I(h) channels contribute to the different functional properties of identified dopaminergic subpopulations in the midbrain

The cation channel mechanisms of subthreshold inward depolarizing currents in the mice VTA dopaminergic neurons and their roles in the chronic-stress-induced depression-like behavior

The slow-intrinsic-pacemaker dopaminergic (DA) neurons originating in the ventral tegmental area (VTA) are implicated in various mood- and emotion-related disorders, such as anxiety, fear, stress and depression. Abnormal activity of projection-specific VTA DA neurons is the key factor in the development of these disorders. Here, we describe the crucial role of the NALCN and TRPC6, non-selective cation channels in mediating the subthreshold inward depolarizing current and driving the firing of action potentials of VTA DA neurons in physiological conditions. Furthermore, we demonstrate that down-regulation of TRPC6 protein expression in the VTA DA neurons likely contributes to the reduced activity of projection-specific VTA DA neurons in chronic mild unpredictable stress (CMUS) depressive mice. In consistent with these, selective knockdown of TRPC6 channels in the VTA DA neurons conferred mice with depression-like behavior. This current study suggests down-regulation of TRPC6 expression/function is involved in reduced VTA DA neuron firing and chronic stress-induced depression-like behavior of mice.

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

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

Mechanisms of pacemaking in mammalian neurons

Many neurons in the mammalian brain show pacemaking activity: rhythmic generation of action potentials in the absence of sensory or synaptic input. Slow pacemaking of neurons releasing modulatory transmitters is easy to rationalize. More surprisingly, many neurons in the motor system also show pacemaking activity, often rapid, including cerebellar Purkinje neurons that fire spontaneously at 20-100 Hz, as well as key neurons in the basal ganglia, including subthalamic nucleus neurons and globus pallidus neurons. Although the spontaneous rhythmic firing of pacemaking neurons is phenomenologically similar to cardiac pacemaking, the underlying ionic mechanism in most neurons is quite different than for cardiac pacemaking. Few spontaneously active neurons rely on HCN 'pacemaker' channels for their activity. Most commonly, a central element is 'persistent' sodium current, steady-state subthreshold current carried by the same voltage-dependent sodium channels that underlie fast action potentials. Persistent sodium current is a steeply voltage-dependent current with a midpoint near -60 mV, which results in regenerative spontaneous depolarization once it produces a net inward current when summed with all other background currents, often at voltages as negative as -70 mV. This 'engine' of pacemaking is present in almost all neurons and must be held in check in non-pacemaking neurons by sufficiently large competing outward currents from background potassium channels. The intrinsic propensity of neurons to fire spontaneously underlies key normal functions such as respiration and generates the complex background oscillatory circuits revealed in EEGs, but can also produce out-of-control oscillations of overall brain function in epilepsy, ataxia and tremor.

How do HCN channels play a part in Alzheimer's and Parkinson's disease?

Neurodegenerative diseases like Alzheimer's and Parkinson's disease (AD and PD) are well-known, yet their underlying causes remain unclear. Recent studies have suggested that disruption of ion channels contribute to their pathogenesis. Among these channels, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, encoded by HCN1-4 genes, are of particular interest due to their role in generating hyperpolarization-activated current (Ih), which is crucial in various neural activities impacting memory and motor functions. A growing body of evidence underscores the pivotal role of HCN in Aβ generation, glial cell function, and ischemia-induced dementia; while HCN is expressed in various regions of the basal ganglia, modulating their functions and influencing motor disorders in PD; neuroinflammation triggered by microglial activation represents a shared pathological mechanism in both AD and PD, in which HCN also plays a significant part. This review delves into the neuronal functions governed by HCN, its roles in the aforementioned pathogenesis, its expression patterns in AD and PD, and discusses potential therapeutic drugs targeting HCN for the treatment of these diseases, aiming to offer a novel perspective and inspire future research endeavors.

Opto-seq reveals input-specific immediate-early gene induction in ventral tegmental area cell types

The ventral tegmental area (VTA) is a critical node in circuits governing motivated behavior and is home to diverse populations of neurons that release dopamine, gamma-aminobutyric acid (GABA), glutamate, or combinations of these neurotransmitters. The VTA receives inputs from many brain regions, but a comprehensive understanding of input-specific activation of VTA neuronal subpopulations is lacking. To address this, we combined optogenetic stimulation of select VTA inputs with single-nucleus RNA sequencing (snRNA-seq) and highly multiplexed in situ hybridization to identify distinct neuronal clusters and characterize their spatial distribution and activation patterns. Quantification of immediate-early gene (IEG) expression revealed that different inputs activated select VTA subpopulations, which demonstrated cell-type-specific transcriptional programs. Within dopaminergic subpopulations, IEG induction levels correlated with differential expression of ion channel genes. This new transcriptomics-guided circuit analysis reveals the diversity of VTA activation driven by distinct inputs and provides a resource for future analysis of VTA cell types.

Muscarinic receptor activation preferentially inhibits rebound in vulnerable dopaminergic neurons

Dopaminergic subpopulations of the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's disease and are characterized by unique electrophysiological properties. The vulnerable population expresses a T-type calcium channel-mediated afterdepolarization (ADP) and shows rebound activity upon release from inhibition, whereas the resilient population does not have an ADP and is slower to fire after hyperpolarization. This rebound activity can trigger dopamine release in the striatum, an important component of basal ganglia function. Using whole-cell patch clamp electrophysiology on ex vivo slices from adult mice of both sexes, we find that muscarinic activation with the non-selective muscarinic agonist Oxotremorine inhibits rebound activity more strongly in vulnerable vs resilient SNc neurons. Here, we show that this effect depends on the direct activation of muscarinic receptors on the SNc dopaminergic neurons. Through a series of pharmacological and transgenic knock-out experiments, we tested whether the muscarinic inhibition of rebound was mediated through the canonical rebound-related ion channels: T-type calcium channels, hyperpolarization-activated cation channels (HCN), and A-type potassium channels. We find that muscarinic receptor activation inhibits HCN-mediated current (Ih) in vulnerable SNc neurons, but that Ih activity is not necessary for the muscarinic inhibition of rebound activity. Similarly, we find that Oxotremorine inhibits rebound activity independently of T-type calcium channels and A-type potassium channels. Together these findings reveal new principles governing acetylcholine and dopamine interactions, showing that muscarinic receptors directly affect SNc rebound activity in the midbrain at the somatodendritic level and differentially modify information processing in distinct SNc subpopulations.

Dopaminergic amacrine cells express HCN channels in the developing and adult mouse retina

Purpose

To determine the molecular and functional expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in developing and mature dopaminergic amacrine cells (DACs), the sole source of ocular dopamine that plays a vital role in visual function and eye development.

Methods

HCN channels are encoded by isoforms 1-4. HCN1, HCN2, and HCN4 were immunostained in retinal slices obtained from mice at postnatal day 4 (P4), P8, and P12 as well as in adults. Each HCN channel isoform was also immunostained with tyrosine hydroxylase, a marker for DACs, at P12 and adult retinas. Genetically-marked DACs were recorded in flat-mount retina preparation using a whole-cell current-clamp technique.

Results

HCN1 was expressed in rods/cones, amacrine cells, and retinal ganglion cells (RGCs) at P4, along with bipolar cells by P12. Different from HCN1, HCN2 and HCN4 were each expressed in amacrine cells and RGCs at P4, along with bipolar cells by P8, and in rods/cones by P12. Double immunostaining shows that each of the three isoforms was expressed in approximately half of DACs at P12 but in almost all DACs in adults. Electrophysiology results demonstrate that HCN channel isoforms form functional HCN channels, and the pharmacological blockade of HCN channels reduced the spontaneous firing frequency in most DACs.

Conclusions

Each class of retinal neurons may use different isoforms of HCN channels to function during development. HCN1, HCN2, and HCN4 form functional HCN channels in DACs, which appears to modulate their spontaneous firing activity.

Serotonergic and dopaminergic neurons in the dorsal raphe are differentially altered in a mouse model for parkinsonism

Parkinson's disease (PD) is characterized by motor impairments caused by degeneration of dopamine neurons in the substantia nigra pars compacta. In addition to these symptoms, PD patients often suffer from non-motor comorbidities including sleep and psychiatric disturbances, which are thought to depend on concomitant alterations of serotonergic and noradrenergic transmission. A primary locus of serotonergic neurons is the dorsal raphe nucleus (DRN), providing brain-wide serotonergic input. Here, we identified electrophysiological and morphological parameters to classify serotonergic and dopaminergic neurons in the murine DRN under control conditions and in a PD model, following striatal injection of the catecholamine toxin, 6-hydroxydopamine (6-OHDA). Electrical and morphological properties of both neuronal populations were altered by 6-OHDA. In serotonergic neurons, most changes were reversed when 6-OHDA was injected in combination with desipramine, a noradrenaline (NA) reuptake inhibitor, protecting the noradrenergic terminals. Our results show that the depletion of both NA and dopamine in the 6-OHDA mouse model causes changes in the DRN neural circuitry.

