Quantification of alcohol drinking patterns in mice

Home-Cage Sipper Devices Reveal Age and Sex Differences in Ethanol Consumption Patterns

Free-choice paradigms such as two-bottle choice (2BC) are commonly used to characterize ethanol consumption and preference of rodent models used to study alcohol use disorder (AUD). However, these assays are limited by low temporal resolution that misses finer patterns of drinking behavior, including circadian drinking patterns that are known to vary with age and sex and are affected in AUD pathogenesis. Modern, cost-effective tools are becoming widely available that could elucidate these patterns, including open-source, Arduino-based home-cage sipper devices. We hypothesized that adaptation of these home-cage sipper devices would uncover distinct age- and sex-related differences in temporal drinking patterns. To test this hypothesis, we used the sipper devices in a continuous 2BC paradigm using water and ethanol (10%; v/v) for 14 days to measure drinking patterns of male and female adolescent (3-week), young adult (6-week), and mature adult (18-week) C57BL/6J mice. Daily grams of fluid consumption were manually recorded at the beginning of the dark cycle, while home-cage sipper devices continuously recorded the number of sips. Consistent with prior studies, females consumed more ethanol than males, and adolescent mice consumed the most out of any age group. Correlation analyses of manually recorded fluid consumption versus home-cage sipper activity revealed a statistically significant prediction of fluid consumption across all experimental groups. Sipper activity was able to capture subtle circadian differences between experimental groups, as well as distinct individual variation in drinking behavior among animals. Blood ethanol concentrations were significantly correlated with sipper data, suggesting that home-cage sipper devices can accurately determine individual timing of ethanol consumption. Overall, our studies show that augmenting the 2BC drinking paradigm with automated home-cage sipper devices can accurately measure ethanol consumption across sexes and age groups, revealing individual differences and temporal patterns of ethanol drinking behavior. Future studies utilizing these home-cage sipper devices will further dissect circadian patterns for age and sex relevant to the pathogenesis of AUD, as well as underlying molecular mechanisms for patterns in ethanol consumption.

Highlights

Female mice consume more ethanol than males in a continuous access paradigmAdolescent male and female mice consume more ethanol than young or mature adult miceAutomated home-cage sipper devices accurately measure ethanol consumptionDevices reveal sex- and age-dependent differences in circadian drinking patternsDevices reveal distinct individual variation in circadian drinking patterns.

Development of eight wireless automated cages system with two lick-o-meters each for rodents

Drinking behavior has been used in basic research to study metabolism, motivation, decision-making and different aspects of health problems, such as anhedonia and alcohol use disorders. In the majority of studies, liquid intake is measured by weighing the bottles before and after the experiment. This method does not tell much about the drinking microstructure, e.g., licking bouts and periods of preference for each liquid, which could be valuable to understand drinking behavior. To improve the data acquisition of drinking microstructure, companies have developed lick-o-meters devices that acquire timestamps when animals approach or drink from a specific sipper. Nevertheless, commercially available devices have elevated costs. Here, we present a low-cost alternative for a lick-o-meter system that allows wireless data acquisition of licking from eight cages with two sippers each. We run a three-phase validation protocol to ensure 1) proper choice of the sensor to detect licks; 2) adaptation of the device to a wireless transmission and realistic in silico tests; and 3) in vivo tests to correlate the amount of licks measured by the prototype and the bottle weight. The capacitive sensor presented appropriate recall and precision for our device. After adaptation to wireless transmission, the in silico validation demonstrated low reading and transmission errors for the device even when tested in extreme simultaneous licking conditions. Finally, a positive correlation between volume consumption and lick's count in the in vivo test was observed, showing that the prototype can be used for in vivo studies interested in rodent drinking microstructure.Significant StatementThis study presents an innovative and low-cost solution for drinking behavioral studies: a lick-o-meter system based on an open-source hardware platform with a user-friendly interface software, capable of simultaneously receiving data from eight automated cages with two drinking bottles each. The lick-o-meter brings an accessible device to acquire high-quality and detailed data. This device also has the possibility to be adaptable to new types of sensors or other neuroscience tools capable of measuring brain activity simultaneously to the behavior.

