent sources that contribute to the heterogeneity among VTA dopamine subpopulations that likely underlies their distinct encoding of reward-related associations and motivation that defines their functions in these contexts.

The neuronal density in the rostral pole of substantia nigra pars compacta in Wistar Albino rats from Rijswijk rats: A link to spike-wave seizures

This study aimed to investigate the role of the nigrostriatal dopaminergic system in the modulation of absence epilepsy. Immunochemical analysis of the rostral pole of the substantia nigra pars compacta (SNpc) was conducted on 13 adult male Wistar Albino rats from Rijswijk rats. The rostral pole of the SNpc included the dorsal and lateral parts. The neuronal density in the dorsal part was higher than in the lateral part. The ratio of dopaminergic to non-dopaminergic neurons in the lateral part of the SNpc was 1:1, while in the dorsal part, it was around 1.9:1. All rats exhibited spontaneous spike-wave discharges (SWDs) on their electrocorticograms. SWDs are known to be a hallmark of absence seizures in both human patients and rat models. In this study, we found that the number and duration of SWDs were negatively correlated with dopaminergic and non-dopaminergic neurons only in the lateral part of the SNpc. However, in the dorsal part of the SNpc, no correlations were found between neuronal density and the severity of absence epilepsy. Our findings suggest that the lateral SNpc may be involved in modulating the severity of absence epilepsy in genetically prone subjects. This contributes to a better understanding of the role of the nigrostriatal dopaminergic system in the absence of epilepsy.

Calbindin and Girk2/Aldh1a1 define resilient vs vulnerable dopaminergic neurons in a primate Parkinson's disease model

The differential vulnerability of dopaminergic neurons of the substantia nigra pars compacta (SNc) is a critical and unresolved question in Parkinson´s disease. Studies in mice show diverse susceptibility of subpopulations of nigral dopaminergic neurons to various toxic agents. In the primate midbrain, the molecular phenotypes of dopaminergic neurons and their differential vulnerability are poorly characterized. We performed a detailed histological study to determine the anatomical distribution of different molecular phenotypes within identified midbrain neurons and their selective vulnerability in control and MPTP-treated monkeys. In the ventral tier of the SNc (nigrosome), neurons rich in Aldh1a1 and Girk2 are intermingled, whereas calbindin is the marker that best identifies the most resilient neurons located in the dorsal tier and ventral tegmental area, recapitulating the well-defined dorsoventral axis of susceptibility to degeneration of dopaminergic neurons. In particular, a loss of Aldh1a1+ neurons in the ventral SNc was observed in parallel to the progressive development of parkinsonism. Aldh1a1+ neurons were the main population of vulnerable dopaminergic nigrostriatal-projecting neurons, while Aldh1a1- neurons giving rise to nigropallidal projections remained relatively preserved. Moreover, bundles of entwined Aldh1a1+ dendrites with long trajectories extending towards the substantia nigra pars reticulata emerged from clusters of Aldh1a1+ neurons and colocalized with dense cannabinoid receptor 1 afferent fibers likely representing part of the striatonigral projection that is affected in human disorders, including Parkinson´s disease. In conclusion, vulnerable nigrostriatal-projecting neurons can be identified by using Aldh1a1 and Girk2. Further studies are needed to define the afferent/efferent projection patterns of these most vulnerable neurons.

Distinct cholinergic circuits underlie discrete effects of reward on attention

Attention and reward are functions that are critical for the control of behavior, and massive multi-region neural systems have evolved to support the discrete computations associated with each. Previous research has also identified that attention and reward interact, though our understanding of the neural mechanisms that mediate this interplay is incomplete. Here, we review the basic neuroanatomy of attention, reward, and cholinergic systems. We then examine specific contexts in which attention and reward computations interact. Building on this work, we propose two discrete neural circuits whereby acetylcholine, released from cell groups located in different parts of the brain, mediates the impact of stimulus-reward associations as well as motivation on attentional control. We conclude by examining these circuits as a potential shared loci of dysfunction across diseases states associated with deficits in attention and reward.

Populations of Hindbrain Glucagon-Like Peptide 1 (GLP1) Neurons That Innervate the Hypothalamic PVH, Thalamic PVT, or Limbic Forebrain BST Have Axon Collaterals That Reach All Central Regions Innervated by GLP1 Neurons

Neurons in the caudal nucleus of the solitary tract (cNTS) and intermediate reticular nucleus (IRt) that express the glucagon gene (Gcg) give rise to glucagon-like peptide 1 (GLP1)-immunopositive axons in the spinal cord and many subcortical brain regions. Central GLP1 receptor signaling contributes to motivated behavior and stress responses in rats and mice, in which hindbrain GLP1 neurons are activated to express c-Fos in a metabolic state-dependent manner. The present study examined whether GLP1 inputs to distinct brain regions arise from distinct subsets of Gcg-expressing neurons, and mapped the distribution of axon collaterals arising from projection-defined GLP1 neural populations. Using our Gcg-Cre knock-in rat model, Cre-dependent adeno-associated virus (AAV) tracing was conducted in adult male and female rats to compare axonal projections of IRt versus cNTS GLP1 neurons. Overlapping projections were observed in all brain regions that receive GLP1 input, with the caveat that cNTS injections produced Cre-dependent labeling of some IRt neurons, and vice versa. In additional experiments, specific diencephalic or limbic forebrain nuclei were microinjected with Cre-dependent retrograde AAVs (AAVrg) that expressed reporters to fully label the axon collaterals of transduced GLP1 neurons. AAVrg injected into each forebrain site labeled Gcg-expressing neurons in both the cNTS and IRt. The collective axon collaterals of labeled neurons entered the spinal cord and every brain region previously reported to contain GLP1-positive axons. These results indicate that the axons of GLP1 neural populations that innervate the thalamic paraventricular nucleus, paraventricular nucleus of the hypothalamus, and/or bed nucleus of the stria terminalis collectively innervate all central regions that receive GLP1 axonal input.