High-Resolution Proteomics Unravel a Native Functional Complex of Cav1.3, SK3, and Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels in Midbrain Dopaminergic Neurons

Pacemaking activity in substantia nigra dopaminergic neurons is generated by the coordinated activity of a variety of distinct somatodendritic voltage- and calcium-gated ion channels. We investigated whether these functional interactions could arise from a common localization in macromolecular complexes where physical proximity would allow for efficient interaction and co-regulations. For that purpose, we immunopurified six ion channel proteins involved in substantia nigra neuron autonomous firing to identify their molecular interactions. The ion channels chosen as bait were Cav1.2, Cav1.3, HCN2, HCN4, Kv4.3, and SK3 channel proteins, and the methods chosen to determine interactions were co-immunoprecipitation analyzed through immunoblot and mass spectrometry as well as proximity ligation assay. A macromolecular complex composed of Cav1.3, HCN, and SK3 channels was unraveled. In addition, novel potential interactions between SK3 channels and sclerosis tuberous complex (Tsc) proteins, inhibitors of mTOR, and between HCN4 channels and the pro-degenerative protein Sarm1 were uncovered. In order to demonstrate the presence of these molecular interactions in situ, we used proximity ligation assay (PLA) imaging on midbrain slices containing the substantia nigra, and we could ascertain the presence of these protein complexes specifically in substantia nigra dopaminergic neurons. Based on the complementary functional role of the ion channels in the macromolecular complex identified, these results suggest that such tight interactions could partly underly the robustness of pacemaking in dopaminergic neurons.

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

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

The Formation and Function of the VTA Dopamine System

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

PI4KIIIβ-Mediated Phosphoinositides Metabolism Regulates Function of the VTA Dopaminergic Neurons and Depression-Like Behavior

Phosphoinositides, including phosphatidylinositol-4,5-bisphosphate (PIP2), play a crucial role in controlling key cellular functions such as membrane and vesicle trafficking, ion channel, and transporter activity. Phosphatidylinositol 4-kinases (PI4K) are essential enzymes in regulating the turnover of phosphoinositides. However, the functional role of PI4Ks and mediated phosphoinositide metabolism in the central nervous system has not been fully revealed. In this study, we demonstrated that PI4KIIIβ, one of the four members of PI4Ks, is an important regulator of VTA dopaminergic neuronal activity and related depression-like behavior of mice by controlling phosphoinositide turnover. Our findings provide new insights into possible mechanisms and potential drug targets for neuropsychiatric diseases, including depression. Both sexes were studied in basic behavior tests, but only male mice could be used in the social defeat depression model.

Inhibition of LRRK2 kinase activity rescues deficits in striatal dopamine physiology in VPS35 p.D620N knock-in mice

Dysregulation of dopamine neurotransmission profoundly affects motor, motivation and learning behaviors, and can be observed during the prodromal phase of Parkinson's disease (PD). However, the mechanism underlying these pathophysiological changes remains to be elucidated. Mutations in vacuolar protein sorting 35 (VPS35) and leucine-rich repeat kinase 2 (LRRK2) both lead to autosomal dominant PD, and VPS35 and LRRK2 may physically interact to govern the trafficking of synaptic cargos within the endo-lysosomal network in a kinase-dependent manner. To better understand the functional role of VPS35 and LRRK2 on dopamine physiology, we examined Vps35 haploinsufficient (Haplo) and Vps35 p.D620N knock-in (VKI) mice and how their behavior, dopamine kinetics and biochemistry are influenced by LRRK2 kinase inhibitors. We found Vps35 p.D620N significantly elevates LRRK2-mediated phosphorylation of Rab10, Rab12 and Rab29. In contrast, Vps35 haploinsufficiency reduces phosphorylation of Rab12. While striatal dopamine transporter (DAT) expression and function is similarly impaired in both VKI and Haplo mice, that physiology is normalized in VKI by treatment with the LRRK2 kinase inhibitor, MLi-2. As a corollary, VKI animals show a significant increase in amphetamine induced hyperlocomotion, compared to Haplo mice, that is also abolished by MLi-2. Taken together, these data show Vps35 p.D620N confers a gain-of-function with respect to LRRK2 kinase activity, and that VPS35 and LRRK2 functionally interact to regulate DAT function and striatal dopamine transmission.

Deep learning-based image analysis identifies a DAT-negative subpopulation of dopaminergic neurons in the lateral Substantia nigra

Here we present a deep learning-based image analysis platform (DLAP), tailored to autonomously quantify cell numbers, and fluorescence signals within cellular compartments, derived from RNAscope or immunohistochemistry. We utilised DLAP to analyse subtypes of tyrosine hydroxylase (TH)-positive dopaminergic midbrain neurons in mouse and human brain-sections. These neurons modulate complex behaviour, and are differentially affected in Parkinson's and other diseases. DLAP allows the analysis of large cell numbers, and facilitates the identification of small cellular subpopulations. Using DLAP, we identified a small subpopulation of TH-positive neurons (~5%), mainly located in the very lateral Substantia nigra (SN), that was immunofluorescence-negative for the plasmalemmal dopamine transporter (DAT), with ~40% smaller cell bodies. These neurons were negative for aldehyde dehydrogenase 1A1, with a lower co-expression rate for dopamine-D2-autoreceptors, but a ~7-fold higher likelihood of calbindin-d28k co-expression (~70%). These results have important implications, as DAT is crucial for dopamine signalling, and is commonly used as a marker for dopaminergic SN neurons.

The PFC-LH-VTA pathway contributes to social deficits in IRSp53-mutant mice

Dopamine (DA) neurons in the ventral tegmental area (VTA) promote social brain functions by releasing DA onto nucleus accumbens neurons, but it remains unclear how VTA neurons communicate with cortical neurons. Here, we report that the medial prefrontal cortex (mPFC)-lateral hypothalamus (LH)-VTA pathway contributes to social deficits in mice with IRSp53 deletion restricted to cortical excitatory neurons (Emx1-Cre;Irsp53fl/fl mice). LH-projecting mutant mPFC neurons display abnormally increased excitability involving decreased potassium channel gene expression, leading to excessive excitatory synaptic input to LH-GABA neurons. A circuit-specific IRSp53 deletion in LH-projecting mPFC neurons also increases neuronal excitability and induces social deficits. LH-GABA neurons with excessive mPFC excitatory synaptic input show a compensatory decrease in excitability, weakening the inhibitory LHGABA-VTAGABA pathway and subsequently over-activating VTA-GABA neurons and over-inhibiting VTA-DA neurons. Accordingly, optogenetic activation of the LHGABA-VTAGABA pathway improves social deficits in Emx1-Cre;Irsp53fl/fl mice. Therefore, the mPFC-LHGABA-VTAGABA-VTADA pathway contributes to the social deficits in Emx1-Cre;Irsp53fl/fl mice.

PLCγ1 in dopamine neurons critically regulates striatal dopamine release via VMAT2 and synapsin III

Dopamine neurons are essential for voluntary movement, reward learning, and motivation, and their dysfunction is closely linked to various psychological and neurodegenerative diseases. Hence, understanding the detailed signaling mechanisms that functionally modulate dopamine neurons is crucial for the development of better therapeutic strategies against dopamine-related disorders. Phospholipase Cγ1 (PLCγ1) is a key enzyme in intracellular signaling that regulates diverse neuronal functions in the brain. It was proposed that PLCγ1 is implicated in the development of dopaminergic neurons, while the physiological function of PLCγ1 remains to be determined. In this study, we investigated the physiological role of PLCγ1, one of the key effector enzymes in intracellular signaling, in regulating dopaminergic function in vivo. We found that cell type-specific deletion of PLCγ1 does not adversely affect the development and cellular morphology of midbrain dopamine neurons but does facilitate dopamine release from dopaminergic axon terminals in the striatum. The enhancement of dopamine release was accompanied by increased colocalization of vesicular monoamine transporter 2 (VMAT2) at dopaminergic axon terminals. Notably, dopamine neuron-specific knockout of PLCγ1 also led to heightened expression and colocalization of synapsin III, which controls the trafficking of synaptic vesicles. Furthermore, the knockdown of VMAT2 and synapsin III in dopamine neurons resulted in a significant attenuation of dopamine release, while this attenuation was less severe in PLCγ1 cKO mice. Our findings suggest that PLCγ1 in dopamine neurons could critically modulate dopamine release at axon terminals by directly or indirectly interacting with synaptic machinery, including VMAT2 and synapsin III.

