Sexually dimorphic mechanisms of VGLUT-mediated protection from dopaminergic neurodegeneration

Parkinson's disease (PD) targets some dopamine (DA) neurons more than others. Sex differences offer insights, with females more protected from DA neurodegeneration. The mammalian vesicular glutamate transporter VGLUT2 and Drosophila ortholog dVGLUT have been implicated as modulators of DA neuron resilience. However, the mechanisms by which VGLUT2/dVGLUT protects DA neurons remain unknown. We discovered DA neuron dVGLUT knockdown increased mitochondrial reactive oxygen species in a sexually dimorphic manner in response to depolarization or paraquat-induced stress, males being especially affected. DA neuron dVGLUT also reduced ATP biosynthetic burden during depolarization. RNA sequencing of VGLUT+ DA neurons in mice and flies identified candidate genes that we functionally screened to further dissect VGLUT-mediated DA neuron resilience across PD models. We discovered transcription factors modulating dVGLUT-dependent DA neuroprotection and identified dj-1β as a regulator of sex-specific DA neuron dVGLUT expression. Overall, VGLUT protects DA neurons from PD-associated degeneration by maintaining mitochondrial health.

Dopamine transporter blockade during adolescence increases adult dopamine function, impulsivity, and aggression

Sensitive developmental periods shape neural circuits and enable adaptation. However, they also engender vulnerability to factors that can perturb developmental trajectories. An understanding of sensitive period phenomena and mechanisms separate from sensory system development is still lacking, yet critical to understanding disease etiology and risk. The dopamine system is pivotal in controlling and shaping adolescent behaviors, and it undergoes heightened plasticity during that time, such that interference with dopamine signaling can have long-lasting behavioral consequences. Here we sought to gain mechanistic insight into this dopamine-sensitive period and its impact on behavior. In mice, dopamine transporter (DAT) blockade from postnatal (P) day 22 to 41 increases aggression and sensitivity to amphetamine (AMPH) behavioral stimulation in adulthood. Here, we refined this sensitive window to P32-41 and identified increased firing of dopaminergic neurons in vitro and in vivo as a neural correlate to altered adult behavior. Aggression can result from enhanced impulsivity and cognitive dysfunction, and dopamine regulates working memory and motivated behavior. Hence, we assessed these behavioral domains and found that P32-41 DAT blockade increases impulsivity but has no effect on cognition, working memory, or motivation in adulthood. Lastly, using optogenetics to drive dopamine neurons, we find that increased VTA but not SNc dopaminergic activity mimics the increase in impulsive behavior in the Go/NoGo task observed after adolescent DAT blockade. Together our data provide insight into the developmental origins of aggression and impulsivity that may ultimately improve diagnosis, prevention, and treatment strategies for related neuropsychiatric disorders.

The dopamine neuron synaptic map in the striatum

Dopamine neurons project to the striatum to control movement, cognition, and motivation via slower volume transmission as well as faster dopamine, glutamate, and GABA synaptic actions capable of conveying the temporal information in dopamine neuron firing. To define the scope of these synaptic actions, recordings of dopamine-neuron-evoked synaptic currents were made in four major striatal neuron types, spanning the entire striatum. This revealed that inhibitory postsynaptic currents are widespread, while excitatory postsynaptic currents are localized to the medial nucleus accumbens and the anterolateral-dorsal striatum, and that all synaptic actions are weak in the posterior striatum. Synaptic actions in cholinergic interneurons are the strongest, variably mediating inhibition throughout the striatum and excitation in the medial accumbens, capable of controlling their activity. This mapping shows that dopamine neuron synaptic actions extend throughout the striatum, preferentially target cholinergic interneurons, and define distinct striatal subregions.

Vulnerable and Resilient Phenotypes in a Mouse Model of Anorexia Nervosa

BACKGROUND

Increased physical activity is a common feature of anorexia nervosa (AN). Although high activity levels are associated with greater risk of developing AN, particularly when combined with dieting, most individuals who diet and exercise maintain a healthy body weight. It is unclear why some individuals develop AN while most do not. A rodent model of resilience and vulnerability to AN would be valuable to research. Dopamine, which is believed to play a crucial role in AN, regulates both reward and activity and may modulate vulnerability.

METHODS

Adolescent and young adult female C57BL/6N mice were tested in the activity-based anorexia (ABA) model, with an extended period of food restriction in adult mice. ABA was also tested in dopamine transporter knockdown mice and wild-type littermates. Mice that adapted to conditions and maintained a stable body weight were characterized as resilient.

RESULTS

In adults, vulnerable and resilient phenotypes emerged in both the ABA and food-restricted mice without wheels. Vulnerable mice exhibited a pronounced increase in running throughout the light cycle, which dramatically peaked prior to requiring removal from the experiment. Resilient mice exhibited an adaptive decrease in total running, appropriate food anticipatory activity, and increased consumption, thereby achieving stable body weight. Hyperdopaminergia accelerated progression of the vulnerable phenotype.

CONCLUSIONS

Our demonstration of distinct resilient and vulnerable phenotypes in mouse ABA significantly advances the utility of the model for identifying genes and neural substrates mediating AN risk and resilience. Modulation of dopamine may play a central role in the underlying circuit.

Non-canonical glutamine transamination sustains efferocytosis by coupling redox buffering to oxidative phosphorylation

Macrophages rely on tightly integrated metabolic rewiring to clear dying neighboring cells by efferocytosis during homeostasis and disease. Here we reveal that glutaminase-1-mediated glutaminolysis is critical to promote apoptotic cell clearance by macrophages during homeostasis in mice. In addition, impaired macrophage glutaminolysis exacerbates atherosclerosis, a condition during which, efficient apoptotic cell debris clearance is critical to limit disease progression. Glutaminase-1 expression strongly correlates with atherosclerotic plaque necrosis in patients with cardiovascular diseases. High-throughput transcriptional and metabolic profiling reveals that macrophage efferocytic capacity relies on a non-canonical transaminase pathway, independent from the traditional requirement of glutamate dehydrogenase to fuel ɑ-ketoglutarate-dependent immunometabolism. This pathway is necessary to meet the unique requirements of efferocytosis for cellular detoxification and high-energy cytoskeletal rearrangements. Thus, we uncover a role for non-canonical glutamine metabolism for efficient clearance of dying cells and maintenance of tissue homeostasis during health and disease in mouse and humans.

Single Dose of Amphetamine Induces Delayed Subregional Attenuation of Cholinergic Interneuron Activity in the Striatum

Psychostimulants such as amphetamine (AMPH) target dopamine (DA) neuron synapses to engender drug-induced plasticity. While DA neurons modulate the activity of striatal (Str) cholinergic interneurons (ChIs) with regional heterogeneity, how AMPH affects ChI activity has not been elucidated. Here, we applied quantitative fluorescence imaging approaches to map the dose-dependent effects of a single dose of AMPH on ChI activity at 2.5 and 24 h after injection across the mouse Str using the activity-dependent marker phosphorylated ribosomal protein S6 (p-rpS6240/244). AMPH did not affect the distribution or morphology of ChIs in any Str subregion. While AMPH at either dose had no effect on ChI activity after 2.5 h, ChI activity was dose dependently reduced after 24 h specifically in the ventral Str/nucleus accumbens (NAc), a critical site of psychostimulant action. AMPH at either dose did not affect the spontaneous firing of ChIs. Altogether this work demonstrates that a single dose of AMPH has delayed regionally heterogeneous effects on ChI activity, which most likely involves extra-Str synaptic input.

