Distinct regulation of dopamine D2S and D2L autoreceptor signaling by calcium

Alterations in surface-based amplitude of low-frequency fluctuations primary open-angle glaucoma link to neurotransmitter profiling and visual impairment severity

The study aimed to examine alterations in surface-based amplitude of low-frequency fluctuations (ALFF) and fractional amplitude of low-frequency fluctuations (fALFF) in primary open-angle glaucoma (POAG) patients using resting-state functional magnetic resonance imaging (rs-fMRI), and to investigate their relationships with visual function and molecular profiling. A total of 70 POAG patients and 45 age- and sex-matched healthy controls (HCs) underwent rs-fMRI scans. The differences between POAG and HCs groups were compared by two-sample t-test. Spearman's correlation analyses assessed the relationship between ALFF/fALFF values and ophthalmic parameters. Spatial correlation analysis of the patients-control difference map with brain imaging data further explores underlying neurobiological mechanisms. POAG patients displayed altered brain activity compared to HCs, including decreased ALFF/fALFF in the visual network and increased in the frontoparietal and default mode networks. They exhibited reduced fALFF in the somatomotor network and increased ALFF in the dorsal and ventral attention networks. These changes are linked to neurotransmitter systems, with fALFF particularly associated with the dopamine system. Moreover, the altered ALFF/fALFF in brain regions related to vision and attention - the occipital lobe, temporal lobe, parietal lobe, paracentral lobule, and frontal lobe correlated with ophthalmic examination parameters. Surface-based ALFF/fALFF in POAG decreased in visual processing regions and increased in brain regions related to cognitive control, working memory, and attention. These changes were linked to neurotransmitter distributions important for emotional stability and mental health, potentially informing treatment approaches for POAG patients.

Morphological and functional decline of the SNc in a model of progressive parkinsonism

The motor symptoms of Parkinson's Disease are attributed to the degeneration of dopamine neurons in the substantia nigra pars compacta (SNc). Previous work in the MCI-Park mouse model has suggested that the loss of somatodendritic dopamine transmission predicts the development of motor deficits. In the current study, brain slices from MCI-Park mice were used to investigate dopamine signaling in the SNc prior to and through the onset of movement deficits. Electrophysiological properties were impaired by p30 and somatic volume was decreased at all time points. The D2 receptor activated potassium current evoked by quinpirole was present initially, but declined after p30. In contrast, D2-IPSCs were absent at all time points. The decrease in GPCR-mediated inhibition was met with increased spontaneous GABAA signaling. Dendro-dendritic synapses are identified as an early locus of dysfunction in response to bioenergetic decline and suggest that dendritic release sites may contribute to the induction of degeneration.

A mathematical model for the role of dopamine-D2 self-regulation in the production of ultradian rhythms

Many self-motivated and goal-directed behaviours display highly flexible, approximately 4 hour ultradian (shorter than a day) oscillations. Despite lacking direct correspondence to physical cycles in the environment, these ultradian rhythms may be involved in optimizing functional interactions with the environment and reflect intrinsic neural dynamics. Current evidence supports a role of mesostriatal dopamine (DA) in the expression and propagation of ultradian rhythmicity, however, the biochemical processes underpinning these oscillations remain to be identified. Here, we use a mathematical model to investigate D2 autoreceptor-dependent DA self-regulation as the source of ultradian behavioural rhythms. DA concentration at the midbrain-striatal synapses is governed through a dual-negative feedback-loop structure, which naturally gives rise to rhythmicity. This model shows the propensity of striatal DA to produce an ultradian oscillation characterized by a flexible period that is highly sensitive to parameter variations. Circadian (approximately 24 hour) regulation consolidates the ultradian oscillations and alters their response to the phase-dependent, rapid-resetting effect of a transient excitatory stimulus. Within a circadian framework, the ultradian rhythm orchestrates behavioural activity and enhances responsiveness to an external stimulus. This suggests a role for the circadian-ultradian timekeeping hierarchy in governing organized behaviour and shaping daily experience through coordinating the motivation to engage in recurring, albeit not highly predictable events, such as social interactions.

The influence of dopamine autoreceptors on temperament and addiction risk

As a major regulator of dopamine (DA), DA autoreceptors (DAARs) exert substantial influence over DA-mediated behaviors. This paper reviews the physiological and behavioral impact of DAARs. Individual differences in DAAR functioning influences temperamental traits such as novelty responsivity and impulsivity, both of which are associated with vulnerability to addictive behavior in animal models and a broad array of externalizing behaviors in humans. DAARs additionally impact the response to psychostimulants and other drugs of abuse. Human PET studies of D2-like receptors in the midbrain provide evidence for parallels to the animal literature. These data lead to the proposal that weak DAAR regulation is a risk factor for addiction and externalizing problems. The review highlights the potential to build translational models of the functional role of DAARs in behavior. It also draws attention to key limitations in the current literature that would need to be addressed to further advance a weak DAAR regulation model of addiction and externalizing risk.

G Protein-Coupled Receptor Kinase 2 Selectively Enhances β-Arrestin Recruitment to the D2 Dopamine Receptor through Mechanisms That Are Independent of Receptor Phosphorylation

The D2 dopamine receptor (D2R) signals through both G proteins and β-arrestins to regulate important physiological processes, such as movement, reward circuitry, emotion, and cognition. β-arrestins are believed to interact with G protein-coupled receptors (GPCRs) at the phosphorylated C-terminal tail or intracellular loops. GPCR kinases (GRKs) are the primary drivers of GPCR phosphorylation, and for many receptors, receptor phosphorylation is indispensable for β-arrestin recruitment. However, GRK-mediated receptor phosphorylation is not required for β-arrestin recruitment to the D2R, and the role of GRKs in D2R-β-arrestin interactions remains largely unexplored. In this study, we used GRK knockout cells engineered using CRISPR-Cas9 technology to determine the extent to which β-arrestin recruitment to the D2R is GRK-dependent. Genetic elimination of all GRK expression decreased, but did not eliminate, agonist-stimulated β-arrestin recruitment to the D2R or its subsequent internalization. However, these processes were rescued upon the re-introduction of various GRK isoforms in the cells with GRK2/3 also enhancing dopamine potency. Further, treatment with compound 101, a pharmacological inhibitor of GRK2/3 isoforms, decreased β-arrestin recruitment and receptor internalization, highlighting the importance of this GRK subfamily for D2R-β-arrestin interactions. These results were recapitulated using a phosphorylation-deficient D2R mutant, emphasizing that GRKs can enhance β-arrestin recruitment and activation independently of receptor phosphorylation.

