Differential regulation of the period genes in striatal regions following cocaine exposure

Sex- and Substance-Specific Associations of Circadian-Related Genes with Addiction in the UK Biobank Cohort Implicate Neuroplasticity Pathways

BACKGROUND/OBJECTIVES

The circadian clockwork is implicated in the etiology of addiction, with circadian rhythm disruptions bidirectionally linked to substance abuse, but the molecular mechanisms that underlie this connection are not well known.

METHODS

Here, we use machine learning to reveal sex- and substance-specific associations with addiction in variants from 51 circadian-related genes (156,702 SNPs) in 98,800 participants from a UK Biobank cohort. We further analyze SNP associations in a subset of the cohort for substance-specific addictions (alcohol, illicit drugs (narcotics), and prescription drugs (opioids)).

RESULTS

We find robust (OR > 10) and novel sex-specific and substance-specific associations with variants in synaptic transcription factors (ZBTB20, CHRNB3) and hormone receptors (RORA), particularly in individuals addicted to narcotics and opioids. Circadian-related gene variants associated with male and female addiction were non-overlapping; variants in males primarily involve dopaminergic pathways, while variants in females factor in metabolic and inflammation pathways, with a novel gene association of female addiction with DELEC1, a gene of unknown function.

CONCLUSIONS

Our findings underscore the complexity of genetic pathways associated with addiction, involving core clock genes and circadian-regulated pathways, and reveal novel circadian-related gene associations that will aid the development of targeted, sex-specific therapeutic interventions for substance abuse.

A Complex Relationship Among the Circadian Rhythm, Reward Circuit and Substance Use Disorder (SUD)

The human brain not only controls the various physiological functions but is also the prime regulator of circadian rhythms, rewards, and behaviors. Environmental factors, professional stress, and social disintegration are regarded as the initial causative factors of addiction behavior. Shift work, artificial light exposure at night, and chronic and acute jet lag influence circadian rhythm dysfunction. The result is impaired neurotransmitter release, dysfunction of neural circuits, endocrine disturbance, and metabolic disorder, leading to advancement in substance use disorder. There is a bidirectional relationship between chronodisruption and addiction behavior. Circadian rhythm dysfunction, neuroadaptation in the reward circuits, and alteration in clock gene expression in the mesolimbic areas influence substance use disorder (SUD), and chronotherapy has potential benefits in the treatment strategies. This review explores the relationship among the circadian rhythm dysfunction, reward circuit, and SUD. The impact of chronotherapy on SUD has also been discussed.

Differential Effects of Cocaine and Morphine on the Diurnal Regulation of the Mouse Nucleus Accumbens Proteome

Substance use disorders are associated with disruptions in sleep and circadian rhythms that persist during abstinence and may contribute to relapse risk. Repeated use of substances such as psychostimulants and opioids may lead to significant alterations in molecular rhythms in the nucleus accumbens (NAc), a brain region central to reward and motivation. Previous studies have identified rhythm alterations in the transcriptome of the NAc and other brain regions following the administration of psychostimulants or opioids. However, little is known about the impact of substance use on the diurnal rhythms of the proteome in the NAc. We used liquid chromatography coupled to tandem mass spectrometry-based quantitative proteomics, along with a data-independent acquisition analysis pipeline, to investigate the effects of cocaine or morphine administration on diurnal rhythms of proteome in the mouse NAc. Overall, our data reveal cocaine and morphine differentially alter diurnal rhythms of the proteome in the NAc, with largely independent differentially expressed proteins dependent on time-of-day. Pathways enriched from cocaine altered protein rhythms were primarily associated with glucocorticoid signaling and metabolism, whereas morphine was associated with neuroinflammation. Collectively, these findings are the first to characterize the diurnal regulation of the NAc proteome and demonstrate a novel relationship between the phase-dependent regulation of protein expression and the differential effects of cocaine and morphine on the NAc proteome. The proteomics data in this study are available via ProteomeXchange with identifier PXD042043.

Differential Effects of Cocaine and Morphine on the Diurnal Regulation of the Mouse Nucleus Accumbens Proteome

Substance use disorders (SUDs) are associated with disruptions in sleep and circadian rhythms that persist during abstinence and may contribute to relapse risk. Repeated use of substances such as psychostimulants and opioids may lead to significant alterations in molecular rhythms in the nucleus accumbens (NAc), a brain region central to reward and motivation. Previous studies have identified rhythm alterations in the transcriptome of the NAc and other brain regions following the administration of psychostimulants or opioids. However, little is known about the impact of substance use on the diurnal rhythms of the proteome in the NAc. We used liquid chromatography coupled to tandem mass spectrometry-based (LC-MS/MS) quantitative proteomics, along with a data-independent acquisition (DIA) analysis pipeline, to investigate the effects of cocaine or morphine administration on diurnal rhythms of proteome in the mouse NAc. Overall, our data reveals cocaine and morphine differentially alters diurnal rhythms of the proteome in the NAc, with largely independent differentially expressed proteins dependent on time-of-day. Pathways enriched from cocaine altered protein rhythms were primarily associated with glucocorticoid signaling and metabolism, whereas morphine was associated with neuroinflammation. Collectively, these findings are the first to characterize the diurnal regulation of the NAc proteome and demonstrate a novel relationship between phase-dependent regulation of protein expression and the differential effects of cocaine and morphine on the NAc proteome.

The circadian neurobiology of reward

Circadian clocks are important regulators of physiology and behavior. In the brain, circadian clocks have been described in many centers of the central reward system. They affect neurotransmitter signaling, neuroendocrine circuits, and the sensitivity to external stimulation. Circadian disruption affects reward signaling, promoting the development of behavioral and substance use disorders. In this review, we summarize our current knowledge of circadian clock-reward crosstalk. We show how chronodisruption affects reward signaling in different animal models. We then translate these findings to circadian aspects of human reward (dys-) function and its clinical implications. Finally, we devise approaches to and challenges in implementing the concepts of circadian medicine in the therapy of substance use disorders.

Targeting the Maladaptive Effects of Binge Drinking on Circadian Gene Expression

Previous studies (1) support a role of circadian genes in regulating alcohol intake, and (2) reveal that harmful alcohol use alters circadian rhythms. However, there is minimal knowledge of the effects of chronic alcohol processes on rhythmic circadian gene expression across brain regions important for circadian biology and alcohol intake. Therefore, the present study sought to test the effects of chronic binge-like drinking on diurnal circadian gene expression patterns in the master circadian pacemaker (SCN), the ventral tegmental area (VTA), and the nucleus accumbens (NAc) in High Drinking in the Dark-1 (HDID-1) mice, a unique genetic risk model for drinking to intoxication. Consistent with earlier findings, we found that 8 weeks of binge-like drinking reduced the amplitude of several core circadian clock genes in the NAc and SCN, but not the VTA. To better inform the use of circadian-relevant pharmacotherapies in reducing harmful drinking and ameliorating alcohol's effects on circadian gene expression, we tested whether the casein kinase-1 inhibitor, PF-67046, or the phosphodiesterase type-4 (an upstream regulator of circadian signalling) inhibitor, apremilast, would reduce binge-like intake and mitigate circadian gene suppression. PF-67046 did not reduce intake but did have circadian gene effects. In contrast, apremilast reduced drinking, but had no effect on circadian expression patterns.

