Regulation of the dopamine transporter: aspects relevant to psychostimulant drugs of abuse

Dual DAT and sigma receptor inhibitors attenuate cocaine effects on nucleus accumbens dopamine dynamics in rats

Psychostimulant use disorders (PSUD) are prevalent; however, no FDA-approved medications have been made available for treatment. Previous studies have shown that dual inhibitors of the dopamine transporter (DAT) and sigma receptors significantly reduce the behavioral/reinforcing effects of cocaine, which have been associated with stimulation of extracellular dopamine (DA) levels resulting from DAT inhibition. Here, we employ microdialysis and fast scan cyclic voltammetry (FSCV) procedures to investigate the effects of dual inhibitors of DAT and sigma receptors in combination with cocaine on nucleus accumbens shell (NAS) DA dynamics in naïve male Sprague Dawley rats. In microdialysis studies, administration of rimcazole (3, 10 mg/kg; i.p.) or its structural analog SH 3-24 (1, 3 mg/kg; i.p.), compounds that are dual inhibitors of DAT and sigma receptors, significantly reduced NAS DA efflux stimulated by increasing doses of cocaine (0.1, 0.3, 1.0 mg/kg; i.v.). Using the same experimental conditions, in FSCV tests, we show that rimcazole pretreatments attenuated cocaine-induced stimulation of evoked NAS DA release but produced no additional effect on DA clearance rate. Under the same conditions, JJC8-091, a modafinil analog and dual inhibitor of DAT and sigma receptors, similarly attenuated cocaine-induced stimulation of evoked NAS DA release but produced no additional effect on DA clearance rate. Our results provide the neurochemical groundwork towards understanding actions of dual inhibitors of DAT and sigma receptors on DA dynamics that likely mediate the behavioral effects of psychostimulants like cocaine.

Series of (([1,1'-Biphenyl]-2-yl)methyl)sulfinylalkyl Alicyclic Amines as Novel and High Affinity Atypical Dopamine Transporter Inhibitors with Reduced hERG Activity

Currently, there are no FDA-approved medications for the treatment of psychostimulant use disorders (PSUD). We have previously discovered "atypical" dopamine transporter (DAT) inhibitors that do not display psychostimulant-like behaviors and may be useful as medications to treat PSUD. Lead candidates (e.g., JJC8-091, 1) have shown promising in vivo profiles in rodents; however, reducing hERG (human ether-à-go-go-related gene) activity, a predictor of cardiotoxicity, has remained a challenge. Herein, a series of 30 (([1,1'-biphenyl]-2-yl)methyl)sulfinylalkyl alicyclic amines was synthesized and evaluated for DAT and serotonin transporter (SERT) binding affinities. A subset of analogues was tested for hERG activity, and the IC50 values were compared to those predicted by our hERG QSAR models, which showed robust predictive power. Multiparameter optimization scores (MPO > 3) indicated central nervous system (CNS) penetrability. Finally, comparison of affinities in human DAT and its Y156F and Y335A mutants suggested that several compounds prefer an inward facing conformation indicating an atypical DAT inhibitor profile.

Modifications to 1-(4-(2-Bis(4-fluorophenyl)methyl)sulfinyl)alkyl Alicyclic Amines That Improve Metabolic Stability and Retain an Atypical DAT Inhibitor Profile

Atypical dopamine transporter (DAT) inhibitors have shown therapeutic potential in the preclinical models of psychostimulant use disorders (PSUD). In rats, 1-(4-(2-((bis(4-fluorophenyl)methyl)sulfinyl)ethyl)-piperazin-1-yl)-propan-2-ol (JJC8-091, 3b) was effective in reducing the reinforcing effects of both cocaine and methamphetamine but did not exhibit psychostimulant behaviors itself. Improvements in DAT affinity and metabolic stability were desirable for discovering pipeline drug candidates. Thus, a series of 1-(4-(2-bis(4-fluorophenyl)methyl)sulfinyl)alkyl alicyclic amines were synthesized and evaluated for binding affinities at DAT and the serotonin transporter (SERT). Replacement of the piperazine with either a homopiperazine or a piperidine ring system was well tolerated at DAT (Ki range = 3-382 nM). However, only the piperidine analogues (20a-d) showed improved metabolic stability in rat liver microsomes as compared to the previously reported analogues. Compounds 12b and 20a appeared to retain an atypical DAT inhibitor profile, based on negligible locomotor activity in mice and molecular modeling that predicts binding to an inward-facing conformation of DAT.

Human Butyrylcholinesterase Mutants for (-)-Cocaine Hydrolysis: A Correlation Relationship between Catalytic Efficiency and Total Hydrogen Bonding Energy with an Oxyanion Hole

A combined computational and experimental study has been carried out to explore and test a quantitative correlation relationship between the relative catalytic efficiency (RCE) of human butyrylcholinesrase (BChE) mutant-catalyzed hydrolysis of substrate (-)-cocaine and the total hydrogen bonding energy (tHBE) of the carbonyl oxygen of the substrate with the oxyanion hole of the enzyme in the modeled transition-state structure (TS1), demonstrating a satisfactory linear correlation relationship between ln(RCE) and tHBE. The satisfactory correlation relationship has led us to computationally predict and experimentally confirm new human BChE mutants that have a further improved catalytic activity against (-)-cocaine, including the most active one (the A199S/F227S/S287G/A328W/Y332G mutant) with a 2790-fold improved catalytic efficiency (kcat/KM = 2.5 × 109 min-1 M-1) compared to the wild-type human BChE. Compared to the reference mutant (the A199S/S287G/A328W/Y332G mutant) tested in the reported clinical development of an enzyme therapy for cocaine dependence treatment, this new mutant (with a newly predicted additional F227S mutation) has an improved catalytic efficiency against (-)-cocaine by ∼2.6-fold. The good agreement between the computational and experimental ln(RCE) values suggests that the obtained correlation relationship is robust for computational prediction. A similar correlation relationship could also be explored in studying BChE or other serine hydrolases/esterases with an oxyanion hole stabilizing the carbonyl oxygen in the rate-determining reaction step of the enzymatic hydrolysis of other substrates.

Dopamine transporter membrane mobility is bidirectionally regulated by phosphorylation and palmitoylation

The primary regulator of dopamine availability in the brain is the dopamine transporter (DAT), a plasma membrane protein that drives reuptake of released dopamine from the extracellular space into the presynaptic neuron. DAT activity is regulated by post-translational modifications that establish clearance capacity through impacts on transport kinetics, and dysregulation of these events may underlie dopaminergic imbalances in mood and psychiatric disorders. Here, using fluorescence recovery after photobleaching, we show that phosphorylation and palmitoylation induce opposing effects on DAT lateral membrane mobility, which may influence functional outcomes by regulating subcellular localization and binding partner interactions. Membrane mobility was also impacted by amphetamine and in polymorphic variant A559V in directions consistent with enhanced phosphorylation. These findings grow the list of DAT properties controlled by these post-translational modifications and highlight their role in establishment of dopaminergic tone in physiological and pathophysiological states.

Hypocretin / Orexin Receptor 1 Knockdown in GABA or Dopamine Neurons in the Ventral Tegmental Area Differentially Impact Mesolimbic Dopamine and Motivation for Cocaine

The hypocretins/orexins (HCRT) have been demonstrated to influence motivation for cocaine through actions on dopamine (DA) transmission. Pharmacological or genetic disruption of the hypocretin receptor 1 (Hcrtr1) reduces cocaine self-administration, blocks reinstatement of cocaine seeking, and decreases conditioned place preference for cocaine. These effects are likely mediated through actions in the ventral tegmental area (VTA) and resulting alterations in DA transmission. For example, HCRT drives VTA DA neuron activity and enhances the effects of cocaine on DA transmission, while disrupting Hcrtr1 attenuates DA responses to cocaine. These findings have led to the perspective that HCRT exerts its effects through Hcrtr1 actions in VTA DA neurons. However, this assumption is complicated by the observation that Hcrtr1 are present on both DA and GABA neurons in the VTA and HCRT drives the activity of both neuronal populations. To address this issue, we selectively knocked down Hcrtr1 on either DA or GABA neurons in the VTA and examined alterations in DA transmission and cocaine self-administration in female and male rats. We found that Hcrtr1 knockdown in DA neurons decreased DA responses to cocaine, increased days to acquire cocaine self-administration, and reduced motivation for cocaine. Although, Hcrtr1 knockdown in GABA neurons enhanced DA responses to cocaine, this manipulation did not affect cocaine self-administration. These observations indicate that while Hcrtr1 on DA versus GABA neurons exert opposing effects on DA transmission, only Hcrtr1 on DA neurons affected acquisition or motivation for cocaine - suggesting a complex interplay between DA transmission and behavior.

