Striatal dopamine neurotransmission: regulation of release and uptake

Erasing "bad memories": reversing aberrant synaptic plasticity as therapy for neurological and psychiatric disorders

Dopamine modulates corticostriatal plasticity in both the direct and indirect pathways of the cortico-striato-thalamo-cortical (CSTC) loops. These gradual changes in corticostriatal synaptic strengths produce long-lasting changes in behavioral responses. Under normal conditions, these mechanisms enable the selection of the most appropriate responses while inhibiting others. However, under dysregulated dopamine conditions, including a lack of dopamine release or dopamine signaling, these mechanisms could lead to the selection of maladaptive responses and/or the inhibition of appropriate responses in an experience-dependent and task-specific manner. In this review, we propose that preventing or reversing such maladaptive synaptic strengths and erasing such aberrant "memories" could be a disease-modifying therapeutic strategy for many neurological and psychiatric disorders. We review evidence from Parkinson's disease, drug-induced parkinsonism, L-DOPA-induced dyskinesia, obsessive-compulsive disorder, substance use disorders, and depression as well as research findings on animal disease models. Altogether, these studies allude to an emerging theme in translational neuroscience and promising new directions for therapy development. Specifically, we propose that combining pharmacotherapy with behavioral therapy or with deep brain stimulation (DBS) could potentially cause desired changes in specific neural circuits. If successful, one important advantage of correcting aberrant synaptic plasticity is long-lasting therapeutic effects even after treatment has ended. We will also discuss the potential molecular targets for these therapeutic approaches, including the cAMP pathway, proteins involved in synaptic plasticity as well as pathways involved in new protein synthesis. We place special emphasis on RNA binding proteins and epitranscriptomic mechanisms, as they represent a new frontier with the distinct advantage of rapidly and simultaneously altering the synthesis of many proteins locally.

An axonal brake on striatal dopamine output by cholinergic interneurons

Depolarization of axons is necessary for somatic action potentials to trigger axonal neurotransmitter release. Here we show that striatal cholinergic interneurons (ChIs) and nicotinic receptors (nAChRs) on mouse dopamine axons interrupt this relationship. After nAChR-mediated depolarization, dopamine release by subsequent depolarization events was suppressed for ~100 ms. This suppression was not due to depletion of dopamine or acetylcholine, but to a limited reactivation of dopamine axons after nAChR-mediated depolarization, and is more prominent in dorsal than in ventral striatum. In vivo, nAChRs predominantly depressed dopamine release, as nAChR antagonism in dorsal striatum elevated dopamine detected with optic-fiber photometry of dopamine sensor GRABDA2m and promoted conditioned place preference. Our findings reveal that ChIs acting via nAChRs transiently limit the reactivation of dopamine axons for subsequent action potentials in dopamine neurons and therefore generate a dynamic inverse scaling of dopamine release according to ChI activity.

Molecular and Behavioral Neuroprotective Effects of Clavulanic Acid and Crocin in Haloperidol-Induced Tardive Dyskinesia in Rats

Clavulanic acid (ClvA), a beta-lactamase inhibitor, is being explored for its significant neuroprotective potential. The effects of ClvA were assessed both individually and in combination with crocin (Cr), an antioxidant derived from saffron, in the context of tardive dyskinesia (TD). In rat haloperidol (Hp)-induced-TD (1 mg/kg, i.p. 21 days), the effects of ClvA (50, 100, 150 mg/kg) and Cr (10, 20, 40 mg/kg) were assessed via vacuous chewing movements (VCM) and tongue protrusion (TP). Striatal malondialdehyde (MDA) and glutathione (GSH) were measured spectrophotometrically. Based on the results, ClvA (100 mg/kg) and Cr (10 mg/kg) were determined with sub-effective doses. Glutamate transporter-subtype1 (GLT1), dopamine active transporter (DAT), vesicular monoamine transporter-type2 (VMAT2), Bax/Bcl2, cleaved Caspase3, phosphorylated AKT/AKT, IL1β, and TNFα levels were quantified using western blotting in sub-effective doses and their combination. The behavioral results of catalepsy and orofacial dyskinesia demonstrated model establishment. Hp decreased GLT1 (p < 0.05), DAT (p < 0.01), VMAT2 (p < 0.001), GSH and pAKT/AKT (p < 0.0001); increased TNFα (p < 0.05), IL1β, cleaved Caspase3 (p < 0.001); MDA and Bax/Bcl2 (p < 0.0001). ClvA 100 mg/kg reversed the decreased GLT1 and VMAT2 (p < 0.01), alongside the increased MDA (p < 0.0001) and VCM (p < 0.05). It also increased AKT phosphorylation (p < 0.05). No effects were noted on DAT, GSH, Bax/Bcl2, or inflammatory factors. However, the combination with Cr at 10 mg/kg influenced ClvA on DAT (p < 0.01) and resulted in a significant increase in GSH (p < 0.0001). Additionally, there was a marked decrease in TNFα (p < 0.0001) and IL1β (p < 0.001), enhancing its effects on reducing MDA and increasing pAKT/AKT (p < 0.0001). The combination adversely affected GLT1. ClvA protects against TD via GLT1 and VMAT2; combined with Cr, it enhances antioxidant effects, improves DAT, and requires dose optimization for GLT1 disruption.

Dopamine-driven increase in IL-1β in myeloid cells is mediated by differential dopamine receptor expression and exacerbated by HIV

The catecholamine neurotransmitter dopamine is classically known for regulation of central nervous system (CNS) functions such as reward, movement, and cognition. Increasing evidence also indicates that dopamine regulates critical functions in peripheral organs and is an important immunoregulatory factor. We have previously shown that dopamine increases NF-κB activity, inflammasome activation, and the production of inflammatory cytokines such as IL-1β in human macrophages. As myeloid lineage cells are central to the initiation and resolution of acute inflammatory responses, dopamine-mediated dysregulation of these functions could both impair the innate immune response and exacerbate chronic inflammation. However, the exact pathways by which dopamine drives myeloid inflammation are not well defined, and studies in both rodent and human systems indicate that dopamine can impact the production of inflammatory mediators through both D1-like dopamine receptors (DRD1, DRD5) and D2-like dopamine receptors (DRD2, DRD3, and DRD4). Therefore, we hypothesized that dopamine-mediated production of IL-1β in myeloid cells is regulated by the ratio of different dopamine receptors that are activated. Our data in primary human monocyte-derived macrophages (hMDM) indicate that DRD1 expression is necessary for dopamine-mediated increases in IL-1β, and that changes in the expression of DRD2 and other dopamine receptors can alter the magnitude of the dopamine-mediated increase in IL-1β. Mature hMDM have a high D1-like to D2-like receptor ratio, which is different relative to monocytes and peripheral blood mononuclear cells (PBMCs). We further confirm in human microglia cell lines that a high ratio of D1-like to D2-like receptors promotes dopamine-induced increases in IL-1β gene and protein expression using pharmacological inhibition or overexpression of dopamine receptors. RNA-sequencing of dopamine-treated microglia shows that genes encoding functions in IL-1β signaling pathways, microglia activation, and neurotransmission increased with dopamine treatment. Finally, using HIV as an example of a chronic inflammatory disease that is substantively worsened by comorbid substance use disorders (SUDs) that impact dopaminergic signaling, we show increased effects of dopamine on inflammasome activation and IL-1β in the presence of HIV in both human macrophages and microglia. These data suggest that use of addictive substances and dopamine-modulating therapeutics could dysregulate the innate inflammatory response and exacerbate chronic neuroimmunological conditions like HIV. Thus, a detailed understanding of dopamine-mediated changes in inflammation, in particular pathways regulating IL-1β, will be critical to effectively tailor medication regimens.

Memantine-induced functional rewiring of the glutamate synapse in the striatum of dopamine transporter knockout rats

BACKGROUND AND PURPOSE

Slow-acting biogenic amines, such as dopamine, are known to modulate fast neurotransmitters e.g. glutamate. In the striatum, dopamine (DA) interacts with glutamate, influencing neural excitability and promoting synaptic plasticity. The exact mechanism of such interaction is not fully understood. This study investigates, in detail, how dopamine overactivity in dopamine transporter knockout (DAT-/-) rats, alters the homeostasis of the striatal glutamate synapse from a molecular, behavioural and functional point of view.