Bmal1-knockout mice exhibit reduced cocaine-seeking behaviour and cognitive impairments

Brain and Muscle Arnt-like Protein 1 (BMAL1) is an essential component of the molecular clock underlying circadian rhythmicity. Its function has been recently associated with mood and reward processing alterations. We investigated the behavioural and neurobiological impact of Bmal1 gene deletion in mice, and how this could affect rewarding effects of cocaine. Additionally, key clock genes and components of the dopamine system were assessed in several brain areas. Our results evidence behavioural alterations in Bmal1-KO mice, including changes in locomotor activity with impaired habituation to environments, short-term memory and social recognition impairments. In addition, Bmal1-KO mice experienced reduced cocaine-induced sensitisation and rewarding effects of cocaine as well as reduced cocaine-seeking behaviour. Furthermore, Bmal1 deletion influenced the expression of other clock-related genes in the mPFC and striatum, as well as alterations in the expression of dopaminergic elements. Overall, the present article offers a novel and extensive characterisation of Bmal1-KO animals. We suggest that reduced cocaine's rewarding effects in these mutant mice might be related to Bmal1 role as an expression regulator of MAO and TH, two essential enzymes involved in dopamine metabolism.

Arousal-Mediated Sleep Disturbance Persists During Cocaine Abstinence in Male Mice

Acute cocaine disturbs sleep on a dose-dependent basis; however, the consequences of chronic cocaine remain unclear. While the arousal promotion following cocaine has been well-established, effects of cocaine on sleep after termination of chronic cocaine exposure appear variable in human subjects with few studies in non-human subjects. Here, a within-subjects design (outcomes normalized to baseline, undisturbed behavior) and between-subjects design (repeated experimenter-administered cocaine vs. experimenter-administered saline) was used to investigate sleep homeostasis and sleep/waking under repeated cocaine/saline exposure and prolonged forced abstinence conditions in mice. Overall, during the forced abstinence period increases in arousal, as determined by sleep latency and gamma energy, persisted for 2 weeks. However, the sleep response to externally enforced sleep deprivation was unchanged suggesting that sleep disruptions during the forced abstinence period were driven by enhancement of arousal in the absence of changes in sleep homeostatic responses.

The Ventral Tegmental Area and Nucleus Accumbens as Circadian Oscillators: Implications for Drug Abuse and Substance Use Disorders

Circadian rhythms convergently evolved to allow for optimal synchronization of individuals’ physiological and behavioral processes with the Earth’s 24-h periodic cycling of environmental light and temperature. Whereas the suprachiasmatic nucleus (SCN) is considered the primary pacemaker of the mammalian circadian system, many extra-SCN oscillatory brain regions have been identified to not only exhibit sustainable rhythms in circadian molecular clock function, but also rhythms in overall region activity/function and mediated behaviors. In this review, we present the most recent evidence for the ventral tegmental area (VTA) and nucleus accumbens (NAc) to serve as extra-SCN oscillators and highlight studies that illustrate the functional significance of the VTA’s and NAc’s inherent circadian properties as they relate to reward-processing, drug abuse, and vulnerability to develop substance use disorders (SUDs).

Activation of dopamine D2 receptors in the medial shell region of the nucleus accumbens increases Per1 expression to enhance alcohol consumption

Circadian genes, including Per1, in the medial shell region of nucleus accumbens (mNAcSh), regulate binge alcohol consumption. However, the upstream mechanism regulating circadian genes-induced alcohol consumption is not known. Since activation of dopamine D2 receptors (D2R) increases Per1 gene expression, we hypothesised that local infusion of quinpirole, a D2R agonist, by increasing Per1 gene expression in the mNAcSh, will increase binge alcohol consumption in mice. We performed two experiments on male C57BL/6J mice, instrumented with bilateral guide cannulas above the mNAcSh, and exposed to a 4-day drinking-in-dark (DID) paradigm. The first experiment determined the effects of bilateral infusion of quinpirole (100 ng/300 nl/site) or DMSO (Vehicle group) in the mNAcSh on Per1 gene expression and alcohol consumption. The second experiment determined the effect of antisense-induced downregulation of Per1 in the mNAcSh on the quinpirole-induced increase in alcohol consumption. Control experiments were performed by exposing the animals to sucrose (10% w/v). After the experiment, animals were euthanised, brains removed and processed for localisation of injection sites and analysis of Per1 gene expression in the mNAcSh. As compared with the DMSO, local bilateral infusion of quinpirole significantly increased the expression of Per1 in the mNAcSh along with an increase in the amount of alcohol consumed in mice exposed to DID paradigm. In addition, local antisense-induced downregulation of Per1 significantly attenuated the effects of intro-accumbal infusion of quinpirole on alcohol consumption. Our results suggest that Per1 in the mNAcSh mediates D2R activation-induced increase in alcohol consumption.

Circadian transcription factor NPAS2 and the NAD+ -dependent deacetylase SIRT1 interact in the mouse nucleus accumbens and regulate reward

Substance use disorders are associated with disruptions to both circadian rhythms and cellular metabolic state. At the molecular level, the circadian molecular clock and cellular metabolic state may be interconnected through interactions with the nicotinamide adenine dinucleotide (NAD⁺ )-dependent deacetylase, sirtuin 1 (SIRT1). In the nucleus accumbens (NAc), a region important for reward, both SIRT1 and the circadian transcription factor neuronal PAS domain protein 2 (NPAS2) are highly enriched, and both are regulated by the metabolic cofactor NAD⁺ . Substances of abuse, like cocaine, greatly disrupt cellular metabolism and promote oxidative stress; however, their effects on NAD⁺ in the brain remain unclear. Interestingly, cocaine also induces NAc expression of both NPAS2 and SIRT1, and both have independently been shown to regulate cocaine reward in mice. However, whether NPAS2 and SIRT1 interact in the NAc and/or whether together they regulate reward is unknown. Here, we demonstrate diurnal expression of Npas2, Sirt1 and NAD⁺ in the NAc, which is altered by cocaine-induced upregulation. Additionally, co-immunoprecipitation reveals NPAS2 and SIRT1 interact in the NAc, and cross-analysis of NPAS2 and SIRT1 chromatin immunoprecipitation sequencing reveals several reward-relevant and metabolic-related pathways enriched among shared gene targets. Notably, NAc-specific Npas2 knock-down or a functional Npas2 mutation in mice attenuates SIRT1-mediated increases in cocaine preference. Together, our data reveal an interaction between NPAS2 and SIRT1 in the NAc, which may serve to integrate cocaine's effects on circadian and metabolic factors, leading to regulation of drug reward.

