Lack of cocaine self-administration in mice expressing a cocaine-insensitive dopamine transporter

Glucagon-like peptide-1 receptors in nucleus accumbens, ventral hippocampus, and lateral septum reduce alcohol reinforcement in mice

Glucagon-like peptide 1 (GLP-1) receptor agonists can decrease alcohol intake by central mechanisms that are still poorly understood. The lateral septum (LS) and the ventral/caudal part of the hippocampus are enriched in GLP-1 receptors, and activity in these regions was shown to modulate reward-related behaviors. Using microinfusions of the GLP-1 receptor agonist exendin-4 in mice trained to self-administer oral alcohol in an operant assay, we tested whether pharmacological stimulation of GLP-1 receptors in hippocampus and LS decrease alcohol self-administration. We report that infusion of exendin-4 in the ventral hippocampus or LS was sufficient to reduce alcohol self-administration with as large effect sizes as we previously reported with systemic exendin-4 administration. Infusion of exendin-4 into the nucleus accumbens also reduced alcohol self-administration, as anticipated based on earlier reports, while infusion of exendin-4 into the caudate-putamen (dorsal striatum) had little effect, consistent with lack of GLP-1 receptor expression in this region. The distribution of exendin-4 after infusion into the LS or caudate putamen was visualized using a fluorescently labeled ligand. These findings add to our understanding of the circuit-level mechanisms underlying the ability of GLP-1 receptor agonists to reduce alcohol self-administration. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Cocaine induces locomotor sensitization through a dopamine-dependent VTA-mPFC-FrA cortico-cortical pathway in male mice

As a central part of the mammalian brain, the prefrontal cortex (PFC) has been implicated in regulating cocaine-induced behaviors including compulsive seeking and reinstatement. Although dysfunction of the PFC has been reported in animal and human users with chronic cocaine abuse, less is known about how the PFC is involved in cocaine-induced behaviors. By using two-photon Ca²⁺ imaging to simultaneously record tens of intact individual networking neurons in the frontal association cortex (FrA) in awake male mice, here we report that a systematic acute cocaine exposure decreased the FrA neural activity in mice, while the chemogenetic intervention blocked the cocaine-induced locomotor sensitization. The hypoactivity of FrA neurons was critically dependent on both dopamine transporters and dopamine transmission in the ventromedial PFC (vmPFC). Both dopamine D1R and D2R neurons in the vmPFC projected to and innervated FrA neurons, the manipulation of which changed the cocaine-induced hypoactivity of the FrA and locomotor sensitization. Together, this work demonstrates acute cocaine-induced hypoactivity of FrA neurons in awake mice, which defines a cortico-cortical projection bridging dopamine transmission and cocaine sensitization.

Establishment of multi-stage intravenous self-administration paradigms in mice

Genetically tractable animal models provide needed strategies to resolve the biological basis of drug addiction. Intravenous self-administration (IVSA) is the gold standard for modeling psychostimulant and opioid addiction in animals, but technical limitations have precluded the widespread use of IVSA in mice. Here, we describe IVSA paradigms for mice that capture the multi-stage nature of the disorder and permit predictive modeling. In these paradigms, C57BL/6J mice with long-standing indwelling jugular catheters engaged in cocaine- or remifentanil-associated lever responding that was fixed ratio-dependent, dose-dependent, extinguished by withholding the drug, and reinstated by the presentation of drug-paired cues. The application of multivariate analysis suggested that drug taking in both paradigms was a function of two latent variables we termed incentive motivation and discriminative control. Machine learning revealed that vulnerability to drug seeking and relapse were predicted by a mouse's a priori response to novelty, sensitivity to drug-induced locomotion, and drug-taking behavior. The application of these behavioral and statistical-analysis approaches to genetically-engineered mice will facilitate the identification of neural circuits driving addiction susceptibility and relapse and focused therapeutic development.

Jugular Vein Catheter Design and Cocaine Self-Administration Using Mice: A Comprehensive Method

Intravenous self-administration (IVSA) is a behavioral method of voluntary drug intake in animal models which is used to study the reinforcing effects of drugs of abuse. It is considered to have greater face validity in the study of substance use and abuse than other assays, and thus, allows for valuable insight into the neurobiological basis of addiction, and the development of substance abuse disorders. The technique typically involves surgically inserting a catheter into the jugular vein, which enables the infusion of drug solution after the performance of a desired operant behavior. Two nose- poke ports or levers are offered as manipulanda and are randomly assigned as active (reinforced) or inactive (non-reinforced) to allow for the examination of discrimination in the assessment of learning. Here, we describe our methodological approach to this assay in a mouse model, including construction and surgical implantation of a jugular vein catheter, set up of operant chambers, and considerations during each phase of the operant task.

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.

Identification of the Risk Genes Associated With Vulnerability to Addiction: Major Findings From Transgenic Animals

Understanding risk factors for substance use disorders (SUD) can facilitate medication development for SUD treatment. While a rich literature exists discussing environmental factors that influence SUD, fewer articles have focused on genetic factors that convey vulnerability to drug use. Methods to identify SUD risk genes include Genome-Wide Association Studies (GWAS) and transgenic approaches. GWAS have identified hundreds of gene variants or single nucleotide polymorphisms (SNPs). However, few genes identified by GWAS have been verified by clinical or preclinical studies. In contrast, significant progress has been made in transgenic approaches to identify risk genes for SUD. In this article, we review recent progress in identifying candidate genes contributing to drug use and addiction using transgenic approaches. A central hypothesis is if a particular gene variant (e.g., resulting in reduction or deletion of a protein) is associated with increases in drug self-administration or relapse to drug seeking, this gene variant may be considered a risk factor for drug use and addiction. Accordingly, we identified several candidate genes such as those that encode dopamine D₂ and D₃ receptors, mGluR₂, M₄ muscarinic acetylcholine receptors, and α₅ nicotinic acetylcholine receptors, which appear to meet the risk-gene criteria when their expression is decreased. Here, we describe the role of these receptors in drug reward and addiction, and then summarize major findings from the gene-knockout mice or rats in animal models of addiction. Lastly, we briefly discuss future research directions in identifying addiction-related risk genes and in risk gene-based medication development for the treatment of addiction.

Disruption of the PDZ domain-binding motif of the dopamine transporter uniquely alters nanoscale distribution, dopamine homeostasis, and reward motivation

The dopamine (DA) transporter (DAT) is part of a presynaptic multiprotein network involving interactions with scaffold proteins via its C-terminal PDZ domain-binding sequence. Using a mouse model expressing DAT with mutated PDZ-binding sequence (DAT-AAA), we previously demonstrated the importance of this binding sequence for striatal expression of DAT. Here, we show by application of direct stochastic reconstruction microscopy not only that the striatal level of transporter is reduced in DAT-AAA mice but also that the nanoscale distribution of this transporter is altered with a higher propensity of DAT-AAA to localize to irregular nanodomains in dopaminergic terminals. In parallel, we observe mesostriatal DA adaptations and changes in DA-related behaviors distinct from those seen in other genetic DAT mouse models. DA levels in the striatum are reduced to ∼45% of that of WT, accompanied by elevated DA turnover. Nonetheless, fast-scan cyclic voltammetry recordings on striatal slices reveal a larger amplitude and prolonged clearance rate of evoked DA release in DAT-AAA mice compared with WT mice. Autoradiography and radioligand binding show reduced DA D2 receptor levels, whereas immunohistochemistry and autoradiography show unchanged DA D1 receptor levels. In behavioral experiments, we observe enhanced self-administration of liquid food under both a fixed ratio of one and progressive ratio schedule of reinforcement but a reduction compared with WT when using cocaine as reinforcer. In summary, our data demonstrate how disruption of PDZ domain interactions causes changes in DAT expression and its nanoscopic distribution that in turn alter DA clearance dynamics and related behaviors.

