Midbrain Gene Screening Identifies a New Mesoaccumbal Glutamatergic Pathway and a Marker for Dopamine Cells Neuroprotected in Parkinson's Disease

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.

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. However there has been increasing attention on other VTA and SNc cell types that release GABA, glutamate, or a combination of these 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.

Sex differences in distribution and identity of aromatase gene expressing cells in the young adult rat brain

BACKGROUND

Aromatase catalyzes the synthesis of estrogens from androgens. Knowledge on its regional expression in the brain is of relevance to the behavioral implications of these hormones that might be linked to sex differences in mental health. The present study investigated the distribution of cells expressing the aromatase coding gene (Cyp19a1) in limbic regions of young adult rats of both sexes, and characterized the cell types expressing this gene.

METHODS

Cyp19a1 mRNA was mapped using fluorescent in situ hybridization (FISH). Co-expression with specific cell markers was assessed with double FISH; glutamatergic, gamma-aminobutyric acid (GABA)-ergic, glial, monoaminergic, as well as interneuron markers were tested. Automated quantification of the cells expressing the different genes was performed using CellProfiler. Sex differences in the number of cells expressing Cyp19a1 was tested non-parametrically, with the effect size indicated by the rank-biserial correlation. FDR correction for multiple testing was applied.

RESULTS

In the male brain, the highest percentage of Cyp19a1+ cells was found in the medial amygdaloid nucleus and the bed nucleus of stria terminalis, followed by the medial preoptic area, the CA2/3 fields of the hippocampus, the cortical amygdaloid nucleus and the amygdalo-hippocampal area. A lower percentage was detected in the caudate putamen, the nucleus accumbens, and the ventromedial hypothalamus. In females, the distribution of Cyp19a1+ cells was similar but at a lower percentage. In most regions, the majority of Cyp19a1+ cells were GABAergic, except for in the cortical-like regions of the amygdala where most were glutamatergic. A smaller fraction of cells co-expressed Slc1a3, suggesting expression of Cyp19a1 in astrocytes; monoaminergic markers were not co-expressed. Moreover, sex differences were detected regarding the identity of Cyp19a1+ cells.

CONCLUSIONS

Females show overall a lower number of cells expressing Cyp19a1 in the limbic brain. In both sexes, aromatase is expressed in a region-specific manner in GABAergic and glutamatergic neurons. These findings call for investigations of the relevance of sex-specific and region-dependent expression of Cyp19a1 in the limbic brain to sex differences in behavior and mental health.

Development, wiring and function of dopamine neuron subtypes

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviours and are linked to various brain diseases. Considerable progress has been made in identifying mDA neuron subtypes, and recent work has begun to unveil how these neuronal subtypes develop and organize into functional brain structures. This progress is important for further understanding the disparate physiological functions of mDA neurons and their selective vulnerability in disease, and will ultimately accelerate therapy development. This Review discusses recent advances in our understanding of molecularly defined mDA neuron subtypes and their circuits, ranging from early developmental events, such as neuron migration and axon guidance, to their wiring and function, and future implications for therapeutic strategies.

Increased sucrose consumption in mice gene-targeted for Vmat2 selectively in NeuroD6-positive neurons of the ventral tegmental area

Ventral tegmental area (VTA) dopamine (DA) neurons are implicated in reward processing, motivation, reward prediction error, and in substance use disorder. Recent studies have identified distinct neuronal subpopulations within the VTA that can be clustered based on their molecular identity, neurotransmitter profile, physiology, projections and behavioral role. One such subpopulation is characterized by expression of the NeuroD6 gene, and projects primarily to the nucleus accumbens medial shell. We recently showed that optogenetic stimulation of these neurons induces real-time place preference while their targeted deletion of the Vmat2 gene caused altered response to rewarding substances, including ethanol and psychostimulants. Based on these recent findings, we wanted to further investigate the involvement of the NeuroD6-positive VTA subpopulation in reward processing. Using the same NeuroD6^Cre+/wt ;Vmat2^flox/flox mice as in our prior study, we now addressed the ability of the mice to process sucrose reward. In order to assess appetitive behavior and motivation to obtain sucrose reward, we tested conditional knockout (cKO) and control littermate mice in an operant sucrose self-administration paradigm. We observed that cKO mice demonstrate higher response rates to the operant task and consume more sucrose rewards than control mice. However, their motivation to obtain sucrose is identical to that of control mice. Our results highlight previous observations that appetitive behavior and motivation to obtain rewards can be served by distinct neuronal circuits, and demonstrate that the NeuroD6 VTA subpopulation is involved in mediating the former, but not the latter. Together with previous studies on the NeuroD6 subpopulation, our findings pinpoint the importance of unraveling the molecular and functional role of VTA subpopulations in order to better understand normal behavior and psychiatric disease.

Therapeutic Molecular Insights into the Active Engagement of Cannabinoids in the Therapy of Parkinson's Disease: A Novel and Futuristic Approach

Parkinson's disease is a neurodegenerative disorder which is characterised mostly by loss of dopaminergic nerve cells throughout the nigral area mainly as a consequence of oxidative stress. Muscle stiffness, disorganised bodily responses, disturbed sleep, weariness, amnesia, and voice impairment are all symptoms of dopaminergic neuron degeneration and existing symptomatic treatments are important to arrest additional neuronal death. Some cannabinoids have recently been demonstrated as robust antioxidants that might protect the nerve cells from degeneration even when cannabinoid receptors are not triggered. Cannabinoids are likely to have property to slow or presumably cease the steady deterioration of the brain's dopaminergic systems, a condition for which there is now no treatment. The use of cannabinoids in combination with currently available drugs has the potential to introduce a radically new paradigm for treatment of Parkinson's disease, making it immensely useful in the treatment of such a debilitating illness.

