Behavior of knock-in mice with a cocaine-insensitive dopamine transporter after virogenetic restoration of cocaine sensitivity in the striatum

The SERT Met172 Mouse: An Engineered Model To Elucidate the Contributions of Serotonin Signaling to Cocaine Action

Cocaine abuse and addiction remain highly prevalent and, unfortunately, poorly treated. It is well-known that essential aspects of cocaine's addictive actions involve the drug's ability to block the presynaptic dopamine (DA) transporter (DAT), thereby elevating extracellular levels of DA in brain circuits that subserve reward, reinforcement, and habit. Less well appreciated are the multiple DA-independent actions of cocaine, activities that we and others believe contribute key pieces to the puzzle of cocaine addiction, treatment, and relapse. In particular, a significant body of work points to altered serotonin (5-HT) signaling as one such component, not surprising given that, relative to DAT, cocaine acts as potently to block the 5-HT transporter (SERT) as to block DAT, and thereby elevates extracellular 5-HT levels throughout the brain when reward-eliciting DA elevations occur. To elucidate the contribution of SERT antagonism to the actions of cocaine, we engineered a mouse model that significantly reduces cocaine potency at SERT without disrupting the expression or function of SERT in vivo. In this short Perspective, we review the rationale for development of the SERT Met172 model, the studies that document the pharmacological impact of the Ile172Met substitution in vitro and in vivo, and our findings with the model that demonstrate serotonergic contributions to the genetic, physiological, and behavioral actions of cocaine.

A microfluidic platform for continuous monitoring of dopamine homeostasis in dopaminergic cells

Homeostasis of dopamine, a classical neurotransmitter, is a key indicator of neuronal health. Dysfunction in the regulation of dopamine is implicated in a long list of neurological disorders, including addiction, depression, and neurodegeneration. The existing methods used to evaluate dopamine homeostasis in vitro are inconvenient and do not allow for continuous non-destructive measurement. In response to this challenge, we introduce an integrated microfluidic system that combines dopaminergic cell culture and differentiation with electroanalytical measurements of extracellular dopamine in real-time at any point during an assay. We used the system to examine the behavior of differentiated SH-SY5Y cells upon exposure to four dopamine transporter ant/agonists (cocaine, ketamine, epigallocatechin gallate, and amphetamine) and study their pharmacokinetics. The IC₅₀ values of cocaine, ketamine, and epigallocatechin gallate were determined to be (average ± standard deviation) 3.7 ± 1.1 µM, 51.4 ± 17.9 µM, and 2.6 ± 0.8 µM, respectively. Furthermore, we used the new system to study amphetamine-mediated dopamine release to probe the related phenomena of dopamine transporter-mediated reverse-transport and dopamine release from vesicles. We propose that this platform, which is the first platform to simultaneously evaluate uptake and release, could be useful to screen for drugs and other agents that target dopaminergic neurons and the function of the dopamine transporter. More broadly, this platform should be adaptable for any application that could benefit from high-temporal resolution electroanalysis combined with multi-day cell culture using small numbers of cells.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Melatonin receptors limit dopamine reuptake by regulating dopamine transporter cell-surface exposure

Melatonin, a neuro-hormone released by the pineal gland, has multiple effects in the central nervous system including the regulation of dopamine (DA) levels, but how melatonin accomplishes this task is not clear. Here, we show that melatonin MT1 and MT2 receptors co-immunoprecipitate with the DA transporter (DAT) in mouse striatal synaptosomes. Increased DA re-uptake and decreased amphetamine-induced locomotor activity were observed in the striatum of mice with targeted deletion of MT1 or MT2 receptors. In vitro experiments confirmed the interactions and recapitulated the inhibitory effect of melatonin receptors on DA re-uptake. Melatonin receptors retained DAT in the endoplasmic reticulum in its immature non-glycosylated form. In conclusion, we reveal one of the first molecular complexes between G protein-coupled receptors (MT1 and MT2) and transporters (DAT) in which melatonin receptors regulate the availability of DAT at the plasma membrane, thus limiting the striatal DA re-uptake capacity in mice.