Contributions of the Sodium Leak Channel NALCN to Pacemaking of Medial Ventral Tegmental Area and Substantia Nigra Dopaminergic Neurons

We tested the role of the sodium leak channel, NALCN, in pacemaking of dopaminergic neuron (DAN) subpopulations from adult male and female mice. In situ hybridization revealed NALCN RNA in all DANs, with lower abundance in medial ventral tegmental area (VTA) relative to substantia nigra pars compacta (SNc). Despite lower relative abundance of NALCN, we found that acute pharmacological blockade of NALCN in medial VTA DANs slowed pacemaking by 49.08%. We also examined the electrophysiological properties of projection-defined VTA DAN subpopulations identified by retrograde labeling. Inhibition of NALCN reduced pacemaking in DANs projecting to medial nucleus accumbens (NAc) and others projecting to lateral NAc by 70.74% and 31.98%, respectively, suggesting that NALCN is a primary driver of pacemaking in VTA DANs. In SNc DANs, potentiating NALCN by lowering extracellular calcium concentration speeded pacemaking in wildtype but not NALCN conditional knockout mice, demonstrating functional presence of NALCN. In contrast to VTA DANs, however, pacemaking in SNc DANs was unaffected by inhibition of NALCN. Instead, we found that inhibition of NALCN increased the gain of frequency-current plots at firing frequencies slower than spontaneous firing. Similarly, inhibition of the hyperpolarization-activated cyclic nucleotide-gated (HCN) conductance increased gain but had little effect on pacemaking. Interestingly, simultaneous inhibition of NALCN and HCN resulted in significant reduction in pacemaker rate. Thus, we found NALCN makes substantial contributions to driving pacemaking in VTA DAN subpopulations. In SNc DANs, NALCN is not critical for pacemaking but inhibition of NALCN makes cells more sensitive to hyperpolarizing stimuli.SIGNIFICANCE STATEMENT Pacemaking in midbrain dopaminergic neurons (DAN) relies on multiple subthreshold conductances, including a sodium leak. Whether the sodium leak channel, NALCN, contributes to pacemaking in DANs located in the VTA and the SNc has not yet been determined. Using electrophysiology and pharmacology, we show that NALCN plays a prominent role in driving pacemaking in projection-defined VTA DAN subpopulations. By contrast, pacemaking in SNc neurons does not rely on NALCN. Instead, the presence of NALCN regulates the excitability of SNc DANs by reducing the gain of the neuron's response to inhibitory stimuli. Together, these findings will inform future efforts to obtain DAN subpopulation-specific treatments for use in neuropsychiatric disorders.

Deficiency of Cullin 3, a Protein Encoded by a Schizophrenia and Autism Risk Gene, Impairs Behaviors by Enhancing the Excitability of Ventral Tegmental Area (VTA) DA Neurons

The dopaminergic neuromodulator system is fundamental to brain functions. Abnormal dopamine (DA) pathway is implicated in psychiatric disorders, including schizophrenia (SZ) and autism spectrum disorder (ASD). Mutations in Cullin 3 (CUL3), a core component of the Cullin-RING ubiquitin E3 ligase complex, have been associated with SZ and ASD. However, little is known about the function and mechanism of CUL3 in the DA system. Here, we show that CUL3 is critical for the function of DA neurons and DA-relevant behaviors in male mice. CUL3-deficient mice exhibited hyperactive locomotion, deficits in working memory and sensorimotor gating, and increased sensitivity to psychostimulants. In addition, enhanced DA signaling and elevated excitability of the VTA DA neurons were observed in CUL3-deficient animals. Behavioral impairments were attenuated by dopamine D2 receptor antagonist haloperidol and chemogenetic inhibition of DA neurons. Furthermore, we identified HCN2, a hyperpolarization-activated and cyclic nucleotide-gated channel, as a potential target of CUL3 in DA neurons. Our study indicates that CUL3 controls DA neuronal activity by maintaining ion channel homeostasis and provides insight into the role of CUL3 in the pathogenesis of psychiatric disorders.SIGNIFICANCE STATEMENT This study provides evidence that Cullin 3 (CUL3), a core component of the Cullin-RING ubiquitin E3 ligase complex that has been associated with autism spectrum disorder and schizophrenia, controls the excitability of dopamine (DA) neurons in mice. Its DA-specific heterozygous deficiency increased spontaneous locomotion, impaired working memory and sensorimotor gating, and elevated response to psychostimulants. We showed that CUL3 deficiency increased the excitability of VTA DA neurons, and inhibiting D2 receptor or DA neuronal activity attenuated behavioral deficits of CUL3-deficient mice. We found HCN2, a hyperpolarization-activated channel, as a target of CUL3 in DA neurons. Our findings reveal CUL3's role in DA neurons and offer insights into the pathogenic mechanisms of autism spectrum disorder and schizophrenia.

HCN channel inhibitor induces ketamine-like rapid and sustained antidepressant effects in chronic social defeat stress model

Repeated, long-term (weeks to months) exposure to standard antidepressant medications is required to achieve treatment efficacy. In contrast, acute ketamine quickly improves mood for an extended time. Recent work implicates that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are involved in mediating ketamine's antidepressant effects. In this study, we directly targeted HCN channels and achieved ketamine-like rapid and sustained antidepressant efficacy. Our in vitro electrophysiological recordings first showed that HCN inhibitor DK-AH 269 (also called cilobradine) decreased the pathological HCN-mediated current (Ih) and abnormal hyperactivity of ventral tegmental area (VTA) dopamine (DA) neurons in a depressive-like model produced by chronic social defeat stress (CSDS). Our in vivo studies further showed that acute intra-VTA or acute systemic administration of DK-AH 269 normalized social behavior and rescued sucrose preference in CSDS-susceptible mice. The single-dose of DK-AH 269, both by intra-VTA microinfusion and intraperitoneal (ip) approaches, could produce an extended 13-day duration of antidepressant-like efficacy. Animals treated with acute DK-AH 269 spent less time immobile than vehicle-treated mice during forced swim test. A social behavioral reversal lasted up to 13 days following the acute DK-AH 269 ip injection, and this rapid and sustained antidepressant-like response is paralleled with a single-dose treatment of ketamine. This study provides a novel ion channel target for acutely acting, long-lasting antidepressant-like effects.

A glutamatergic DRN-VTA pathway modulates neuropathic pain and comorbid anhedonia-like behavior in mice

Chronic pain causes both physical suffering and comorbid mental symptoms such as anhedonia. However, the neural circuits and molecular mechanisms underlying these maladaptive behaviors remain elusive. Here using a mouse model, we report a pathway from vesicular glutamate transporter 3 neurons in the dorsal raphe nucleus to dopamine neurons in the ventral tegmental area (VGluT3DRN→DAVTA) wherein population-level activity in response to innocuous mechanical stimuli and sucrose consumption is inhibited by chronic neuropathic pain. Mechanistically, neuropathic pain dampens VGluT3DRN → DAVTA glutamatergic transmission and DAVTA neural excitability. VGluT3DRN → DAVTA activation alleviates neuropathic pain and comorbid anhedonia-like behavior (CAB) by releasing glutamate, which subsequently promotes DA release in the nucleus accumbens medial shell (NAcMed) and produces analgesic and anti-anhedonia effects via D2 and D1 receptors, respectively. In addition, VGluT3DRN → DAVTA inhibition produces pain-like reflexive hypersensitivity and anhedonia-like behavior in intact mice. These findings reveal a crucial role for VGluT3DRN → DAVTA → D2/D1NAcMed pathway in establishing and modulating chronic pain and CAB.

Calcium dysregulation combined with mitochondrial failure and electrophysiological maturity converge in Parkinson's iPSC-dopamine neurons

Parkinson's disease (PD) is characterized by a progressive deterioration of motor and cognitive functions. Although death of dopamine neurons is the hallmark pathology of PD, this is a late-stage disease process preceded by neuronal dysfunction. Here we describe early physiological perturbations in patient-derived induced pluripotent stem cell (iPSC)-dopamine neurons carrying the GBA-N370S mutation, a strong genetic risk factor for PD. GBA-N370S iPSC-dopamine neurons show an early and persistent calcium dysregulation notably at the mitochondria, followed by reduced mitochondrial membrane potential and oxygen consumption rate, indicating mitochondrial failure. With increased neuronal maturity, we observed decreased synaptic function in PD iPSC-dopamine neurons, consistent with the requirement for ATP and calcium to support the increase in electrophysiological activity over time. Our work demonstrates that calcium dyshomeostasis and mitochondrial failure impair the higher electrophysiological activity of mature neurons and may underlie the vulnerability of dopamine neurons in PD.