Endocytosis of dopamine receptor: Signaling in brain

This chapter describes the physiological significance of dopamine receptor endocytosis and the consequence of the receptor signaling. Endocytosis of dopamine receptors is regulated by many components such as clathrin, β-arrestin, caveolin, and Rab family proteins. The dopamine receptors escape from lysosomal digestion, and their recycling occurs rapidly, reinforcing the dopaminergic signal transduction. In addition, the pathological impact of the receptors interacting with specific proteins has been the focus of much attention. Based on this background, this chapter provides an in-depth understanding of the mechanisms of molecules interacting with dopamine receptors and discusses the potential pharmacotherapeutic targets for α-synucleinopathies and neuropsychiatric disorders.

Co-activation of GPCRs facilitate GIRK-dependent current

The activity of dopamine neurons is dependent on both intrinsic properties and afferent projections. One potent form of inhibition is mediated by the activation of two inhibitory G protein-coupled receptors, D2 and GABA_B receptors. Each of these receptors activates G protein-coupled inwardly rectifying potassium (GIRK) channels. Recordings in brain slices have shown that co-activation using saturating concentrations of agonists results in occlusion of the GIRK current. The present study examined the interaction between D2 and GABA_B receptors using transient applications of sub-saturating concentrations of agonists where the co-application of one agonist resulted in both facilitation and inhibition (desensitization) of the other. The heterologous facilitation was modelled based on the known cooperative interaction between the G protein βγ subunits and GIRK channels. The results indicate that a low tonic level of G βγ results in facilitation of GIRK current and a high level of G βγ results in occlusion. The kinetics of the current induced by transient receptor activation is prolonged in each case. The results suggest that the cooperative interaction between G βγ subunits and GIRK channels determines both the amplitude and kinetics of GPCR-dependent current. KEY POINTS: Inhibitory D2 and GABA_B receptors modulate dopamine neuron activity through shared G protein-coupled inwardly rectifying potassium (GIRK) channels. This study reports robust bidirectional interactions between these two converging receptor pathways. Coincident activation of D2 and GABA_B receptors leads to facilitation of GIRK channel currents, augmenting both amplitude and prolonging the duration of phasic responses. Activation of either D2 or GABA_B receptors also acutely desensitized the GIRK channel current induced by D2 receptor activation that rapidly recovers following termination of desensitizing stimulus. Results demonstrate that the activity of either G protein-coupled receptor system must be considered in the context of other G protein-coupled receptors.

Diversity and specificity in location-based signaling outputs of neuronal GPCRs

The common mechanisms by which members of the G protein-coupled receptor (GPCR) family respond to neurotransmitters in the brain have been well studied. However, it is becoming increasingly clear that GPCRs show great diversity in their intracellular location, interacting partners and effectors, and signaling consequences. Here we will discuss recent studies on the diversity of location, effectors, and signaling of GPCRs, and how these could interact to generate specific spatiotemporal patterns of GPCR signaling in cells.

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.

G protein-coupled receptor kinase-2 confers isoform-specific calcium sensitivity to dopamine D2 receptor desensitization

The dopamine D₂ receptor (D₂ R) functions as an autoreceptor on dopaminergic cell bodies and terminals and as a postsynaptic receptor on a variety of neurons in the central nervous system. As a result of alternative splicing, the D₂ R is expressed as two isoforms: long (D_2L R) and short (D_2S R) differing by a stretch of 29 residues in the third intracellular loop, with D_2S R being the predominant presynaptic isoform. Recent reports described a Ca²⁺ sensitivity of the desensitization time course of potassium currents elicited via D_2S R, but not via D_2L R, when either isoform was selectively expressed in dopaminergic neurons. Here, we aimed to study the mechanism behind this subtype-specific Ca²⁺ sensitivity. Thus, we measured the desensitization of potassium channel responses evoked by D_2L R and D_2S R using two-electrode voltage clamp in Xenopus oocytes in the absence and presence of different amounts of β-arrestin2 and G protein-coupled receptor kinase-2 (GRK2), both of which are known to play important roles in D₂ R desensitization in native cells. We found that co-expression of both GRK2 and β-arrestin2 was necessary for reconstitution of the Ca²⁺ sensitivity of D_2S R desensitization, while D_2L R did not display Ca²⁺ sensitivity under these conditions. The effect of Ca²⁺ chelation by BAPTA-AM to slow the rate of D_2S R desensitization was mimicked by the GRK2 inhibitor, Cmpd101, and by the kinase-inactivating GRK2 mutation, K220R, but not by the PKC inhibitor, Gö6976, nor by the calmodulin antagonist, KN-93. Thus, Ca²⁺ -sensitive desensitization of D_2S R appears to be mediated via a GRK2 phosphorylation-dependent mechanism.

