Distinct roles of D1 and D5 dopamine receptors in motor activity and striatal synaptic plasticity

Dopaminergic Input Regulates the Sensitivity of Indirect Pathway Striatal Spiny Neurons to Brain-Derived Neurotrophic Factor

Motor dysfunction in Parkinson's disease (PD) is closely linked to the dopaminergic depletion of striatal neurons and altered synaptic plasticity at corticostriatal synapses. Dopamine receptor D1 (DRD1) stimulation is a crucial step in the formation of long-term potentiation (LTP), whereas dopamine receptor D2 (DRD2) stimulation is needed for the formation of long-term depression (LTD) in striatal spiny projection neurons (SPNs). Tropomyosin receptor kinase B (TrkB) and its ligand brain-derived neurotrophic factor (BDNF) are centrally involved in plasticity regulation at the corticostriatal synapses. DRD1 activation enhances TrkB's sensitivity for BDNF in direct pathway spiny projection neurons (dSPNs). In this study, we showed that the activation of DRD2 in cultured striatal indirect pathway spiny projection neurons (iSPNs) and cholinergic interneurons causes the retraction of TrkB from the plasma membrane. This provides an explanation for the opposing synaptic plasticity changes observed upon DRD1 or DRD2 stimulation. In addition, TrkB was found within intracellular structures in dSPNs and iSPNs from Pitx3-/- mice, a genetic model of PD with early onset dopaminergic depletion in the dorsolateral striatum (DLS). This dysregulated BDNF/TrkB signaling might contribute to the pathophysiology of direct and indirect pathway striatal projection neurons in PD.

Sensory Reinforced Corticostriatal Plasticity

BACKGROUND

Regional changes in corticostriatal transmission induced by phasic dopaminergic signals are an essential feature of the neural network responsible for instrumental reinforcement during discovery of an action. However, the timing of signals that are thought to contribute to the induction of corticostriatal plasticity is difficult to reconcile within the framework of behavioural reinforcement learning, because the reinforcer is normally delayed relative to the selection and execution of causally-related actions.

OBJECTIVE

While recent studies have started to address the relevance of delayed reinforcement signals and their impact on corticostriatal processing, our objective was to establish a model in which a sensory reinforcer triggers appropriately delayed reinforcement signals relayed to the striatum via intact neuronal pathways and to investigate the effects on corticostriatal plasticity.

METHODS

We measured corticostriatal plasticity with electrophysiological recordings using a light flash as a natural sensory reinforcer, and pharmacological manipulations were applied in an in vivo anesthetized rat model preparation.

RESULTS

We demonstrate that the spiking of striatal neurons evoked by single-pulse stimulation of the motor cortex can be potentiated by a natural sensory reinforcer, operating through intact afferent pathways, with signal timing approximating that required for behavioural reinforcement. The pharmacological blockade of dopamine receptors attenuated the observed potentiation of corticostriatal neurotransmission.

CONCLUSION

This novel in vivo model of corticostriatal plasticity offers a behaviourally relevant framework to address the physiological, anatomical, cellular, and molecular bases of instrumental reinforcement learning.

The Impact of Muscarinic Antagonism on Psychosis-Relevant Behaviors and Striatal [11C] Raclopride Binding in Tau Mouse Models of Alzheimer's Disease

Psychosis that occurs over the course of Alzheimer's disease (AD) is associated with increased caregiver burden and a more rapid cognitive and functional decline. To find new treatment targets, studies modeling psychotic conditions traditionally employ agents known to induce psychosis, utilizing outcomes with cross-species relevance, such as locomotive activity and sensorimotor gating, in rodents. In AD, increased burdens of tau pathology (a diagnostic hallmark of the disease) and treatment with anticholinergic medications have, separately, been reported to increase the risk of psychosis. Recent evidence suggests that muscarinic antagonists may increase extracellular tau. Preclinical studies in AD models have not previously utilized muscarinic cholinergic antagonists as psychotomimetic agents. In this report, we utilize a human-mutant-tau model (P301L/COMTKO) and an over-expressed non-mutant human tau model (htau) in order to compare the impact of antimuscarinic (scopolamine 10 mg/kg/day) treatment with dopaminergic (reboxetine 20 mg/kg/day) treatment, for 7 days, on locomotion and sensorimotor gating. Scopolamine increased spontaneous locomotion, while reboxetine reduced it; neither treatment impacted sensorimotor gating. In the P301L/COMTKO, scopolamine treatment was associated with decreased muscarinic M4 receptor expression, as quantified with RNA-seq, as well as increased dopamine receptor D2 signaling, as estimated with Micro-PET [11C] raclopride binding. Scopolamine also increased soluble tau in the striatum, an effect that partially mediated the observed increases in locomotion. Studies of muscarinic agonists in preclinical tau models are warranted to determine the impact of treatment-on both tau and behavior-that may have relevance to AD and other tauopathies.

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

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

Chlorpromazine, an Inverse Agonist of D1R-Like, Differentially Targets Voltage-Gated Calcium Channel (CaV) Subtypes in mPFC Neurons

The dopamine receptor type 1 (D1R) and the dopamine receptor type 5 (D5R), which are often grouped as D1R-like due to their sequence and signaling similarities, exhibit high levels of constitutive activity. The molecular basis for this agonist-independent activation has been well characterized through biochemical and mutagenesis in vitro studies. In this regard, it was reported that many antipsychotic drugs act as inverse agonists of D1R-like constitutive activity. On the other hand, D1R is highly expressed in the medial prefrontal cortex (mPFC), a brain area with important functions such as working memory. Here, we studied the impact of D1R-like constitutive activity and chlorpromazine (CPZ), an antipsychotic drug and D1R-like inverse agonist, on various neuronal Ca_V conductances, and we explored its effect on calcium-dependent neuronal functions in the mouse medial mPFC. Using ex vivo brain slices containing the mPFC and transfected HEK293T cells, we found that CPZ reduces Ca_V2.2 currents by occluding D1R-like constitutive activity, in agreement with a mechanism previously reported by our lab, whereas CPZ directly inhibits Ca_V1 currents in a D1R-like activity independent manner. In contrast, CPZ and D1R constitutive activity did not affect Ca_V2.1, Ca_V2.3, or Ca_V3 currents. Finally, we found that CPZ reduces excitatory postsynaptic responses in mPFC neurons. Our results contribute to understanding CPZ molecular targets in neurons and describe a novel physiological consequence of CPZ non-canonical action as a D1R-like inverse agonist in the mouse brain.

Dichotomous regulation of striatal plasticity by dynorphin

Modulation of corticostriatal plasticity alters the information flow throughout basal ganglia circuits and represents a fundamental mechanism for motor learning, action selection, and reward. Synaptic plasticity in the striatal direct- and indirect-pathway spiny projection neurons (dSPNs and iSPNs) is regulated by two distinct networks of GPCR signaling cascades. While it is well-known that dopamine D2 and adenosine A2a receptors bi-directionally regulate iSPN plasticity, it remains unclear how D1 signaling modulation of synaptic plasticity is counteracted by dSPN-specific Gi signaling. Here, we show that striatal dynorphin selectively suppresses long-term potentiation (LTP) through Kappa Opioid Receptor (KOR) signaling in dSPNs. Both KOR antagonism and conditional deletion of dynorphin in dSPNs enhance LTP counterbalancing with different levels of D1 receptor activation. Behaviorally, mice lacking dynorphin in D1 neurons show comparable motor behavior and reward-based learning, but enhanced flexibility during reversal learning. These findings support a model in which D1R and KOR signaling bi-directionally modulate synaptic plasticity and behavior in the direct pathway.