The impact of the mesoprefrontal dopaminergic system on the maturation of interneurons in the murine prefrontal cortex

The prefrontal cortex (PFC) undergoes a protracted maturation process. This is true both for local interneurons and for innervation from midbrain dopaminergic (mDA) neurons. In the striatum, dopaminergic (DA) neurotransmission is required for the maturation of medium spiny neurons during a critical developmental period. To investigate whether DA innervation influences the maturation of interneurons in the PFC, we used a conditional knockout (cKO) mouse model in which innervation from mDA neurons to the mPFC (mesoprefrontal innnervation) is not established during development. In this mouse model, the maturation of parvalbumin (PV) and calbindin (CB) interneuron populations in the PFC is dysregulated during a critical period in adolescence with changes persisting into adulthood. PV interneurons are particularly vulnerable to lack of mesoprefrontal input, showing an inability to maintain adequate PV expression with a concomitant decrease in Gad1 expression levels. Interestingly, lack of mesoprefrontal innervation does not appear to induce compensatory changes such as upregulation of DA receptor expression in PFC neurons or increased innervation density of other neuromodulatory (serotonergic and noradrenergic) innervation. In conclusion, our study shows that adolescence is a sensitive period during which mesoprefrontal input plays a critical role in promoting the maturation of specific interneuron subgroups. The results of this study will help to understand how a dysregulated mesoprefrontal DA system contributes to the pathophysiology of neurodevelopmental disorders.

Volume electron microscopy for genetically and molecularly defined neural circuits

The brain networks responsible for adaptive behavioral changes are based on the physical connections between neurons. Light and electron microscopy have long been used to study neural projections and the physical connections between neurons. Volume electron microscopy has recently expanded its scale of analysis due to methodological advances, resulting in complete wiring maps of neurites in a large volume of brain tissues and even entire nervous systems in a growing number of species. However, structural approaches frequently suffer from inherent limitations in which elements in images are identified solely by morphological criteria. Recently, an increasing number of tools and technologies have been developed to characterize cells and cellular components in the context of molecules and gene expression. These advancements include newly developed probes for visualization in electron microscopic images as well as correlative integration methods for the same elements across multiple microscopic modalities. Such approaches advance our understanding of interactions between specific neurons and circuits and may help to elucidate novel aspects of the basal ganglia network involving dopamine neurons. These advancements are expected to reveal mechanisms for processing adaptive changes in specific neural circuits that modulate brain functions.

The transcriptomic and spatial organization of telencephalic GABAergic neuronal types

The telencephalon of the mammalian brain comprises multiple regions and circuit pathways that play adaptive and integrative roles in a variety of brain functions. There is a wide array of GABAergic neurons in the telencephalon; they play a multitude of circuit functions, and dysfunction of these neurons has been implicated in diverse brain disorders. In this study, we conducted a systematic and in-depth analysis of the transcriptomic and spatial organization of GABAergic neuronal types in all regions of the mouse telencephalon and their developmental origins. This was accomplished by utilizing 611,423 single-cell transcriptomes from the comprehensive and high-resolution transcriptomic and spatial cell type atlas for the adult whole mouse brain we have generated, supplemented with an additional single-cell RNA-sequencing dataset containing 99,438 high-quality single-cell transcriptomes collected from the pre- and postnatal developing mouse brain. We present a hierarchically organized adult telencephalic GABAergic neuronal cell type taxonomy of 7 classes, 52 subclasses, 284 supertypes, and 1,051 clusters, as well as a corresponding developmental taxonomy of 450 clusters across different ages. Detailed charting efforts reveal extraordinary complexity where relationships among cell types reflect both spatial locations and developmental origins. Transcriptomically and developmentally related cell types can often be found in distant and diverse brain regions indicating that long-distance migration and dispersion is a common characteristic of nearly all classes of telencephalic GABAergic neurons. Additionally, we find various spatial dimensions of both discrete and continuous variations among related cell types that are correlated with gene expression gradients. Lastly, we find that cortical, striatal and some pallidal GABAergic neurons undergo extensive postnatal diversification, whereas septal and most pallidal GABAergic neuronal types emerge simultaneously during the embryonic stage with limited postnatal diversification. Overall, the telencephalic GABAergic cell type taxonomy can serve as a foundational reference for molecular, structural and functional studies of cell types and circuits by the entire community.

Salience signaling and stimulus scaling of ventral tegmental area glutamate neuron subtypes