Signaling-Biased and Constitutively Active Dopamine D2 Receptor Variant

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. Compared to the wild type D2 receptor, the novel allelic variant D2-I²¹²F activates a Gα_i1β₁γ₂ heterotrimer with higher potency and modestly enhanced basal activity in human embryonic kidney (HEK) 293 cells and has decreased capacity to recruit arrestin3. We now report that omitting overexpressed G protein-coupled receptor kinase-2 (GRK2) decreased the potency and efficacy of quinpirole for arrestin recruitment. The relative efficacy of quinpirole for arrestin recruitment to D2-I²¹²F compared to D2-WT was considerably lower without overexpressed GRK2 than with added GRK2. D2-I²¹²F exhibited higher basal activation of Gα_oA than Gα_i1 but little or no increase in the potency of quinpirole relative to D2-WT. Other signs of D2-I²¹²F constitutive activity for G protein-mediated signaling, in addition to basal activation of Gα_i/o, were enhanced basal inhibition of forskolin-stimulated cyclic AMP accumulation that was reversed by the inverse agonists sulpiride and spiperone and a ∼4-fold increase in the apparent affinity of D2-I²¹²F for quinpirole, determined from competition binding assays. In mouse midbrain slices, inhibition of tonic current by the inverse agonist sulpiride in dopamine neurons expressing D2-I²¹²F was consistent with our hypothesis of enhanced constitutive activity and sensitivity to dopamine relative to D2-WT. Molecular dynamics simulations with D2 receptor models suggested that an ionic lock between the cytoplasmic ends of the third and sixth α-helices that constrains many G protein-coupled receptors in an inactive conformation spontaneously breaks in D2-I²¹²F. Overall, these results confirm that D2-I²¹²F is a constitutively active and signaling-biased D2 receptor mutant and also suggest that the effect of the likely pathogenic variant in a given brain region will depend on the nature of G protein and GRK expression.

Cortical mediation of relationships between dopamine receptor D2 and cognition is absent in youth at risk of bipolar disorder

Bipolar disorder is associated with cognitive deficits and cortical changes for which the developmental dynamics are not well understood. The dopamine D2 receptor (DRD2) gene has been associated with both psychiatric disorders and cognitive variability. Here we examined the mediating role of brain structure in the relationship between DRD2 genomic variation and cognitive performance, with target cortical regions selected based on evidence of association with DRD2, bipolar disorder and/or cognition from prior literature. Participants (n = 143) were aged 12-30 years and comprised 62 first-degree relatives of bipolar patients (deemed 'at-risk'), 55 controls, and 26 patients with established bipolar disorder; all were unrelated Caucasian individuals with complete data across the three required modalities (structural magnetic resonance imaging, neuropsychological and genetic data). A DRD2 haplotype was derived from three functional polymorphisms (rs1800497, rs1076560, rs2283265) associated with alternative splicing (i.e., D2-short/-long isoforms). Moderated mediation analyses explored group differences in relationships between this DRD2 haplotype, three structural brain networks which subsume the identified cortical regions of interest (frontoparietal, dorsal-attention, and ventral-attention), and three cognitive indices (intelligence, attention, and immediate memory). Controls who were homozygous for the DRD2 major haplotype demonstrated greater cognitive performance as a result of dorsal-attention network mediation. However, this association was absent in the 'at-risk' group. This study provides the first evidence of a functional DRD2-brain-cognition pathway. The absence of typical brain-cognition relationships in young 'at-risk' individuals may reflect biological differences that precede illness onset. Further insight into early pathogenic processes may facilitate targeted early interventions.

Differential roles of two isoforms of dopamine D2 receptors in l-dopa-induced abnormal involuntary movements in mice

l-dopa and dopamine D2 receptor (D2R) agonists are commonly used to relieve the motor deficits of Parkinson's disease. However, long-term treatment with l-dopa or D2R agonists can induce adverse effects such as abnormal involuntary movements (AIMs), which are major limiting factors in achieving long-term control of parkinsonian syndromes. The pathophysiological mechanisms involved in the development of dopaminergic agonist-induced adverse effects are not well understood. Here, we examined the role of two D2R isoforms, D2S and D2L, in l-dopa-induced AIMs using dopamine D2L knockout (D2L KO) mice (expressing purely D2S) and wild-type mice (expressing predominantly D2L). We found that D2L KO mice displayed markedly enhanced AIMs in response to chronic treatment of l-dopa compared to wild-type mice. The l-dopa-induced enhancement of AIMs in D2L KO mice was significantly reduced by the D2R antagonist eticlopride. D2L KO mice also displayed markedly enhanced AIMs in response to chronic treatment with quinpirole, a preferential D2R agonist. These results suggest that D2S contributes more than D2L to dopaminergic agonist-induced AIMs. Our findings may uncover a new factor that contributes to the pathophysiology of dopaminergic drug-induced AIMs, a characteristic manifestation of dyskinesia and also present in psychosis. There is a possibility that the increased ratio of D2S to D2L in the brain plays a significant role in the development of AIM side effects induced by l-dopa or D2R agonists. See Video Abstract, http://links.lww.com/WNR/A622.

Multi-Electrode Array Analysis Identifies Complex Dopamine Responses and Glucose Sensing Properties of Substantia Nigra Neurons in Mouse Brain Slices

