Sade Spencer

Cocaine-induced adaptations in D1 and D2 accumbens projection neurons (a dichotomy not necessarily synonymous with direct and indirect pathways)

Cocaine exposure causes enduring neuroadaptations in ventral striatum, or nucleus accumbens (NAc), an area critically involved in reward learning and relapse of drug seeking. Medium spiny neurons (MSNs) in striatum are dichotomous in their expression of either D1 or D2 dopamine receptors, along with other receptors and neuropeptides. In dorsal striatum, these two subpopulations show non-overlapping innervation of distinct terminal fields via the direct or indirect pathways. However, NAc D1-MSNs and D2-MSNs are not fully segregated in this manner, with both cell types innervating ventral pallidum. Recent studies show that D1-MSNs and D2-MSNs play opposing roles in cocaine-associated behaviors. Further, cocaine induces differential adaptations in these two subpopulations in NAc, including changes to synaptic plasticity, glutamatergic signaling, and spine morphology.

Specific Role of VTA Dopamine Neuronal Firing Rates and Morphology in the Reversal of Anxiety-Related, but not Depression-Related Behavior in the ClockΔ19 Mouse Model of Mania

Lithium has been used extensively for mood stabilization, and it is particularly efficacious in the treatment of bipolar mania. Like other drugs used in the treatment of psychiatric diseases, it has little effect on the mood of healthy individuals. Our previous studies found that mice with a mutation in the Clock gene (ClockΔ19) have a complete behavioral profile that is very similar to human mania, which can be reversed with chronic lithium treatment. However, the cellular and physiological effects that underlie its targeted therapeutic efficacy remain unknown. Here we find that ClockΔ19 mice have an increase in dopaminergic activity in the ventral tegmental area (VTA), and that lithium treatment selectively reduces the firing rate in the mutant mice with no effect on activity in wild-type mice. Furthermore, lithium treatment reduces nucleus accumbens (NAc) dopamine levels selectively in the mutant mice. The increased dopaminergic activity in the Clock mutants is associated with cell volume changes in dopamine neurons, which are also rescued by lithium treatment. To determine the role of dopaminergic activity and morphological changes in dopamine neurons in manic-like behavior, we manipulated the excitability of these neurons by overexpressing an inwardly rectifying potassium channel subunit (Kir2.1) selectively in the VTA of ClockΔ19 mice and wild-type mice using viral-mediated gene transfer. Introduction of this channel mimics the effects of lithium treatment on the firing rate of dopamine neurons in ClockΔ19 mice and leads to a similar change in dopamine cell volume. Furthermore, reduction of dopaminergic firing rates in ClockΔ19 animals results in a normalization of locomotor- and anxiety-related behavior that is very similar to lithium treatment; however, it is not sufficient to reverse depression-related behavior. These results suggest that abnormalities in dopamine cell firing and associated morphology underlie alterations in anxiety-related behavior in bipolar mania, and that the therapeutic effects of lithium come from a reversal of these abnormal phenotypes.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Circadian genes Period 1 and Period 2 in the nucleus accumbens regulate anxiety‐related behavior

It has been suggested for some time that circadian rhythm abnormalities underlie the development of multiple psychiatric disorders. However, it is unclear how disruptions in individual circadian genes might regulate mood and anxiety. Here we found that mice lacking functional mPeriod 1 (mPer1) or mPeriod 2 (mPer2) individually did not have consistent behavioral abnormalities in measures of anxiety‐related behavior. However, mice deficient in both mPer1 and mPer2 had an increase in levels of anxiety‐like behavior in multiple measures. Moreover, we found that mPer1 and mPer2 expression was reduced in the nucleus accumbens (NAc) after exposure to chronic social defeat stress, a paradigm that led to increased anxiety‐related behavior. Following social defeat, chronic treatment with fluoxetine normalized Per gene expression towards wild‐type levels. Knockdown of both mPer1 and mPer2 expression via RNA interference specifically in the NAc led to a similar increase in anxiety‐like behavior as seen in the mutant animals. Taken together, these results implicate the Per genes in the NAc in response to stress and the development of anxiety.