Aberrant somatic calcium channel function in cNurr1 and LRRK2-G2019S mice

In Parkinson's disease (PD), axons of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc) degenerate before their cell bodies. Calcium influx during pacemaker firing might contribute to neuronal loss, but it is not known if dysfunctions of voltage-gated calcium channels (VGCCs) occur in DA neurons somata and axon terminals. We investigated T-type and L-type VGCCs in SNc-DA neurons of two mouse models of PD: mice with a deletion of the Nurr1 gene in DA neurons from an adult age (cNurr1 mice), and mice bearing the G2019S mutation in the gene coding for LRRK2 (G2019S mice). Adult cNurr1 mice displayed motor and DA deficits, while middle-aged G2019S mice did not. The number and morphology of SNc-DA neurons, most of their intrinsic membrane properties and pacemaker firing were unaltered in cNurr1 and G2019S mice compared to their control and wild-type littermates. L-type VGCCs contributed to the pacemaker firing of SNc-DA neurons in G2019S mice, but not in control, wild-type, and cNurr1 mice. In cNurr1 mice, but not G2019S mice, the contribution of T-type VGCCs to the pacemaker firing of SNc-DA neurons was reduced, and somatic dopamine-D2 autoreceptors desensitized more. Altered contribution of L-type and T-type VGCCs to the pacemaker firing was not observed in the presence of a LRRK2 kinase inhibitor in G2019S mice, and in the presence of a flavonoid with antioxidant activity in G2019S and cNurr1 mice. The role of L-type and T-type VGCCs in controlling dopamine release from axon terminals in the striatum was unaltered in cNurr1 and G2019S mice. Our findings uncover opposite changes, linked to oxidative stress, in the function of two VGCCs in DA neurons somata, but not axon terminals, in two different experimental PD models.

Disease mechanisms as Subtypes: Mitochondrial and bioenergetic dysfunction

Parkinson disease (PD) is the second most common neurodegenerative disease in the world. Despite its enormous human and societal cost, there is no disease-modifying therapy for PD. This unmet medical need reflects our limited understanding of PD pathogenesis. One of the most important clues comes from the recognition that PD motor symptoms arises from the dysfunction and degeneration of a very select group of neurons in the brain. These neurons have a distinctive set of anatomic and physiologic traits that reflect their role in brain function. These traits elevate mitochondrial stress, potentially making them particularly vulnerable to age, as well as to genetic mutations and environmental toxins linked to PD incidence. In this chapter, the literature supporting this model is outlined, along with gaps in our knowledge base. The translational implications of this hypothesis are then discussed, with a focus on why disease-modification trials have failed to date and what this means for the development of new strategies for altering disease course.

Estrous Cycle Mediates Midbrain Neuron Excitability Altering Social Behavior upon Stress

The estrous cycle is a potent modulator of neuron physiology. In rodents, in vivo ventral tegmental area (VTA) dopamine (DA) activity has been shown to fluctuate across the estrous cycle. Although the behavioral effect of fluctuating sex steroids on the reward circuit is well studied in response to drugs of abuse, few studies have focused on the molecular adaptations in the context of stress and motivated social behaviors. We hypothesized that estradiol fluctuations across the estrous cycle acts on the dopaminergic activity of the VTA to alter excitability and stress response. We used whole-cell slice electrophysiology of VTA DA neurons in naturally cycling, adult female C57BL/6J mice to characterize the effects of the estrous cycle and the role of 17β-estradiol on neuronal activity. We show that the estrous phase alters the effect of 17β-estradiol on excitability in the VTA. Behaviorally, the estrous phase during a series of acute variable social stressors modulates subsequent reward-related behaviors. Pharmacological inhibition of estrogen receptors in the VTA before stress during diestrus mimics the stress susceptibility found during estrus, whereas increased potassium channel activity in the VTA before stress reverses stress susceptibility found during estrus as assessed by social interaction behavior. This study identifies one possible potassium channel mechanism underlying the increased DA activity during estrus and reveals estrogen-dependent changes in neuronal function. Our findings demonstrate that the estrous cycle and estrogen signaling changes the physiology of DA neurons resulting in behavioral differences when the reward circuit is challenged with stress.SIGNIFICANCE STATEMENT The activity of the ventral tegmental area encodes signals of stress and reward. Dopaminergic activity has been found to be regulated by both local synaptic inputs as well as inputs from other brain regions. Here, we provide evidence that cycling sex steroids also play a role in modulating stress sensitivity of dopaminergic reward behavior. Specifically, we reveal a correlation of ionic activity with estrous phase, which influences the behavioral response to stress. These findings shed new light on how estrous cycle may influence dopaminergic activity primarily during times of stress perturbation.

Morpho-Functional Changes of Nigral Dopamine Neurons in an α-Synuclein Model of Parkinson's Disease

BACKGROUND

The accumulation of α-synuclein (α-syn) fibrils in intraneuronal inclusions called Lewy bodies and Lewy neurites is a pathological signature of Parkinson's disease (PD). Although several aspects linked to α-syn-dependent pathology (concerning its spreading, aggregation, and activation of inflammatory and neurodegenerative processes) have been under intense investigation, less attention has been devoted to the real impact of α-syn overexpression on structural and functional properties of substantia nigra pars compacta (SNpc) dopamine (DA) neurons, particularly at tardive stages of α-syn buildup, despite this has obvious relevance to comprehending mechanisms beyond PD progression.

OBJECTIVES

We aimed to determine the consequences of a prolonged α-syn overexpression on somatodendritic morphology and functions of SNpc DA neurons.

METHODS

We performed immunohistochemistry, stereological DA cell counts, analyses of dendritic arborization, ex vivo patch-clamp recordings, and in vivo DA microdialysis measurements in a 12- to 13-month-old transgenic rat model overexpressing the full-length human α-syn (Snca^+/+ ) and age-matched wild-type rats.

RESULTS

Aged Snca^+/+ rats have mild loss of SNpc DA neurons and decreased basal DA levels in the SN. Residual nigral DA neurons display smaller soma and compromised dendritic arborization and, in parallel, increased firing activity, switch in firing mode, and hyperexcitability associated with hypofunction of fast activating/inactivating voltage-gated K⁺ channels and Ca²⁺ - and voltage-activated large conductance K⁺ channels. These intrinsic currents underlie the repolarization/afterhyperpolarization phase of action potentials, thus affecting neuronal excitability.

CONCLUSIONS

Besides clarifying α-syn-induced pathological landmarks, such evidence reveals compensatory functional mechanisms that nigral DA neurons could adopt during PD progression to counteract neurodegeneration. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Preservation of dendritic D2 receptor transmission in substantia nigra dopamine neurons with age

Substantia nigra pars compacta (SNc) dopamine neurons are required for voluntary movement and reward learning, and advanced age is associated with motor and cognitive decline. In the midbrain, D2-type dopamine receptors located at dendrodendritic synapses between dopamine neurons control cell firing through G protein-activated potassium (GIRK) channels. We previously showed that aging disrupts dopamine neuron pacemaker firing in mice, but only in males. Here we show that the amplitude of D2-receptor inhibitory postsynaptic currents (D2-IPSCs) are moderately reduced in aged male mice. Local application of dopamine revealed a reduction in the amplitude of the D2-receptor currents in old males compared to young, pointing to a postsynaptic mechanism. Further experiments indicated that reduced D2 receptor signaling was not due to a general reduction in GIRK channel currents or degeneration of the dendritic arbor. Kinetic analysis showed no differences in D2-IPSC shape in old versus young mice or between sexes. Potentiation of D2-IPSCs by corticotropin releasing factor (CRF) was also not affected by age, indicating preservation of one mechanism of plasticity. These findings have implications for understanding dopamine transmission in aging, and reduced D2 receptor inhibition could contribute to increased susceptibility of males to SNc dopamine neuron degeneration in Parkinson's disease.

Changed firing activity of nigra dopaminergic neurons in Parkinson's disease

Parkinson's disease is the second most common neurodegenerative disease which is characterized by selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. The intrinsic neuronal firing activity is critical for the functional organization of brain and the specific deficits of neuronal firing activity may be associated with different brain disorders. It is known that the surviving nigra dopaminergic neurons exhibit altered firing activity, such as decreased spontaneous firing frequency, reduced number of firing neurons and increased burst firing in Parkinson's disease. Several ionic mechanisms are involved in changed firing activity of dopaminergic neurons under parkinsonian state. In this review, we summarize the changes of spontaneous firing activity as well as the possible mechanisms of nigra dopaminergic neurons in Parkinson's disease. This review may let us clearly understand the involvement of neuronal firing activity of nigra dopaminergic neurons in Parkinson's disease.

