Activity-dependent somatodendritic dopamine release in the substantia nigra autoinhibits the releasing neuron

The association of genetic polymorphisms within the dopaminergic system with nicotine dependence: A narrative review

Nicotine, the main compound in cigarettes, leads to smoking addiction. Nicotine acts on the limbic dopamine reward loop in the midbrain by binding to nicotinic acetylcholine receptors, promoting the release of dopamine, and resulting in a rewarding effect or satisfaction. This satisfaction is essential for continued and compulsive tobacco use, and therefore dopamine plays a crucial role in nicotine dependence. Numerous studies have identified genetic polymorphisms of dopaminergic pathways which may influence susceptibility to nicotine addiction. Dopamine levels are greatly influenced by synthesis, storage, release, degradation, and reuptake-related genes, including genes encoding tyrosine hydroxylase, dopamine decarboxylase, dopamine transporter, dopamine receptor, dopamine 3-hydroxylase, catechol-O-methyltransferase, and monoamine oxidase. In this paper, we review research progress on the effects of polymorphisms in the above genes on downstream smoking behavior and nicotine dependence, to offer a theoretical basis for the elucidation of the genetic mechanism underlying nicotine dependence and future personalized treatment for smoking cessation.

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

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

Dopamine transmission in the tail striatum: Regional variation and contribution of dopamine clearance mechanisms

The striatum can be divided into four anatomically and functionally distinct domains: the dorsolateral, dorsomedial, ventral and the more recently identified caudolateral (tail) striatum. Dopamine transmission in these striatal domains underlies many important behaviours, yet little is known about this phenomenon in the tail striatum. Furthermore, the tail is divided anatomically into four divisions (dorsal, medial, intermediate and lateral) based on the profile of D1 and D2 dopamine receptor-expressing medium spiny neurons, something that is not seen elsewhere in the striatum. Considering this organisation, how dopamine transmission occurs in the tail striatum is of great interest. We recorded evoked dopamine release in the four tail divisions, with comparison to the dorsolateral striatum, using fast-scan cyclic voltammetry in rat brain slices. Contributions of clearance mechanisms were investigated using dopamine transporter knockout (DAT-KO) rats, pharmacological transporter inhibitors and dextran. Evoked dopamine release in all tail divisions was smaller in amplitude than in the dorsolateral striatum and, importantly, regional variation was observed: dorsolateral ≈ lateral > medial > dorsal ≈ intermediate. Release amplitudes in the lateral division were 300% of that in the intermediate division, which also exhibited uniquely slow peak dopamine clearance velocity. Dopamine clearance in the intermediate division was most dependent on DAT, and no alternative dopamine transporters investigated (organic cation transporter-3, norepinephrine transporter and serotonin transporter) contributed significantly to dopamine clearance in any tail division. Our findings confirm that the tail striatum is not only a distinct dopamine domain but also that each tail division has unique dopamine transmission characteristics. This supports that the divisions are not only anatomically but also functionally distinct. How this segregation relates to the overall function of the tail striatum, particularly the processing of multisensory information, is yet to be determined.

Dopamine transporter positron emission tomography in patients with Alzheimer's disease with Lewy body disease features

In 36 normal controls (NC), 37 patients with Alzheimer's disease (AD) without parkinsonism (ADP-), 31 AD with parkinsonism (ADP+), and 40 AD with dementia with Lewy bodies (ADDLB), dual-phase dopamine transporter (DAT) positron emission tomography (PET) were performed to evaluate the diagnostic performance of DAT and early-to-delayed uptake ratios (E/Ds) in the anterior caudate (AC), posterior caudate (PC), anterior putamen (AP), posterior putamen (PP), and substantia nigra (SN) to differentiate ADP+/ADDLB from NC, and their effects on parkinsonism and cognition. DAT-SN and E/D-PP showed higher accuracies to differentiate ADP+/ADDLB from NC than DAT-PP. Among AD patients, lower DAT in the putamen and PC and higher E/Ds in the striatum were associated with severe parkinsonism, while higher E/Ds in the putamen, PC, and SN were associated with executive dysfunction. Our results suggest that decreased DAT-SN and increased E/D-PP could be biomarkers differentiating ADP+/ADDLB from pure AD and controls. Meanwhile, increased E/Ds in the putamen could reflect the severity of DLB presenting with parkinsonism and executive dysfunction among AD patients.