Probing the Skin–Brain Axis: New Vistas Using Mouse Models

Inflammatory diseases of the skin, including atopic dermatitis and psoriasis, have gained increasing attention with rising incidences in developed countries over the past decades. While bodily properties, such as immunological responses of the skin, have been described in some detail, interactions with the brain via different routes are less well studied. The suggested routes of the skin–brain axis comprise the immune system, HPA axis, and the peripheral and central nervous system, including microglia responses and structural changes. They provide starting points to investigate the molecular mechanisms of neuropsychiatric comorbidities in AD and psoriasis. To this end, mouse models exist for AD and psoriasis that could be tested for relevant behavioral entities. In this review, we provide an overview of the current mouse models and assays. By combining an extensive behavioral characterization and state-of-the-art genetic interventions with the investigation of underlying molecular pathways, insights into the mechanisms of the skin–brain axis in inflammatory cutaneous diseases are examined, which will spark further research in humans and drive the development of novel therapeutic strategies.

Circadian gene × environment perturbations influence alcohol drinking in Cryptochrome-deficient mice

Alcohol use disorder (AUD) is a widespread addiction disorder with severe consequences for health. AUD patients often suffer from sleep disturbances and irregular daily patterns. Conversely, disruptions of circadian rhythms are considered a risk factor for AUD and alcohol relapses. In this study, we investigated the extent to which circadian genetic and environmental disruptions and their interaction alter alcohol drinking behaviour in mice. As a model of genetic circadian disruption, we used Cryptochrome1/2-deficient (Cry1/2^-/- ) mice with strongly suppressed circadian rhythms and found that they exhibit significantly reduced preference for alcohol but increased incentive motivation to obtain it. Similarly, we found that low circadian SCN amplitude correlates with reduced alcohol preference in WT mice. Moreover, we show that the low alcohol preference of Cry1/2^-/- mice concurs with high corticosterone and low levels of the orexin precursor prepro-orexin and that WT and Cry1/2^-/- mice respond differently to alcohol withdrawal. As a model of environmentally induced disruption of circadian rhythms, we exposed mice to a "shift work" light/dark regimen, which also leads to a reduction in their alcohol preference. Interestingly, this effect is even more pronounced when genetic and environmental circadian perturbations interact in Cry1/2^-/- mice under "shift work" conditions. In conclusion, our study demonstrates that in mice, disturbances in circadian rhythms have pronounced effects on alcohol consumption as well as on physiological factors and other behaviours associated with AUD and that the interaction between circadian genetic and environmental disturbances further alters alcohol consumption behaviour.

NMDA Receptors in Accumbal D1 Neurons Influence Chronic Sugar Consumption and Relapse

Glutamatergic input via NMDA and AMPA receptors within the mesolimbic dopamine (DA) pathway plays a critical role in the development of addictive behavior and relapse toward drugs of abuse. Although well-established for drugs of abuse, it is not clear whether glutamate receptors within the mesolimbic system are involved in mediating chronic consumption and relapse following abstinence from a non-drug reward. Here, we evaluated the contribution of mesolimbic glutamate receptors in mediating chronic sugar consumption and the sugar-deprivation effect (SDE), which is used as a measure of relapse-like behavior following abstinence. We studied four inducible mutant mouse lines lacking the GluA1 or GluN1 subunit in either DA transporter (DAT) or D1R-expressing neurons in an automated monitoring system for free-choice sugar drinking in the home cage. Mice lacking either GluA1 or GluN1 in D1R-expressing neurons (GluA1^D1CreERT2 or GluN1^D1CreERT2mice) have altered sugar consumption in both sexes, whereas GluA1^DATCreERT2 and GluN1^DATCreERT2do not differ from their respective littermate controls. In terms of relapse-like behavior, female GluN1^D1CreERT2mice show a more pronounced SDE. Given that glutamate receptors within the mesolimbic system play a critical role in mediating relapse behavior of alcohol and other drugs of abuse, it is surprising that these receptors do not mediate the SDE, or in the case of female GluN1^D1CreERT2 mice, show an opposing effect. We conclude that a relapse-like phenotype of sugar consumption differs from that of drugs of abuse on the molecular level, at least with respect to the contribution of mesolimbic glutamate receptors.

Female mice are more prone to develop an addictive-like phenotype for sugar consumption

The concept of “sugar addiction” is gaining increasing attention in both the lay media and scientific literature. However, the concept of sugar addiction is controversial and only a few studies to date have attempted to determine the “addictive” properties of sugar using rigorous scientific criteria. Here we set out to systematically test the addictive properties of sugar in male and female mice using established paradigms and models from the drug addiction field. Male and female C57BL/6N (8–10 weeks old) were evaluated in 4 experimental procedures to study the addictive properties of sugar: (i) a drinking in the dark (DID) procedure to model sugar binging; (ii) a long-term free choice home cage drinking procedure measuring the sugar deprivation effect (SDE) following an abstinence phase; (iii) a long-term operant sugar self-administration with persistence, motivation and compulsivity measures and (iv) intracranial self-stimulation (ICSS). Female mice were more vulnerable to the addictive properties of sugar than male mice, showing higher binge and long-term, excessive drinking, a more pronounced relapse-like drinking following deprivation, and higher persistence and motivation for sugar. No sex differences were seen in a compulsivity test or reward sensitivity measured using ICSS following extended sugar consumption. This study demonstrates the occurrence of an addictive-like phenotype for sugar in male and female mice, similar to drugs of abuse, and suggests sex-dependent differences in the development of sugar addiction.