Upregulation of Ca2+-binding proteins contributes to VTA dopamine neuron survival in the early phases of Alzheimer's disease in Tg2576 mice

BACKGROUND

Recent clinical and experimental studies have highlighted the involvement of Ventral Tegmental Area (VTA) dopamine (DA) neurons for the early pathogenesis of Alzheimer's Disease (AD). We have previously described a progressive and selective degeneration of these neurons in the Tg2576 mouse model of AD, long before amyloid-beta plaque formation. The degenerative process in DA neurons is associated with an autophagy flux impairment, whose rescue can prevent neuronal loss. Impairments in autophagy can be the basis for accumulation of damaged mitochondria, leading to disturbance in calcium (Ca²⁺) homeostasis, and to functional and structural deterioration of DA neurons.

METHODS

In Tg2576 mice, we performed amperometric recordings of DA levels and analysis of dopaminergic fibers in the Nucleus Accumbens - a major component of the ventral striatum precociously affected in AD patients - together with retrograde tracing, to identify the most vulnerable DA neuron subpopulations in the VTA. Then, we focused on these neurons to analyze mitochondrial integrity and Apoptosis-inducing factor (AIF) localization by electron and confocal microscopy, respectively. Stereological cell count was also used to evaluate degeneration of DA neuron subpopulations containing the Ca²⁺-binding proteins Calbindin-D28K and Calretinin. The expression levels for these proteins were analyzed by western blot and confocal microscopy. Lastly, using electrophysiology and microfluorometry we analyzed VTA DA neuron intrinsic properties and cytosolic free Ca²⁺ levels.

RESULTS

We found a progressive degeneration of mesolimbic DA neurons projecting to the ventral striatum, located in the paranigral nucleus and parabrachial pigmented subnucleus of the VTA. At the onset of degeneration (3 months of age), the vulnerable DA neurons in the Tg2576 accumulate damaged mitochondria, while AIF translocates from the mitochondria to the nucleus. Although we describe an age-dependent loss of the DA neurons expressing Calbindin-D28K or Calretinin, we observed that the remaining cells upregulate the levels of Ca²⁺-binding proteins, and the free cytosolic levels of Ca²⁺ in these neurons are significantly decreased. Coherently, TUNEL-stained Tg2576 DA neurons express lower levels of Calbindin-D28K when compared with non-apoptotic cells.

CONCLUSION

Overall, our results suggest that the overexpression of Ca²⁺-binding proteins in VTA DA neurons might be an attempt of cells to survive by increasing their ability to buffer free Ca²⁺. Exploring strategies to overexpress Ca²⁺-binding proteins could be fundamental to reduce neuronal suffering and improve cognitive and non-cognitive functions in AD.

Effect of Renal Ischemia Reperfusion on Brain Neuroinflammation

Acute kidney injury (AKI) is an inflammatory sequence. It can lead to distant organ injury, including damage to the central nervous system (CNS), mediated by increased circulating cytokines and other inflammatory mediators. It can also lead to increased blood-brain barrier (BBB) permeability. However, the effect of AKI on the inflammatory response of the brain has not yet been investigated. Therefore, we observed the effect of AKI on BBB permeability, microglia and astrocyte activation, and neuronal toxicity in the brain. The striatum and ventral midbrain, known to control overall movement, secrete the neurotransmitter dopamine. The activation of microglia and astrocytes present in this area causes neuro-degenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). The activation of astrocytes and microglia in the hippocampus and cerebral cortex, which are responsible for important functions, including memory, learning, concentration, and language, can trigger nerve cell apoptosis. The activation of astrocytes and microglia at this site is also involved in the inflammatory response associated with the accumulation of beta-amyloid. In the situation of kidney ischemia reperfusion (IR)-induced AKI, activation of microglia and astrocytes were observed in the striatum, ventral midbrain, hippocampus, and cortex. However, neuronal cell death was not observed until 48 h.

Dendritic involvement in inhibition and disinhibition of vulnerable dopaminergic neurons in healthy and pathological conditions

Dopaminergic neurons in the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's Disease, with the ventral region degenerating more severely than the dorsal region. Compared with the dorsal neurons, the ventral neurons in the SNc have distinct dendritic morphology, electrophysiological characteristics, and circuit connections with the basal ganglia. These characteristics shape information processing in the ventral SNc and structure the balance of inhibition and disinhibition in the striatonigral circuitry. In this paper, I review foundational studies and recent work comparing the circuitry of the ventral and dorsal SNc neurons and discuss how loss of the ventral neurons early in Parkinson's Disease could affect the overall balance of inhibition and disinhibition of dopamine signals.

Neuropeptide apelin presented in the dopaminergic neurons modulates the neuronal excitability in the substantia nigra pars compacta

The dopaminergic neurons in the substantia nigra pars compacta are characterized by autonomous pacemaking activity. The spontaneous firing activity of nigral dopaminergic neurons plays an important role in physiological function and is essential for their survival. Importantly, the spontaneous firing activity may also be involved in the preferential vulnerability of the nigral dopaminergic neurons in Parkinson's disease (PD). The neuropeptide apelin was reported to exert neuroprotective effects in neurodegenerative diseases, including PD. And it was noticed that apelin modulates neuronal activity in some brain regions. The present study investigated the electrophysiological and behavioral effects of apelin in the substantia nigra. Double-labeling immunofluorescence showed that apelin was present in nigral dopaminergic neurons and that these neurons expressed apelin receptor APJ. Further single unit in vivo electrophysiological recordings revealed that endogenous apelin tonically increased the firing rate of nigral dopaminergic neurons in both normal and parkinsonian animals. Exogenous apelin-13 exerted excitatory effects on the majority of nigral dopaminergic neurons, yet reduced excitability in a subset of neurons. In addition, nigral application of apelin-13 increased motor activity in normal rats and blocking endogenous apelin reduced motor activity. Considering the involvement of the spontaneous firing activity of nigral dopaminergic neurons in the development of PD and the possibility that apelin acts in an autocrine manner on apelin receptors expressed by nigral dopaminergic neurons, the modulation of the spontaneous firing activity of nigral dopaminergic neurons by apelin may serve as a neuroprotective factor in PD.

Neurotensin Release from Dopamine Neurons Drives Long-Term Depression of Substantia Nigra Dopamine Signaling

Midbrain dopamine neurons play central physiological roles in voluntary movement, reward learning, and motivated behavior. Inhibitory signaling at somatodendritic dopamine D2 receptor (D2R) synapses modulates excitability of dopamine neurons. The neuropeptide neurotensin is expressed by many inputs to the midbrain and induces LTD of D2R synaptic currents (LTD_DA); however, the source of neurotensin that is responsible for LTD_DA is not known. Here we show, in brain slices from male and female mice, that LTD_DA is driven by neurotensin released by dopamine neurons themselves. Optogenetic stimulation of dopamine neurons was sufficient to induce LTD_DA in the substantia nigra, but not the VTA, and was dependent on neurotensin receptor signaling, postsynaptic calcium, and vacuolar-type H⁺-ATPase activity in the postsynaptic cell. These findings reveal a novel form of signaling between dopamine neurons involving release of the peptide neurotensin, which may act as a feedforward mechanism to increase dopamine neuron excitability.SIGNIFICANCE STATEMENT Dopamine neurons in the midbrain play a critical role in reward learning and the initiation of movement. Aberrant dopamine neuron function is implicated in a range of diseases and disorders, including Parkinson's disease, schizophrenia, obesity, and substance use disorders. D2 receptor-mediated PSCs are produced by a rare form of dendrodendritic synaptic transmission between dopamine neurons. These D2 receptor-mediated PSCs undergo LTD following application of the neuropeptide neurotensin. Here we show that release of neurotensin by dopamine neurons themselves is sufficient to induce LTD of dopamine transmission in the substantia nigra. Neurotensin signaling therefore mediates a second form of interdopamine neuron communication and may provide a mechanism by which dopamine neurons maintain excitability when nigral dopamine is elevated.