Genomic profile of Parkinson's disease in Asians

Parkinson's Disease (PD) has witnessed an alarming rise in prevalence, highlighting the suboptimal nature of early diagnostic and therapeutic strategies. To address this issue, genetic testing has emerged as a potential avenue. In this comprehensive review, we have meticulously summarized the variants associated with PD in Asian populations. Our review reveals that these variants exert their influence on diverse biological pathways, encompassing the autophagy-lysosome pathway, cholesterol metabolism, circadian rhythm regulation, immune system response, and synaptic function. Conventionally, PD has been linked to other diseases; however, our findings shed light on a shared genetic susceptibility among these conditions, implying an underlying pathophysiological mechanism that unifies them. Moreover, it is noteworthy that these PD-associated variants can significantly impact drug responses during therapeutic interventions. This review not only provides a consolidated overview of the genetic variants associated with PD in Asian populations but also contributes novel insights into the intricate relationships between PD and other diseases by elucidating shared genetic components. These findings underscore the importance of personalized approaches in diagnosing and treating PD based on individual genetic profiles to optimize patient outcomes.

Emotion in action: When emotions meet motor circuits

The brain is a remarkably complex organ responsible for a wide range of functions, including the modulation of emotional states and movement. Neuronal circuits are believed to play a crucial role in integrating sensory, cognitive, and emotional information to ultimately guide motor behavior. Over the years, numerous studies employing diverse techniques such as electrophysiology, imaging, and optogenetics have revealed a complex network of neural circuits involved in the regulation of emotional or motor processes. Emotions can exert a substantial influence on motor performance, encompassing both everyday activities and pathological conditions. The aim of this review is to explore how emotional states can shape movements by connecting the neural circuits for emotional processing to motor neural circuits. We first provide a comprehensive overview of the impact of different emotional states on motor control in humans and rodents. In line with behavioral studies, we set out to identify emotion-related structures capable of modulating motor output, behaviorally and anatomically. Neuronal circuits involved in emotional processing are extensively connected to the motor system. These circuits can drive emotional behavior, essential for survival, but can also continuously shape ongoing movement. In summary, the investigation of the intricate relationship between emotion and movement offers valuable insights into human behavior, including opportunities to enhance performance, and holds promise for improving mental and physical health. This review integrates findings from multiple scientific approaches, including anatomical tracing, circuit-based dissection, and behavioral studies, conducted in both animal and human subjects. By incorporating these different methodologies, we aim to present a comprehensive overview of the current understanding of the emotional modulation of movement in both physiological and pathological conditions.

Dopaminergic dysfunction: Role for genetic & epigenetic testing in the new psychiatry

Reward Deficiency Syndrome (RDS), particularly linked to addictive disorders, costs billions of dollars globally and has resulted in over one million deaths in the United States (US). Illicit substance use has been steadily rising and in 2021 approximately 21.9% (61.2 million) of individuals living in the US aged 12 or older had used illicit drugs in the past year. However, only 1.5% (4.1 million) of these individuals had received any substance use treatment. This increase in use and failure to adequately treat or provide treatment to these individuals resulted in 106,699 overdose deaths in 2021 and increased in 2022. This article presents an alternative non-pharmaceutical treatment approach tied to gene-guided therapy, the subject of many decades of research. The cornerstone of this paradigm shift is the brain reward circuitry, brain stem physiology, and neurotransmitter deficits due to the effects of genetic and epigenetic insults on the interrelated cascade of neurotransmission and the net release of dopamine at the Ventral Tegmental Area -Nucleus Accumbens (VTA-NAc) reward site. The Genetic Addiction Risk Severity (GARS) test and pro-dopamine regulator nutraceutical KB220 were combined to induce "dopamine homeostasis" across the brain reward circuitry. This article aims to encourage four future actionable items: 1) the neurophysiologically accurate designation of, for example, "Hyperdopameism /Hyperdopameism" to replace the blaming nomenclature like alcoholism; 2) encouraging continued research into the nature of dysfunctional brainstem neurotransmitters across the brain reward circuitry; 3) early identification of people at risk for all RDS behaviors as a brain check (cognitive testing); 4) induction of dopamine homeostasis using "precision behavioral management" along with the coupling of GARS and precision Kb220 variants; 5) utilization of promising potential treatments include neuromodulating modalities such as Transmagnetic stimulation (TMS) and Deep Brain Stimulation(DBS), which target different areas of the neural circuitry involved in addiction and even neuroimmune agents like N-acetyl-cysteine.