Biochemical Neuroadaptations in the Rat Striatal Dopaminergic System after Prolonged Exposure to Methamphetamine Self-Administration

Perturbations in striatal dopamine (DA) homeostasis might underlie the behavioral and pathobiological consequences of METH use disorder in humans. To identify potential consequences of long-term METH exposure, we modeled the adverse consequence DSM criterion of substance use disorders by giving footshocks to rats that had escalated their intake of METH during a drug self-administration procedure. Next, DA D1 receptor antagonist, SCH23390 was injected. Thereafter, rats were euthanized to measure several indices of the striatal dopaminergic system. Footshocks split the METH rats into two phenotypes: (i) shock-sensitive that decreased their METH-intake and (ii) shock-resistant that continued their METH intake. SCH23390 caused substantial dose-dependent reduction of METH taking in both groups. Stopping SCH23390 caused re-emergence of compulsive METH taking in shock-resistant rats. Compulsive METH takers also exhibited greater incubation of METH seeking than non-compulsive rats during withdrawal from METH SA. Analyses of DA metabolism revealed non-significant decreases (about 35%) in DA levels in resistant and sensitive rats. However, striatal contents of the deaminated metabolites, DOPAL and DOPAC, were significantly increased in sensitive rats. VMAT2 and DAT protein levels were decreased in both phenotypes. Moreover, protein expression levels of the D1-like DA receptor, D5R, and D2-like DA receptors, D3R and D4R, were significantly decreased in the compulsive METH takers. Our results parallel findings in post-mortem striatal tissues of human METH users who develop Parkinsonism after long-term METH intake and support the use of this model to investigate potential therapeutic interventions for METH use disorder.

Chronic early-life lead exposure sensitizes adolescent rats to cocaine: Role of the dopaminergic system

Exposure to heavy metals has been associated with psychiatric disorders and recent studies suggest an association between childhood lead (Pb²⁺) intoxication and schizophrenia (SZ). In animal models, Pb²⁺ exposure recapitulates key neuropathological and dopaminergic system alterations present in SZ. Given the high comorbidity of mental disorders such as SZ and substance abuse, coupled with evidence showing that Pb²⁺ exposure affects addiction circuits, we hypothesized that early life Pb²⁺ exposure could sensitize neuronal systems relevant to SZ and substance abuse. To this goal, we examined the effects of chronic developmental Pb²⁺ exposure on the acute locomotor response to cocaine (0, 5, and 15 mg kg^–1) and behavioral sensitization. We also examined the role of the dopaminergic system in the psychostimulant effects of cocaine, and measured D1-dopamine receptor (D1R) levels in the rat brain using [³H]-SCH23390 quantitative receptor autoradiography, as well as the ability of the D1R antagonist SCH23390 to block the cocaine effects on locomotor activation. These studies were performed in male and female rats at different developmental ages consisting of juveniles (postnatal, PN14), early-adolescent (PN28), late adolescent (PN50), and adults (PN120). Our results show that chronic developmental Pb²⁺ exposure increases the acute locomotor response to the higher dose of cocaine in Pb²⁺-exposed male adolescent (PN28 and PN50) rats, and to the lower dose of cocaine in adolescent female rats. No changes in the locomotor activity were detected in adult rats. Behavioral sensitization experiments showed a sustained sensitization in early adolescent Pb²⁺-exposed male but not female rats. The cocaine-induced effects on locomotor activity were abrogated by injection of a D1R antagonist suggesting the involvement of this dopamine receptor subtype. Furthermore, Pb²⁺-induced increases D1R levels in several brain regions were prominent in juveniles and early adolescence but not in late adolescence or in adults. In summary, early chronic developmental Pb²⁺ exposure results in age and sex-dependent effect on the locomotor response to cocaine, suggesting differential susceptibilities to the neurotoxic effects of Pb²⁺ exposure. Our data provides further support to the notion that Pb²⁺ exposure is an environmental risk factor for psychiatric disorders and substance abuse.

Dopamine Receptors: Is It Possible to Become a Therapeutic Target for Depression?

Dopamine and its receptors are currently recognized targets for the treatment of several neuropsychiatric disorders, including Parkinson’s disease, schizophrenia, some drug use addictions, as well as depression. Dopamine receptors are widely distributed in various regions of the brain, but their role and exact contribution to neuropsychiatric diseases has not yet been thoroughly studied. Based on the types of dopamine receptors and their distribution in different brain regions, this paper reviews the current research status of the molecular, cellular and circuit mechanisms of dopamine and its receptors involved in depression. Multiple lines of investigation of these mechanisms provide a new future direction for understanding the etiology and treatment of depression and potential new targets for antidepressant treatments.

ADHD co-morbidities: A review of implication of gene × environment effects with dopamine-related genes

ADHD is a major burden in adulthood, where co-morbid conditions such as depression, substance use disorder and obesity often dominate the clinical picture. ADHD has substantial shared heritability with other mental disorders, contributing to comorbidity. However, environmental risk factors exist but their interaction with genetic makeup, especially in relation to comorbid disorders, remains elusive. This review for the first time summarizes present knowledge on gene x environment (GxE) interactions regarding the dopamine system. Hitherto, mainly candidate (GxE) studies were performed, focusing on the genes DRD4, DAT1 and MAOA. Some evidence suggest that the variable number tandem repeats in DRD4 and MAOA may mediate GxE interactions in ADHD generally, and comorbid conditions specifically. Nevertheless, even for these genes, common variants are bound to suggest risk only in the context of gender and specific environments. For other polymorphisms, evidence is contradictory and less convincing. Particularly lacking are longitudinal studies testing the interaction of well-defined environmental with polygenic risk scores reflecting the dopamine system in its entirety.

Frequency-dependency of the involvement of dopamine D1/D5 and beta-adrenergic receptors in hippocampal LTD triggered by locus coeruleus stimulation

Patterned stimulation of the locus coeruleus (LC, 100 Hz), in conjunction with test-pulse stimulation of hippocampal afferents, results in input-specific long-term depression (LTD) of synaptic plasticity in the hippocampus. Effects are long-lasting and have been described in Schaffer-collateral-CA1 and perforant path-dentate gyrus synapses in behaving rats. To what extent LC-mediated hippocampal LTD (LC-LTD) is frequency-dependent is unclear. Here, we report that LC-LTD can be triggered by LC stimulation with 2 and 5 Hz akin to tonic activity, 10 Hz equivalent to phasic activity, and 100 Hz akin to high-phasic activity in the dentate gyrus (DG) of freely behaving rats. LC-LTD at both 2 and 100 Hz can be significantly prevented by an NMDA receptor antagonist. The LC releases both noradrenaline (NA) and dopamine (DA) from its hippocampal terminals and may also trigger hippocampal DA release by activating the ventral tegmental area (VTA). Unclear is whether both neurotransmitters contribute equally to hippocampal LTD triggered by LC stimulation (LC-LTD). Both DA D1/D5 receptors (D1/D5R) and beta-adrenergic receptors (β-AR) are critically required for hippocampal LTD that is induced by patterned stimulation of hippocampal afferents, or is facilitated by spatial learning. We, therefore, explored to what extent these receptor subtypes mediate frequency-dependent hippocampal LC-LTD. LC-LTD elicited by 2, 5, and 10 Hz stimulation was unaffected by antagonism of β-AR with propranolol, whereas LC-LTD induced by these frequencies was prevented by D1/D5R-antagonism using SCH23390. By contrast, LC-LTD evoked at 100 Hz was prevented by β-AR-antagonism and only mildly affected by D1/D5R-antagonism. Taken together, these findings support that LC-LTD can be triggered by LC activity at a wide range of frequencies. Furthermore, the contribution of D1/D5R and β-AR to hippocampal LTD that is triggered by LC activity is frequency-dependent and suggests that D1/D5R may be involved in LC-mediated hippocampal tonus.