Ventral tegmental area (VTA) glutamatergic neurons participate in reward, aversion, drug-seeking, and stress. Subsets of VTA VGluT2+ neurons are capable of co-transmitting glutamate and GABA (VGluT2+VGaT+ neurons), transmitting glutamate without GABA (VGluT2+VGaT- neurons), or co-transmitting glutamate and dopamine (VGluT2+TH+ neurons), but whether these molecularly distinct subpopulations show behavior-related differences is not wholly understood. We identified that neuronal activity of each VGluT2+ subpopulation is sensitive to reward value but signaled this in different ways. The phasic maximum activity of VGluT2+VGaT+ neurons increased with sucrose concentration, whereas VGluT2+VGaT- neurons increased maximum and sustained activity with sucrose concentration, and VGluT2+TH+ neurons increased sustained but not maximum activity with sucrose concentration. Additionally, VGluT2+ subpopulations signaled consummatory preferences in different ways. VGluT2+VGaT- neurons and VGluT2+TH+ neurons showed a signaling preference for a behaviorally-preferred fat reward over sucrose, but in temporally-distinct ways. In contrast, VGluT2+VGaT+ neurons uniquely signaled a less behaviorally-preferred sucrose reward compared with fat. Further experiments suggested that VGluT2+VGaT+ consummatory reward-related activity was related to sweetness, partially modulated by hunger state, and not dependent on caloric content or behavioral preference. All VGluT2+ subtypes increased neuronal activity following aversive stimuli but VGluT2+VGaT+ neurons uniquely scaled their magnitude and sustained activity with footshock intensity. Optogenetic activation of VGluT2+VGaT+ neurons during low intensity footshock enhanced fear-related behavior without inducing place preference or aversion. We interpret these data such that VTA glutamatergic subpopulations signal different elements of rewarding and aversive experiences and highlight the unique role of VTA VGluT2+VGaT+ neurons in enhancing the salience of behavioral experiences.

Aging disrupts the coordination between mRNA and protein expression in mouse and human midbrain

Age-related dopamine (DA) neuron loss is a primary feature of Parkinson's disease. However, it remains unclear whether similar biological processes occur during healthy aging, albeit to a lesser degree. We therefore determined whether midbrain DA neurons degenerate during aging in mice and humans. In mice, we identified no changes in midbrain neuron numbers throughout aging. Despite this, we found age-related decreases in midbrain mRNA expression of tyrosine hydroxylase (Th), the rate limiting enzyme of DA synthesis. Among midbrain glutamatergic cells, we similarly identified age-related declines in vesicular glutamate transporter 2 (Vglut2) mRNA expression. In co-transmitting Th +/Vglut2 + neurons, Th and Vglut2 transcripts decreased with aging. Importantly, striatal Th and Vglut2 protein expression remained unchanged. In translating our findings to humans, we found no midbrain neurodegeneration during aging and identified age-related decreases in TH and VGLUT2 mRNA expression similar to mouse. Unlike mice, we discovered diminished density of striatal TH+ dopaminergic terminals in aged human subjects. However, TH and VGLUT2 protein expression were unchanged in the remaining striatal boutons. Finally, in contrast to Th and Vglut2 mRNA, expression of most ribosomal genes in Th + neurons was either maintained or even upregulated during aging. This suggests a homeostatic mechanism where age-related declines in transcriptional efficiency are overcome by ongoing ribosomal translation. Overall, we demonstrate species-conserved transcriptional effects of aging in midbrain dopaminergic and glutamatergic neurons that are not accompanied by marked cell death or lower striatal protein expression. This opens the door to novel therapeutic approaches to maintain neurotransmission and bolster neuronal resilience.

Ventral pallidum GABA and glutamate neurons drive approach and avoidance through distinct modulation of VTA cell types

The ventral pallidum (VP) contains GABA and glutamate neurons projecting to ventral tegmental area (VTA) whose stimulation drives approach and avoidance, respectively. Yet little is known about the mechanisms by which VP cell types shape VTA activity and drive behavior. Here, we found that both VP GABA and glutamate neurons were activated during approach to reward or by delivery of an aversive stimulus. Stimulation of VP GABA neurons inhibited VTA GABA, but activated dopamine and glutamate neurons. Remarkably, stimulation-evoked activation was behavior-contingent such that VTA recruitment was inhibited when evoked by the subject's own action. Conversely, VP glutamate neurons activated VTA GABA, as well as dopamine and glutamate neurons, despite driving aversion. However, VP glutamate neurons evoked dopamine in aversion-associated ventromedial nucleus accumbens (NAc), but reduced dopamine release in reward-associated dorsomedial NAc. These findings show how heterogeneous VP projections to VTA can be engaged to shape approach and avoidance behaviors.

Functional diversity of dopamine axons in prefrontal cortex during classical conditioning

Midbrain dopamine neurons impact neural processing in the prefrontal cortex (PFC) through mesocortical projections. However, the signals conveyed by dopamine projections to the PFC remain unclear, particularly at the single-axon level. Here, we investigated dopaminergic axonal activity in the medial PFC (mPFC) during reward and aversive processing. By optimizing microprism-mediated two-photon calcium imaging of dopamine axon terminals, we found diverse activity in dopamine axons responsive to both reward and aversive stimuli. Some axons exhibited a preference for reward, while others favored aversive stimuli, and there was a strong bias for the latter at the population level. Long-term longitudinal imaging revealed that the preference was maintained in reward- and aversive-preferring axons throughout classical conditioning in which rewarding and aversive stimuli were paired with preceding auditory cues. However, as mice learned to discriminate reward or aversive cues, a cue activity preference gradually developed only in aversive-preferring axons. We inferred the trial-by-trial cue discrimination based on machine learning using anticipatory licking or facial expressions, and found that successful discrimination was accompanied by sharper selectivity for the aversive cue in aversive-preferring axons. Our findings indicate that a group of mesocortical dopamine axons encodes aversive-related signals, which are modulated by both classical conditioning across days and trial-by-trial discrimination within a day.