Dopaminergic (DA) midbrain neurons within the substantia nigra (SN) display an autonomous pacemaker activity that is crucial for dopamine release and voluntary movement control. Their progressive degeneration is a hallmark of Parkinson's disease. Their metabolically demanding activity-mode affects Ca²⁺ homeostasis, elevates metabolic stress, and renders SN DA neurons particularly vulnerable to degenerative stressors. Accordingly, their activity is regulated by complex mechanisms, notably by dopamine itself, via inhibitory D2-autoreceptors and the neuroprotective neuronal Ca²⁺ sensor NCS-1. Analyzing regulation of SN DA neuron activity-pattern is complicated by their high vulnerability. We studied this activity and its control by dopamine, NCS-1, and glucose with extracellular multi-electrode array (MEA) recordings from midbrain slices of juvenile and adult mice. Our tailored MEA- and spike sorting-protocols allowed high throughput and long recording times. According to individual dopamine-responses, we identified two distinct SN cell-types, in similar frequency: dopamine-inhibited and dopamine-excited neurons. Dopamine-excited neurons were either silent in the absence of dopamine, or they displayed pacemaker-activities, similar to that of dopamine-inhibited neurons. Inhibition of pacemaker-activity by dopamine is typical for SN DA neurons, and it can undergo prominent desensitization. We show for adult mice, that the number of SN DA neurons with desensitized dopamine-inhibition was increased (~60–100%) by a knockout of NCS-1, or by prevention of NCS-1 binding to D2-autoreceptors, while time-course and degrees of desensitization were not altered. The number of neurons with desensitized D2-responses was also higher (~65%) at high glucose-levels (25 mM), compared to lower glucose (2.5 mM), while again desensitization-kinetics were unaltered. However, spontaneous firing-rates were significantly higher at high glucose-levels (~20%). Moreover, transient glucose-deprivation (1 mM) induced a fast and fully-reversible pacemaker frequency reduction. To directly address and quantify glucose-sensing properties of SN DA neurons, we continuously monitored their electrical activity, while altering extracellular glucose concentrations stepwise from 0.5 mM up to 25 mM. SN DA neurons were excited by glucose, with EC₅₀ values ranging from 0.35 to 2.3 mM. In conclusion, we identified a novel, common subtype of dopamine-excited SN neurons, and a complex, joint regulation of dopamine-inhibited neurons by dopamine and glucose, within the range of physiological brain glucose-levels.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Dopamine D2 autoreceptor interactome: Targeting the receptor complex as a strategy for treatment of substance use disorder

Dopamine D2 autoreceptors (D2ARs), located in somatodendritic and axon terminal compartments of dopamine (DA) neurons, function to provide a negative feedback regulatory control on DA neuron firing, DA synthesis, reuptake and release. Dysregulation of D2AR-mediated DA signaling is implicated in vulnerability to substance use disorder (SUD). Due to the extreme low abundance of D2ARs compared to postsynaptic D2 receptors (D2PRs) and the lack of experimental tools to differentiate the signaling of D2ARs from D2PRs, the regulation of D2ARs by drugs of abuse is poorly understood. The recent availability of conditional D2AR knockout mice and newly developed virus-mediated gene delivery approaches have provided means to specifically study the function of D2ARs at the molecular, cellular and behavioral levels. There is a growing revelation of novel mechanisms and new proteins that mediate D2AR activity, suggesting that D2ARs act cooperatively with an array of membrane and intracellular proteins to tightly control DA transmission. This review highlights D2AR-interacting partners including transporters, G-protein-coupled receptors, ion channels, intracellular signaling modulators, and protein kinases. The complexity of the D2AR interaction network illustrates the functional divergence of D2ARs. Pharmacological targeting of multiple D2AR-interacting partners may be more effective to restore disrupted DA homeostasis by drugs of abuse.

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.

Attenuated dopamine receptor signaling in nucleus accumbens core in a rat model of chemically-induced neuropathy

Neuropathy is major source of chronic pain that can be caused by mechanically or chemically induced nerve injury. Intraplantar formalin injection produces local necrosis over a two-week period and has been used to model neuropathy in rats. To determine whether neuropathy alters dopamine (DA) receptor responsiveness in mesolimbic brain regions, we examined dopamine D₁-like and D₂-like receptor (D_1/2R) signaling and expression in male rats 14 days after bilateral intraplantar formalin injections into both rear paws. D₂R-mediated G-protein activation and expression of the D₂R long, but not short, isoform were reduced in nucleus accumbens (NAc) core, but not in NAc shell, caudate-putamen or ventral tegmental area of formalin- compared to saline-treated rats. In addition, D₁R-stimulated adenylyl cyclase activity was also reduced in NAc core, but not in NAc shell or prefrontal cortex, of formalin-treated rats, whereas D₁R expression was unaffected. Other proteins involved in dopamine neurotransmission, including dopamine uptake transporter and tyrosine hydroxylase, were unaffected by formalin treatment. In behavioral tests, the potency of a D₂R agonist to suppress intracranial self-stimulation (ICSS) was decreased in formalin-treated rats, whereas D₁R agonist effects were not altered. The combination of reduced D₂R expression and signaling in NAc core with reduced suppression of ICSS responding by a D₂R agonist suggest a reduction in D₂ autoreceptor function. Altogether, these results indicate that intraplantar formalin produces attenuation of highly specific DA receptor signaling processes in NAc core of male rats and suggest the development of a neuropathy-induced allostatic state in both pre- and post-synaptic DA receptor function.

AMP-activated protein kinase slows D2 dopamine autoreceptor desensitization in substantia nigra neurons

Dopamine neurons in the substantia nigra zona compacta (SNC) are well known to express D2 receptors. When dopamine is released from somatodendritic sites, activation of D2 autoreceptors suppresses dopamine neuronal activity through activation of G protein-coupled K⁺ channels. AMP-activated protein kinase (AMPK) is a master enzyme that acts in somatic tissues to suppress energy expenditure and encourage energy production. We hypothesize that AMPK may also conserve energy in central neurons by reducing desensitization of D2 autoreceptors. We used whole-cell patch-clamp recordings to study the effects of AMPK activators and inhibitors on D2 autoreceptor-mediated current in SNC neurons in midbrain slices from rat pups (11-23 days post-natal). Slices were superfused with 100 μM dopamine or 30 μM quinpirole for 25 min, which evoked outward currents that decayed slowly over time. Although the AMPK activators A769662 and ZLN024 significantly slowed rundown of dopamine-evoked current, slowing of quinpirole-evoked current required the presence of a D1-like agonist (SKF38393). Moreover, the D1-like agonist also slowed the rundown of quinpirole-induced current even in the absence of an AMPK activator. Pharmacological antagonist experiments showed that the D1-like agonist effect required activation of either protein kinase A (PKA) or exchange protein directly activated by cAMP 2 (Epac2) pathways. In contrast, the effect of AMPK on rundown of current evoked by quinpirole plus SKF38393 required PKA but not Epac2. We conclude that AMPK slows D2 autoreceptor desensitization by augmenting the effect of D1-like receptors.