Ethanol actions on the ventral tegmental area: novel potential targets on reward pathway neurons

The ventral tegmental area (VTA) evaluates salience of environmental stimuli and provides dopaminergic innervation to many brain areas affected by acute and chronic ethanol exposure. While primarily associated with rewarding and reinforcing stimuli, recent evidence indicates a role for the VTA in aversion as well. Ethanol actions in the VTA may trigger neuroadaptation resulting in reduction of the aversive responses to alcohol and a relative increase in the rewarding responses. In searching for effective pharmacotherapies for the treatment of alcohol abuse and alcoholism, recognition of this imbalance may reveal novel strategies. In addition to conventional receptor/ion channel pharmacotherapies, epigenetic factors that control neuroadaptation to chronic ethanol treatment can be targeted as an avenue for development of therapeutic approaches to restore the balance. Furthermore, when exploring therapies to address reward/aversion imbalance in the action of alcohol in the VTA, sex differences have to be taken into account to ensure effective treatment for both men and women. These principles apply to a VTA-centric approach to therapies, but should hold true when thinking about the overall approach in the development of neuroactive drugs to treat alcohol use disorders. Although the functions of the VTA itself are complex, it is a useful model system to evaluate the reward/aversion imbalance that occurs with ethanol exposure and could be used to provide new leads in the efforts to develop novel drugs to treat alcoholism.

Dopamine transporter mutant animals: a translational perspective

The dopamine transporter (DAT) plays an important homeostatic role in the control of both the extracellular and intraneuronal concentrations of dopamine, thereby providing effective control over activity of dopaminergic transmission. Since brain dopamine is known to be involved in numerous neuropsychiatric disorders, investigations using mice with genetically altered DAT function and thus intensity of dopamine-mediated signaling have provided numerous insights into the pathology of these disorders and novel pathological mechanisms that could be targeted to provide new therapeutic approaches for these disorders. In this brief overview, we discuss recent investigations involving animals with genetically altered DAT function, particularly focusing on translational studies providing new insights into pathology and pharmacology of dopamine-related disorders. Perspective applications of these and newly developed models of DAT dysfunction are also discussed.

Poly (ADP-Ribose) Polymerase-1 (PARP-1) Induction by Cocaine Is Post-Transcriptionally Regulated by miR-125b

Cocaine exposure alters gene expression in the brain via methylation and acetylation of histones along with methylation of DNA. Recently, poly (ADP-ribose) polymerase-1 (PARP-1) catalyzed PARylation has been reported as an important regulator of cocaine-mediated gene expression. In this study, we report that the cellular microRNA “miR-125b” plays a key role for cocaine-induced PARP-1 expression. Acute and chronic cocaine exposure resulted in the downregulation of miR-125b concurrent with upregulation of PARP-1 in dopaminergic neuronal cells and nucleus accumbens (NAc) of mice but not in the medial prefrontal cortex (PFC) or ventral tegmental area (VTA). In silico analysis predicted a binding site of miR-125b in a conserved 3’-untranslated region (3’UTR) of the PARP-1 mRNA. Knockdown and overexpression studies showed that miR-125b levels negatively correlate with PARP-1 protein expression. Luciferase reporter assay using a vector containing the 3’UTR of PARP-1 mRNA confirmed regulation of PARP-1 by miR-125b. Specific nucleotide mutations within the binding site abrogated miR-125b’s regulatory effect on PARP-1 3’UTR. Finally, we established that downregulation of miR-125b and concurrent upregulation of PARP-1 is dependent on binding of cocaine to the dopamine transporter (DAT). Collectively, these results identify miR-125b as a post-transcriptional regulator of PARP-1 expression and establish a novel mechanism underlying the molecular effects of cocaine action.

Blockade of the 5-HT transporter contributes to the behavioural, neuronal and molecular effects of cocaine

BACKGROUND AND PURPOSE

The psychostimulant cocaine induces complex molecular, cellular and behavioural responses as a consequence of inhibiting presynaptic dopamine, noradrenaline and 5-HT transporters. To elucidate 5-HT transporter (SERT)-specific contributions to cocaine action, we evaluated cocaine effects in the SERT Met172 knock-in mouse, which expresses a SERT coding substitution that eliminates high-affinity cocaine recognition.

EXPERIMENTAL APPROACH

We measured the effects of SERT Met172 on cocaine antagonism of 5-HT re-uptake using ex vivo synaptosome preparations and in vivo microdialysis. We assessed SERT dependence of cocaine actions behaviourally through acute and chronic locomotor activation, sensitization, conditioned place preference (CPP) and oral cocaine consumption. We used c-Fos, quantitative RT-PCR and RNA sequencing methods for insights into cellular and molecular networks supporting SERT-dependent cocaine actions.