EXPERIMENTAL APPROACH

The expression, localisation, retention and electrophysiological properties of N-methyl-D-aspartate (NMDA) receptors as well as dendritic spine density and morphology were investigated in the striatum of DAT-/- rats, at baseline and after treatment with the non-competitive NMDA receptor antagonist memantine (30 mg kg-1).

KEY RESULTS

Dopamine overactivity dramatically reorganises the striatal glutamate synapse, redistributing NMDA receptors in the synapse as typified by reduced synaptic availability and reduced expression of NMDA scaffolding proteins, as well as by increased GluN2B-containing NMDA receptors in the extra synapse. Such changes are accompanied by reduced spine density, suggesting dopamine-induced structural rearrangements. These results converge into a compromised plasticity, as shown by the impaired ability to promote long-term depression (LTD) in the striatum of DAT-/-rats. Notably, memantine counteracts hyperlocomotion, reverses spine alterations and abolishes the extrasynaptic movements of NMDA receptors in the striatum of DAT-/- rats, thus restoring functional LTD.

CONCLUSION AND IMPLICATIONS

A hyperdopaminergic condition seems to alter striatal homeostasis by increasing extrasynaptic NMDA receptors. These findings may be relevant to manipulate disorders characterised by elevated dopaminergic activity.

Quantitative chemoproteomics reveals dopamine's protective modification of Tau

Dopamine (DA) is one of the most important neurotransmitters. Its oxidation leads to electrophilic quinone, which covalently modifies nucleophilic residues in proteins, resulting in 'dopamination'. Individual dopaminated proteins have been studied, most of which were functionally damaged by dopamination. Here, we developed a quantitative chemoproteomic strategy to site-specifically measure proteins' dopamination. More than 6,000 dopamination sites were quantified. Half-maximal inhibitory concentration values for 63 hypersensitive sites were measured. Among them, hypersensitive dopamination of two cysteines in microtubule-associated protein Tau was biochemically validated and functionally characterized to prevent Tau's amyloid fibrillation and promote Tau-mediated assembly of microtubules. In addition, endogenous dopamination of Tau in mouse brain was detected through targeted mass spectrometry analysis. Our study not only provides a global portrait of dopamination but also discovers a protective role of DA in regulating the function of Tau, which will enhance our understanding of the physiological and pathological functions of DA in human brain.

Extrinsic and intrinsic control of striatal cholinergic interneuron activity

The striatum is an integrated component of the basal ganglia responsible for associative learning and response. Besides the presence of the most abundant γ-aminobutyric acid (GABA-ergic) medium spiny neurons (MSNs), the striatum also contains distributed populations of cholinergic interneurons (ChIs), which bidirectionally communicate with many of these neuronal subtypes. Despite their sparse distribution, ChIs provide the largest source of acetylcholine (ACh) to striatal cells, have a prominent level of arborization and activity, and are potent modulators of striatal output and play prominent roles in plasticity underlying associative learning and reinforcement. Deviations from this tonic activity, including phasic bursts or pauses caused by region-selective excitatory input, neuromodulator, or neuropeptide release can exert strong influences on intrinsic activity and synaptic plasticity via diverse receptor signaling. Recent studies and new tools have allowed improved identification of factors driving or suppressing cholinergic activity, including peptides. This review aims to outline our current understanding of factors that control tonic and phasic ChI activity, specifically focusing on how neuromodulators and neuropeptides interact to facilitate or suppress phasic ChI responses underlying learning and plasticity.

Advances in hydrogel for diagnosis and treatment for Parkinson's disease

Currently, few symptomatic and palliative care options are available for patients with Parkinson's disease (PD). Interdisciplinary research in materials engineering and regenerative medicine has stimulated the development of innovative therapeutic strategy for patients with PD. Hydrogels, which are versatile and accessible to modify, have garnered considerable interests. Hydrogels are a kind of three-dimensional hydrophilic network structure gels that are widely employed in biological materials. Hydrogels are conspicuous in many therapeutic applications, including neuron regeneration, neuroprotection, and diagnosis. This review focuses on the advantageous applications of hydrogel-based biomaterials in diagnosing and treating the patients with PD, including cell culture, disease modeling, carriers for cells, medications and proteins, as well as diagnostic and monitoring biosensors.

The Neuroprotective Effects of Primary Functional Components Mulberry Leaf Extract in Diabetes-Induced Oxidative Stress and Inflammation

Diabetes-associated neurodegeneration may result from increased oxidative stress in the brain under hyperglycemic conditions, which leads to neuronal cell death. The current study employs the neuroblastoma cell line SH-SY5Y and db/db mouse model of diabetes maintained on a high-fat diet to investigate the neuroprotective effects of the primary functional components of mulberry (Morus alba Linn) leaf extract (MLE), chlorogenic acid (CGA), and neochlorogenic acid (NCGA). CGA and NCGA demonstrated the ability to enhance the activities of the antioxidant enzymes superoxide dismutase and glutathione peroxidase, and attenuate inflammation via regulating nuclear factor erythroid 2-related factor 2 (Nrf2), nuclear factor-κB (NFκB), and inflammatory cytokines, thereby protecting SH-SY5Y cells from oxidative damage induced by palmitic acid and high glucose. CGA and NCGA were found to decrease the expression of proinflammatory proteins α-synuclein and amyloid-β (Aβ). In addition, CGA and NCGA treatments increased the expression of tyrosine hydroxylase (TH) and brain-derived neurotrophic factor (BDNF). Furthermore, MLE supplementation in the animal model resulted in decreased levels of α-synuclein and Aβ concomitant with an elevated expression of TH. These experimental findings suggest that the neuroprotective effects of CGA and NCGA may be mediated via three pathways: reducing oxidative stress, decreasing neuronal inflammation, and enhancing BDNF expression.

Regulation of Dopamine Release by Tonic Activity Patterns in the Striatal Brain Slice

Voluntary movement, motivation, and reinforcement learning depend on the activity of ventral midbrain neurons, which extend axons to release dopamine (DA) in the striatum. These neurons exhibit two patterns of action potential activity: low-frequency tonic activity that is intrinsically generated and superimposed high-frequency phasic bursts that are driven by synaptic inputs. Ex vivo acute striatal brain preparations are widely employed to study the regulation of evoked DA release but exhibit very different DA release kinetics than in vivo recordings. To investigate the relationship between phasic and tonic neuronal activity, we stimulated the slice in patterns intended to mimic tonic activity, which were interrupted by a series of burst stimuli. Conditioning the striatal slice with low-frequency activity altered DA release triggered by high-frequency bursts and produced kinetic parameters that resemble those in vivo. In the absence of applied tonic activity, nicotinic acetylcholine receptor and D2 DA receptor antagonists had no significant effect on neurotransmitter release, driven by repeated burst activity in the striatal brain slice. In contrast, in tonically stimulated slices, the D2 receptor blockade decreased the amount of DA released during a single-burst and facilitated DA release in subsequent bursts. This experimental system provides a means to reconcile the difference in the kinetics of DA release ex vivo and in vivo and provides a novel approach to more accurately emulate pre- and postsynaptic mechanisms that control axonal DA release in vivo.

Action Potential Firing Patterns Regulate Dopamine Release via Voltage-Sensitive Dopamine D2 Autoreceptors in Mouse Striatum In Vivo

Dopamine (DA) in the striatum is vital for motor and cognitive behaviors. Midbrain dopaminergic neurons generate both tonic and phasic action potential (AP) firing patterns in behavior mice. Besides AP numbers, whether and how different AP firing patterns per se modulate DA release remain largely unknown. Here by using in vivo and ex vivo models, it is shown that the AP frequency per se modulates DA release through the D2 receptor (D2R), which contributes up to 50% of total DA release. D2R has a voltage-sensing site at D131 and can be deactivated in a frequency-dependent manner by membrane depolarization. This voltage-dependent D2R inhibition of DA release is mediated via the facilitation of voltage-gated Ca2+ channels (VGCCs). Collectively, this work establishes a novel mechanism that APs per se modulate DA overflow by disinhibiting the voltage-sensitive autoreceptor D2R and thus the facilitation of VGCCs, providing a pivotal pathway and insight into mammalian DA-dependent functions in vivo.