Spatial Transcriptome Profiling of Mouse Hippocampal Single Cell Microzone in Parkinson's Disease

The hippocampus is an important part of the limbic system in the human brain that has essential roles in spatial navigation and cognitive functions. It is still unknown how gene expression changes in single-cell in different spatial locations of the hippocampus of Parkinson's disease. The purpose of this study was to analyze the gene expression features of single cells in different spatial locations of mouse hippocampus, and to explore the effects of gene expression regulation on learning and memory mechanisms. Here, we obtained 74 single-cell samples from different spatial locations in a mouse hippocampus through microdissection technology, and used single-cell RNA-sequencing and spatial transcriptome sequencing to visualize and quantify the single-cell transcriptome features of tissue sections. The results of differential expression analysis showed that the expression of Sv2b, Neurod6, Grp and Stk32b genes in a hippocampus single cell at different locations was significantly different, and the marker genes of CA1, CA3 and DG subregions were identified. The results of gene function enrichment analysis showed that the up-regulated differentially expressed genes Tubb2a, Eno1, Atp2b1, Plk2, Map4, Pex5l, Fibcd1 and Pdzd2 were mainly involved in neuron to neuron synapse, vesicle-mediated transport in synapse, calcium signaling pathway and neurodegenerative disease pathways, thus affecting learning and memory function. It revealed the transcriptome profile and heterogeneity of spatially located cells in the hippocampus of PD for the first time, and demonstrated that the impaired learning and memory ability of PD was affected by the synergistic effect of CA1 and CA3 subregions neuron genes. These results are crucial for understanding the pathological mechanism of the Parkinson's disease and making precise treatment plans.

Identification of hub genes and construction of diagnostic nomogram model in schizophrenia

Schizophrenia (SCZ), which is characterized by debilitating neuropsychiatric disorders with significant cognitive impairment, remains an etiological and therapeutic challenge. Using transcriptomic profile analysis, disease-related biomarkers linked with SCZ have been identified, and clinical outcomes can also be predicted. This study aimed to discover diagnostic hub genes and investigate their possible involvement in SCZ immunopathology. The Gene Expression Omnibus (GEO) database was utilized to get SCZ Gene expression data. Differentially expressed genes (DEGs) were identified and enriched by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and disease ontology (DO) analysis. The related gene modules were then examined using integrated weighted gene co-expression network analysis. Single-sample gene set enrichment (GSEA) was exploited to detect immune infiltration. SVM-REF, random forest, and least absolute shrinkage and selection operator (LASSO) algorithms were used to identify hub genes. A diagnostic model of nomogram was constructed for SCZ prediction based on the hub genes. The clinical utility of nomogram prediction was evaluated, and the diagnostic utility of hub genes was validated. mRNA levels of the candidate genes in SCZ rat model were determined. Finally, 24 DEGs were discovered, the majority of which were enriched in biological pathways and activities. Four hub genes (NEUROD6, NMU, PVALB, and NECAB1) were identified. A difference in immune infiltration was identified between SCZ and normal groups, and immune cells were shown to potentially interact with hub genes. The hub gene model for the two datasets was verified, showing good discrimination of the nomogram. Calibration curves demonstrated valid concordance between predicted and practical probabilities, and the nomogram was verified to be clinically useful. According to our research, NEUROD6, NMU, PVALB, and NECAB1 are prospective biomarkers in SCZ and that a reliable nomogram based on hub genes could be helpful for SCZ risk prediction.

Dopamine neurons exhibit emergent glutamatergic identity in Parkinson's disease

Loss of midbrain dopamine neurons causes the cardinal symptoms of Parkinson's disease. However, not all dopamine neurons are equally vulnerable and a better understanding of the cell-type specific properties relating to selective dopamine neuron degeneration is needed. Most midbrain dopamine neurons express the vesicular glutamate transporter VGLUT2 during development and a subset continue to express low levels of VGLUT2 in adulthood, enabling the co-release of glutamate. Moreover, VGLUT2 expression in dopamine neurons can be neuroprotective since its genetic disruption was shown to sensitize dopamine neurons to neurotoxins. Here, we show that in response to toxic insult, and in two distinct models of alpha-synuclein stress, VGLUT2 dopamine neurons were resilient to degeneration. Dopamine neurons expressing VGLUT2 were enriched whether or not insult induced dopamine neuron loss, suggesting that while VGLUT2 dopamine neurons are more resilient, VGLUT2 expression can also be transcriptionally upregulated by injury. Finally, we observed that VGLUT2 expression was enhanced in surviving dopamine neurons from post-mortem Parkinson's disease individuals. These data indicate that emergence of a glutamatergic identity in dopamine neurons may be part of a neuroprotective response in Parkinson's disease.

An atlas of transcriptionally defined cell populations in the rat ventral tegmental area

The ventral tegmental area (VTA) is a complex brain region that is essential for reward function and frequently implicated in neuropsychiatric disease. While decades of research on VTA function have focused on dopamine neurons, recent evidence has identified critical roles for GABAergic and glutamatergic neurons in reward processes. Additionally, although subsets of VTA neurons express genes involved in the synthesis and transport of multiple neurotransmitters, characterization of these combinatorial populations has largely relied on low-throughput methods. To comprehensively define the molecular architecture of the VTA, we performed single-nucleus RNA sequencing on 21,600 cells from the rat VTA. Analysis of neuronal subclusters identifies selective markers for dopamine and combinatorial neurons, reveals expression profiles for receptors targeted by drugs of abuse, and demonstrates population-specific enrichment of gene sets linked to brain disorders. These results highlight the heterogeneity of the VTA and provide a resource for further exploration of VTA gene expression.