Characterizing enteric neurons in dopamine transporter (DAT)-Cre reporter mice reveals dopaminergic subtypes with dual-transmitter content

The enteric nervous system (ENS) comprises a complex network of neurons whereby a subset appears to be dopaminergic although the characteristics, roles, and implications in disease are less understood. Most investigations relating to enteric dopamine (DA) neurons rely on immunoreactivity to tyrosine hydroxylase (TH)-the rate-limiting enzyme in the production of DA. However, TH immunoreactivity is likely to provide an incomplete picture. This study herein provides a comprehensive characterization of DA neurons in the gut using a reporter mouse line, expressing a fluorescent protein (tdTomato) under control of the DA transporter (DAT) promoter. Our findings confirm a unique localization of DA neurons in the gut and unveil the discrete subtypes of DA neurons in this organ, which we characterized using both immunofluorescence and single-cell transcriptomics, as well as validated using in situ hybridization. We observed distinct subtypes of DAT-tdTomato neurons expressing co-transmitters and modulators across both plexuses; some of them likely co-releasing acetylcholine, while others were positive for a slew of canonical DAergic markers (TH, VMAT2 and GIRK2). Interestingly, we uncovered a seemingly novel population of DA neurons unique to the ENS which was ChAT/DAT-tdTomato-immunoreactive and expressed Grp, Calcb, and Sst. Given the clear heterogeneity of DAergic gut neurons, further investigation is warranted to define their functional signatures and decipher their implication in disease.

Role of dopamine neurons in familiarity

Dopamine neurons signal the salience of environmental stimuli and influence learning, although it is less clear if these neurons also determine the salience of memories. Ventral tegmental area (VTA) dopamine neurons increase their firing in the presence of new objects and reduce it upon repeated, inconsequential exposures, marking the shift from novelty to familiarity. This study investigates how dopamine neuron activity during repeated familiar object exposure affects an animal's preference for new objects in a subsequent novel object recognition (NOR) test. We hypothesize that a single familiarization session will not sufficiently lower dopamine activity, such that the memory of a familiar object remains salient, leading to equal exploration of familiar and novel objects and weaker NOR discrimination. In contrast, multiple familiarization sessions likely suppress dopamine activity more effectively, reducing the salience of the familiar object and enhancing subsequent novelty discrimination. Our experiments in mice indicated that multiple familiarization sessions reduce VTA dopamine neuron activation, as measured by c-Fos expression, and enhance novelty discrimination compared with a single familiarization session. Dopamine neurons that show responsiveness to novelty were primarily located in the paranigral nucleus of the VTA and expressed vesicular glutamate transporter 2 transcripts, marking them as dopamine-glutamate neurons. Chemogenetic inhibition of dopamine neurons during a single session paralleled the effects of multiple sessions, improving NOR. These findings suggest that a critical role of dopamine neurons during the transition from novelty to familiarity is to modulate the salience of an object's memory.

Distributional coding of associative learning in discrete populations of midbrain dopamine neurons

Midbrain dopamine neurons are thought to play key roles in learning by conveying the difference between expected and actual outcomes. Recent evidence suggests diversity in dopamine signaling, yet it remains poorly understood how heterogeneous signals might be organized to facilitate the role of downstream circuits mediating distinct aspects of behavior. Here, we investigated the organizational logic of dopaminergic signaling by recording and labeling individual midbrain dopamine neurons during associative behavior. Our findings show that reward information and behavioral parameters are not only heterogeneously encoded but also differentially distributed across populations of dopamine neurons. Retrograde tracing and fiber photometry suggest that populations of dopamine neurons projecting to different striatal regions convey distinct signals. These data, supported by computational modeling, indicate that such distributional coding can maximize dynamic range and tailor dopamine signals to facilitate specialized roles of different striatal regions.

Transcriptomic atlas of midbrain dopamine neurons uncovers differential vulnerability in a Parkinsonism lesion model

Midbrain dopamine (mDA) neurons comprise diverse cells with unique innervation targets and functions. This is illustrated by the selective sensitivity of mDA neurons of the substantia nigra compacta (SNc) in patients with Parkinson's disease, while those in the ventral tegmental area (VTA) are relatively spared. Here, we used single nuclei RNA sequencing (snRNA-seq) of approximately 70,000 mouse midbrain cells to build a high-resolution atlas of mouse mDA neuron diversity at the molecular level. The results showed that differences between mDA neuron groups could best be understood as a continuum without sharp differences between subtypes. Thus, we assigned mDA neurons to several 'territories' and 'neighborhoods' within a shifting gene expression landscape where boundaries are gradual rather than discrete. Based on the enriched gene expression patterns of these territories and neighborhoods, we were able to localize them in the adult mouse midbrain. Moreover, because the underlying mechanisms for the variable sensitivities of diverse mDA neurons to pathological insults are not well understood, we analyzed surviving neurons after partial 6-hydroxydopamine (6-OHDA) lesions to unravel gene expression patterns that correlate with mDA neuron vulnerability and resilience. Together, this atlas provides a basis for further studies on the neurophysiological role of mDA neurons in health and disease.

A dual-omics approach on the effects of fibroblast growth factor-2 (FGF-2) on ventral tegmental area dopaminergic neurons in response to alcohol consumption in mice

Harmful alcohol consumption is a major socioeconomic burden to the health system, as it can be the cause of mortality of heavy alcohol drinkers. The dopaminergic (DAergic) system is thought to play an important role in the pathogenesis of alcohol drinking behaviour; however, its exact role remains elusive. Fibroblast growth factor 2 (FGF-2), a neurotrophic factor, associated with both the DAergic system and alcohol consumption, may play an important role in DAergic neuroadaptations during alcohol abuse. Within this study, we aimed to clarify the role of endogenous FGF-2 on the DAergic system and whether there is a possible link to alcohol consumption. We found that lack of FGF-2 reduces the alcohol intake of mice. Transcriptome analysis of DAergic neurons revealed that FGF-2 knockout (FGF-2 KO) shifts the molecular fingerprint of midbrain dopaminergic (mDA) neurons to DA subtypes of the ventral tegmental area (VTA). In line with this, proteomic changes predominantly appear also in the VTA. Interestingly, these changes led to an altered regulation of the FGF-2 signalling cascades and DAergic pathways in a region-specific manner, which was only marginally affected by voluntary alcohol consumption. Thus, lack of FGF-2 not only affects the gene expression but also the proteome of specific brain regions of mDA neurons. Our study provides new insights into the neuroadaptations of the DAergic system during alcohol abuse and, therefore, comprises novel targets for future pharmacological interventions.

