Free-running rhythms of cocaine self-administration in rats held under constant lighting conditions

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

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

A buprenorphine-validated rat model of opioid use disorder optimized to study sex differences in vulnerability to relapse

RATIONALE

Opioid use disorder (OUD) is a major epidemic in the USA. Despite evidence indicating that OUD may be particularly severe for women, preclinical models have yet to establish sex as a major factor in OUD.

OBJECTIVES

Here, we examined sex differences in vulnerability to relapse following intermittent access fentanyl self-administration and protracted abstinence and used buprenorphine, the FDA-approved treatment for OUD, to test the validity of our model.

METHODS

Following acquisition of fentanyl self-administration under one of two training conditions, male and female rats were given extended, 24-h/day access to fentanyl (0.25 μg/kg/infusion, 10 days) using an intermittent access procedure. Vulnerability to relapse was assessed using an extinction/cue-induced reinstatement procedure following 14 days of abstinence; buprenorphine (0 or 3 mg/kg/day) was administered throughout abstinence.

RESULTS

Levels of drug-seeking were high following extended-access fentanyl self-administration and abstinence; buprenorphine markedly decreased drug-seeking supporting the validity of our relapse model. Females self-administered more fentanyl and responded at higher levels during subsequent extinction testing. Buprenorphine was effective in both sexes and eliminated sex and estrous phase differences in drug-seeking. Interestingly, the inclusion of a time-out during training had a major impact on later fentanyl self-administration in females, but not males, indicating that the initial exposure conditions can persistently impact vulnerability in females.

CONCLUSIONS

These findings demonstrate the utility of this rat model for determining sex and hormonal influences on the development and treatment of OUD.

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.

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.

Circadian neurons in the lateral habenula: Clocking motivated behaviors

The main circadian clock in mammals is located in the hypothalamic suprachiasmatic nucleus (SCN), however, central timing mechanisms are also present in other brain structures beyond the SCN. The lateral habenula (LHb), known for its important role in the regulation of the monoaminergic system, contains such a circadian clock whose molecular and cellular mechanisms as well as functional role are not well known. However, since monoaminergic systems show circadian activity, it is possible that the LHb-clock's role is to modulate the rhythmic activity of the dopamine, serotonin and norephinephrine systems, and associated behaviors. Moreover, the LHb is involved in different pathological states such as depression, addiction and schizophrenia, states in which sleep and circadian alterations have been reported. Thus, perturbations of circadian activity in the LHb might, in part, be a cause of these rhythmic alterations in psychiatric ailments. In this review the current state of the LHb clock and its possible implications in the control of monoaminergic systems rhythms, motivated behaviors (e.g., feeding, drug intake) and depression (with circadian disruptions and altered motivation) will be discussed.

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.

Toward an evolutionary basis for resilience to drug addiction

According to Müller & Schumann (M&S), people would have evolved adaptations for learning to use psychoactive plants and drugs as instruments that reveal particularly advantageous in modern urban environments. Here I "instrumentalize" this framework to propose an evolutionary basis for the existence of a biological resilience to drug addiction in people.

Effects of circadian disruption on methamphetamine consumption in methamphetamine-exposed rats

RATIONALE

A substantial number of clinical studies indicate associations between sleep abnormalities and drug abuse; however, the role played by the circadian system in the development of addiction is largely unknown.

OBJECTIVE

The aim of this study was to examine the effects of experimentally induced chronic jet lag on methamphetamine consumption in a rat model of methamphetamine drinking.

METHODS

Male Sprague-Dawley rats (n = 32) were housed in running wheel cages in a 12:12 h light:dark cycle. One group of rats (n = 16) was given 2 weeks of forced methamphetamine consumption (0.01 % in drinking water; meth pre-exposed) while a second group (n = 16, not pre-exposed) received water only. This was followed by a 2-week abstinence period during which half of the animals from each group were exposed to four consecutive 6-h advancing phase shifts of the light:dark cycle, while the other half remained on the original light:dark cycle. Methamphetamine consumption was assessed in all rats following the deprivation period using a two-bottle choice paradigm.

RESULTS

Methamphetamine consumption was initially lower in methamphetamine pre-exposed versus not pre-exposed rats. However, during the second week following abstinence, consumption was significantly higher in phase-shifted rats of the methamphetamine pre-exposed group compared to all other groups.

CONCLUSIONS

These data reveal an effect of circadian rhythm disturbance on methamphetamine consumption and suggest that dysregulation of the circadian system be considered in the etiology of relapse and addiction.

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.

Insights into the Role of the Habenular Circadian Clock in Addiction

Drug addiction is a brain disease involving alterations in anatomy and functional neural communication. Drug intake and toxicity show daily rhythms in both humans and rodents. Evidence concerning the role of clock genes in drug intake has been previously reported. However, the implication of a timekeeping brain locus is much less known. The epithalamic lateral habenula (LHb) is now emerging as a key nucleus in drug intake and addiction. This brain structure modulates the activity of dopaminergic neurons from the ventral tegmental area, a central part of the reward system. Moreover, the LHb has circadian properties: LHb cellular activity (i.e., firing rate and clock genes expression) oscillates in a 24-h range, and the nucleus is affected by photic stimulation and has anatomical connections with the main circadian pacemaker, the suprachiasmatic nucleus. Here, we describe the current insights on the role of the LHb as a circadian oscillator and its possible implications on the rhythmic regulation of the dopaminergic activity and drug intake. These data could inspire new strategies to treat drug addiction, considering circadian timing as a principal factor.