Reduced Alcohol Consumption Following Ablation of Cholinergic Interneurons in the Nucleus Accumbens of Wistar Rats

Alcohol use disorder is a severe mental health condition causing medical consequences and preterm death. Alcohol activates the mesolimbic dopamine system leading to an increase of extracellular dopamine (DA) in the nucleus accumbens, an event that is associated with the reinforcing effects of alcohol. Cholinergic interneurons (CIN) are important modulators of accumbal DA signalling, and depletion of accumbal CIN attenuates the alcohol-induced increase in extracellular DA. The aim of this study was to explore the functional role of accumbal CIN in alcohol-related behaviour. To this end, ablation of CIN was induced by local administration of anticholine acetyltransferase-saporin bilaterally into the nucleus accumbens of male Wistar rats. Alcohol consumption in ablated and sham-treated rats was studied using a two-bottle-choice intermittent alcohol consumption paradigm. Rats with depleted CIN consumed significantly less alcohol than sham-treated controls. No differences in sucrose preference, motor activity, water intake or weight gain were noted between treatment groups, suggesting that the ablation selectively affected alcohol-related behaviour. In conclusion, this study further supports a role for accumbal CIN in regulating alcohol-consummatory behaviour.

Synaptic vesicle characterization of iPSC-derived dopaminergic neurons provides insight into distinct secretory vesicle pools

The dysfunction of dopaminergic (DA) neurons is central to Parkinson's disease. Distinct synaptic vesicle (SV) populations, differing in neurotransmitter content (dopamine vs. glutamate), may vary due to differences in trafficking and exocytosis. However, the structural organization of these vesicles remains unclear. In this study, we examined axonal varicosities in human iPSC-derived DA and glutamatergic neurons (i3Neurons). i3Neurons primarily contained small, clear SVs (40-50 nm), whereas DA neurons contained larger, pleiomorphic vesicles including dense core and empty vesicles, in addition to the classical SVs. VMAT2-positive vesicles in DA neurons, which load dopamine, were spatially segregated from VGLUT1/2-positive vesicles in an SV-like reconstitution system. These vesicles also colocalized with SV markers (e.g., VAMP2, SV2C), and can be clustered by synapsin. Moreover, DA axonal terminals in mouse striata showed similar vesicle pool diversity. These findings reveal structural differences in DA neurons' vesicles, highlighting iPSC-derived neurons as effective models for studying presynaptic structures.

Dysregulated acetylcholine-mediated dopamine neurotransmission in the eIF4E Tg mouse model of autism spectrum disorders

Autism spectrum disorder (ASD) consists of diverse neurodevelopmental conditions where core behavioral symptoms are critical for diagnosis. Altered dopamine (DA) neurotransmission in the striatum has been suggested to contribute to the behavioral features of ASD. Here, we examine DA neurotransmission in a mouse model of ASD characterized by elevated expression of eukaryotic initiation factor 4E (eIF4E), a key regulator of cap-dependent translation, using a comprehensive approach that encompasses genetics, behavior, synaptic physiology, and imaging. The results indicate that increased eIF4E expression leads to behavioral inflexibility and impaired striatal DA release. The loss of normal DA neurotransmission is due to a defect in nicotinic receptor signaling that regulates calcium dynamics in dopaminergic axons. These findings provide a mechanistic understanding of ASD symptoms and offer a foundation for targeted therapeutic interventions by revealing the intricate interplay between eIF4E, DA neurotransmission, and behavioral flexibility.

Volatile oil from Acori graminei Rhizoma affected the synaptic plasticity of rats with tic disorders by modulating dopaminergic and glutamatergic systems

ETHNOPHARMACOLOGICAL RELEVANCE

Acori graminei Rhizoma is a commonly used traditional Chinese medicine for treating TD, with its main component being calamus volatile oil. Volatile Oil from Acori graminei Rhizoma (VOA)can protect nerve cells and alleviate learning and memory disorders. However, the mechanism of anti-tic of VOA is still unclear.

AIM OF THE STUDY

We aimed to explore the effects of Volatile Oil from Acori Tatarinowii Rhizoma (VOA) on striatal dopaminergic and glutamatergic systems and synaptic plasticity of rats with Tic Disorder (TD), as well as its pharmaceutical mechanism against TD.

MATERIALS AND METHODS

This study involved 48 (three-week-old) Sprague Dawley (SD) rats, which were randomly divided into two primary groups: Control (8) and TD (40). Rats in the TD group were injected intraperitoneally with 3,3-iminodipropionitrile (IDPN) to construct the TD rat model. They were divided into five subgroups: Model, Tiapride, VOA-high, VOA-medium, and VOA-low (N = 8). After modeling, VOA was administrated to rats in the VOA groups through gavage (once/day for four consecutive weeks), while rats in the blank control and model groups received normal saline of the same volume. The animals' behavioral changes were reflected using the stereotypic and motor behavior scores. After interferences, patterns of striatal neurons and the density of dendritic spines were investigated using H&E and Golgi staining, and the ultrastructure of striatal synapses was examined using Transmission Electron Microscopy (TEM). Furthermore, Ca2+ content was determined using the Ca2+ detector, and Dopamine (DA) and Glutamate (GLU) contents in serum and striatum were detected through ELISA. Finally, DRD1, DRD2, AMPAR1, NMPAR1, DAT, VMAT2, CAMKⅡ, and CREB expression in the striatum was detected using Quantitative real-time PCR (qRT-PCR), Western Blotting (WB) and Immunohistochemical (IHC) methods.

RESULTS

Compared to rats in the blank control and model groups, rats in the VOA groups showed lower stereotypic behavior scores. Furthermore, rats in the VOA groups exhibited relieved, neuron damage and increased quantities of neuronal dendrites and dendritic spines Additionally, based on TEM images show that, the VOA groups showed a clear synaptic structure and increased amounts of postsynaptic dense substances and synaptic vesicles. The VOA groups also exhibited reduced Ca2+ contents, and upregulation of DRD1, DRD2, DAT, AMPAR1, and NMPAR1 and downregulation of VMAT-2, CAMKⅡ, and CREB in the striatum.

CONCLUSIONS

In summary, VOA could influence synaptic plasticity by tuning the dopaminergic and glutamatergic systems, thus relieving TD.

Clinicians' viewpoints on current paradigms of care and research in Parkinson's disease

Aim of this review is to discuss the value of current ongoing research initiatives in Parkinson's disease from the clinicians' point of view. The repeat, recent failures on progress slowing reflect the drifting apart between initially promising experimental and then disappointing clinical outcomes in the translational trials with well selected Parkinson's disease patients. A similar development concerns the emerging gap between novel developed drugs with improved pharmacokinetic behaviour and their limited use in the clinical practice following approval. Restricted regional different worldwide availability and direct, respectively indirect budget limitations for neurologists in private practice are essential hurdles. They prevent the widespread prescription of these compounds. As a result return of investment for the pharmaceutical industry becomes more and more uncertain. The interest for research on novel treatment approaches for the amelioration of motor and non motor symptoms declines. Clinicians crucially scrutinize the claim for an optimum patient care by payers and regulators.

Modeling early phenotypes of Parkinson's disease by age-induced midbrain-striatum assembloids

Parkinson's disease, an aging-associated neurodegenerative disorder, is characterised by nigrostriatal pathway dysfunction caused by the gradual loss of dopaminergic neurons in the substantia nigra pars compacta of the midbrain. Human in vitro models are enabling the study of the dopaminergic neurons' loss, but not the dysregulation within the dopaminergic network in the nigrostriatal pathway. Additionally, these models do not incorporate aging characteristics which potentially contribute to the development of Parkinson's disease. Here we present a nigrostriatal pathway model based on midbrain-striatum assembloids with inducible aging. We show that these assembloids can develop characteristics of the nigrostriatal connectivity, with catecholamine release from the midbrain to the striatum and synapse formation between midbrain and striatal neurons. Moreover, Progerin-overexpressing assembloids acquire aging traits that lead to early neurodegenerative phenotypes. This model shall help to reveal the contribution of aging as well as nigrostriatal connectivity to the onset and progression of Parkinson's disease.