Oscillation/Coincidence-Detection Models of Reward-Related Timing in Corticostriatal Circuits

The major tenets of beat-frequency/coincidence-detection models of reward-related timing are reviewed in light of recent behavioral and neurobiological findings. This includes the emphasis on a core timing network embedded in the motor system that is comprised of a corticothalamic-basal ganglia circuit. Therein, a central hub provides timing pulses (i.e., predictive signals) to the entire brain, including a set of distributed satellite regions in the cerebellum, cortex, amygdala, and hippocampus that are selectively engaged in timing in a manner that is more dependent upon the specific sensory, behavioral, and contextual requirements of the task. Oscillation/coincidence-detection models also emphasize the importance of a tuned ‘perception’ learning and memory system whereby target durations are detected by striatal networks of medium spiny neurons (MSNs) through the coincidental activation of different neural populations, typically utilizing patterns of oscillatory input from the cortex and thalamus or derivations thereof (e.g., population coding) as a time base. The measure of success of beat-frequency/coincidence-detection accounts, such as the Striatal Beat-Frequency model of reward-related timing (SBF), is their ability to accommodate new experimental findings while maintaining their original framework, thereby making testable experimental predictions concerning diagnosis and treatment of issues related to a variety of dopamine-dependent basal ganglia disorders, including Huntington’s and Parkinson’s disease.

β2-subunit alternative splicing stabilizes Cav2.3 Ca2+ channel activity during continuous midbrain dopamine neuron-like activity

In dopaminergic (DA) Substantia nigra (SN) neurons Cav2.3 R-type Ca2+-currents contribute to somatodendritic Ca2+-oscillations. This activity may contribute to the selective degeneration of these neurons in Parkinson's disease (PD) since Cav2.3-knockout is neuroprotective in a PD mouse model. Here, we show that in tsA-201-cells the membrane-anchored β2-splice variants β2a and β2e are required to stabilize Cav2.3 gating properties allowing sustained Cav2.3 availability during simulated pacemaking and enhanced Ca2+-currents during bursts. We confirmed the expression of β2a- and β2e-subunit transcripts in the mouse SN and in identified SN DA neurons. Patch-clamp recordings of mouse DA midbrain neurons in culture and SN DA neurons in brain slices revealed SNX-482-sensitive R-type Ca2+-currents with voltage-dependent gating properties that suggest modulation by β2a- and/or β2e-subunits. Thus, β-subunit alternative splicing may prevent a fraction of Cav2.3 channels from inactivation in continuously active, highly vulnerable SN DA neurons, thereby also supporting Ca2+ signals contributing to the (patho)physiological role of Cav2.3 channels in PD.

Calcium-binding proteins typify the dopaminergic neuronal subtypes in the ventral tegmental area of zebra finch, Taeniopygia guttata

Calcium-binding proteins (CBPs) regulate neuronal function in midbrain dopamine (DA)-ergic neurons in mammals by buffering and sensing the intracellular Ca²⁺ , and vesicular release. In birds, the equivalent set of neurons are important in song learning, directed singing, courtship, and energy balance, yet the status of CBPs in these neurons is unknown. Herein, for the first time, we probe the nature of CBPs, namely, Calbindin-, Calretinin-, Parvalbumin-, and Secretagogin-expressing DA neurons in the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain of zebra finch, Taeniopygia guttata. qRT-PCR analysis of ventral midbrain tissue fragment revealed higher Calbindin- and Calretinin-mRNA levels compared to Parvalbumin and Secretagogin. Application of immunofluorescence showed CBP-immunoreactive (-i) neurons in VTA (anterior [VTAa], mid [VTAm], caudal [VTAc]), SN (compacta [SNc], and reticulata [SNr]). Compared to VTAa, higher Calbindin- and Parvalbumin-immunoreactivity (-ir), and lower Calretinin-ir were observed in VTAm and VTAc. Secretagogin-ir was highly localized to VTAa. In SN, Calbindin- and Calretinin-ir were higher in SNc, SNr was Parvalbumin enriched, and Secretagogin-ir was not detected. Weak, moderate, and intense tyrosine hydroxylase (TH)-i VTA neurons were demarcated as subtypes 1, 2, and 3, respectively. While subtype 1 TH-i neurons were neither Calbindin- nor Calretinin-i, ∼80 and ∼65% subtype 2 and ∼30 and ∼45% subtype 3 TH-i neurons co-expressed Calbindin and Calretinin, respectively. All TH-i neuronal subtypes co-expressed Parvalbumin with reciprocal relationship with TH-ir. We suggest that the CBPs may determine VTA DA neuronal heterogeneity and differentially regulate their activity in T. guttata.

Human midbrain dopaminergic neuronal differentiation markers predict cell therapy outcome in a Parkinson's disease model

Human pluripotent stem cell (hPSC)-based replacement therapy holds great promise in treating Parkinson's disease (PD). However, the heterogeneity of hPSC-derived donor cells and the low yield of midbrain dopaminergic (mDA) neurons after transplantation hinder its broad clinical application. Here, we depicted the single-cell molecular landscape during mDA neuron differentiation. We found that this process recapitulated the development of multiple but adjacent fetal brain regions including ventral midbrain, isthmus, and ventral hindbrain, resulting in heterogenous donor cell population. We reconstructed the differentiation trajectory of mDA lineage and identified CLSTN2 and PTPRO as specific surface markers of mDA progenitors, which were predictive of mDA neuron differentiation and could facilitate highly enriched mDA neurons (up to 80%) following progenitor sorting and transplantation. Marker sorted progenitors exhibited higher therapeutic potency in correcting motor deficits of PD mice. Different marker sorted grafts had a strikingly consistent cellular composition, in which mDA neurons were enriched, while off-target neuron types were mostly depleted, suggesting stable graft outcomes. Our study provides a better understanding of cellular heterogeneity during mDA neuron differentiation, and establishes a strategy to generate highly purified donor cells to achieve stable and predictable therapeutic outcomes, raising the prospect of hPSC-based PD cell replacement therapies.

Midbrain projection to the basolateral amygdala encodes anxiety-like but not depression-like behaviors

Anxiety disorders are complex diseases, and often co-occur with depression. It is as yet unclear if a common neural circuit controls anxiety-related behaviors in both anxiety-alone and comorbid conditions. Here, utilizing the chronic social defeat stress (CSDS) paradigm that induces singular or combined anxiety- and depressive-like phenotypes in mice, we show that a ventral tegmental area (VTA) dopamine circuit projecting to the basolateral amygdala (BLA) selectively controls anxiety- but not depression-like behaviors. Using circuit-dissecting ex vivo electrophysiology and in vivo fiber photometry approaches, we establish that expression of anxiety-like, but not depressive-like, phenotypes are negatively correlated with VTA → BLA dopamine neuron activity. Further, our optogenetic studies demonstrate a causal link between such neuronal activity and anxiety-like behaviors. Overall, these data establish a functional role for VTA → BLA dopamine neurons in bi-directionally controlling anxiety-related behaviors not only in anxiety-alone, but also in anxiety-depressive comorbid conditions in mice.

Moderate ethanol drinking is sufficient to alter Ventral Tegmental Area dopamine neurons activity via functional and structural remodeling of GABAergic transmission

Earlier studies have shown a major involvement of Ventral Tegmental Area (VTA) dopamine (DA) neurons in mediating the rewarding effects of ethanol (EtOH). Much less is known on the role of this system in mediating the transition from moderate to excessive drinking and abuse. Here we sought to explore the hypothesis that early stage drinking in rodents, resembling recreational EtOH use in humans, is sufficient to dysregulate VTA DA transmission thus increasing the propensity to use over time. To this purpose, midbrain slice recordings in mice previously exposed to an escalating (3, 6 and 12%) 18-day voluntary EtOH drinking paradigm was used. By recording from DA and γ-aminobutyric acid (GABA) VTA neurons in midbrain slices, we found that moderate EtOH drinking leads to a significant suppression of the spontaneous activity of VTA DA neurons, while increasing their response to acute EtOH application. We also found that chronic EtOH leads to the enhancement of GABA input frequency onto a subset of DA neurons. Structurally, chronic EtOH induced a significant increase in the number of GABA axonal boutons contacting DA neurons, suggesting deep rewiring of the GABA network. This scenario is consistent with a downmodulation of the reward DA system induced by moderate EtOH drinking, a neurochemical state defined as "hypodopaminergic" and previously associated with advanced stages of drug use in humans. In this context, increased sensitivity of DA neurons towards acute EtOH may represent the neurophysiological correlate of increased unitary rewarding value, possibly driving progression to addiction.