Dopamine across timescales and cell types: Relevance for phenotypes in Parkinson's disease progression

Dopamine neurons in the substantia nigra pars compacta (SNc) synthesize and release dopamine, a critical neurotransmitter for movement and learning. SNc dopamine neurons degenerate in Parkinson's Disease (PD), causing a host of motor and non-motor symptoms. Here, we review recent conceptual advances in our basic understanding of the dopamine system - including our rapidly advancing knowledge of dopamine neuron heterogeneity - with special attention to their importance for understanding PD. In PD patients, dopamine neuron degeneration progresses from lateral SNc to medial SNc, suggesting clinically relevant heterogeneity in dopamine neurons. With technical advances in dopamine system interrogation, we can understand the relevance of this heterogeneity for PD progression and harness it to develop new treatments.

Different electrophysiological profiles of genetically labelled dopaminergic neurons in the mouse midbrain and olfactory bulb

Dopaminergic (DA) neurons play pivotal roles in diverse brain functions, spanning movement, reward processing and sensory perception. DA neurons are most abundant in the midbrain (Substantia Nigra pars compacta [SNC] and Ventral Tegmental Area [VTA]) and the olfactory bulb (OB) in the forebrain. Interestingly, a subtype of OB DA neurons is capable of regenerating throughout life, while a second class is exclusively born during embryonic development. Compelling evidence in SNC and VTA also indicates substantial heterogeneity in terms of morphology, connectivity and function. To further investigate this heterogeneity and directly compare form and function of midbrain and forebrain bulbar DA neurons, we performed immunohistochemistry and whole-cell patch-clamp recordings in ex vivo brain slices from juvenile DAT-tdTomato mice. After confirming the penetrance and specificity of the dopamine transporter (DAT) Cre line, we compared soma shape, passive membrane properties, voltage sags and action potential (AP) firing across midbrain and forebrain bulbar DA subtypes. We found that each DA subgroup within midbrain and OB was highly heterogeneous, and that DA neurons across the two brain areas are also substantially different. These findings complement previous work in rats as well as gene expression and in vivo datasets, further questioning the existence of a single "dopaminergic" neuronal phenotype.

The Formation and Function of the VTA Dopamine System

The midbrain dopamine system is a sophisticated hub that integrates diverse inputs to control multiple physiological functions, including locomotion, motivation, cognition, reward, as well as maternal and reproductive behaviors. Dopamine is a neurotransmitter that binds to G-protein-coupled receptors. Dopamine also works together with other neurotransmitters and various neuropeptides to maintain the balance of synaptic functions. The dysfunction of the dopamine system leads to several conditions, including Parkinson's disease, Huntington's disease, major depression, schizophrenia, and drug addiction. The ventral tegmental area (VTA) has been identified as an important relay nucleus that modulates homeostatic plasticity in the midbrain dopamine system. Due to the complexity of synaptic transmissions and input-output connections in the VTA, the structure and function of this crucial brain region are still not fully understood. In this review article, we mainly focus on the cell types, neurotransmitters, neuropeptides, ion channels, receptors, and neural circuits of the VTA dopamine system, with the hope of obtaining new insight into the formation and function of this vital brain region.

GABA co-released from striatal dopamine axons dampens phasic dopamine release through autoregulatory GABAA receptors

Striatal dopamine axons co-release dopamine and gamma-aminobutyric acid (GABA), using GABA provided by uptake via GABA transporter-1 (GAT1). Functions of GABA co-release are poorly understood. We asked whether co-released GABA autoinhibits dopamine release via axonal GABA type A receptors (GABAARs), complementing established inhibition by dopamine acting at axonal D2 autoreceptors. We show that dopamine axons express α3-GABAAR subunits in mouse striatum. Enhanced dopamine release evoked by single-pulse optical stimulation in striatal slices with GABAAR antagonism confirms that an endogenous GABA tone limits dopamine release. Strikingly, an additional inhibitory component is seen when multiple pulses are used to mimic phasic axonal activity, revealing the role of GABAAR-mediated autoinhibition of dopamine release. This autoregulation is lost in conditional GAT1-knockout mice lacking GABA co-release. Given the faster kinetics of ionotropic GABAARs than G-protein-coupled D2 autoreceptors, our data reveal a mechanism whereby co-released GABA acts as a first responder to dampen phasic-to-tonic dopamine signaling.

Dopamine neuron activity encodes the length of upcoming contralateral movement sequences

Dopaminergic neurons (DANs) in the substantia nigra pars compacta (SNc) have been related to movement speed, and loss of these neurons leads to bradykinesia in Parkinson's disease (PD). However, other aspects of movement vigor are also affected in PD; for example, movement sequences are typically shorter. However, the relationship between the activity of DANs and the length of movement sequences is unknown. We imaged activity of SNc DANs in mice trained in a freely moving operant task, which relies on individual forelimb sequences. We uncovered a similar proportion of SNc DANs increasing their activity before either ipsilateral or contralateral sequences. However, the magnitude of this activity was higher for contralateral actions and was related to contralateral but not ipsilateral sequence length. In contrast, the activity of reward-modulated DANs, largely distinct from those modulated by movement, was not lateralized. Finally, unilateral dopamine depletion impaired contralateral, but not ipsilateral, sequence length. These results indicate that movement-initiation DANs encode more than a general motivation signal and invigorate aspects of contralateral movements.