Neuroprotective effect of L-DOPA-induced interleukin-13 on striatonigral degeneration in cerebral ischemia

Levodopa (L-DOPA) treatment is a clinically effective strategy for improving motor function in patients with ischemic stroke. However, the mechanisms by which modulating the dopamine system relieves the pathology of the ischemic brain remain unclear. Emerging evidence from an experimental mouse model of ischemic stroke, established by middle cerebral artery occlusion (MCAO), suggested that L-DOPA has the potential to modulate the inflammatory and immune response that occurs during a stroke. Here, we aimed to demonstrate the therapeutic effect of L-DOPA in regulating the systemic immune response and improving functional deficits in mice with ischemia. Transient MCAO led to progressive degeneration of nigrostriatal dopamine neurons and significant rotational behavior in mice. Exogenous L-DOPA treatment attenuated the striatonigral degeneration and reversed motor behavioral impairment. Notably, treatment with L-DOPA significantly increased IL-13 but reduced IFN-γ in infarct lesions. To investigate the role of IL-13 in motor behavior, we stereotaxically injected anti-IL-13 antibodies into the infarct area of the mouse brain one week after MCAO, followed by L-DOPA treatment. The intervention reduced dopamine, IL-13, and IL-10 levels and exacerbated motor function. IL-13 is potentially expressed on CD4 T cells, while IL-10 is mainly expressed on microglia rather than astrocytes. Finally, IL-13 activates the phagocytosis of microglia, which may contribute to neuroprotection by eliminating degenerating neurons. Our study provides evidence that the L-DOPA-activated dopamine system modulates peripheral immune cells, resulting in the expression of anti-inflammatory and neuroprotective cytokines in mice with ischemic stroke.

Rapid modulation of striatal cholinergic interneurons and dopamine release by satellite astrocytes

Astrocytes are increasingly appreciated to possess underestimated and important roles in modulating neuronal circuits. Astrocytes in striatum can regulate dopamine transmission by governing the extracellular tone of axonal neuromodulators, including GABA and adenosine. However, here we reveal that striatal astrocytes occupy a cell type-specific anatomical and functional relationship with cholinergic interneurons (ChIs), through which they rapidly excite ChIs and govern dopamine release via nicotinic acetylcholine receptors on subsecond timescales. We identify that ChI somata are in unexpectedly close proximity to astrocyte somata, in mouse and human, forming a "soma-to-soma" satellite-like configuration not typically observed for other striatal neurons. We find that transient depolarization of astrocytes in mouse striatum reversibly regulates ChI excitability by decreasing extracellular calcium. These findings reveal a privileged satellite astrocyte-interneuron interaction for striatal ChIs operating on subsecond timescales via regulation of extracellular calcium dynamics to shape downstream striatal circuit activity and dopamine signaling.

Effects of Acute Haloperidol Treatment on Dopaminergic Markers, GAD67, and A2A Receptors in Rats with High and Low VCMs

Vacuous chewing movements (VCM) have been utilized as an experimental model of orofacial dyskinesia (OD) in rodents to study the underlying molecular mechanisms related to tardive dyskinesia (TD). This study aimed to investigate if the acute treatment with haloperidol can alter components of the dopaminergic synapse or its modulators such as glutamic acid decarboxylase (GAD67) and adenosine 2A (A2A) receptor. Furthermore, to evaluate if changes in molecular markers are associated with the number of VCMs induced by haloperidol in rats it is proposing a method to classify the animals into High and Low VCM groups. Here, we treated rats with haloperidol decanoate (single injection, intramuscularly, 28 mg/Kg of unconjugated haloperidol) and evaluated the number of VCMs after 4 weeks. Haloperidol-treated rats were divided into three groups (Low, High, and Spontaneous VCM) according to the evaluation of the VCM profile proposed here. After, dopamine (DA) levels, monoamine oxidase (MAO) activity, and the immunoreactivity of tyrosine hydroxylase (TH), dopamine transporter (DAT), D2 receptor, GAD67, and A2A were determined in brain structures. No significant differences were found in DA levels, MAO activity, and immunoreactivity of the TH, DAT, D2 receptor, GAD67, and A2A receptor in brain structures. VCM intensity was correlated with TH immunoreactivity in Sn in the High VCM group while it was inversely correlated with the immunoreactivity of the A2A receptor in the striatum of the Spontaneous VCM group. Other significant correlations were found considering the VCM profile suggesting that High VCM after acute haloperidol treatment seems to be associated with the lack of ability to reorganize the neurotransmission in the nigrostriatal pathway. Further studies could clarify the main targets involved in the motor side effects of antipsychotics. The present study demonstrated an easy way to separate the animals into High and Low VCMs.

G protein-coupled receptor modulation of striatal dopamine transmission: Implications for psychoactive drug effects

Dopamine transmission in the striatum is a critical mediator of the rewarding and reinforcing effects of commonly misused psychoactive drugs. G protein-coupled receptors (GPCRs) that bind a variety of neuromodulators including dopamine, endocannabinoids, acetylcholine and endogenous opioid peptides regulate dopamine release by acting on several components of dopaminergic circuitry. Striatal dopamine release can be driven by both somatic action potential firing and local mechanisms that depend on acetylcholine released from striatal cholinergic interneurons. GPCRs that primarily regulate somatic firing of dopamine neurons via direct effects or modulation of synaptic inputs are likely to affect distinct aspects of behaviour and psychoactive drug actions compared with those GPCRs that primarily regulate local acetylcholine-dependent dopamine release in striatal regions. This review will highlight mechanisms by which GPCRs modulate dopaminergic transmission and the relevance of these findings to psychoactive drug effects on physiology and behaviour.

Mechanisms of neuromodulatory volume transmission

A wealth of neuromodulatory transmitters regulate synaptic circuits in the brain. Their mode of signaling, often called volume transmission, differs from classical synaptic transmission in important ways. In synaptic transmission, vesicles rapidly fuse in response to action potentials and release their transmitter content. The transmitters are then sensed by nearby receptors on select target cells with minimal delay. Signal transmission is restricted to synaptic contacts and typically occurs within ~1 ms. Volume transmission doesn't rely on synaptic contact sites and is the main mode of monoamines and neuropeptides, important neuromodulators in the brain. It is less precise than synaptic transmission, and the underlying molecular mechanisms and spatiotemporal scales are often not well understood. Here, we review literature on mechanisms of volume transmission and raise scientific questions that should be addressed in the years ahead. We define five domains by which volume transmission systems can differ from synaptic transmission and from one another. These domains are (1) innervation patterns and firing properties, (2) transmitter synthesis and loading into different types of vesicles, (3) architecture and distribution of release sites, (4) transmitter diffusion, degradation, and reuptake, and (5) receptor types and their positioning on target cells. We discuss these five domains for dopamine, a well-studied monoamine, and then compare the literature on dopamine with that on norepinephrine and serotonin. We include assessments of neuropeptide signaling and of central acetylcholine transmission. Through this review, we provide a molecular and cellular framework for volume transmission. This mechanistic knowledge is essential to define how neuromodulatory systems control behavior in health and disease and to understand how they are modulated by medical treatments and by drugs of abuse.

Functional PET/MRI reveals active inhibition of neuronal activity during optogenetic activation of the nigrostriatal pathway

The dopaminergic system is a central component of the brain's neurobiological framework, governing motor control and reward responses and playing an essential role in various brain disorders. Within this complex network, the nigrostriatal pathway represents a critical circuit for dopamine neurotransmission from the substantia nigra to the striatum. However, stand-alone functional magnetic resonance imaging is unable to study the intricate interplay between brain activation and its molecular underpinnings. In our study, the use of a functional [fluorine-18]2-fluor-2-deoxy-d-glucose positron emission tomography approach, simultaneously with blood oxygen level-dependent functional magnetic resonance imaging, provided an important insight that demonstrates an active suppression of the nigrostriatal activity during optogenetic stimulation. This result increases our understanding of the molecular mechanisms of brain function and provides an important perspective on how dopamine influences hemodynamic responses in the brain.