Dopamine and Dopamine-Related Ligands Can Bind Not Only to Dopamine Receptors

The dopaminergic system is one of the most important neurotransmitter systems in the central nervous system (CNS). It acts mainly by activation of the D₁-like receptor family at the target cell. Additionally, fine-tuning of the signal is achieved via pre-synaptic modulation by the D₂-like receptor family. Some dopamine drugs (both agonists and antagonists) bind in addition to DRs also to α₂-ARs and 5-HT receptors. Unfortunately, these compounds are often considered subtype(s) specific. Thus, it is important to consider the presence of these receptor subtypes in specific CNS areas as the function virtually elicited by one receptor type could be an effect of other—or the co-effect of multiple receptors. However, there are enough molecules with adequate specificity. In this review, we want to give an overview of the most common off-targets for established dopamine receptor ligands. To give an overall picture, we included a discussion on subtype selectivity. Molecules used as antipsychotic drugs are reviewed too. Therefore, we will summarize reported affinities and give an outline of molecules sufficiently specific for one or more subtypes (i.e., for subfamily), the presence of DR, α₂-ARs, and 5-HT receptors in CNS areas, which could help avoid ambiguous results.

DAT-truncated epigenetics: heterozigosity of the grand-mother rat temperates the vulnerable phenotype in second-generation offspring

Behavioral phenotype differs among epigenotypes of dopamine-transporter heterozygous (DAT-HET) rats. Epigenetic regulations act through trans-generational effects, referring to phenotypic variations emerging at second or third generation. To investigate trans-generational influences exerted by maternal grandmothers, we developed breeding schemes where only the genotype of maternal grandmothers varied. Heterozygous females, to serve as MAT vs MIX mothers, were generated, respectively, from WTxKO=MAT and MATxKO=MIX breeding, with KO males acting as grandfather. The HET experimental groups, generated from either MAT or MIX mothers, were called MIX-by-MAT and MIX² (male-fathers KO; asset-M: wild\healthy-allele from dam); or SOT and SIX (male-fathers WT; asset-P: mutated-allele from dam). Thus, sequelae of first-encounter between wild\healthy and mutated DAT-alleles (in maternal-lineage) were compared at first- (MAT-dam, WT-grandmother) vs. at second- (MIX-dam, HET-grandmother) generation. We characterized, within these epigenotypes, (1) circadian home-cage activity; (2) preference for social stimuli. Marked alterations of circadian activity appeared in HETs, if offspring of MAT-dams, compared to MIX² (HET offspring of MIX-dams) which, in turn, were undistinguishable from WT-controls. A clear-cut social preference by WT-rats was expressed towards SIX compared to SOT stimulus-rats, confirming reduced social motivations. In conclusion, significant epigenetic modulations took place in DAT-HET rats, as a function of maternal grandmother's genotype.

Keeping Track of the Genealogy of Heterozygotes Using Epigenetic Reference Codes and Breeding Tables

Studying neurobehavioral consequences of the hypofunctional dopamine transporter (DAT) across several generations entails the need to monitor allelic transmission to offspring, taking into account both maternal and paternal inheritance. Since each type of heterozygote expresses differential phenotypes, based on lineage of inheritance for wild and mutated alleles (from male or female ancestors), it is important to track transgenerational epigenetic effects. We deemed it essential to assign specific abbreviations identifying their characteristics. Therefore, we devised a Mendelian-inspired table to keep track of these. Starting from two progenitors (WT and KO) we named resulting heterozygous progenies MAT and PAT to differentiate them based on inheritance of the wild allele (from the mother or father). Tracing subsequent generations, similar logic has been followed: if coupling HET dams with KO males, initials “M” [(grand)maternal] and “P” [(grand)paternal] are kept, but “AT” is turned into “IX” (MIX and PIX), while if breeding HETs with WTs, “M” is changed to “W” resembling an upside down “M” and “P” to “S” for “sperm” (WAT and SOT). To underline the development within “hyperdopaminergic-uterus” a central letter “U” is added (MUX, PUX, and QULL), while a Greek initial (μAT, μIX, and νIX) underlines the uterine-worsened origin of the allele. In HET × HET breeding (GIX and DIX), the mutated allele can be inherited from both sides of the genealogical line. However, when the mother is MAT, wild and mutated alleles encounter for the first time, causing putative anomalies in the progeny. Replacing dam with a second-generation female (MIX and MUX) may mitigate epigenetic effects on third-generation offspring; therefore suffixes (“-f,” “-fu,” “-ϕ,” and “-ϕu”) emphasize that subsequent-generation dams imply that the alleles already encountered in HET (rather than WT) grand-dams.

Basic mechanisms of plasticity and learning

The last century was characterized by a significant scientific effort aimed at unveiling the neurobiological basis of learning and memory. Thanks to the characterization of the mechanisms regulating the long-term changes of neuronal synaptic connections, it was possible to understand how specific neural networks shape themselves during the acquisition of memory traces or complex motor tasks. In this chapter, we will summarize the mechanisms underlying the main forms of synaptic plasticity taking advantage of the studies performed in the hippocampus and in the nucleus striatum, key brain structures that play a crucial role in cognition. Moreover, we will discuss how the molecular pathways involved in the induction of physiologic synaptic long-term changes could be disrupted during neurodegenerative and neuroinflammatory disorders, highlighting the translational relevance of this intriguing research field.

Effects of methamphetamine-induced neurotoxicity on striatal long-term potentiation

RATIONALE

Methamphetamine (METH) exposure is associated with damage to central monoamine systems, particularly dopamine signaling. Rodent models of such damage have revealed a decrease in the amplitude of phasic dopamine signals and significant striatal dysfunction, including changes in the molecular, system, and behavioral functions of the striatum. Dopamine signaling through D1 receptors promotes corticostriatal long-term potentiation (LTP), a critical substrate of these striatal functions.

OBJECTIVES

Therefore, the purpose of this study was to determine if METH-induced dopamine neurotoxicity would impair D1 receptor-dependent striatal LTP in mice.

METHODS

Mice were treated with a METH binge regimen (4 × 10 mg/kg d,l-methamphetamine, s.c.) that recapitulates all of the known METH-induced neurotoxic effects observed in humans, including dopamine toxicity. Three weeks later, acute brain slices containing either the dorsomedial striatum (DMS) or dorsolateral striatum (DLS) were prepared, and plasticity was assessed using white matter, high-frequency stimulation (HFS), and striatal extracellular electrophysiology.

RESULTS

Under these conditions, LTP was induced in brain slices containing the DMS from saline-pretreated mice, but not mice with METH-induced neurotoxicity. Furthermore, the LTP observed in DMS slices from saline-pretreated mice was blocked by the dopamine D1 receptor antagonist SCH23390, indicating that this LTP is dopamine D1 receptor-dependent. Finally, acute in vivo treatment of METH-pretreated mice with bupropion (50 mg/kg, i.p.) promoted LTP in DMS slices.

CONCLUSIONS

Together, these studies demonstrate that METH-induced neurotoxicity impairs dopamine D1 receptor-dependent LTP within the DMS and that the FDA-approved drug bupropion restores induction of striatal LTP in mice with METH-induced dopamine neurotoxicity.