Perinatal interference with the serotonergic system affects VTA function in the adult via glutamate co-transmission

Serotonin and dopamine are associated with multiple psychiatric disorders. How they interact during development to affect subsequent behavior remains unknown. Knockout of the serotonin transporter or postnatal blockade with selective serotonin reuptake inhibitors (SSRIs) leads to novelty-induced exploration deficits in adulthood, potentially involving the dopamine system. Here, we show in the mouse that raphe nucleus serotonin neurons activate ventral tegmental area dopamine neurons via glutamate co-transmission and that this co-transmission is reduced in animals exposed postnatally to SSRIs. Blocking serotonin neuron glutamate co-transmission mimics this SSRI-induced hypolocomotion, while optogenetic activation of dopamine neurons reverses this hypolocomotor phenotype. Our data demonstrate that serotonin neurons modulate dopamine neuron activity via glutamate co-transmission and that this pathway is developmentally malleable, with high serotonin levels during early life reducing co-transmission, revealing the basis for the reduced novelty-induced exploration in adulthood due to postnatal SSRI exposure.

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Identifying strategies to target the metabolic flexibility of tumours

Plasticity of cancer metabolism can be a major obstacle to efficient targeting of tumour-specific metabolic vulnerabilities. Here, we identify the compensatory mechanisms following the inhibition of major pathways of central carbon metabolism in c-MYC-induced liver tumours. We find that, while inhibition of both glutaminase isoforms (Gls1 and Gls2) in tumours considerably delays tumourigenesis, glutamine catabolism continues, owing to the action of amidotransferases. Synergistic inhibition of both glutaminases and compensatory amidotransferases is required to block glutamine catabolism and proliferation of mouse and human tumour cells in vitro and in vivo. Gls1 deletion is also compensated for by glycolysis. Thus, co-inhibition of Gls1 and hexokinase 2 significantly affects Krebs cycle activity and tumour formation. Finally, the inhibition of biosynthesis of either serine (Psat1-KO) or fatty acid (Fasn-KO) is compensated for by uptake of circulating nutrients, and dietary restriction of both serine and glycine or fatty acids synergistically suppresses tumourigenesis. These results highlight the high flexibility of tumour metabolism and demonstrate that either pharmacological or dietary targeting of metabolic compensatory mechanisms can improve therapeutic outcomes.

Glutaminase activity in GLS1 Het mouse brain compared to putative pharmacological inhibition by ebselen using ex vivo MRS

Glutaminase mediates the recycling of neurotransmitter glutamate, supporting most excitatory neurotransmission in the mammalian central nervous system. A constitutive heterozygous reduction in GLS1 engenders in mice a model of schizophrenia resilience and associated increases in Gln, reductions in Glu and activity-dependent attenuation of excitatory synaptic transmission. Hippocampal brain slices from GLS1 heterozygous mice metabolize less Gln to Glu. Whether glutaminase activity is diminished in the intact brain in GLS1 heterozygous mice has not been assessed, nor the regional impact. Moreover, it is not known whether pharmacological inhibition would mimic the genetic reduction. We addressed this using magnetic resonance spectroscopy to assess amino acid content and ¹³C-acetate loading to assess glutaminase activity, in multiple brain regions. Glutaminase activity was reduced significantly in the hippocampus of GLS1 heterozygous mice, while acute treatment with the putative glutaminase inhibitor ebselen did not impact glutaminase activity, but did significantly increase GABA. This approach identifies a molecular imaging strategy for testing target engagement by comparing genetic and pharmacological inhibition, across brain regions.

Glutamate Dehydrogenase-Deficient Mice Display Schizophrenia-Like Behavioral Abnormalities and CA1-Specific Hippocampal Dysfunction

Brain imaging has revealed that the CA1 subregion of the hippocampus is hyperactive in prodromal and diagnosed patients with schizophrenia (SCZ), and that glutamate is a driver of this hyperactivity. Strikingly, mice deficient in the glutamate synthetic enzyme glutaminase have CA1 hypoactivity and a SCZ-resilience profile, implicating glutamate-metabolizing enzymes. To address this further, we examined mice with a brain-wide deficit in the glutamate-metabolizing enzyme glutamate dehydrogenase (GDH), encoded by Glud1, which should lead to glutamate excess due to reduced glutamate metabolism in astrocytes. We found that Glud1-deficient mice have behavioral abnormalities in the 3 SCZ symptom domains, with increased baseline and amphetamine-induced hyperlocomotion as a positive symptom proxy, nest building and social preference as a negative symptom proxy, and reversal/extradimensional set shifting in the water T-maze and contextual fear conditioning as a cognitive symptom proxy. Neuroimaging of cerebral blood volume revealed hippocampal hyperactivity in CA1, which was associated with volume reduction. Parameters of hippocampal synaptic function revealed excess glutamate release and an elevated excitatory/inhibitory balance in CA1. Finally, in a direct clinical correlation using imaging-guided microarray, we found a significant SCZ-associated postmortem reduction in GLUD1 expression in CA1. These findings advance GLUD1 deficiency as a driver of excess hippocampal excitatory transmission and SCZ symptoms, and identify GDH as a target for glutamate modulation pharmacotherapy for SCZ. More broadly, these findings point to the likely involvement of alterations in glutamate metabolism in the pathophysiology of SCZ.

Molecular Imaging of mGluR5 Availability with [11C]ABP68 in Glutaminase Heterozygous Mice

Many PET tracers enable determination of fluctuations in neurotransmitter release, yet glutamate specifically can not be visualized in a noninvasive manner. Several studies point to the possibility of visualizing fluctuations in glutamate release by changes in affinity of the mGluR5 radioligand [¹¹C]ABP688. These studies use pharmacological challenges to alter glutamate levels, and so probe release, but have not measured chronic alterations in receptor occupancy due to altered neurotransmission relevant to chronic neuropsychiatric disorders or their treatment. In this regard, the GLS1 heterozygous mouse has known reductions in activity of the glutamate-synthetic enzyme glutaminase, brain glutamate levels and release. We imaged this model to elucidate glutamatergic systems. Dynamic [¹¹C]ABP688 microPET scans were performed for mGluR5. Western blot was used as an ex vivo validation. No significant differences were found in BP_ND between WT and GLS1 Hets. SPM showed voxel-wise increased in BP_ND in GLS1 Hets compared to WT consistent with lower synaptic glutamate. This was not due to alterations in mGluR5 levels, as western blot results showed lower mGluR5 levels in GLS1 Hets. We conclude that because of the chronic glutaminase deficiency and subsequent decrease in glutamate, the mGluR5 protein levels are lowered. Due to these decreased endogenous glutamate levels, however, there is increased [¹¹C]ABP688 binding to the allosteric site in selected regions. We speculate that lower endogenous glutamate leads to less conformational change to the receptors, and thus higher availability of the binding site. The lower mGluR5 levels, however, lessen [¹¹C]ABP688 binding in GLS1 Hets, in part masking the increase in binding due to diminished endogenous glutamate levels as confirmed with voxel-wise analysis.

Dopamine neuron glutamate cotransmission evokes a delayed excitation in lateral dorsal striatal cholinergic interneurons

Dopamine neurons have different synaptic actions in the ventral and dorsal striatum (dStr), but whether this heterogeneity extends to dStr subregions has not been addressed. We have found that optogenetic activation of dStr dopamine neuron terminals in mouse brain slices pauses the firing of cholinergic interneurons in both the medial and lateral subregions, while in the lateral subregion the pause is shorter due to a subsequent excitation. This excitation is mediated mainly by metabotropic glutamate receptor 1 (mGluR1) and partially by dopamine D1-like receptors coupled to transient receptor potential channel 3 and 7. DA neurons do not signal to spiny projection neurons in the medial dStr, while they elicit ionotropic glutamate responses in the lateral dStr. The DA neurons mediating these excitatory signals are in the substantia nigra (SN). Thus, SN dopamine neurons engage different receptors in different postsynaptic neurons in different dStr subregions to convey strikingly different signals.