Cocaine induces paradigm-specific changes to the transcriptome within the ventral tegmental area

During the initial stages of drug use, cocaine-induced neuroadaptations within the ventral tegmental area (VTA) are critical for drug-associated cue learning and drug reinforcement processes. These neuroadaptations occur, in part, from alterations to the transcriptome. Although cocaine-induced transcriptional mechanisms within the VTA have been examined, various regimens and paradigms have been employed to examine candidate target genes. In order to identify key genes and biological processes regulating cocaine-induced processes, we employed genome-wide RNA-sequencing to analyze transcriptional profiles within the VTA from male mice that underwent one of four commonly used paradigms: acute home cage injections of cocaine, chronic home cage injections of cocaine, cocaine-conditioning, or intravenous-self administration of cocaine. We found that cocaine alters distinct sets of VTA genes within each exposure paradigm. Using behavioral measures from cocaine self-administering mice, we also found several genes whose expression patterns corelate with cocaine intake. In addition to overall gene expression levels, we identified several predicted upstream regulators of cocaine-induced transcription shared across all paradigms. Although distinct gene sets were altered across cocaine exposure paradigms, we found, from Gene Ontology (GO) term analysis, that biological processes important for energy regulation and synaptic plasticity were affected across all cocaine paradigms. Coexpression analysis also identified gene networks that are altered by cocaine. These data indicate that cocaine alters networks enriched with glial cell markers of the VTA that are involved in gene regulation and synaptic processes. Our analyses demonstrate that transcriptional changes within the VTA depend on the route, dose and context of cocaine exposure, and highlight several biological processes affected by cocaine. Overall, these findings provide a unique resource of gene expression data for future studies examining novel cocaine gene targets that regulate drug-associated behaviors.

Locomotor sensitization modulates voluntary self-administration of methamphetamine in Drosophila melanogaster

As complexities of addictive behaviors cannot be fully captured in laboratory studies, scientists use simple addiction-associated phenotypes and measure them in laboratory animals. Locomotor sensitization, characterized by an increased behavioral response to the same dose of the drug, has been extensively used to elucidate the genetic basis and molecular mechanisms of neuronal plasticity. However, to what extent it contributes to the development of addiction is not completely clear. We tested if the development of locomotor sensitization to methamphetamine affects voluntary self-administration, and vice versa, in order to investigate how two drug-associated phenotypes influence one another. In our study, we used the genetically tractable model organism, Drosophila melanogaster, and quantified locomotor sensitization and voluntary self-administration to methamphetamine using behavioral tests that were developed and adapted in our laboratory. We show that flies express robust locomotor sensitization to the second dose of volatilized methamphetamine, which significantly lowers preferential self-administration of methamphetamine. Naive flies preferentially self-administer food with methamphetamine over plain food. Exposing flies to volatilized methamphetamine after voluntary self-administration abolishes locomotor sensitization. We tested period null (per⁰¹ ) mutant flies and showed that they do not develop locomotor sensitization, nor do they show preferential self-administration of methamphetamine. Our results suggest that there may be partially overlapping neural circuitry that regulates the expression of locomotor sensitization and preferential self-administration to methamphetamine and that this circuitry requires a functional per gene.

Antisense-Induced Downregulation of Clock Genes in the Shell Region of the Nucleus Accumbens Reduces Binge Drinking in Mice

INTRODUCTIONS

Binge drinking is a deadly pattern of alcohol consumption. Evidence suggests that genetic variation in clock genes is strongly associated with alcohol misuse; however, the neuroanatomical basis for such a relationship is unknown. The shell region of the nucleus accumbens (NAcSh) is well known to play a role in binge drinking. Hence, we examined whether clock genes in the NAcSh regulate binge drinking.

METHODS

To address this question, 2 experiments were performed on male C57BL/6J mice. In the first experiment, mice exposed to alcohol or sucrose under the 4-day drinking-in-the-dark (DID) paradigm were euthanized at 2 different time points on day 4 [7 hours after light (pre-binge drinking) or dark (post-binge drinking) onset]. The brains were processed for RT-PCR to examine the expression of circadian clock genes (Clock, Per1, and Per2) in the NAcSh and suprachiasmatic nucleus (SCN). In the second experiment, mice were exposed to alcohol, sucrose, or water as described above. On day 4, 1 hour prior to the onset of alcohol exposure, mice were bilaterally infused with either a mixture of circadian clock gene antisense oligodeoxynucleotides (AS-ODNs; antisense group) or nonsense/random ODNs (R-ODNs; control group) through surgically implanted cannulas above the NAcSh. Alcohol/sucrose/water consumption was measured for 4 hours. Blood alcohol concentration was measured to confirm binge drinking. Microinfusion sites were histologically verified using cresyl violet staining.

RESULTS

As compared to sucrose, mice euthanized post-binge drinking (not pre-binge drinking) on day 4 displayed a greater expression of circadian genes in the NAcSh but not in the SCN. Knockdown of clock genes in the NAcSh caused a significantly lower volume of alcohol to be consumed on day 4 than in the control treatment. No differences were found in sucrose or water consumption.

CONCLUSIONS

Our results suggest that clock genes in the NAcSh play a crucial role in binge drinking.

Circadian-Dependent and Sex-Dependent Increases in Intravenous Cocaine Self-Administration in Npas2 Mutant Mice

Substance use disorder (SUD) is associated with disruptions in circadian rhythms. The circadian transcription factor neuronal PAS domain protein 2 (NPAS2) is enriched in reward-related brain regions and regulates reward, but its role in SU is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration (acquisition, dose-response, progressive ratio, extinction, cue-induced reinstatement) in wild-type (WT) and Npas2 mutant mice at different times of day. In the light (inactive) phase, cocaine self-administration, reinforcement, motivation and extinction responding were increased in all Npas2 mutants. Sex differences emerged during the dark (active) phase with Npas2 mutation increasing self-administration, extinction responding, and reinstatement only in females as well as reinforcement and motivation in males and females. To determine whether circulating hormones are driving these sex differences, we ovariectomized WT and Npas2 mutant females and confirmed that unlike sham controls, ovariectomized mutant mice showed no increase in self-administration. To identify whether striatal brain regions are activated in Npas2 mutant females, we measured cocaine-induced ΔFosB expression. Relative to WT, ΔFosB expression was increased in D1+ neurons in the nucleus accumbens (NAc) core and dorsolateral (DLS) striatum in Npas2 mutant females after dark phase self-administration. We also identified potential target genes that may underlie the behavioral responses to cocaine in Npas2 mutant females. These results suggest NPAS2 regulates reward and activity in specific striatal regions in a sex and time of day (TOD)-specific manner. Striatal activation could be augmented by circulating sex hormones, leading to an increased effect of Npas2 mutation in females.SIGNIFICANCE STATEMENT Circadian disruptions are a common symptom of substance use disorders (SUDs) and chronic exposure to drugs of abuse alters circadian rhythms, which may contribute to subsequent SU. Diurnal rhythms are commonly found in behavioral responses to drugs of abuse with drug sensitivity and motivation peaking during the dark (active) phase in nocturnal rodents. Emerging evidence links disrupted circadian genes to SU vulnerability and drug-induced alterations to these genes may augment drug-seeking. The circadian transcription factor neuronal PAS domain protein 2 (NPAS2) is enriched in reward-related brain regions and regulates reward, but its role in SU is unclear. To examine the role of NPAS2 in drug taking, we measured intravenous cocaine self-administration in wild-type (WT) and Npas2 mutant mice at different times of day.