Are There Prevalent Sex Differences in Psychostimulant Use Disorder? A Focus on the Potential Therapeutic Efficacy of Atypical Dopamine Uptake Inhibitors

Psychostimulant use disorders (PSUD) affect a growing number of men and women and exert sizable public health and economic burdens on our global society. Notably, there are some sex differences in the onset of dependence, relapse rates, and treatment success with PSUD observed in preclinical and clinical studies. The subtle sex differences observed in the behavioral aspects of PSUD may be associated with differences in the neurochemistry of the dopaminergic system between sexes. Preclinically, psychostimulants have been shown to increase synaptic dopamine (DA) levels and may downregulate the dopamine transporter (DAT). This effect is greatest in females during the high estradiol phase of the estrous cycle. Interestingly, women have been shown to be more likely to begin drug use at younger ages and report higher levels of desire to use cocaine than males. Even though there is currently no FDA-approved medication, modafinil, a DAT inhibitor approved for use in the treatment of narcolepsy and sleep disorders, has shown promise in the treatment of PSUD among specific populations of affected individuals. In this review, we highlight the therapeutic potential of modafinil and other atypical DAT inhibitors focusing on the lack of sex differences in the actions of these agents.

Dopamine Transmission Imbalance in Neuroinflammation: Perspectives on Long-Term COVID-19

Dopamine (DA) is a key neurotransmitter in the basal ganglia, implicated in the control of movement and motivation. Alteration of DA levels is central in Parkinson’s disease (PD), a common neurodegenerative disorder characterized by motor and non-motor manifestations and deposition of alpha-synuclein (α-syn) aggregates. Previous studies have hypothesized a link between PD and viral infections. Indeed, different cases of parkinsonism have been reported following COVID-19. However, whether SARS-CoV-2 may trigger a neurodegenerative process is still a matter of debate. Interestingly, evidence of brain inflammation has been described in postmortem samples of patients infected by SARS-CoV-2, which suggests immune-mediated mechanisms triggering the neurological sequelae. In this review, we discuss the role of proinflammatory molecules such as cytokines, chemokines, and oxygen reactive species in modulating DA homeostasis. Moreover, we review the existing literature on the possible mechanistic interplay between SARS-CoV-2-mediated neuroinflammation and nigrostriatal DAergic impairment, and the cross-talk with aberrant α-syn metabolism.

Zwitterionic Polymer Coated and Aptamer Functionalized Flexible Micro-Electrode Arrays for In Vivo Cocaine Sensing and Electrophysiology

The number of people aged 12 years and older using illicit drugs reached 59.3 million in 2020, among which 5.2 million are cocaine users based on the national data. In order to fully understand cocaine addiction and develop effective therapies, a tool is needed to reliably measure real-time cocaine concentration and neural activity in different regions of the brain with high spatial and temporal resolution. Integrated biochemical sensing devices based upon flexible microelectrode arrays (MEA) have emerged as a powerful tool for such purposes; however, MEAs suffer from undesired biofouling and inflammatory reactions, while those with immobilized biologic sensing elements experience additional failures due to biomolecule degradation. Aptasensors are powerful tools for building highly selective sensors for analytes that have been difficult to detect. In this work, DNA aptamer-based electrochemical cocaine sensors were integrated on flexible MEAs and protected with an antifouling zwitterionic poly (sulfobetaine methacrylate) (PSB) coating, in order to prevent sensors from biofouling and degradation by the host tissue. In vitro experiments showed that without the PSB coating, both adsorption of plasma protein albumin and exposure to DNase-1 enzyme have detrimental effects on sensor performance, decreasing signal amplitude and the sensitivity of the sensors. Albumin adsorption caused a 44.4% sensitivity loss, and DNase-1 exposure for 24 hr resulted in a 57.2% sensitivity reduction. The PSB coating successfully protected sensors from albumin fouling and DNase-1 enzyme digestion. In vivo tests showed that the PSB coated MEA aptasensors can detect repeated cocaine infusions in the brain for 3 hrs after implantation without sensitivity degradation. Additionally, the same MEAs can record electrophysiological signals at different tissue depths simultaneously. This novel flexible MEA with integrated cocaine sensors can serve as a valuable tool for understanding the mechanisms of cocaine addiction, while the PSB coating technology can be generalized to improve all implantable devices suffering from biofouling and inflammatory host responses.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

A hypothalamic dopamine locus for psychostimulant-induced hyperlocomotion in mice

The lateral septum (LS) has been implicated in the regulation of locomotion. Nevertheless, the neurons synchronizing LS activity with the brain’s clock in the suprachiasmatic nucleus (SCN) remain unknown. By interrogating the molecular, anatomical and physiological heterogeneity of dopamine neurons of the periventricular nucleus (PeVN; A14 catecholaminergic group), we find that Th⁺/Dat1⁺ cells from its anterior subdivision innervate the LS in mice. These dopamine neurons receive dense neuropeptidergic innervation from the SCN. Reciprocal viral tracing in combination with optogenetic stimulation ex vivo identified somatostatin-containing neurons in the LS as preferred synaptic targets of extrahypothalamic A14 efferents. In vivo chemogenetic manipulation of anterior A14 neurons impacted locomotion. Moreover, chemogenetic inhibition of dopamine output from the anterior PeVN normalized amphetamine-induced hyperlocomotion, particularly during sedentary periods. Cumulatively, our findings identify a hypothalamic locus for the diurnal control of locomotion and pinpoint a midbrain-independent cellular target of psychostimulants.

The psychostimulant-sensitive neural mechanism linking the circadian clock to locomotion is unknown. Here, hypothalamic A14 neurons are shown to time diurnal activity by entraining the lateral septum, and their activity is shown to be sensitive to amphetamine.

Cellular and neuronal mechanisms that underlie addiction - literature review

Introduction: Addictive substances act on a number of neurotransmitter systems, and the end result of this action is the activation of the reward system in the brain. The cellular and neuronal mechanisms that underlie addiction have long been searched for. One of such neurotransmitters is dopamine, a catecholamine synthesized in neurons located mainly in the midbrain.

Material and method: The available literature was reviewed on the Pubmed platform and from other sources. The analysis included original studies, reviews.

The aim of the study was to review the literature on the relationship between the DRD2 gene and the occurrence of substance addiction.

Discussion: This work presents several currently discussed biological mechanisms, especially at the molecular and genetic level, involved in the process of addiction to various psychoactive substances. They discovered the brain structures that are most at risk, as well as other neurotransmitter systems and receptor proteins through which they can exert their pathological effects. It has also been established that exposure to psychoactive substances causes significant changes in expression in over 100 genes (including genes for dopaminergic, serotonergic and signaling pathways). The DRD2 receptor (present, among others, in the nucleus accumbens) plays an important role in the reward system, in the transmission of information. The weakening of this conductivity is a significant risk factor for the onset of clinical features that are associated with reward system deficiency syndrome. The expression of the D2 receptor gene may take up to 2 isoforms: short D2S and long D2L.

Conclusions: Further research at the molecular level may result in the modification of psychotherapy and pharmacotherapy in terms of their personalization.

Cannabidiol inhibits methamphetamine-induced dopamine release via modulation of the DRD1-MeCP2-BDNF-TrkB signaling pathway

RATIONALE

Adaptive alteration of dopamine (DA) system in mesocorticolimbic circuits is an extremely intricate and dynamic process, which contributes to maintaining methamphetamine (METH)-related disorders. There are no approved pharmacotherapies for METH-related disorders. Cannabidiol (CBD), a major non-psychoactive constituent of cannabis, has received attention for its therapeutic potential in treating METH-related disorders. However, the major research obstacles of CBD are the yet to be clarified mechanisms behind its therapeutic potential. Recent evidence showed that DA system may be active target of CBD. CBD could be a promising dopaminergic medication for METH-related disorders.

OBJECTIVES

We investigated the role of the DA receptor D1 (DRD1)-methyl-CpG-binding protein 2 (MeCP2)-brain-derived neurotrophic factor (BDNF)-tyrosine receptor kinase B (TrkB) signaling pathway in DA release induced by METH. Investigating the intervention effects of CBD on the DRD1-MeCP2-BDNF-TrkB signaling pathway could help clarify the underlying mechanisms and therapeutic potential of CBD in METH-related disorders.