Editorial note

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Neuronal Depolarization Drives Increased Dopamine Synaptic Vesicle Loading via VGLUT

The ability of presynaptic dopamine terminals to tune neurotransmitter release to meet the demands of neuronal activity is critical to neurotransmission. Although vesicle content has been assumed to be static, in vitro data increasingly suggest that cell activity modulates vesicle content. Here, we use a coordinated genetic, pharmacological, and imaging approach in Drosophila to study the presynaptic machinery responsible for these vesicular processes in vivo. We show that cell depolarization increases synaptic vesicle dopamine content prior to release via vesicular hyperacidification. This depolarization-induced hyperacidification is mediated by the vesicular glutamate transporter (VGLUT). Remarkably, both depolarization-induced dopamine vesicle hyperacidification and its dependence on VGLUT2 are seen in ventral midbrain dopamine neurons in the mouse. Together, these data suggest that in response to depolarization, dopamine vesicles utilize a cascade of vesicular transporters to dynamically increase the vesicular pH gradient, thereby increasing dopamine vesicle content.

Dopamine neuron dependent behaviors mediated by glutamate cotransmission

Dopamine neurons in the ventral tegmental area use glutamate as a cotransmitter. To elucidate the behavioral role of the cotransmission, we targeted the glutamate-recycling enzyme glutaminase (gene Gls1). In mice with a dopamine transporter (Slc6a3)-driven conditional heterozygous (cHET) reduction of Gls1 in their dopamine neurons, dopamine neuron survival and transmission were unaffected, while glutamate cotransmission at phasic firing frequencies was reduced, enabling a selective focus on the cotransmission. The mice showed normal emotional and motor behaviors, and an unaffected response to acute amphetamine. Strikingly, amphetamine sensitization was reduced and latent inhibition potentiated. These behavioral effects, also seen in global GLS1 HETs with a schizophrenia resilience phenotype, were not seen in mice with an Emx1-driven forebrain reduction affecting most brain glutamatergic neurons. Thus, a reduction in dopamine neuron glutamate cotransmission appears to mediate significant components of the GLS1 HET schizophrenia resilience phenotype, and glutamate cotransmission appears to be important in attribution of motivational salience.

Role of glutamine and interlinked asparagine metabolism in vessel formation

Endothelial cell (EC) metabolism is emerging as a regulator of angiogenesis, but the precise role of glutamine metabolism in ECs is unknown. Here, we show that depriving ECs of glutamine or inhibiting glutaminase 1 (GLS1) caused vessel sprouting defects due to impaired proliferation and migration, and reduced pathological ocular angiogenesis. Inhibition of glutamine metabolism in ECs did not cause energy distress, but impaired tricarboxylic acid (TCA) cycle anaplerosis, macromolecule production, and redox homeostasis. Only the combination of TCA cycle replenishment plus asparagine supplementation restored the metabolic aberrations and proliferation defect caused by glutamine deprivation. Mechanistically, glutamine provided nitrogen for asparagine synthesis to sustain cellular homeostasis. While ECs can take up asparagine, silencing asparagine synthetase (ASNS, which converts glutamine-derived nitrogen and aspartate to asparagine) impaired EC sprouting even in the presence of glutamine and asparagine. Asparagine further proved crucial in glutamine-deprived ECs to restore protein synthesis, suppress ER stress, and reactivate mTOR signaling. These findings reveal a novel link between endothelial glutamine and asparagine metabolism in vessel sprouting.

Heterogeneity in Dopamine Neuron Synaptic Actions Across the Striatum and Its Relevance for Schizophrenia

Brain imaging has revealed alterations in dopamine uptake, release, and receptor levels in patients with schizophrenia that have been resolved on the scale of striatal subregions. However, the underlying synaptic mechanisms are on a finer scale. Dopamine neuron synaptic actions vary across the striatum, involving variations not only in dopamine release but also in dopamine neuron connectivity, cotransmission, modulation, and activity. Optogenetic studies have revealed that dopamine neurons release dopamine in a synaptic signal mode, and that the neurons also release glutamate and gamma-aminobutyric acid as cotransmitters, with striking regional variation. Fast glutamate and gamma-aminobutyric acid cotransmission convey discrete patterns of dopamine neuron activity to striatal neurons. Glutamate may function not only in a signaling role at a subset of dopamine neuron synapses, but also in mediating vesicular synergy, contributing to regional differences in loading of dopamine into synaptic vesicles. Regional differences in dopamine neuron signaling are likely to be differentially involved in the schizophrenia disease process and likely determine the subregional specificity of the action of psychostimulants that exacerbate the disorder, and antipsychotics that ameliorate the disorder. Elucidating dopamine neuron synaptic signaling offers the potential for achieving greater pharmacological specificity through intersectional pharmacological actions targeting subsets of dopamine neuron synapses.

Genetic Pharmacotherapy as an Early CNS Drug Development Strategy: Testing Glutaminase Inhibition for Schizophrenia Treatment in Adult Mice

Genetic pharmacotherapy is an early drug development strategy for the identification of novel CNS targets in mouse models prior to the development of specific ligands. Here for the first time, we have implemented this strategy to address the potential therapeutic value of a glutamate-based pharmacotherapy for schizophrenia involving inhibition of the glutamate recycling enzyme phosphate-activated glutaminase. Mice constitutively heterozygous for GLS1, the gene encoding glutaminase, manifest a schizophrenia resilience phenotype, a key dimension of which is an attenuated locomotor response to propsychotic amphetamine challenge. If resilience is due to glutaminase deficiency in adulthood, then glutaminase inhibitors should have therapeutic potential. However, this has been difficult to test given the dearth of neuroactive glutaminase inhibitors. So, we used genetic pharmacotherapy to ask whether adult induction of GLS1 heterozygosity would attenuate amphetamine responsiveness. We generated conditional floxGLS1 mice and crossed them with global CAG^ERT2cre∕+ mice to produce GLS1 iHET mice, susceptible to tamoxifen induction of GLS1 heterozygosity. One month after tamoxifen treatment of adult GLS1 iHET mice, we found a 50% reduction in GLS1 allelic abundance and glutaminase mRNA levels in the brain. While GLS1 iHET mice showed some recombination prior to tamoxifen, there was no impact on mRNA levels. We then asked whether induction of GLS heterozygosity would attenuate the locomotor response to propsychotic amphetamine challenge. Before tamoxifen, control and GLS1 iHET mice did not differ in their response to amphetamine. One month after tamoxifen treatment, amphetamine-induced hyperlocomotion was blocked in GLS1 iHET mice. The block was largely maintained after 5 months. Thus, a genetically induced glutaminase reduction—mimicking pharmacological inhibition—strongly attenuated the response to a propsychotic challenge, suggesting that glutaminase may be a novel target for the pharmacotherapy of schizophrenia. These results demonstrate how genetic pharmacotherapy can be implemented to test a CNS target in advance of the development of specific neuroactive inhibitors. We discuss further the advantages, limitations, and feasibility of the wider application of genetic pharmacotherapy for neuropsychiatric drug development.

Metabolic reprogramming induces resistance to anti-NOTCH1 therapies in T cell acute lymphoblastic leukemia

Activating mutations in NOTCH1 are common in T-cell acute lymphoblastic leukemia (TALL). Here we identify glutaminolysis as a critical pathway for leukemia cell growth downstream of NOTCH1 and a key determinant of clinical response to anti-NOTCH1 therapies. Mechanistically, inhibition of NOTCH1 signaling in T-ALL induces a metabolic shutdown with prominent inhibition of glutaminolysis and triggers autophagy as a salvage pathway supporting leukemia cell metabolism. Consequently, both inhibition of glutaminolysis and inhibition of autophagy strongly and synergistically enhance the antileukemic effects of anti-NOTCH1 therapies. Moreover, we demonstrate that Pten loss induces increased glycolysis and consequently rescues leukemic cell metabolism abrogating the antileukemic effects of NOTCH1 inhibition. Overall, these results identify glutaminolysis as a major node in cancer metabolism controlled by NOTCH1 and as therapeutic target for the treatment of T-ALL.