Wideband ratiometric measurement of tonic and phasic dopamine release in the striatum

Reward learning, cognition, and motivation are supported by changes in neurotransmitter levels across multiple timescales. Current measurement technologies for various neuromodulators (such as dopamine and serotonin) do not bridge timescales of fluctuations, limiting the ability to define the behavioral significance, regulation, and relationship between fast (phasic) and slow (tonic) dynamics. To help resolve longstanding debates about the behavioral significance of dopamine across timescales, we developed a novel quantification strategy, augmenting extensively used carbon-fiber Fast Scan Cyclic Voltammetry (FSCV). We iteratively engineered the FSCV scan sequence to rapidly modify electrode sensitivity within a sampling window and applied ratiometric analysis for wideband dopamine measurement. This allowed us to selectively eliminate artifacts unrelated to electrochemical detection (i.e., baseline drift), overcoming previous limitations that precluded wideband dopamine detection from milliseconds to hours. We extensively characterize this approach in vitro, validate performance in vivo with simultaneous microdialysis, and deploy this technique to measure wideband dopamine changes across striatal regions under pharmacological, optogenetic, and behavioral manipulations. We demonstrate that our approach can extend to additional analytes, including serotonin and pH, providing a robust platform to assess the contributions of multi-timescale neuromodulator fluctuations to cognition, learning, and motivation.

Seasonal variation in D2/3 dopamine receptor availability in the human brain

PURPOSE

Brain functional and physiological plasticity is essential to combat dynamic environmental challenges. The rhythmic dopamine signaling pathway, which regulates emotion, reward and learning, shows seasonal patterns with higher capacity of dopamine synthesis and lower number of dopamine transporters during dark seasons. However, seasonal variation of the dopamine receptor signaling remains to be characterized.

METHODS

Based on a historical database of healthy human brain [11C]raclopride PET scans (n = 291, 224 males and 67 females), we investigated the seasonal patterns of D2/3 dopamine receptor signaling. Daylength at the time of scanning was used as a predictor for brain regional non-displaceable binding of the radiotracer, while controlling for age and sex.

RESULTS

Daylength was negatively correlated with availability of D2/3 dopamine receptors in the striatum. The largest effect was found in the left caudate, and based on the primary sample, every 4.26 h (i.e., one standard deviation) increase of daylength was associated with a mean 2.8% drop (95% CI -0.042 to -0.014) of the receptor availability.

CONCLUSIONS

Seasonally varying D2/3 receptor signaling may also underlie the seasonality of mood, feeding, and motivational processes. Our finding suggests that in future studies of brain dopamine signaling, especially in high-latitude regions, the effect of seasonality should be considered.

A neural circuit architecture for rapid learning in goal-directed navigation

Anchoring goals to spatial representations enables flexible navigation but is challenging in novel environments when both representations must be acquired simultaneously. We propose a framework for how Drosophila uses internal representations of head direction (HD) to build goal representations upon selective thermal reinforcement. We show that flies use stochastically generated fixations and directed saccades to express heading preferences in an operant visual learning paradigm and that HD neurons are required to modify these preferences based on reinforcement. We used a symmetric visual setting to expose how flies' HD and goal representations co-evolve and how the reliability of these interacting representations impacts behavior. Finally, we describe how rapid learning of new goal headings may rest on a behavioral policy whose parameters are flexible but whose form is genetically encoded in circuit architecture. Such evolutionarily structured architectures, which enable rapidly adaptive behavior driven by internal representations, may be relevant across species.

A chemogenetic approach for dopamine imaging with tunable sensitivity

Genetically-encoded dopamine (DA) sensors enable high-resolution imaging of DA release, but their ability to detect a wide range of extracellular DA levels, especially tonic versus phasic DA release, is limited by their intrinsic affinity. Here we show that a human-selective dopamine receptor positive allosteric modulator (PAM) can be used to boost sensor affinity on-demand. The PAM enhances DA detection sensitivity across experimental preparations (in vitro, ex vivo and in vivo) via one-photon or two-photon imaging. In vivo photometry-based detection of optogenetically-evoked DA release revealed that DETQ administration produces a stable 31 minutes window of potentiation without effects on animal behavior. The use of the PAM revealed region-specific and metabolic state-dependent differences in tonic DA levels and enhanced single-trial detection of behavior-evoked phasic DA release in cortex and striatum. Our chemogenetic strategy can potently and flexibly tune DA imaging sensitivity and reveal multi-modal (tonic/phasic) DA signaling across preparations and imaging approaches.

PEDOT/CNT Flexible MEAs Reveal New Insights into the Clock Gene's Role in Dopamine Dynamics

Substantial evidence has shown that the Circadian Locomotor Output Cycles Kaput (Clock) gene is a core transcription factor of circadian rhythms that regulates dopamine (DA) synthesis. To shed light on the mechanism of this interaction, flexible multielectrode arrays (MEAs) are developed that can measure both DA concentrations and electrophysiology chronically. The dual functionality is enabled by conducting polymer PEDOT doped with acid-functionalized carbon nanotubes (CNT). The PEDOT/CNT microelectrode coating maintained stable electrochemical impedance and DA detection by square wave voltammetry for 4 weeks in vitro. When implanted in wild-type (WT) and Clock mutation (MU) mice, MEAs measured tonic DA concentration and extracellular neural activity with high spatial and temporal resolution for 4 weeks. A diurnal change of DA concentration in WT is observed, but not in MU, and a higher basal DA concentration and stronger cocaine-induced DA increase in MU. Meanwhile, striatal neuronal firing rate is found to be positively correlated with DA concentration in both animal groups. These findings offer new insights into DA dynamics in the context of circadian rhythm regulation, and the chronically reliable performance and dual measurement capability of this technology hold great potential for a broad range of neuroscience research.

Presynaptic GABAA receptors control integration of nicotinic input onto dopaminergic axons in the striatum

Axons of dopaminergic neurons express gamma-aminobutyric acid type-A receptors (GABAARs) and nicotinic acetylcholine receptors (nAChRs) which are both independently positioned to shape striatal dopamine release. Using electrophysiology and calcium imaging, we investigated how interactions between GABAARs and nAChRs influence dopaminergic axon excitability. Direct axonal recordings showed that benzodiazepine application suppresses subthreshold axonal input from cholinergic interneurons (CINs). In imaging experiments, we used the first temporal derivative of presynaptic calcium signals to distinguish between direct- and nAChR-evoked activity in dopaminergic axons. We found that GABAAR antagonism with gabazine selectively enhanced nAChR-evoked axonal signals. Acetylcholine release was unchanged in gabazine suggesting that GABAARs located on dopaminergic axons, but not CINs, mediated this enhancement. Unexpectedly, we found that a widely used GABAAR antagonist, picrotoxin, inhibits axonal nAChRs and should be used cautiously for striatal circuit analysis. Overall, we demonstrate that GABAARs on dopaminergic axons regulate integration of nicotinic input to shape presynaptic excitability.

Striatal GDNF Neurons Chemoattract RET-Positive Dopamine Axons at Seven Times Farther Distance Than Medium Spiny Neurons

Glial cell line-derived neurotrophic factor (GDNF) is among the strongest dopamine neuron function- and survival-promoting factors known. Due to this reason, it has clinical relevance in dopamine disorders such as Parkinson's disease and schizophrenia. In the striatum, GDNF is exclusively expressed in interneurons, which make up only about 0.6% of striatal cells. Despite clinical significance, histological analysis of striatal GDNF system arborization and relevance to incoming dopamine axons, which bear its receptor RET, has remained enigmatic. This is mainly due to the lack of antibodies able to visualize GDNF- and RET-positive cellular processes; here, we overcome this problem by using knock-in marker alleles. We find that GDNF neurons chemoattract RET+ axons at least seven times farther in distance than medium spiny neurons (MSNs), which make up 95% of striatal neurons. Furthermore, we provide evidence that tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, is enriched towards GDNF neurons in the dopamine axons. Finally, we find that GDNF neuron arborizations occupy approximately only twelve times less striatal volume than 135 times more abundant MSNs. Collectively, our results improve our understanding of how endogenous GDNF affects striatal dopamine system function.

Dissociable effects of dopaminergic medications on depression symptom dimensions in Parkinson disease

Depression in Parkinson disease (PD) is common, is disabling and responds poorly to standard antidepressants. Motivational symptoms of depression are particularly prevalent in PD and emerge with loss of dopaminergic innervation of the striatum. Optimizing dopaminergic treatment for PD can improve depressive symptoms. However, the differential effect of antiparkinsonian medication on symptom dimensions of depression is not known. Using data from a large (n = 412) longitudinal study of patients with newly diagnosed PD followed over 5 years, we investigated whether there are dissociable effects of dopaminergic medications on different depression symptom dimensions in PD. Previously validated 'motivation' and 'depression' dimensions were derived from the 15-item geriatric depression scale. Dopaminergic neurodegeneration was measured using repeated striatal dopamine transporter imaging. We identified dissociable associations between dopaminergic medications and different dimensions of depression in PD. Dopamine agonists were shown to be effective for treatment of motivational symptoms of depression. In contrast, monoamine oxidase-B inhibitors improved both depressive and motivation symptoms, albeit the latter effect is attenuated in patients with more severe striatal dopaminergic neurodegeneration.