RESULTS

METH (400 μM) significantly increased DA release from primary neurons in vitro, which was blocked by CBD (1 μM) pretreatment. METH (400 μM) significantly increased the expression levels of DRD1, BDNF, and TrkB, but decreased the expression of MeCP2 in the neurons, whereas CBD (1 μM) pretreatment notably inhibited the protein changes induced by METH. In addition, DRD1 antagonist SCH23390 (10 μM) inhibited the DA release and protein change induced by METH in vitro. However, DRD1 agonist SKF81297 (10 μM) induced DA release and protein change in vitro, which was also blocked by CBD (1 μM) pretreatment. METH (2 mg/kg) significantly increased the DA level in the nucleus accumbens (NAc) of rats with activation of the DRD1-MeCP2-BDNF-TrkB signaling pathway, but these changes were blocked by CBD (40 or 80 mg/kg) pretreatment.

CONCLUSIONS

This study indicates that METH induces DA release via the DRD1-MeCP2-BDNF-TrkB signaling pathway. Furthermore, CBD significantly inhibits DA release induced by METH through modulation of this pathway.

A high-affinity cocaine binding site associated with the brain acid soluble protein 1

Cocaine is a monoamine transport inhibitor. Current models attributing pharmacologic actions of cocaine to inhibiting the activity of the amine transporters alone failed to translate to the clinic. Cocaine inhibition of the dopamine, serotonin, and norepinephrine transporters is relatively weak, suggesting that blockade of the amine transporters alone cannot account for the actions of cocaine, especially at low doses. There is evidence for significantly more potent actions of cocaine, suggesting the existence of a high-affinity receptor(s) for the drug. Identifying and characterizing such receptors will deepen our understanding of cocaine pharmacologic actions and pave the way for therapeutic development. Here we identify a high-affinity cocaine binding site associated with BASP1 that is involved in mediating the drug’s psychotropic actions.

Cocaine exerts its stimulant effect by inhibiting dopamine (DA) reuptake, leading to increased dopamine signaling. This action is thought to reflect the binding of cocaine to the dopamine transporter (DAT) to inhibit its function. However, cocaine is a relatively weak inhibitor of DAT, and many DAT inhibitors do not share cocaine’s behavioral actions. Further, recent reports show more potent actions of the drug, implying the existence of a high-affinity receptor for cocaine. We now report high-affinity binding of cocaine associated with the brain acid soluble protein 1 (BASP1) with a dissociation constant (K_d) of 7 nM. Knocking down BASP1 in the striatum inhibits [³H]cocaine binding to striatal synaptosomes. Depleting BASP1 in the nucleus accumbens but not the dorsal striatum diminishes locomotor stimulation in mice. Our findings imply that BASP1 is a pharmacologically relevant receptor for cocaine.

The role of the SLC6A3 3' UTR VNTR in nicotine effects on cognitive, affective, and motor function

RATIONALE

Nicotine has been widely studied for its pro-dopaminergic effects. However, at the behavioural level, past investigations have yielded heterogeneous results concerning effects on cognitive, affective, and motor outcomes, possibly linked to individual differences at the level of genetics. A candidate polymorphism is the 40-base-pair variable number of tandem repeats polymorphism (rs28363170) in the SLC6A3 gene coding for the dopamine transporter (DAT). The polymorphism has been associated with striatal DAT availability (9R-carriers > 10R-homozygotes), and 9R-carriers have been shown to react more strongly to dopamine agonistic pharmacological challenges than 10R-homozygotes.

OBJECTIVES

In this preregistered study, we hypothesized that 9R-carriers would be more responsive to nicotine due to genotype-related differences in DAT availability and resulting dopamine activity.

METHODS

N=194 non-smokers were grouped according to their genotype (9R-carriers, 10R-homozygotes) and received either 2-mg nicotine or placebo gum in a between-subject design. Spontaneous blink rate (SBR) was obtained as an indirect measure of striatal dopamine activity and smooth pursuit, stop signal, simple choice and affective processing tasks were carried out in randomized order.

RESULTS

Reaction times were decreased under nicotine compared to placebo in the simple choice and stop signal tasks, but nicotine and genotype had no effects on any of the other task outcomes. Conditional process analyses testing the mediating effect of SBR on performance and how this is affected by genotype yielded no significant results.

CONCLUSIONS

Overall, we could not confirm our main hypothesis. Individual differences in nicotine response could not be explained by rs28363170 genotype.

In vivo hydrogen peroxide diffusivity in brain tissue supports volume signaling activity

Hydrogen peroxide is a major redox signaling molecule underlying a novel paradigm of cell function and communication. A role for H₂O₂ as an intercellular signaling molecule and neuromodulator in the brain has become increasingly apparent, with evidence showing this biological oxidant to regulate neuronal polarity, connectivity, synaptic transmission and tuning of neuronal networks. This notion is supported by its ability to diffuse in the extracellular space, from source of production to target. It is, thus, crucial to understand extracellular H₂O₂ concentration dynamics in the living brain and the factors which shape its diffusion pattern and half-life. To address this issue, we have used a novel microsensor to measure H₂O₂ concentration dynamics in the brain extracellular matrix both in an ex vivo model using rodent brain slices and in vivo. We found that exogenously applied H₂O₂ is removed from the extracellular space with an average half-life of t_1/2 = 2.2 s in vivo. We determined the in vivo effective diffusion coefficient of H₂O₂ to be D* = 2.5 × 10⁻⁵ cm² s⁻¹. This allows it to diffuse over 100 μm in the extracellular space within its half-life. Considering this, we can tentatively place H₂O₂ within the class of volume neurotransmitters, connecting all cell types within the complex network of brain tissue, regardless of whether they are physically connected. These quantitative details of H₂O₂ diffusion and half-life in the brain allow us to interpret the physiology of the redox signal and lay the pavement to then address dysregulation in redox homeostasis associated with disease processes.

Cocaine hydrolase blocks cocaine-induced dopamine transporter trafficking to the plasma membrane

Cocaine blocks dopamine uptake via dopamine transporter (DAT) on plasma membrane of neuron cells and, as a result, produces the high and induces DAT trafficking to plasma membrane which contributes to the drug seeking or craving. In this study, we first examined the dose dependence of cocaine-induced DAT trafficking and hyperactivity in rats, demonstrating that cocaine at an intraperitoneal dose of 10 mg/kg or higher led to redistribution of most DAT to the plasma membrane while inducing significant hyperactivity in rats. However, administration of 5-mg/kg cocaine (ip) did not significantly induce DAT trafficking or hyperactivity in rats. So the threshold (intraperitoneal) dose of cocaine that can significantly induce DAT trafficking or hyperactivity should be between 5 and 10 mg/kg. These data suggest that when a cocaine dose is high enough to induce significant hyperactivity, it can also significantly induce DAT trafficking to the plasma membrane. Further, the threshold brain cocaine concentration required to induce significant hyperactivity and DAT trafficking was estimated to be ~2.0 ± 0.8 μg/g. Particularly, for treatment of cocaine abuse, previous studies demonstrated that an exogenous cocaine-metabolizing enzyme, for example, CocH3-Fc(M3), can effectively block cocaine-induced hyperactivity. However, it was unknown whether an enzyme could also effectively block cocaine-induced DAT trafficking to the plasma membrane. This study demonstrates, for the first time, that the enzyme is also capable of effectively blocking cocaine from reaching the brain even with a lethal dose of 60-mg/kg cocaine (ip) and, thus, powerfully preventing cocaine-induced physiological effects such as the hyperactivity and DAT trafficking.

Dopamine Transporter Is a Master Regulator of Dopaminergic Neural Network Connectivity

Dopaminergic neurons of the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) exhibit spontaneous firing activity. The dopaminergic neurons in these regions have been shown to exhibit differential sensitivity to neuronal loss and psychostimulants targeting dopamine transporter. However, it remains unclear whether these regional differences scale beyond individual neuronal activity to regional neuronal networks. Here, we used live-cell calcium imaging to show that network connectivity greatly differs between SNC and VTA regions with higher incidence of hub-like neurons in the VTA. Specifically, the frequency of hub-like neurons was significantly lower in SNC than in the adjacent VTA, consistent with the interpretation of a lower network resilience to SNC neuronal loss. We tested this hypothesis, in DAT-cre/loxP-GCaMP6f mice of either sex, when activity of an individual dopaminergic neuron is suppressed, through whole-cell patch clamp electrophysiology, in either SNC or VTA networks. Neuronal loss in the SNC increased network clustering, whereas the larger number of hub-neurons in the VTA overcompensated by decreasing network clustering in the VTA. We further show that network properties are regulatable via a dopamine transporter but not a D2 receptor dependent mechanism. Our results demonstrate novel regulatory mechanisms of functional network topology in dopaminergic brain regions.SIGNIFICANCE STATEMENT In this work, we begin to untangle the differences in complex network properties between the substantia nigra pars compacta (SNC) and VTA, that may underlie differential sensitivity between regions. The methods and analysis employed provide a springboard for investigations of network topology in multiple deep brain structures and disorders.