The fragile X mental retardation protein promotes adjustments in cocaine self-administration that preserve reinforcement level

The fragile X mental retardation protein (FMRP), an RNA-binding protein, regulates cocaine-induced neuronal plasticity and is critical for the normal development of drug-induced locomotor sensitization, as well as reward-related learning in the conditioned place preference assay. However, it is unknown whether FMRP impacts behaviors that are used to more closely model substance use disorders. Utilizing a cocaine intravenous self-administration (IVSA) assay in Fmr1 knockout (KO) and wild-type (WT) littermate mice, we find that, despite normal acquisition and extinction learning, Fmr1 KO mice fail to make a normal upward shift in responding during dose-response testing. Later, when given access to the original acquisition dose under increasing fixed ratio (FR) schedules of reinforcement (FR1, FR3, and FR5), Fmr1 KO mice earn significantly fewer cocaine infusions than WT mice. Importantly, similar deficits are not present in operant conditioning using a palatable food reinforcer, indicating that our results do not represent broad learning or reward-related deficits in Fmr1 KO mice. Additionally, we find an FMRP target, the activity-regulated cytoskeleton-associated protein (Arc), to be significantly reduced in synaptic cellular fractions prepared from the nucleus accumbens of Fmr1 KO, compared with WT, mice following operant tasks reinforced with cocaine but not food. Overall, our findings suggest that FMRP facilitates adjustments in drug self-administration behavior that generally serve to preserve reinforcement level, and combined with our similar IVSA findings in Arc KO mice may implicate Arc, along with FMRP, in behavioral shifts that occur in drug taking when drug availability is altered.

Kappa Opioid Receptor Mediated Differential Regulation of Serotonin and Dopamine Transporters in Mood and Substance Use Disorder

Dynorphin (DYN) is an endogenous neurosecretory peptide which exerts its activity by binding to the family of G protein-coupled receptors, namely the kappa opioid receptor (KOR). Opioids are associated with pain, analgesia, and drug abuse, which play a central role in mood disorders with monoamine neurotransmitter interactions. Growing evidence demonstrates the cellular signaling cascades linked to KOR-mediated monoamine transporters regulation in cell models and native brain tissues. This chapter will review DYN/KOR role in mood and addiction in relevance to dopaminergic and serotonergic neurotransmissions. Also, we discuss the recent findings on KOR-mediated differential regulation of serotonin and dopamine transporters (SERT and DAT). These findings led to a better understanding of the role of DYN/KOR system in aminergic neurotransmission via its modulatory effect on both amine release and clearance. Detailed knowledge of these processes at the molecular level enables designing novel pharmacological reagents to target transporter motifs to treat mood and addiction and reduce unwanted side effects such as aversion, dysphoria, sedation, and psychomimesis.

The Streetlight Effect: Reappraising the Study of Addiction in Light of the Findings of Genome-wide Association Studies

Drug dependence has long been thought to have a genetic component. Research seeking to identify the genetic basis of addiction has gone through important transitions over its history, in part based upon the emergence of new technologies, but also as the result of changing perspectives. Early research approaches were largely dictated by available technology, with technological advancements having highly transformative effects on genetic research, but the limitations of technology also affected modes of thinking about the genetic causes of disease. This review explores these transitions in thinking about the genetic causes of addiction in terms of the "streetlight effect," which is a type of observational bias whereby people search for something only where it is easiest to search. In this way, the genes that were initially studied in the field of addiction genetics were chosen because they were the most "obvious," and formed current understanding of the biological mechanisms underlying the actions of drugs of abuse and drug dependence. The problem with this emphasis is that prior to the genomic era the vast majority of genes and proteins had yet to be identified, much less studied. This review considers how these initial choices, as well as subsequent choices that were also driven by technological limitations, shaped the study of the genetic basis of drug dependence. While genome-wide approaches overcame the initial biases regarding which genes to choose to study inherent in candidate gene studies and other approaches, genome-wide approaches necessitated other assumptions. These included additive genetic causation and limited allelic heterogeneity, which both appear to be incorrect. Thus, the next stage of advancement in this field must overcome these shortcomings through approaches that allow the examination of complex interactive effects, both gene × gene and gene × environment interactions. Techniques for these sorts of studies have recently been developed and represent the next step in our understanding of the genetic basis of drug dependence.

Caenorhabditis elegans as an in vivo Model to Assess Amphetamine Tolerance

Amphetamine is a potent psychostimulant also used to treat attention deficit/hyperactivity disorder and narcolepsy. In vivo and in vitro data have demonstrated that amphetamine increases the amount of extra synaptic dopamine by both inhibiting reuptake and promoting efflux of dopamine through the dopamine transporter. Previous studies have shown that chronic use of amphetamine causes tolerance to the drug. Thus, since the molecular mechanisms underlying tolerance to amphetamine are still unknown, an animal model to identify the neurochemical mechanisms associated with drug tolerance is greatly needed. Here we took advantage of a unique behavior caused by amphetamine in Caenorhabditis elegans to investigate whether this simple, but powerful, genetic model develops tolerance following repeated exposure to amphetamine. We found that at least 3 treatments with 0.5 mM amphetamine were necessary to see a reduction in the amphetamine-induced behavior and, thus, to promote tolerance. Moreover, we found that, after intervals of 60/90 minutes between treatments, animals were more likely to exhibit tolerance than animals that underwent 10-minute intervals between treatments. Taken together, our results show that C. elegans is a suitable system to study tolerance to drugs of abuse such as amphetamines.

Novelty-induced hyperactivity and suppressed cocaine induced locomotor activation in mice lacking threonine 53 phosphorylation of dopamine transporter

Dopamine (DA) transporter (DAT) is dynamically regulated by several protein kinases and the Thr53 phosphorylation of DAT (pT53-DAT) is documented in heterologous cell models and in rat brain. However, the role of endogenous pT53-DAT in living animals has never been addressed. Here we generated and studied the pT53-lacking DAT mouse model (DAT-Ala53) by CRISPR/Cas9 technology. DAT-Ala53 mice showed normal growth, body weight, body temperature, grip strength, and sucrose preference while pT53-DAT was completely absent. However, DAT-Ala53 mice showed hyperlocomotion, pronounced vertical exploratory behavior, and stereotypy in a novel environment compared to wild-type littermates (WT). DAT-Ala53 mice displayed unaltered levels of monoamines, glutamate, and GABA in the striatum compared to WT. There were also no significant differences between DAT-Ala53 mice and WT in tyrosine hydroxylase (TH) and phospho-TH levels, or in total and surface DAT levels, or in DA-transport kinetic parameters V_max and K_m. Immunohistochemical and colocalization analyses of TH and DAT in caudate-putamen and nucleus accumbens revealed no significant differences between DAT-Ala53 and WT mice. Interestingly, cocaine's potency to inhibit striatal DA transport and cocaine-induced locomotor activation were significantly reduced in the DAT-Ala53 mice. Also, ERK1/2 inhibitors completely failed to inhibit striatal DA uptake in DAT-Ala53 mice. Collectively, our findings reveal that the mice lacking pT53-DAT display novelty-induced hyperactive phenotype despite having normal transporter protein expression, DA-transport kinetics and DA-linked markers. The results also reveal that the lack of endogenous pT53-DAT renders DAT resistant to ERK1/2 inhibition and also less susceptible to cocaine inhibition and cocaine-evoked locomotor stimulation.

Sex-Specific Role for Egr3 in Nucleus Accumbens D2-Medium Spiny Neurons Following Long-Term Abstinence From Cocaine Self-administration

BACKGROUND

We previously showed that the transcription factor Egr3 (early growth response 3) is oppositely regulated in nucleus accumbens (NAc) cell subtypes 24 hours following cocaine exposure and bidirectionally mediates cocaine-related behaviors in male rodents. Overexpressing Egr3 in D₂ receptor-containing medium spiny neurons (D2-MSNs) before drug exposure reduces the rewarding and psychomotor sensitization effects of cocaine. However, it is unknown if Egr3 plays a role in long-term neuroadaptations in the NAc and relapse to cocaine seeking.