Synaptotagmin-1-dependent phasic axonal dopamine release is dispensable for basic motor behaviors in mice

In Parkinson's disease (PD), motor dysfunctions only become apparent after extensive loss of DA innervation. This resilience has been hypothesized to be due to the ability of many motor behaviors to be sustained through a diffuse basal tone of DA; but experimental evidence for this is limited. Here we show that conditional deletion of the calcium sensor synaptotagmin-1 (Syt1) in DA neurons (Syt1 cKODA mice) abrogates most activity-dependent axonal DA release in the striatum and mesencephalon, leaving somatodendritic (STD) DA release intact. Strikingly, Syt1 cKODA mice showed intact performance in multiple unconditioned DA-dependent motor tasks and even in a task evaluating conditioned motivation for food. Considering that basal extracellular DA levels in the striatum were unchanged, our findings suggest that activity-dependent DA release is dispensable for such tasks and that they can be sustained by a basal tone of extracellular DA. Taken together, our findings reveal the striking resilience of DA-dependent motor functions in the context of a near-abolition of phasic DA release, shedding new light on why extensive loss of DA innervation is required to reveal motor dysfunctions in PD.

Synaptotagmin-1 is a Ca2+ sensor for somatodendritic dopamine release

Modes of somatodendritic transmission range from rapid synaptic signaling to protracted regulation over distance. Somatodendritic dopamine secretion in the midbrain leads to D2 receptor-induced modulation of dopamine neurons on the timescale of seconds. Temporally imprecise release mechanisms are often presumed to be at play, and previous work indeed suggested roles for slow Ca2+ sensors. We here use mouse genetics and whole-cell electrophysiology to establish that the fast Ca2+ sensor synaptotagmin-1 (Syt-1) is important for somatodendritic dopamine release. Syt-1 ablation from dopamine neurons strongly reduces stimulus-evoked D2 receptor-mediated inhibitory postsynaptic currents (D2-IPSCs) in the midbrain. D2-IPSCs evoked by paired stimuli exhibit less depression, and high-frequency trains restore dopamine release. Spontaneous somatodendritic dopamine secretion is independent of Syt-1, supporting that its exocytotic mechanisms differ from evoked release. We conclude that somatodendritic dopamine transmission relies on the fast Ca2+ sensor Syt-1, leading to synchronous release in response to the initial stimulus.

Somatodendritic Release of Cholecystokinin Potentiates GABAergic Synapses Onto Ventral Tegmental Area Dopamine Cells

BACKGROUND

Neuropeptides are contained in nearly every neuron in the central nervous system and can be released not only from nerve terminals but also from somatodendritic sites. Cholecystokinin (CCK), among the most abundant neuropeptides in the brain, is expressed in the majority of midbrain dopamine neurons. Despite this high expression, CCK function within the ventral tegmental area (VTA) is not well understood.

METHODS

We confirmed CCK expression in VTA dopamine neurons through immunohistochemistry and in situ hybridization and detected optogenetically induced CCK release using an enzyme-linked immunosorbent assay. To investigate whether CCK modulates VTA circuit activity, we used whole-cell patch clamp recordings in mouse brain slices. We infused CCK locally in vivo and tested food intake and locomotion in fasted mice. We also used in vivo fiber photometry to measure Ca²⁺ transients in dopamine neurons during feeding.

RESULTS

Here we report that VTA dopamine neurons release CCK from somatodendritic regions, where it triggers long-term potentiation of GABAergic (gamma-aminobutyric acidergic) synapses. The somatodendritic release occurs during trains of optogenetic stimuli or prolonged but modest depolarization and is dependent on synaptotagmin-7 and T-type Ca²⁺ channels. Depolarization-induced long-term potentiation is blocked by a CCK₂ receptor antagonist and mimicked by exogenous CCK. Local infusion of CCK in vivo inhibits food consumption and decreases distance traveled in an open field test. Furthermore, intra-VTA-infused CCK reduced dopamine cell Ca²⁺ signals during food consumption after an overnight fast and was correlated with reduced food intake.

CONCLUSIONS

Our experiments introduce somatodendritic neuropeptide release as a previously unknown feedback regulator of VTA dopamine cell excitability and dopamine-related behaviors.

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.

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.