Iron status, immune system, and expression of brain divalent metal transporter 1 and dopamine receptors D1 interrelationship in Parkinson’s disease and the role of grape seed and green coffee bean extracts and quercetin in mitigating the disease in rats

Introduction: Parkinson’s disease (PD) is a neurodegenerative disease with a prevalence of 1% in the elderly worldwide. The aim of the research is to study the interrelationship of iron status, the immune system including inflammatory cytokines, brain divalent metal transporter 1 (DMT1), and dopamine receptors D1 (DRD1) in a PD rat model. The potential protective effects of grape seed and green coffee bean ethanol extracts and quercetin were also studied. Methods: Phenolic and flavonoid contents of grape seed and green coffee bean and in vitro free radicals scavenging activities of the extracts and quercetin were determined. Male rats were divided into five groups. Group 1 served as normal control (NC), group 2 represented Parkinsonian control (PC). Groups 3, 4, and 5 were the test groups treated by daily oral green coffee bean, grape seed extracts, and quercetin, respectively. PD was induced by rotenone in groups 2 to 5. Brain oxidative stress, DMT1, and DRD1 expressions, and histopathology were assessed. Parameters of the immune system, represented by plasma interferon-gamma (IFNγ) and CD4, and brain tumor necrosis factor-alpha (TNF-α) along with iron status were also determined. Results: Phenolic and flavonoid contents of green coffee bean were high compared to grape seed (P < 0.05). Quercetin experienced the highest in-vitro free radicals scavenging activities. Iron deficiency anemia, together with elevated IFNγ, TNF-α, DMT1 expressions, and brain malondialdehyde (MDA) were demonstrated in PC compared to NC (P < 0.05). Also, reduction in CD4 and brain reduced-glutathione (GSH) (P < 0.05) were noticed in PC with brain histopathological alterations. Different treatments showed variable improvements in the majority of parameters (P < 0.05) and brain histopathology. Conclusion: Iron deficiency anemia might result from cytokine elevation in PD. Reduced DRD1 and altered immune system including cytokines together with increased brain DMT1 might induce neurodegeneration in PD. Different treatments showed variable neuroprotective effects through modulation of inflammation, oxidative stress, immune system, iron status, DMT1, and DRD1.

Pharmacological perturbation reveals deficits in D2 receptor responses in Thap1 null mice

The primary dystonia DYT6 is caused by mutations in the transcription factor Thanatos‐associated protein 1 (THAP1). To understand THAP1’s functions, we generated mice lacking THAP1 in the nervous system. THAP1 loss causes locomotor deficits associated with transcriptional changes. Since many of the genes misregulated involve dopaminergic signaling, we pharmacologically challenged the two striatal canonical dopamine pathways: the direct, regulated by the D1 receptor, and the indirect, regulated by the D2 receptor. We discovered that depleting THAP1 specifically interferes with the D2 receptor responses, pointing to a selective misregulation of the indirect pathway in DYT6 with implications for pathogenesis and treatment.

Dopamine receptor D2 on CD4+ T cells is protective against neuroinflammation and neurodegeneration in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disease. Recently, neuroinflammation driven by CD4⁺ T cells has been involved in PD pathophysiology. Human and murine lymphocytes express all the five subtypes of dopamine receptors (DRs), DRD1 to DRD5. However, roles of DRs particularly DRD2 expressed on CD4⁺ T cells in PD remain elucidated. Global Drd1- or Drd2-knockout (Drd1^-/- or Drd2^-/-) mice or CD4⁺ T cell-specific Drd2-knockout (Drd2^fl/fl/CD4^Cre) mice were intraperitoneally injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to induce PD with the different mutants. On the 7th day following MPTP injection, mice were assessed for dopaminergic neurodegeneration, locomotor impairments, microglial activation, as well as CD4⁺ T-cell differentiation and function. Furthermore, in vitro CD4⁺ T cells were exposed to DRD2 agonist and antagonist and then differentiation and function of the cells were determined. MPTP induced dopaminergic neuronal loss in the nigrostriatal system, motor coordinative and behavioral impairments, microglial activation, and CD4⁺ T-cell polarization to pro-inflammatory T-helper (Th)1 and Th17 phenotypes. Importantly, either Drd2^-/- or Drd2^fl/fl/CD4^Cre mice manifested more severe dopaminergic neurodegeneration, motor deficits, microglial activation, and CD4⁺ T-cell bias towards Th1 and Th17 phenotypes in response to MPTP, but Drd1^-/- did not further alter MPTP intoxication. DRD2 agonist sumanirole inhibited shift of CD4⁺ T cells obtained from MPTP-intoxicated mice to Th1 and Th17 phenotypes and DRD2 antagonist L-741,626 reversed sumanirole effects. These findings suggest that DRD2 expressed on CD4⁺ T cells is protective against neuroinflammation and neurodegeneration in PD. Thus, developing a therapeutic strategy of stimulating DRD2 may be promising for mitigation of PD.

Autophagy as a gateway for the effects of methamphetamine: From neurotransmitter release and synaptic plasticity to psychiatric and neurodegenerative disorders

As a major eukaryotic cell clearing machinery, autophagy grants cell proteostasis, which is key for neurotransmitter release, synaptic plasticity, and neuronal survival. In line with this, besides neuropathological events, autophagy dysfunctions are bound to synaptic alterations that occur in mental disorders, and early on, in neurodegenerative diseases. This is also the case of methamphetamine (METH) abuse, which leads to psychiatric disturbances and neurotoxicity. While consistently altering the autophagy machinery, METH produces behavioral and neurotoxic effects through molecular and biochemical events that can be recapitulated by autophagy blockade. These consist of altered physiological dopamine (DA) release, abnormal stimulation of DA and glutamate receptors, as well as oxidative, excitotoxic, and neuroinflammatory events. Recent molecular insights suggest that METH early impairs the autophagy machinery, though its functional significance remains to be investigated. Here we discuss evidence suggesting that alterations of DA transmission and autophagy are intermingled within a chain of events underlying behavioral alterations and neurodegenerative phenomena produced by METH. Understanding how METH alters the autophagy machinery is expected to provide novel insights into the neurobiology of METH addiction sharing some features with psychiatric disorders and parkinsonism.

Optogenetic induction of orbitostriatal long-term potentiation in the dorsomedial striatum elicits a persistent reduction of alcohol-seeking behavior in rats

Uncontrolled drug-seeking and -taking behaviors are generally driven by maladaptive corticostriatal synaptic plasticity. The orbital frontal cortex (OFC) and its projections to the dorsomedial striatum (DMS) have been extensively implicated in drug-seeking and relapse behaviors. The influence of the synaptic plasticity of OFC projections to the DMS (OFC→DMS) on drug-seeking and -taking behaviors has not been fully characterized. To investigate this, we trained rats to self-administer 20% alcohol and then delivered an in vivo optogenetic protocol designed to induce long-term potentiation (LTP) selectively at OFC→DMS synapses. We selected LTP induction because we found that voluntary alcohol self-administration suppressed OFC→DMS transmission and LTP may normalize this transmission, consequently reducing alcohol-seeking behavior. Importantly, ex vivo slice electrophysiology studies confirmed that this in vivo optical stimulation protocol resulted in a significant increase in excitatory OFC→DMS transmission strength on day two after stimulation, suggesting that LTP was induced in vivo. Rat alcohol-seeking and -taking behaviors were significantly reduced on days 1-3, but not on days 7-11, after LTP induction. Striatal synaptic plasticity is modulated by several critical neurotransmitter receptors, including dopamine D1 receptors (D1Rs) and adenosine A2A receptors (A2ARs). We found that delivery of in vivo optical stimulation in the presence of a D1R antagonist abolished the LTP-associated decrease in alcohol-seeking behavior, whereas delivery in the presence of an A2AR antagonist may facilitate this LTP-induced behavioral change. These results demonstrate that alcohol-seeking behavior was negatively regulated by the potentiation of excitatory OFC→DMS neurotransmission. Our findings provide direct evidence that the OFC exerts "top-down" control of alcohol-seeking behavior via the DMS.