TRPV1 blockers as potential new treatments for psychiatric disorders

The transient receptor potential vanilloid-1 channel (TRPV1) is responsible for decoding physical and chemical stimuli. TRPV1 is activated by capsaicin (a compound from chili peppers), heat (above 43°C) and acid environment, playing a major role in pain, inflammation and body temperature. Molecular and histological studies have suggested TRPV1 expression in specific brain regions, where it can be activated primarily by the endocannabinoid anandamide, fostering studies on its potential role in psychiatric disorders. TRPV1 blockers are effective in various animal models predictive of anxiolytic and antipanic activities, in addition to reducing conditioned fear. In models of antidepressant activity, these compounds reduce behavioral despair and promote active stress-coping behavior. TRPV1 blockers also reduce the effects of certain drugs of abuse and revert behavioral changes in animal models of neurodevelopmental disorders. The main limiting factor in developing TRPV1 blockers as therapeutic agents concerns their effects on body temperature, particularly hyperthermia. New compounds, which block specific states of the channel, could represent an alternative. Moreover, compounds blocking both TRPV1 and the anandamide-hydrolyzing enzyme, fatty acid amide hydrolase (FAAH), termed dual TRPV1/FAAH blockers, have been investigated with promising results. Overall, preclinical studies yield favorable results with TRPV1 blockers in animal models of psychiatric disorders.

Midbrain Dopamine Neurons Defined by TrpV1 Modulate Psychomotor Behavior

Dopamine (DA) neurons of the ventral tegmental area (VTA) continue to gain attention as far more heterogeneous than previously realized. Within the medial aspect of the VTA, the unexpected presence of TrpV1 mRNA has been identified. TrpV1 encodes the Transient Receptor Potential cation channel subfamily V member 1, TRPV1, also known as the capsaicin receptor, well recognized for its role in heat and pain processing by peripheral neurons. In contrast, the brain distribution of TrpV1 has been debated. Here, we hypothesized that the TrpV1+ identity defines a distinct subpopulation of VTA DA neurons. To explore these brain TrpV1+ neurons, histological analyses and Cre-driven mouse genetics were employed. TrpV1 mRNA was most strongly detected at the perinatal stage forming a band of scattered neurons throughout the medial VTA, reaching into the posterior hypothalamus. Within the VTA, the majority of TrpV1 co-localized with both Tyrosine hydroxylase (Th) and Vesicular monoamine transporter 2 (Vmat2), confirming a DA phenotype. However, TrpV1 also co-localized substantially with Vesicular glutamate transporter 2 (Vglut2), representing the capacity for glutamate (GLU) release. These TrpV1+/Th+/Vglut2+/Vmat2+ neurons thus constitute a molecularly and anatomically distinct subpopulation of DA-GLU co-releasing neurons. To assess behavioral impact, a TrpV1Cre -driven strategy targeting the Vmat2 gene in mice was implemented. This manipulation was sufficient to alter psychomotor behavior induced by amphetamine. The acute effect of the drug was accentuated above control levels, suggesting super-sensitivity in the drug-na ve state resembling a "pre-sensitized" phenotype. However, no progressive increase with repeated injections was observed. This study identifies a distinct TrpV1+ VTA subpopulation as a critical modulatory component in responsiveness to amphetamine. Moreover, expression of the gene encoding TRPV1 in selected VTA neurons opens up for new possibilities in pharmacological intervention of this heterogeneous, but clinically important, brain area.

Generation of a DAT-P2A-Flpo mouse line for intersectional genetic targeting of dopamine neuron subpopulations

Dopaminergic projections exert widespread influence over multiple brain regions and modulate various behaviors including movement, reward learning, and motivation. It is increasingly appreciated that dopamine neurons are heterogeneous in their gene expression, circuitry, physiology, and function. Current approaches to target dopamine neurons are largely based on single gene drivers, which either label all dopamine neurons or mark a subset but concurrently label non-dopaminergic neurons. Here, we establish a mouse line with Flpo recombinase expressed from the endogenous Slc6a3 (dopamine active transporter [DAT]) locus. DAT-P2A-Flpo mice can be used together with Cre-expressing mouse lines to efficiently and selectively label dopaminergic subpopulations using Cre/Flp-dependent intersectional strategies. We demonstrate the utility of this approach by generating DAT-P2A-Flpo;NEX-Cre mice that specifically label Neurod6-expressing dopamine neurons, which project to the nucleus accumbens medial shell. DAT-P2A-Flpo mice add to a growing toolbox of genetic resources that will help parse the diverse functions mediated by dopaminergic circuits.

Kramer et al. generate a DAT-P2A-Flpo mouse line that enables intersectional genetic targeting of dopamine neuron subpopulations using Flp/Cre-dependent constructs. They show that ventral tegmental area dopamine neurons expressing Neurod6 give rise to the majority of dopaminergic projections to the nucleus accumbens medial shell and olfactory tubercle.

Uncovering Statistical Links Between Gene Expression and Structural Connectivity Patterns in the Mouse Brain

Finding links between genes and structural connectivity is of the utmost importance for unravelling the underlying mechanism of the brain connectome. In this study we identify links between the gene expression and the axonal projection density in the mouse brain, by applying a modified version of the Linked ICA method to volumetric data from the Allen Institute for Brain Science for identifying independent sources of information that link both modalities at the voxel level. We performed separate analyses on sets of projections from the visual cortex, the caudoputamen and the midbrain reticular nucleus, and we determined those brain areas, injections and genes that were most involved in independent components that link both gene expression and projection density data, while we validated their biological context through enrichment analysis. We identified representative and literature-validated cortico-midbrain and cortico-striatal projections, whose gene subsets were enriched with annotations for neuronal and synaptic function and related developmental and metabolic processes. The results were highly reproducible when including all available projections, as well as consistent with factorisations obtained using the Dictionary Learning and Sparse Coding technique. Hence, Linked ICA yielded reproducible independent components that were preserved under increasing data variance. Taken together, we have developed and validated a novel paradigm for linking gene expression and structural projection patterns in the mouse mesoconnectome, which can power future studies aiming to relate genes to brain function.