METHODS

We measured EGR3 protein levels in the NAc following 20 days of forced abstinence from intravenous cocaine self-administration in 10-week-old Sprague Dawley rats and C57BL/6 mice. In 8- to 10-week-old A2A-Cre mice, we used virally mediated Egr3 overexpression in NAc D2-MSNs to test the role of Egr3 on operant responding during seeking, extinction, and drug-induced reinstatement of cocaine self-administration. To evaluate if Egr3 contributed to sex differences to cocaine relapse, we conducted these procedures in both male and female rodents.

RESULTS

We found that EGR3 expression was reduced only in female rodents after 20 days of forced abstinence. Additionally, we showed that our self-administration paradigm in mice recapitulated the sex differences in cocaine intake and relapse demonstrated in humans and rats. Finally, whereas Egr3 overexpression in D2-MSNs during forced abstinence facilitated extinction and blunted drug-induced reinstatement in female mice, it had the opposite effect in male mice.

CONCLUSIONS

We showed that the immediate early gene Egr3 has long-term effects on drug-related behaviors. Our work suggests that changes in Egr3 expression in D2-MSNs contributes to sex differences in cocaine relapse.

Characterization of genetically complex Collaborative Cross mouse strains that model divergent locomotor activating and reinforcing properties of cocaine

RATIONALE

Few effective treatments exist for cocaine use disorders due to gaps in knowledge about its complex etiology. Genetically defined animal models provide a useful tool for advancing our understanding of the biological and genetic underpinnings of addiction-related behavior and evaluating potential treatments. However, many attempts at developing mouse models of behavioral disorders were based on overly simplified single gene perturbations, often leading to inconsistent and misleading results in pre-clinical pharmacology studies. A genetically complex mouse model may better reflect disease-related behaviors.

OBJECTIVES

Screening defined, yet genetically complex, intercrosses of the Collaborative Cross (CC) mice revealed two lines, RIX04/17 and RIX41/51, with extreme high and low behavioral responses to cocaine. We characterized these lines as well as their CC parents, CC004/TauUnc and CC041/TauUnc, to evaluate their utility as novel model systems for studying the biological and genetic mechanisms underlying behavioral responses to cocaine.

METHODS

Behavioral responses to acute (initial locomotor sensitivity) and repeated (behavioral sensitization, conditioned place preference, intravenous self-administration) exposures to cocaine were assessed. We also examined the monoaminergic system (striatal tissue content and in vivo fast scan cyclic voltammetry), HPA axis reactivity, and circadian rhythms as potential mechanisms for the divergent phenotypic behaviors observed in the two strains, as these systems have a previously known role in mediating addiction-related behaviors.

RESULTS

RIX04/17 and 41/51 show strikingly divergent initial locomotor sensitivity to cocaine with RIX04/17 exhibiting very high and RIX41/51 almost no response. The lines also differ in the emergence of behavioral sensitization with RIX41/51 requiring more exposures to exhibit a sensitized response. Both lines show conditioned place preference for cocaine. We determined that the cocaine sensitivity phenotype in each RIX line was largely driven by the genetic influence of one CC parental strain, CC004/TauUnc and CC041/TauUnc. CC004 demonstrates active operant cocaine self-administration and CC041 is unable to acquire under the same testing conditions, a deficit which is specific to cocaine as both strains show operant response for a natural food reward. Examination of potential mechanisms driving differential responses to cocaine show strain differences in molecular and behavioral circadian rhythms. Additionally, while there is no difference in striatal dopamine tissue content or dynamics, there are selective differences in striatal norepinephrine and serotonergic tissue content.

CONCLUSIONS

These CC strains offer a complex polygenic model system to study underlying mechanisms of cocaine response. We propose that CC041/TauUnc and CC004/TauUnc will be useful for studying genetic and biological mechanisms underlying resistance or vulnerability to the stimulatory and reinforcing effects of cocaine.

In Situ Regulated Dopamine Transporter Trafficking: There's No Place Like Home

Dopamine (DA) is critical for motivation, reward, movement initiation, and learning. Mechanisms that control DA signaling have a profound impact on these important behaviors, and additionally play a role in DA-related neuropathologies. The presynaptic SLC6 DA transporter (DAT) limits extracellular DA levels by clearing released DA, and is potently inhibited by addictive and therapeutic psychostimulants. Decades of evidence support that the DAT is subject to acute regulation by a number of signaling pathways, and that endocytic trafficking strongly regulates DAT availability and function. DAT trafficking studies have been performed in a variety of model systems, including both in vitro and ex vivo preparations. In this review, we focus on the breadth of DAT trafficking studies, with specific attention to, and comparison of, how context may influence DAT's response to different stimuli. In particular, this overview highlights that stimulated DAT trafficking not only differs between in vitro and ex vivo environments, but also is influenced by both sex and anatomical subregions.

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.

Variability in the Drug Response of M4 Muscarinic Receptor Knockout Mice During Day and Night Time

Mice are nocturnal animals. Surprisingly, the majority of physiological/pharmacological studies are performed in the morning, i.e., in the non-active phase of their diurnal cycle. We have shown recently that female (not male) mice lacking the M₄ muscarinic receptors (MR, M₄KO) did not differ substantially in locomotor activity from their wild-type counterparts (C57Bl/6Tac) during the inactive period. Increased locomotion has been shown in the active phase of their diurnal cycle. We compared the effects of scopolamine, oxotremorine, and cocaine on locomotor response, hypothermia and spontaneous behavior in the open field arena in the morning (9:00 AM) and in the evening (9:00 PM) in WT and in C57Bl/6NTac mice lacking the M₄ MR. Furthermore, we also studied morning vs. evening densities of muscarinic, GABA_A, D₁-like, D₂-like, NMDA and kainate receptors using autoradiography in the motor, somatosensory and visual cortex and in the striatum, thalamus, hippocampus, pons, and medulla oblongata. At 9:00 AM, scopolamine induced an increase in motor activity in WT and in M₄KO, yet no significant increase was observed at 9:00 PM. Oxotremorine induced hypothermic effects in both WT and M₄KO. Hypothermic effects were more evident in WT than in M₄KO. Hypothermia in both cases was more pronounced at 9:00 AM than at 9:00 PM. Cocaine increased motor activity when compared to saline. There was no difference in behavior in the open field between WT and M₄KO when tested at 9:00 AM; however, at 9:00 PM, activity of M₄KO was doubled in comparison to that of WT. Both WT and KO animals spent less time climbing in their active phase. Autoradiography revealed no significant morning vs. evening difference. Altogether, our results indicate the necessity of comparing morning vs. evening drug effects

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.

Differential expression of VGLUT3 in laboratory mouse strains: Impact on drug-induced hyperlocomotion and anxiety-related behaviors

The atypical vesicular glutamate transporter VGLUT3 is present in subpopulations of GABAergic interneurons in the cortex and the hippocampus, in subgroups of serotoninergic neurons in raphe nuclei, and in cholinergic interneurons in the striatum. C56BL/6N mice that no longer express VGLUT3 (VGLUT3^-/- ) display anxiety-associated phenotype, increased spontaneous and cocaine-induced locomotor activity and decreased haloperidol-induced catalepsy. Inbred mouse strains differ markedly in their sensitivity to anxiety and behavioral responses elicited by drugs. The purpose of this study was to investigate strain differences in VGLUT3 expression levels and its potential correlates with anxiety and reward-guided behaviors. Five inbred mouse lines were chosen according to their contrasted anxiety and drugs sensitivity: C57BL/6N, C3H/HeN, DBA/2J, 129/Sv, and BALB/c. VGLUT3 protein expression was measured in different brain areas involved in reward or mood regulation (such as the striatum, the hippocampus, and raphe nuclei) and genetic variations in Slc17a8, the gene encoding for VGLUT3, have been explored. These five inbred mouse strains express very different levels of VGLUT3, which cannot be attributed to the genetic variation of the Slc17a8 locus. Furthermore, mice behavior in the open field, elevated plus maze, spontaneous- and cocaine-induced locomotor was highly heterogeneous and only partially correlated to VGLUT3 levels. These data highlight the fact that one single gene polymorphism could not account for VGLUT3 expression variations, and that region specific VGLUT3 expression level variations might play a key role in the modulation of discrete behaviors.