Which Dopamine Polymorphisms Are Functional? Systematic Review and Meta-analysis of COMT, DAT, DBH, DDC, DRD1-5, MAOA, MAOB, TH, VMAT1, and VMAT2

BACKGROUND

Many polymorphisms in dopamine genes are reported to affect cognitive, imaging, or clinical phenotypes. It is often inferred or assumed that such associations are causal, mediated by a direct effect of the polymorphism on the gene product itself. However, the supporting evidence is not always clear.

METHODS

We conducted systematic reviews and meta-analyses to assess the empirical evidence for functional polymorphisms in genes encoding dopaminergic enzymes (COMT, DBH, DDC, MAOA, MAOB, and TH), dopamine receptors (DRD1, DRD2, DRD3, DRD4, and DRD5), the dopamine transporter (DAT), and vesicular transporters (VMAT1 and VMAT2). We defined functionality as an effect of the polymorphism on the expression, abundance, activity, or affinity of the gene product.

RESULTS

We screened 22,728 articles and identified 255 eligible studies. We found robust and medium to large effects for polymorphisms in 4 genes. For catechol-O-methyltransferase (COMT), the Val¹⁵⁸Met polymorphism (rs4680) markedly affected enzyme activity, protein abundance, and protein stability. Dopamine β-hydroxylase (DBH) activity was associated with rs1611115, rs2519152, and the DBH-STR polymorphism. Monoamine oxidase A (MAOA) activity was associated with a 5' VNTR polymorphism. Dopamine D₂ receptor (DRD2) binding was influenced by the Taq1A (rs1800497) polymorphism, and rs1076560 affected DRD2 splicing.

CONCLUSIONS

Some widely studied dopaminergic polymorphisms clearly and substantially affect the abundance or activity of the encoded gene product. However, for other polymorphisms, evidence of such an association is negative, inconclusive, or lacking. These findings are relevant when selecting polymorphisms as "markers" of dopamine function, and for interpreting the biological plausibility of associations between these polymorphisms and aspects of brain function or dysfunction.

Calcium, Dopamine and Neuronal Calcium Sensor 1: Their Contribution to Parkinson's Disease

Parkinson’s disease (PD) is a debilitating neurodegenerative disorder characterized by loss of dopaminergic neurons in the substantia nigra pars compacta. The causes of PD in humans are still unknown, although metabolic characteristics of the neurons affected by the disease have been implicated in their selective susceptibility. Mitochondrial dysfunction and proteostatic stress are recognized to be important in the pathogenesis of both familial and sporadic PD, and they both culminate in bioenergetic deficits. Exposure to calcium overload has recently emerged as a key determinant, and pharmacological treatment that inhibits Ca²⁺ entry diminishes neuronal damage in chemical models of PD. In this review, we first introduce general concepts on neuronal Ca²⁺ signaling and then summarize the current knowledge on fundamental properties of substantia nigra pars compacta dopaminergic neurons, on the role of the interplay between Ca²⁺ and dopamine signaling in neuronal activity and susceptibility to cell death. We also discuss the possible involvement of a “neglected” player, the Neuronal Calcium Sensor-1 (NCS-1), which has been shown to participate to dopaminergic signaling by regulating dopamine dependent receptor desensitization in normal brain but, data supporting a direct role in PD pathogenesis are still missing. However, it is intriguing to speculate that the Ca²⁺-dependent modulation of NCS-1 activity could eventually counteract dopaminergic neurons degeneration.

Hypofunctional Dopamine Uptake and Antipsychotic Treatment-Resistant Schizophrenia

Antipsychotic treatment resistance in schizophrenia remains a major issue in psychiatry. Nearly 30% of patients with schizophrenia do not respond to antipsychotic treatment, yet the underlying neurobiological causes are unknown. All effective antipsychotic medications are thought to achieve their efficacy by targeting the dopaminergic system. Here we review early literature describing the fundamental mechanisms of antipsychotic drug efficacy, highlighting mechanistic concepts that have persisted over time. We then reconsider the original framework for understanding antipsychotic efficacy in light of recent advances in our scientific understanding of the dopaminergic effects of antipsychotics. Based on these new insights, we describe a role for the dopamine transporter in the genesis of both antipsychotic therapeutic response and primary resistance. We believe that this discussion will help delineate the dopaminergic nature of antipsychotic treatment-resistant schizophrenia.

Gi/o protein-coupled receptors in dopamine neurons inhibit the sodium leak channel NALCN

Dopamine (D2) receptors provide autoinhibitory feedback onto dopamine neurons through well-known interactions with voltage-gated calcium channels and G protein-coupled inwardly-rectifying potassium (GIRK) channels. Here, we reveal a third major effector involved in D2R modulation of dopaminergic neurons - the sodium leak channel, NALCN. We found that activation of D2 receptors robustly inhibits isolated sodium leak currents in wild-type mice but not in NALCN conditional knockout mice. Intracellular GDP-βS abolished the inhibition, indicating a G protein-dependent signaling mechanism. The application of dopamine reliably slowed pacemaking even when GIRK channels were pharmacologically blocked. Furthermore, while spontaneous activity was observed in nearly all dopaminergic neurons in wild-type mice, neurons from NALCN knockouts were mainly silent. Both observations demonstrate the critical importance of NALCN for pacemaking in dopaminergic neurons. Finally, we show that GABA-B receptor activation also produces inhibition of NALCN-mediated currents. Therefore, we identify NALCN as a core effector of inhibitory G protein-coupled receptors.