Targeted inhibition of tumor-specific glutaminase diminishes cell-autonomous tumorigenesis

Glutaminase (GLS), which converts glutamine to glutamate, plays a key role in cancer cell metabolism, growth, and proliferation. GLS is being explored as a cancer therapeutic target, but whether GLS inhibitors affect cancer cell-autonomous growth or the host microenvironment or have off-target effects is unknown. Here, we report that loss of one copy of Gls blunted tumor progression in an immune-competent MYC-mediated mouse model of hepatocellular carcinoma. Compared with results in untreated animals with MYC-induced hepatocellular carcinoma, administration of the GLS-specific inhibitor bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide (BPTES) prolonged survival without any apparent toxicities. BPTES also inhibited growth of a MYC-dependent human B cell lymphoma cell line (P493) by blocking DNA replication, leading to cell death and fragmentation. In mice harboring P493 tumor xenografts, BPTES treatment inhibited tumor cell growth; however, P493 xenografts expressing a BPTES-resistant GLS mutant (GLS-K325A) or overexpressing GLS were not affected by BPTES treatment. Moreover, a customized Vivo-Morpholino that targets human GLS mRNA markedly inhibited P493 xenograft growth without affecting mouse Gls expression. Conversely, a Vivo-Morpholino directed at mouse Gls had no antitumor activity in vivo. Collectively, our studies demonstrate that GLS is required for tumorigenesis and support small molecule and genetic inhibition of GLS as potential approaches for targeting the tumor cell-autonomous dependence on GLS for cancer therapy.

Dopamine neurons control striatal cholinergic neurons via regionally heterogeneous dopamine and glutamate signaling

Midbrain dopamine neurons fire in bursts conveying salient information. Bursts are associated with pauses in tonic firing of striatal cholinergic interneurons. Although the reciprocal balance of dopamine and acetylcholine in the striatum is well known, how dopamine neurons control cholinergic neurons has not been elucidated. Here, we show that dopamine neurons make direct fast dopaminergic and glutamatergic connections with cholinergic interneurons, with regional heterogeneity. Dopamine neurons drive a burst-pause firing sequence in cholinergic interneurons in the medial shell of the nucleus accumbens, mixed actions in the accumbens core, and a pause in the dorsal striatum. This heterogeneity is due mainly to regional variation in dopamine-neuron glutamate cotransmission. A single dose of amphetamine attenuates dopamine neuron connections to cholinergic interneurons with dose-dependent regional specificity. Overall, the present data indicate that dopamine neurons control striatal circuit function via discrete, plastic connections with cholinergic interneurons.

Dopamine D2 receptors regulate the anatomical and functional balance of basal ganglia circuitry

Structural plasticity in the adult brain is essential for adaptive behavior. We have found a remarkable anatomical plasticity in the basal ganglia of adult mice that is regulated by dopamine D2 receptors (D2Rs). By modulating neuronal excitability, striatal D2Rs bidirectionally control the density of direct pathway collaterals in the globus pallidus that bridge the direct pathway with the functionally opposing indirect pathway. An increase in bridging collaterals is associated with enhanced inhibition of pallidal neurons in vivo and disrupted locomotor activation after optogenetic stimulation of the direct pathway. Chronic blockade with haloperidol, an antipsychotic medication used to treat schizophrenia, decreases the extent of bridging collaterals and rescues the locomotor imbalance. These findings identify a role for bridging collaterals in regulating the concerted balance of striatal output and may have important implications for understanding schizophrenia, a disease involving excessive activation of striatal D2Rs that is treated with D2R blockers.

Modeling resilience to schizophrenia in genetically modified mice: a novel approach to drug discovery

Complex psychiatric disorders, such as schizophrenia, arise from a combination of genetic, developmental, environmental and social factors. These vulnerabilities can be mitigated by adaptive factors in each of these domains engendering resilience. Modeling resilience in mice using transgenic approaches offers a direct path to intervention, as resilience mutations point directly to therapeutic targets. As prototypes for this approach, we discuss the three mouse models of schizophrenia resilience, all based on modulating glutamatergic synaptic transmission. This motivates the broader development of schizophrenia resilience mouse models independent of specific pathophysiological hypotheses as a strategy for drug discovery. Three guiding validation criteria are presented. A resilience-oriented approach should identify pharmacologically tractable targets and in turn offer new insights into pathophysiological mechanisms.

Synaptic underpinnings of altered hippocampal function in glutaminase-deficient mice during maturation

Glutaminase-deficient mice (GLS1 hets), with reduced glutamate recycling, have a focal reduction in hippocampal activity, mainly in CA1, and manifest behavioral and neurochemical phenotypes suggestive of schizophrenia resilience. To address the basis for the hippocampal hypoactivity, we examined synaptic plastic mechanisms and glutamate receptor expression. Although baseline synaptic strength was unaffected in Schaffer collateral inputs to CA1, we found that long-term potentiation was attenuated. In wild-type (WT) mice, GLS1 gene expression was highest in the hippocampus and cortex, where it was reduced by about 50% in GLS1 hets. In other brain regions with lower WT GLS1 gene expression, there were no genotypic reductions. In adult GLS1 hets, NMDA receptor NR1 subunit gene expression was reduced, but not AMPA receptor GluR1 subunit gene expression. In contrast, juvenile GLS1 hets showed no reductions in NR1 gene expression. In concert with this, adult GLS1 hets showed a deficit in hippocampal-dependent contextual fear conditioning, whereas juvenile GLS1 hets did not. These alterations in glutamatergic synaptic function may partly explain the hippocampal hypoactivity seen in the GLS1 hets. The maturity-onset reduction in NR1 gene expression and in contextual learning supports the premise that glutaminase inhibition in adulthood should prove therapeutic in schizophrenia.

Vesicular glutamate transport promotes dopamine storage and glutamate corelease in vivo

Dopamine neurons in the ventral tegmental area (VTA) play an important role in the motivational systems underlying drug addiction, and recent work has suggested that they also release the excitatory neurotransmitter glutamate. To assess a physiological role for glutamate corelease, we disrupted the expression of vesicular glutamate transporter 2 selectively in dopamine neurons. The conditional knockout abolishes glutamate release from midbrain dopamine neurons in culture and severely reduces their excitatory synaptic output in mesoaccumbens slices. Baseline motor behavior is not affected, but stimulation of locomotor activity by cocaine is impaired, apparently through a selective reduction of dopamine stores in the projection of VTA neurons to ventral striatum. Glutamate co-entry promotes monoamine storage by increasing the pH gradient that drives vesicular monoamine transport. Remarkably, low concentrations of glutamate acidify synaptic vesicles more slowly but to a greater extent than equimolar Cl(-), indicating a distinct, presynaptic mechanism to regulate quantal size.