Circadian rhythms and substance use disorders: A bidirectional relationship

The circadian system organizes circadian rhythms (biological cycles that occur around 24 h) that couple environmental cues (zeitgebers) with internal functions of the organism. The misalignment between circadian rhythms and external cues is known as chronodisruption and contributes to the development of mental, metabolic and other disorders, including cancer, cardiovascular diseases and addictive disorders. Drug addiction represents a global public health concern and affects the health and well-being of individuals, families and communities. In this manuscript, we reviewed evidence indicating a bidirectional relationship between the circadian system and the development of addictive disorders. We provide information on the interaction between the circadian system and drug addiction for each drug or drug class (alcohol, cannabis, hallucinogens, psychostimulants and opioids). We also describe evidence showing that drug use follows a circadian pattern, which changes with the progression of addiction. Furthermore, clock gene expression is also altered during the development of drug addiction in many brain areas related to drug reward, drug seeking and relapse. The regulation of the glutamatergic and dopaminergic neurocircuitry by clock genes is postulated to be the main circadian mechanism underlying the escalation of drug addiction. The bidirectional interaction between the circadian system and drug addiction seems to be mediated by the effects caused by each drug or class of drugs of abuse. These studies provide new insights on the development of successful strategies aimed at restoring/stabilizing circadian rhythms to reduce the risk for addiction development and relapse.

Depression-like behavior is associated with lower Per2 mRNA expression in the lateral habenula of rats

Circadian rhythm dysfunction is primary symptom of depression and is closely related to depression onset. The role of the lateral habenula (LHb) of the thalamus in the pathogenesis of depression has been a research topic of great interest. The neuronal activity of this structure has circadian characteristics, which are related to the regulation of circadian rhythms. However, in depression model of rats, the role of clock genes in the LHb has not been assessed. To address this gap, we used a clomipramine (CLI) injection-induced depression model in rats to assess the daily expression of rhythmic genes in the LHb and depression-like behavior in rats at multiple time points. In determining the role of the Per2 gene in the development of depression-like behavior in the LHb, we found that the expression of this clock gene differed in a circadian manner. Per2 expression was also significantly decreased in CLI-treated rats in late afternoon (17:00) and in the middle of the night (1:00). Furthermore, silencing Per2 in the LHb of normal rats induced depression-like behavior at night, suggesting that Per2 may play an important role in the pathogenesis of depression. Collectively, these results indicate that decreased Per2 expression in the LHb may be related to increased depression-like behavior at night in depression model of rats.

A regulatory pathway linking caffeine action, mood and the diurnal clock

Depression is a leading cause of disability worldwide. Circadian abnormalities and mood changes are symptoms of depression. The psychostimulant caffeine alters wakefulness and alleviates other depression-related symptoms during chronic intake, but the underlying mechanisms are unclear. It is not known, whether and how acute caffeine administration affects mood. Molecular approaches, transgenic mouse models, pharmacological intervention and behavioral analysis were combined to uncover a regulatory pathway, which connects caffeine action with diurnal signaling via the key dopaminergic protein DARPP-32 and alters mood-related phenotypes in mice, which are often assessed in the context of antidepressant action. We observed that Thr75-DARPP-32 binds to the circadian regulator CLOCK and disrupts CLOCK:BMAL1 chromatin binding, thereby affecting gene expression. T75A-DARPP-32 mutant mice show reduced caffeine effects on CLOCK:BMAL1 and lack caffeine-induced effects on mood. This study provides a link between caffeine, diurnal signaling and mood-related behaviors, which may open new perspectives for our understanding of antidepressant mechanisms in the mouse brain.

Regulation and function of extra-SCN circadian oscillators in the brain

Most organisms evolved endogenous, so called circadian clocks as internal timekeeping mechanisms allowing them to adapt to recurring changes in environmental demands brought about by 24-hour rhythms such as the light-dark cycle, temperature variations or changes in humidity. The mammalian circadian clock system is based on cellular oscillators found in all tissues of the body that are organized in a hierarchical fashion. A master pacemaker located in the suprachiasmatic nucleus (SCN) synchronizes peripheral tissue clocks and extra-SCN oscillators in the brain with each other and with external time. Different time cues (so called Zeitgebers) such as light, food intake, activity and hormonal signals reset the clock system through the SCN or by direct action at the tissue clock level. While most studies on non-SCN clocks so far have focused on peripheral tissues, several extra-SCN central oscillators were characterized in terms of circadian rhythm regulation and output. Some of them are directly innervated by the SCN pacemaker, while others receive indirect input from the SCN via other neural circuits or extra-brain structures. The specific physiological function of these non-SCN brain oscillators as well as their role in the regulation of the circadian clock network remains understudied. In this review we summarize our current knowledge about the regulation and function of extra-SCN circadian oscillators in different brain regions and devise experimental approaches enabling us to unravel the organization of the circadian clock network in the central nervous system.

Effects of Kappa opioid receptor blockade by LY2444296 HCl, a selective short-acting antagonist, during chronic extended access cocaine self-administration and re-exposure in rat

RATIONALE

Cocaine addiction is a chronic brain disease characterized by compulsive drug intake and dysregulation of brain reward systems. Few preclinical studies have modeled the natural longitudinal course of cocaine addiction. Extended access self-administration protocols are powerful tools for modeling the advanced stages of addiction; however, few studies have duration of drug access longer than 12 h/session, potentially limiting their construct validity. Identification of changes in cocaine intake patterns during the development of addictive-like states may allow better treatments for vulnerable subjects. The kappa opioid receptor (KOPr) system has been implicated in the neurobiological regulation of addictive states as well as mood and stress disorders, with selective KOPr antagonists proposed as possible pharmacotherapeutic agents. Chronic cocaine exposure increases the expression of KOPr and its endogenous agonists, the dynorphins, in several brain areas in rodents.

OBJECTIVES

To examine the behavioral pattern of intake during chronic (14 days) 18 h intravenous cocaine self-administration (0.5 mg/kg/infusion) and the effect of a novel short-acting KOPr antagonist LY2444296 HCl (3 mg/kg) administered during sessions 8 to 14 of chronic 18 h/day cocaine self-administration and prior to a single re-exposure session after 2 cocaine-free withdrawal days.