KEY RESULTS

SERT Met172 mice demonstrated functional insensitivity for cocaine at SERT. Although they displayed wild-type levels of acute cocaine-induced hyperactivity or chronic sensitization, the pattern of acute motor activation was different, with a bias toward thigmotaxis. CPP was increased, and a time-dependent elevation in oral cocaine consumption was observed. SERT Met172 mice displayed relatively higher levels of neuronal activation in the hippocampus, piriform cortex and prelimbic cortex (PrL), accompanied by region-dependent changes in immediate early gene expression. Distinct SERT-dependent gene expression networks triggered by acute and chronic cocaine administration were identified, including PrL Akt and nucleus accumbens ERK1/2 signalling.

CONCLUSION AND IMPLICATIONS

Our studies reveal distinct SERT contributions to cocaine action, reinforcing the possibility of targeting specific aspects of cocaine addiction by modulation of 5-HT signalling.

Interactions of Cathinone NPS with Human Transporters and Receptors in Transfected Cells

Pharmacological assays carried out in transfected cells have been very useful for describing the mechanism of action of cathinone new psychoactive substances (NPS). These in vitro characterizations provide fast and reliable information on psychoactive substances soon after they emerge for recreational use. Well-investigated comparator compounds, such as methamphetamine, 3,4-methylenedioxymethamphetamine, cocaine, and lysergic acid diethylamide, should always be included in the characterization to enhance the translation of the in vitro data into clinically useful information. We classified cathinone NPS according to their pharmacology at monoamine transporters and receptors. Cathinone NPS are monoamine uptake inhibitors and most induce transporter-mediated monoamine efflux with weak to no activity at pre- or postsynaptic receptors. Cathinones with a nitrogen-containing pyrrolidine ring emerged as NPS that are extremely potent transporter inhibitors but not monoamine releasers. Cathinones exhibit clinically relevant differences in relative potencies at serotonin vs. dopamine transporters. Additionally, cathinone NPS have more dopaminergic vs. serotonergic properties compared with their non-β-keto amphetamine analogs, suggesting more stimulant and reinforcing properties. In conclusion, in vitro pharmacological assays in heterologous expression systems help to predict the psychoactive and toxicological effects of NPS.

Inhibition of the Serotonin Transporter Is Altered by Metabolites of Selective Serotonin and Norepinephrine Reuptake Inhibitors and Represents a Caution to Acute or Chronic Treatment Paradigms

Previous studies of transgenic mice carrying a single isoleucine to methionine substitution (I172M) in the serotonin transporter (SERT) demonstrated a loss of sensitivity to multiple antidepressants (ADs) at SERT. However, the ability of AD metabolites to antagonize SERT was not assessed. Here, we evaluated the selectivity and potency of these metabolites for inhibition of SERT in mouse brain-derived synaptosomes and blood platelets from wild-type (I172 mSERT) and the antidepressant-insensitive mouse M172 mSERT. The metabolites norfluoxetine and desmethylsertraline lost the selectivity demonstrated by the parent compounds for inhibition of wild-type mSERT over M172 mSERT, whereas desvenlafaxine and desmethylcitalopram retained selectivity. Furthermore, we show that the metabolite desmethylcitalopram accumulates in the brain and that the metabolites desmethylcitalopram, norfluoxetine, and desvenlafaxine inhibit serotonin uptake in wild-type mSERT at potencies similar to those of their parent compounds, suggesting that metabolites may play a role in effects observed following AD administration in wild-type and M172 mice.

A Single Amphetamine Infusion Reverses Deficits in Dopamine Nerve-Terminal Function Caused by a History of Cocaine Self-Administration

There are ∼ 1.6 million people who meet the criteria for cocaine addiction in the United States, and there are currently no FDA-approved pharmacotherapies. Amphetamine-based dopamine-releasing drugs have shown efficacy in reducing the motivation to self-administer cocaine and reducing intake in animals and humans. It is hypothesized that amphetamine acts as a replacement therapy for cocaine through elevation of extracellular dopamine levels. Using voltammetry in brain slices, we tested the ability of a single amphetamine infusion in vivo to modulate dopamine release, uptake kinetics, and cocaine potency in cocaine-naive animals and after a history of cocaine self-administration (1.5 mg/kg/infusion, fixed-ratio 1, 40 injections/day × 5 days). Dopamine kinetics were measured 1 and 24 h after amphetamine infusion (0.56 mg/kg, i.v.). Following cocaine self-administration, dopamine release, maximal rate of uptake (Vmax), and membrane-associated dopamine transporter (DAT) levels were reduced, and the DAT was less sensitive to cocaine. A single amphetamine infusion reduced Vmax and membrane DAT levels in cocaine-naive animals, but fully restored all aspects of dopamine terminal function in cocaine self-administering animals. Here, for the first time, we demonstrate pharmacologically induced, immediate rescue of deficits in dopamine nerve-terminal function in animals with a history of high-dose cocaine self-administration. This observation supports the notion that the DAT expression and function can be modulated on a rapid timescale and also suggests that the pharmacotherapeutic actions of amphetamine for cocaine addiction go beyond that of replacement therapy.

