The mammalian circadian clock exhibits acute tolerance to ethanol

The Mammalian Circadian Clock Exhibits Chronic Ethanol Tolerance and Withdrawal-Induced Glutamate Hypersensitivity, Accompanied by Changes in Glutamate and TrkB Receptor Proteins

BACKGROUND

Alcohol tolerance and withdrawal-induced effects are criteria for alcohol use disorders listed by the DSM-V. Although tolerance and withdrawal have been studied over many decades, there is still uncertainty regarding mechanistic distinctions that characterize these different forms of ethanol (EtOH)-induced plasticity. Previously, we demonstrated that the suprachiasmatic nucleus (SCN) circadian clock develops both acute and rapid tolerance to EtOH inhibition of glutamate-induced circadian phase shifts. Here, we demonstrate that chronic EtOH tolerance and withdrawal-induced glutamate hypersensitivity occur in vitro and that rapid tolerance, chronic tolerance, and glutamate hypersensitivity have distinct cellular changes.

METHODS

We use single-unit extracellular electrophysiological recordings to determine whether chronic tolerance to EtOH inhibition of glutamatergic phase shifts and withdrawal-induced glutamate hypersensitivity develop in the SCN. We use Western blotting to compare phosphorylation state and total expression of N-methyl-D-aspartate (NMDA) receptor subunits and associated proteins in the SCN after mice were exposed to varying EtOH consumption paradigms.

RESULTS

Chronic tolerance developed after a minimum of 8 days of 4 h/d EtOH access, as indicated by a decreased sensitivity to EtOH inhibition of glutamate-induced phase shifts. We also observed an increased sensitivity to glutamate-induced phase shifts in SCN tissue following withdrawal. We demonstrated an increase in the ratio of NR2B:NR2A NMDA receptor subunit expression after 21 days, but not after 10 days of EtOH drinking. This increase persisted during EtOH withdrawal, along with an increase in NR2B Y1472 phosphorylation, mature brain-derived neurotrophic factor, and phosphorylated TrkB.

CONCLUSIONS

These results demonstrate that multiple tolerance forms and withdrawal-induced glutamate hypersensitivity occur in the SCN and that these different forms of EtOH-induced plasticity are accompanied by distinct changes in cellular physiology. Importantly, this study further demonstrates the power of using the SCN as a model system to investigate EtOH-induced plasticity.

Assessing ethanol's actions in the suprachiasmatic circadian clock using in vivo and in vitro approaches

Research over the past decade has demonstrated substantial interactions between the circadian system and the processes through which alcohol affects behavior and physiology. Here we summarize the results of our collaborative efforts focused on this intersection. Using a combination of in vivo and in vitro approaches, we have shown that ethanol affects many aspects of the mammalian circadian system, both acutely as well as after chronic administration. Conversely, we have shown circadian influences on ethanol consumption. Importantly, we are beginning to delve into the cellular mechanisms associated with these effects. We are also starting to form a picture of the neuroanatomical bases for many of these actions. Finally, we put our current findings into perspective by suggesting new avenues of inquiry for our future efforts.

Circadian clock genes: effects on dopamine, reward and addiction

Addiction is a widespread public health issue with social and economic ramifications. Substance abuse disorders are often accompanied by disruptions in circadian rhythms including sleep/wake cycles, which can exacerbate symptoms of addiction and dependence. Additionally, genetic disturbance of circadian molecular mechanisms can predispose some individuals to substance abuse disorders. In this review, we will discuss how circadian genes can regulate midbrain dopaminergic activity and subsequently, drug intake and reward. We will also suggest future directions for research on circadian genes and drugs of abuse.

Circadian activity rhythms and voluntary ethanol intake in male and female ethanol-preferring rats: effects of long-term ethanol access