Intravenous cocaine self-administration in a panel of inbred mouse strains differing in acute locomotor sensitivity to cocaine

RATIONALE

Initial sensitivity to drugs of abuse often predicts subsequent use and abuse, but this relationship is not always observed in human studies. Moreover, studies examining the relationship between initial locomotor sensitivity and the rewarding and reinforcing effects of drugs in animal models have also been equivocal. Understanding the relationship between initial drug effects and propensity to continue use, potentially resulting in the development of a substance use disorder, may help to identify key targets for prevention and treatment.

OBJECTIVES

We examined intravenous cocaine self-administration in a set of mouse strains that were previously identified to be at the phenotypic extremes for cocaine-induced locomotor activation to determine if initial locomotor sensitivity predicted acquisition, extinction, dose response, or progressive ratio (PR) breakpoint.

METHODS

We selected eight inbred mouse strains based on locomotor sensitivity to 20 mg/kg cocaine. These strains, designated as low and high responders, were tested in an intravenous self-administration paradigm that included acquisition of 0.5 mg/(kg*inf) under a FR1 schedule, extinction, re-acquisition, dose response to 0.125, 0.25, 0.5, 1, and 2 mg/(kg*inf), and progressive ratio.

RESULTS

We observed overall differences in self-administration behavior between high and low responders. Low responders self-administered less cocaine and had lower breakpoints under the PR schedule. However, we also observed strain differences within each group. Self-administration in the low responder, LG/J, more closely resembled the behavior of the high-responding group, and the high responder, P/J, had self-administration behavior that more closely resembled the low-responding group.

CONCLUSIONS

We conclude that acute cocaine-induced locomotor activation does predict self-administration behavior, but in a strain-specific manner. These data support the idea that genetic background influences the relationship among addiction-related behaviors.

Effects of muscarinic M1 and M4 acetylcholine receptor stimulation on extinction and reinstatement of cocaine seeking in male mice, independent of extinction learning

RATIONALE

Stimulating muscarinic M₁/M₄ receptors can blunt reinforcing and other effects of cocaine. A hallmark of addiction is continued drug seeking/craving after abstinence and relapse.

OBJECTIVES

We tested whether stimulating M₁ and/or M₄ receptors could facilitate extinction of cocaine seeking, and whether this was mediated via memory consolidation.

METHODS

Experimentally naïve C57BL/6J mice were allowed to acquire self-administration of intravenous cocaine (1 mg/kg/infusion) under a fixed-ratio 1 schedule of reinforcement. Then, saline was substituted for cocaine until responding extinguished to ≤30% of cocaine-reinforced responding. Immediately after each extinction session, mice received saline, the M₁/M₄ receptor-preferring agonist xanomeline, the M₁ receptor-selective allosteric agonist VU0357017, the M₄ receptor-selective positive allosteric modulator VU0152100, or VU0357017 + VU0152100. In additional experiments, xanomeline was administered delayed after the session or in the home cage before extinction training began. In the latter group, reinstatement of responding by a 10-mg/kg cocaine injection was also tested.

RESULTS

Stimulating M₁ + M₄ receptors significantly expedited extinction from 17.2 sessions to 8.3 using xanomeline or 7.8 using VU0357017 + VU0152100. VU0357017 alone and VU0152100 alone did not significantly modify rates of extinction (12.6 and 14.6 sessions). The effect of xanomeline was fully preserved when administered delayed after or unpaired from extinction sessions (7.5 and 6.4 sessions). Xanomeline-treated mice showed no cocaine-induced reinstatement.

CONCLUSIONS

These findings show that M₁/M₄ receptor stimulation can decrease cocaine seeking in mice. The effect lasted beyond treatment duration and was not dependent upon extinction learning. This suggests that M₁/M₄ receptor stimulation modulated or reversed some neurochemical effects of cocaine exposure.

Pronounced Hyperactivity, Cognitive Dysfunctions, and BDNF Dysregulation in Dopamine Transporter Knock-out Rats

Dopamine (DA) controls many vital physiological functions and is critically involved in several neuropsychiatric disorders such as schizophrenia and attention deficit hyperactivity disorder. The major function of the plasma membrane dopamine transporter (DAT) is the rapid uptake of released DA into presynaptic nerve terminals leading to control of both the extracellular levels of DA and the intracellular stores of DA. Here, we present a newly developed strain of rats in which the gene encoding DAT knockout Rats (DAT-KO) has been disrupted by using zinc finger nuclease technology. Male and female DAT-KO rats develop normally but weigh less than heterozygote and wild-type rats and demonstrate pronounced spontaneous locomotor hyperactivity. While striatal extracellular DA lifetime and concentrations are significantly increased, the total tissue content of DA is markedly decreased demonstrating the key role of DAT in the control of DA neurotransmission. Hyperactivity of DAT-KO rats can be counteracted by amphetamine, methylphenidate, the partial Trace Amine-Associated Receptor 1 (TAAR1) agonist RO5203648 ((S)-4-(3,4-Dichloro-phenyl)-4,5-dihydro-oxazol-2-ylamine) and haloperidol. DAT-KO rats also demonstrate a deficit in working memory and sensorimotor gating tests, less propensity to develop obsessive behaviors and show strong dysregulation in frontostriatal BDNF function. DAT-KO rats could provide a novel translational model for human diseases involving aberrant DA function and/or mutations affecting DAT or related regulatory mechanisms.SIGNIFICANCE STATEMENT Here, we present a newly developed strain of rats in which the gene encoding the dopamine transporter (DAT) has been disrupted (Dopamine Transporter Knockout rats [DAT-KO rats]). DAT-KO rats display functional hyperdopaminergia accompanied by pronounced spontaneous locomotor hyperactivity. Hyperactivity of DAT-KO rats can be counteracted by amphetamine, methylphenidate, and a few other compounds exerting inhibitory action on dopamine-dependent hyperactivity. DAT-KO rats also demonstrate cognitive deficits in working memory and sensorimotor gating tests, less propensity to develop compulsive behaviors, and strong dysregulation in frontostriatal BDNF function. These observations highlight the key role of DAT in the control of brain dopaminergic transmission. DAT-KO rats could provide a novel translational model for human diseases involving aberrant dopamine functions.

Innate immune signaling in the ventral tegmental area contributes to drug-primed reinstatement of cocaine seeking

Cocaine addiction is a chronic relapsing disorder characterized by persistent perturbations to an organism's homeostatic processes that result in maladaptive drug seeking. Although considerable attention has been directed at the consequences of neuronal changes following chronic cocaine taking, few studies have examined the role of microglia, the brain's resident immune cells, following chronic cocaine administration. Toll-Like Receptor 4 (TLR4) is a molecular pattern receptor that recognizes pathogens, danger signals, and xenobiotics and induces proinflammatory signaling in the central nervous system. TLR4 is generally considered to be expressed primarily by microglia. Here, we used a rodent model of cocaine addiction to investigate the role of TLR4 in the ventral tegmental area (VTA) in cocaine seeking. Male Sprague-Dawley rats were trained to self-administer cocaine in daily 2-h sessions for 15days. Following self-administration, rats underwent extinction training and were tested in a drug-primed reinstatement paradigm. Pharmacological antagonism of TLR4 in the VTA using lipopolysaccharide from the bacterium Rhodobacter sphaeroides (LPS-RS) significantly reduced cocaine-primed reinstatement of drug seeking but had no effect on sucrose seeking. TLR4 activation within the VTA using the TLR4 activator, lipopolysaccharide, was sufficient to moderately reinstate cocaine seeking. We also assessed changes in proinflammatory cytokine expression in the VTA following cocaine self-administration. Cocaine self-administration increased the expression of mRNA for the proinflammatory cytokine interleukin-1ß, but not tumor necrosis factor alpha, in the VTA. Pharmacological antagonism of the interleukin-1 receptor in the VTA reduced cocaine-primed drug seeking. These results are consistent with the hypothesis that chronic cocaine produces inflammatory signaling that contributes to cocaine seeking.

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.