Dopaminergic Modulation of Forced Running Performance in Adolescent Rats: Role of Striatal D1 and Extra-striatal D2 Dopamine Receptors

Improving exercise capacity during adolescence impacts positively on cognitive and motor functions. However, the neural mechanisms contributing to enhance physical performance during this sensitive period remain poorly understood. Such knowledge could help to optimize exercise programs and promote a healthy physical and cognitive development in youth athletes. The central dopamine system is of great interest because of its role in regulating motor behavior through the activation of D1 and D2 receptors. Thus, the aim of the present study is to determine whether D1 or D2 receptor signaling contributes to modulate the exercise capacity during adolescence and if this modulation takes place through the striatum. To test this, we used a rodent model of forced running wheel that we implemented recently to assess the exercise capacity. Briefly, rats were exposed to an 8-day period of habituation in the running wheel before assessing their locomotor performance in response to an incremental exercise test, in which the speed was gradually increased until exhaustion. We found that systemic administration of D1-like (SCH23390) and/or D2-like (raclopride) receptor antagonists prior to the incremental test reduced the duration of forced running in a dose-dependent manner. Similarly, locomotor activity in the open field was decreased by the dopamine antagonists. Interestingly, this was not the case following intrastriatal infusion of an effective dose of SCH23390, which decreased motor performance during the incremental test without disrupting the behavioral response in the open field. Surprisingly, intrastriatal delivery of raclopride failed to impact the duration of forced running. Altogether, these results indicate that the level of locomotor response to incremental loads of forced running in adolescent rats is dopamine dependent and mechanistically linked to the activation of striatal D1 and extra-striatal D2 receptors.

Modulation of dendritic spines by protein phosphatase-1

Protein phosphatase-1 (PP-1), a highly conserved multifunctional serine/threonine phosphatase, is enriched in dendritic spines where it plays a major role in modulating excitatory synaptic activity. In addition to established functions in spine maturation and development, multi-subunit holoenzyme forms of PP-1 modulate higher-order cognitive functions such learning and memory. Mechanisms involved in regulating PP-1 activity and localization in spines include interactions with neurabin and spinophilin, structurally related synaptic scaffolding proteins associated with the actin cytoskeleton. Since PP-1 is a critical element in synaptic development, signaling, and plasticity, alterations in PP-1 signaling in dendritic spines are implicated in various neurological and psychiatric disorders. The effects of PP-1 depend on its isoform-specific association with regulatory proteins and activation of downstream signaling pathways. Here we review the role of PP-1 and its binding proteins neurabin and spinophilin in both developing and established dendritic spines, as well as some of the disorders that result from its dysregulation.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

Dopamine D1 Receptors Regulate Spines in Striatal Direct-Pathway and Indirect-Pathway Neurons

BACKGROUND

Dopamine transmission is involved in the maintenance of the structural plasticity of direct-pathway and indirect-pathway striatal projection neurons (d-SPNs and i-SPNs, respectively). The lack of dopamine in Parkinson's disease produces synaptic remodeling in both types of SPNs, reducing the length of the dendritic arbor and spine density and increasing the intrinsic excitability. Meanwhile, the elevation of dopamine levels by levodopa recovers these alterations selectively in i-SPNs. However, little is known about the specific role of the D1 receptor (D1R) in these alterations.

METHODS

To explore the specific role of D1R in the synaptic remodeling of SPNs, we used knockout D1R mice (D1R^-/- ) and wild-type mice crossed with drd2-enhanced green fluorescent protein (eGFP) to identify d-SPNs and i-SPNs. Corticostriatal slices were used for reconstruction of the dendritic arbors after Lucifer yellow intracellular injection and for whole-cell recordings in naïve and parkinsonian mice treated with saline or levodopa.

RESULTS

The genetic inactivation of D1R reduces the length of the dendritic tree and the spine density in all SPNs, although more so in d-SPNs, which also increases their spiking. In parkinsonian D1R^-/- mice, the spine density decreases in i-SPNs, and this spine loss recovers after chronic levodopa.

CONCLUSIONS

D1R is essential for the maintenance of spine plasticity in d-SPNs but also affects i-SPNs, indicating an important crosstalk between these 2 types of neurons. © 2020 International Parkinson and Movement Disorder Society.

Beneficial effects of the phytocannabinoid Δ9-THCV in L-DOPA-induced dyskinesia in Parkinson's disease

The antioxidant and CB₂ receptor agonist properties of Δ⁹-tetrahydrocannabivarin (Δ⁹-THCV) afforded neuroprotection in experimental Parkinson's disease (PD), whereas its CB₁ receptor antagonist profile at doses lower than 5 mg/kg caused anti-hypokinetic effects. In the present study, we investigated the anti-dyskinetic potential of Δ⁹-THCV (administered i.p. at 2 mg/kg for two weeks), which had not been investigated before. This objective was investigated after inducing dyskinesia by repeated administration of L-DOPA (i.p. at 10 mg/kg) in a genetic model of dopaminergic deficiency, Pitx3^ak mutant mice, which serves as a useful model for testing anti-dyskinetic agents. The daily treatment of these mice with L-DOPA for two weeks progressively increased the time spent in abnormal involuntary movements (AIMs) and elevated their horizontal and vertical activities (as measured in a computer-aided actimeter), signs that reflected the dyskinetic state of these mice. Interestingly, when combined with L-DOPA from the first injection, Δ⁹-THCV delayed the appearance of all these signs and decreased their intensity, with a reduction in the levels of FosB protein and the histone pAcH3 (measured by immunohistochemistry), which had previously been found to be elevated in the basal ganglia in L-DOPA-induced dyskinesia. In addition to the anti-dyskinetic effects of Δ⁹-THCV when administered at the onset of L-DOPA treatment, Δ⁹-THCV was also effective in attenuating the intensity of dyskinesia when administered for three consecutive days once these signs were already present (two weeks after the onset of L-DOPA treatment). In summary, our data support the anti-dyskinetic potential of Δ⁹-THCV, both to delay the occurrence and to attenuate the magnitude of dyskinetic signs. Although further studies are clearly required to determine the clinical significance of these data in humans, the results nevertheless situate Δ⁹-THCV in a promising position for developing a cannabinoid-based therapy for patients with PD.

The Dopamine D5 receptor contributes to activation of cholinergic interneurons during L-DOPA induced dyskinesia

The dopamine D5 receptor (D5R) is a Gαs-coupled dopamine receptor belonging to the dopamine D1-like receptor family. Together with the dopamine D2 receptor it is highly expressed in striatal cholinergic interneurons and therefore is poised to be a positive regulator of cholinergic activity in response to L-DOPA in the dopamine-depleted parkinsonian brain. Tonically active cholinergic interneurons become dysregulated during chronic L-DOPA administration and participate in the expression of L-DOPA induced dyskinesia. The molecular mechanisms involved in this process have not been elucidated, however a correlation between dyskinesia severity and pERK expression in cholinergic cells has been described. To better understand the function of the D5 receptor and how it affects cholinergic interneurons in L-DOPA induced dyskinesia, we used D5R knockout mice that were rendered parkinsonian by unilateral 6-OHDA injection. In the KO mice, expression of pERK was strongly reduced indicating that activation of these cells is at least in part driven by the D5 receptor. Similarly, pS6, another marker for the activity status of cholinergic interneurons was also reduced. However, mice lacking D5R exhibited slightly worsened locomotor performance in response to L-DOPA and enhanced LID scores. Our findings suggest that D5R can modulate L-DOPA induced dyskinesia and is a critical activator of CINs via pERK and pS6.