How high-resolution basal-state functional imaging can guide the development of new pharmacotherapies for schizophrenia

We describe here a coordinated brain imaging and animal models approach in which we have shown that the hippocampal CA1 region is a principal node in schizophrenia pathogenesis and have identified a novel treatment approach to the disorder based on inhibition of glutamate release. To identify biomarkers, we have focused on the putative prodromal period, typically lasting a few years, preceding the first onset of psychosis. About one-third of a high-risk cohort followed prospectively for 2.5 years will progress to threshold psychosis, making it possible to perform a relatively short prospective study. We have utilized a technological development in functional imaging techniques in which we measure cerebral blood volume (CBV), which allows for interrogation of subregions of the brain in the basal state at submillimeter resolution. Measurements of CBV in schizophrenia as well as in high-risk or prodromal stages can then pinpoint brain subregions differentially targeted during the earliest stages of the disorder. Our data suggest that the CA1 subfield of the hippocampal formation is most consistently implicated across disease stages, identifying a putative biomarker suitable for guiding drug development. Our studies in transgenic mice mutant in the glutamate synthetic enzyme glutaminase support the hypothesis that CA1 hyperfunction is due to altered glutamatergic neurotransmission. As a proof of principle, the glutaminase-deficient mice suggest that pharmacotherapies that reduce glutamatergic neurotransmission in the CA1 subfield may be a uniquely effective therapeutic strategy in schizophrenia and preventative in prodromal stages of the disorder.

Dopamine neuron glutamate cotransmission: frequency-dependent modulation in the mesoventromedial projection

Mesoventromedial dopamine neurons projecting from the medial ventral tegmental area to the ventromedial shell of the nucleus accumbens play a role in attributing incentive salience to environmental stimuli that predict important events, and appear to be particularly sensitive to the effects of psychostimulant drugs. Despite the observation that these dopamine neurons make up almost the entire complement of neurons in the projection, stimulating their cell bodies evokes a fast glutamatergic response in accumbens neurons. This is apparently due to dopamine neuron glutamate cotransmission, suggested by the extensive coexpression of vesicular glutamate transporter 2 (VGLUT2) in the neurons. To examine the interplay between the dopamine and glutamate signals, we used acute quasi-horizontal brain slices made from DAT-YFP mice in which the intact mesoventromedial projection can be visualized. Under current clamp, when dopamine neurons were stimulated repeatedly, dopamine neuron glutamate transmission showed dopamine-mediated facilitation, solely at higher, burst-firing frequencies. Facilitation was diminished under voltage clamp and flipped to inhibition by intracellular Cs(+) or GDPbetaS, indicating that it was mediated postsynaptically. Postsynaptic facilitation was D1 mediated, required activation of NMDA receptors and closure of voltage gated K(+)-channels. When postsynaptic facilitation was blocked, D2-mediated presynaptic inhibition became apparent. These counterbalanced pre- and postsynaptic actions determine the frequency dependence of dopamine modulation; at lower firing frequencies dopamine modulation is not apparent, while at burst firing frequency postsynaptic facilitation dominates and dopamine becomes facilitatory. Dopamine neuron glutamate cotransmission may play an important role in encoding the incentive salience value of conditioned stimuli that activate goal-directed behaviors, and may be an important subtract for enduring drug-seeking behaviors.

Distinct roles of presynaptic dopamine receptors in the differential modulation of the intrinsic synapses of medium-spiny neurons in the nucleus accumbens

Background

In both schizophrenia and addiction, pathological changes in dopamine release appear to induce alterations in the circuitry of the nucleus accumbens that affect coordinated thought and motivation. Dopamine acts principally on medium-spiny GABA neurons, which comprise 95% of accumbens neurons and give rise to the majority of inhibitory synapses in the nucleus. To examine dopamine action at single medium-spiny neuron synapses, we imaged Ca²⁺ levels in their presynaptic varicosities in the acute brain slice using two-photon microscopy.

Results

Presynaptic Ca²⁺ rises were differentially modulated by dopamine. The D1/D5 selective agonist SKF81297 was exclusively facilitatory. The D2/D3 selective agonist quinpirole was predominantly inhibitory, but in some instances it was facilitatory. Studies using D2 and D3 receptor knockout mice revealed that quinpirole inhibition was either D2 or D3 receptor-mediated, while facilitation was mainly D3 receptor-mediated. Subsets of varicosities responded to both D1 and D2 agonists, showing that there was significant co-expression of these receptor families in single medium-spiny neurons. Neighboring presynaptic varicosities showed strikingly heterogeneous responses to DA agonists, suggesting that DA receptors may be differentially trafficked to individual varicosities on the same medium-spiny neuron axon.

Conclusion

Dopamine receptors are present on the presynaptic varicosities of medium-spiny neurons, where they potently control GABAergic synaptic transmission. While there is significant coexpression of D1 and D2 family dopamine receptors in individual neurons, at the subcellular level, these receptors appear to be heterogeneously distributed, potentially explaining the considerable controversy regarding dopamine action in the striatum, and in particular the degree of dopamine receptor segregation on these neurons. Assuming that post-receptor signaling is restricted to the microdomains of medium-spiny neuron varicosities, the heterogeneous distribution of dopamine receptors on individual varicosities is likely to encode patterns in striatal information processing.

Dopamine presynaptically and heterogeneously modulates nucleus accumbens medium-spiny neuron GABA synapses in vitro

Background

The striatal complex is the major target of dopamine action in the CNS. There, medium-spiny GABAergic neurons, which constitute about 95% of the neurons in the area, form a mutually inhibitory synaptic network that is modulated by dopamine. When put in culture, the neurons reestablish this network. In particular, they make autaptic connections that provide access to single, identified medium-spiny to medium-spiny neuron synaptic connections.

Results

We examined medium-spiny neuron autaptic connections in postnatal cultures from the nucleus accumbens, the ventral part of the striatal complex. These connections were subject to presynaptic dopamine modulation. D1-like receptors mediated either inhibition or facilitation, while D2-like receptors predominantly mediated inhibition. Many connections showed both D1 and D2 modulation, consistent with a significant functional colocalization of D1 and D2-like receptors at presynaptic sites. These same connections were subject to GABA_A, GABA_B, norepinephrine and serotonin modulation, revealing a multiplicity of modulatory autoreceptors and heteroreceptors on individual varicosities. In some instances, autaptic connections had two components that were differentially modulated by dopamine agonists, suggesting that dopamine receptors could be distributed heterogeneously on the presynaptic varicosities making up a single synaptic (i.e. autaptic) connection.

Conclusion

Differential trafficking of dopamine receptors to different presynaptic varicosities could explain the many controversial studies reporting widely varying degrees of dopamine receptor colocalization in medium-spiny neurons, as well as more generally the diversity of dopamine actions in target areas. Longer-term changes in the modulatory actions of dopamine in the striatal complex could be due to plasticity in the presynaptic distribution of dopamine receptors on medium-spiny neuron varicosities.

Mice lacking brain/kidney phosphate-activated glutaminase have impaired glutamatergic synaptic transmission, altered breathing, disorganized goal-directed behavior and die shortly after birth

Neurotransmitter glutamate has been thought to derive mainly from glutamine via the action of glutaminase type 1 (GLS1). To address the importance of this pathway in glutamatergic transmission, we knocked out GLS1 in mice. The insertion of a STOP cassette by homologous recombination produced a null allele that blocked transcription, encoded no immunoreactive protein, and abolished GLS1 enzymatic activity. Null mutants were slightly smaller, were deficient in goal-directed behavior, hypoventilated, and died in the first postnatal day. No gross or microscopic defects were detected in peripheral organs or in the CNS. In cultured neurons from the null mutants, miniature EPSC amplitude and duration were normal; however, the amplitude of evoked EPSCs decayed more rapidly with sustained 10 Hz stimulation, consistent with an observed reduction in depolarization-evoked glutamate release. Because of this activity-dependent impairment in glutamatergic transmission, we surmised that respiratory networks, which require temporal summation of synaptic input, would be particularly affected. We found that the amplitude of inspirations was decreased in vivo, chemosensitivity to CO2 was severely altered, and the frequency of pacemaker activity recorded in the respiratory generator in the pre-Bötzinger complex, a glutamatergic brainstem network that can be isolated in vitro, was increased. Our results show that although alternate pathways to GLS1 glutamate synthesis support baseline glutamatergic transmission, the GLS1 pathway is essential for maintaining the function of active synapses, and thus the mutation is associated with impaired respiratory function, abnormal goal-directed behavior, and neonatal demise.