Synaptic and intrinsic plasticity in the ventral tegmental area after chronic cocaine

Cocaine exposure induces persistent changes in synaptic transmission and intrinsic properties of ventral tegmental area (VTA) dopamine neurons. Despite significant progress in understanding cocaine-induced plasticity, an effective treatment of cocaine addiction is lacking. Chronic cocaine potentiates excitatory and alters inhibitory transmission to dopamine neurons, induces dopamine neuron hyperexcitability, and reduces dopamine release in projection areas. Understanding how intrinsic and synaptic plasticity interact to control dopamine neuron firing and dopamine release could prove useful in the development of new therapeutics. In this review, we examine recent literature discussing cocaine-induced plasticity in the VTA and highlight potential therapeutic interventions.

The Evolving Understanding of Dopamine Neurons in the Substantia Nigra and Ventral Tegmental Area

In recent years, the population of neurons in the ventral tegmental area (VTA) and substantia nigra (SN) has been examined at multiple levels. The results indicate that the projections, neurochemistry, and receptor and ion channel expression in this cell population vary widely. This review centers on the intrinsic properties and synaptic regulation that control the activity of dopamine neurons. Although all dopamine neurons fire action potentials in a pacemaker pattern in the absence of synaptic input, the intrinsic properties that underlie this activity differ considerably. Likewise, the transition into a burst/pause pattern results from combinations of intrinsic ion conductances, inhibitory and excitatory synaptic inputs that differ among this cell population. Finally, synaptic plasticity is a key regulator of the rate and pattern of activity in different groups of dopamine neurons. Through these fundamental properties, the activity of dopamine neurons is regulated and underlies the wide-ranging functions that have been attributed to dopamine.

Differential regulation of striatal motor behavior and related cellular responses by dopamine D2L and D2S isoforms

The dopamine D2 receptor (D2R) is a major component of the dopamine system. D2R-mediated signaling in dopamine neurons is involved in the presynaptic regulation of dopamine levels. Postsynaptically, i.e., in striatal neurons, D2R signaling controls complex functions such as motor activity through regulation of cell firing and heterologous neurotransmitter release. The presence of two isoforms, D2L and D2S, which are generated by a mechanism of alternative splicing of the Drd2 gene, raises the question of whether both isoforms may equally control presynaptic and postsynaptic events. Here, we addressed this question by comparing behavioral and cellular responses of mice with the selective ablation of either D2L or D2S isoform. We establish that the presence of either D2L or D2S can support postsynaptic functions related to the control of motor activity in basal conditions. On the contrary, absence of D2S but not D2L prevents the inhibition of tyrosine hydroxylase phosphorylation and, thereby, of dopamine synthesis, supporting a major presynaptic role for D2S. Interestingly, boosting dopamine signaling in the striatum by acute cocaine administration reveals that absence of D2L, but not of D2S, strongly impairs the motor and cellular response to the drug, in a manner similar to the ablation of both isoforms. These results suggest that when the dopamine system is challenged, D2L signaling is required for the control of striatal circuits regulating motor activity. Thus, our findings show that D2L and D2S share similar functions in basal conditions but not in response to stimulation of the dopamine system.

Calcium Release from Stores Inhibits GIRK

Synaptic transmission is mediated by ionotropic and metabotropic receptors that together regulate the rate and pattern of action potential firing. Metabotropic receptors can activate ion channels and modulate other receptors and channels. The present paper examines the interaction between group 1 mGluR-mediated calcium release from stores and GABA_B/D2-mediated GIRK currents in rat dopamine neurons of the Substantia Nigra. Transient activation of mGluRs decreased the GIRK current evoked by GABA_B and D2 receptors, although less efficaciously for D2. The mGluR-induced inhibition of GIRK current peaked in 1 s and recovered to baseline after 5 s. The inhibition was dependent on release of calcium from stores, was larger for transient than for tonic currents, and was unaffected by inhibitors of PLC, PKC, PLA₂, or calmodulin. This inhibition of GABA_B IPSCs through release of calcium from stores is a postsynaptic mechanism that may broadly reduce GIRK-dependent inhibition of many central neurons.

Cocaine-induced adaptation of dopamine D2S, but not D2L autoreceptors

The dopamine D2 receptor has two splice variants, D2S (Short) and D2L (Long). In dopamine neurons, both variants can act as autoreceptors to regulate neuronal excitability and dopamine release, but the roles of each variant are incompletely characterized. In a previous study we used viral receptor expression in D2 receptor knockout mice to show distinct effects of calcium signaling on D2S and D2L autoreceptor function (Gantz et al., 2015). However, the cocaine-induced plasticity of D2 receptor desensitization observed in wild type mice was not recapitulated with this method of receptor expression. Here we use mice with genetic knockouts of either the D2S or D2L variant to investigate cocaine-induced plasticity in D2 receptor signaling. Following a single in vivo cocaine exposure, the desensitization of D2 receptors from neurons expressing only the D2S variant was reduced. This did not occur in D2L-expressing neurons, indicating differential drug-induced plasticity between the variants.

Junk food diet-induced obesity increases D2 receptor autoinhibition in the ventral tegmental area and reduces ethanol drinking

Similar to drugs of abuse, the hedonic value of food is mediated, at least in part, by the mesostriatal dopamine (DA) system. Prolonged intake of either high calorie diets or drugs of abuse both lead to a blunting of the DA system. Most studies have focused on DAergic alterations in the striatum, but little is known about the effects of high calorie diets on ventral tegmental area (VTA) DA neurons. Since high calorie diets produce addictive-like DAergic adaptations, it is possible these diets may increase addiction susceptibility. However, high calorie diets consistently reduce psychostimulant intake and conditioned place preference in rodents. In contrast, high calorie diets can increase or decrease ethanol drinking, but it is not known how a junk food diet (cafeteria diet) affects ethanol drinking. In the current study, we administered a cafeteria diet consisting of bacon, potato chips, cheesecake, cookies, breakfast cereals, marshmallows, and chocolate candies to male Wistar rats for 3–4 weeks, producing an obese phenotype. Prior cafeteria diet feeding reduced homecage ethanol drinking over 2 weeks of testing, and transiently reduced sucrose and chow intake. Importantly, cafeteria diet had no effect on ethanol metabolism rate or blood ethanol concentrations following 2g/kg ethanol administration. In midbrain slices, we showed that cafeteria diet feeding enhances DA D2 receptor (D2R) autoinhibition in VTA DA neurons. These results show that junk food diet-induced obesity reduces ethanol drinking, and suggest that increased D2R autoinhibition in the VTA may contribute to deficits in DAergic signaling and reward hypofunction observed with obesity.