RESULTS

Both daily and hourly cocaine intake patterns changed over 14 days of 18 h self-administration. LY pretreatment affected the pattern of self-administration across the second week of extended access cocaine self-administration and prevented the increase in cocaine intake during re-exposure.

CONCLUSIONS

Overall, the KOPr antagonist attenuated escalated cocaine consumption in a rat model of extended access cocaine self-administration.

Melatonin decreases cocaine-induced locomotor activity in pinealectomized rats

Objective:

Several studies have shown that the time of day regulates the reinforcing effects of cocaine. Additionally, melatonin and its MT₁ and MT₂ receptors have been found to participate in modulation of the reinforcing effects of such addictive drugs as cocaine. Loss of the diurnal variation in cocaine-induced locomotor sensitization and cocaine-induced place preference has been identified in pinealectomized mice. In addition, several studies in rodents have shown that administration of melatonin decreased the reinforcing effects of cocaine. The objective of this study was to evaluate the effect of melatonin on cocaine-induced locomotor activity in pinealectomized rats at different times of day (zeitgeber time [ZT]4, ZT10, ZT16, and ZT22).

Methods:

Naïve, pinealectomized Wistar rats received cocaine at different times of day. Melatonin was administered 30 min before cocaine; luzindole was administered 15 min prior to melatonin and 45 min before cocaine. After administration of each treatment, locomotor activity for each animal was recorded for a total of 30 min. Pinealectomy was confirmed at the end of the experiment through melatonin quantitation by ELISA.

Results:

Cocaine-induced locomotor activity varied according to the time of day. Continuous lighting and pinealectomy increased cocaine-induced locomotor activity. Melatonin administration decreased cocaine-induced locomotor activity in naïve and pinealectomized rats at different times of day. Luzindole blocked the melatonin-induced reduction in cocaine-induced locomotor activity in pinealectomized rats.

Conclusion:

Given its ability to mitigate various reinforcing effects of cocaine, melatonin could be a useful therapy for cocaine abuse.

Cannabidiol affects circadian clock core complex and its regulation in microglia cells

Cannabis is often used by consumers for sleep disorders. Studies show that circadian rhythm could be affected by a misuse of cannabis. Recent research has connected the role of microglial cells with psychiatric disorders such as substance abuse. The aim was to show the effect of two major components of cannabis on circadian genes regulation in microglial cells. In BV-2 microglial cells, cannabidiol (CBD) induces a deregulation of circadian genes with (P-value = 0.039) or without (P-value = 0.0015) lipopolisaccharides stimulation. CBD up regulated Arntl (P = 9.72E-5) and down regulated Clock (P = 0.0034) in BV-2 cells. Temporal expression of Arntl (light and dark P = 0.0054) and Clock (light and dark P = 0.047) was confirmed to have 24 hours light and dark rhythmic regulation in dissected suprachiasmatic nucleus as well as of Cb1 cannabinoid receptor (light and dark P = 0.019). In BV-2 microglia cells, CBD also up regulated CRY2 (P = 0.0473) and PER1 (P = 0.0131). Other nuclear molecules show a deregulation of circadian rhythm in microglial cells by CBD, such as RORA, RevErbα, RORB, CREBBP, AFT4, AFT5 and NFIL3. Our study suggests that circadian rhythm in microglial cells is deregulated by CBD but not by THC. It is consistent with clinical observations of the use of therapeutic cannabis to treat insomnia.

Differential Effects of Addictive Drugs on Sleep and Sleep Stages

Addictive drugs affect sleep both in individuals currently using drugs and in individuals who have withdrawn from drugs. In fact, sleep disturbances are reported by individuals for some drugs long after they have quit taking them and after other withdrawal symptoms have subsided. This suggests that addictive drugs and sleep share some of the same neurobiological mechanisms. Sleep researchers may be studying the neurobiology of addictive drugs without knowing it. The purpose of this survey is to summarize the effects that addictive drugs have on sleep and stages of sleep. We demonstrate that different addictive drugs have differential effects on disturbance of sleep, in general, and on specific stages of sleep either while the drug is on board or after withdrawal. Accordingly, these results are intended to encourage sleep researchers to use their knowledge of sleep mechanisms to offer researchers of addictive drugs new insights of how addictive drugs might affect brain mechanisms. Also, these results should alert researchers of addiction that treatment for drug effects needs to consider treatment for sleep disturbances as well. Treatment for addiction is rarely accompanied by treatment for sleep disturbances even though this survey demonstrates they are clearly related.

Cell-Type-Specific Regulation of Nucleus Accumbens Synaptic Plasticity and Cocaine Reward Sensitivity by the Circadian Protein, NPAS2

The circadian transcription factor neuronal PAS domain 2 (NPAS2) is linked to psychiatric disorders associated with altered reward sensitivity. The expression of Npas2 is preferentially enriched in the mammalian forebrain, including the nucleus accumbens (NAc), a major neural substrate of motivated and reward behavior. Previously, we demonstrated that downregulation of NPAS2 in the NAc reduces the conditioned behavioral response to cocaine in mice. We also showed that Npas2 is preferentially enriched in dopamine receptor 1 containing medium spiny neurons (D1R-MSNs) of the striatum. To extend these studies, we investigated the impact of NPAS2 disruption on accumbal excitatory synaptic transmission and strength, along with the behavioral sensitivity to cocaine reward in a cell-type-specific manner. Viral-mediated knockdown of Npas2 in the NAc of male and female C57BL/6J mice increased the excitatory drive onto MSNs. Using Drd1a-tdTomato mice in combination with viral knockdown, we determined these synaptic adaptations were specific to D1R-MSNs relative to non-D1R-MSNs. Interestingly, NAc-specific knockdown of Npas2 blocked cocaine-induced enhancement of synaptic strength and glutamatergic transmission specifically onto D1R-MSNs. Last, we designed, validated, and used a novel Cre-inducible short-hairpin RNA virus for MSN-subtype-specific knockdown of Npas2 Cell-type-specific Npas2 knockdown in D1R-MSNs, but not D2R-MSNs, in the NAc reduced cocaine conditioned place preference. Together, our results demonstrate that NPAS2 regulates excitatory synapses of D1R-MSNs in the NAc and cocaine reward-related behavior.SIGNIFICANCE STATEMENT Drug addiction is a widespread public health concern often comorbid with other psychiatric disorders. Disruptions of the circadian clock can predispose or exacerbate substance abuse in vulnerable individuals. We demonstrate a role for the core circadian protein, NPAS2, in mediating glutamatergic neurotransmission at medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a region critical for reward processing. We find that NPAS2 negatively regulates functional excitatory synaptic plasticity in the NAc and is necessary for cocaine-induced plastic changes in MSNs expressing the dopamine 1 receptor (D1R). We further demonstrate disruption of NPAS2 in D1R-MSNs produces augmented cocaine preference. These findings highlight the significance of cell-type-specificity in mechanisms underlying reward regulation by NPAS2 and extend our knowledge of its function.