Implication of 5-HT7 receptor in prefrontal circuit assembly and detrimental emotional effects of SSRIs during development

Altered development of prefrontal cortex (PFC) circuits can have long-term consequences on adult emotional behavior. Changes in serotonin homeostasis during critical periods produced by genetic or pharmacological inactivation of the serotonin transporter (SERT, or Slc6a4), have been involved in such developmental effects. In mice, selective serotonin reuptake inhibitors (SSRIs), administered during postnatal development cause exuberant synaptic connectivity of the PFC to brainstem dorsal raphe nucleus (DRN) circuits, and increase adult risk for developing anxiety and depressive symptoms. SERT is transiently expressed in the glutamate neurons of the mouse PFC, that project to the DRN. Here, we find that 5-HTR7 is transiently co-expressed with SERT by PFC neurons, and it plays a key role in the maturation of PFC-to-DRN synaptic circuits during early postnatal life. 5-HTR7-KO mice show reduced PFC-to-DRN synaptic density (as measured by array-tomography and VGLUT1/synapsin immunocytochemistry). Conversely, 5-HTR7 over-expression in the developing PFC increased PFC-to-DRN synaptic density. Long-term consequences on depressive-like and anxiogenic behaviors were observed in adults. 5-HTR7 over-expression in the developing PFC, results in depressive-like symptoms in adulthood. Importantly, the long-term depressive-like and anxiogenic effects of SSRIs (postnatal administration of fluoxetine from P2 to P14) were not observed in 5-HTR7-KO mice, and were prevented by co-administration of the selective inhibitor of 5-HTR7, SB269970. This study identifies a new role 5-HTR7 in the postnatal maturation of prefrontal descending circuits. Furthermore, it shows that 5-HTR7 in the PFC is crucially required for the detrimental emotional effects caused by SSRI exposure during early postnatal life.

Selective Knockout of the Vesicular Monoamine Transporter 2 (Vmat2) Gene in Calbindin2/Calretinin-Positive Neurons Results in Profound Changes in Behavior and Response to Drugs of Abuse

The vesicular monoamine transporter 2 (VMAT2) has a range of functions in the central nervous system, from sequestering toxins to providing conditions for the quantal release of monoaminergic neurotransmitters. Monoamine signaling regulates diverse functions from arousal to mood, movement, and motivation, and dysregulation of VMAT2 function is implicated in various neuropsychiatric diseases. While all monoamine-releasing neurons express the Vmat2 gene, only a subset is positive for the calcium-binding protein Calbindin 2 (Calb2; aka Calretinin, 29 kDa Calbindin). We recently showed that about half of the dopamine neurons in the mouse midbrain are positive for Calb2 and that Calb2 is an early developmental marker of midbrain dopamine cells. Calb2-positive neurons have also been identified in other monoaminergic areas, yet the role of Calb2-positive monoaminergic neurons is poorly understood. To selectively address the impact of Calb2-positive monoaminergic neurons in behavioral regulation, we took advantage of the Cre-LoxP system to create a new conditional knockout (cKO) mouse line in which Vmat2 expression is deleted selectively in Calb2-Cre-positive neurons. In this Vmat2^{lox/lox;Calb2−Cre} cKO mouse line, gene targeting of Vmat2 was observed in several distinct monoaminergic areas. By comparing control and cKO mice in a series of behavioral tests, specific dissimilarities were identified. In particular, cKO mice were smaller than control mice and showed heightened sensitivity to the stereotypy-inducing effects of amphetamine and slight reductions in preference toward sucrose and ethanol, as well as a blunted response in the elevated plus maze test. These data uncover new knowledge about the role of genetically defined subtypes of neurons in the brain’s monoaminergic systems.

Classification of Midbrain Dopamine Neurons Using Single-Cell Gene Expression Profiling Approaches

Dysfunctional dopamine (DA) signaling has been associated with a broad spectrum of neuropsychiatric disorders, prompting investigations into how midbrain DA neuron heterogeneity may underpin this variety of behavioral symptoms. Emerging literature indeed points to functional heterogeneity even within anatomically defined DA clusters. Recognizing the need for a systematic classification scheme, several groups have used single-cell profiling to catalog DA neurons based on their gene expression profiles. We aim here not only to synthesize points of congruence but also to highlight key differences between the molecular classification schemes derived from these studies. In doing so, we hope to provide a common framework that will facilitate investigations into the functions of DA neuron subtypes in the healthy and diseased brain.

Nanoparticles Mediating the Sustained Puerarin Release Facilitate Improved Brain Delivery to Treat Parkinson's Disease

Recent work has highlighted the potential of puerarin (PU) as a valuable compound to treat Parkinson's disease (PD), but its undesirable water solubility and bioavailability have constrained its utility. In this study, we sought to develop nanoparticles (NPs) that could be used to encapsulate PU, thereby extending its in vivo half-life and improving its bioavailability and accumulation in the brain to treat the symptoms of PD. We prepared spherical NPs (88.36 ± 1.67 nm) from six-armed star-shaped poly(lactide-co-glycolide) (6-s-PLGA) NPs that were used to encapsulate PU (PU-NPs) with 89.52 ± 1.74% encapsulation efficiency, 42.97 ± 1.58% drug loading, and a 48 h sustained drug release. NP formation and drug loading were largely mediated by hydrophobic interactions, while changes in the external environment led these NPs to become increasingly hydrophilic, thereby leading to drug release. Relative to PU alone, PU-NPs exhibited significantly improved cellular internalization, permeation, and neuroprotective effects. Upon the basis of Förster resonance energy transfer (FRET) of NPs-administered zebrafish, we were able to determine that these NPs were rapidly absorbed into circulation whereupon they were able to access the brain. We further conducted oral PU-NPs administration to rats, revealing significant improvements in PU accumulation within the plasma and brain relative to rats administered free PU. In MPTP-mediated neurotoxicity in mice, we found that PU-NPs treatment improved disease-associated behavioral deficits and depletion of dopamine and its metabolites. These findings indicated that PU-NPs represent a potentially viable approach to enhancing PU oral absorption, thus improving its delivery to the brain wherein it can aid in the treatment of PD.