Dopamine transporter function fluctuates across sleep/wake state: potential impact for addiction

The dopamine transporter (DAT) has been implicated in a variety of arousal-related processes including the regulation of motor activity, learning, motivated behavior, psychostimulant abuse, and, more recently, sleep/wake state. We previously demonstrated that DAT uptake regulates fluctuations in extracellular dopamine (DA) in the striatum across the light/dark cycle with DA levels at their highest during the dark phase and lowest during the light phase. Despite this evidence, whether fluctuations in DA uptake across the light/dark cycle are associated with changes in sleep/wake has not been tested. To address this, we employed a combination of sleep/wake recordings, fast scan cyclic voltammetry, and western blotting to examine whether sleep/wake state and/or light/dark phase impact DA terminal neurotransmission in male rats. Further, we assessed whether variations in plasma membrane DAT levels and/or phosphorylation of the threonine 53 site on the DAT accounts for fluctuations in DA neurotransmission. Given the extensive evidence indicating that psychostimulants increase DA through interactions with the DAT, we also examined to what degree the effects of cocaine at inhibiting the DAT vary across sleep/wake state. Results demonstrated a significant association between individual sleep/wake states and DA terminal neurotransmission, with higher DA uptake rate, increased phosphorylation of the DAT, and enhanced cocaine potency observed after periods of sleep. These findings suggest that sleep/wake state influences DA neurotransmission in a manner that is likely to impact a host of DA-dependent processes including a variety of neuropsychiatric disorders.

A study of the dopamine transporter using the TRACT assay, a novel in vitro tool for solute carrier drug discovery

Members of the solute carrier (SLC) transporter protein family are increasingly recognized as therapeutic drug targets. The majority of drug screening assays for SLCs are based on the uptake of radiolabeled or fluorescent substrates. Thus, these approaches often have limitations that compromise on throughput or the physiological environment of the SLC. In this study, we report a novel application of an impedance-based biosensor, xCELLigence, to investigate dopamine transporter (DAT) activity via substrate-induced activation of G protein-coupled receptors (GPCRs). The resulting assay, which is coined the 'transporter activity through receptor activation' (TRACT) assay, is based on the hypothesis that DAT-mediated removal of extracellular dopamine directly affects the ability of dopamine to activate cognate membrane-bound GPCRs. In two human cell lines with heterologous DAT expression, dopamine-induced GPCR signaling was attenuated. Pharmacological inhibition or the absence of DAT restored the apparent potency of dopamine for GPCR activation. The inhibitory potencies for DAT inhibitors GBR12909 (pIC50 = 6.2, 6.6) and cocaine (pIC50 = 6.3) were in line with values from reported orthogonal transport assays. Conclusively, this study demonstrates the novel use of label-free whole-cell biosensors to investigate DAT activity using GPCR activation as a readout. This holds promise for other SLCs that share their substrate with a GPCR.

New Drugs, Old Targets: Tweaking the Dopamine System to Treat Psychostimulant Use Disorders

The abuse of illicit psychostimulants such as cocaine and methamphetamine continues to pose significant health and societal challenges. Despite considerable efforts to develop medications to treat psychostimulant use disorders, none have proven effective, leaving an underserved patient population and unanswered questions about what mechanism(s) of action should be targeted for developing pharmacotherapies. As both cocaine and methamphetamine rapidly increase dopamine (DA) levels in mesolimbic brain regions, leading to euphoria that in some can lead to addiction, targets in which this increased dopaminergic tone may be mitigated have been explored. Further, understanding and targeting mechanisms underlying relapse are fundamental to the success of discovering medications that reduce the reinforcing effects of the drug of abuse, decrease the negative reinforcement or withdrawal/negative affect that occurs during abstinence, or both. Atypical inhibitors of the DA transporter and partial agonists/antagonists at DA D₃ receptors are described as two promising targets for future drug development.

Optogenetically-induced multimerization of the dopamine transporter increases uptake and trafficking to the plasma membrane

The dopamine transporter (DAT) is essential for the reuptake of the released neurotransmitter dopamine (DA) in the brain. Psychostimulants, methamphetamine and cocaine, have been reported to induce the formation of DAT multimeric complexes in vivo and in vitro. The interpretation of DAT multimer function has been primarily in the context of compounds that induce structural and functional modifications of the DAT, complicating the understanding of the significance of DAT multimers. To examine multimerization in the absence of DAT ligands as well as in their presence, we developed a novel, optogenetic fusion chimera of cryptochrome 2 and DAT with an mCherry fluorescent reporter (Cry2-DAT). Using blue light to induce Cry2-DAT multimeric protein complex formation, we were able to simultaneously test the functional contributions of DAT multimerization in the absence or presence of substrates or inhibitors with high spatiotemporal precision. We found that blue light-stimulated Cry2-DAT multimers significantly increased IDT307 uptake and MFZ 9-18 binding in the absence of ligands as well as after methamphetamine and nomifensine treatment. Blue light-induced Cry2-DAT multimerization increased colocalization with recycling endosomal marker Rab11 and had decreased presence in Rab5-positive early endosomes and Rab7-positive late endosomes. Our data suggest that the increased uptake and binding results from induced and rapid trafficking of DAT multimers to the plasma membrane. Our data suggest that DAT multimers may function to help maintain DA homeostasis.

Structure-activity relationships for a series of (Bis(4-fluorophenyl)methyl)sulfinylethyl-aminopiperidines and -piperidine amines at the dopamine transporter: Bioisosteric replacement of the piperazine improves metabolic stability

Despite considerable efforts to develop medications to treat psychostimulant use disorders, none have proven effective, leaving an underserved patient population and unanswered questions as to what mechanism(s) of action should be targeted for developing pharmacotherapies. Atypical dopamine transporter (DAT) inhibitors, based on (±)modafinil, have shown therapeutic potential in preclinical models of psychostimulant abuse. However, metabolic instability among other limitations to piperazine analogues 1-3 have impeded further development. Herein, bioisosteric substitutions of the piperazine ring were explored with a series of aminopiperidines (A) and piperidine amines (B) wherein compounds with either a terminal tertiary amine or amide were synthesized. Several lead compounds showed high to moderate DAT affinities and metabolic stability in rat liver microsomes. Aminopiperidines 7 (DAT Ki = 50.6 nM), 21b (DAT Ki = 77.2 nM) and 33 (DAT Ki = 30.0 nM) produced only minimal stimulation of ambulatory activity in mice, compared to cocaine, suggesting an atypical DAT inhibitor profile.

Blockade of D3 receptor prevents changes in DAT and D3R expression in the mesolimbic dopaminergic circuit produced by social stress- and cocaine prime-induced reinstatement of cocaine-CPP

BACKGROUND

Cocaine may cause persistent changes in the brain, which are more apparent in DA transporter (DAT) and DA receptor availability within the nucleus accumbens (NAc). On the other hand, the DA D3 receptor (D3R) has emerged as a promising pharmacotherapeutic target for substance use disorders.

AIMS

This study aims to assess the impact of selective D3R antagonism on DAT and D3R after reinstatement of cocaine preference (CPP) induced by an acute session of social defeat stress (SDS) and a cocaine prime in mice after a period of abstinence.

METHODS

Male mice were conditioned with 25 mg/kg of cocaine for 4 days. After 60 days of extinction training mice were pretreated with the selective D3R antagonist SB-277011A before the re-exposure to a priming dose of cocaine or to a single SDS session. CPP scores were determined and levels of DAT, D3R, phospho Akt (pAkt) and phospho mTOR (pmTOR) were assessed in the NAc shell.

RESULTS

An increase in DAT and D3R expression was seen in the NAc after both a cocaine prime- and SDS-induced reinstatement of CPP. Pretreatment with SB-277011A blocked elevated DAT and D3R expression as well as SDS-induced reinstatement. By contrast, the blockade of D3R did not modified the cocaine prime-induced CPP. Changes in DAT and D3R expression do not seem to occur via the canonic pathway involving Akt/mTOR.

CONCLUSIONS

Our results suggest that the selective D3R antagonist ability to inhibit DAT and D3R up-regulation could represent a possible mechanism for its behavioral effects in cocaine-memories reinstatement induced by social stress.