Dopamine-driven Increase in IL-1β in Myeloid Cells is Mediated by Differential Dopamine Receptor Expression and Exacerbated by HIV

The catecholamine neurotransmitter dopamine is classically known for regulation of central nervous system (CNS) functions such as reward, movement, and cognition. Increasing evidence also indicates that dopamine regulates critical functions in peripheral organs and is an important immunoregulatory factor. We have previously shown that dopamine increases NF-κB activity, inflammasome activation, and the production of inflammatory cytokines such as IL-1β in human macrophages. As myeloid lineage cells are central to the initiation and resolution of acute inflammatory responses, dopamine-mediated dysregulation of these functions could both impair the innate immune response and exacerbate chronic inflammation. However, the exact pathways by which dopamine drives myeloid inflammation are not well defined, and studies in both rodent and human systems indicate that dopamine can impact the production of inflammatory mediators through both D1-like dopamine receptors (DRD1, DRD5) and D2-like dopamine receptors (DRD2, DRD3, and DRD4). Therefore, we hypothesized that dopamine-mediated production of IL-1β in myeloid cells is regulated by the ratio of different dopamine receptors that are activated. Our data in primary human monocyte-derived macrophages (hMDM) indicate that DRD1 expression is necessary for dopamine-mediated increases in IL-1β, and that changes in the expression of DRD2 and other dopamine receptors can alter the magnitude of the dopamine-mediated increase in IL-1β. Mature hMDM have a high D1-like to D2-like receptor ratio, which is different relative to monocytes and peripheral blood mononuclear cells (PBMCs). We further confirm in human microglia cell lines that a high ratio of D1-like to D2-like receptors promotes dopamine-induced increases in IL-1β gene and protein expression using pharmacological inhibition or overexpression of dopamine receptors. RNA-sequencing of dopamine-treated microglia shows that genes encoding functions in IL-1β signaling pathways, microglia activation, and neurotransmission increased with dopamine treatment. Finally, using HIV as an example of a chronic inflammatory disease that is substantively worsened by comorbid substance use disorders (SUDs) that impact dopaminergic signaling, we show increased effects of dopamine on inflammasome activation and IL-1β in the presence of HIV in both human macrophages and microglia. These data suggest that use of addictive substances and dopamine-modulating therapeutics could dysregulate the innate inflammatory response and exacerbate chronic neuroimmunological conditions like HIV. Thus, a detailed understanding of dopamine-mediated changes in inflammation, in particular pathways regulating IL-1β, will be critical to effectively tailor medication regimens.

(+)-Borneol Protects Dopaminergic Neuronal Loss in Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Induced Parkinson's Disease Mice: A Study of Dopamine Level using In Vivo Brain Microdialysis

Considerable research efforts have been directed toward the symptom relief of Parkinson's disease (PD) by attenuating dopamine (DA) depletion. One common feature of these existing therapies is their unavailability of preventing the neurodegenerative process of dopaminergic neurons. (+)-Borneol, a natural highly lipid-soluble bicyclic monoterpene, has been reported to regulate the levels of monoamine neurotransmitters in the central nervous system and exhibit neuroprotective effects. However, the effect of (+)-borneol on the dopaminergic neuronal loss of methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mice is not defined. Herein, we first report that 30 mg/kg (+)-borneol significantly attenuated the motor deficits of PD mice, which benefits from markedly increasing the level of DA and decreasing the metabolic rate of DA in the striatum of conscious and freely moving mouse detected by ultraperformance liquid chromatography tandem mass spectrometry online combined with in vivo brain microdialysis sampling. It is worth noting that the enhanced level of DA by (+)-borneol was enabled by the reduction in loss of tyrosine hydroxylase-immunoreactive dopaminergic neurons in the substantia nigra and striatum and promotion of reserpine- or nomifensine-induced DA release in PD mice. Interestingly, (+)-borneol evidently inhibited the decreased expression levels of DA transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) on the MPTP mouse model of PD. Moreover, (+)-borneol suppressed the neuroinflammation by inhibiting the production of IL-1β, IL-6, and TNF-α and attenuated oxidative stress by decreasing the level of MDA and increasing the activities of SOD and GSH-px in PD mice. These findings demonstrate that (+)-borneol protects DA neurons by inhibiting neuroinflammation and oxidative stress. Further research work for the neuroprotection mechanism of (+)-borneol will focus on reactive oxygen species-mediated apoptosis. Therefore, (+)-borneol is a potential therapeutic candidate for retarding the neurodegenerative process of PD.

Midbrain organoids for Parkinson's disease (PD) - A powerful tool to understand the disease pathogenesis

Brain Organiods (BOs) are a promising technique for researching disease progression in the human brain. These organoids, which are produced from human induced pluripotent stem cells (HiPSCs), can construct themselves into structured frameworks. In the context of Parkinson's disease (PD), recent advancements have been made in the development of Midbrain organoids (MBOs) models that consider key pathophysiological mechanisms such as alpha-synuclein (α-Syn), Lewy bodies, dopamine loss, and microglia activation. However, there are limitations to the current use of BOs in disease modelling and drug discovery, such as the lack of vascularization, long-term differentiation, and absence of glial cells. To address these limitations, researchers have proposed the use of spinning bioreactors to improve oxygen and nutrient perfusion. Modelling PD utilising modern experimental in vitro models is a valuable tool for studying disease mechanisms and elucidating previously unknown features of PD. In this paper, we exclusively review the unique methods available for cultivating MBOs using a pumping system that mimics the circulatory system. This mechanism may aid in delivering the required amount of oxygen and nutrients to all areas of the organoids, preventing cell death, and allowing for long-term culture and using co-culturing techniques for developing glial cell in BOs. Furthermore, we emphasise some of the significant discoveries about the BOs and the potential challenges of using BOs will be discussed.

Comment on 'Accumbens cholinergic interneurons dynamically promote dopamine release and enable motivation'

Acetylcholine is widely believed to modulate the release of dopamine in the striatum of mammals. Experiments in brain slices clearly show that synchronous activation of striatal cholinergic interneurons is sufficient to drive dopamine release via axo-axonal stimulation of nicotinic acetylcholine receptors. However, evidence for this mechanism in vivo has been less forthcoming. Mohebi, Collins and Berke recently reported that, in awake behaving rats, optogenetic activation of striatal cholinergic interneurons with blue light readily evokes dopamine release measured with the red fluorescent sensor RdLight1 (Mohebi et al., 2023). Here, we show that blue light alone alters the fluorescent properties of RdLight1 in a manner that may be misconstrued as phasic dopamine release, and that this artefactual photoactivation can account for the effects attributed to cholinergic interneurons. Our findings indicate that measurements of dopamine using the red-shifted fluorescent sensor RdLight1 should be interpreted with caution when combined with optogenetics. In light of this and other publications that did not observe large acetylcholine-evoked dopamine transients in vivo, the conditions under which such release occurs in behaving animals remain unknown.

Ivermectin increases striatal cholinergic activity to facilitate dopamine terminal function

Ivermectin (IVM) is a commonly prescribed antiparasitic treatment with pharmacological effects on invertebrate glutamate ion channels resulting in paralysis and death of invertebrates. However, it can also act as a modulator of some vertebrate ion channels and has shown promise in facilitating L-DOPA treatment in preclinical models of Parkinson's disease. The pharmacological effects of IVM on dopamine terminal function were tested, focusing on the role of two of IVM's potential targets: purinergic P2X4 and nicotinic acetylcholine receptors. Ivermectin enhanced electrochemical detection of dorsal striatum dopamine release. Although striatal P2X4 receptors were observed, IVM effects on dopamine release were not blocked by P2X4 receptor inactivation. In contrast, IVM attenuated nicotine effects on dopamine release, and antagonizing nicotinic receptors prevented IVM effects on dopamine release. IVM also enhanced striatal cholinergic interneuron firing. L-DOPA enhances dopamine release by increasing vesicular content. L-DOPA and IVM co-application further enhanced release but resulted in a reduction in the ratio between high and low frequency stimulations, suggesting that IVM is enhancing release largely through changes in terminal excitability and not vesicular content. Thus, IVM is increasing striatal dopamine release through enhanced cholinergic activity on dopamine terminals.