Amphetamine Reverses Escalated Cocaine Intake via Restoration of Dopamine Transporter Conformation

Cocaine abuse disrupts dopamine system function, and reduces cocaine inhibition of the dopamine transporter (DAT), which results in tolerance. Although tolerance is a hallmark of cocaine addiction and a DSM-V criterion for substance abuse disorders, the molecular adaptations producing tolerance are unknown, and testing the impact of DAT changes on drug taking behaviors has proven difficult. In regard to treatment, amphetamine has shown efficacy in reducing cocaine intake; however, the mechanisms underlying these effects have not been explored. The goals of this study were twofold; we sought to (1) identify the molecular mechanisms by which cocaine exposure produces tolerance and (2) determine whether amphetamine-induced reductions in cocaine intake are connected to these mechanisms. Using cocaine self-administration and fast-scan cyclic voltammetry in male rats, we show that low-dose, continuous amphetamine treatment, during self-administration or abstinence, completely reversed cocaine tolerance. Amphetamine treatment also reversed escalated cocaine intake and decreased motivation to obtain cocaine as measured in a behavioral economics task, thereby linking tolerance to multiple facets of cocaine use. Finally, using fluorescence resonance energy transfer imaging, we found that cocaine tolerance is associated with the formation of DAT-DAT complexes, and that amphetamine disperses these complexes. In addition to extending our basic understanding of DATs and their role in cocaine reinforcement, we serendipitously identified a novel therapeutic target: DAT oligomer complexes. We show that dispersion of oligomers is concomitant with reduced cocaine intake, and propose that pharmacotherapeutics aimed at these complexes may have potential for cocaine addiction treatment.SIGNIFICANCE STATEMENT Tolerance to cocaine's subjective effects is a cardinal symptom of cocaine addiction and a DSM-V criterion for substance abuse disorders. However, elucidating the molecular adaptions that produce tolerance and determining its behavioral impact have proven difficult. Using cocaine self-administration in rats, we link tolerance to cocaine effects at the dopamine transporter (DAT) with aberrant cocaine-taking behaviors. Further, tolerance was associated with multi-DAT complexes, which formed after cocaine exposure. Treatment with amphetamine deconstructed DAT complexes, reversed tolerance, and decreased cocaine seeking. These data describe the behavioral consequence of cocaine tolerance, provide a putative mechanism for its development, and suggest that compounds that disperse DAT complexes may be efficacious treatments for cocaine addiction.

Cross-talk between the epigenome and neural circuits in drug addiction

Drug addiction is a behavioral disorder characterized by dysregulated learning about drugs and associated cues that result in compulsive drug seeking and relapse. Learning about drug rewards and predictive cues is a complex process controlled by a computational network of neural connections interacting with transcriptional and molecular mechanisms within each cell to precisely guide behavior. The interplay between rapid, temporally specific neuronal activation, and longer-term changes in transcription is of critical importance in the expression of appropriate, or in the case of drug addiction, inappropriate behaviors. Thus, these factors and their interactions must be considered together, especially in the context of treatment. Understanding the complex interplay between epigenetic gene regulation and circuit connectivity will allow us to formulate novel therapies to normalize maladaptive reward behaviors, with a goal of modulating addictive behaviors, while leaving natural reward-associated behavior unaffected.

Atypical dopamine transporter inhibitors R-modafinil and JHW 007 differentially affect D2 autoreceptor neurotransmission and the firing rate of midbrain dopamine neurons

Abuse of psychostimulants like cocaine that inhibit dopamine (DA) reuptake through the dopamine transporter (DAT) represents a major public health issue, however FDA-approved pharmacotherapies have yet to be developed. Recently a class of ligands termed "atypical DAT inhibitors" has gained attention due to their range of effectiveness in increasing extracellular DA levels without demonstrating significant abuse liability. These compounds not only hold promise as therapeutic agents to treat stimulant use disorders but also as experimental tools to improve our understanding of DAT function. Here we used patch clamp electrophysiology in mouse brain slices to explore the effects of two atypical DAT inhibitors (R-modafinil and JHW 007) on the physiology of single DA neurons in the substantia nigra and ventral tegmental area. Despite their commonalities of being DAT inhibitors that lack cocaine-like behavioral profiles, these compounds exhibited surprisingly divergent cellular effects. Similar to cocaine, R-modafinil slowed DA neuron firing in a D2 receptor-dependent manner and rapidly enhanced the amplitude and duration of D2 receptor-mediated currents in the midbrain. In contrast, JHW 007 exhibited little effect on firing, slow DAT blockade, and an unexpected inhibition of D2 receptor-mediated currents that may be due to direct D2 receptor antagonism. Furthermore, pretreatment with JHW 007 blunted the cellular effects of cocaine, suggesting that it may be valuable to investigate similar DAT inhibitors as potential therapeutic agents. Further exploration of these and other atypical DAT inhibitors may reveal important cellular effects of compounds that will have potential as pharmacotherapies for treating cocaine use disorders.

Cocaine Potency at the Dopamine Transporter Tracks Discrete Motivational States During Cocaine Self-Administration

Although the dopamine transporter (DAT) is the primary site of action for cocaine, and the dopamine system is known to mediate the reinforcing effects of cocaine, the dopaminergic variations underlying individual differences in cocaine self-administration behaviors are not fully understood. Recent advances in the application of economic principles to operant tasks in rodents have allowed for the within-subject, within-session determination of both consummatory and appetitive responding for reinforcers. Here we combined a behavioral economics approach with cocaine self-administration and ex vivo voltammetric recording of dopamine signaling in the core of the nucleus accumbens of rats to determine the relationship between dopamine signaling and discrete aspects of cocaine taking and seeking. We found neither dopamine release or uptake tracked individual differences in cocaine consumption or the reinforcing efficacy of cocaine. Cocaine potency at the DAT was correlated with reinforcing efficacy, but was not related to cocaine consumption. Further, we introduce a novel analysis that determines perseverative responding within the same procedure, and find that cocaine potency at the DAT also tracks differences in perseverative responding. Together, we demonstrate that cocaine effects at the DAT determine the reinforcing efficacy of cocaine, and perseverative responding for sub-threshold doses of cocaine that do not maintain responding when presented in isolation. Surprisingly, we find that variations in cocaine potency do not account for differences in cocaine consumption, suggesting that satiation for cocaine is determined by other targets or mechanisms. Finally, we outline a novel approach for relating drug-target interactions and potency to discrete motivational states during a single self-administration session.

Allosteric modulatory effects of SRI-20041 and SRI-30827 on cocaine and HIV-1 Tat protein binding to human dopamine transporter

Dopamine transporter (DAT) is the target of cocaine and HIV-1 transactivator of transcription (Tat) protein. Identifying allosteric modulatory molecules with potential attenuation of cocaine and Tat binding to DAT are of great scientific and clinical interest. We demonstrated that tyrosine 470 and 88 act as functional recognition residues in human DAT (hDAT) for Tat-induced inhibition of DA transport and transporter conformational transitions. Here we investigated the allosteric modulatory effects of two allosteric ligands, SRI-20041 and SRI-30827 on cocaine binding on wild type (WT) hDAT, Y470 H and Y88 F mutants. Effect of SRI-30827 on Tat-induced inhibition of [³H]WIN35,428 binding was also determined. Compared to a competitive DAT inhibitor indatraline, both SRI-compounds displayed a similar decrease (30%) in IC₅₀ for inhibition of [³H]DA uptake by cocaine in WT hDAT. The addition of SRI-20041 or SRI-30827 following cocaine slowed the dissociation rate of [³H]WIN35,428 binding in WT hDAT relative to cocaine alone. Moreover, Y470H and Y88F hDAT potentiate the inhibitory effect of cocaine on DA uptake and attenuate the effects of SRI-compounds on cocaine-mediated dissociation rate. SRI-30827 attenuated Tat-induced inhibition of [³H]WIN35,428 binding. These observations demonstrate that tyrosine 470 and 88 are critical for allosteric modulatory effects of SRI-compounds on the interaction of cocaine with hDAT.