The Intertwined Roles of Circadian Rhythms and Neuronal Metabolism Fueling Drug Reward and Addiction

Drug addiction is a highly prevalent and devastating disorder with few effective treatments, resulting in enormous burdens on family and society. The cellular and behavioral effects of drugs of abuse are related to their abilities to elevate synaptic dopamine levels. Midbrain dopaminergic neurons projecting from the ventral tegmental area to the nucleus accumbens play crucial roles in substance-induced neural and behavioral plasticity. Significantly, increasing work suggests that interplay between the brain circadian system and the cellular bioenergetic machinery in these dopamine neurons plays a critical role in mediating the actions of drugs of abuse. Here, we describe recent progress in elucidating the interconnections between circadian and metabolic systems at the molecular and cellular levels and their relationships to modulation of drug reward and addiction.

The role of CLOCK gene in psychiatric disorders: Evidence from human and animal research

The circadian clock system drives daily rhythms in physiology, metabolism, and behavior in mammals. Molecular mechanisms of this system consist of multiple clock genes, with Circadian Locomotor Output Cycles Kaput (CLOCK) as a core member that plays an important role in a wide range of behaviors. Alterations in the CLOCK gene are associated with common psychiatric disorders as well as with circadian disturbances comorbidities. This review addresses animal, molecular, and genetic studies evaluating the role of the CLOCK gene on many psychiatric conditions, namely autism spectrum disorder, schizophrenia, attention-deficit/hyperactivity disorder, major depressive disorder, bipolar disorder, anxiety disorder, and substance use disorder. Many animal experiments focusing on the effects of the Clock gene in behavior related to psychiatric conditions have shown consistent biological plausibility and promising findings. In humans, genetic and gene expression studies regarding disorder susceptibility, sleep disturbances related comorbidities, and response to pharmacological treatment, in general, are in agreement with animal studies. However, the number of controversial results is high. Literature suggests that the CLOCK gene exerts important influence on these conditions, and influences the susceptibility to phenotypes of psychiatric disorders.

Neural Mechanisms of Circadian Regulation of Natural and Drug Reward

Circadian rhythms are endogenously generated near 24-hour variations of physiological and behavioral functions. In humans, disruptions to the circadian system are associated with negative health outcomes, including metabolic, immune, and psychiatric diseases, such as addiction. Animal models suggest bidirectional relationships between the circadian system and drugs of abuse, whereby desynchrony, misalignment, or disruption may promote vulnerability to drug use and the transition to addiction, while exposure to drugs of abuse may entrain, disrupt, or perturb the circadian timing system. Recent evidence suggests natural (i.e., food) and drug rewards may influence overlapping neural circuitry, and the circadian system may modulate the physiological and behavioral responses to these stimuli. Environmental disruptions, such as shifting schedules or shorter/longer days, influence food and drug intake, and certain mutations of circadian genes that control cellular rhythms are associated with altered behavioral reward. We highlight the more recent findings associating circadian rhythms to reward function, linking environmental and genetic evidence to natural and drug reward and related neural circuitry.

Behavioral Deficits Following Withdrawal from Chronic Ethanol Are Influenced by SLO Channel Function in Caenorhabditis elegans

Symptoms of withdrawal from chronic alcohol use are a driving force for relapse in alcohol dependence. Thus, uncovering molecular targets to lessen their severity is key to breaking the cycle of dependence. Using the nematode Caenorhabditis elegans, we tested whether one highly conserved ethanol target, the large-conductance, calcium-activated potassium channel (known as the BK channel or Slo1), modulates ethanol withdrawal. Consistent with a previous report, we found that C. elegans displays withdrawal-related behavioral impairments after cessation of chronic ethanol exposure. We found that the degree of impairment is exacerbated in worms lacking the worm BK channel, SLO-1, and is reduced by selective rescue of this channel in the nervous system. Enhanced SLO-1 function, via gain-of-function mutation or overexpression, also dramatically reduced behavioral impairment during withdrawal. Consistent with these results, we found that chronic ethanol exposure decreased SLO-1 expression in a subset of neurons. In addition, we found that the function of a distinct, conserved Slo family channel, SLO-2, showed an inverse relationship to withdrawal behavior, and this influence depended on SLO-1 function. Together, our findings show that modulation of either Slo family ion channel bidirectionally regulates withdrawal behaviors in worm, supporting further exploration of the Slo family as targets for normalizing behaviors during alcohol withdrawal.

Circadian Plasticity of Mammalian Inhibitory Interneurons

Inhibitory interneurons participate in all neuronal circuits in the mammalian brain, including the circadian clock system, and are indispensable for their effective function. Although the clock neurons have different molecular and electrical properties, their main function is the generation of circadian oscillations. Here we review the circadian plasticity of GABAergic interneurons in several areas of the mammalian brain, suprachiasmatic nucleus, neocortex, hippocampus, olfactory bulb, cerebellum, striatum, and in the retina.

Circadian Rhythms and Substance Abuse: Chronobiological Considerations for the Treatment of Addiction

Reward-related learning, including that associated with drugs of abuse, is largely mediated by the dopaminergic mesolimbic pathway. Mesolimbic neurophysiology and motivated behavior, in turn, are modulated by the circadian timing system which generates ∼24-h rhythms in cellular activity. Both drug taking and seeking and mesolimbic dopaminergic neurotransmission can vary widely over the day. Moreover, circadian clock genes are expressed in ventral tegmental area dopaminergic cells and in mesolimbic target regions where they can directly modulate reward-related neurophysiology and behavior. There also exists a reciprocal influence between drug taking and circadian timing as the administration of drugs of abuse can alter behavioral rhythms and circadian clock gene expression in mesocorticolimbic structures. These interactions suggest that manipulations of the circadian timing system may have some utility in the treatment of substance abuse disorders. Here, the literature on bidirectional interactions between the circadian timing system and drug taking is briefly reviewed, and potential chronotherapeutic considerations for the treatment of addiction are discussed.

Social Isolation Modulates CLOCK Protein and Beta-Catenin Expression Pattern in Gonadotropin-Inhibitory Hormone Neurons in Male Rats

Postweaning social isolation reduces the amplitude of the daily variation of CLOCK protein in the brain and induces lower reproductive activity. Gonadotropin-inhibitory hormone (GnIH) acts as an inhibitor in the reproductive system and has been linked to stress. Social isolation has been shown to lower neuronal activity of GnIH-expressing neurons in the dorsomedial hypothalamus (DMH). The exact mechanism by which social isolation may affect GnIH is still unclear. We investigated the impact of social isolation on regulatory cellular mechanisms in GnIH neurons. We examined via immunohistochemistry the expression of CLOCK protein at four different times throughout the day in GnIH cells tagged with enhanced fluorescent green protein (EGFP-GnIH) in 9-week-old adult male rats that have been raised for 6 weeks under postweaning social isolation and compared them with group-raised control rats of the same age. We also studied the expression of β-catenin-which has been shown to be affected by circadian proteins such as Bmal1-in EGFP-GnIH neurons to determine whether it could play a role in linking CLOCK in GnIH neurons. We found that social isolation modifies the pattern of CLOCK expression in GnIH neurons in the DMH. Socially isolated rats displayed greater CLOCK expression in the dark phase, while control rats displayed increased CLOCK expression in the light phase. Furthermore, β-catenin expression pattern in GnIH cells was disrupted by social isolation. This suggests that social isolation triggers changes in CLOCK and GnIH expression, which may be associated with an increase in nuclear β-catenin during the dark phase.