Local regulation of striatal dopamine: A diversity of circuit mechanisms for a diversity of behavioral functions?

Striatal dopamine governs a wide range of behavioral functions, yet local dopamine concentrations can be dissociated from somatic activity. Here, we discuss how dopamine's diverse roles in behavior may be driven by local circuit mechanisms shaping dopamine release. We first look at historical and recent work demonstrating that striatal circuits interact with dopaminergic terminals to either initiate the release of dopamine or modulate the release of dopamine initiated by spiking in midbrain dopamine neurons, with particular attention to GABAergic and cholinergic local circuit mechanisms. Then we discuss some of the first in vivo studies of acetylcholine-dopamine interactions in striatum and broadly discuss necessary future work in understanding the roles of midbrain versus striatal dopamine regulation.

Cholecystokinin (CCK): a neuromodulator with therapeutic potential in Alzheimer's and Parkinson's disease

Cholecystokinin (CCK) is a neuropeptide modulating digestion, glucose levels, neurotransmitters and memory. Recent studies suggest that CCK exhibits neuroprotective effects in Alzheimer's disease (AD) and Parkinson's disease (PD). Thus, we review the physiological function and therapeutic potential of CCK. The neuropeptide facilitates hippocampal glutamate release and gates GABAergic basket cell activity, which improves declarative memory acquisition, but inhibits consolidation. Cortical CCK alters recognition memory and enhances audio-visual processing. By stimulating CCK-1 receptors (CCK-1Rs), sulphated CCK-8 elicits dopamine release in the substantia nigra and striatum. In the mesolimbic pathway, CCK release is triggered by dopamine and terminates reward responses via CCK-2Rs. Importantly, activation of hippocampal and nigral CCK-2Rs is neuroprotective by evoking AMPK activation, expression of mitochondrial fusion modulators and autophagy. Other benefits include vagus nerve/CCK-1R-mediated expression of brain-derived neurotrophic factor, intestinal protection and suppression of inflammation. We also discuss caveats and the therapeutic combination of CCK with other peptide hormones.

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.

Putative neural mechanisms underlying release-mode-specific abnormalities in dopamine neural activity in a schizophrenia-like model: The distinct roles of glutamate and serotonin in the impaired regulation of dopamine neurons

Abnormalities in dopamine function might be related to psychiatric disorders such as schizophrenia. Even at the same concentration, dopamine exerts opposite effects on information processing in the prefrontal cortex depending on independent dopamine release modes known as tonic and phasic releases. This duality of dopamine prevents a blanket interpretation of the implications of dopamine abnormalities for diseases on the basis of absolute dopamine levels. Moreover, the mechanisms underlying the mode-specific dopamine abnormalities are not clearly understood. Here, I show that the two modes of dopamine release in the prefrontal cortex of a schizophrenia-like model are disrupted by different mechanisms. In the schizophrenia-like model established by perinatal exposure to inflammatory cytokine, epidermal growth factor, tonic release was enhanced and phasic release was decreased in the prefrontal cortex. I examined the activity of dopamine neurons in the ventral tegmental area (VTA), which sends dopamine projections to the prefrontal cortex, under anaesthesia. The activation of VTA dopamine neurons during excitatory stimulation (local application of glutamate or N-methyl-d-aspartic acid [NMDA]), which is associated with phasic activity, was blunt in this model. Dopaminergic neuronal activity in the resting state related to tonic release was increased by disinhibition of the dopamine neurons due to the impairment of 5HT2 (5HT2A) receptor-regulated GABAergic inputs. Moreover, chronic administration of risperidone ameliorated this disinhibition of dopaminergic neurons. These results provide an idea about the mechanism of dopamine disturbance in schizophrenia and may be informative in explaining the effects of atypical antipsychotics as distinct from those of typical drugs.

Dysregulated acetylcholine-mediated dopamine neurotransmission in the eIF4E Tg mouse model of autism spectrum disorders

Autism Spectrum Disorders (ASD) consist of diverse neurodevelopmental conditions where core behavioral symptoms are critical for diagnosis. Altered dopamine neurotransmission in the striatum has been suggested to contribute to the behavioral features of ASD. Here, we examine dopamine neurotransmission in a mouse model of ASD characterized by elevated expression of the eukaryotic initiation factor 4E (eIF4E), a key regulator of cap-dependent translation, using a comprehensive approach that encompasses genetics, behavior, synaptic physiology, and imaging. The results indicate that increased eIF4E expression leads to behavioral inflexibility and impaired striatal dopamine release. The loss of normal dopamine neurotransmission is due to a defective nicotinic receptor signaling that regulates calcium dynamics in dopaminergic axons. These findings reveal an intricate interplay between eIF4E, DA neurotransmission, and behavioral flexibility, provide a mechanistic understanding of ASD symptoms and offer a foundation for targeted therapeutic interventions.

Apathy and Motivation: Biological Basis and Drug Treatment

Apathy is a disabling syndrome associated with poor functional outcomes that is common across a broad range of neurological and psychiatric conditions. Currently, there are no established therapies specifically for the condition, and safe and effective treatments are urgently needed. Advances in the understanding of motivation and goal-directed behavior in humans and animals have shed light on the cognitive and neurobiological mechanisms contributing to apathy, providing an important foundation for the development of new treatments. Here, we review the cognitive components, neural circuitry, and pharmacology of apathy and motivation, highlighting converging evidence of shared transdiagnostic mechanisms. Though no pharmacological treatments have yet been licensed, we summarize trials of existing and novel compounds to date, identifying several promising candidates for clinical use and avenues of future drug development.

Comment on 'Accumbens cholinergic interneurons dynamically promote dopamine release and enable motivation'

Acetylcholine is widely believed to modulate the release of dopamine in the striatum of mammals. Experiments in brain slices clearly show that synchronous activation of striatal cholinergic interneurons is sufficient to drive dopamine release via axo-axonal stimulation of nicotinic acetylcholine receptors. However, evidence for this mechanism in vivo has been less forthcoming. A recent paper in eLife (Mohebi et al., 2023) reported that, in awake behaving rats, optogenetic activation of striatal cholinergic interneurons with blue light readily evokes dopamine release measured with the red fluorescent sensor RdLight1. Here, we show that blue light alone alters the fluorescent properties of RdLight1 in a manner that may be misconstrued as phasic dopamine release, and that this artefactual photoactivation can account for the effects attributed to cholinergic interneurons. Our findings indicate that measurements of dopamine using the red-shifted fluorescent sensor RdLight1 should be interpreted with caution when combined with optogenetics. In light of this and other publications that did not observe large acetylcholine-evoked dopamine transients in vivo, the conditions under which such release occurs in behaving animals remain unknown.

Enhancing the Study of Quantal Exocytotic Events: Combining Diamond Multi-Electrode Arrays with Amperometric PEak Analysis (APE) an Automated Analysis Code

MicroGraphited-Diamond-Multi Electrode Arrays (μG-D-MEAs) can be successfully used to reveal, in real time, quantal exocytotic events occurring from many individual neurosecretory cells and/or from many neurons within a network. As μG-D-MEAs arrays are patterned with up to 16 sensing microelectrodes, each of them recording large amounts of data revealing the exocytotic activity, the aim of this work was to support an adequate analysis code to speed up the signal detection. The cutting-edge technology of microGraphited-Diamond-Multi Electrode Arrays (μG-D-MEAs) has been implemented with an automated analysis code (APE, Amperometric Peak Analysis) developed using Matlab R2022a software to provide easy and accurate detection of amperometric spike parameters, including the analysis of the pre-spike foot that sometimes precedes the complete fusion pore dilatation. Data have been acquired from cultured PC12 cells, either collecting events during spontaneous exocytosis or after L-DOPA incubation. Validation of the APE code was performed by comparing the acquired spike parameters with those obtained using Quanta Analysis (Igor macro) by Mosharov et al.