Preclinical Evidence That 5-HT1B Receptor Agonists Show Promise as Medications for Psychostimulant Use Disorders

BACKGROUND

5-HT1B receptor agonists enhance cocaine intake during daily self-administration sessions but decrease cocaine intake when tested after prolonged abstinence. We examined if 5-HT1B receptor agonists produce similar abstinence-dependent effects on methamphetamine intake.

METHODS

Male rats were trained to self-administer methamphetamine (0.1 mg/kg, i.v.) on low (fixed ratio 5 and variable ratio 5) and high (progressive ratio) effort schedules of reinforcement until intake was stable. Rats were then tested for the effects of the selective 5-HT1B receptor agonist, CP 94,253 (5.6 or 10 mg/kg), or the less selective but clinically available 5-HT1B/1D receptor agonist, zolmitriptan (10 mg/kg), on methamphetamine self-administration both before and after a 21-day forced abstinence period during which the rats remained in their home cages.

RESULTS

The inverted U-shaped, methamphetamine dose-response function for intake on the fixed ratio 5 schedule was shifted downward by CP 94,253 both before and after abstinence. The CP 94,253-induced decrease in methamphetamine intake was replicated in rats tested on a variable ratio 5 schedule, and the 5-HT1B receptor antagonist SB 224,289 (10 mg/kg) reversed this effect. CP 94,253 also attenuated methamphetamine intake on a progressive ratio schedule both pre- and postabstinence. Similarly, zolmitriptan attenuated methamphetamine intake on a variable ratio 5 schedule both pre- and postabstinence, and the latter effect was sustained after each of 2 more treatments given every 2 to 3 days prior to daily sessions.

CONCLUSIONS

Unlike the abstinence-dependent effect of 5-HT1B receptor agonists on cocaine intake reported previously, both CP 94,253 and zolmitriptan decreased methamphetamine intake regardless of abstinence. These findings suggest that 5-HT1B receptor agonists may have clinical efficacy for psychostimulant use disorders.

Dopaminergic control of anxiety in young and aged zebrafish

Changes in the expression of the dopamine transporter (DAT), or the sensitivity of dopamine receptors, are associated with aging and substance abuse and may underlie some of the symptoms common to both conditions. In this study, we explored the role of the dopaminergic system in the anxiogenic effects of aging and acute cocaine exposure by comparing the behavioral phenotypes of wild type (WT) and DAT knockout zebrafish (DAT-KO) of different ages. To determine the involvement of specific dopamine receptors in anxiety states, antagonists to D1 (SCH23390) and D2/D3 (sulpiride) were employed. We established that DAT-KO results in a chronic anxiety-like state, seen as an increase in bottom-dwelling and thigmotaxis. Similar effects were produced by aging and acute cocaine administration, both leading to reduction in DAT mRNA abundance (qPCR). Inhibition of D1 activity counteracted the anxiety-like effects associated with DAT deficit, independent of its origin. Inhibition of D2/D3 receptors reduced anxiety in young DAT-KO, and enhanced the anxiogenic effects of cocaine in WT, but did not affect aged WT or DAT-KO fish. These findings provide new evidence that the dopaminergic system plays a critical role in anxiety-like states, and suggest that adult zebrafish provide a sensitive diurnal vertebrate model for elucidating the molecular mechanisms of anxiety and a platform for anxiolytic drug screens.

Ack1 is a dopamine transporter endocytic brake that rescues a trafficking-dysregulated ADHD coding variant

The dopamine (DA) transporter (DAT) facilitates high-affinity presynaptic DA reuptake that temporally and spatially constrains DA neurotransmission. Aberrant DAT function is implicated in attention-deficit/hyperactivity disorder and autism spectrum disorder. DAT is a major psychostimulant target, and psychostimulant reward strictly requires binding to DAT. DAT function is acutely modulated by dynamic membrane trafficking at the presynaptic terminal and a PKC-sensitive negative endocytic mechanism, or "endocytic brake," controls DAT plasma membrane stability. However, the molecular basis for the DAT endocytic brake is unknown, and it is unknown whether this braking mechanism is unique to DAT or common to monoamine transporters. Here, we report that the cdc42-activated, nonreceptor tyrosine kinase, Ack1, is a DAT endocytic brake that stabilizes DAT at the plasma membrane and is released in response to PKC activation. Pharmacologic and shRNA-mediated Ack1 silencing enhanced basal DAT internalization and blocked PKC-stimulated DAT internalization, but had no effects on SERT endocytosis. Both cdc42 activation and PKC stimulation converge on Ack1 to control Ack1 activity and DAT endocytic capacity, and Ack1 inactivation is required for stimulated DAT internalization downstream of PKC activation. Moreover, constitutive Ack1 activation is sufficient to rescue the gain-of-function endocytic phenotype exhibited by the ADHD DAT coding variant, R615C. These findings reveal a unique endocytic control switch that is highly specific for DAT. Moreover, the ability to rescue the DAT(R615C) coding variant suggests that manipulating DAT trafficking mechanisms may be a potential therapeutic approach to correct DAT coding variants that exhibit trafficking dysregulation.

Psychostimulants affect dopamine transmission through both dopamine transporter-dependent and independent mechanisms

The precise mechanisms by which cocaine and amphetamine-like psychostimulants exert their reinforcing effects are not yet fully defined. It is widely believed, however, that these drugs produce their effects by enhancing dopamine neurotransmission in the brain, especially in limbic areas such as the nucleus accumbens, by inducing dopamine transporter-mediated reverse transport and/or blocking dopamine reuptake though the dopamine transporter. Here, we present the evidence that aside from dopamine transporter, non-dopamine transporter-mediated mechanisms also participate in psychostimulant-induced dopamine release and contribute to the behavioral effects of these drugs, such as locomotor activation and reward. Accordingly, psychostimulants could increase norepinephrine release in the prefrontal cortex, the latter then alters the firing pattern of dopamine neurons resulting in changes in action potential-dependent dopamine release. These alterations would further affect the temporal pattern of dopamine release in the nucleus accumbens, thereby modifying information processing in that area. Hence, a synaptic input to a nucleus accumbens neuron may be enhanced or inhibited by dopamine depending on its temporal relationship to dopamine release. Specific temporal patterns of dopamine release may also be required for certain forms of synaptic plasticity in the nucleus accumbens. Together, these effects induced by psychostimulants, mediated through a non-dopamine transporter-mediated mechanism involving norepinephrine and the prefrontal cortex, may also contribute importantly to the reinforcing properties of these drugs.

Cocaine-Induced Endocannabinoid Mobilization in the Ventral Tegmental Area

Cocaine is a highly addictive drug that acts upon the brain’s reward circuitry via the inhibition of mono-amine uptake. Endogenous cannabinoids (eCB) are lipid molecules released from midbrain dopamine (DA) neurons that modulate cocaine’s effects through poorly understood mechanisms. We find that cocaine stimulates release of the eCB, 2-arach-idonoylglycerol (2-AG), in the rat ventral midbrain to suppress GABAergic inhibition of DA neurons, through activation of presynaptic cannabinoid CB1 receptors. Cocaine mobilizes 2-AG via inhibition of norepinephrine uptake and promotion of a cooperative interaction between G_q/11-coupled type-1 metabotropic glutamate and α₁-adrenergic receptors to stimulate internal calcium stores and activate phospholipase C. The disinhibition of DA neurons by cocaine-mobilized 2-AG is also functionally relevant because it augments DA release in the nucleus accumbens in vivo. Our results identify a mechanism through which the eCB system can regulate the rewarding and addictive properties of cocaine.

Differential Roles for Dopamine D1-Like and D2-Like Receptors in Mediating the Reinforcing Effects of Cocaine: Convergent Evidence from Pharmacological and Genetic Studies

A series of studies by Drs. Barak Caine, James Woods, Gregory Collins, Jonathan Katz and Takato Hiranita demonstrated a novel and unique reinforcing effect using dopamine (DA) D2-like receptor [D2-like R: D₂, D₃, and D₄ receptor subtypes (respectively, D₂R, D₃R, and D₄R)] agonists in rats and genetically modified mice. In order to understand how important their findings are, a comparison was made regarding the reinforcing effects of DA D2-like R full agonists with those of DA uptake inhibitors and of a DA D1-like receptor [D1-like R, D₁ and D₅ receptor subtypes (D₁R and D₅R)] full agonist (±)-SKF 82958.