Reduced sensitivity to ethanol and excessive drinking in a mouse model of neuropathic pain

The co-occurrence of chronic pain and alcohol use disorders (AUDs) involves complex interactions between genetic and neurophysiological aspects, and the research has reported mixed findings when they both co-occur. There is also an indication of a gender-dependent effect; males are more likely to use alcohol to cope with chronic pain problems than females. Recently, a new conceptualization has emerged, proposing that the negative affective component of pain drives and maintains alcohol-related behaviors. We studied in a longitudinal fashion alterations in alcohol drinking patterns and pain thresholds in a mouse model of chronic neuropathic pain in a sex-dependent manner. Following partial denervation (spared nerve injury [SNI]), stimulus-evoked pain responses were measured before chronic alcohol consumption, during drinking, during a deprivation phase, and following an episode of excessive drinking. During the course of alcohol drinking, we observed pronounced sex differences in pain thresholds. Male mice showed a strong increase in pain thresholds, suggesting an analgesic effect induced by alcohol over time, an effect that was not observed in female mice. SNI mice did not differ from sham-operated controls in baseline alcohol consumption. However, following a deprivation phase and the reintroduction of ethanol, male SNI mice but not female mice showed more pronounced excessive drinking than controls. Finally, we observed decreased central ethanol sensitivity in male SNI mice but not in females. Together with our finding, that ethanol is able to decrease a pain-induced negative affective memory we come to following conclusion. We propose that a lower sensitivity to the intoxicating effects of alcohol together with the ability of alcohol to reduce the negative affective component of pain may explain the higher co-occurrence of AUD in male chronic pain patients.

Metabotropic Glutamate Receptor Subtype 5 in Alcohol-Induced Negative Affect

Allosteric modulators of metabotropic glutamate 5 receptors (mGlu5 receptors) have been identified as a promising treatment to independently alleviate both negative affective states and ethanol-seeking and intake. However, these conditions are often comorbid and might precipitate one another. Acute and protracted ethanol withdrawal can lead to negative affective states. In turn, these states are primary drivers of alcohol relapse, particularly among women. The current review synthesizes preclinical studies that have observed the role of mGlu5 receptor modulation in negative affective states following ethanol exposure. The primary behavioral assays discussed are ethanol-seeking and intake, development and extinction of ethanol-associated cues and contexts, behavioral despair, and anxiety-like activity. The work done to-date supports mGlu5 receptor modulation as a promising target for mediating negative affective states to reduce ethanol intake or prevent relapse. Limitations in interpreting these data include the lack of models that use alcohol-dependent animals, limited use of adolescent and female subjects, and a lack of comprehensive evaluations of negative affective-like behavior.

Nonphosphorylatable Src Ser75 Mutation Increases Ethanol Preference and Consumption in Mice

Src is highly expressed in CNS neurons and contributes not only to developmental proliferation and differentiation but also to high-order brain functions, such as those contributing to alcohol consumption. Src knock-out mice exhibit no CNS abnormalities, presumably due to compensation by other Src family kinases (SFKs), but have a shortened lifespan and osteopetrosis-associated defects, impeding investigations of the role of Src on behavior in adult mice. However, the Unique domain of Src differs from those in other SFKs and is phosphorylated by cyclin-dependent kinase 1 (Cdk1) and Cdk5 at Ser75, which influences its postmitotic function in neurons. Therefore, ethanol consumption in mice harboring nonphosphorylatable (Ser75Ala) or phosphomimetic (Ser75Asp) Src mutants was investigated. Mice harboring the Ser75Ala Src mutant, but not the Ser75Asp mutant, had a higher preference for and consumption of solutions containing 5% and 10% ethanol than wild-type mice. However, plasma ethanol concentrations and sensitivities to the sedative effects of ethanol were not different among the groups. In mice harboring the Ser75Ala Src mutant, the activity of Rho-associated kinase (ROCK) in the striatum was significantly lower and Akt Ser473 phosphorylation was significantly higher than in wild-type mice. These results suggest that Src regulates voluntary ethanol drinking in a manner that depends on Ser75 phosphorylation.