Developmental Co-expression of Vglut2 and Nurr1 in a Mes-Di-Encephalic Continuum Preceeds Dopamine and Glutamate Neuron Specification

Midbrain dopamine (DA) neurons exist as several subtypes and are found in a heterogeneous environment including GABAergic and glutamatergic neurons as well as various types of co-releasing neurons. Developmental programs underlying this heterogeneity have remained elusive. In this study, combinatorial mRNA analysis was performed at stages when neuronal phenotypes are first specified. Vesicular transporters for dopamine and other monoamines (VMAT2), GABA (VIAAT), and glutamate (VGLUT2) were assessed by systematically applying fluorescent in situ hybridization through the mes-di-encephalon of the mouse embryo at embryonal days (E) 9.5–14.5. The results show that early differentiating dopamine neurons express the gene encoding VGLUT2 before onset of any dopaminergic markers. Prior to its down-regulation in maturing dopamine neurons, Vglut2 mRNA co-localizes extensively with Tyrosine hydroxylase (Th) and Nurr1, commonly used as markers for DA neurons. Further, Vglut2 and Nurr1 mRNAs are shown to overlap substantially in diencephalic neurons that maintain a glutamatergic phenotype. The results suggest that Vglut2/Nurr1-double positive cells give rise both to dopaminergic and glutamatergic neurons within the mes-di-encephalic area. Finally, analysis of markers representing subtypes of dopamine neurons, including the newly described NeuroD6 subtype, shows that certain subtype specifications arise early. Histological findings are outlined in the context of neuroanatomical concepts and the prosomeric model of brain development. The study contributes to the current decoding of the recently discovered heterogeneity among neurons residing along the cephalic flexure.

Modulation of epileptogenesis: A paradigm for the integration of enzyme-based microelectrode arrays and optogenetics

BACKGROUND

Genesis of acquired epilepsy includes transformations spanning genetic-to- network-level modifications, disrupting the regional excitatory/inhibitory balance. Methodology concurrently tracking changes at multiple levels is lacking. Here, viral vectors are used to differentially express two opsin proteins in neuronal populations within dentate gyrus (DG) of hippocampus. When activated, these opsins induced excitatory or inhibitory neural output that differentially affected neural networks and epileptogenesis. In vivo measures included behavioral observation coupled to real-time measures of regional glutamate flux using ceramic-based amperometric microelectrode arrays (MEAs).

RESULTS

Using MEA technology, phasic increases of extracellular glutamate were recorded immediately upon application of blue light/488 nm to DG of rats previously transfected with an AAV 2/5 vector containing an (excitatory) channelrhodopsin-2 transcript. Rats receiving twice-daily 30-sec light stimulation to DG ipsilateral to viral transfection progressed through Racine seizure stages. AAV 2/5 (inhibitory) halorhodopsin-transfected rats receiving concomitant amygdalar kindling and DG light stimuli were kindled significantly more slowly than non-stimulated controls. In in vitro slice preparations, both excitatory and inhibitory responses were independently evoked in dentate granule cells during appropriate light stimulation. Latency to response and sensitivity of responses suggest a degree of neuron subtype-selective functional expression of the transcripts.

CONCLUSIONS

This study demonstrates the potential for coupling MEA technology and optogenetics for real-time neurotransmitter release measures and modification of seizure susceptibility in animal models of epileptogenesis. This microelectrode/optogenetic technology could prove useful for characterization of network and system level dysfunction in diseases involving imbalanced excitatory/inhibitory control of neuron populations and guide development of future treatment strategies.

Neurocircuitry of Reward and Addiction: Potential Impact of Dopamine-Glutamate Co-release as Future Target in Substance Use Disorder

Dopamine-glutamate co-release is a unique property of midbrain neurons primarily located in the ventral tegmental area (VTA). Dopamine neurons of the VTA are important for behavioral regulation in response to rewarding substances, including natural rewards and addictive drugs. The impact of glutamate co-release on behaviors regulated by VTA dopamine neurons has been challenging to probe due to lack of selective methodology. However, several studies implementing conditional knockout and optogenetics technologies in transgenic mice have during the past decade pointed towards a role for glutamate co-release in multiple physiological and behavioral processes of importance to substance use and abuse. In this review, we discuss these studies to highlight findings that may be critical when considering mechanisms of importance for prevention and treatment of substance abuse.