Chronic alcohol (ethanol) intake alters fundamental properties of the circadian clock. While previous studies have reported significant alterations in free-running circadian period during chronic ethanol access, these effects are typically subtle and appear to require high levels of intake. In the present study we examined the effects of long-term voluntary ethanol intake on ethanol consumption and free-running circadian period in male and female, selectively bred ethanol-preferring P and HAD2 rats. In light of previous reports that intermittent access can result in escalated ethanol intake, an initial 2-week water-only baseline was followed by either continuous or intermittent ethanol access (i.e., alternating 15-day epochs of ethanol access and ethanol deprivation) in separate groups of rats. Thus, animals were exposed to either 135 days of continuous ethanol access or to five 15-day access periods alternating with four 15-day periods of ethanol deprivation. Animals were maintained individually in running-wheel cages under continuous darkness throughout the experiment to allow monitoring of free-running activity and drinking rhythms, and 10% (v/v) ethanol and plain water were available continuously via separate drinking tubes during ethanol access. While there were no initial sex differences in ethanol drinking, ethanol preference increased progressively in male P and HAD2 rats under both continuous and intermittent-access conditions, and eventually exceeded that seen in females. Free-running period shortened during the initial ethanol-access epoch in all groups, but the persistence of this effect showed complex dependence on sex, breeding line, and ethanol-access schedule. Finally, while females of both breeding lines displayed higher levels of locomotor activity than males, there was little evidence for modulation of activity level by ethanol access. These results are consistent with previous findings that chronic ethanol intake alters free-running circadian period, and show further that the development of chronobiological tolerance to ethanol may vary by sex and genotype.

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.

The mammalian circadian clock in the suprachiasmatic nucleus exhibits rapid tolerance to ethanol in vivo and in vitro

BACKGROUND

Ethanol (EtOH) triggers cellular adaptations that induce tolerance in many brain areas, including the suprachiasmatic nucleus (SCN), the site of the master circadian clock. EtOH inhibits light-induced phase shifts in the SCN in vivo and glutamate-induced phase shifts in vitro. The in vitro phase shifts develop acute tolerance to EtOH, occurring within minutes of initial exposure, while the in vivo phase shifts exhibit no evidence of chronic tolerance. An intermediate form, rapid tolerance, is not well studied but may predict subsequent chronic tolerance. Here, we investigated rapid tolerance in the SCN clock.

METHODS

Adult C57BL/6 mice were provided 15% EtOH or water for one 12-hour lights-off period. For in vitro experiments, SCN-containing brain slices were prepared in the morning and treated for 10 minutes with glutamate +/- EtOH the following night. Single-cell neuronal firing rates were recorded extracellularly during the subsequent day to determine SCN clock phase. For in vivo experiments, mice receiving EtOH 24 hours previously were exposed to a 30-minute light pulse immediately preceded by intraperitoneal saline or 2 g/kg EtOH injection. Mice were then placed in constant darkness and their phase-shifting responses measured.

RESULTS

In vitro, the SCN clock from EtOH-exposed mice exhibited rapid tolerance, with a 10-fold increase in EtOH needed to inhibit glutamate-induced phase shifts. Co-application of brain-derived neurotrophic factor prevented EtOH inhibition, consistent with experiments using EtOH-naïve mice. Rapid tolerance lasts 48 to 96 hours, depending on whether assessing in vitro phase advances or phase delays. Similarly, in vivo, prior EtOH consumption prevented EtOH's acute blockade of photic phase delays. Finally, immunoblot experiments showed no changes in SCN glutamate receptor subunit (NR2B) expression or phosphorylation in response to rapid tolerance induction.

CONCLUSIONS

The SCN circadian clock develops rapid tolerance to EtOH as assessed both in vivo and in vitro, and the tolerance lasts for several days. These data demonstrate the utility of the circadian system as a model for investigating cellular mechanisms through which EtOH acts in the brain.

The role of clock in ethanol-related behaviors

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

Chronic ethanol consumption disrupts the core molecular clock and diurnal rhythms of metabolic genes in the liver without affecting the suprachiasmatic nucleus

Chronic ethanol consumption disrupts several metabolic pathways including β-oxidation and lipid biosynthesis, facilitating the development of alcoholic fatty liver disease. Many of these same metabolic pathways are directly regulated by cell autonomous circadian clocks, and recent studies suggest that disruption of daily rhythms in metabolism contributes to multiple common cardiometabolic diseases (including non-alcoholic fatty liver disease). However, it is not known whether ethanol disrupts the core molecular clock in the liver, nor whether this, in turn, alters rhythms in lipid metabolism. Herein, we tested the hypothesis that chronic ethanol consumption disrupts the molecular circadian clock in the liver and potentially changes the diurnal expression patterns of lipid metabolism genes. Consistent with previous studies, male C57BL/6J mice fed an ethanol-containing diet exhibited higher levels of liver triglycerides compared to control mice, indicating hepatic steatosis. Further, the diurnal oscillations of core clock genes (Bmal1, Clock, Cry1, Cry2, Per1, and Per2) and clock-controlled genes (Dbp, Hlf, Nocturnin, Npas2, Rev-erbα, and Tef) were altered in livers from ethanol-fed mice. In contrast, ethanol had only minor effects on the expression of core clock genes in the suprachiasmatic nucleus (SCN). These results were confirmed in Per2^Luciferase knock-in mice, in which ethanol induced a phase advance in PER2::LUC bioluminescence oscillations in liver, but not SCN. Further, there was greater variability in the phase of PER2::LUC oscillations in livers from ethanol-fed mice. Ethanol consumption also affected the diurnal oscillations of metabolic genes, including Adh1, Cpt1a, Cyp2e1, Pck1, Pdk4, Ppargc1a, Ppargc1b and Srebp1c, in the livers of C57BL/6J mice. In summary, chronic ethanol consumption alters the function of the circadian clock in liver. Importantly, these results suggest that chronic ethanol consumption, at levels sufficient to cause steatosis, disrupts the core hepatic clock as well as the diurnal rhythms of key lipid metabolism genes.