Optogenetic action potentials and intrinsic pacemaker interplay in retrogradely identified midbrain dopamine neurons

Dissecting the diversity of midbrain dopamine (DA) neurons by optotagging is a promising addition to better identify their functional properties and contribution to motivated behavior. Retrograde molecular targeting of DA neurons with specific axonal projection allows further refinement of this approach. Here, we focus on adult mouse DA neurons in the substantia nigra pars compacta (SNc) projecting to dorsal striatum (DS) by demonstrating the selectivity of a floxed AAV9-based retrograde channelrhodopsin-eYFP (ChR-eYFP) labeling approach in DAT-cre mice. Furthermore, we show the utility of a sparse labeling version for anatomical single-cell reconstruction and demonstrate that ChR-eYFR expressing DA neurons retain intrinsic functional properties indistinguishable from conventionally retrogradely red-beads-labeled neurons. We systematically explore the properties of optogenetically evoked action potentials (oAPs) and their interaction with intrinsic pacemaking in this defined subpopulation of DA neurons. We found that the shape of the oAP and its first derivative, as a proxy for extracellularly recorded APs, is highly distinct from spontaneous APs (sAPs) of the same neurons and systematically varies across the pacemaker duty cycle. The timing of the oAP also affects the backbone oscillator of the intrinsic pacemaker by introducing transient "compensatory pauses". Characterizing this systematic interplay between oAPs and sAPs in defined DA neurons will also facilitate a refinement of DA neuron optotagging in vivo.

Heterogeneity of mesencephalic dopaminergic neurons: From molecular classifications, electrophysiological properties to functional connectivity

The mesencephalic dopamine (DA) system is composed of neuronal subtypes that are molecularly and functionally distinct, are responsible for specific behaviors, and are closely associated with numerous brain disorders. Existing research has made significant advances in identifying the heterogeneity of mesencephalic DA neurons, which is necessary for understanding their diverse physiological functions and disease susceptibility. Moreover, there is a conflict regarding the electrophysiological properties of the distinct subsets of midbrain DA neurons. This review aimed to elucidate recent developments in the heterogeneity of midbrain DA neurons, including subpopulation categorization, electrophysiological characteristics, and functional connectivity to provide new strategies for accurately identifying distinct subtypes of midbrain DA neurons and investigating the underlying mechanisms of these neurons in various diseases.

Prediction error in dopamine neurons during associative learning

Dopamine neurons have long been thought to facilitate learning by broadcasting reward prediction error (RPE), a teaching signal used in machine learning, but more recent work has advanced alternative models of dopamine's computational role. Here, I revisit this critical issue and review new experimental evidences that tighten the link between dopamine activity and RPE. First, I introduce the recent observation of a gradual backward shift of dopamine activity that had eluded researchers for over a decade. I also discuss several other findings, such as dopamine ramping, that were initially interpreted to conflict but later found to be consistent with RPE. These findings improve our understanding of neural computation in dopamine neurons.

Neuromelanin-sensitive MRI as a promising biomarker of catecholamine function

Disruptions to dopamine and noradrenergic neurotransmission are noted in several neurodegenerative and psychiatric disorders. Neuromelanin-sensitive (NM)-MRI offers a non-invasive approach to visualize and quantify the structural and functional integrity of the substantia nigra and locus coeruleus. This method may aid in the diagnosis and quantification of longitudinal changes of disease and could provide a stratification tool for predicting treatment success of pharmacological interventions targeting the dopaminergic and noradrenergic systems. Given the growing clinical interest in NM-MRI, understanding the contrast mechanisms that generate this signal is crucial for appropriate interpretation of NM-MRI outcomes and for the continued development of quantitative MRI biomarkers that assess disease severity and progression. To date, most studies associate NM-MRI measurements to the content of the neuromelanin pigment and/or density of neuromelanin-containing neurons, while recent studies suggest that the main source of the NM-MRI contrast is not the presence of neuromelanin but the high-water content in the dopaminergic and noradrenergic neurons. In this review, we consider the biological and physical basis for the NM-MRI contrast and discuss a wide range of interpretations of NM-MRI. We describe different acquisition and image processing approaches and discuss how these methods could be improved and standardized to facilitate large-scale multisite studies and translation into clinical use. We review the potential clinical applications in neurological and psychiatric disorders and the promise of NM-MRI as a biomarker of disease, and finally, we discuss the current limitations of NM-MRI that need to be addressed before this technique can be utilized as a biomarker and translated into clinical practice and offer suggestions for future research.

Targeting the Actions of Muscarinic Receptors on Dopamine Systems: New Strategies for Treating Neuropsychiatric Disorders

Cholinergic regulation of dopamine (DA) signaling has significant implications for numerous disorders, including schizophrenia, substance use disorders, and mood-related disorders. The activity of midbrain DA neurons and DA release patterns in terminal regions are tightly regulated by cholinergic neurons found in both the striatum and the hindbrain. These cholinergic neurons can modulate DA circuitry by activating numerous receptors, including muscarinic acetylcholine receptor (mAChR) subtypes. This review specifically focuses on the complex role of M2, M4, and M5 mAChR subtypes in regulating DA neuron activity and DA release and the potential clinical implications of targeting these mAChR subtypes.