A Role for p38 Mitogen-activated Protein Kinase-mediated Threonine 30-dependent Norepinephrine Transporter Regulation in Cocaine Sensitization and Conditioned Place Preference

The noradrenergic and p38 mitogen-activated protein kinase (p38 MAPK) systems are implicated in cocaine-elicited behaviors. Previously, we demonstrated a role for p38 MAPK-mediated norepinephrine transporter (NET) Thr(30) phosphorylation in cocaine-induced NET up-regulation (Mannangatti, P., Arapulisamy, O., Shippenberg, T. S., Ramamoorthy, S., and Jayanthi, L. D. (2011) J. Biol. Chem. 286, 20239-20250). The present study explored the functional interaction between p38 MAPK-mediated NET regulation and cocaine-induced behaviors. In vitro cocaine treatment of mouse prefrontal cortex synaptosomes resulted in enhanced NET function, surface expression, and phosphorylation. Pretreatment with PD169316, a p38 MAPK inhibitor, completely blocked cocaine-mediated NET up-regulation and phosphorylation. In mice, in vivo administration of p38 MAPK inhibitor SB203580 completely blocked cocaine-induced NET up-regulation and p38 MAPK activation in the prefrontal cortex and nucleus accumbens. When tested for cocaine-induced locomotor sensitization and conditioned place preference (CPP), mice receiving SB203580 on cocaine challenge day or on postconditioning test day exhibited significantly reduced cocaine sensitization and CPP. A transactivator of transcription (TAT) peptide strategy was utilized to test the involvement of the NET-Thr(30) motif. In vitro treatment of synaptosomes with TAT-NET-Thr(30) (wild-type peptide) completely blocked cocaine-mediated NET up-regulation and phosphorylation. In vivo administration of TAT-NET-Thr(30) peptide but not TAT-NET-T30A (mutant peptide) completely blocked cocaine-mediated NET up-regulation and phosphorylation. In the cocaine CPP paradigm, mice receiving TAT-NET-Thr(30) but not TAT-NET-T30A on postconditioning test day exhibited significantly reduced cocaine CPP. Following extinction, TAT-NET-Thr(30) when given prior to cocaine challenge significantly reduced reinstatement of cocaine CPP. These results demonstrate that the direct inhibition of p38 MAPK or the manipulation of NET-Thr(30) motif/phosphorylation via a TAT peptide strategy prevents cocaine-induced NET up-regulation, locomotor sensitization, and CPP, suggesting a role for Thr(30)-linked NET regulation in cocaine-elicited behaviors.

Restoration of cocaine stimulation and reward by reintroducing wild type dopamine transporter in adult knock-in mice with a cocaine-insensitive dopamine transporter

In previous studies, we generated knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) and found cocaine does not stimulate locomotion or produce reward in these mice, indicating DAT inhibition is necessary for cocaine stimulation and reward. However, DAT uptake is reduced in DAT-CI mice and thus the lack of cocaine responses could be due to adaptive changes. To test this, we used adeno-associated virus (AAV) to reintroduce the cocaine-sensitive wild type DAT (AAV-DATwt) back into adult DAT-CI mice, which restores cocaine inhibition of DAT in affected brain regions but does not reverse the adaptive changes. In an earlier study we showed that AAV-DATwt injections in regions covering the lateral nucleus accumbens (NAc) and lateral caudate-putamen (CPu) restored cocaine stimulation but not cocaine reward. In the current study, we expanded the AAV-DATwt infected areas to cover the olfactory tubercle (Tu) and the ventral midbrain (vMB) containing the ventral tegmental area (VTA) and substantia nigra (SN) in addition to CPu and NAc with multiple injections. These mice displayed the restoration of both locomotor stimulation and cocaine reward. We further found that AAV-DATwt injection in the vMB alone was sufficient to restore both cocaine stimulation and reward in DAT-CI mice. AAV injected in the VTA and SN resulted in DATwt expression and distribution to the DA terminal regions. In summary, cocaine induced locomotion and reward can be restored in fully developed DAT-CI mice, and cocaine inhibition of DAT expressed in dopaminergic neurons originated from the ventral midbrain mediates cocaine reward and stimulation.