Synaptic-like axo-axonal transmission from striatal cholinergic interneurons onto dopaminergic fibers

Transmission from striatal cholinergic interneurons (CINs) controls dopamine release through nicotinic acetylcholine receptors (nAChRs) on dopaminergic axons. Anatomical studies suggest that cholinergic terminals signal predominantly through non-synaptic volume transmission. However, the influence of cholinergic transmission on electrical signaling in axons remains unclear. We examined axo-axonal transmission from CINs onto dopaminergic axons using perforated-patch recordings, which revealed rapid spontaneous EPSPs with properties characteristic of fast synapses. Pharmacology showed that axonal EPSPs (axEPSPs) were mediated primarily by high-affinity α6-containing receptors. Remarkably, axEPSPs triggered spontaneous action potentials, suggesting that these axons perform integration to convert synaptic input into spiking, a function associated with somatodendritic compartments. We investigated the cross-species validity of cholinergic axo-axonal transmission by recording dopaminergic axons in macaque putamen and found similar axEPSPs. Thus, we reveal that synaptic-like neurotransmission underlies cholinergic signaling onto dopaminergic axons, supporting the idea that striatal dopamine release can occur independently of somatic firing to provide distinct signaling.

Leptin Promotes Striatal Dopamine Release via Cholinergic Interneurons and Regionally Distinct Signaling Pathways

Dopamine (DA) is a critical regulator of striatal network activity and is essential for motor activation and reward-associated behaviors. Previous work has shown that DA is influenced by the reward value of food, as well as by hormonal factors that reguate food intake and energy expenditure. Changes in striatal DA signaling also have been linked to aberrant eating patterns. Here we test the effect of leptin, an adipocyte-derived hormone involved in feeding and energy homeostasis regulation, on striatal DA release and uptake. Immunohistochemical evaluation identified leptin receptor (LepR) expression throughout mouse striatum, including on striatal cholinergic interneurons (ChIs) and their extensive processes. Using fast-scan cyclic voltammetry (FSCV), we found that leptin causes a concentration-dependent increase in evoked extra-cellular DA concentration ([DA]o) in dorsal striatum (dStr) and nucleus accumbens (NAc) core and shell in male mouse striatal slices, and also an increase in the rate of DA uptake. Further, we found that leptin increases ChI excitability, and that the enhancing effect of leptin on evoked [DA]o is lost when nicotinic acetylcholine (ACh) receptors are antagonized or when examined in striatal slices from mice lacking ACh synthesis. Evaluation of signaling pathways underlying leptin's action revealed a requirement for intracellular Ca2+, and the involvement of different downstream pathways in dStr and NAc core versus NAc shell. These results provide the first evidence for dynamic regulation of DA release and uptake by leptin within brain motor and reward pathways, and highlight the involvement of ChIs in this process.SIGNIFICANCE STATEMENT Given the importance of striatal dopamine (DA) in reward, motivation, motor behavior and food intake, identifying the actions of metabolic hormones on DA release in striatal subregions should provide new insight into factors that influence DA-dependent motivated behaviors. We find that one of these hormones, leptin, boosts striatal DA release through a process involving striatal cholinergic interneurons (ChIs) and nicotinic acetylcholine (ACh) receptors. Moreover, we find that the intracellular cascades downstream from leptin receptor (LepR) activation that lead to enhanced DA release differ among striatal subregions. Thus, we not only show that leptin regulates DA release, but also identify characteristics of this process that could be harnessed to alter pathologic eating behaviors.

Cocaine increases quantal norepinephrine secretion through NET-dependent PKC activation in locus coeruleus neurons

The norepinephrine neurons in locus coeruleus (LC-NE neurons) are essential for sleep arousal, pain sensation, and cocaine addiction. According to previous studies, cocaine increases NE overflow (the profile of extracellular NE level in response to stimulation) by blocking the NE reuptake. NE overflow is determined by NE release via exocytosis and reuptake through NE transporter (NET). However, whether cocaine directly affects vesicular NE release has not been directly tested. By recording quantal NE release from LC-NE neurons, we report that cocaine directly increases the frequency of quantal NE release through regulation of NET and downstream protein kinase C (PKC) signaling, and this facilitation of NE release modulates the activity of LC-NE neurons and cocaine-induced stimulant behavior. Thus, these findings expand the repertoire of mechanisms underlying the effects of cocaine on NE (pro-release and anti-reuptake), demonstrate NET as a release enhancer in LC-NE neurons, and provide potential sites for treatment of cocaine addiction.