Potential of a unique antibody gene signature to predict conversion to clinically definite multiple sclerosis

We identified a unique antibody gene mutation pattern (i.e. signature) in cerebrospinal fluid (CSF) B cells from multiple sclerosis (MS) patients not present in control populations. Prevalence of the signature in CSF B cells of patients at risk to develop MS predicted conversion to MS with 91% accuracy in a small cohort of clinically isolated syndrome patients. If confirmed, signature prevalence would be a novel genetic diagnostic tool candidate for patients with early demyelinating disease of the central nervous system.

The Role of Clock in Ethanol-Related Behaviors

Mice with a mutation in the Clock gene (ClockΔ19) exhibit increased preference for stimulant rewards and sucrose. They also have an increase in dopaminergic activity in the ventral tegmental area (VTA) and a general increase in glutamatergic tone that might underlie these behaviors. However, it is unclear if their phenotype would extend to a very different class of drug (ethanol), and if so, whether these systems might be involved in their response. Continuous access voluntary ethanol intake was evaluated in ClockΔ19 mutants and wild-type (WT) mice. We found that ClockΔ19 mice exhibited significantly increased ethanol intake in a two-bottle choice paradigm. Interestingly, this effect was more robust in female mice. Moreover, chronic ethanol experience resulted in a long-lasting decrease in VTA Clock expression. To determine the importance of VTA Clock expression in ethanol intake, we knocked down Clock expression in the VTA of WT mice via RNA interference. We found that reducing Clock expression in the VTA resulted in significantly increased ethanol intake similar to the ClockΔ19 mice. Interestingly, we also discovered that ClockΔ19 mice exhibit significantly augmented responses to the sedative effects of ethanol and ketamine, but not pentobarbital. However, their drinking behavior was not affected by acamprosate, an FDA-approved drug for the treatment of alcoholism, suggesting that their increased glutamatergic tone might underlie the increased sensitivity to the sedative/hypnotic properties of ethanol but not the rewarding properties of ethanol. Taken together, we have identified a significant role for Clock in the VTA as a negative regulator of ethanol intake and implicate the VTA dopamine system in this response.

Direct Regulation of Diurnal Drd3 Expression and Cocaine Reward by NPAS2

BACKGROUNDnCircadian gene disruptions are associated with the development of psychiatric disorders, including addiction. However, the mechanisms by which circadian genes regulate reward remain poorly understood.nnnMETHODSnWe used mice with a mutation in Npas2 and adeno-associated virus-short hairpin RNA mediated knockdown of Npas2 and Clock in the nucleus accumbens (NAc). We performed conditioned place preference assays. We utilized cell sorting quantitative real-time polymerase chain reaction, and chromatin immunoprecipitation followed by deep sequencing.nnnRESULTSnNpas2 mutants exhibit decreased sensitivity to cocaine reward, which is recapitulated with a knockdown of neuronal PAS domain protein 2 (NPAS2) specifically in the NAc, demonstrating the importance of NPAS2 in this region. Interestingly, reducing circadian locomotor output cycles kaput (CLOCK) (a homologue of NPAS2) in the NAc had no effect, suggesting an important distinction in NPAS2 and CLOCK function. Furthermore, we found that NPAS2 expression is restricted to Drd1 expressing neurons while CLOCK is ubiquitous. Moreover, NPAS2 and CLOCK have distinct temporal patterns of DNA binding, and we identified novel and unique binding sites for each protein. We identified the Drd3 dopamine receptor as a direct transcriptional target of NPAS2 and found that NPAS2 knockdown in the NAc disrupts its diurnal rhythm in expression. Chronic cocaine treatment likewise disrupts the normal rhythm in Npas2 and Drd3 expression in the NAc, which may underlie behavioral plasticity in response to cocaine.nnnCONCLUSIONSnTogether, these findings identify an important role for the circadian protein, NPAS2, in the NAc in the regulation of dopamine receptor expression and drug reward.

