Presented here is a protocol for the 2BC/CIE model of alcohol dependence in mice to study alcohol use disorder.
Alcohol use disorder (AUD) is a chronic alcohol-related disorder that typically presents as uncontrolled drinking and preoccupation with alcohol. A key component of AUD research is using translationally relevant preclinical models. Over the past several decades, a variety of animal models have been used to study AUD. One prominent model of AUD is the chronic intermittent ethanol vapor exposure (CIE) model, which is a well-established approach for inducing alcohol dependence in rodents through repeated cycles of ethanol exposure via inhalation. To model AUD in mice, the CIE exposure is paired with a voluntary two-bottle choice (2BC) of alcohol drinking and water to measure the escalation of alcohol drinking. The 2BC/CIE procedure involves alternating weeks of 2BC drinking and CIE, which repeat until the escalation of alcohol drinking is achieved. In the present study, we outline the procedures for performing 2BC/CIE, including the daily use of the CIE vapor chamber, and provide an example of escalated alcohol drinking in C57BL/6J mice using this approach.
Alcohol use disorder (AUD), which involves chronic excessive alcohol consumption, is one of the most common psychiatric disorders and is a global problem. AUD symptoms involve repeated cycles of intoxication, withdrawal, and cravings and are characterized by the constant consumption of alcohol without regard for the social, occupational, and health consequences1,2,3,4,5,6,7. Alcohol use disorder often occurs in conjunction with other pervasive, persistent, and impairing mental disorders8, such as ADHD9, anxiety10, or depression11 and is responsible for approximately 88,000 deaths annually in the United States alone2. Excessive or frequent alcohol use can affect a person's work status and social relationships12 and may lead to increased violence13. Physically, acute withdrawal from alcohol can result in anxiety, agitation, tremor, excessive sweating, altered consciousness, and hallucinations14,15. Furthermore, people may feel withdrawal symptoms when cutting down or stopping drinking and become irritable or cranky16. Additionally, chronic alcohol consumption can cause memory loss17 and can result in thiamine deficiency, also known as Wernicke-Korsakoff syndrome (WKS), which contributes significantly to alcohol-induced dementia18.
To further advance AUD research, it is necessary to have translationally relevant animal models of the disease. The most common model of AUD in rodents is chronic intermittent ethanol vapor exposure (CIE), which is a well-established approach for inducing alcohol dependence through repeated inhalation of alcohol vapor4,19,20,21,22,23,24,25,26,27,28,29,30. Rodent CIE procedures induce withdrawal symptoms such as handling-induced convulsions31, hyperexcitability, irritability-like behavior, anxiety-like behavior, and sleep disorders and result in an escalation of alcohol drinking22,32,33,34,35, thus meaning the CIE model has translational validity to human AUD.
In rats, the CIE model often involves the operant self-administration of alcohol to measure the escalation of intake36,37,38, whereas the mouse model involves CIE and two-bottle choice (2BC) drinking39,40. Preclinical models of alcohol dependence have consistently shown that animals increase their ethanol intake after chronic ethanol vapor exposure23,41,42,43. In mice specifically, repeated cycles of CIE have been shown to escalate voluntary ethanol intake3,21,44,45,46. Overall, prior studies demonstrate that the CIE model is sufficient to increase ethanol consumption and model AUD in rodents.
This study aims to highlight the CIE method for studying AUD and, more specifically, focus on the 2BC/CIE mouse model. We go through a detailed process of the steps necessary for performing 2BC/CIE and present an example of the escalation of alcohol drinking after CIE.
All procedures were approved by the Purdue University Animal Care and Use Committee. The present study used 8 week old C57BL/6J mice. The CIE group had 5 mice (3 male, 2 female), and the Air group had 10 mice (5 male and 5 female). The animals were obtained from a commercial source (see Table of Materials) and were group-housed on a 12 h light-dark cycle with access to food. The body weights of the mice were measured once per week throughout the experiment.
1. General experimental design
NOTE: Two-bottle choice (2BC)/chronic intermittent ethanol vapor exposure (CIE) is a preclinical mouse model to explore ethanol dependence47.
2. Experimental preparation
3. Animal habituation
4. Two-bottle choice with 15% w/v ethanol and water
5. Chronic intermittent ethanol vapor (CIE) exposure
In the present representative study, ethanol intake (g/kg) during 2BC is reported during baseline drinking and after weeks of CIE (post vapor). Briefly, as described in the protocol, during 2BC, the mice had access to two bottles: one containing water and the other containing 15% (w/v) ethanol. After the baseline intakes were determined, the subjects were split and evenly assigned to the CIE or Air group. The initial baseline ethanol intake during the 3 week period stabilized at 2.00 g/kg ± 0.21 g/kg (n = 15) before the CIE (Figure 1A,B).