Loss of feedback inhibition via D2 autoreceptors enhances acquisition of cocaine taking and reactivity to drug-paired cues

A prominent aspect of drug addiction is the ability of drug-associated cues to elicit craving and facilitate relapse. Understanding the factors that regulate cue reactivity will be vital for improving treatment of addictive disorders. Low availability of dopamine (DA) D2 receptors (D2Rs) in the striatum is associated with high cocaine intake and compulsive use. However, the role of D2Rs of nonstriatal origin in cocaine seeking and taking behavior and cue reactivity is less understood and possibly underestimated. D2Rs expressed by midbrain DA neurons function as autoreceptors, exerting inhibitory feedback on DA synthesis and release. Here, we show that selective loss of D2 autoreceptors impairs the feedback inhibition of DA release and amplifies the effect of cocaine on DA transmission in the nucleus accumbens (NAc) in vitro. Mice lacking D2 autoreceptors acquire a cued-operant self-administration task for cocaine faster than littermate control mice but acquire similarly for a natural reward. Furthermore, although mice lacking D2 autoreceptors were able to extinguish self-administration behavior in the absence of cocaine and paired cues, they exhibited perseverative responding when cocaine-paired cues were present. This enhanced cue reactivity was selective for cocaine and was not seen during extinction of sucrose self-administration. We conclude that low levels of D2 autoreceptors enhance the salience of cocaine-paired cues and can contribute to the vulnerability for cocaine use and relapse.

The neurosteroid dehydroepiandrosterone sulfate, but not androsterone, enhances the antidepressant effect of cocaine examined in the forced swim test--Possible role of serotonergic neurotransmission

One of the mechanisms of cocaine's actions in the central nervous system is its antidepressant action. This effect might be responsible for increased usage of the drug by individuals with mood disorders. Higher endogenous levels of the excitatory neurosteroid dehydroepiandrosterone sulfate (DHEAS) were reported to correlate with successful abstinence from cocaine use in addicts, but a clinical trial showed that supplementation with a high dose of DHEA increased cocaine usage instead. Such ambiguous effects of DHEA(S) could potentially be linked to its influence on the antidepressant effect of cocaine. In this study we tested DHEAS and its metabolite, androsterone, for interactions with cocaine in animal model of depression (forced swim test) and examined the effects of both steroids and cocaine on serotoninergic neurotransmission. All substances were also tested for influence on locomotor activity. A cocaine dose of 5mg/kg, which had no significant effect on locomotor activity, was chosen for the forced swim test. Neither DHEAS nor androsterone showed any antidepressant action in this test, while cocaine manifested a clear antidepressant effect. Androsterone slightly reduced the antidepressant influence of cocaine while DHEAS markedly, dose-dependently enhanced it. Such an effect might be caused by the influence of DHEAS on serotonin neurotransmission, as this steroid decreased serotonin concentration and turnover in the striatum. When DHEAS and cocaine were administered together, the levels of serotonin in the striatum and hippocampus remained unchanged. This phenomenon may explain the additive antidepressant action of DHEAS and cocaine and why co-administration of DHEAS and cocaine increases drug use.

Repeated exposure of adult rats to transient oxidative stress induces various long-lasting alterations in cognitive and behavioral functions

Exposure of neonates to oxidative stress may increase the risk of psychiatric disorders such as schizophrenia in adulthood. However, the effects of moderate oxidative stress on the adult brain are not completely understood. To address this issue, we systemically administrated 2-cyclohexen-1-one (CHX) to adult rats to transiently reduce glutathione levels. Repeated administration of CHX did not affect the acquisition or motivation of an appetitive instrumental behavior (lever pressing) rewarded by a food outcome under a progressive ratio schedule. In addition, response discrimination and reversal learning were not affected. However, acute CHX administration blunted the sensitivity of the instrumental performance to outcome devaluation, and this effect was prolonged in rats with a history of repeated CHX exposure, representing pro-depression-like phenotypes. On the other hand, repeated CHX administration reduced immobility in forced swimming tests and blunted acute cocaine-induced behaviors, implicating antidepressant-like effects. Multivariate analyses segregated a characteristic group of behavioral variables influenced by repeated CHX administration. Taken together, these findings suggest that repeated administration of CHX to adult rats did not cause a specific mental disorder, but it induced long-term alterations in behavioral and cognitive functions, possibly related to specific neural correlates.

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.

Changes in endocannabinoid and N-acylethanolamine levels in rat brain structures following cocaine self-administration and extinction training

Preclinical investigations have demonstrated that drugs of abuse alter the levels of lipid-based signalling molecules, including endocannabinoids (eCBs) and N-acylethanolamines (NAEs), in the rodent brain. In addition, several drugs targeting eCBs and/or NAEs are implicated in reward and/or seeking behaviours related to the stimulation of dopamine systems in the brain. In our study, the brain levels of eCBs (anandamide (AEA) and 2-arachidonoylglycerol (2-AG)) and NAEs (oleoylethanolamide (OEA) and palmitoylethanolamide (PEA)) were analyzed via an LC-MS/MS method in selected brain structures of rats during cocaine self-administration and after extinction training according to the "yoked" control procedure. Repeated (14days) cocaine (0.5mg/kg/infusion) self-administration and yoked drug delivery resulted in a significant decrease (ca. 52%) in AEA levels in the cerebellum, whereas levels of 2-AG increased in the frontal cortex, the hippocampus and the cerebellum and decreased in the hippocampus and the dorsal striatum. In addition, we detected increases (>150%) in the levels of OEA and PEA in the limbic areas in both cocaine treated groups, as well as an increase in the tissue levels of OEA in the dorsal striatum in only the yoked cocaine group and increases in the tissue levels of PEA in the dorsal striatum (both cocaine groups) and the nucleus accumbens (yoked cocaine group only). Compared to the yoked saline control group, extinction training (10days) resulted in a potent reduction in AEA levels in the frontal cortex, the hippocampus and the nucleus accumbens and in 2-AG levels in the hippocampus, the dorsal striatum and the cerebellum. The decreases in the limbic and subcortical areas were more apparent for rats that self-administered cocaine. Following extinction, there was a region-specific change in the levels of NAEs in rats previously injected with cocaine; a potent increase (ca. 100%) in the levels of OEA and PEA was detected in the prefrontal cortex and the hippocampus, whilst a drop was noted in the striatal areas versus yoked saline yoked animals. Our findings support the previous pharmacological evidence that the eCB system and NAEs are involved in reinforcement and extinction of positively reinforced behaviours and that these lipid-derived molecules may represent promising targets for the development of new treatments for drug addiction.

Lesions of cholinergic pedunculopontine tegmental nucleus neurons fail to affect cocaine or heroin self-administration or conditioned place preference in rats

Cholinergic input to the ventral tegmental area (VTA) is known to contribute to reward. Although it is known that the pedunculopontine tegmental nucleus (PPTg) provides an important source of excitatory input to the dopamine system, the specific role of PPTg cholinergic input to the VTA in cocaine reward has not been previously determined. We used a diphtheria toxin conjugated to urotensin-II (Dtx::UII), the endogenous ligand for urotensin-II receptors expressed by PPTg cholinergic but not glutamatergic or GABAergic cells, to lesion cholinergic PPTg neurons. Dtx::UII toxin infusion resulted in the loss of 95.78 (±0.65)% of PPTg cholinergic cells but did not significantly alter either cocaine or heroin self-administration or the development of cocaine or heroin conditioned place preferences. Thus, cholinergic cells originating in PPTg do not appear to be critical for the rewarding effects of cocaine or of heroin.