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.

Circadian insights into dopamine mechanisms

Almost every physiological or behavioral process in mammals follows rhythmic patterns, which depend mainly on a master circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). The dopaminergic (DAergic) system in the brain is principally implicated in motor functions, motivation and drug intake. Interestingly, DA-related parameters and behaviors linked to the motivational and arousal states, show daily rhythms that could be regulated by the SCN or by extra-SCN circadian oscillator(s) modulating DAergic systems. Here we examine what is currently understood about the anatomical and functional central multi-oscillatory circadian system, highlighting how the main SCN clock communicates timing information with other brain clocks to regulate the DAergic system and conversely, how DAergic cues may have feedback effects on the SCN. These studies give new insights into the role of the brain circadian system in DA-related neurologic pathologies, such as Parkinson's disease, attention deficit/hyperactive disorder and drug addiction.

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.

Circadian discrimination of reward: evidence for simultaneous yet separable food- and drug-entrained rhythms in the rat

A unique extra-suprachiasmatic nucleus (SCN) oscillator, operating independently of the light-entrainable oscillator, has been hypothesized to generate feeding and drug-related rhythms. To test the validity of this hypothesis, sham-lesioned (Sham) and SCN-lesioned (SCNx) rats were housed in constant dim-red illumination (LL(red)) and received a daily cocaine injection every 24 h for 7 d (Experiment 1). In a second experiment, rats underwent 3-h daily restricted feeding (RF) followed 12 d later by the addition of daily cocaine injections given every 25 h in combination with RF until the two schedules were in antiphase. In both experiments, body temperature and total activity were monitored continuously. Results from Experiment 1 revealed that cocaine, but not saline, injections produced anticipatory increases in temperature and activity in SCNx and Sham rats. Following withdrawal from cocaine, free-running temperature rhythms persisted for 2-10 d in SCNx rats. In Experiment 2, robust anticipatory increases in temperature and activity were associated with RF and cocaine injections; however, the feeding periodicity (23.9 h) predominated over the cocaine periodicity. During drug withdrawal, the authors observed two free-running rhythms of temperature and activity that persisted for >14 d in both Sham and SCNx rats. The periods of the free-running rhythms were similar to the feeding entrainment (period = 23.7 and 24.0 h, respectively) and drug entrainment (period = 25.7 and 26.1 h, respectively). Also during withdrawal, the normally close correlation between activity and temperature was greatly disrupted in Sham and SCNx rats. Taken together, these results do not support the existence of a single oscillator mediating the rewarding properties of both food and cocaine. Rather, they suggest that these two highly rewarding behaviors can be temporally isolated, especially during drug withdrawal. Under stable dual-entrainment conditions, food reward appears to exhibit a slightly greater circadian influence than drug reward. The ability to generate free-running temperature rhythms of different frequencies following combined food and drug exposures could reflect a state of internal desynchrony that may contribute to the addiction process and drug relapse.

Photoperiodic suppression of drug reinstatement

The rewarding influence of drugs of abuse varies with time of day and appears to involve interactions between the circadian and the mesocorticolimbic dopamine systems. The circadian system is also intimately involved in measuring daylength. Thus, the present study examined the impact of changing daylength (photoperiod) on cocaine-seeking behaviors. Male Sprague-Dawley rats were trained and tested on a 12L:12D light:dark schedule for cocaine-induced reinstatement of conditioned place preference (CPP) at three times of day (Zeitgeber time (ZT): 4, 12, and 20) to determine a preference score. Rats were then shifted to either shorter (6L:18D) or longer (18L:6D) photoperiods and then to constant conditions, re-tested for cocaine-induced reinstatement under each different condition, and then returned to their original photoperiod (12L:12D) and tested once more. Rats exhibited a circadian profile of preference score in constant darkness with a peak at 12 h after lights-off. At both ZT4 and ZT20, but not at ZT12, shorter photoperiods profoundly suppressed cocaine reinstatement, which did not recover even after switching back to 12L:12D. In contrast, longer photoperiods did not alter reinstatement. Separate studies showed that the suppression of cocaine reinstatement was not due to repeated testing. In an additional experiment, we examined the photoperiodic regulation of tyrosine hydroxylase (TH) and dopamine transporter (DAT) proteins in drug-naive rats. These results revealed photoperiodic modulation of proteins in the prefrontal cortex and dorsal striatum, but not in the nucleus accumbens or ventral tegmental area. Together, these findings add further support to the circadian genesis of cocaine-seeking behaviors and demonstrate that drug-induced reinstatement is modulated by photoperiod. Furthermore, the results suggest that photoperiod partly contributes to the seasonal expression of certain drug-related behaviors in humans living at different latitudes and thus our findings may have implications for novel targeting of circadian rhythms in the treatment of addiction.