Involvement of Endocannabinoids in Alcohol "Binge" Drinking: Studies of Mice with Human Fatty Acid Amide Hydrolase Genetic Variation and After CB1 Receptor Antagonists

Abstract

Background: The endocannabinoid system has been found to play an important role in modulating alcohol intake. Inhibition or genetic deletion of fatty acid amide hydrolase (FAAH; a key catabolic enzyme for endocannabinoids) leads to increased alcohol consumption and preference in rodent models. A common human single-nucleotide polymorphism (SNP; C385A, rs324420) in the FAAH gene is associated with decreased enzymatic activity of FAAH, resulting in increased anandamide levels in both humans and FAAH C385A knock-in mice.

Methods: As this FAAH SNP has been reported to be associated with altered alcohol abuse, the present study used these genetic knock-in mice containing the human SNP C385A to determine the impact of variant FAAH gene on alcohol "binge" drinking in the drinking-in-the-dark (DID) model.

Results: We found that the FAAH(A/A) mice had greater alcohol intake and preference than the wild-type FAAH(C/C) mice, suggesting that increased endocannabinoid signaling in FAAH(A/A) mice led to increased alcohol "binge" consumption. The specificity on alcohol vulnerability was suggested by the lack of any FAAH genotype difference on sucrose or saccharin intake. Using the "binge" DID model, we confirmed that selective CB1 receptor antagonist AM251 reduced alcohol intake in the wild-type mice.

Conclusions: These data suggest that there is direct and selective involvement of the human FAAH C385A SNP and CB1 receptors in alcohol "binge" drinking.

Keywords: Alcohol Drinking; CB1 Receptor; Endocannabinoid System; Human Fatty Acid Amide Hydrolase C385A Knock-In.

Figure 1

Genotypic difference in alcohol (15%) intake in 4-day drinking-in-the-dark (DID) model (A) and alcohol preference (15% alcohol vs. water) on day 5 (B) in FAAH knock-in mice. (A) On the test days, alcohol at 15% concentration was presented 3 hours after the beginning of dark cycle, and then alcohol intake values were recorded at 4 hour time point for 4 consecutive days in the FAAH^A/A, FAAH^C/A and FAAH^C/C mice; and (B) the mice were tested on day 5 for 15% alcohol vs. water choice using 2-bottle 4-hour paradigm after the 4-day DID drinking test. Genotypic difference: *p<0.05 vs. FAAH^C/C mice. The values presented in the figure are mean ± SEM.

Figure 2

No genotype differences in 4% (A) or 8% (B) sucrose intake in 4-day drinking-in-the-dark (DID) model, or sucrose preference (8% sucrose vs. water) on day 5 (C) in FAAH knock-in mice. (A and B) On the test days, sucrose was presented 3 hours after the beginning of dark cycle, and then their intake values were recorded at 4 hours for 4 consecutive days in the FAAH^A/A, FAAH^C/A and FAAH^C/C mice; and (C) the mice were tested on day 5 for 8% sucrose vs. water choice using 2-bottle paradigm, and then their intake values were recorded at 4 hours. The values presented in the figure are mean ± SEM.

Figure 3

No genotype differences in 0.1% (A) or 0.2% (B) saccharin intake in 4-day drinking-in-the-dark (DID) model, or saccharin preference (0.1% saccharin vs. water) on day 5 (C) in FAAH knock-in mice. (A and B) On the test days, saccharin was presented 3 hours after the beginning of dark cycle, and then their intake values were recorded at 4 hours for 4 consecutive days in the FAAH^A/A, FAAH^C/A and FAAH^C/C mice; and (C) the mice were tested on day 5 for 0.1% saccharin vs. water choice using 2-bottle paradigm, and then their intake values were recorded at 4 hours. The values presented in the figure are mean ± SEM.

Figure 4

Effect of acute AM251 administration at 0.3 or 1 mg/kg on alcohol (15%) intake on day 5 after 4-day drinking-in-the-dark (DID) model in C57BL/6J mice. The mice were assigned to one of 3 treatment groups: (1) vehicle as control; (2) 0.3 mg/kg AM251; and (3) 1 mg/kg AM251. AM251 effect: *p<0.05, **p<0.01 vs. vehicle. The values presented in the figure are mean ± SEM.