Ih blockade reduces cocaine-induced firing patterns of putative dopaminergic neurons of the ventral tegmental area in the anesthetized rat

The hyperpolarization-activated cation current (Ih) is a determinant of intrinsic excitability in various cells, including dopaminergic neurons (DA) of the ventral tegmental area (VTA). In contrast to other cellular conductances, Ih is activated by hyperpolarization negative to -55 mV and activating Ih produces a time-dependent depolarizing current. Our laboratory demonstrated that cocaine sensitization, a chronic cocaine behavioral model, significantly reduces Ih amplitude in VTA DA neurons. Despite this reduction in Ih, the spontaneous firing of VTA DA cells after cocaine sensitization remained similar to control groups. Although the role of Ih in controlling VTA DA excitability is still poorly understood, our hypothesis is that Ih reduction could play a role of a homeostatic controller compensating for cocaine-induced change in excitability. Using in vivo single-unit extracellular electrophysiology in isoflurane anesthetized rats, we explored the contribution of Ih on spontaneous firing patterns of VTA DA neurons. A key feature of spontaneous excitability is bursting activity; bursting is defined as trains of two or more spikes occurring within a short interval and followed by a prolonged period of inactivity. Burst activity increases the reliability of information transfer. To elucidate the contribution of Ih to spontaneous firing patterns of VTA DA neurons, we locally infused an Ih blocker (ZD 7288, 8.3 μM) and evaluated its effect. Ih blockade significantly reduced firing rate, bursting frequency, and percent of spikes within a burst. In addition, Ih blockade significantly reduced acute cocaine-induced spontaneous firing rate, bursting frequency, and percent of spikes within a burst. Using whole-cell patch-clamp, we determine the progressive reduction of Ih after acute and chronic cocaine administration (15 mg/k.g intraperitoneally). Our data show a significant reduction (~25%) in Ih amplitude after 24 but not 2 h of acute cocaine administration. These results suggest that a progressive reduction of Ih could serve as a homeostatic regulator of cocaine-induced spontaneous firing patterns related to VTA DA excitability.

Chronic methamphetamine-induced neurodegeneration: Differential vulnerability of ventral tegmental area and substantia nigra pars compacta dopamine neurons

Methamphetamine (meth) increases monoamine oxidase (MAO)-dependent mitochondrial stress in substantia nigra pars compacta (SNc) axons; chronic administration produces SNc degeneration that is prevented by MAO inhibition suggesting that MAO-dependent axonal mitochondrial stress is a causal factor. To test whether meth similarly increases mitochondrial stress in ventral tegmental area (VTA) axons, we used a genetically encoded redox biosensor to assess mitochondrial stress ex vivo. Meth increased MAO-dependent mitochondrial stress in both SNc and VTA axons. However, despite having the same meth-induced stress as SNc neurons, VTA neurons were resistant to chronic meth-induced degeneration indicating that meth-induced MAO-dependent mitochondrial stress in axons was necessary but not sufficient for degeneration. To determine whether L-type Ca2+ channel-dependent stress differentiates SNc and VTA axons, as reported in the soma, the L-type Ca2+ channel activator Bay K8644 was used. Opening L-type Ca2+ channels increased axonal mitochondrial stress in SNc but not VTA axons. To first determine whether mitochondrial stress was necessary for SNc degeneration, mice were treated with the mitochondrial antioxidant mitoTEMPO. Chronic meth-induced SNc degeneration was prevented by mitoTEMPO thereby confirming the necessity of mitochondrial stress. Similar to results with the antioxidant, both MAO inhibition and L-type Ca2+ channel inhibition also prevented SNc degeneration. Taken together the presented data demonstrate that both MAO- and L-type Ca2+ channel-dependent mitochondrial stress is necessary for chronic meth-induced degeneration.

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

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

Ketamine induces opposite changes in AMPA receptor calcium permeability in the ventral tegmental area and nucleus accumbens

Ketamine elicits rapid and durable antidepressant actions in treatment-resistant patients with mood disorders such as major depressive disorder and bipolar depression. The mechanisms might involve the induction of metaplasticity in brain regions associated with reward-related behaviors, mood, and hedonic drive, particularly the ventral tegmental area (VTA) and the nucleus accumbens (NAc). We have examined if ketamine alters the insertion of the GluA2 subunit of AMPA receptors (AMPAR), which determines calcium permeability of the channel, at glutamatergic synapses onto dopamine (DA) neurons in the VTA and spiny projection neurons (SPNs) in the Core region of the NAc. Mice received one injection of either saline or a low dose of ketamine 24 h before electrophysiological recordings were performed. We found that GluA2-lacking calcium-permeable (CP) AMPARs were present in DA neurons in the VTA of mice treated with saline, and that ketamine-induced the removal of a fraction of these receptors. In NAc SPNs, ketamine induced the opposite change, i.e., GluA2-lacking CP-AMPARs were inserted at glutamatergic synapses. Ketamine-induced metaplasticity was independent of group I metabotropic glutamate receptors (mGluRs) because an agonist of these receptors had similar effects on glutamatergic transmission in mice treated with saline and in mice treated with ketamine in both VTA DA neurons and in the NAc. Thus, ketamine reduces the insertion of CP-AMPARs in VTA DA neurons and induces their insertion in the NAc. The mechanism by which ketamine elicits antidepressant actions might thus involve an alteration in the contribution of GluA2 to AMPARs thereby modulating synaptic plasticity in the mesolimbic circuit.

Unraveling the Role of Dopaminergic and Calretinin Interneurons in the Olfactory Bulb

The perception and discriminating of odors are sensory activities that are an integral part of our daily life. The first brain region where odors are processed is the olfactory bulb (OB). Among the different cell populations that make up this brain area, interneurons play an essential role in this sensory activity. Moreover, probably because of their activity, they represent an exception compared to other parts of the brain, since OB interneurons are continuously generated in the postnatal and adult period. In this review, we will focus on periglomerular (PG) cells which are a class of interneurons found in the glomerular layer of the OB. These interneurons can be classified into distinct subtypes based on their neurochemical nature, based on the neurotransmitter and calcium-binding proteins expressed by these cells. Dopaminergic (DA) periglomerular cells and calretinin (CR) cells are among the newly generated interneurons and play an important role in the physiology of OB. In the OB, DA cells are involved in the processing of odors and the adaptation of the bulbar network to external conditions. The main role of DA cells in OB appears to be the inhibition of glutamate release from olfactory sensory fibers. Calretinin cells are probably the best morphologically characterized interneurons among PG cells in OB, but little is known about their function except for their inhibitory effect on noisy random excitatory signals arriving at the main neurons. In this review, we will mainly describe the electrophysiological properties related to the excitability profiles of DA and CR cells, with a particular view on the differences that characterize DA mature interneurons from cells in different stages of adult neurogenesis.

The Resilient Phenotype Induced by Prophylactic Ketamine Exposure During Adolescence Is Mediated by the Ventral Tegmental Area-Nucleus Accumbens Pathway

BACKGROUND

Major depressive disorder is prevalent in children and adolescents and is associated with a high degree of morbidity throughout life, with potentially devastating personal consequences and public health impact. The efficacy of ketamine (KET) as an antidepressant has been demonstrated in adolescent rodents; however, the neurobiological mechanisms underlying these effects are unknown. Recent evidence showed that KET reverses stress-induced (i.e., depressive-like) deficits within major mesocorticolimbic regions, such as the prefrontal cortex, nucleus accumbens (NAc), and hippocampus, in adult rodents. However, little is known about KET's effect in the ventral tegmental area (VTA), which provides the majority of dopaminergic input to these brain regions.

METHODS

We characterized behavioral, biochemical, and electrophysiological effects produced by KET treatment in C57BL/6J male mice during adolescence (n = 7-10 per condition) within the VTA and its major projection regions, namely, the NAc and prefrontal cortex. Subsequently, molecular targets within the VTA-NAc projection were identified for viral gene transfer manipulations to recapitulate the effects of stress or KET treatment.

RESULTS

Repeated KET treatment produced a robust proresilient response to chronic social defeat stress. This effect was largely driven by Akt signaling activity within the VTA and NAc, and it could be blocked or recapitulated through direct Akt-viral-mediated manipulation. Additionally, we found that the KET-induced resilient phenotype is dependent on VTA-NAc, but not VTA-prefrontal cortex, pathway activity.

CONCLUSIONS

These findings indicate that KET exposure during adolescence produces a proresilient phenotype mediated by changes in Akt intracellular signaling and altered neuronal activity within the VTA-NAc pathway.