Toxicokinetics and Toxicodynamics of Ayahuasca Alkaloids N,N-Dimethyltryptamine (DMT), Harmine, Harmaline and Tetrahydroharmine: Clinical and Forensic Impact

Ayahuasca is a hallucinogenic botanical beverage originally used by indigenous Amazonian tribes in religious ceremonies and therapeutic practices. While ethnobotanical surveys still indicate its spiritual and medicinal uses, consumption of ayahuasca has been progressively related with a recreational purpose, particularly in Western societies. The ayahuasca aqueous concoction is typically prepared from the leaves of the N,N-dimethyltryptamine (DMT)-containing Psychotria viridis, and the stem and bark of Banisteriopsis caapi, the plant source of harmala alkaloids. Herein, the toxicokinetics and toxicodynamics of the psychoactive DMT and harmala alkaloids harmine, harmaline and tetrahydroharmine, are comprehensively covered, particularly emphasizing the psychological, physiological, and toxic effects deriving from their concomitant intake. Potential therapeutic utility, particularly in mental and psychiatric disorders, and forensic aspects of DMT and ayahuasca are also reviewed and discussed. Following administration of ayahuasca, DMT is rapidly absorbed and distributed. Harmala alkaloids act as potent inhibitors of monoamine oxidase A (MAO-A), preventing extensive first-pass degradation of DMT into 3-indole-acetic acid (3-IAA), and enabling sufficient amounts of DMT to reach the brain. DMT has affinity for a variety of serotonergic and non-serotonergic receptors, though its psychotropic effects are mainly related with the activation of serotonin receptors type 2A (5-HT_2A). Mildly to rarely severe psychedelic adverse effects are reported for ayahuasca or its alkaloids individually, but abuse does not lead to dependence or tolerance. For a long time, the evidence has pointed to potential psychotherapeutic benefits in the treatment of depression, anxiety, and substance abuse disorders; and although misuse of ayahuasca has been diverting attention away from such clinical potential, research onto its therapeutic effects has now strongly resurged.

Chemogenetic Manipulation of Dopamine Neurons Dictates Cocaine Potency at Distal Dopamine Transporters

The reinforcing efficacy of cocaine is largely determined by its capacity to inhibit the dopamine transporter (DAT), and emerging evidence suggests that differences in cocaine potency are linked to several symptoms of cocaine use disorder. Despite this evidence, the neural processes that govern cocaine potency in vivo remain unclear. In male rats, we used chemogenetics with intra-VTA microinfusions of the agonist clozapine-n-oxide to bidirectionally modulate dopamine neurons. Using ex vivo fast scan cyclic voltammetry, pharmacological probes of the DAT, biochemical assessments of DAT membrane availability and phosphorylation, and cocaine self-administration, we tested the effects of chemogenetic manipulations on cocaine potency at distal DATs in the nucleus accumbens as well as the behavioral economics of cocaine self-administration. We discovered that chemogenetic manipulation of dopamine neurons produced rapid, bidirectional modulation of cocaine potency at DATs in the nucleus accumbens. We then provided evidence that changes in cocaine potency are associated with alterations in DAT affinity for cocaine and demonstrated that this change in affinity coincides with DAT conformation biases and changes in DAT phosphorylation state. Finally, we showed that chemogenetic manipulation of dopamine neurons alters cocaine consumption in a manner consistent with changes in cocaine potency at distal DATs. Based on the spatial and temporal constraints inherent to our experimental design, we posit that changes in cocaine potency are driven by alterations in dopamine neuron activity. When considered together, these observations provide a novel mechanism through which GPCRs regulate cocaine's pharmacological and behavioral effects.SIGNIFICANCE STATEMENT Differences in the pharmacological effects of cocaine are believed to influence the development and progression of cocaine use disorder. However, the biological and physiological processes that determine sensitivity to cocaine remain unclear. In this work, we use a combination of chemogenetics, fast scan cyclic voltammetry, pharmacology, biochemistry, and cocaine self-administration with economic demand analysis to demonstrate a novel mechanism by which cocaine potency is determined in vivo These studies identify a novel process by which the pharmacodynamics of cocaine are derived in vivo, and thus this work has widespread implications for understanding the mechanisms that regulate cocaine consumption across stages of addiction.

Methamphetamine-Induced Neuronal Damage: Neurotoxicity and Neuroinflammation

Methamphetamine (METH) is a highly addictive psychostimulant and one of the most widely abused drugs worldwide. The continuous use of METH eventually leads to drug addiction and causes serious health complications, including attention deficit, memory loss and cognitive decline. These neurological complications are strongly associated with METH-induced neurotoxicity and neuroinflammation, which leads to neuronal cell death. The current review investigates the molecular mechanisms underlying METH-mediated neuronal damages. Our analysis demonstrates that the process of neuronal impairment by METH is closely related to oxidative stress, transcription factor activation, DNA damage, excitatory toxicity and various apoptosis pathways. Thus, we reach the conclusion here that METH-induced neuronal damages are attributed to the neurotoxic and neuroinflammatory effect of the drug. This review provides an insight into the mechanisms of METH addiction and contributes to the discovery of therapeutic targets on neurological impairment by METH abuse.

Clinical potential of a rationally engineered enzyme for treatment of cocaine dependence: Long-lasting blocking of the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine

It is a grand challenge to develop a truly effective treatment of substance use disorder (SUD), particularly for cocaine and other drugs without an FDA-approved treatment available, because a truly effective therapy must effectively block the drug's physiological and reinforcing effects during the entire period of treatment in order to achieve the long-time abstinence required by the FDA. Whether a biologic, such as monoclonal antibody, vaccine, or therapeutic enzyme, can be truly effective for SUD treatment or not has been the subject of extensive debate. The main debate question is whether a biologic, particularly an exogenous enzyme, can effectively block the drug's reinforcing effect. In this report, we demonstrate that a modest dose of a recently redesigned long-acting cocaine hydrolase, CocH3-Fc(M6), can be used to effectively block the psychostimulant, discriminative stimulus, and reinforcing effects of cocaine for a sufficiently long period of time. For example, a dose of 3 mg/kg CocH3-Fc(M6) completely blocked the discriminative stimulus and reinforcing effects for 24/25 days and continued to significantly attenuate/decrease the cocaine effects for at least 29 days in rats. All the animal data consistently suggest that the long-acting cocaine hydrolase is a truly promising candidate of enzyme therapy for treatment of cocaine use disorder.

Ficus exasperata Vahl leaves extract attenuates motor deficit in vanadium-induced parkinsonism mice

Medicinal herbs have played significant roles in the treatment of various diseases in humans and animals. Sodium metavanadate is a potentially toxic environmental pollutant that induces oxidative damage, neurological disorder, Parkinsonism and Parkinson-like disease upon excessive exposure. This study is designed to investigate the impact of saponin fraction of Ficus exasperata Vahl leaf extract (at 50 and 100 mg/kg body weight for 14 days at different animal groupings) on vanadium treated mice. Animals were randomly grouped into five groups. Control (normal saline), NaVO₃ (10 mg/kg for 7 days), withdrawal group, NaVO₃+Vahl (low dose) and NaVO₃+Vahl (high dose). The animals were screened for motor coordination using rotarod and PBTs and a post mortem study was conducted by quantitatively assessing the markers of oxidative stress such as lipid peroxidation, catalase, glutathione activities, and also through immunohistochemistry via glia fibrillary acidic protein, tyrosine hydroxylase and dopamine transporter to study the integrity of astrocytes and dopaminergic neurons of the substantia nigra (SNc). Vanadium-exposed group showed a decreased motor activity on the neurobehavioural tests as well as an increase in markers of oxidative stress. Saponin fraction of F. exasperata Vahl leaves extract produced a statistically significant motor improvement which may be due to high antioxidant activities of saponin, thereby providing an ameliorative effect on the histoarchitecture of the SNc. It can be inferred that the saponin fraction of F. exasperata Vahl leaves extract to possesses ameliorative, motor-enhancing and neurorestorative benefit on motor deficit in vanadium-induced parkinsonism mice.

Promiscuity of in Vitro Secondary Pharmacology Assays and Implications for Lead Optimization Strategies

We conducted an analysis on screening data generated from 1445 compounds against a panel of 130 enzymes, ion channels, and receptors to assess secondary pharmacological risks. Hit rates of these targets as well as physicochemical properties for those hits were evaluated. A majority of targets yielded hits with higher clogP, molecular weight, and more basic character than inactive compounds. Although most targets favored lipophilic hits, the average clogP of hits at a given target did not correlate with its hit rate. Furthermore, a matched pair analysis was completed to determine structural changes that impacted off-target activities. A correlation of binding assays used in this analysis illustrated that some pharmacologically related binding assays are highly correlative and may be substituted for a smaller set of surrogate assays.

