Background Circadian gene disruptions are associated with the development of psychiatric disorders, including addiction. However, the mechanisms by which circadian genes regulate drug reward remain poorly understood. Methods We used mice with a mutation in Npas2, and AAV-shRNA mediated knock-down of Npas2 and Clock in the nucleus accumbens (NAc). We performed conditioned place preference (CPP) assays for cocaine. We utilized cell sorting techniques, qPCR and chromatin immunoprecipitation (ChIP) assays followed by deep sequencing (ChIP-seq). Results Npas2 mutants exhibit decreased sensitivity to cocaine reward which can be recapitulated with a knock-down of NPAS2 specifically in the NAc, demonstrating the functional importance of NPAS2 in this region. Interestingly, reducing CLOCK (a homologue of NPAS2) expression in the NAc had no effect, suggesting an important distinction in NPAS2 and CLOCK function. Furthermore, we find that NPAS2 expression is restricted to Drd1 expressing neurons, (i.e. “direct” pathway circuitry) 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 find that NPAS2 knock-down 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. Conclusions Together, these findings identify an important and novel role for the circadian protein, NPAS2, in the NAc in the regulation of dopamine receptor expression and drug reward.

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 (ClockD19) 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 ClockD19 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 ClockD19 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 ClockD19 mice and leads to a similar change in dopamine cell volume. Furthermore, reduction of dopaminergic firing rates in ClockD19 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.

Chronic cocaine use reduces glutamate levels in the nucleus accumbens (NAc), and is associated with experience-dependent changes in (±)-a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor membrane expression in NAc neurons. These changes accompany behavioral sensitization to cocaine and increased susceptibility to cocaine relapse. The functional relationship between neuroplasticity in AMPA receptors and the behavioral manifestation of cocaine addiction remains unclear. Thus, we examined the behavioral effects of up-and downregulating basal AMPA receptor function in the NAc core and shell using viral-mediated gene transfer of wild-type glutamate receptor 1 (wt-GluR1) or a dominant-negative pore-dead GluR1 (pd-GluR1), respectively. Transient increases in wt-GluR1 during or after cocaine treatments diminished the development of cocaine sensitization, while pd-GluR1 expression exacerbated cocaine sensitization. Parallel changes were found in D2, but not D1, receptormediated behavioral responses. As a correlate of the sensitization experiments, we overexpressed wt-or pd-GluR1 in the NAc core during cocaine self-administration, and tested the effects on subsequent drug-seeking behavior 3 weeks after overexpression declined. wt-GluR1 overexpression during self-administration had no effect on cocaine intake, but subsequently reduced cocaine seeking in extinction and cocaine-induced reinstatement, whereas pd-GluR1 facilitated cocaine-induced reinstatement. When overexpressed during reinstatement tests, wt-GluR1 directly attenuated cocaine-and D2 agonist-induced reinstatement, while pd-GluR1 enhanced reinstatement. In both experimental procedures, neither wt-nor pd-GluR1 expression affected cue-induced reinstatement. Together, these results suggest that degrading basal AMPA receptor function in NAc neurons is sufficient to facilitate relapse via sensitization in D2 receptor responses, whereas elevating basal AMPA receptor function attenuates these behaviors.

SummaryDiurnal and circadian rhythms are prominent in nearly all bodily functions. Chronic disruptions in normal sleep wake and social schedules can lead to serious health problems such as those seen in shift worker's syndrome. Moreover, genetic disruptions in normal circadian gene functions have recently been linked to a variety of psychiatric conditions including depression, bipolar disorder, seasonal affective disorder and alcoholism. Recent studies are beginning to determine how these circadian genes and rhythms are involved in the development of drug addiction. Several of these studies suggest an important role for these genes in limbic regions of the brain, outside of the central circadian pacemaker in the suprachiasmatic nucleus (SCN). This review summarizes some of the basic research into the importance of circadian genes in drug addiction.

Rationale Buprenorphine (BPN) has been shown to rapidly improve mood in treatment-resistant depressed patients in small clinical studies. However, BPN’s effects in preclinical tests for mood and antidepressant efficacy are largely unexplored. Objective The current study examined the effects of BPN in the forced swim test (FST) and novelty-induced hypophagia (NIH) test as measures of antidepressant and anxiolytic-like effects in C57BL/6J mice. Microdialysis was used to measure whether BPN engaged KORs in the nucleus accumbens shell (NAcSh) at a behaviorally active dose (0.25 mg/kg). Methods BPN was tested in the FST at both 30 min and 24 h post administration. Also measured in the FST at 24 h post administration were the KOR antagonist norbinaltorphimine (nor-BNI), the MOR agonist morphine and the reference antidepressant desipramine. The anxiolytic effects of BPN were examined in the NIH test 24 h after treatment. The effects of acute injection of BPN and the KOR agonist U50,488 were measured on extracellular DA levels in the NAcSh. Results BPN produced significant reductions in FST immobility without changing locomotor activity and reduced approach latencies in the novel environment of the NIH test when tested 24 h after treatment. Repeated daily BPN injections for 6 d did not produce tolerance to these behavioral effects. nor-BNI produced a similar antidepressant-like response in the FST 24 h postinjection but morphine and desipramine were ineffective. BPN (0.25 mg/kg) did not alter DA levels when given alone but prevented the KOR agonist U50,488 from reducing DA levels. Conclusions Acute and subchronic treatment with BPN produced antidepressant and anxiolytic-like responses in mice at doses that engage KORs. These studies support the clinical evidence that BPN may be a novel rapid-acting antidepressant medication and provides rodent models for investigating associated neurochemical mechanisms.

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.

Mice with a mutation in the Clock gene (ClockD19) 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 ClockD19 mutants and wild-type (WT) mice. We found that ClockD19 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 ClockD19 mice. Interestingly, we also discovered that ClockD19 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.

Previous studies have identified potential antidepressant effects of buprenorphine (BPN), a drug with high affinity for mu opioid receptor (MORs) and kappa opioid receptors (KORs) and some affinity at delta opioid receptor (DOR) and opioid receptor-like 1 (ORL-1) receptors. Therefore, these studies examined which opioid receptors were involved in BPN's effects on animal behavior tests sensitive to antidepressant drugs. The acute effects of BPN were tested in the forced swim test (FST) using mice with genetic deletion of individual opioid receptors or after pharmacological blockade of receptors. For evaluating the effects of BPN on chronic stress, separate groups of mice were exposed to unpredictable chronic mild stress (UCMS) for 3 weeks and treated with BPN for at least 7 days before behavioral assessment and subsequent measurement of Oprk1, Oprm1, and Pdyn mRNA expression in multiple brain regions. BPN did not reduce immobility in mice with KOR deletion or after pretreatment with norbinaltorphimine, even though desipramine remained effective. In contrast, BPN reduced immobility in MOR and DOR knockout mice and in mice pretreated with the ORL-1 antagonist JTC-801. UCMS reduced sucrose preference, decreased time in the light side of the light/dark box, increased immobility in the FST and induced regionspecific alterations in Oprk1, Oprm1, and PDYN mRNA expression in the frontal cortex and striatum. All of these changes were normalized following BPN treatment. The KOR was identified as a key player mediating the effects of BPN in tests sensitive to antidepressant drugs in mice. These studies support further development of BPN as a novel antidepressant.