Animal models of addiction

In recent years, animal models in psychiatric research have been criticized for their limited translational value to the clinical situation. Failures in clinical trials have thus often been attributed to the lack of predictive power of preclinical animal models. Here, I argue that animal models of voluntary drug intake—under nonoperant and operant conditions—and addiction models based on the Diagnostic and Statistical Manual of Mental Disorders are crucial and informative tools for the identification of pathological mechanisms, target identification, and drug development. These models provide excellent face validity, and it is assumed that the neurochemical and neuroanatomical substrates involved in drug-intake behavior are similar in laboratory rodents and humans. Consequently, animal models of drug consumption and addiction provide predictive validity. This predictive power is best illustrated in alcohol research, in which three approved medications—acamprosate, naltrexone, and nalmefene—were developed by means of animal models and then successfully translated into the clinical situation.

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.

Enhanced motivation to alcohol in transgenic mice expressing human α-synuclein

α-Synuclein (αSYN) is the neuropathological hallmark protein of Parkinson's disease (PD) and related neurodegenerative disorders. Moreover, the gene encoding αSYN (SNCA) is a major genetic contributor to PD. Interestingly, independent genome-wide association studies also identified SNCA as the most important candidate gene for alcoholism. Furthermore, single-nucleotide-polymorphisms have been associated with alcohol-craving behavior and alcohol-craving patients showed augmented αSYN expression in blood. To investigate the effect of αSYN on the addictive properties of chronic alcohol use, we examined consumption, motivation, and seeking responses induced by environmental stimuli and relapse behavior in transgenic mice expressing the human mutant [A30P]αSYN throughout the brain. The primary reinforcing effects of alcohol under operant self-administration conditions were increased, while consumption and the alcohol deprivation effect were not altered in the transgenic mice. The same mice were subjected to immunohistochemical measurements of immediate-early gene inductions in brain regions involved in addiction-related behaviors. Acute ethanol injection enhanced immunostaining for the phosphorylated form of cAMP response element binding protein in both amygdala and nucleus accumbens of αSYN transgenic mice, while in wild-type mice no effect was visible. However, at the same time, levels of cFos remain unchanged in both genotypes. These results provide experimental confirmation of SNCA as a candidate gene for alcoholism in addition to its known link to PD.

The glucagon-like peptide 1 receptor agonist Exendin-4 decreases relapse-like drinking in socially housed mice

Glucagon-like peptide-1 (GLP-1) is a gut peptide that regulates food intake and glucose metabolism. GLP-1 is also produced and released in the brain, and GLP-1 receptors are expressed in brain regions important for alcohol and drug reward, and for the development of addiction. GLP-1 receptor agonists can decrease alcohol intake acutely in rodents. However, alcohol use disorder is a chronic condition that requires treatments to be effective in promoting abstinence from excessive alcohol consumption over time. Here, we assessed the effect of daily treatment with the GLP-1 receptor agonist Exendin-4 in an assay of relapse-like drinking in socially housed mice. Male C57BL/6NTac mice were allowed continuous access to alcohol without tastant in the home cage for 37days. Then, alcohol bottles were removed and Exendin-4 (1.5μg/kg/day) or saline was administered subcutaneously for 8days during alcohol deprivation. Treatment continued for 8 additional days after reintroducing access to alcohol. A high-precision automated fluid consumption system was used to monitor intake of alcohol and water, drinking kinetics, and locomotor activity. Exendin-4 prevented the deprivation-induced increase in alcohol intake observed in control mice, without significantly affecting total fluid intake, body weight, or locomotor activity. The reduced alcohol intake was caused by a protracted latency to the first drink of alcohol and a reduced number of drinking bouts, while bout size and duration were not affected. The effect was maintained undiminished throughout the treatment period. These findings support the possible use of GLP-1 receptor agonists in the treatment of alcohol use disorder.

Metabolic shift of the kynurenine pathway impairs alcohol and cocaine seeking and relapse

RATIONALE

The glutamatergic system plays a key role in the maintenance of drug use and development of drug-related conditioned behaviours. In particular, hyper-glutamatergic activity and N-methyl-D-aspartate receptor (NMDAR) activation may drive drug craving and relapse. Inhibition of kynurenine-3-monooxygenase (KMO) shifts the metabolic kynurenine pathway towards production of kynurenic acid, which leads to a reduction of glutamatergic/NMDAR activity via different mechanisms.

OBJECTIVES

In this study, we investigated whether drug-seeking and relapse behaviour could be modified by the metabolic shift of endogenous kynurenine pathway.