Circadian influences on dopamine circuits of the brain: regulation of striatal rhythms of clock gene expression and implications for psychopathology and disease

Circadian clock proteins form an autoregulatory feedback loop that is central to the endogenous generation and transmission of daily rhythms in behavior and physiology. Increasingly, circadian rhythms in clock gene expression are being reported in diverse tissues and brain regions that lie outside of the suprachiasmatic nucleus (SCN), the master circadian clock in mammals. For many of these extra-SCN rhythms, however, the region-specific implications are still emerging. In order to gain important insights into the potential behavioral, physiological, and psychological relevance of these daily oscillations, researchers have begun to focus on describing the neurochemical, hormonal, metabolic, and epigenetic contributions to the regulation of these rhythms. This review will highlight important sites and sources of circadian control within dopaminergic and striatal circuitries of the brain and will discuss potential implications for psychopathology and disease . For example, rhythms in clock gene expression in the dorsal striatum are sensitive to changes in dopamine release, which has potential implications for Parkinson’s disease and drug addiction. Rhythms in the ventral striatum and limbic forebrain are sensitive to psychological and physical stressors, which may have implications for major depressive disorder. Collectively, a rich circadian tapestry has emerged that forces us to expand traditional views and to reconsider the psychopathological, behavioral, and physiological importance of these region-specific rhythms in brain areas that are not immediately linked with the regulation of circadian rhythms.

Implications of circadian rhythm and stress in addiction vulnerability

In the face of chronic stress, some individuals can maintain normal function while others go on to develop mental illness. Addiction, affecting one in every twelve people in America, is a substance use disorder long associated with stressful life events and disruptions in the sleep/wake cycle. The circadian and stress response systems have evolved to afford adaptability to environmental changes and allow for maintenance of functional stability, or homeostasis. This mini-review will discuss how circadian rhythms and stress individually affect drug response, affect each other, and how their interactions may regulate reward-related behavior. In particular, we will focus on the interactions between the circadian clock and the regulation of glucocorticoids by the hypothalamic-pituitary-adrenal (HPA) axis. Determining how these two systems act on dopaminergic reward circuitry may not only reveal the basis for vulnerability to addiction, but may also illuminate potential therapeutic targets for future investigation.

Circadian Mechanisms Underlying Reward-Related Neurophysiology and Synaptic Plasticity

Evidence from clinical and preclinical research provides an undeniable link between disruptions in the circadian clock and the development of psychiatric diseases, including mood and substance abuse disorders. The molecular clock, which controls daily patterns of physiological and behavioral activity in living organisms, when desynchronized, may exacerbate or precipitate symptoms of psychiatric illness. One of the outstanding questions remaining in this field is that of cause and effect in the relationship between circadian rhythm disruption and psychiatric disease. Focus has recently turned to uncovering the role of circadian proteins beyond the maintenance of homeostatic systems and outside of the suprachiasmatic nucleus (SCN), the master pacemaker region of the brain. In this regard, several groups, including our own, have sought to understand how circadian proteins regulate mechanisms of synaptic plasticity and neurotransmitter signaling in mesocorticolimbic brain regions, which are known to be critically involved in reward processing and mood. This regulation can come in the form of direct transcriptional control of genes central to mood and reward, including those associated with dopaminergic activity in the midbrain. It can also be seen at the circuit level through indirect connections of mesocorticolimbic regions with the SCN. Circadian misalignment paradigms as well as genetic models of circadian disruption have helped to elucidate some of the complex interactions between these systems and neural activity influencing behavior. In this review, we explore findings that link circadian protein function with synaptic adaptations underlying plasticity as it may contribute to the development of mood disorders and addiction. In light of recent advances in technology and sophisticated methods for molecular and circuit-level interrogation, we propose future directions aimed at teasing apart mechanisms through which the circadian system modulates mood and reward-related behavior.

NIDA-Drug Addiction Treatment Outcome Study (DATOS) Relapse as a Function of Spirituality/Religiosity

BACKGROUND

The connection between religion/spirituality and deviance, like substance abuse, was first made by Durkheim who defined socially expected behaviors as norms. He explained that deviance is due in large part to their absence (called anomie), and concluded that spirituality lowers deviance by preserving norms and social bonds. Impairments in brain reward circuitry, as observed in Reward Deficiency Syndrome (RDS), may also result in deviance and as such we wondered if stronger belief in spirituality practice and religious belief could lower relapse from drugs of abuse.

METHODS

The NIDA Drug Addiction Treatment Outcome Study data set was used to examine post hoc relapse rates among 2,947 clients who were interviewed at 12 months after intake broken down by five spirituality measures.

RESULTS

Our main findings strongly indicate, that those with low spirituality have higher relapse rates and those with high spirituality have higher remission rates with crack use being the sole exception. We found significant differences in terms of cocaine, heroin, alcohol, and marijuana relapse as a function of strength of religious beliefs (x2 = 15.18, p = 0.028; logistic regression = 10.65, p = 0.006); frequency of attending religious services (x2 = 40.78, p < 0.0005; logistic regression = 30.45, p < 0.0005); frequency of reading religious books (x2 = 27.190, p < 0.0005; logistic regression = 17.31, p < 0.0005); frequency of watching religious programs (x2 = 19.02, p = 0.002; logistic regression = ns); and frequency of meditation/prayer (x2 = 11.33, p = 0.045; logistic regression = 9.650, p = 0.002). Across the five measures of spirituality, the spiritual participants reported between 7% and 21% less alcohol, cocaine, heroin, and marijuana use than the non-spiritual subjects. However, the crack users who reported that religion was not important reported significantly less crack use than the spiritual participants. The strongest association between remission and spirituality involves attending religious services weekly, the one marker of the five that involves the highest social interaction/social bonding consistent with Durkheim's social bond theory.

CONCLUSIONS

Stronger spiritual/religious beliefs and practices are directly associated with remission from abused drugs except crack. Much like the value of having a sponsor, for clients who abuse drugs, regular spiritual practice, particularly weekly attendance at the religious services of their choice is associated with significantly higher remission. These results demonstrate the clinically significant role of spirituality and the social bonds it creates in drug treatment programs.