Structure-Activity Relationships for a Series of (Bis(4-fluorophenyl)methyl)sulfinyl Alkyl Alicyclic Amines at the Dopamine Transporter: Functionalizing the Terminal Nitrogen Affects Affinity, Selectivity, and Metabolic Stability

Atypical dopamine transporter (DAT) inhibitors have shown therapeutic potential in preclinical models of psychostimulant abuse. In rats, 1-(4-(2-((bis(4-fluorophenyl)methyl)sulfinyl)ethyl)-piperazin-1-yl)-propan-2-ol (3b) was effective in reducing the reinforcing effects of both cocaine and methamphetamine but did not exhibit psychostimulant behaviors itself. While further development of 3b is ongoing, diastereomeric separation, as well as improvements in potency and pharmacokinetics were desirable for discovering pipeline drug candidates. Thus, a series of bis(4-fluorophenyl)methyl)sulfinyl)alkyl alicyclic amines, where the piperazine-2-propanol scaffold was modified, were designed, synthesized, and evaluated for binding affinities at DAT, as well as the serotonin transporter and σ₁ receptors. Within the series, 14a showed improved DAT affinity (K_i = 23 nM) over 3b (K_i = 230 nM), moderate metabolic stability in human liver microsomes, and a hERG/DAT affinity ratio = 28. While 14a increased locomotor activity relative to vehicle, it was significantly lower than activity produced by cocaine. These results support further investigation of 14a as a potential treatment for psychostimulant use disorders.

Epigenetic Regulation of the Ontogenic Expression of the Dopamine Transporter

The dopamine transporter (DAT) is a plasma membrane transport protein responsible for regulating the duration and intensity of dopaminergic signaling. Altered expression of DAT is linked to neurodevelopmental disorders, including attention deficit hyperactivity disorder and autism spectrum disorder, and is shown to contribute to the response of psychotropic drugs and neurotoxicants. Although the postnatal levels of DAT have been characterized, there are few data regarding the mechanisms that regulate postnatal DAT expression. Here, we examine the ontogeny of DAT mRNA from postnatal days 0 to 182 in the rat brain and define a role for epigenetic mechanisms regulating DAT expression. DAT mRNA and protein significantly increased between PND 0 and 6 months in rat midbrain and striatum, respectively. The epigenetic modifiers Dnmt1, Dnmt3a, Dnmt3b, and Hdac2 demonstrated age associated decreases in mRNA expression whereas Hdac5 and Hdac8 showed increased mRNA expression with age. Chromatin immunoprecipitation studies revealed increased protein enrichment of acetylated histone 3 at lysines 9 and 14 and the dopaminergic transcription factors Nurr1 and Pitx3 within the DAT promoter in an age-related manner. Together these studies provide evidence for the role of epigenetic modifications in the regulation of DAT during development. The identification of these mechanisms may contribute to potential therapeutic interventions aimed at neurodevelopmental disorders of the dopaminergic system.

The striatal-enriched protein Rhes is a critical modulator of cocaine-induced molecular and behavioral responses

Previous evidence pointed out a role for the striatal-enriched protein Rhes in modulating dopaminergic transmission. Based on the knowledge that cocaine induces both addiction and motor stimulation, through its ability to enhance dopaminergic signaling in the corpus striatum, we have now explored the involvement of Rhes in the effects associated with this psychostimulant. Our behavioral data showed that a lack of Rhes in knockout animals caused profound alterations in motor stimulation following cocaine exposure, eliciting a significant leftward shift in the dose-response curve and triggering a dramatic hyperactivity. We also found that Rhes modulated either short- or long-term motor sensitization induced by cocaine, since lack of this protein prevents both of them in mutants. Consistent with this in vivo observation, we found that lack of Rhes in mice caused a greater increase in striatal cocaine-dependent D1R/cAMP/PKA signaling, along with considerable enhancement of Arc, zif268, and Homer1 mRNA expression. We also documented that lack of Rhes in mice produced cocaine-related striatal alterations in proteomic profiling, with a differential expression of proteins clustering in calcium homeostasis and cytoskeletal protein binding categories. Despite dramatic striatal alterations associated to cocaine exposure, our data did not reveal any significant changes in midbrain dopaminergic neurons as a lack of Rhes did not affect: (i) DAT activity; (ii) D2R-dependent regulation of GIRK; and (iii) D2R-dependent regulation of dopamine release. Collectively, our results strengthen the view that Rhes acts as a pivotal physiological “molecular brake” for striatal dopaminergic system overactivation induced by psychostimulants, thus making this protein of interest in regulating the molecular mechanism underpinning cocaine-dependent motor stimulatory effects.

Ayahuasca: Psychological and Physiologic Effects, Pharmacology and Potential Uses in Addiction and Mental Illness

Background:

Ayahuasca, a traditional Amazonian decoction with psychoactive properties, is made from bark of the Banisteriopsis caapi vine (containing beta-carboline alkaloids) and leaves of the Psychotria viridis bush (supplying the hallucinogen N,N-dimethyltryptamine, DMT). Originally used by indigenous shamans for the purposes of spirit communi-cation, magical experiences, healing, and religious rituals across several South American countries, ayahuasca has been in-corporated into folk medicine and spiritual healing, and several Brazilian churches use it routinely to foster a spiritual experi-ence. More recently, it is being used in Europe and North America, not only for religious or healing reasons, but also for rec-reation.

Objective:

To review ayahuasca’s behavioral effects, possible adverse effects, proposed mechanisms of action and potential clinical uses in mental illness.

Method:

We searched Medline, in English, using the terms ayahuasca, dimethyltryptamine, Banisteriopsis caapi, and Psy-chotria viridis and reviewed the relevant publications.

Results:

The following aspects of ayahuasca are summarized: Political and legal factors; acute and chronic psychological ef-fects; electrophysiological studies and imaging; physiological effects; safety and adverse effects; pharmacology; potential psychiatric uses.

Conclusion:

Many years of shamanic wisdom have indicated potential therapeutic uses for ayahuasca, and several present day studies suggest that it may be useful for treating various psychiatric disorders and addictions. The side effect profile ap-pears to be relatively mild, but more detailed studies need to be done. Several prominent researchers believe that government regulations with regard to ayahuasca should be relaxed so that it could be provided more readily to recognized, credible re-searchers to conduct comprehensive clinical trials.

No effect of sex on ethanol intake and preference after dopamine transporter (DAT) knockdown in adult mice

RATIONALE

Dopamine levels are controlled in part by transport across the cell membrane by the dopamine transporter (DAT), and recent evidence showed that a polymorphism in the gene encoding DAT is associated with alcoholism. However, research in animal models using DAT knockout mice has yielded conflicting results.

OBJECTIVES

The present study was planned to evaluate the effects of DAT knockdown in the nucleus accumbens (Nacc) on voluntary ethanol consumption and preference in male and female C57BL/6J mice.

METHODS

For this purpose, animals were stereotaxically injected with DAT siRNA-expressing lentiviral vectors in the Nacc, and using a voluntary, continuous access two-bottle choice model of alcohol, we investigated the importance of accumbal DAT expression in voluntary alcohol intake and preference. We also investigated the effects of DAT knockdown on saccharin and quinine consumption and ethanol metabolism.

RESULTS

We show that females consumed more alcohol than males. Interestingly, DAT knockdown in the Nacc significantly decreased alcohol intake and preference in both groups, but no significant sex by group interaction was observed. Also, DAT knockdown did not alter total fluid consumption, saccharin or quinine consumption, or blood ethanol concentrations. Using Pearson correlation, results indicated a strong positive relationship between DAT mRNA expression and ethanol consumption and preference.

CONCLUSIONS

Taken together, these data provide further evidence that DAT plays an important role in controlling ethanol intake and that accumbal DAT contributes in the modulation of the reinforcing effects of ethanol. Overall, the results suggest that DAT inhibitors may be valuable in the pharmacotherapy of alcoholism.

Effects of MDPV on dopamine transporter regulation in male rats. Comparison with cocaine

RATIONALE

MDPV (3,4-methylenedioxypyrovalerone) is a synthetic cathinone present in bath salts. It is a powerful psychostimulant and blocker of the dopamine transporter (DAT), like cocaine. It is known that acute exposure to psychostimulants induces rapid changes in DAT function.

OBJECTIVES

To investigate the effects of MDPV on DAT function comparing with cocaine.

METHODS

Binding of [³H]WIN 35428 was performed on PC 12 cells treated with MDPV and washed. Rat striatal synaptosomes were incubated with MDPV or cocaine (1 μM) for 1 h and [³H]dopamine (DA) uptake was performed. Also, different treatments with MDPV or cocaine were performed in Sprague-Dawley rats to assess locomotor activity and ex vivo [³H]DA uptake.