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.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

Synaptotagmins 1 and 7 Play Complementary Roles in Somatodendritic Dopamine Release

The molecular mechanisms underlying somatodendritic dopamine (DA) release remain unresolved, despite the passing of decades since its discovery. Our previous work showed robust release of somatodendritic DA in submillimolar extracellular Ca2+ concentration ([Ca2+]o). Here we tested the hypothesis that the high-affinity Ca2+ sensor synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca2+ dependence. Somatodendritic DA release from SNc DA neurons was assessed using whole-cell recording in midbrain slices from male and female mice to monitor evoked DA-dependent D2 receptor-mediated inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse train-evoked D2ICs, revealing a functional role for Syt7. The assessment of the Ca2+ dependence of pulse train-evoked D2ICs confirmed robust DA release in submillimolar [Ca2+]o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knock-out (KO) mice. In millimolar [Ca2+]o, pulse train-evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca2+]o than seen in WT mice; the effect on single pulse-evoked DA release, however, did not differ between genotypes. Single-cell application of a Syt1 Ab had no effect on train-evoked D2ICs in WT SNc DA neurons, but did cause a decrease in D2IC amplitude in Syt7 KOs, indicating a functional substitution of Syt1 for Syt7. In addition, Syt1 Ab decreased single pulse-evoked D2ICs in WT cells, indicating the involvement of Syt1 in tonic DA release. Thus, Syt7 and Syt1 play complementary roles in somatodendritic DA release from SNc DA neurons.SIGNIFICANCE STATEMENT The respective Ca2+ dependence of somatodendritic and axonal dopamine (DA) release differs, resulting in the persistence of somatodendritic DA release in submillimolar Ca2+ concentrations too low to support axonal release. We demonstrate that synaptotagmin7 (Syt7), a high-affinity Ca2+ sensor, underlies phasic somatodendritic DA release and its Ca2+ sensitivity in the substantia nigra pars compacta. In contrast, we found that synaptotagmin 1 (Syt1), the Ca2+ sensor underlying axonal DA release, plays a role in tonic, but not phasic, somatodendritic DA release in wild-type mice. However, Syt1 can facilitate phasic DA release after Syt7 deletion. Thus, we show that both Syt1 and Syt7 act as Ca2+ sensors subserving different aspects of somatodendritic DA release processes.

Optogenetic Stimulation of Midbrain Dopamine Neurons Produces Striatal Serotonin Release

Targeting neurons with light-driven opsins is widely used to investigate cell-specific responses. We transfected midbrain dopamine neurons with the excitatory opsin Chrimson. Extracellular basal and stimulated neurotransmitter levels in the dorsal striatum were measured by microdialysis in awake mice. Optical activation of dopamine cell bodies evoked terminal dopamine release in the striatum. Multiplexed analysis of dialysate samples revealed that the evoked dopamine was accompanied by temporally coupled increases in striatal 3-methoxytyramine, an extracellular dopamine metabolite, and in serotonin. We investigated a mechanism for dopamine-serotonin interactions involving striatal dopamine receptors. However, the evoked serotonin associated with optical stimulation of dopamine neurons was not abolished by striatal D1- or D2-like receptor inhibition. Although the mechanisms underlying the coupling of striatal dopamine and serotonin remain unclear, these findings illustrate advantages of multiplexed measurements for uncovering functional interactions between neurotransmitter systems. Furthermore, they suggest that the output of optogenetic manipulations may extend beyond opsin-expressing neuronal populations.

Neuroprotection and Disease Modification by Astrocytes and Microglia in Parkinson Disease