The NeuroD6 Subtype of VTA Neurons Contributes to Psychostimulant Sensitization and Behavioral Reinforcement

Reward-related behavior is complex and its dysfunction correlated with neuropsychiatric illness. Dopamine (DA) neurons of the ventral tegmental area (VTA) have long been associated with different aspects of reward function, but it remains to be disentangled how distinct VTA DA neurons contribute to the full range of behaviors ascribed to the VTA. Here, a recently identified subtype of VTA neurons molecularly defined by NeuroD6 (NEX1M) was addressed. Among all VTA DA neurons, less than 15% were identified as positive for NeuroD6. In addition to dopaminergic markers, sparse NeuroD6 neurons expressed the vesicular glutamate transporter 2 (Vglut2) gene. To achieve manipulation of NeuroD6 VTA neurons, NeuroD6(NEX)-Cre-driven mouse genetics and optogenetics were implemented. First, expression of vesicular monoamine transporter 2 (VMAT2) was ablated to disrupt dopaminergic function in NeuroD6 VTA neurons. Comparing Vmat2^{lox/lox;NEX-Cre} conditional knock-out (cKO) mice with littermate controls, it was evident that baseline locomotion, preference for sugar and ethanol, and place preference upon amphetamine-induced and cocaine-induced conditioning were similar between genotypes. However, locomotion upon repeated psychostimulant administration was significantly elevated above control levels in cKO mice. Second, optogenetic activation of NEX-Cre VTA neurons was shown to induce DA release and glutamatergic postsynaptic currents within the nucleus accumbens. Third, optogenetic stimulation of NEX-Cre VTA neurons in vivo induced significant place preference behavior, while stimulation of VTA neurons defined by Calretinin failed to cause a similar response. The results show that NeuroD6 VTA neurons exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.

The role of the neuropeptide PEN receptor, GPR83, in the reward pathway: Relationship to sex-differences

GPR83, the receptor for the neuropeptide PEN, exhibits high expression in the nucleus accumbens of the human and rodent brain, suggesting that it plays a role in modulating the mesolimbic reward pathway. However, the cell-type specific expression of GPR83, its functional impact in the reward pathway, and in drug reward-learning has not been fully explored. Using GPR83/eGFP mice, we show high GPR83 expression on cholinergic interneurons in the nucleus accumbens and moderate expression on ventral tegmental area dopamine neurons. In GPR83 knockout mice, baseline dopamine release in the nucleus accumbens is enhanced which disrupts the ratio of tonic vs phasic release. Additionally, GPR83 knockout leads to changes in the expression of dopamine-related genes. Using the morphine conditioned place preference model, we identify sex differences in morphine reward-learning, show that GPR83 is upregulated in the nucleus accumbens following morphine conditioned place preference, and show that shRNA-mediated knockdown of GPR83 in the nucleus accumbens leads to attenuation morphine reward. Together, these findings detect GPR83 expression in the reward-pathway, and show its involvement in dopamine release and morphine reward-learning.

Functional neuroanatomical review of the ventral tegmental area

The ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) are assumed to play a key role in dopamine-related functions such as reward-related behaviour, motivation, addiction and motor functioning. Although dopamine-producing midbrain structures are bordering, they show significant differences in structure and function that argue for a distinction when studying the functions of the dopaminergic midbrain, especially by means of neuroimaging. First, unlike the SNc, the VTA is not a nucleus, which makes it difficult to delineate the structure due to lack of clear anatomical borders. Second, there is no consensus in the literature about the anatomical nomenclature to describe the VTA. Third, these factors in combination with limitations in magnetic resonance imaging (MRI) complicate VTA visualization. We suggest that developing an MRI-compatible probabilistic atlas of the VTA will help to overcome these issues. Such an atlas can be used to identify the individual VTA and serve as region-of-interest for functional MRI.

Cartography of hevin-expressing cells in the adult brain reveals prominent expression in astrocytes and parvalbumin neurons

Hevin, also known as SPARC-like 1, is a member of the secreted protein acidic and rich in cysteine family of matricellular proteins, which has been implicated in neuronal migration and synaptogenesis during development. Unlike previously characterized matricellular proteins, hevin remains strongly expressed in the adult brain in both astrocytes and neurons, but its precise pattern of expression is unknown. The present study provides the first systematic description of hevin mRNA distribution in the adult mouse brain. Using isotopic in situ hybridization, we showed that hevin is strongly expressed in the cortex, hippocampus, basal ganglia complex, diverse thalamic nuclei and brainstem motor nuclei. To identify the cellular phenotype of hevin-expressing cells, we used double fluorescent in situ hybridization in mouse and human adult brains. In the mouse, hevin mRNA was found in the majority of astrocytes but also in specific neuronal populations. Hevin was expressed in almost all parvalbumin-positive projection neurons and local interneurons. In addition, hevin mRNA was found in: (1) subsets of other inhibitory GABAergic neuronal subtypes, including calbindin, cholecystokinin, neuropeptide Y, and somatostatin-positive neurons; (2) subsets of glutamatergic neurons, identified by the expression of the vesicular glutamate transporters VGLUT1 and VGLUT2; and (3) the majority of cholinergic neurons from motor nuclei. Hevin mRNA was absent from all monoaminergic neurons and cholinergic neurons of the ascending pathway. A similar cellular profile of expression was observed in human, with expression of hevin in parvalbumin interneurons and astrocytes in the cortex and caudate nucleus as well as in cortical glutamatergic neurons. Furthermore, hevin transcript was enriched in ribosomes of astrocytes and parvalbumin neurons providing a direct evidence of hevin mRNAs translation in these cell types. This study reveals the unique and complex expression profile of the matricellular protein hevin in the adult brain. This distribution is compatible with a role of hevin in astrocytic-mediated adult synaptic plasticity and in the regulation of network activity mediated by parvalbumin-expressing neurons.