Antagonism of kappa opioid receptors accelerates the development of L-DOPA-induced dyskinesia in a preclinical model of moderate dopamine depletion

Levels of the opioid peptide dynorphin, an endogenous ligand selective for kappa-opioid receptors (KORs), its mRNA and pro-peptide precursors are differentially dysregulated in Parkinson's disease (PD) and following the development of l-DOPA-induced dyskinesia (LID). It remains unclear whether these alterations contribute to the pathophysiological mechanisms underlying PD motor impairment and the subsequent development of LID, or whether they are part of compensatory mechanisms. We sought to investigate nor-BNI, a KOR antagonist, 1) in the dopamine (DA)-depleted PD state, 2) during the development phase of LID, and 3) via measuring of tonic levels of striatal DA. While nor-BNI (3 mg/kg; s.c.) did not lead to functional restoration in the DA-depleted state, it affected the dose-dependent development of abnormal voluntary movements (AIMs) in response to escalating doses of l-DOPA in a rat PD model with a moderate striatal 6-hydroxdopamine (6-OHDA) lesion. We tested five escalating doses of l-DOPA (6, 12, 24, 48, 72 mg/kg; i.p.), and nor-BNI significantly increased the development of AIMs at the 12 and 24 mg/kg l-DOPA doses. However, after reaching the 72 mg/kg l-DOPA, AIMs were not significantly different between control and nor-BNI groups. In summary, while blocking KORs significantly increased the rate of development of LID induced by chronic, escalating doses of l-DOPA in a moderate-lesioned rat PD model, it did not contribute further once the overall severity of LID was established. While we observed an increase of tonic DA levels in the moderately lesioned dorsolateral striatum, there was no tonic DA change following administration of nor-BNI.

Application of OpenArray Technology to Assess Changes in the Expression of Functionally Significant Genes in the Substantia Nigra of Mice in a Model of Parkinson's Disease

Studying the molecular mechanisms of the pathogenesis of Parkinson's disease (PD) is critical to improve PD treatment. We used OpenArray technology to assess gene expression in the substantia nigra (SN) cells of mice in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD and in controls. Among the 11 housekeeping genes tested, Rps27a was taken as the reference gene due to its most stable expression in normal and experimental conditions. From 101 genes encoding functionally significant proteins of nigrostriatal dopaminergic neurons, 57 highly expressed genes were selected to assess their expressions in the PD model and in the controls. The expressions of Th, Ddc, Maoa, Comt, Slc6a3, Slc18a2, Drd2, and Nr4a2 decreased in the experiment compared to the control, indicating decreases in the synthesis, degradation, and transport of dopamine and the impaired autoregulation of dopaminergic neurons. The expressions of Tubb3, Map2, Syn1, Syt1, Rab7, Sod1, Cib1, Gpx1, Psmd4, Ubb, Usp47, and Ctsb genes were also decreased in the MPTP-treated mice, indicating impairments of axonal and vesicular transport and abnormal functioning of the antioxidant and ubiquitin-proteasome systems in the SN. The detected decreases in the expressions of Snca, Nsf, Dnm1l, and Keap1 may serve to reduce pathological protein aggregation, increase dopamine release in the striatum, prevent mitophagy, and restore the redox status of SN cells.

Characterization of Selective M5 Acetylcholine Muscarinic Receptor Modulators on Dopamine Signaling in the Striatum

The type-5 muscarinic acetylcholine receptor (mAChR, M5) is almost exclusively expressed in dopamine (DA) neurons of the ventral tegmental area and substantia nigra pars compacta; therefore, they are ideally located to modulate DA signaling and underlying behaviors. However, the role of M5 in shaping DA release is still poorly characterized. In this study, we first quantitatively mapped the expression of M5 in different neurons of the mouse midbrain, then used voltammetry in mouse striatum to evaluate the effect of M5-selective modulators on DA release. The M5 negative allosteric modulator ML375 significantly decreased electrically evoked DA release and blocked the effect of Oxotremorine-M (Oxo-M; nonselective mAChR agonist) on DA release in the presence of an acetylcholine nicotinic receptor blocker. Conversely, the M5 positive allosteric modulator VU 0365114 significantly increased electrically evoked DA release and the Oxo-M effect on DA release. We then assessed M5's impact on mesolimbic circuit function in vivo. Although psychostimulant-induced locomotor activity models in knockout mice have previously been used to characterize the role of M5 in DA transmission, the results of these studies conflict, leading us to select a different in vivo model, namely a cocaine self-administration paradigm. In contrast to a previous study that also used this model, in the current study, administration of ML375 did not decrease cocaine self-administration in rats (using fixed and progressive ratio). These conflicting results illustrate the complexity of M5 modulation and the need to further characterize its involvement in the regulation of dopamine signaling, central to multiple neuropsychiatric diseases. SIGNIFICANCE STATEMENT: This work describes the type-5 muscarinic receptor (M5) pattern of expression within the midbrain as well as its physiological modulation by selective compounds at the axon terminal level in the striatum, where M5 directly shapes dopamine transmission. It offers the first direct readout of mesolimbic dopamine release modulation by M5, highlighting its role in regulating neurocircuits implicated in the pathophysiology of neuropsychiatric disorders such as substance use disorders, major depressive disorder, and schizophrenia.

Developmental exposure to the Parkinson's disease-associated organochlorine pesticide dieldrin alters dopamine neurotransmission in α-synuclein pre-formed fibril (PFF)-injected mice

Parkinson's disease (PD) is the fastest-growing neurological disease worldwide, with increases outpacing aging and occurring most rapidly in recently industrialized areas, suggesting a role of environmental factors. Epidemiological, post-mortem, and mechanistic studies suggest that persistent organic pollutants, including the organochlorine pesticide dieldrin, increase PD risk. In mice, developmental dieldrin exposure causes male-specific exacerbation of neuronal susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and synucleinopathy. Specifically, in the α-synuclein (α-syn) pre-formed fibril (PFF) model, exposure leads to increased deficits in striatal dopamine (DA) turnover and motor deficits on the challenging beam. Here, we hypothesized that alterations in DA handling contribute to the observed changes and assessed vesicular monoamine transporter 2 (VMAT2) function and DA release in this dieldrin/PFF 2-hit model. Female C57BL/6 mice were exposed to 0.3 mg/kg dieldrin or vehicle every 3 days by feeding, starting at 8 weeks of age and continuing throughout breeding, gestation, and lactation. Male offspring from independent litters underwent unilateral, intrastriatal injections of α-syn PFFs at 12 weeks of age, and vesicular 3H-DA uptake assays and fast-scan cyclic voltammetry were performed 4 months post-PFF injection. Dieldrin-induced an increase in DA release in striatal slices in PFF-injected animals, but no change in VMAT2 activity. These results suggest that developmental dieldrin exposure increases a compensatory response to synucleinopathy-triggered striatal DA loss. These findings are consistent with silent neurotoxicity, where developmental exposure to dieldrin primes the nigrostriatal striatal system to have an exacerbated response to synucleinopathy in the absence of observable changes in typical markers of nigrostriatal dysfunction and degeneration.

Adolescent neurostimulation of dopamine circuit reverses genetic deficits in frontal cortex function

Dopamine system dysfunction is implicated in adolescent-onset neuropsychiatric disorders. Although psychosis symptoms can be alleviated by antipsychotics, cognitive symptoms remain unresponsive and novel paradigms investigating the circuit substrates underlying cognitive deficits are critically needed. The frontal cortex and its dopaminergic input from the midbrain are implicated in cognitive functions and undergo maturational changes during adolescence. Here, we used mice carrying mutations in Arc or Disc1 to model mesofrontal dopamine circuit deficiencies and test circuit-based neurostimulation strategies to restore cognitive functions. We found that in a memory-guided spatial navigation task, frontal cortical neurons were activated coordinately at the decision-making point in wild-type but not Arc-/- mice. Chemogenetic stimulation of midbrain dopamine neurons or optogenetic stimulation of frontal cortical dopamine axons in a limited adolescent period consistently reversed genetic defects in mesofrontal innervation, task-coordinated neuronal activity, and memory-guided decision-making at adulthood. Furthermore, adolescent stimulation of dopamine neurons also reversed the same cognitive deficits in Disc1+/- mice. Our findings reveal common mesofrontal circuit alterations underlying the cognitive deficits caused by two different genes and demonstrate the feasibility of adolescent neurostimulation to reverse these circuit and behavioral deficits. These results may suggest developmental windows and circuit targets for treating cognitive deficits in neurodevelopmental disorders.