Amygdalo-nigral inputs target dopaminergic and GABAergic neurons in the primate: a view from dendrites and soma

The central nucleus (CeN) of the amygdala is an important afferent to the DA system that mediates motivated learning. We previously found that CeN terminals in nonhuman primates primarily overlap the elongated lateral VTA (parabrachial pigmented nucleus, PBP, A10), and retrorubral field(A8) subregion. Here, we examined CeN afferent contacts on cell somata and proximal dendrites of DA and GABA neurons, and distal dendrites of each, using confocal and electron microscopy (EM) methods, respectively. At the soma/proximal dendrites, the proportion of TH+ and GAD1+ cells receiving at least one CeN afferent contact was surprisingly similar (TH = 0.55: GAD1=0.55 in PBP; TH = 0.56; GAD1 =0.51 in A8), with the vast majority of contacted TH+ and GAD1+ soma/proximal dendrites received 1-2 contacts. Similar numbers of tracer-labeled terminals also contacted TH-positive and GAD1-positive small dendrites and/or spines (39% of all contacted dendrites were either TH- or GAD1-labeled). Overall, axon terminals had more symmetric (putative inhibitory) axonal contacts with no difference in the relative distribution in the PBP versus A8, or onto TH+ versus GAD1+ dendrites/spines in either region. The striking uniformity in the amygdalonigral projection across the PBP-A8 terminal field suggests that neither neurotransmitter phenotype nor midbrain location dictates likelihood of a terminal contact. We discuss how this afferent uniformity can play out in recently discovered differences in DA:GABA cell densities between the PBP and A8, and affect specific outputs.

Significance statement

The amygdala's central nucleus (CeN) channels salient cues to influence both appetitive and aversive responses via DA outputs. In higher species, the broad CeN terminal field overlaps the parabrachial pigmented nucleus ('lateral A10') and the retrorubral field (A8). We quantified terminal contacts in each region on DA and GABAergic soma/proximal dendrites and small distal dendrites. There was striking uniformity in contacts on DA and GABAergic cells, regardless of soma and dendritic compartment, in both regions. Most contacts were symmetric (putative inhibitory) with little change in the ratio of inhibitory to excitatory contacts by region.We conclude that post-synaptic shifts in DA-GABA ratios are key to understanding how these relatively uniform inputs can produce diverse effects on outputs.

Molecular profiling of human substantia nigra identifies diverse neuron types associated with vulnerability in Parkinson's disease

Parkinson's disease (PD) is characterized pathologically by the loss of dopaminergic (DA) neurons in the substantia nigra (SN). Whether cell types beyond DA neurons in the SN show vulnerability in PD remains unclear. Through transcriptomic profiling of 315,867 high-quality single nuclei in the SN from individuals with and without PD, we identified cell clusters representing various neuron types, glia, endothelial cells, pericytes, fibroblasts, and T cells and investigated cell type-dependent alterations in gene expression in PD. Notably, a unique neuron cluster marked by the expression of RIT2, a PD risk gene, also displayed vulnerability in PD. We validated RIT2-enriched neurons in midbrain organoids and the mouse SN. Our results demonstrated distinct transcriptomic signatures of the RIT2-enriched neurons in the human SN and implicated reduced RIT2 expression in the pathogenesis of PD. Our study sheds light on the diversity of cell types, including DA neurons, in the SN and the complexity of molecular and cellular changes associated with PD pathogenesis.

Dopamine pathways mediating affective state transitions after sleep loss

The pathophysiology of affective disorders-particularly circuit-level mechanisms underlying bidirectional, periodic affective state transitions-remains poorly understood. In patients, disruptions of sleep and circadian rhythm can trigger transitions to manic episodes, whereas depressive states are reversed. Here, we introduce a hybrid automated sleep deprivation platform to induce transitions of affective states in mice. Acute sleep loss causes mixed behavioral states, featuring hyperactivity, elevated social and sexual behaviors, and diminished depressive-like behaviors, where transitions depend on dopamine (DA). Using DA sensor photometry and projection-targeted chemogenetics, we reveal that elevated DA release in specific brain regions mediates distinct behavioral changes in affective state transitions. Acute sleep loss induces DA-dependent enhancement in dendritic spine density and uncaging-evoked dendritic spinogenesis in the medial prefrontal cortex, whereas optically mediated disassembly of enhanced plasticity reverses the antidepressant effects of sleep deprivation on learned helplessness. These findings demonstrate that brain-wide dopaminergic pathways control sleep-loss-induced polymodal affective state transitions.

Inhibitory Pedunculopontine Neurons Gate Dopamine-Mediated Motor Actions of Unsigned Valence

BACKGROUND

The pedunculopontine nucleus (PPN) maintains a bidirectional connectivity with the basal ganglia that supports their shared roles in the selection and execution of motor actions. Previous studies identified a role for PPN neurons in goal-directed behavior, but the cellular substrates underlying this function have not been elucidated. We recently revealed the existence of a monosynaptic GABAergic input from the PPN that inhibits dopamine neurons of the substantia nigra. Activation of this pathway interferes with the execution of learned motor sequences when the actions are rewarded, even though the inhibition of dopamine neurons did not shift the value of the action, hence suggesting executive control over the gating of behavior.

OBJECTIVE

To test the attributes of the inhibition of dopamine neurons by the PPN in the context of goal-directed behavior regardless of whether the outcome is positively or negatively reinforced.

METHODS

We delivered optogenetic stimulation to PPN GABAergic axon terminals in the substantia nigra during a battery of behavioral tasks with positive and negative valence.

RESULTS

Inhibition of dopamine neurons by PPN optogenetic activation during an appetitive task impaired the initiation and overall execution of the behavioral sequence without affecting the consumption of reward. During an active avoidance task, the same activation impaired the ability of mice to avoid a foot shock, but their escape response was unaffected. In addition, responses to potential threats were significantly attenuated.