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

Activation of the mGlu1 metabotropic glutamate receptor has antipsychotic-like effects and is required for efficacy of M4 muscarinic receptor allosteric modulators

Recent clinical and preclinical studies suggest that selective activators of the M₄ muscarinic acetylcholine receptor have potential as a novel treatment for schizophrenia. M₄ activation inhibits striatal dopamine release by mobilizing endocannabinoids, providing a mechanism for local effects on dopamine signaling in the striatum but not in extrastriatal areas. G protein-coupled receptors (GPCRs) typically induce endocannabinoid release through activation of Gα_q/11-type G proteins whereas M₄ transduction occurs through Gα_i/o-type G proteins. We now report that the ability of M₄ to inhibit dopamine release and induce antipsychotic-like effects in animal models is dependent on co-activation of the Gα_q/11-coupled mGlu₁ subtype of metabotropic glutamate (mGlu) receptor. This is especially interesting in light of recent findings that multiple loss of function single nucleotide polymorphisms (SNPs) in the human gene encoding mGlu₁ (GRM1) are associated with schizophrenia, and points to GRM1/mGlu₁ as a gene within the "druggable genome" that could be targeted for the treatment of schizophrenia. Herein, we report that potentiation of mGlu₁ signaling following thalamo-striatal stimulation is sufficient to inhibit striatal dopamine release, and that a novel mGlu₁ positive allosteric modulator (PAM) exerts robust antipsychotic-like effects through an endocannabinoid-dependent mechanism. However, unlike M₄, mGlu₁ does not directly inhibit dopamine D₁ receptor signaling and does not reduce motivational responding. Taken together, these findings highlight a novel mechanism of cross talk between mGlu₁ and M₄ and demonstrate that highly selective mGlu₁ PAMs may provide a novel strategy for the treatment of positive symptoms associated with schizophrenia.

Dopamine D1 and muscarinic acetylcholine receptors in dorsal striatum are required for high speed running

Dopamine (DA) signaling in the basal ganglia plays important roles in motor control. Motor deficiencies were previously reported in dopamine receptor D1 (D1R) and D2 (D2R) knockout mice. While these results indicate the involvement of DA receptors in motor execution, the null knockout (KO) mouse lacks the specificity necessary to determine when and where in the brain D1R and D2R function in motor execution. To address these questions, we restricted the loss of function temporally and spatially by using D1R conditional knockdown (cKD) mice and mice injected with antagonists against DA receptors directly into the dorsal striatum. In addition, we address the DA and acetylcholine (ACh) balance hypothesis by using antagonists against ACh receptors. We tested the motor ability of the mice with a newly devised task named the accelerating step-wheel. In this task, the maximum running speed was measured in a situation where the wheel rotation speed was gradually accelerated in one trial. We found significant decreases in the maximum running speed of D1R cKD mice and the mice injected with the antagonist against D1R or muscarinic ACh receptor. These results indicated that D1R and muscarinic ACh receptor in the dorsal striatum play pivotal roles in the execution of walking/running.

The Neuroscience of Drug Reward and Addiction

Drug consumption is driven by a drug's pharmacological effects, which are experienced as rewarding, and is influenced by genetic, developmental, and psychosocial factors that mediate drug accessibility, norms, and social support systems or lack thereof. The reinforcing effects of drugs mostly depend on dopamine signaling in the nucleus accumbens, and chronic drug exposure triggers glutamatergic-mediated neuroadaptations in dopamine striato-thalamo-cortical (predominantly in prefrontal cortical regions including orbitofrontal cortex and anterior cingulate cortex) and limbic pathways (amygdala and hippocampus) that, in vulnerable individuals, can result in addiction. In parallel, changes in the extended amygdala result in negative emotional states that perpetuate drug taking as an attempt to temporarily alleviate them. Counterintuitively, in the addicted person, the actual drug consumption is associated with an attenuated dopamine increase in brain reward regions, which might contribute to drug-taking behavior to compensate for the difference between the magnitude of the expected reward triggered by the conditioning to drug cues and the actual experience of it. Combined, these effects result in an enhanced motivation to "seek the drug" (energized by dopamine increases triggered by drug cues) and an impaired prefrontal top-down self-regulation that favors compulsive drug-taking against the backdrop of negative emotionality and an enhanced interoceptive awareness of "drug hunger." Treatment interventions intended to reverse these neuroadaptations show promise as therapeutic approaches for addiction.

Histamine-induced plasticity and gene expression in corticostriatal pathway under hyperammonemia

Aims

Histamine H3 receptor (H3R) antagonists/inverse agonists increase vigilance. We studied brain histaminergic pathways under hyperammonemia and the transcriptome of receptors and their signaling cascades to provide a rationale for wake‐promoting therapies.

Methods

We analyzed histamine‐induced long‐lasting depression of corticostriatal synaptic transmission (LLDhist). As the expression of dopamine 1 receptors (D1R) is upregulated in LGS‐KO striatum where D1R‐H3R dimers may exist, we investigated actions of H3R and D1R agonists and antagonists. We analyzed transcription of selected genes in cortex and dorsal striatum in a mouse model of inborn hyperammonemia (liver‐specific glutamine synthetase knockout: LGS‐KO) and compared it with human hepatic encephalopathy.

Results

LGS‐KO mice showed significant reduction of the direct depression (DD) but not the long‐lasting depression (LLD) by histamine. Neither pharmacological activation nor inhibition of D1R significantly affected DDhist and LLDhist in WT striatum, while in LGS‐KO mice D1R activation suppressed LLDhist. Histaminergic signaling was found unchanged at the transcriptional level except for the H2R. A study of cAMP‐regulated genes indicated a significant reduction in the molecular signature of wakefulness in the diseased cortex.

Conclusions

Our findings provide a rationale for the development of aminergic wake‐promoting therapeutics in hyperammonemic disorders.

Dopamine D1 and D2 receptors mediate neuropeptide S-induced antinociception in the mouse formalin test

Neuropeptide S (NPS) is the endogenous ligand of a G-protein coupled receptor named NPS receptor. The NPS system controls several biological functions, including anxiety, wakefulness, locomotor activity, food intake, and pain transmission. A growing body of evidence supports facilitatory effects for NPS over dopaminergic neurotransmission. The present study was aimed to investigate the role of dopamine receptors signaling in the antinociceptive effects of NPS in the mouse formalin test. The following dopamine receptor antagonists were employed: SCH 23390 (selective dopamine D₁ antagonist, 0.05 mg/kg, ip), haloperidol (non-selective dopamine D₂-like receptor antagonist; 0.03 mg/kg, ip), and sulpiride (selective dopamine D₂-like receptor antagonist; 25 mg/kg, ip). Mice were pretreated with dopamine antagonists before the supraspinal administration of NPS (0.1 nmol, icv). Morphine (5 mg/kg, sc) and indomethacin (10 mg/kg, ip) were used as positive controls to set up the experimental conditions. Morphine-induced antinociceptive effects were observed during phases 1 and 2 of the test, while indomethacin was only active at phase 2. Central NPS significantly reduced formalin-induced nociception during both phases. The systemic administration of SCH 23390 slightly blocked the effects of NPS only during phase 2. Haloperidol prevented NPS-induced antinociceptive effects. Similar to haloperidol, sulpiride also counteracted the antinociceptive effects of NPS in both phases of the formalin test. In conclusion, the present findings suggest that the analgesic effects of NPS are linked with dopaminergic neurotransmission mainly through dopamine D₂-like receptor signaling.