When looking at all the weeks of alcohol drinking, a two-way repeated measures ANOVA revealed a significant effect of CIE treatment, F(13,78) =7.471, p < 0.0001, and week, F(6,78) = 14.07, p < 0.0001, on ethanol intake, as well as a significant week treatment interaction effect, F(6,78) = 5.135, p = 0.0002, on ethanol intake. A Bonferroni post hoc test51 revealed that the ethanol-dependent (CIE) mice increased their ethanol intake significantly at post-vapor week (PV) 6 compared to their baseline drinking volume. The Bonferroni post hoc test further revealed that the dependent mice consumed significantly more ethanol than the non-dependent (Air) mice at PV6 (6.9772 g/kg ± 1.94 g/kg for CIE and 3.637 g/kg ± 0.52 g/kg for Air, p < 0.0001) (Figure 1A).
When looking at a direct comparison of baseline intake and the last week of alcohol drinking post-CIE (PV6), a two-way repeated measures ANOVA also revealed a significant effect of CIE treatment, F(1,13) = 3.858, p = 0.0713, a significant effect of week, F (1,13) = 25.77, p = 0.0002, and a significant week treatment interaction effect, F(1,13) = 5.716, p = 0.0326, on ethanol intake. The Bonferroni post hoc test also showed that at PV6, the ethanol-dependent mice had significantly higher ethanol intake compared to their own baseline and non-dependent mice (Figure 1B).
To assess the BECs during CIE, blood from the dependent mice was collected at the end of the CIE every week. An ideal range for the BECs of the mice during the CIE procedure is 150-250 mL/dL. A representative average weekly BEC (173.12 mg/dL ± 20.786 mg/dL) is shown in Figure 1C.
Figure 1: Ethanol intake during two-bottle choice drinking and blood ethanol concentrations post-CIE in male and female mice. (A) Two-bottle choice ethanol intake of the dependent (CIE; n = 5; gold circles) and non-dependent (Air; n = 10; black circles) mice over all the weeks. The ethanol intake of the CIE group was significantly higher than baseline levels after post-vapor week 6. The dependent mice also had higher alcohol intake compared to the Air exposure mice on post-vapor week 6. (B) Comparison of ethanol intake between baseline and post-vapor week 6. The dependent mice (gold bars) showed significantly higher ethanol intake than the non-dependent mice (black bars) on post-vapor week 6. The dependent mice also had significantly higher ethanol intake compared to their baseline intake during post-vapor week 6. (C) Representative blood ethanol concentrations for all the dependent mice. Data are shown as mean ± SEM. * = significant (p < 0.05) difference between non-dependent (black) and dependent group (gold) in PV6; ** p < 0.01; ## = significant (p = 0.001) difference between baseline and PV6 within the dependent group; #### p < 0.0001. Abbreviation: PV = post-vapor week. Please click here to view a larger version of this figure.
Supplementary Figure 1: Drinking bottles. Image of the hand-crafted drinking bottles for water (top bottle) and alcohol (bottom bottle). Each contains a rubber stopper on the end where the liquid is filled. f Please click here to download this File.
Supplementary Figure 2: Cage setup for the two-bottle choice procedure with ethanol and water. The bottle positions are rotated daily. (A) Top view. (B) Side view. Note that a clip is used to secure the ball-bearing sipper tube to the wire cage lid, and the sipper tube calibration markings on the bottles are oriented upward to enable the experimenter to record measures while the mice have access to ethanol. Please click here to download this File.
Supplementary Figure 3: Representation of the vapor chambers holding (A) mouse cages and (B) the overall ethanol inhalation system setup. Please click here to download this File.
Alcohol use disorder represents a global public health problem with high prevalence and cost to society52. To study AUD in preclinical animal models, a common method in mice is 2BC/CIE20,34,39,40,47,53,54,55. Here, this established model of alcohol dependence was used to present representative methods and results from a standard 2BC/CIE study. As expected, this study showed that mice increased their intake of alcohol after several weeks of CIE compared to the Air controls and their own previous baseline alcohol intake. Following the protocol described in this manuscript, researchers can achieve sustained alcohol exposure (i.e., persistent blood and brain levels of alcohol) that leads to the development of withdrawal symptoms and, eventually, the escalation of alcohol intake. Animal withdrawal can manifest as increased irritability, aggression, sensory overreaction, abnormal activity, and central nervous system overexcitation33. Importantly, the withdrawal symptoms observed in animal models correspond to the symptoms of alcohol withdrawal in humans to a large extent3,4,33,56,57,58.