CONCLUSION

Our results show that PPN GABAergic neurons modulate learned, goal-directed behavior of unsigned valence without affecting overall motor behavior.

A nigroincertal projection mediates aversion and enhances coping responses to potential threat

Recent studies have shown that the non-DA neurons in the ventral tegmental area (VTA) and substantia nigra (SN) not only modulate motivational behaviors but also regulate defensive behaviors. While zona incerta (ZI) is a threat-responsive substrate and receives innervations from the ventral midbrain, the function of the ventral midbrain-to-ZI connection remains poorly defined. Here, we demonstrate that the ZI receives heterogenous innervations from the ventral midbrain. By utilizing a retrograde AAV preferentially labeling non-DA neurons in the ventral midbrain, we found that ZI-projecting non-DA cells in the ventral midbrain are activated by restraint stress. We focused on the SN and found that SN-to-ZI GABAergic input is engaged by a predatory odor. Sustained pan-neuronal SN-to-ZI activation results in aversion and enhances defensive behaviors, likely through a disinhibition mechanism to recruit downstream brain regions that regulate defensive behaviors. Collectively, our results reveal a novel role of nigroincertal projection in mediating negative valence and regulating defensive behaviors.

Combinatorial genetic strategies for dissecting cell lineages, cell types, and gene function in the mouse brain

Research in neuroscience has greatly benefited from the development of genetic approaches that enable lineage tracing, cell type targeting, and conditional gene regulation. Recent advances in combinatorial strategies, which integrate multiple cellular features, have significantly enhanced the spatiotemporal precision and flexibility of these manipulations. In this minireview, we introduce the concept and design of these strategies and provide a few examples of their application in genetic fate mapping, cell type targeting, and reversible conditional gene regulation. These advancements have facilitated in-depth investigation into the developmental principles underlying the assembly of brain circuits, granting experimental access to highly specific cell lineages and subtypes, as well as offering valuable new tools for modeling and studying neurological diseases. Additionally, we discuss future directions aimed at expanding and improving the existing genetic toolkit for a better understanding of the development, structure, and function of healthy and diseased brains.

Alterations in neurotransmitter co-release in Parkinson's disease

Parkinson's disease is a neurological disorder characterized by degeneration of midbrain dopamine neurons, which results in numerous adaptations in basal ganglia circuits. Research over the past twenty-five years has identified that midbrain dopamine neurons of the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) co-release multiple other transmitters including glutamate and GABA, in addition to their canonical transmitter, dopamine. This review summarizes previous work characterizing neurotransmitter co-release from dopamine neurons, work examining potential changes in co-release dynamics that result in animal models of Parkinson's disease, and future opportunities for determining how dysfunction in co-release may contribute to circuit dysfunction in Parkinson's disease.

Distributed dopaminergic signaling in the basal ganglia and its relationship to motor disability in Parkinson's disease

The degeneration of mesencephalic dopaminergic neurons that innervate the basal ganglia is responsible for the cardinal motor symptoms of Parkinson's disease (PD). It has been thought that loss of dopaminergic signaling in one basal ganglia region - the striatum - was solely responsible for the network pathophysiology causing PD motor symptoms. While our understanding of dopamine (DA)'s role in modulating striatal circuitry has deepened in recent years, it also has become clear that it acts in other regions of the basal ganglia to influence movement. Underscoring this point, examination of a new progressive mouse model of PD shows that striatal dopamine DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. This review summarizes recent advances in the effort to understand basal ganglia circuitry, its modulation by DA, and how its dysfunction drives PD motor symptoms.

Distribution Patterns of Subgroups of Inhibitory Neurons Divided by Calbindin 1

The inhibitory neurons in the brain play an essential role in neural network firing patterns by releasing γ-aminobutyric acid (GABA) as the neurotransmitter. In the mouse brain, based on the protein molecular markers, inhibitory neurons are usually to be divided into three non-overlapping groups: parvalbumin (PV), neuropeptide somatostatin (SST), and vasoactive intestinal peptide (VIP)-expressing neurons. Each neuronal group exhibited unique properties in molecule, electrophysiology, circuitry, and function. Calbindin 1 (Calb1), a ubiquitous calcium-binding protein, often acts as a "divider" in excitatory neuronal classification. Based on Calb1 expression, the excitatory neurons from the same brain region can be classified into two subgroups with distinct properties. Besides excitatory neurons, Calb1 also expresses in part of inhibitory neurons. But, to date, little research focused on the intersectional relationship between inhibitory neuronal subtypes and Calb1. In this study, we genetically targeted Calb1-expression (Calb1+) and Calb1-lacking (Calb1-) subgroups of PV and SST neurons throughout the mouse brain by flexibly crossing transgenic mice relying on multi-recombinant systems, and the distribution patterns and electrophysiological properties of each subgroup were further demonstrated. Thus, this study provided novel insights and strategies into inhibitory neuronal classification.

Cell type specificity for circuit output in the midbrain dopaminergic system

Midbrain dopaminergic neurons are a relatively small group of neurons in the mammalian brain controlling a wide range of behaviors. In recent years, increasingly sophisticated tracing, imaging, transcriptomic, and machine learning approaches have provided substantial insights into the anatomical, molecular, and functional heterogeneity of dopaminergic neurons. Despite this wealth of new knowledge, it remains unclear whether and how the diverse features defining dopaminergic subclasses converge to delineate functional ensembles within the dopaminergic system. Here, we review recent studies investigating various aspects of dopaminergic heterogeneity and discuss how development, behavior, and disease influence subtype characteristics. We then outline what further approaches could be pursued to gain a more inclusive picture of dopaminergic diversity, which could be crucial to understanding the functional architecture of this system.