RESULTS

MDPV increased surface [³H]WIN 35428 binding on PC 12 cells. In vitro incubation of synaptosomes with MDPV produced significant increases in V_max and K_M for [³H]DA uptake. In synaptosomes from MDPV- (1.5 mg/kg, s.c.) and cocaine- (30 mg/kg, i.p.) treated rats, there was a significantly higher and more persistent increase in [³H]DA uptake in the case of MDPV than cocaine. Repeated doses of MDPV developed tolerance to this DAT upregulation and 24 h after the 5-day treatment with MDPV, [³H]DA uptake was reduced. However, a challenge with the same drugs after withdrawal recovered the DAT upregulation by both drugs and showed an increased response to MDPV vs the first dose. At the same time, animals were sensitized to the stereotypies induced by both psychostimulants.

CONCLUSIONS

MDPV induces a rapid and reversible functional upregulation of DAT more powerfully and lasting than cocaine.

Development of a Semimechanistic Pharmacokinetic-Pharmacodynamic Model Describing Dextroamphetamine Exposure and Striatal Dopamine Response in Rats and Nonhuman Primates following a Single Dose of Dextroamphetamine

Acute central nervous system exposure to dextroamphetamine (d-amphetamine) elicits a multitude of effects, including dual action on the dopamine transporter (DAT) to increase extracellular dopamine, and induction of a negative feedback response to limit the dopamine increase. A semimechanistic pharmacokinetic and pharmacodynamic (PK/PD) model with consideration of these multiple effects as a basis was developed. Integrated pharmacokinetics of d-amphetamine in plasma, brain extracellular fluid (ECF) via microdialysis, and cerebrospinal fluid were characterized using a population approach. This PK model was then linked to an indirect-response pharmacodynamic model using as a basis the measurement of extracellular striatal dopamine, also via microdialysis. In both rats and nonhuman primates (NHPs), d-amphetamine stimulation of dopamine outflow (reverse transport) through DAT was primarily responsible for the dose-linear increase in dopamine. As well, in both species a moderator function was needed to account for loss of the dopamine response in the presence of a relatively sustained d-amphetamine ECF exposure, presumptive of an acute tolerance response. PK/PD model structure was consistent between species; however, there was a 10-fold faster return to baseline dopamine in NHPs in response to an acute d-amphetamine challenge. These results suggest preservation from rodents to NHPs regarding the mechanism by which amphetamine increases extracellular dopamine, but a faster system response in NHPs to tolerate this increase. This microdialysis-based PK/PD model suggests greater value in directing preclinical discovery of novel approaches that modify reverse transport stimulation to treat amphetamine abuse. General value regarding insertion of an NHP model in paradigm rodent-to-human translational research is also suggested.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

Drugs for Insomnia beyond Benzodiazepines: Pharmacology, Clinical Applications, and Discovery

Although the GABAergic benzodiazepines (BZDs) and Z-drugs (zolpidem, zopiclone, and zaleplon) are FDA-approved for insomnia disorders with a strong evidence base, they have many side effects, including cognitive impairment, tolerance, rebound insomnia upon discontinuation, car accidents/falls, abuse, and dependence liability. Consequently, the clinical use of off-label drugs and novel drugs that do not target the GABAergic system is increasing. The purpose of this review is to analyze the neurobiological and clinical evidence of pharmacological treatments of insomnia, excluding the BZDs and Z-drugs. We analyzed the melatonergic agonist drugs, agomelatine, prolonged-release melatonin, ramelteon, and tasimelteon; the dual orexin receptor antagonist suvorexant; the modulators of the α₂δ subunit of voltage-sensitive calcium channels, gabapentin and pregabalin; the H₁ antagonist, low-dose doxepin; and the histamine and serotonin receptor antagonists, amitriptyline, mirtazapine, trazodone, olanzapine, and quetiapine. The pharmacology and mechanism of action of these treatments and the evidence-base for the use of these drugs in clinical practice is outlined along with novel pipelines. There is evidence to recommend suvorexant and low-dose doxepin for sleep maintenance insomnia; there is also sufficient evidence to recommend ramelteon for sleep onset insomnia. Although there is limited evidence for the use of the quetiapine, trazodone, mirtazapine, amitriptyline, pregabalin, gabapentin, agomelatine, and olanzapine as treatments for insomnia disorder, these drugs may improve sleep while successfully treating comorbid disorders, with a different side effect profile than the BZDs and Z-drugs. The unique mechanism of action of each drug allows for a more personalized and targeted medical management of insomnia.

Trends in use of prescription stimulants in the United States and Territories, 2006 to 2016

BACKGROUND

Stimulants are considered the first-line treatment for Attention Deficit Hyperactivity Disorder (ADHD) in the US and they are used in other indications. Stimulants are also diverted for non-medical purposes. Ethnic and regional differences in ADHD diagnosis and in stimulant use have been identified in earlier research. The objectives of this report were to examine the pharmacoepidemiological pattern of these controlled substances over the past decade and to conduct a regional analysis.

METHODS

Data (drug weights) reported to the US Drug Enforcement Administration's Automation of Reports and Consolidated Orders System for four stimulants (amphetamine, methylphenidate, lisdexamfetamine, and methamphetamine) were obtained from 2006 to 2016 for Unites States/Territories. Correlations between state level use (mg/person) and Hispanic population were completed.

RESULTS

Amphetamine use increased 2.5 fold from 2006 to 2016 (7.9 to 20.0 tons). Methylphenidate use, at 16.5 tons in 2006, peaked in 2012 (19.4 tons) and subsequently showed a modest decline (18.6 tons in 2016). The consumption per municipality significantly increased 7.6% for amphetamine and 5.5% for lisdexamfetamine but decreased 2.7% for methylphenidate (all p < .0005) from 2015 to 2016. Pronounced regional differences were also observed. Lisdexamfetamine use in 2016 was over thirty-fold higher in the Southern US (43.8 mg/person) versus the Territories (1.4 mg/person). Amphetamine use was about one-third lower in the West (48.1 mg/person) relative to the Northeastern (75.4 mg/person, p < .05) or the Midwestern (69.9 mg/person, p ≤ .005) states. States with larger Hispanic populations had significantly lower methylphenidate (r(49) = -0.63), lisdexamfetamine (B, r(49) = -0.49), and amphetamine (r(49) = -0.43) use.

CONCLUSIONS

Total stimulant usage doubled in the last decade. There were dynamic changes but also regional disparities in the use of stimulant medications. Future research is needed to better understand the reasons for the sizable regional and ethnic variations in use of these controlled substances.

Activation of RHO-1 in cholinergic motor neurons competes with dopamine signalling to control locomotion

The small GTPase RhoA plays a crucial role in the regulation of neuronal signalling to generate behaviour. In the developing nervous system RhoA is known to regulate the actin cytoskeleton, however the effectors of RhoA-signalling in adult neurons remain largely unidentified. We have previously shown that activation of the RhoA ortholog (RHO-1) in C. elegans cholinergic motor neurons triggers hyperactivity of these neurons and loopy locomotion with exaggerated body bends. This is achieved in part through increased diacylglycerol (DAG) levels and the recruitment of the synaptic vesicle protein UNC-13 to synaptic release sites, however other pathways remain to be identified. Dopamine, which is negatively regulated by the dopamine re-uptake transporter (DAT), has a central role in modulating locomotion in both humans and C. elegans. In this study we identify a new pathway in which RHO-1 regulates locomotory behaviour by repressing dopamine signalling, via DAT-1, linking these two pathways together. We observed an upregulation of dat-1 expression when RHO-1 is activated and show that loss of DAT-1 inhibits the loopy locomotion phenotype caused by RHO-1 activation. Reducing dopamine signalling in dat-1 mutants through mutations in genes involved in dopamine synthesis or in the dopamine receptor DOP-1 restores the ability of RHO-1 to trigger loopy locomotion in dat-1 mutants. Taken together, we show that negative regulation of dopamine signalling via DAT-1 is necessary for the neuronal RHO-1 pathway to regulate locomotion.

Downregulation of thioredoxin-1 in the ventral tegmental area delays extinction of methamphetamine-induced conditioned place preference

BACKGROUND

Drug addiction is characterized by compulsive drug use and relapse. Thioredoxin-1 is emerging as an important modulator involved in the cellular protective response against a variety of toxic stressors. Previous study has reported that thioredoxin-1 overexpression prevents the acquisition of methamphetamine-conditioned place preference. Here, we aimed to investigate the effect of thioredoxin-1 on methamphetamine-conditioned place preference extinction and the possible mechanism.