Clock genes × stress × reward interactions in alcohol and substance use disorders

Adverse life events and highly stressful environments have deleterious consequences for mental health. Those environmental factors can potentiate alcohol and drug abuse in vulnerable individuals carrying specific genetic risk factors, hence producing the final risk for alcohol- and substance-use disorders development. The nature of these genes remains to be fully determined, but studies indicate their direct or indirect relation to the stress hypothalamo-pituitary-adrenal (HPA) axis and/or reward systems. Over the past decade, clock genes have been revealed to be key-players in influencing acute and chronic alcohol/drug effects. In parallel, the influence of chronic stress and stressful life events in promoting alcohol and substance use and abuse has been demonstrated. Furthermore, the reciprocal interaction of clock genes with various HPA-axis components, as well as the evidence for an implication of clock genes in stress-induced alcohol abuse, have led to the idea that clock genes, and Period genes in particular, may represent key genetic factors to consider when examining gene × environment interaction in the etiology of addiction. The aim of the present review is to summarize findings linking clock genes, stress, and alcohol and substance abuse, and to propose potential underlying neurobiological mechanisms.

Direct regulation of diurnal Drd3 expression and cocaine reward by NPAS2

BACKGROUND

Circadian gene disruptions are associated with the development of psychiatric disorders, including addiction. However, the mechanisms by which circadian genes regulate reward remain poorly understood.

METHODS

We 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.

RESULTS

Npas2 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.

CONCLUSIONS

Together, these findings identify an important role for the circadian protein, NPAS2, in the NAc in the regulation of dopamine receptor expression and drug reward.

Diurnal and circadian regulation of reward-related neurophysiology and behavior

Here, we review work over the past two decades that has indicated drug reward is modulated by the circadian system that generates daily (i.e., 24h) rhythms in physiology and behavior. Specifically, drug-self administration, psychomotor stimulant-induced conditioned place preference, and locomotor sensitization vary widely across the day in various species. These drug-related behavioral rhythms are associated with rhythmic neural activity and dopaminergic signaling in the mesocorticolimbic pathways, with a tendency toward increased activity during the species typical wake period. While the mechanisms responsible for such cellular rhythmicity remain to be fully identified, circadian clock genes are expressed in these brain areas and can function locally to modulate both dopaminergic neurotransmission and drug-associated behavior. In addition, neural and endocrine inputs to these brain areas contribute to cellular and reward-related behavioral rhythms, with the medial prefrontal cortex playing a pivotal role. Acute or chronic administration of drugs of abuse can also alter clock gene expression in reward-related brain regions. Emerging evidence suggests that drug craving in humans is under a diurnal regulation and that drug reward may be influenced by clock gene polymorphisms. These latter findings, in particular, indicate that the development of therapeutic strategies to modulate the circadian influence on drug reward may prove beneficial in the treatment of substance abuse disorders.

Chronic cocaine causes long-term alterations in circadian period and photic entrainment in the mouse

The disruptive effects of cocaine on physiological, behavioral and genetic processes are well established. However, few studies have focused on the actions of cocaine on the adult circadian timekeeping system, and none have explored the circadian implications of long-term (weeks to months) cocaine exposure. The present study was undertaken to explore the actions of such long-term cocaine administration on core circadian parameters in mice, including rhythm period, length of the nocturnal activity period and photic entrainment. For cocaine dosing over extended periods, cocaine was provided in drinking water using continuous and scheduled regimens. The impact of chronic cocaine on circadian regulation was evidenced by disruptions of the period of circadian entrainment and intrinsic free-running circadian period. Specifically, mice under a skeleton photoperiod (1-min pulse of dim light delivered daily) receiving continuous ad libitum cocaine entrained rapidly to the light pulse at activity onset. Conversely, water controls entrained more slowly at activity offset through a process of phase-delays, which resulted in their activity rhythms being entrained 147° out of phase with the cocaine group. This pattern persisted after cocaine withdrawal. Next, mice exposed to scheduled daily cocaine presentations exhibited free-running periods under constant darkness that were significantly longer than water controls and which also persisted after cocaine withdrawal. These cocaine-induced perturbations of clock timing could produce chronic psychological and physiological stress, contributing to increased cocaine use and dependence.

Cocaine modulates mammalian circadian clock timing by decreasing serotonin transport in the SCN

Cocaine abuse disrupts reward and homeostatic processes through diverse processes, including those involved in circadian clock regulation. Recently we showed that cocaine administration to mice disrupts nocturnal photic phase resetting of the suprachiasmatic (SCN) circadian clock, whereas administration during the day induces non-photic phase shifts. Importantly, the same effects are seen when cocaine is applied to the SCN in vitro, where it blocks photic-like (glutamate-induced) phase shifts at night and induces phase advances during the day. Furthermore, our previous data suggest that cocaine acts in the SCN by enhancing 5-HT signaling. For example, the in vitro actions of cocaine mimic those of 5-HT and are blocked by the 5-HT antagonist, metergoline, but not the dopamine receptor antagonist, fluphenazine. Although our data are consistent with cocaine acting through enhanced 5-HT signaling, the nonselective actions of cocaine as an antagonist of monoamine transporters raises the question of whether inhibition of the 5-HT transporter (SERT) is key to its circadian effects. Here we investigate this issue using transgenic mice expressing a SERT that exhibits normal 5-HT recognition and transport but significantly reduced cocaine potency (SERT Met172). Circadian patterns of SCN behavioral and neuronal activity did not differ between wild-type (WT) and SERT Met172 mice, nor did they differ in the ability of the 5-HT1A,2,7 receptor agonist, 8-OH-DPAT to reset SCN clock phase, consistent with the normal SERT expression and activity in the transgenic mice. However, (1) cocaine administration does not induce phase advances when administered in vivo or in vitro in SERT Met172 mice; (2) cocaine does not block photic or glutamate-induced phase shifts in SERT Met172 mice; and (3) cocaine does not induce long-term changes in free-running period in SERT Met172 mice. We conclude that SERT antagonism is required for the phase shifting of the SCN circadian clock induced by cocaine.

Circadian rhythms and addiction: mechanistic insights and future directions

Circadian rhythms are prominent in many physiological and behavioral functions. Circadian disruptions either by environmental or molecular perturbation can have profound health consequences, including the development and progression of addiction. Both animal and humans studies indicate extensive bidirectional relationships between the circadian system and drugs of abuse. Addicted individuals display disrupted rhythms, and chronic disruption or particular chronotypes may increase the risk for substance abuse and relapse. Moreover, polymorphisms in circadian genes and an evening chronotype have been linked to mood and addiction disorders, and recent efforts suggest an association with the function of reward neurocircuitry. Animal studies are beginning to determine how altered circadian gene function results in drug-induced neuroplasticity and behaviors. Many studies suggest a critical role for circadian rhythms in reward-related pathways in the brain and indicate that drugs of abuse directly affect the central circadian pacemaker. In this review, we highlight key findings demonstrating the importance of circadian rhythms in addiction and how future studies will reveal important mechanistic insights into the involvement of circadian rhythms in drug addiction.