Prefrontal mRNA expression of long and short isoforms of D2 dopamine receptor: Possible role in delayed learning deficit caused by early life interleukin-1β treatment

Long (D2L) and short (D2S) isoform of the D2 dopamine receptor are believed to play different roles in behavioral regulation. However, little is known about differential regulation of these isoforms mRNA expression during the process of learning in physiological and pathological states. In this study, we have investigated the combined effect of training in active avoidance (AA) paradigm and chronic early life treatment with pro-inflammatory cytokine interleukin (IL)-1β (1μg/kg i.p., P15-21) on D2S and D2L dopamine receptor mRNA expression in the medial prefrontal cortex (mPFC) of adult rats. We have shown differential regulation of D2 short and long mRNA isoform expression in the mPFC. There was no effect of AA-training on D2S mRNA expression, while D2L mRNA was downregulated in AA-trained control (intact and saline-treated) animals, and this effect was not observed in rats treated with IL-1β. D2S mRNA expression level negatively correlated with learning ability within control (saline-treated and intact) groups but not in IL-1β-treated animals. Thus, prefrontal expression of distinct D2 dopamine receptor splice variants is supposed to be implicated in cognitive decline caused by early life immune challenge.

Desensitized D2 autoreceptors are resistant to trafficking

Dendritic release of dopamine activates dopamine D2 autoreceptors, which are inhibitory G protein-coupled receptors (GPCRs), to decrease the excitability of dopamine neurons. This study used tagged D2 receptors to identify the localization and distribution of these receptors in living midbrain dopamine neurons. GFP-tagged D2 receptors were found to be unevenly clustered on the soma and dendrites of dopamine neurons within the substantia nigra pars compacta (SNc). Physiological signaling and desensitization of the tagged receptors were not different from wild type receptors. Unexpectedly, upon desensitization the tagged D2 receptors were not internalized. When tagged D2 receptors were expressed in locus coeruleus neurons, a desensitizing protocol induced significant internalization. Likewise, when tagged µ-opioid receptors were expressed in dopamine neurons they too were internalized. The distribution and lack of agonist-induced internalization of D2 receptors on dopamine neurons indicate a purposefully regulated localization of these receptors.

Restructuring of basal ganglia circuitry and associated behaviors triggered by low striatal D2 receptor expression: implications for substance use disorders

Dopamine D2 receptors (D2Rs) consistently emerge as a critical substrate for the etiology of some major psychiatric disorders. Indeed, a central theory of substance use disorders (SUDs) postulates that a reduction in D2R levels in the striatum is a determining factor that confers vulnerability to abuse substances. A large number of clinical and preclinical studies strongly support this link between SUDs and D2Rs; however, identifying the mechanism by which low D2Rs facilitate SUDs has been hindered by the complexity of circuit connectivity, the heterogeneity of D2R expression and the multifaceted constellation of phenotypes observed in SUD patient. Animal models are well‐suited for understanding the mechanisms because they allow access to the circuitry and the genetic tools that enable a dissection of the D2R heterogeneity. This review discusses recent findings on the functional role of D2Rs and highlights the distinctive contributions of D2Rs expressed on specific neuronal subpopulations to the behavioral responses to stimulant drugs. A circuit‐wide restructuring of local and long‐range inhibitory connectivity within the basal ganglia is observed in response to manipulation of striatal D2R levels and is accompanied by multiple alterations in dopamine‐dependent behaviors. Collectively, these new findings provide compelling evidence for a critical role of striatal D2Rs in shaping basal ganglia connectivity; even among neurons that do not express D2Rs. These findings from animal models have deep clinical implications for SUD patients with low levels D2R availability where a similar restructuring of basal ganglia circuitry is expected to take place.

AT1-receptor response to non-saturating Ang-II concentrations is amplified by calcium channel blockers

BACKGROUND

Blockers of angiotensin II type 1 receptor (AT₁R) and the voltage gated calcium channel 1.2 (Ca_V1.2) are commonly used for treatment of hypertension. Yet there is little information about the effect of physiological concentrations of angiotensin II (AngII) on AT₁R signaling and whether there is a reciprocal regulation of AT₁R signaling by Ca_V1.2.

METHODS

To elucidate these questions, we have studied the Ca²⁺ signaling response to physiological and pharmacological AngII doses in HEK293a cells, vascular smooth muscle cells and cardiomyocytes using a Ca²⁺ sensitive dye as the principal sensor. Intra-cellular calcium recordings were performed in presence and absence of Ca_V1.2 blockers. Semi-quantitative imaging methods were used to assess the plasma membrane expression of AT₁R and G-protein activation.

RESULTS

Repeated exposure to pharmacological (100 nM) concentrations of AngII caused, as expected, a down-regulation of the Ca²⁺ response. In contrast, repeated exposure to physiological (1 nM) AngII concentration resulted in an enhancement of the Ca²⁺ response. The up-regulation of the Ca²⁺ response to repeated 1 nM AngII doses and the down-regulation of the Ca²⁺ response to repeated 100 nM Angll doses were not accompanied by a parallel change of the AT₁R plasma membrane expression. The Ca²⁺ response to 1 nM of AngII was amplified in the presence of therapeutic concentrations of the Ca_V1.2 blockers, nifedipine and verapamil, in vascular smooth muscle cells, cardiomyocytes and HEK293a cells. Amplification of the AT₁R response was also observed following inhibition of the calcium permeable transient receptor potential cation channels, suggesting that the activity of AT₁R is sensitive to calcium influx.