Targeted gene expression in dopamine and serotonin neurons of the mouse brain

We used a knock-in strategy to generate two lines of mice expressing Cre recombinase under the transcriptional control of the dopamine transporter promoter (DAT-cre mice) or the serotonin transporter promoter (SERT-cre mice). In DAT-cre mice, immunocytochemical staining of adult brains for the dopamine-synthetic enzyme tyrosine hydroxylase and for Cre recombinase revealed that virtually all dopaminergic neurons in the ventral midbrain expressed Cre. Crossing DAT-cre mice with ROSA26-stop-lacZ or ROSA26-stop-YFP reporter mice revealed a near perfect correlation between staining for tyrosine hydroxylase and beta-galactosidase or YFP. YFP-labeled fluorescent dopaminergic neurons could be readily identified in live slices. Crossing SERT-cre mice with the ROSA26-stop-lacZ or ROSA26-stop-YFP reporter mice similarly revealed a near perfect correlation between staining for serotonin-synthetic enzyme tryptophan hydroxylase and beta-galactosidase or YFP. Additional Cre expression in the thalamus and cortex was observed, reflecting the known pattern of transient SERT expression during early postnatal development. These findings suggest a general strategy of using neurotransmitter transporter promoters to drive selective Cre expression and thus control mutations in specific neurotransmitter systems. Crossed with fluorescent-gene reporters, this strategy tags neurons by neurotransmitter status, providing new tools for electrophysiology and imaging.

Dopamine neurons mediate a fast excitatory signal via their glutamatergic synapses

Dopamine neurons are thought to convey a fast, incentive salience signal, faster than can be mediated by dopamine. A resolution of this paradox may be that midbrain dopamine neurons exert fast excitatory actions. Using transgenic mice with fluorescent dopamine neurons, in which the axonal projections of the neurons are visible, we made horizontal brain slices encompassing the mesoaccumbens dopamine projection. Focal extracellular stimulation of dopamine neurons in the ventral tegmental area evoked dopamine release and early monosynaptic and late polysynaptic excitatory responses in postsynaptic nucleus accumbens neurons. Local superfusion of the ventral tegmental area with glutamate, which should activate dopamine neurons selectively, produced an increase in excitatory synaptic events. Local superfusion of the ventral tegmental area with the D2 agonist quinpirole, which should increase the threshold for dopamine neuron activation, inhibited the early response. So dopamine neurons make glutamatergic synaptic connections to accumbens neurons. We propose that dopamine neuron glutamatergic transmission may be the initial component of the incentive salience signal.

Expression and function of 5-HT3 receptors in the enteric neurons of mice lacking the serotonin transporter

The actions of enteric 5-HT are terminated by 5-HT transporter (SERT)-mediated uptake, and gastrointestinal motility is abnormal in SERT -/- mice. We tested the hypothesis that adaptive changes in enteric 5-HT(3) receptors help SERT -/- mice survive despite inefficient 5-HT inactivation. Expression of mRNA encoding enteric 5-HT(3A) subunits was similar in SERT +/+ and -/- mice, but that of 5-HT(3B) subunits was fourfold less in SERT -/- mice. 5-HT(3B) mRNA was found, by in situ hybridization, in epithelial cells and enteric neurons. 5-HT evoked a fast inward current in myenteric neurons that was pharmacologically identified as 5-HT(3) mediated. The EC(50) of the 5-HT response was lower in SERT +/+ (18 microM) than in SERT -/- (36 microM) mice and desensitized rapidly in a greater proportion of SERT -/- neurons; however, peak amplitudes, steady-state current, and decay time constants were not different. Adaptive changes thus occur in the subunit composition of enteric 5-HT(3) receptors of SERT -/- mice that are reflected in 5-HT(3) receptor affinity and desensitization.

Glutamate is a cotransmitter in ventral midbrain dopamine neurons

Interactions between apparently separate dopaminergic and glutamatergic pathways figure prominently in the pathophysiology of Parkinson's Disease. So it is surprising that the ventral midbrain dopamine neurons, which give rise to the dopaminergic pathway, may themselves also be glutamatergic. We have addressed this idea in both rat and monkey brain and found that most ventral midbrain dopamine neurons exhibit glutamate immunoreactivity. We used postnatal cell culture to examine ventral midbrain dopamine neurons more closely. In vitro most dopamine neurons exhibit glutamate immunoreactivity, as well as immunoreactivity for phosphate-activated glutaminase, the enzyme principally responsible for the synthesis of neurotransmitter glutamate; inhibition of glutaminase reduces glutamate staining. In single cell microcultures, dopamine neurons make both dopaminergic and glutamatergic synaptic varicosities. Stimulation of individual dopamine neurons evokes a fast excitatory synaptic response mediated by glutamate; it also evokes dopamine release that inhibits the excitatory response via presynaptic D2 receptors. Thus, dopamine neurons appear to exert rapid synaptic actions via their glutamatergic synapses and slower modulatory actions via their dopaminergic synapses, including possibly inhibition of their own glutamatergic synapses. So, in the setting of dopamine neuron demise, there will be a loss of both dopaminergic and glutamatergic inputs to the striatum; furthermore, glutamate released by dopamine neurons may contribute to an excitotoxic cascade and the death of neighboring dopamine neurons.

Dale's principle and glutamate corelease from ventral midbrain dopamine neurons

While direct application of dopamine modulates postsynaptic activity, electrical stimulation of dopamine neurons typically evokes excitation. Most of this excitation appears to be due to activation of collateral pathways; however, several lines of evidence have suggested that there is a monosynaptic component due to glutamate corelease by dopamine neurons. Recently, more direct evidence obtained in culture has shown that ventral midbrain dopamine neurons release both dopamine and glutamate. Moreover, they appear to do so from separate release sites, calling into question recent modifications of Dale's Principle. The neurochemical phenotype of a given synapse may be determined by subcellular neurotransmitter levels, uptake, or storage. However, the relationship between dopamine and glutamate release from dopamine neuron synapses in the intact brain--and the mechanisms involved--has yet to be resolved.

Mesoaccumbens dopamine neuron synapses reconstructed in vitro are glutamatergic

The mesoaccumbens projection, formed by ventral tegmental area dopamine neurons synapsing on nucleus accumbens gamma-aminobutyric acid neurons, has been implicated in the pathogenesis of schizophrenia and drug addiction. Despite intensive study, the nature of the signal conveyed by dopamine neurons has not been fully resolved. In addition to several slower, dopamine-mediated, modulatory actions, several lines of evidence suggest that dopamine neurons have fast excitatory actions. To test this, we placed dopamine neurons together with accumbens neurons in microcultures. Surprisingly, most dopamine neurons made excitatory recurrent connections (autapses), which provided a basis for their identification; accumbens gamma-aminobutyric acid neurons were identified by their distinctive size. In 75% of mesoaccumbens cell pairs, stimulation of the dopamine neuron evoked a glutamate-mediated, excitatory synaptic response in the accumbens neuron. Immunostaining revealed dopamine neuron varicosities that were predominantly dopaminergic, ones that were predominantly glutamatergic, and ones that were both dopaminergic and glutamatergic. Despite close appositions of both glutamatergic and dopaminergic varicosities to the dendrites of accumbens neurons, only glutamatergic synaptic responses were seen. In the majority of cell pairs, pharmacologic activation of D2-type dopamine receptors inhibited glutamatergic responses, presumably via immunocytochemically-visualized presynaptic D2 receptors. In some cell pairs, the evoked autaptic and synaptic responses were discordant, suggesting that D2 receptors may be differentially trafficked to different presynaptic varicosities.Thus, dopamine neurons appear to mediate both slow dopaminergic and fast glutamatergic actions via separate sets of synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, these results add a new dimension to monoamine neuron signaling that may have important implications for neuropsychiatric disorders.