Disentangling the diverse roles of dopamine D2 receptors in striatal function and behavior

Dopamine D2 receptors (D2Rs) mediate many of the actions of dopamine in the striatum, ranging from movement to the effortful pursuit of reward. Yet despite significant advances in linking D2Rs to striatal functions with pharmacological and genetic strategies in animals, how dopamine orchestrates its myriad actions on different cell populations -each expressing D2Rs- remains unclear. Furthermore, brain imaging and genetic studies in humans have consistently associated striatal D2R alterations with various neurological and neuropsychiatric disorders, but how and which D2Rs are involved in each case is poorly understood. Therefore, a critical first step is to engage in a refined and systematic investigation of the impact of D2R function on specific striatal cells, circuits, and behaviors. Here, I will review recent efforts, primarily in animal models, aimed at unlocking the complex and heterogeneous roles of D2Rs in striatum.

Cholinergic modulation of striatal microcircuits

The purpose of this review is to bridge the gap between earlier literature on striatal cholinergic interneurons and mechanisms of microcircuit interaction demonstrated with the use of newly available tools. It is well known that the main source of the high level of acetylcholine in the striatum, compared to other brain regions, is the cholinergic interneurons. These interneurons provide an extensive local innervation that suggests they may be a key modulator of striatal microcircuits. Supporting this idea requires the consideration of functional properties of these interneurons, their influence on medium spiny neurons, other interneurons, and interactions with other synaptic regulators. Here, we underline the effects of intrastriatal and extrastriatal afferents onto cholinergic interneurons and discuss the activation of pre‐ and postsynaptic muscarinic and nicotinic receptors that participate in the modulation of intrastriatal neuronal interactions. We further address recent findings about corelease of other transmitters in cholinergic interneurons and actions of these interneurons in striosome and matrix compartments. In addition, we summarize recent evidence on acetylcholine‐mediated striatal synaptic plasticity and propose roles for cholinergic interneurons in normal striatal physiology. A short examination of their role in neurological disorders such as Parkinson's, Huntington's, and Tourette's pathologies and dystonia is also included.

Modulatory effect of dopamine receptor 5 on the neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for dopamine has been reported for magnocellular neuroendocrine cells in the mammalian hypothalamus. We examined its potential role as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the euryhaline flounder Paralichthys olivaceus. In vitro application of dopamine (DA) caused an increase in electrical activity (firing frequency, recorded extracellularly) of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The dopamine precursor, levodopa (L-DOPA), also increased firing frequency, cell recruitment and enhanced bursting and tonic activity. The effect of dopamine was blocked by the D1, D5 receptor antagonist SCH23390, but not by the D2, D3, D4 receptor antagonist amisulpride. Transcriptome sequencing revealed that all DA receptors (D1, D2, D3, D4, and D5) were present in the flounder CNSS. However, quantitative RT-PCR revealed that D5 receptor mRNA expression was significantly increased in the CNSS following dopamine superfusion. These findings suggest that dopamine may modulate CNSS activity in vivo, and therefore neurosecretory output, through D5 receptors.

The Intersection of Central Dopamine System and Stroke: Potential Avenues Aiming at Enhancement of Motor Recovery

Dopamine, a major neurotransmitter, plays a role in a wide range of brain sensorimotor functions. Parkinson's disease and schizophrenia are two major human neuropsychiatric disorders typically associated with dysfunctional dopamine activity levels, which can be alleviated through the druggability of the dopaminergic systems. Meanwhile, several studies suggest that optimal brain dopamine activity levels are also significantly impacted in other serious neurological conditions, notably stroke, but this has yet to be fully appreciated at both basic and clinical research levels. This is of utmost importance as there is a need for better treatments to improve recovery from stroke. Here, we discuss the state of knowledge regarding the modulation of dopaminergic systems following stroke, and the use of dopamine boosting therapies in animal stroke models to improve stroke recovery. Indeed, studies in animals and humans show stroke leads to changes in dopamine functioning. Moreover, evidence from animal stroke models suggests stimulation of dopamine receptors may be a promising therapeutic approach for enhancing motor recovery from stroke. With respect to the latter, we discuss the evidence for several possible receptor-linked mechanisms by which improved motor recovery may be mediated. One avenue of particular promise is the subtype-selective stimulation of dopamine receptors in conjunction with physical therapy. However, results from clinical trials so far have been more mixed due to a number of potential reasons including, targeting of the wrong patient populations and use of drugs which modulate a wide array of receptors. Notwithstanding these issues, it is hoped that future research endeavors will assist in the development of more refined dopaminergic therapeutic approaches to enhance stroke recovery.

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

Pharmacological analysis of zebrafish lphn3.1 morphant larvae suggests that saturated dopaminergic signaling could underlie the ADHD-like locomotor hyperactivity

Polymorphisms in the gene coding for the adhesion G-protein coupled receptor LPHN3 are a risk factor for attention-deficit/hyperactivity disorder (ADHD). Transient down-regulation of latrophilin3.1 (lphn3.1), the zebrafish LPHN3 homologue, causes hyperactivity. Zebrafish injected with a lphn3.1-specific morpholino are hyperactive and display an impairment in dopaminergic neuron development. In the present study we used lphn3.1 morphants to further characterize the changes to dopaminergic signaling that trigger hyperactivity. We applied dopamine agonists (Apomorphine, Quinpirole, SKF-38393) and antagonists (Haloperidol, Eticlopride, SCH-23390) to Lphn3.1 morpholino-injected or control-injected animals. The percentage of change in locomotor activity was then determined at three different time periods (10-20 min, 30-40 min and 60-70 min). Our results show that drugs targeting dopamine receptors appear to elicit similar effects on locomotion in zebrafish larvae and mammals. In addition, we observed that lphn3.1 morphants have an overall hyposensitivity to dopamine agonists and antagonists compared to control fish. These results are compatible with a model whereby dopaminergic neurotransmission is saturated in lphn3.1 morphants.

Basal ganglia mechanisms in action selection, plasticity, and dystonia

Basal ganglia circuits are organized to selected desired actions and to inhibit potentially competing unwanted actions. This is accomplished through a complex circuitry that is modified through development and learning. Mechanisms of neural plasticity underlying these modifications are increasingly understood, but new mechanisms continue to be discovered. Dystonia, a movement disorder characterized by involuntary muscle contractions that cause abnormal postures and movements. Emerging evidence points to important links between mechanisms of plasticity and the manifestations of dystonia. Investigation of these mechanisms has improved understanding of the action of currently used medication and is informing the development of new treatments.

Regulation of Striatal Neuron Activity by Cyclic Nucleotide Signaling and Phosphodiesterase Inhibition: Implications for the Treatment of Parkinson's Disease

Cyclic nucleotide phosphodiesterase (PDE) enzymes catalyze the hydrolysis and inactivation of cyclic nucleotides (cAMP/cGMP) in the brain. Several classes of PDE enzymes with distinct tissue distributions, cyclic nucleotide selectivity, and regulatory factors are highly expressed in brain regions subserving cognitive and motor processes known to be disrupted in neurodegenerative diseases such as Parkinson's disease (PD). Furthermore, small-molecule inhibitors of several different PDE family members alter cyclic nucleotide levels and favorably enhance motor performance and cognition in animal disease models. This chapter will explore the roles and therapeutic potential of non-selective and selective PDE inhibitors on neural processing in fronto-striatal circuits in normal animals and models of DOPA-induced dyskinesias (LIDs) associated with PD. The impact of selective PDE inhibitors and augmentation of cAMP and cGMP signaling on the membrane excitability of striatal medium-sized spiny projection neurons (MSNs) will be discussed. The effects of cyclic nucleotide signaling and PDE inhibitors on synaptic plasticity of striatonigral and striatopallidal MSNs will be also be reviewed. New data on the efficacy of PDE10A inhibitors for reversing behavioral and electrophysiological correlates of L-DOPA-induced dyskinesias in a rat model of PD will also be presented. Together, these data will highlight the potential of novel PDE inhibitors for treatment of movement disorders such as PD which are associated with abnormal corticostriatal transmission.