Cocaine modulates pathways for photic and nonphotic entrainment of the mammalian SCN circadian clock

Cocaine abuse is highly disruptive to circadian physiological and behavioral rhythms. The present study was undertaken to determine whether such effects are manifest through actions on critical photic and nonphotic regulatory pathways in the master circadian clock of the mouse suprachiasmatic nucleus (SCN). Impairment of SCN photic signaling by systemic (intraperitoneal) cocaine injection was evidenced by strong (60%) attenuation of light-induced phase-delay shifts of circadian locomotor activity during the early night. A nonphotic action of cocaine was apparent from its induction of 1-h circadian phase-advance shifts at midday. The serotonin receptor antagonist, metergoline, blocked shifting by 80%, implicating a serotonergic mechanism. Reverse microdialysis perfusion of the SCN with cocaine at midday induced 3.7 h phase-advance shifts. Control perfusions with lidocaine and artificial cerebrospinal fluid had little shifting effect. In complementary in vitro experiments, photic-like phase-delay shifts of the SCN circadian neuronal activity rhythm induced by glutamate application to the SCN were completely blocked by cocaine. Cocaine treatment of SCN slices alone at subjective midday, but not the subjective night, induced 3-h phase-advance shifts. Lidocaine had no shifting effect. Cocaine-induced phase shifts were completely blocked by metergoline, but not by the dopamine receptor antagonist, fluphenazine. Finally, pretreatment of SCN slices for 2 h with a low concentration of serotonin agonist (to block subsequent serotonergic phase resetting) abolished cocaine-induced phase shifts at subjective midday. These results reveal multiple effects of cocaine on adult circadian clock regulation that are registered within the SCN and involve enhanced serotonergic transmission.

Effects of withdrawal from chronic intermittent ethanol vapor on the level and circadian periodicity of running-wheel activity in C57BL/6J and C3H/HeJ mice

BACKGROUND

Alcohol withdrawal is associated with behavioral and chronobiological disturbances that may persist during protracted abstinence. We previously reported that C57BL/6J (B6) mice show marked but temporary reductions in running-wheel activity, and normal free-running circadian rhythms, following a 4-day chronic intermittent ethanol (CIE) vapor exposure (16 hours of ethanol vapor exposure alternating with 8 hours of withdrawal). In the present experiments, we extend these observations in 2 ways: (i) by examining post-CIE locomotor activity in C3H/HeJ (C3H) mice, an inbred strain characterized by high sensitivity to ethanol withdrawal, and (ii) by directly comparing the responses of B6 and C3H mice to a longer-duration CIE protocol.

METHODS

In Experiment 1, C3H mice were exposed to the same 4-day CIE protocol used in our previous study with B6 mice (referred to here as the 1-cycle CIE protocol). In Experiment 2, C3H and B6 mice were exposed to 3 successive 4-day CIE cycles, each separated by 2 days of withdrawal (the 3-cycle CIE protocol). Running-wheel activity was monitored prior to and following CIE, and post-CIE activity was recorded in constant darkness to allow assessment of free-running circadian period and phase.

RESULTS

C3H mice displayed pronounced reductions in running-wheel activity that persisted for the duration of the recording period (up to 30 days) following both 1-cycle (Experiment 1) and 3-cycle (Experiment 2) CIE protocols. In contrast, B6 mice showed reductions in locomotor activity that persisted for about 1 week following the 3-cycle CIE protocol, similar to the results of our previous study using a 1-cycle protocol in this strain. Additionally, C3H mice showed significant shortening of free-running period following the 3-cycle, but not the 1-cycle, CIE protocol, while B6 mice showed normal free-running rhythms.