Chronic Administration of the Methylxanthine Propentofylline Impairs Reinstatement to Cocaine by a GLT-1-Dependent Mechanism

In recent years, interactions between neurons and glia have been evaluated as mediators of neuropsychiatric diseases, including drug addiction. In particular, compounds that increase expression of the astroglial glutamate transporter GLT-1 (N-acetylcysteine and ceftriaxone) can decrease measures of drug seeking. However, it is unknown whether the compounds that influence broad measures of glial physiology can influence behavioral measures of drug relapse, nor is it clear whether the upregulated GLT-1 is functionally important for suppressing of drug seeking. To address these questions, we sought to determine whether the glial modulator and neuroprotective agent propentofylline (PPF) modifies drug seeking in rats using a reinstatement model of cocaine relapse. We found that 7 days of chronic (but not acute) administration of PPF significantly decreased both cue- and cocaine-induced reinstatement of cocaine seeking. We next determined whether the effect of systemic PPF on reinstatement depended upon its ability to restore expression of GLT-1 in the nucleus accumbens. PPF restored the cocaine-induced decrease in GLT-1 in the accumbens core; then, using an antisense strategy against glutamate transporter GLT-1, we found that restored transporter expression was necessary for PPF to inhibit cue-primed cocaine seeking. These findings indicate that modulating glial physiology with atypical xanthine derivatives like PPF is a potential avenue for developing new medications for cocaine abuse, and support the hypothesis that neuron–glial interactions contribute to mechanisms of psychostimulant addiction, particularly via expression and function of astroglial glutamate transporters.

An important role for Cholecystokinin , a CLOCK target gene, in the development and treatment of manic-like behaviors

Mice with a mutation in the Clock gene (ClockΔ19) have been identified as a model of mania; however, the mechanisms that underlie this phenotype, and the changes in the brain that are necessary for lithium’s effectiveness on these mice remain unclear. Here, we find that cholecystokinin (Cck) is a direct transcriptional target of CLOCK and levels of Cck are reduced in the ventral tegmental area (VTA) of ClockΔ19 mice. Selective knockdown of Cck expression via RNA interference in the VTA of wild-type mice produces a manic-like phenotype. Moreover, chronic treatment with lithium restores Cck expression to near wild-type and this increase is necessary for the therapeutic actions of lithium. The decrease in Cck expression in the ClockΔ19 mice appears to be due to a lack of interaction with the histone methyltransferase, MLL1, resulting in decreased histone H3K4me3 and gene transcription, an effect reversed by lithium. Human postmortem tissue from bipolar subjects reveals a similar increase in Cck expression in the VTA with mood stabilizer treatment. These studies identify a key role for Cck in the development and treatment of mania, and describe some of the molecular mechanisms by which lithium may act as an effective antimanic agent.

A mutation in CLOCK leads to altered dopamine receptor function

Mice with a mutation in the Clock gene (ClockΔ19) have a number of behavioral phenotypes that suggest alterations in dopaminergic transmission. These include hyperactivity, increased exploratory behavior, and increased reward value for drugs of abuse. However, the complex changes in dopaminergic transmission that underlie the behavioral abnormalities in these mice remain unclear. Here we find that a loss of CLOCK function increases dopamine release and turnover in striatum as indicated by increased levels of metabolites HVA and DOPAC, and enhances sensitivity to dopamine receptor antagonists. Interestingly, this enlarged dopaminergic tone results in downstream changes in dopamine receptor (DR) levels with a surprising augmentation of both D1‐ and D2‐type DR protein, but a significant shift in the ratio of D1u2003:u2003D2 receptors in favor of D2 receptor signaling. These effects have functional consequences for both behavior and intracellular signaling, with alterations in locomotor responses to both D1‐type and D2‐type specific agonists and a blunted response to cAMP activation in the ClockΔ19 mutants. Taken together, these studies further elucidate the abnormalities in dopaminergic transmission that underlie mood, activity, and addictive behaviors.