Oxidative stress and neuroinflammation are common bases for disease onset and progression in many neurodegenerative diseases. In Parkinson disease, which is characterized by the degeneration of dopaminergic neurons resulting in dopamine depletion, the pathogenesis differs between hereditary and solitary disease forms and is often unclear. In addition to the pathogenicity of alpha-synuclein as a pathological disease marker, the involvement of dopamine itself and its interactions with glial cells (astrocyte or microglia) have attracted attention. Pacemaking activity, which is a hallmark of dopaminergic neurons, is essential for the homeostatic maintenance of adequate dopamine concentrations in the synaptic cleft, but it imposes a burden on mitochondrial oxidative glucose metabolism, leading to reactive oxygen species production. Astrocytes provide endogenous neuroprotection to the brain by producing and releasing antioxidants in response to oxidative stress. Additionally, the protective function of astrocytes can be modified by microglia. Some types of microglia themselves are thought to exacerbate Parkinson disease by releasing pro-inflammatory factors (M1 microglia). Although these inflammatory microglia may further trigger the inflammatory conversion of astrocytes, microglia may induce astrocytic neuroprotective effects (A2 astrocytes) simultaneously. Interestingly, both astrocytes and microglia express dopamine receptors, which are upregulated in the presence of neuroinflammation. The anti-inflammatory effects of dopamine receptor stimulation are also attracting attention because the functions of astrocytes and microglia are greatly affected by both dopamine depletion and therapeutic dopamine replacement in Parkinson disease. In this review article, we will focus on the antioxidative and anti-inflammatory effects of astrocytes and their synergism with microglia and dopamine.

Subcellular localization of D2 receptors in the murine substantia nigra

G-protein-coupled D2 autoreceptors expressed on dopamine neurons (D2Rs) inhibit transmitter release and cell firing at axonal endings and somatodendritic compartments. Mechanistic details of somatodendritic dopamine release remain unresolved, partly due to insufficient information on the subcellular distribution of D2Rs. Previous studies localizing D2Rs have been hindered by a dearth of antibodies validated for specificity in D2R knockout animals and have been limited by the small sampling areas imaged by electron microscopy. This study utilized sub-diffraction fluorescence microscopy and electron microscopy to examine D2 receptors in a superecliptic pHlourin GFP (SEP) epitope-tagged D2 receptor knockin mouse. Incubating live slices with an anti-SEP antibody achieved the selective labeling of plasma membrane-associated receptors for immunofluorescent imaging over a large area of the substantia nigra pars compacta (SNc). SEP-D2Rs appeared as puncta-like structures along the surface of dendrites and soma of dopamine neurons visualized by antibodies to tyrosine hydroxylase (TH). TH-associated SEP-D2Rs displayed a cell surface density of 0.66 puncta/µm², which corresponds to an average frequency of 1 punctum every 1.50 µm. Separate ultrastructural experiments using silver-enhanced immunogold revealed that membrane-bound particles represented 28% of total D2Rs in putative dopamine cells within the SNc. Structures immediately adjacent to dendritic membrane gold particles were unmyelinated axons or axon varicosities (40%), astrocytes (19%), other dendrites (7%), or profiles unidentified (34%) in single sections. Some apposed profiles also expressed D2Rs. Fluorescent and ultrastructural analyses also provided the first visualization of membrane D2Rs at the axon initial segment, a compartment critical for action potential generation. The punctate appearance of anti-SEP staining indicates there is a population of D2Rs organized in discrete signaling sites along the plasma membrane, and for the first time, a quantitative estimate of spatial frequency is provided.

Rewiring of the Serotonin System in Major Depression

Serotonin is a key neurotransmitter that is implicated in a wide variety of behavioral and cognitive phenotypes. Originating in the raphe nuclei, 5-HT neurons project widely to innervate many brain regions implicated in the functions. During the development of the brain, as serotonin axons project and innervate brain regions, there is evidence that 5-HT plays key roles in wiring the developing brain, both by modulating 5-HT innervation and by influencing synaptic organization within corticolimbic structures. These actions are mediated by 14 different 5-HT receptors, with region- and cell-specific patterns of expression. More recently, the role of the 5-HT system in synaptic re-organization during adulthood has been suggested. The 5-HT neurons have the unusual capacity to regrow and reinnervate brain regions following insults such as brain injury, chronic stress, or altered development that result in disconnection of the 5-HT system and often cause depression, anxiety, and cognitive impairment. Chronic treatment with antidepressants that amplify 5-HT action, such as selective serotonin reuptake inhibitors (SSRIs), appears to accelerate the rewiring of the 5-HT system by mechanisms that may be critical to the behavioral and cognitive improvements induced in these models. In this review, we survey the possible 5-HT receptor mechanisms that could mediate 5-HT rewiring and assess the evidence that 5-HT-mediated brain rewiring is impacting recovery from mental illness. By amplifying 5-HT-induced rewiring processes using SSRIs and selective 5-HT agonists, more rapid and effective treatments for injury-induced mental illness or cognitive impairment may be achieved.