Targeting VGLUT2 in Mature Dopamine Neurons Decreases Mesoaccumbal Glutamatergic Transmission and Identifies a Role for Glutamate Co-release in Synaptic Plasticity by Increasing Baseline AMPA/NMDA Ratio

Expression of the Vglut2/Slc17a6 gene encoding the Vesicular glutamate transporter 2 (VGLUT2) in midbrain dopamine (DA) neurons enables these neurons to co-release glutamate in the nucleus accumbens (NAc), a feature of putative importance to drug addiction. For example, it has been shown that conditional deletion of Vglut2 gene expression within developing DA neurons in mice causes altered locomotor sensitization to addictive drugs, such as amphetamine and cocaine, in adulthood. Alterations in DA neurotransmission in the mesoaccumbal pathway has been proposed to contribute to these behavioral alterations but the underlying molecular mechanism remains largely elusive. Repeated exposure to cocaine is known to cause lasting adaptations of excitatory synaptic transmission onto medium spiny neurons (MSNs) in the NAc, but the putative contribution of VGLUT2-mediated glutamate co-release from the mesoaccumbal projection has never been investigated. In this study, we implemented a tamoxifen-inducible Cre-LoxP strategy to selectively probe VGLUT2 in mature DA neurons of adult mice. Optogenetics-coupled patch clamp analysis in the NAc demonstrated a significant reduction of glutamatergic neurotransmission, whilst behavioral analysis revealed a normal locomotor sensitization to amphetamine and cocaine. When investigating if the reduced level of glutamate co-release from DA neurons caused a detectable post-synaptic effect on MSNs, patch clamp analysis identified an enhanced baseline AMPA/NMDA ratio in DA receptor subtype 1 (DRD1)-expressing accumbal MSNs which occluded the effect of cocaine on synaptic transmission. We conclude that VGLUT2 in mature DA neurons actively contributes to glutamatergic neurotransmission in the NAc, a finding which for the first time highlights VGLUT2-mediated glutamate co-release in the complex mechanisms of synaptic plasticity in drug addiction.

The GABAergic Gudden's dorsal tegmental nucleus: A new relay for serotonergic regulation of sleep-wake behavior in the mouse

Serotonin (5-HT) neurons are involved in wake promotion and exert a strong inhibitory influence on rapid eye movement (REM) sleep. Such effects have been ascribed, at least in part to the action of 5-HT at post-synaptic 5-HT_1A receptors (5-HT_1AR) in the brainstem, a major wake/REM sleep regulatory center. However, the neuroanatomical substrate through which 5-HT_1AR influence sleep remains elusive. We therefore investigated whether a brainstem structure containing a high density of 5-HT_1AR mRNA, the GABAergic Gudden's dorsal tegmental nucleus (DTg), may contribute to 5-HT-mediated regulatory mechanisms of sleep-wake stages. We first found that bilateral lesions of the DTg promote wake at the expense of sleep. In addition, using local microinjections into the DTg in freely moving mice, we showed that local activation of 5-HT_1AR by the prototypical agonist 8-OH-DPAT enhances wake and reduces deeply REM sleep duration. The specific involvement of 5-HT_1AR in the latter effects was further demonstrated by ex vivo extracellular recordings showing that the selective 5-HT_1AR antagonist WAY 100635 prevented DTg neuron inhibition by 8-OH-DPAT. We next found that GABAergic neurons of the ventral DTg exclusively targets glutamatergic neurons of the lateral mammillary nucleus (LM) in the posterior hypothalamus by means of anterograde and retrograde tracing techniques using cre driver mouse lines and a modified rabies virus. Altogether, our findings strongly support the idea that 5-HT-driven enhancement of wake results from 5-HT_1AR-mediated inhibition of DTg GABAergic neurons that would in turn disinhibit glutamatergic neurons in the mammillary bodies. We therefore propose a Raphe→DTg→LM pathway as a novel regulatory circuit underlying 5-HT modulation of arousal.

Combinatorial Expression of Grp and Neurod6 Defines Dopamine Neuron Populations with Distinct Projection Patterns and Disease Vulnerability

Midbrain dopamine neurons project to numerous targets throughout the brain to modulate various behaviors and brain states. Within this small population of neurons exists significant heterogeneity based on physiology, circuitry, and disease susceptibility. Recent studies have shown that dopamine neurons can be subdivided based on gene expression; however, the extent to which genetic markers represent functionally relevant dopaminergic subpopulations has not been fully explored. Here we performed single-cell RNA-sequencing of mouse dopamine neurons and validated studies showing that Neurod6 and Grp are selective markers for dopaminergic subpopulations. Using a combination of multiplex fluorescent in situ hybridization, retrograde labeling, and electrophysiology in mice of both sexes, we defined the anatomy, projection targets, physiological properties, and disease vulnerability of dopamine neurons based on Grp and/or Neurod6 expression. We found that the combinatorial expression of Grp and Neurod6 defines dopaminergic subpopulations with unique features. Grp+/Neurod6+ dopamine neurons reside in the ventromedial VTA, send projections to the medial shell of the nucleus accumbens, and have noncanonical physiological properties. Grp+/Neurod6- dopamine neurons are found in the VTA as well as in the ventromedial portion of the SNc, where they project selectively to the dorsomedial striatum. Grp-/Neurod6+ dopamine neurons represent a smaller VTA subpopulation, which is preferentially spared in a 6-OHDA model of Parkinson's disease. Together, our work provides detailed characterization of Neurod6 and Grp expression in the midbrain and generates new insights into how these markers define functionally relevant dopaminergic subpopulations.

Aversion or Salience Signaling by Ventral Tegmental Area Glutamate Neurons

Ventral tegmental area (VTA) neurons play roles in reward and aversion. The VTA has, in addition to dopamine neurons, glutamatergic neurons expressing VGluT2. Here, by determining the firing patterns of VTA-VGluT2 neurons expressing channelrhodopsin 2, we identified a major subpopulation of VTA-VGluT2 neurons whose firing rates decreased or were unchanged during sucrose consumption and increased during facial airpuff presentation. We identified a small subpopulation of VTA-VGluT2 neurons whose firing rates increased in response to both rewarding and aversive stimuli. We also found that the changes in firing rate of some VTA-VGluT2 neurons were greater following reward delivery compared with reward omission, whereas others did not differ. We conclude that VTA-VGluT2 neurons are responsive to aversive stimuli, but subpopulations of VTA-VGluT2 neurons are differentially affected by sucrose reward. Reward-responsive subpopulations of VTA-VGluT2 neurons are also divided into those affected by reward expectation alone or the real-time delivery of reward.