METHODS

(a) An extinction procedure in mice was employed to investigate the effect of thioredoxin-1 on the extinction of methamphetamine-conditioned place preference. After the acquisition of methamphetamine-conditioned place preference, mice underwent the following procedures: the injection of thioredoxin-1 small interfering RNA in the ventral tegmental area followed by the post-conditioned place preference test, four days of extinction training followed by four days of recovery after surgery. (b) The levels of thioredoxin-1, dopamine D1 receptor, tyrosine hydroxylase, phosphorylated extracellular regulated kinase, and phosphorylated cyclic adenosine monophosphate response element binding protein were examined by using Western blot analysis.

RESULTS

Thioredoxin-1 downregulation in the ventral tegmental area delayed methamphetamine-conditioned place preference extinction. The expression of thioredoxin-1 was decreased in the ventral tegmental area of mice in control and negative groups after methamphetamine-conditioned place preference extinction, but not in the thioredoxin-1 siRNA group. The levels of dopamine D1 receptor, tyrosine hydroxylase, phosphorylated extracellular regulated kinase, and phosphorylated cyclic adenosine monophosphate response element binding protein were decreased in the ventral tegmental area, nucleus accumbens, and prefrontal cortex of mice in the control and negative groups after methamphetamine-conditioned place preference extinction, but were inversely increased in thioredoxin-1 siRNA group.

CONCLUSIONS

The results suggest that downregulation of thioredoxin-1 in the ventral tegmental area may delay methamphetamine-conditioned place preference extinction by regulating the mesocorticolimbic dopaminergic signaling pathway.

Dopaminergic Disturbances in Tourette Syndrome: An Integrative Account

Tourette syndrome (TS) is thought to involve dopaminergic disturbances, but the nature of those disturbances remains controversial. Existing hypotheses suggest that TS involves 1) supersensitive dopamine receptors, 2) overactive dopamine transporters that cause low tonic but high phasic dopamine, 3) presynaptic dysfunction in dopamine neurons, or 4) dopaminergic hyperinnervation. We review evidence that contradicts the first two hypotheses; we also note that the last two hypotheses have traditionally been considered too narrowly, explaining only small subsets of findings. We review all studies that have used positron emission tomography and single-photon emission computerized tomography to investigate the dopaminergic system in TS. The seemingly diverse findings from those studies have typically been interpreted as pointing to distinct mechanisms, as evidenced by the various hypotheses concerning the nature of dopaminergic disturbances in TS. We show, however, that the hyperinnervation hypothesis provides a simple, parsimonious explanation for all such seemingly diverse findings. Dopaminergic hyperinnervation likely causes increased tonic and phasic dopamine. We have previously shown, using a computational model of the role of dopamine in basal ganglia, that increased tonic dopamine and increased phasic dopamine likely increase the propensities to express and learn tics, respectively. There is therefore a plausible mechanistic link between dopaminergic hyperinnervation and TS via increased tonic and phasic dopamine. To further bolster this argument, we review evidence showing that all medications that are effective for TS reduce signaling by tonic dopamine, phasic dopamine, or both.

Efavirenz exposure, alone and in combination with known drugs of abuse, engenders addictive-like bio-behavioural changes in rats

Efavirenz is abused in a cannabis-containing mixture known as Nyaope. The addictive-like effects of efavirenz (5, 10 and 20 mg/kg) was explored using conditioned place preference (CPP) in rats following sub-acute exposure vs. methamphetamine (MA; 1 mg/kg) and Δ⁹-tetrahydrocannabinol (THC; 0.75 mg/kg). The most addictive dose of efavirenz was then compared to THC alone and THC plus efavirenz following sub-chronic exposure using multiple behavioural measures, viz. CPP, sucrose preference test (SPT) and locomotor activity. Peripheral superoxide dismutase (SOD), regional brain lipid peroxidation and monoamines were also determined. Sub-acute efavirenz (5 mg/kg) had a significant rewarding effect in the CPP comparable to MA and THC. Sub-chronic efavirenz (5 mg/kg) and THC + efavirenz were equally rewarding using CPP, with increased cortico-striatal dopamine (DA), and increased lipid peroxidation and SOD. Sub-chronic THC did not produce CPP but significantly increased SOD and decreased hippocampal DA. Sub-chronic THC + efavirenz was hedonic in the SPT and superior to THC alone regarding cortico-striatal lipid peroxidation and sucrose preference. THC + efavirenz increased cortico-striatal DA and decreased serotonin (5-HT). Concluding, efavirenz has dose-dependent rewarding effects, increases oxidative stress and alters regional brain monoamines. Efavirenz is hedonic when combined with THC, highlighting its abuse potential when combined with THC.

Mesolimbic dopamine system and its modulation by vitamin D in a chronic mild stress model of depression in the rat

Depression, a common mood disorder, involves anhedonia and defects in reward circuits and mesolimbic dopamine transmission in the striatum and nucleus accumbens (NAc). Active vitamin-D, (1,25-(OH)₂ vitamin-D3), exerts protective and regulatory effects on the brain dopamine system. In this study, key depression-like symptoms were induced in rats by chronic mild-stress (CMS) and the comparative effect of treatment with 1,25-(OH)₂ vitamin-D3 (5, 10 μg/kg, or vehicle; i.p., twice weekly) or fluoxetine (5 mg/kg or vehicle, i.p., daily) on anhedonic behavior, locomotor activity and anxiety-like behavior was examined using sucrose preference test (SPT), open field test (OFT) and novel object exploration test (NOT), respectively. We also measured serum corticosterone levels and dopamine transporter-immunoreactivity (DAT-ir) levels in NAc shell and core. CMS exposure for 3 weeks was followed by a SPT and thereafter CMS was continued for 5 weeks, along with vitamin-D or fluoxetine treatment and further testing, which was concluded with another SPT. Vitamin-D treatment enhanced sucrose preference (P < 0.01; an hedonic effect) and increased object exploration (P < 0.01) in CMS rats. CMS significantly reduced the level of DAT-ir in NAc (P < 0.0001). Vitamin-D treatment restored/increased DAT-ir levels (P < 0.0001) in CMS rat NAc (core/ shell), compared to levels in fluoxetine treated and non-treated CMS rats. Vitamin-D did not alter locomotor activity or produce an anxiolytic effect in the OFT. These data suggest that similar to the antidepressant, fluoxetine, regular vitamin-D treatment can improve 'anhedonia-like symptoms' in rats subjected to CMS, probably by regulating the effect of dopamine-related actions in the NAc.

Single Quantum Dot Tracking Illuminates Neuroscience at the Nanoscale

The use of nanometer-sized semiconductor crystals, known as quantum dots, allows us to directly observe individual biomolecular transactions through a fluorescence microscope. Here, we review the evolution of single quantum dot tracking over the past two decades, highlight key biophysical discoveries facilitated by quantum dots, briefly discuss biochemical and optical implementation strategies for a single quantum dot tracking experiment, and report recent accomplishments of our group at the interface of molecular neuroscience and nanoscience.

Low Striatal Dopamine D2-type Receptor Availability is Linked to Simulated Drug Choice in Methamphetamine Users

Individuals with drug use disorders seek drugs over other rewarding activities, and exhibit neurochemical deficits related to dopamine, which is involved in value-based learning and decision-making. Thus, a dopaminergic disturbance may underpin drug-biased choice in addiction. Classical drug-choice assessments, which offer drug-consumption opportunities, are inappropriate for addicted individuals seeking treatment or abstaining. Fifteen recently abstinent methamphetamine users and 15 healthy controls completed two laboratory paradigms of 'simulated' drug choice (choice for drug-related vs affectively pleasant, unpleasant, and neutral images), and underwent positron emission tomography measurements of dopamine D2-type receptor availability, indicated by binding potential (BP_ND) for [¹⁸F]fallypride. Thirteen of the methamphetamine users and 10 controls also underwent [¹¹C]NNC112 PET scans to measure dopamine D1-type receptor availability. Group analyses showed that, compared with controls, methamphetamine users chose to view more methamphetamine-related images on one task, with a similar trend on the second task. Regression analyses showed that, on both tasks, the more methamphetamine users chose to view methamphetamine images, specifically vs pleasant images (the most frequently chosen images across all participants), the lower was their D2-type BP_ND in the lateral orbitofrontal cortex, an important region in value-based choice. No associations were observed with D2-type BP_ND in striatal regions, or with D1-type BP_ND in any region. These results identify a neurochemical correlate for a laboratory drug-seeking paradigm that can be administered to treatment-seeking and abstaining drug-addicted individuals. More broadly, these results refine the central hypothesis that dopamine-system deficits contribute to drug-biased decision-making in addiction, here showing a role for the orbitofrontal cortex.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.