METHODS

An inhibitor of kynurenine-3-monooxygenase (KMO) Ro61-8048 (4 and 40 mg/kg) and its prodrug JM6 (100 and 200 mg/kg) were tested in two behavioural rat models for drug seeking and relapse-the alcohol deprivation effect (ADE) model in long-term alcohol-drinking rats and the model of cue-induced reinstatement of alcohol- and cocaine-seeking behaviour.

RESULTS

Our results show that relapse-like alcohol drinking during the ADE was abolished by repeated intraperitoneal administration of Ro61-8048 and significantly reduced by its oral prodrug JM6. Cue-induced reinstatement of both alcohol- and cocaine-seeking behaviour was also abolished by administration of Ro61-8048.

CONCLUSIONS

Pharmacological enhancement of endogenous kynurenic acid levels provides a novel treatment strategy to interfere with glutamatergic/NMDAR activity as well as with craving and relapse in alcohol-dependent patients and drug addicts.

Paradoxical augmented relapse in alcohol-dependent rats during deep-brain stimulation in the nucleus accumbens

Case reports indicate that deep-brain stimulation in the nucleus accumbens may be beneficial to alcohol-dependent patients. The lack of clinical trials and our limited knowledge of deep-brain stimulation call for translational experiments to validate these reports. To mimic the human situation, we used a chronic-continuous brain-stimulation paradigm targeting the nucleus accumbens and other brain sites in alcohol-dependent rats. To determine the network effects of deep-brain stimulation in alcohol-dependent rats, we combined electrical stimulation of the nucleus accumbens with functional magnetic resonance imaging (fMRI), and studied neurotransmitter levels in nucleus accumbens-stimulated versus sham-stimulated rats. Surprisingly, we report here that electrical stimulation of the nucleus accumbens led to augmented relapse behavior in alcohol-dependent rats. Our associated fMRI data revealed some activated areas, including the medial prefrontal cortex and caudate putamen. However, when we applied stimulation to these areas, relapse behavior was not affected, confirming that the nucleus accumbens is critical for generating this paradoxical effect. Neurochemical analysis of the major activated brain sites of the network revealed that the effect of stimulation may depend on accumbal dopamine levels. This was supported by the finding that brain-stimulation-treated rats exhibited augmented alcohol-induced dopamine release compared with sham-stimulated animals. Our data suggest that deep-brain stimulation in the nucleus accumbens enhances alcohol-liking probably via augmented dopamine release and can thereby promote relapse.

Glutamate Receptors within the Mesolimbic Dopamine System Mediate Alcohol Relapse Behavior

Glutamatergic input within the mesolimbic dopamine (DA) pathway plays a critical role in the development of addictive behavior. Although this is well established for some drugs of abuse, it is not known whether glutamate receptors within the mesolimbic system are involved in mediating the addictive properties of chronic alcohol use. Here we evaluated the contribution of mesolimbic NMDARs and AMPARs in mediating alcohol-seeking responses induced by environmental stimuli and relapse behavior using four inducible mutant mouse lines lacking the glutamate receptor genes Grin1 or Gria1 in either DA transporter (DAT) or D1R-expressing neurons. We first demonstrate the lack of GluN1 or GluA1 in either DAT- or D1R-expressing neurons in our mutant mouse lines by colocalization studies. We then show that GluN1 and GluA1 receptor subunits within these neuronal subpopulations mediate the alcohol deprivation effect, while having no impact on context- plus cue-induced reinstatement of alcohol-seeking behavior. We further validated these results pharmacologically by demonstrating similar reductions in the alcohol deprivation effect after infusion of the NMDAR antagonist memantine into the nucleus accumbens and ventral tegmental area of control mice, and a rescue of the mutant phenotype via pharmacological potentiation of AMPAR activity using aniracetam. In conclusion, dopamine neurons as well as D1R-expressing medium spiny neurons and their glutamatergic inputs via NMDARs and AMPARs act in concert to influence relapse responses. These results provide a neuroanatomical and molecular substrate for relapse behavior and emphasize the importance of glutamatergic drugs in modulating relapse behavior.

SIGNIFICANCE STATEMENT

Here we provide genetic and pharmacological evidence that glutamate receptors within the mesolimbic dopamine system play an essential role in alcohol relapse. Using various inducible and site-specific transgenic mouse models and pharmacological validation experiments, we show that critical subunits of NMDARs and AMPARs expressed either in dopamine neurons or in dopamine receptor D1-containing neurons play an important role in the alcohol deprivation effect (the increase in alcohol intake after a period of abstinence) while having no impact on context- plus cue-induced reinstatement of alcohol-seeking responses. Medications targeting glutamatergic neurotransmission by selective inactivation of these glutamate receptors might have therapeutic efficacy.