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The majority of VTA glutamate neurons increase their firing during airpuff

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The majority of VTA glutamate neurons decrease their firing during reward

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A few VTA glutamate neurons increase their firing during both airpuff and reward

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Firing heterogeneity among VTA glutamate neurons during reward delivery or omission

Structural and Functional Characterization of the Interaction of Snapin with the Dopamine Transporter: Differential Modulation of Psychostimulant Actions

The importance of dopamine (DA) neurotransmission is emphasized by its direct implication in several neurological and psychiatric disorders. The DA transporter (DAT), target of psychostimulant drugs, is the key protein that regulates spatial and temporal activity of DA in the synaptic cleft via the rapid reuptake of DA into the presynaptic terminal. There is strong evidence suggesting that DAT-interacting proteins may have a role in its function and regulation. Performing a two-hybrid screening, we identified snapin, a SNARE-associated protein implicated in synaptic transmission, as a new binding partner of the carboxyl terminal of DAT. Our data show that snapin is a direct partner and regulator of DAT. First, we determined the domains required for this interaction in both proteins and characterized the DAT-snapin interface by generating a 3D model. Using different approaches, we demonstrated that (i) snapin is expressed in vivo in dopaminergic neurons along with DAT; (ii) both proteins colocalize in cultured cells and brain and, (iii) DAT and snapin are present in the same protein complex. Moreover, by functional studies we showed that snapin produces a significant decrease in DAT uptake activity. Finally, snapin downregulation in mice produces an increase in DAT levels and transport activity, hence increasing DA concentration and locomotor response to amphetamine. In conclusion, snapin/DAT interaction represents a direct link between exocytotic and reuptake mechanisms and is a potential target for DA transmission modulation.

Validated multi-step approach for in vivo recording and analysis of optogenetically evoked glutamate in the mouse globus pallidus

Precise quantification of extracellular glutamate concentrations upon neuronal activation is crucial for the understanding of brain function and neurological disorders. While optogenetics is an outstanding method for the correlation between distinct neurons and their role in circuitry and behavior, the electrochemically inactive nature of glutamate has proven challenging for recording upon optogenetic stimulations. This difficulty is due to the necessity for using enzyme-coated microelectrodes and the risk for light-induced artifacts. In this study, we establish a method for the combination of in vivo optogenetic stimulation with selective measurement of glutamate concentrations using enzyme-coated multielectrode arrays and amperometry. The glutamatergic subthalamic nucleus (STN), which is the main electrode target site in deep brain stimulation treatment of advanced Parkinson's disease, has recently proven opotogenetically targetable in Pitx2-Cre-transgenic mice and was here used as model system. Upon stereotactic injection of viral Channelrhodopsin2-eYFP constructs into the STN, amperometric recordings were performed at a range of optogenetic stimulation frequencies in the globus pallidus, the main STN target area, in anesthetized mice. Accurate quantification was enabled through a multi-step analysis approach based on self-referencing microelectrodes and repetition of the experimental protocol at two holding potentials, which allowed for the identification, isolation and removal of photoelectric and photoelectrochemical artifacts. This study advances the field of in vivo glutamate detection with combined optogenetics and amperometric recordings by providing a validated analysis framework for application in a wide variety of glutamate-based approaches in neuroscience.

Single-Cell RNA-Seq of Mouse Dopaminergic Neurons Informs Candidate Gene Selection for Sporadic Parkinson Disease

Genetic variation modulating risk of sporadic Parkinson disease (PD) has been primarily explored through genome-wide association studies (GWASs). However, like many other common genetic diseases, the impacted genes remain largely unknown. Here, we used single-cell RNA-seq to characterize dopaminergic (DA) neuron populations in the mouse brain at embryonic and early postnatal time points. These data facilitated unbiased identification of DA neuron subpopulations through their unique transcriptional profiles, including a postnatal neuroblast population and substantia nigra (SN) DA neurons. We use these population-specific data to develop a scoring system to prioritize candidate genes in all 49 GWAS intervals implicated in PD risk, including genes with known PD associations and many with extensive supporting literature. As proof of principle, we confirm that the nigrostriatal pathway is compromised in Cplx1-null mice. Ultimately, this systematic approach establishes biologically pertinent candidates and testable hypotheses for sporadic PD, informing a new era of PD genetic research.

Disrupting Glutamate Co-transmission Does Not Affect Acquisition of Conditioned Behavior Reinforced by Dopamine Neuron Activation

Dopamine neurons in the ventral tegmental area (VTA) were previously found to express vesicular glutamate transporter 2 (VGLUT2) and to co-transmit glutamate in the ventral striatum (VStr). This capacity may play an important role in reinforcement learning. Although it is known that activation of the VTA-VStr dopamine system readily reinforces behavior, little is known about the role of glutamate co-transmission in such reinforcement. By combining electrode recording and optogenetics, we found that stimulation of VTA dopamine neurons in vivo evoked fast excitatory responses in many VStr neurons of adult mice. Whereas conditional knockout of the gene encoding VGLUT2 in dopamine neurons largely eliminated fast excitatory responses, it had little effect on the acquisition of conditioned responses reinforced by dopamine neuron activation. Therefore, glutamate co-transmission appears dispensable for acquisition of conditioned responding reinforced by DA neuron activation.