D1- and D2-like dopamine receptors are co-localized on the presynaptic varicosities of striatal and nucleus accumbens neurons in vitro

The neuromodulatory actions of dopamine in the striatum and nucleus accumbens are likely to depend on the distribution of dopamine receptors on individual postsynaptic cells. To address this, we have visualized D1- and D2-like receptors on living medium-spiny GABAergic neurons in cultures from the striatum and nucleus accumbens using receptor antagonist fluoroprobes. We labeled D1-like receptors with rhodamine-SCH23390, D2-like receptors with rhodamine-N-(p-aminophenethyl)spiperone and synaptic sites with K+-stimulated uptake of the activity-dependent endocytic tracer FM-143. The fluoroprobes were applied in sequence to assess co-localization. We found that D1- or D2-like receptors were present on about two-thirds of the cells, and co-localized on 22+/-3% (mean +/- S.E.M.) of striatal and 38+/-6% of nucleus accumbens cells. On either D1 or D2 labeled cells, postsynaptic labeling continuously outlined the cell body membrane and extended to proximal dendrites, but not axons. About two-thirds of synaptic varicosities showed D1 or D2 labeling. D1- and D2-like receptors were co-localized on 21+/-4% of striatal and 27+/-3% of nucleus accumbens varicosities. Presynaptic labeling was typically more intense than postsynaptic labeling. The distribution of presynaptic dopamine receptors contrasted with that of postsynaptic GABA(A) receptors, which were clustered in longer patches on neighboring postsynaptic membranes. The extensive presence of D1- and D2-like receptors on presynaptic varicosities of medium-spiny neurons suggests that the receptors are likely to play an important and interacting role in the presynaptic modulation of inhibitory synaptic transmission in the striatum and nucleus accumbens. The significant overlap in labeling suggests that D1-D2 interactions, which occur at the level of individual postsynaptic cells, the circuit level and the systems level, may also be mediated at the presynaptic level. Finally, the ability to visualize dopamine, as well as GABA(A), receptors on the individual synapses of living neurons now makes possible physiological studies of individual mesolimbic system synapses with known receptor expression.

Dopamine neurons make glutamatergic synapses in vitro

Interactions between dopamine and glutamate play prominent roles in memory, addiction, and schizophrenia. Several lines of evidence have suggested that the ventral midbrain dopamine neurons that give rise to the major CNS dopaminergic projections may also be glutamatergic. To examine this possibility, we double immunostained ventral midbrain sections from rat and monkey for the dopamine-synthetic enzyme tyrosine hydroxylase and for glutamate; we found that most dopamine neurons immunostained for glutamate, both in rat and monkey. We then used postnatal cell culture to examine individual dopamine neurons. Again, most dopamine neurons immunostained for glutamate; they were also immunoreactive for phosphate-activated glutaminase, the major source of neurotransmitter glutamate. Inhibition of glutaminase reduced glutamate staining. In single-cell microculture, dopamine neurons gave rise to varicosities immunoreactive for both tyrosine hydroxylase and glutamate and others immunoreactive mainly for glutamate, which were found near the cell body. At the ultrastructural level, dopamine neurons formed occasional dopaminergic varicosities with symmetric synaptic specializations, but they more commonly formed nondopaminergic varicosities with asymmetric synaptic specializations. Stimulation of individual dopamine neurons evoked a fast glutamatergic autaptic EPSC that showed presynaptic inhibition caused by concomitant dopamine release. Thus, dopamine neurons may exert rapid synaptic actions via their glutamatergic synapses and slower modulatory actions via their dopaminergic synapses. Together with evidence for glutamate cotransmission in serotonergic raphe neurons and noradrenergic locus coeruleus neurons, the present results suggest that glutamatergic cotransmission may be the rule for central monoaminergic neurons.

Visualization of D1 dopamine receptors on living nucleus accumbens neurons and their colocalization with D2 receptors

To examine the substrate for dopamine (DA) synaptic action in the nucleus accumbens (nAcc), we visualized the cellular and subcellular distribution of DA receptors on postnatal nAcc neurons in culture using fluoroprobe derivatives of DA receptor ligands. Previously, we have shown that rhodamine-N-(p-aminophenethyl)-spiperone (NAPS) (10 nM), a derivative of the D2 antagonist spiperone, labels D2-like receptors on living nAcc neurons. We now show that rhodamine-Sch-23390 (30 nM), a derivative of the D1 antagonist, labels D1-like receptors. Putative specific membrane labeling reached a plateau after about 20 min. Labeling was stereospecific, as it was unaffected by competition with (-)-butaclamol, but blocked with (+)-butaclamol. We found that 52 +/- 7% of nAcc medium-sized neurons showed D1 labeling, which extended onto the dendrites. Labeling was also seen on presynaptic terminals, often abutting D1-positive and D1-negative cell bodies, consistent with a presynaptic modulatory role for D1 receptors. Larger neurons, which may be GABAergic or cholinergic interneurons, were also labeled. By sequential labeling first with rhodamine-Sch-23390 and then rhodamine-NAPS, we found that 38 +/- 6% of medium-sized neurons express both D1- and D2-like receptors, indicating that D1-D2 interactions may occur at the level of single postsynaptic neurons.

Reserpine inhibits amphetamine action in ventral midbrain culture

Although amphetamine releases catecholamines from isolated secretory vesicles, a number of in vivo experiments have indicated that the vesicular amine transport blocker reserpine does not block amphetamine-induced release. To address this paradox, we examined the effect of reserpine on amphetamine-induced dopamine release from postnatal ventral midbrain neurons in culture. These cultures provide a preparation in which intracellular, extracellular, and releasable dopamine pools can be measured simultaneously. We found that 1 microM reserpine for 90 min reduced stimulation-dependent dopamine release by > 95%. In parallel, reserpine reduced amphetamine-induced dopamine release by > 95% compared with cells not exposed to reserpine or by 75% compared with reserpine-treated cultures. This shows that amphetamine acts principally by redistributing dopamine from synaptic vesicles to the cytosol.

Visualization of antipsychotic drug binding to living mesolimbic neurons reveals D2 receptor, acidotropic, and lipophilic components

To examine the binding of antipsychotic drugs to living neurons, we applied fluoroprobe derivatives of the D2 antagonist spiperone to mesolimbic system neurons in postnatal culture. We found that rhodamine-N-(p-aminophenethyl)spiperone (rhodamine-NAPS) stereospecifically labeled the plasma membranes of 38 +/- 6% of ventral tegmental area neurons, 22 +/- 7% of which were dopaminergic, and 50 +/- 6% of medium-sized putatively GABAergic nucleus accumbens neurons, with a time constant of approximately 8 min. In contrast, the BODIPY derivative of NAPS rapidly labeled intracellular sites in all neurons in a punctate pattern, consistent with acidotropic uptake. Native antipsychotics also show acidotropic uptake, which we visualized by their displacement of the fluorescent weak base vital dye acridine orange from acidic intracellular compartments. We found that acidotropic uptake correlated best with the partition coefficients of the drugs. With a time constant of 23 min, rhodamine-NAPS labeled all neurons in a pattern suggestive of lipophilic solvation. Thus, initially rhodamine-NAPS makes possible visualization of D2 receptors on living neurons; however, acidotropic uptake and lipophilic solvation obscure receptor labeling and may account for time-dependent factors in the action of antipsychotic drugs, as well as affect their use as radioreceptor ligands.