Excessive alcohol drinking can lead to alcohol dependence, a maladaptive neurophysiological state that leads to a range of withdrawal symptoms when alcohol consumption is drastically reduced or stopped altogether33,56,59. These symptoms often involve compensatory responses to the effects of alcohol, which are assumed to represent the brain's attempt to re-establish homeostasis during sustained exposure to alcohol58,60,61,62,63,64. Due to the limitations of human studies and the different aspects of alcohol withdrawal, clinical studies have had difficulty identifying the risk factors, vulnerability, and potential withdrawal mechanisms of human alcoholics65,66. Using animal models allows researchers to control and examine the neural, molecular, genetic, and environmental factors that promote alcohol dependence and withdrawal. The 2BC/CIE model has been used to induce alcohol dependence and withdrawal symptoms for the study of AUD. These withdrawal symptoms include hyperexcitability, irritability-like behavior, anxiety-like behavior, and sleep disorders3,14,22,28,44,57,67.
There are several significant advantages to the 2BC/CIE model. First, this is one of the only methods that results in the development of ethanol dependence and a significant escalation of ethanol intake. Although the CIE exposure is forced ethanol intake, there is a component of voluntary ethanol escalation through 2BC, which may be lacking in alternative models of AUD such as injection or oral gavage (i.e., quick models used to study the neural mechanisms of alcohol exposure/dependence)68,69,70. Ideally, mice can reach the target blood ethanol concentration level (150 mg%) within 1 week71. In addition, stable BECs can be maintained over a long period with normal weight regulation and feeding behavior71.
There are a few limitations to the current version of the 2BC/CIE method. Pyrazole has been widely used as an in vitro and in vivo ethanol dehydrogenase inhibitor. Since the metabolism of mice is faster than that of rats, the infusion of pyrazole before the CIE experiment starts can inhibit the function of alcohol dehydrogenase in mice, thus enhancing the level of intoxication in the mice72. However, pyrazole is highly toxic, and the long-term injection of pyrazole can result in weight loss, liver necrosis, and cytotoxicity in other organs, especially when administered with alcohol72. An inherent limitation of the 2BC method is that it is difficult to demonstrate the animal's motivation to obtain alcohol33. On the contrary, in operant models, the effort to achieve the alcohol can be measured using progressive ratio responding34,73,74. However, recent open-source models to design electronic 2BC equipment that counts the number of contacts with the sipper tube over time may help mitigate this issue54,75,76,77. Finally, one major caveat to the use of the 2BC/CIE model is that the development of alcohol dependence occurs through the forced chronic intake of alcohol vapor and not voluntary consumption, although the escalation of alcohol intake is expressed through voluntary 2BC consumption. Recently, differences in neural activity profiles in alcohol-dependent rats when using forced or voluntary vapor exposure methods have been identified74, which suggests that the lack of choice for the induction of dependence may lead to differences in neural responses.
In summary, while the alcohol field has various animal models to investigate the different physiological and behavioral aspects of AUD (for example, alcohol injection or gavage), the protocol described above is one of the most reliable methods for modeling AUD. Here, a protocol for the 2BC/CIE model of alcohol dependence in mice to study AUD is highlighted, which can be used for a variety of research questions, including the examination of the brain-wide neural circuits involved in AUD22,55,78,79. This method will continue to be ideal for the further examination of the neurobiological mechanisms of AUD80.
The authors have nothing to disclose.
This work was supported by National Institutes of Health (NIH) grants AA027301 and AA029985.
500 Eppendorf Tubes | Eppendorf | L203896J | |
95% ethanol | Decon laboratories | 2816 | |
Analox machine | Analox Instruments | Analox-AM1 | |
Animal Weighing Scale | Kent Scientific | SCL-4000 | |
Binder Clips | Office Depot | 560394 | |
C57BL/6J mice | The Jackson Laboratory | 000664 | |
Centrifuge | Eppendorf | 5418R | |
Chronic intermitted vapor chamber | La Jolla Alcohol Research Inc | Custom made materials | |
Heparin/EDTA | Sagent Pharmaceuticals | TS/DRUGS/2/2015 | |
Mouse bedding | Bed-o’Cobs | 8B | fill with 1/8" deep |
Mouse drinking bottle | Custom made materials | ||
Pyrazole | Sigma-Aldrich | bccc6397 | |
Teklad global 18% protein (mouse food) | Teklad global | Envigo 2018 |
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