CONCLUSIONS

These results reveal genetic differences in the persistence of ethanol withdrawal-induced hypo-locomotion. In addition, chronobiological alterations during extended abstinence may depend on both genetic susceptibility and an extended prior withdrawal history. The present data establish a novel experimental model for long-term behavioral and circadian disruptions associated with ethanol withdrawal.

Acute ethanol disrupts photic and serotonergic circadian clock phase-resetting in the mouse

BACKGROUND

Alcohol dependence is associated with impaired circadian rhythms and sleep. Ethanol administration disrupts circadian clock phase-resetting, suggesting a mode for the disruptive effect of alcohol dependence on the circadian timing system. In this study, we extend previous work in C57BL/6J mice to: (i) characterize the suprachiasmatic nucleus (SCN) pharmacokinetics of acute systemic ethanol administration, (ii) explore the effects of acute ethanol on photic and nonphotic phase-resetting, and (iii) determine if the SCN is a direct target for photic effects.

METHODS

First, microdialysis was used to characterize the pharmacokinetics of acute intraperitoneal (i.p.) injections of 3 doses of ethanol (0.5, 1.0, and 2.0 g/kg) in the mouse SCN circadian clock. Second, the effects of acute i.p. ethanol administration on photic phase delays and serotonergic ([+]8-OH-DPAT-induced) phase advances of the circadian activity rhythm were assessed. Third, the effects of reverse-microdialysis ethanol perfusion of the SCN on photic phase-resetting were characterized.

RESULTS

Peak ethanol levels from the 3 doses of ethanol in the SCN occurred within 20 to 40 minutes postinjection with half-lives for clearance ranging from 0.6 to 1.8 hours. Systemic ethanol treatment dose-dependently attenuated photic and serotonergic phase-resetting. This treatment also did not affect basal SCN neuronal activity as assessed by Fos expression. Intra-SCN perfusion with ethanol markedly reduced photic phase delays.

CONCLUSIONS

These results confirm that acute ethanol attenuates photic phase-delay shifts and serotonergic phase-advance shifts in the mouse. This dual effect could disrupt photic and nonphotic entrainment mechanisms governing circadian clock timing. It is also significant that the SCN clock is a direct target for disruptive effects of ethanol on photic shifting. Such actions by ethanol could underlie the disruptive effects of alcohol abuse on behavioral, physiological, and endocrine rhythms associated with alcoholism.

Alcohol usage and abrupt cessation modulate diurnal activity

Alcohol has many effects throughout the body. The effect on circadian rhythms and the correlation of these effects to withdrawal effects of alcohol present interesting findings. By measuring 3 planes of activity of female Sprague-Dawley rats during alcohol usage and continuing study through the first 2 days following withdrawal of alcohol allow for the observation of a drastic modulation of the circadian pattern of activity.

Chronic ethanol disrupts circadian photic entrainment and daily locomotor activity in the mouse

BACKGROUND

Chronic ethanol abuse is associated with disrupted circadian rhythms and sleep. Ethanol administration impairs circadian clock phase-resetting, suggesting a mode for the disruptive effect of alcohol abuse on circadian timing. Here, we extend previous studies to explore the effects of chronic forced ethanol on photic phase-resetting, photic entrainment, and daily locomotor activity patterns in C57BL/6J mice.

METHODS

First, microdialysis was used to characterize the circadian patterns of ethanol uptake in the suprachiasmatic (SCN) circadian clock and correlate this with systemic ethanol levels and episodic drinking of 10 or 15% ethanol. Second, the effects of chronic forced ethanol drinking and withdrawal on photic phase-delays of the circadian activity rhythm were assessed. Third, the effects of chronic ethanol drinking on entrainment to a weak photic zeitgeber (1 minute of 25 lux intensity light per day) were assessed. This method was used to minimize any masking actions of light that could mask ethanol effects on clock entrainment.

RESULTS

Peak ethanol levels in the SCN and periphery occurred during the dark phase and coincided with the time when light normally induces phase-delays in mice. These delays were dose-dependently inhibited by chronic ethanol and its withdrawal. Chronic ethanol did not impede re-entrainment to a shifted light cycle but affected entrainment under the weak photic zeitgeber and disrupted the daily pattern of locomotor activity.

CONCLUSIONS

These results confirm that chronic ethanol consumption and withdrawal markedly impair circadian clock photic phase-resetting. Ethanol also disturbs the temporal structure of nighttime locomotor activity and photic entrainment. Collectively, these results suggest a direct action of ethanol on the SCN clock.