Inactivation mode of sodium channels defines the different maximal firing rates of conventional versus atypical midbrain dopamine neurons

Two subpopulations of midbrain dopamine (DA) neurons are known to have different dynamic firing ranges in vitro that correspond to distinct projection targets: the originally identified conventional DA neurons project to the dorsal striatum and the lateral shell of the nucleus accumbens, whereas an atypical DA population with higher maximum firing frequencies projects to prefrontal regions and other limbic regions including the medial shell of nucleus accumbens. Using a computational model, we show that previously identified differences in biophysical properties do not fully account for the larger dynamic range of the atypical population and predict that the major difference is that originally identified conventional cells have larger occupancy of voltage-gated sodium channels in a long-term inactivated state that recovers slowly; stronger sodium and potassium conductances during action potential firing are also predicted for the conventional compared to the atypical DA population. These differences in sodium channel gating imply that longer intervals between spikes are required in the conventional population for full recovery from long-term inactivation induced by the preceding spike, hence the lower maximum frequency. These same differences can also change the bifurcation structure to account for distinct modes of entry into depolarization block: abrupt versus gradual. The model predicted that in cells that have entered depolarization block, it is much more likely that an additional depolarization can evoke an action potential in conventional DA population. New experiments comparing lateral to medial shell projecting neurons confirmed this model prediction, with implications for differential synaptic integration in the two populations.

We developed a theoretical and mathematical framework that could explain the major electrophysiological differences between the conventional midbrain dopamine (DA) neurons with a low maximum firing rate, and the more recently identified atypical DA neurons. Testable predictions from this framework were then verified with in vitro patch-clamp recordings from DA neurons with identified phenotypes and projection targets. Since different subpopulations of DA neurons participate in different circuits, and these circuits are likely differentially dysregulated in diseases such as addiction, Parkinson disease, and schizophrenia, it is important to identify the differences of their intrinsic electrophysiological properties as a prelude to developing more precisely targeted therapies.

Developmental HCN channelopathy results in decreased neural progenitor proliferation and microcephaly in mice

The development of the cerebral cortex relies on the controlled division of neural stem and progenitor cells. The requirement for precise spatiotemporal control of proliferation and cell fate places a high demand on the cell division machinery, and defective cell division can cause microcephaly and other brain malformations. Cell-extrinsic and -intrinsic factors govern the capacity of cortical progenitors to produce large numbers of neurons and glia within a short developmental time window. In particular, ion channels shape the intrinsic biophysical properties of precursor cells and neurons and control their membrane potential throughout the cell cycle. We found that hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits are expressed in mouse, rat, and human neural progenitors. Loss of HCN channel function in rat neural stem cells impaired their proliferation by affecting the cell-cycle progression, causing G1 accumulation and dysregulation of genes associated with human microcephaly. Transgene-mediated, dominant-negative loss of HCN channel function in the embryonic mouse telencephalon resulted in pronounced microcephaly. Together, our findings suggest a role for HCN channel subunits as a part of a general mechanism influencing cortical development in mammals.

Lewy Body-like Inclusions in Human Midbrain Organoids Carrying Glucocerebrosidase and α-Synuclein Mutations

Objective

We utilized human midbrain‐like organoids (hMLOs) generated from human pluripotent stem cells carrying glucocerebrosidase gene (GBA1) and α‐synuclein (α‐syn; SNCA) perturbations to investigate genotype‐to‐phenotype relationships in Parkinson disease, with the particular aim of recapitulating α‐syn– and Lewy body–related pathologies and the process of neurodegeneration in the hMLO model.

Methods

We generated and characterized hMLOs from GBA1^−/− and SNCA overexpressing isogenic embryonic stem cells and also generated Lewy body–like inclusions in GBA1/SNCA dual perturbation hMLOs and conduritol‐b‐epoxide–treated SNCA triplication hMLOs.

Results

We identified for the first time that the loss of glucocerebrosidase, coupled with wild‐type α‐syn overexpression, results in a substantial accumulation of detergent‐resistant, β‐sheet–rich α‐syn aggregates and Lewy body–like inclusions in hMLOs. These Lewy body–like inclusions exhibit a spherically symmetric morphology with an eosinophilic core, containing α‐syn with ubiquitin, and can also be formed in Parkinson disease patient–derived hMLOs. We also demonstrate that impaired glucocerebrosidase function promotes the formation of Lewy body–like inclusions in hMLOs derived from patients carrying the SNCA triplication.

Interpretation

Taken together, the data indicate that our hMLOs harboring 2 major risk factors (glucocerebrosidase deficiency and wild‐type α‐syn overproduction) of Parkinson disease provide a tractable model to further elucidate the underlying mechanisms for progressive Lewy body formation. ANN NEUROL 2021;90:490–505

The gating pore blocker 1-(2,4-xylyl)guanidinium selectively inhibits pacemaking of midbrain dopaminergic neurons

Although several ionic mechanisms are known to control rate and regularity of the slow pacemaker in dopamine (DA) neurons, the core mechanism of pacing is controversial. Here we tested the hypothesis that pacemaking of SNc DA neurons is enabled by an unconventional conductance. We found that 1-(2,4-xylyl)guanidinium (XG), an established blocker of gating pore currents, selectively inhibits pacemaking of DA neurons. The compound inhibited all slow pacemaking DA neurons that were tested, both in the substantia nigra pars compacta, and in the ventral tegmental area. Interestingly, bursting behavior was not affected by XG. Furthermore, the drug did not affect fast pacemaking of GABAergic neurons from substantia nigra pars reticulata neurons or slow pacemaking of noradrenergic neurons. In DA neurons, current-clamp analysis revealed that XG did not appear to affect ion channels involved in the action potential. Its inhibitory effect persisted during blockade of all ion channels previously suggested to contribute to pacemaking. RNA sequencing and voltage-clamp recordings yielded no evidence for a gating pore current to underlie the conductance. However, we could isolate a small subthreshold XG-sensitive current, which was carried by both Na⁺ and Cl^- ions. Although the molecular target of XG remains to be defined, these observations represent a step towards understanding pacemaking in DA neurons.

Ih from synapses to networks: HCN channel functions and modulation in neurons

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels and the current they carry, I_h, are widely and diversely distributed in the central nervous system (CNS). The distribution of the four subunits of HCN channels is variable within the CNS, within brain regions, and often within subcellular compartments. The precise function of I_h can depend heavily on what other channels are co-expressed. In this review, we give an overview of HCN channel structure, distribution, and modulation by cyclic adenosine monophosphate (cAMP). We then discuss HCN channel and I_h functions, where we have parsed the roles into two main effects: a steady effect on maintaining the resting membrane potential at relatively depolarized values, and slow channel dynamics. Within this framework, we discuss I_h involvement in resonance, synaptic integration, transmitter release, plasticity, and point out a special case, where the effects of I_h on the membrane potential and its slow channel dynamics have dual roles in thalamic neurons.

Mouse Embryonic Stem Cells Expressing GDNF Show Enhanced Dopaminergic Differentiation and Promote Behavioral Recovery After Grafting in Parkinsonian Rats

Parkinson's disease (PD) is characterized by the progressive loss of midbrain dopaminergic neurons (DaNs) of the substantia nigra pars compacta and the decrease of dopamine in the brain. Grafting DaN differentiated from embryonic stem cells (ESCs) has been proposed as an alternative therapy for current pharmacological treatments. Intrastriatal grafting of such DaNs differentiated from mouse or human ESCs improves motor performance, restores DA release, and suppresses dopamine receptor super-sensitivity. However, a low percentage of grafted neurons survive in the brain. Glial cell line-derived neurotrophic factor (GDNF) is a strong survival factor for DaNs. GDNF has proved to be neurotrophic for DaNs in vitro and in vivo, and induces axonal sprouting and maturation. Here, we engineered mouse ESCs to constitutively produce human GDNF, to analyze DaN differentiation and the possible neuroprotection by transgenic GDNF after toxic challenges in vitro, or after grafting differentiated DaNs into the striatum of Parkinsonian rats. GDNF overexpression throughout in vitro differentiation of mouse ESCs increases the proportion of midbrain DaNs. These transgenic cells were less sensitive than control cells to 6-hydroxydopamine in vitro. After grafting control or GDNF transgenic DaNs in hemi-Parkinsonian rats, we observed significant recoveries in both pharmacological and non-pharmacological behavioral tests, as well as increased striatal DA release, indicating that DaNs are functional in the brain. The graft volume, the number of surviving neurons, the number of DaNs present in the striatum, and the proportion of DaNs in the grafts were significantly higher in rats transplanted with GDNF-expressing cells, when compared to control cells. Interestingly, no morphological alterations in the brain of rats were found after grafting of GDNF-expressing cells. This approach is novel, because previous works have use co-grafting of DaNs with other cell types that express GDNF, or viral transduction in the host tissue before or after grafting of DaNs. In conclusion, GDNF production by mouse ESCs contributes to enhanced midbrain differentiation and permits a higher number of surviving DaNs after a 6-hydroxydopamine challenge in vitro, as well as post-grafting in the lesioned striatum. These GDNF-expressing ESCs can be useful to improve neuronal survival after transplantation.