CONCLUSIONS

Our findings have implications for the understanding of hyperactivity of the angiotensin system and for use of Ca²⁺ channel blockers as mono-therapy in hypertension.

Dopamine Inhibition Differentially Controls Excitability of Substantia Nigra Dopamine Neuron Subpopulations through T-Type Calcium Channels

While there is growing appreciation for diversity among ventral tegmental area dopamine neurons, much less is known regarding functional heterogeneity among the substantia nigra pars compacta (SNc) neurons. Here, we show that calbindin-positive dorsal tier and calbindin-negative ventral tier SNc dopaminergic neurons in mice comprise functionally distinct subpopulations distinguished by their dendritic calcium signaling, rebound excitation, and physiological responses to dopamine D2-receptor (D2) autoinhibition. While dopamine is known to inhibit action potential backpropagation, our experiments revealed an unexpected enhancement of excitatory responses and dendritic calcium signals in the presence of D2-receptor inhibition. Specifically, dopamine inhibition and direct hyperpolarization enabled the generation of low-threshold depolarizations that occurred in an all-or-none or graded manner, due to recruitment of T-type calcium channels. Interestingly, these effects occurred selectively in calbindin-negative dopaminergic neurons within the SNc. Thus, calbindin-positive and calbindin-negative SNc neurons differ substantially in their calcium channel composition and efficacy of excitatory inputs in the presence of dopamine inhibition.SIGNIFICANCE STATEMENT Substantia nigra dopaminergic neurons can be divided into two populations: the calbindin-negative ventral tier, which is vulnerable to neurodegeneration in Parkinson's disease, and the calbindin-positive dorsal tier, which is relatively resilient. Although tonic firing is similar in these subpopulations, we find that their responses to dopamine-mediated inhibition are strikingly different. During inhibition, calbindin-negative neurons exhibit increased sensitivity to excitatory inputs, which can then trigger large dendritic calcium transients due to strong expression of T-type calcium channels. Therefore, SNc neurons differ substantially in their calcium channel composition, which may contribute to their differential vulnerability. Furthermore, T-currents increase excitation efficacy onto calbindin-negative cells during dopamine inhibition, suggesting that shared inputs are differentially processed in subpopulations resulting in distinct downstream dopamine signals.

G Protein-Gated Potassium Channels: A Link to Drug Addiction

G protein-gated inwardly rectifying potassium (GIRK) channels are regulators of neuronal excitability in the brain. Knockout mice lacking GIRK channels display altered behavioral responses to multiple addictive drugs, implicating GIRK channels in addictive behaviors. Here, we review the effects of GIRK subunit deletions on the behavioral response to psychostimulants, such as cocaine and methamphetamine. Additionally, exposure of mice to psychostimulants produces alterations in the surface expression of GIRK channels in multiple types of neurons within the reward system of the brain. Thus, we compare the subcellular mechanisms by which drug exposure appears to alter GIRK expression in multiple cell types and provide an outlook on future studies examining the role of GIRK channels in addiction. A greater understanding of how GIRK channels are regulated by addictive drugs may enable the development of therapies to prevent or treat drug abuse.

Electrical and Ca(2+) signaling in dendritic spines of substantia nigra dopaminergic neurons

Little is known about the density and function of dendritic spines on midbrain dopamine neurons, or the relative contribution of spine and shaft synapses to excitability. Using Ca(2+) imaging, glutamate uncaging, fluorescence recovery after photobleaching and transgenic mice expressing labeled PSD-95, we comparatively analyzed electrical and Ca(2+) signaling in spines and shaft synapses of dopamine neurons. Dendritic spines were present on dopaminergic neurons at low densities in live and fixed tissue. Uncaging-evoked potential amplitudes correlated inversely with spine length but positively with the presence of PSD-95. Spine Ca(2+) signals were less sensitive to hyperpolarization than shaft synapses, suggesting amplification of spine head voltages. Lastly, activating spines during pacemaking, we observed an unexpected enhancement of spine Ca(2+) midway throughout the spike cycle, likely involving recruitment of NMDA receptors and voltage-gated conductances. These results demonstrate functionality of spines in dopamine neurons and reveal a novel modulation of spine Ca(2+) signaling during pacemaking.

Gastrointestinal dopamine as an anti-incretin and its possible role in bypass surgery as therapy for type 2 diabetes with associated obesity

The objective of this review was to summarize and integrate specific clinical observations from the field of gastric bypass surgery and recent findings in beta cell biology. When considered together, these data sets suggest a previously unrecognized physiological mechanism which may explain how Roux-en-Y gastric bypass (RYGB) surgery mediates the early rapid reversal of hyperglycemia, observed before weight loss, in certain type 2 diabetes mellitus (T2DM) patients. The novel mechanism is based on a recently recognized inhibitory circuit of glucose stimulated insulin secretion driven by DA stored in β-cell vesicles and the gut. We propose that DA and glucagon-like peptide 1 (GLP-1) represent two opposing arms of a glucose stimulated insulin secretion (GSIS) regulatory system and hypothesize that dopamine represents the "anti-incretin" hypothesized to explain the beneficial effects of bariatric surgery on T2DM. These new hypotheses and the research driven by them may directly impact our understanding of: 1) the mechanisms underlying improved glucose homeostasis seen before weight loss following bariatric surgery; and 2) the regulation of glucose stimulated insulin secretion within islets. On a practical level, these studies may result in the development of novels drugs to modulate insulin secretion and/or methods to quantitatively asses in real time beta cell function and mass.