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

Cocaine's main pharmacological actions are the inhibition of the dopamine, serotonin, and norepinephrine transporters. Its main behavioral effects are reward and locomotor stimulation, potentially leading to addiction. Using knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) we have shown previously that inhibition of the dopamine transporter (DAT) is necessary for both of these behaviors. In this study, we sought to determine brain regions in which DAT inhibition by cocaine stimulates locomotor activity and/or produces reward. We used adeno-associated viral vectors to re-introduce the cocaine-sensitive wild-type DAT in specific brain regions of DAT-CI mice, which otherwise only express a cocaine-insensitive DAT globally. Viral-mediated expression of wild-type DAT in the rostrolateral striatum restored cocaine-induced locomotor stimulation and sensitization in DAT-CI mice. In contrast, the expression of wild-type DAT in the dorsal striatum, or in the medial nucleus accumbens, did not restore cocaine-induced locomotor stimulation. These data help to determine cocaine's molecular actions and anatomical loci that cause hyperlocomotion. Interestingly, cocaine did not produce significant reward - as measured by conditioned place-preference - in any of the three cohorts of DAT-CI mice with the virus injections. Therefore, the locus or loci underlying cocaine-induced reward remain underdetermined. It is possible that multiple dopamine-related brain regions are involved in producing the robust rewarding effect of cocaine.

Monoamine transporter inhibitors and substrates as treatments for stimulant abuse

The acute and chronic effects of abused psychostimulants on monoamine transporters and associated neurobiology have encouraged development of candidate medications that target these transporters. Monoamine transporters, in general, and dopamine transporters, in particular, are critical molecular targets that mediate abuse-related effects of psychostimulants such as cocaine and amphetamine. Moreover, chronic administration of psychostimulants can cause enduring changes in neurobiology reflected in dysregulation of monoamine neurochemistry and behavior. The current review will evaluate evidence for the efficacy of monoamine transporter inhibitors and substrates to reduce abuse-related effects of stimulants in preclinical assays of stimulant self-administration, drug discrimination, and reinstatement. In considering deployment of monoamine transport inhibitors and substrates as agonist-type medications to treat stimulant abuse, the safety and abuse liability of the medications are an obvious concern, and this will also be addressed. Future directions in drug discovery should identify novel medications that retain efficacy to decrease stimulant use but possess lower abuse liability and evaluate the degree to which efficacious medications can attenuate or reverse neurobiological effects of chronic stimulant use.

Dopamine transporter endocytic trafficking in striatal dopaminergic neurons: differential dependence on dynamin and the actin cytoskeleton

Dopaminergic signaling profoundly impacts rewarding behaviors, movement, and executive function. The presynaptic dopamine (DA) transporter (DAT) recaptures released DA, thereby limiting synaptic DA availability and maintaining dopaminergic tone. DAT constitutively internalizes and PKC activation rapidly accelerates DAT endocytosis, resulting in DAT surface loss. Longstanding evidence supports PKC-stimulated DAT trafficking in heterologous expression studies. However, PKC-stimulated DAT internalization is not readily observed in cultured dopaminergic neurons. Moreover, conflicting reports implicate both classic and nonclassic endocytic mechanisms mediating DAT trafficking. Prior DAT trafficking studies relied primarily upon chronic gene disruption and dominant-negative protein expression, or were performed in cell lines and cultured neurons, yielding results difficult to translate to adult dopaminergic neurons. Here, we use newly described dynamin inhibitors to test whether constitutive and PKC-stimulated DAT internalization are dynamin-dependent in adult dopaminergic neurons. Ex vivo biotinylation studies in mouse striatal slices demonstrate that acute PKC activation drives native DAT surface loss, and that surface DAT surprisingly partitions between endocytic-willing and endocytic-resistant populations. Acute dynamin inhibition reveals that constitutive DAT internalization is dynamin-independent, whereas PKC-stimulated DAT internalization is dynamin-dependent. Moreover, total internal reflection fluorescence microscopy experiments demonstrate that constitutive DAT internalization occurs equivalently from lipid raft and nonraft microdomains, whereas PKC-stimulated DAT internalization arises exclusively from lipid rafts. Finally, DAT endocytic recycling relies on a dynamin-dependent mechanism that acts in concert with the actin cytoskeleton. These studies are the first comprehensive investigation of native DAT trafficking in ex vivo adult neurons, and reveal that DAT surface dynamics are governed by complex multimodal mechanisms.

Modafinil as a catecholaminergic agent: empirical evidence and unanswered questions

Modafinil, in its two clinical formulations (Provigil^® and Nuvigil^®), is a widely prescribed wake-promoting therapeutic agent. It binds competitively to the cell-membrane dopamine (DA) transporter and is dependent on catecholaminergic (dopaminergic and adrenergic) signaling for its wake-promoting effects. The clinical spectrum of effects for modafinil is distinct from the effects seen with other catecholaminergic agents. Relative to other commonly used agents that act through catecholaminergic mechanisms, modafinil has a relatively low abuse potential, produces wakefulness with an attenuated compensatory sleep recovery thereafter, and does not ameliorate cataplexy in narcolepsy. These clinically relevant phenomenological differences between modafinil and agents such as amphetamines and cocaine do not eliminate catecholaminergic effects as a possible mediator of its wake-promoting action; they merely reflect its unique pharmacological profile. Modafinil is an exceptionally weak, but apparently very selective, DA transporter inhibitor. The pharmacodynamic response to modafinil, as measured by DA levels in brain microdialyzate, is protracted relative to other agents that act via catecholaminergic mechanisms. The conformational constraints on the interaction of modafinil with the DA transporter – and probably, as a consequence, its effects on trace amine receptor signaling in the catecholaminergic cell – are unique among catecholaminergic agents. These unique pharmacological properties of modafinil should be considered both in seeking to thoroughly understand its putatively elusive mechanism of action and in the design of novel therapeutic agents.

Rats classified as low or high cocaine locomotor responders: a unique model involving striatal dopamine transporters that predicts cocaine addiction-like behaviors

Individual differences are a hallmark of drug addiction. Here, we describe a rat model based on differential initial responsiveness to low dose cocaine. Despite similar brain cocaine levels, individual outbred Sprague-Dawley rats exhibit markedly different magnitudes of acute cocaine-induced locomotor activity and, thereby, can be classified as low or high cocaine responders (LCRs or HCRs). LCRs and HCRs differ in drug-induced, but not novelty-associated, hyperactivity. LCRs have higher basal numbers of striatal dopamine transporters (DATs) than HCRs and exhibit marginal cocaine inhibition of in vivo DAT activity and cocaine-induced increases in extracellular DA. Importantly, lower initial cocaine response predicts greater locomotor sensitization, conditioned place preference and greater motivation to self-administer cocaine following low dose acquisition. Further, outbred Long-Evans rats classified as LCRs, versus HCRs, are more sensitive to cocaine's discriminative stimulus effects. Overall, results to date with the LCR/HCR model underscore the contribution of striatal DATs to individual differences in initial cocaine responsiveness and the value of assessing the influence of initial drug response on subsequent expression of addiction-like behaviors.

Peer influences on drug self-administration: an econometric analysis in socially housed rats

Social-learning theories of substance use propose that members of peer groups influence the drug use of other members by selectively modeling, reinforcing, and punishing either abstinence-related or drug-related behaviors. The objective of the present study was to examine the social influences on cocaine self-administration in isolated and socially housed rats, under conditions where the socially housed rats were tested simultaneously with their partner in the same chamber. To this end, male rats were obtained at weaning and housed in isolated or pair-housed conditions for 6 weeks. Rats were then implanted with intravenous catheters and cocaine self-administration was examined in custom-built operant conditioning chambers that allowed two rats to be tested simultaneously. For some socially housed subjects, both rats had simultaneous access to cocaine; for others, only one rat of the pair had access to cocaine. An econometric analysis was applied to the data, and the reinforcing strength of cocaine was measured by examining consumption (i.e. quantity demanded) and elasticity of demand as a function of price, which was manipulated by varying the dose and ratio requirements on a fixed ratio schedule of reinforcement. Cocaine consumption decreased as a function of price in all groups. Elasticity of demand did not vary across groups, but consumption was significantly lower in socially housed rats paired with a rat without access to cocaine. These data suggest that the presence of an abstaining peer decreases the reinforcing strength of cocaine, thus supporting the development of social interventions in drug abuse prevention and treatment programs.