Nonequivalent modulation of corticospinal excitability by positive and negative outcomes

OBJECTIVE

The difference between positive and negative outcomes is important in trial-and-error decision-making processes and affects corticospinal excitability. This study investigated corticospinal excitability during the performance of trial-and-error decision-making tasks with varying competing behavioral outcomes.

METHODS

Each trial began with one of five colored circles presented as a cue. Each color represented a different reward probability, ranging from 10% to 90%. The subjects were instructed to decide whether to perform wrist flexion in response to the cue. Two seconds after the presentation of the cue, a reward stimulus (picture of a coin) or penalty stimulus (mauve circle) was randomly presented to the subject. If the picture of a coin appeared, the subjects received the coin after the experiment if they had performed wrist flexion, but not if they had not performed wrist flexion. If a mauve circle appeared, a coin was deducted from the total reward if the subjects had performed wrist flexion, but not if they had not performed wrist flexion. One second after the reward or penalty stimulus, transcranial magnetic stimulation was delivered to the primary motor cortex at the midpoint between the centers of gravity of the flexor carpi radialis (agonist) and extensor carpi radialis (antagonist) muscles.

RESULTS

Cumulative wrist flexions were positively correlated with reward probabilities. Motor evoked potential (MEP) amplitudes in agonist muscles were significantly higher when wrist flexion incurred a penalty than when it incurred a reward, but there was no difference in the MEP amplitudes of antagonist muscles.

CONCLUSION

Positive and negative behavioral outcomes differentially altered behavior and corticospinal excitability, and unexpected penalties had a stronger effect on corticospinal excitability for agonist muscles.

Dopamine and Acetylcholine, a Circuit Point of View in Parkinson's Disease

Data from the World Health Organization (National Institute on Aging, 2011) and the National Institutes of Health (He et al., 2016) predicts that while today the worldwide population over 65 years of age is estimated around 8.5%, this number will reach an astounding 17% by 2050. In this framework, solving current neurodegenerative diseases primarily associated with aging becomes more pressing than ever. In 2017, we celebrate a grim 200th anniversary since the very first description of Parkinson’s disease (PD) and its related symptomatology. Two centuries after this debilitating disease was first identified, finding a cure remains a hopeful goal rather than an attainable objective on the horizon. Tireless work has provided insight into the characterization and progression of the disease down to a molecular level. We now know that the main motor deficits associated with PD arise from the almost total loss of dopaminergic cells in the substantia nigra pars compacta. A concomitant loss of cholinergic cells entails a cognitive decline in these patients, and current therapies are only partially effective, often inducing side-effects after a prolonged treatment. This review covers some of the recent developments in the field of Basal Ganglia (BG) function in physiology and pathology, with a particular focus on the two main neuromodulatory systems known to be severely affected in PD, highlighting some of the remaining open question from three main stand points:

- Heterogeneity of midbrain dopamine neurons.

- Pairing of dopamine (DA) sub-circuits.

- Dopamine-Acetylcholine (ACh) interaction.

A vast amount of knowledge has been accumulated over the years from experimental conditions, but very little of it is reflected or used at a translational or clinical level. An initiative to implement the knowledge that is emerging from circuit-based approaches to tackle neurodegenerative disorders like PD will certainly be tremendously beneficial.

Electrical stimulation of the hippocampal fimbria facilitates neuronal nitric oxide synthase activity in the medial shell of the rat nucleus accumbens: Modulation by dopamine D1 and D2 receptor activation

The medial shell region of the nucleus accumbens (msNAc) is a key center for the regulation of goal-directed behavior and is likely to be dysfunctional in neuropsychiatric disorders such as addiction, depression and schizophrenia. Nitric oxide (NO)-producing interneurons in the msNAc are potently modulated by dopamine (DA) and may play an important role in synaptic integration in msNAc networks. In this study, neuronal NO synthase (nNOS) activity was measured in anesthetized rats using amperometric microsensors implanted into the msNAc or via histochemical techniques. In amperometric studies, NO oxidation current was recorded prior to and during electrical stimulation of the ipsilateral fimbria. Fimbria stimulation elicited a frequency and intensity-dependent increase in msNAc NO efflux which was attenuated by systemic administration of the nNOS inhibitor N^G-propyl-l-arginine. Parallel studies using NADPH-diaphorase histochemistry to assay nNOS activity produced highly complementary outcomes. Moreover, systemic administration of either a DA D1 receptor agonist or a DA D2 receptor antagonist potentiated nNOS activity in the msNAc elicited by fimbria stimulation. These observations demonstrate for the first time that NO synthesis in nNOS expressing interneurons in the msNAc is facilitated by robust activation of hippocampal afferents in a manner that is differentially modulated by DA D1 and D2 receptor activation.

Parkinsonism Driven by Antipsychotics Originates from Dopaminergic Control of Striatal Cholinergic Interneurons

Typical antipsychotics can cause disabling side effects. Specifically, antagonism of D2R signaling by the typical antipsychotic haloperidol induces parkinsonism in humans and catalepsy in rodents. Striatal dopamine D2 receptors (D2R) are major regulators of motor activity through their signaling on striatal projection neurons and interneurons. We show that D2R signaling on cholinergic interneurons contributes to an in vitro pause in firing of these otherwise tonically active neurons and to the striatal dopamine/acetylcholine balance. The selective ablation of D2R from cholinergic neurons allows discrimination between the motor-reducing and cataleptic effects of antipsychotics. The cataleptic effect of antipsychotics is triggered by blockade of D2R on cholinergic interneurons and the consequent increase of acetylcholine signaling on striatal projection neurons. These studies illuminate the critical role of D2R-mediated signaling in regulating the activity of striatal cholinergic interneurons and the mechanisms of typical antipsychotic side effects.

Combining Dopaminergic Facilitation with Robot-Assisted Upper Limb Therapy in Stroke Survivors: A Focused Review

Despite aggressive conventional therapy, lasting hemiplegia persists in a large percentage of stroke survivors. The aim of this article is to critically review the rationale behind targeting multiple sites along the motor learning network by combining robotic therapy with pharmacotherapy and virtual reality–based reward learning to alleviate upper extremity impairment in stroke survivors. Methods for personalizing pharmacologic facilitation to each individual’s unique biology are also reviewed. At the molecular level, treatment with levodopa was shown to induce long-term potentiation-like and practice-dependent plasticity. Clinically, trials combining conventional therapy with levodopa in stroke survivors yielded statistically significant but clinically unconvincing outcomes because of limited personalization, standardization, and reproducibility. Robotic therapy can induce neuroplasticity by delivering intensive, reproducible, and functionally meaningful interventions that are objective enough for the rigors of research. Robotic therapy also provides an apt platform for virtual reality, which boosts learning by engaging reward circuits. The future of stroke rehabilitation should target distinct molecular, synaptic, and cortical sites through personalized multimodal treatments to maximize motor recovery.