Amphetamine redistributes dopamine from synaptic vesicles to the cytosol and promotes reverse transport

Whether amphetamine acts principally at the plasma membrane or at synaptic vesicles is controversial. We find that d-amphetamine injection into the Planorbis giant dopamine neuron causes robust dopamine release, demonstrating that specific amphetamine uptake is not required. Arguing for action at vesicles, whole-cell capillary electrophoresis of single Planorbis dopamine neurons shows that amphetamine reduces vesicular dopamine, while amphetamine reduces quantal dopamine release from PC12 cells by > 50% per vesicle. Intracellular injection of dopamine into the Planorbis dopamine neuron produces rapid nomifensine-sensitive release, showing that an increased substrate concentration gradient is sufficient to induce release. These experiments indicate that amphetamine acts at the vesicular level where it redistributes dopamine to the cytosol, promoting reverse transport, and dopamine release.

GABA synapses formed in vitro by local axon collaterals of nucleus accumbens neurons

GABAergic medium-spiny neuron axons not only form the principal projections of the nucleus accumbens (nAcc) but also branch locally in a dense network overlapping their own dendrites, suggesting that their recurrent synapses mediate the major information processing functions of the nAcc. We used postnatal nAcc cultures to study these synapses individually. In culture, as in the intact nAcc, medium-spiny neurons account for over 95% of the cells and are GABAergic. Strikingly, these neurons showed a spike afterhyperpolarization (AHP) that was largely blocked by the GABAA antagonist bicuculline. The bicuculline-sensitive AHP occurred without or with latency, and met criteria for monosynapticity; consistent with this, dye fills showed the presence of recurrent axons and a low incidence of dye coupling. Blockade of Ca2+ influx eliminated this autaptic PSP, while TTX almost completely eliminated it, indicating that it is due to exocytic GABA release principally at axodendritic contacts. While blocking GABAB receptors had no direct effect on the autaptic PSP, activating these receptors with baclofen produced presynaptic inhibition, as well as directly mediated hyperpolarization; together, these actions increased the signal-to-noise ratio in the cellular response to synaptic inputs. Bicuculline also increased the signal-to-noise ratio; in addition, it induced burst firing and depolarization inactivation. In contrast, the indirect GABA agonist flurazepam and the GABA uptake blocker nipecotic acid each enhanced autaptic PSPs. Since autapses formed in vitro appear to be functionally equivalent to synapses between neighboring medium-spiny neurons that receive similar inputs, these results bear on the function of intrinsic GABA synapses in the intact nAcc. Thus, intrinsic GABA synapses are likely to regulate the signal-to-noise ratio in nAcc information processing and may be important targets for the modulatory actions of endogenous neurotransmitters and drugs.

Methamphetamine neurotoxicity involves vacuolation of endocytic organelles and dopamine-dependent intracellular oxidative stress

Methamphetamine (MA) produces selective degeneration of dopamine (DA) neuron terminals without cell body loss. While excitatory amino acids (EAAs) contribute to MA toxicity, terminal loss is not characteristic of excitotoxic lesions nor is excitotoxicity selective for DA fibers; rather, EAAs may modulate MA-induced DA turnover, suggesting that DA-dependent events play a key role in MA neurotoxicity. To examine this possibility, we used postnatal ventral midbrain DA neuron cultures maintained under continuous EAA blockade. As in vivo, MA caused neurite degeneration but minimal cell death. We found that MA is a vacuologenic weak base that induces swelling of endocytic compartments; MA also induces blebbing of the plasma membrane. However, these morphological changes occurred in MA-treated cultures lacking DA neurons. Therefore, while collapse of endosomal and lysosomal pH gradients and vacuolation may contribute to MA neurotoxicity, this does not explain selective DA terminal degeneration. Alternatively, MA could exert its neurotoxic effects by collapsing synaptic vesicle proton gradients and redistributing DA from synaptic vesicles to the cytoplasm. This could cause the formation of DA-derived free radicals and reactive metabolites. To test whether MA induces oxidative stress within living DA neurons, we used 2,7-dichlorofluorescin diacetate (DCF), an indicator of intracellular hydroperoxide production. MA dramatically increased the number of DCF-labeled cells in ventral midbrain cultures, which contain about 30% DA neurons, but not in nucleus accumbens cultures, which do not contain DA neurons. In the DA neuron cultures, intracellular DDF labeling was localized to axonal varicosities, blebs, and endocytic organelles. These results suggest that MA redistributes DA from the reducing environment within synaptic vesicles to extravesicular oxidizing environments, thus generating oxygen radicals and reactive metabolites within DA neurons that may trigger selective DA terminal loss.

Amphetamine and other weak bases act to promote reverse transport of dopamine in ventral midbrain neurons

Amphetamine-like psychostimulants are thought to produce rewarding effects by increasing dopamine levels at mesolimbic synapses. Paradoxically, dopamine uptake blockers, which generally increase extracellular dopamine, inhibit amphetamine-induced dopamine overflow. This effect could be due to either inhibition of amphetamine uptake or inhibition of dopamine efflux through the transporter (reverse transport). We used weak bases and dopamine uptake blockers in ventral midbrain neuron cultures to separate the effects on blockade of amphetamine uptake from reverse transport of dopamine. Amphetamine, ammonium chloride, tributylamine, and monensin, at concentrations that produce similar reductions in acidic pH gradients, increased dopamine release. This effect was inhibited by uptake blockers. Although in the case of amphetamine the inhibition of release could have been due to blockade of amphetamine uptake, inhibition also occurred with weak bases that are not transporter substrates. This suggests that reduction of vesicular pH gradients increases cytoplasmic dopamine which in turn promotes reverse transport. Consistent with this model, extracellular 3,4-dihydroxyphenylacetic acid was increased by ammonium chloride and monensin, as would be expected with elevated cytoplasmic dopamine levels. These findings extend the weak base mechanism of amphetamine action, in which amphetamine reduces vesicular pH gradients resulting in increased cytoplasmic dopamine that promotes reverse transport.

Identified postnatal mesolimbic dopamine neurons in culture: morphology and electrophysiology

To examine the intrinsic properties of postnatal mesolimbic dopamine (DA) neurons, we dissociated the ventral tegmental area (VTA) from postnatal rats, enriched for DA neurons by microdissection or gradient purification, and grew the cells in culture. In these cultures, up to 50% of neurons were dopaminergic. DA neurons resembled their in vivo counterparts in soma shapes, and in showing two levels of tyrosine hydroxylase (TH) expression, axodendritic differentiation, two sizes of synaptic vesicles, nest-like synaptic arrangements with non-DA cells, and synaptic specializations. Electrophysiologically, however, they could not be distinguished from non-DA cells, which could be consistent with heterogeneity in cell properties. To examine a functional subset of VTA DA neurons, we retrogradely labeled VTA neurons projecting to the nucleus accumbens. These mesoaccumbens neurons were 86% TH positive, 56% cholecystokinin positive, and 0% neurotensin positive; they also displayed the soma shapes characteristic of DA neurons more generally and two levels of TH expression. Like their in vivo counterparts, mesoaccumbens cells generally fired single broad spikes that were triggered by slow depolarizations and had robust spike afterhyperpolarizations, low- and high-threshold Ca2+ spikes, rapid accommodation of firing, time-dependent anomalous rectification, and hyperpolarizing autoreceptor responses. Strikingly, the expression of these active properties did not change with time in culture. Mesoaccumbens DA cells could be identified by a distinctive subset of properties that made up an electrophysiological signature; however, unlike their in vivo counterparts, they were less often spontaneously active and never fired in bursts. These results suggest that most DA cell properties are intrinsic to the cells, including a significant heterogeneity that is maintained in postnatal culture; their level and mode of activity, however, appear to require afferent input. Culturing identified postnatal VTA DA neurons now makes possible examination of the impact of their individual properties on synaptic function.