The α5 neuronal nicotinic acetylcholine receptor subunit plays an important role in the sedative effects of ethanol but does not modulate consumption in mice

Abstract

Background: Alcohol use disorders (AUDs) are a major public health problem, and the few treatment options available to those seeking treatment offer only modest success rates. There remains a need to identify novel targets for the treatment of AUDs. The neuronal nicotinic acetylcholine receptors (nAChRs) represent a potential therapeutic target in the brain, as recent human genetic studies have implicated gene variants in the α5 nAChR subunit as high risk factors for developing alcohol dependence.

Methods: Here, we evaluate the role of the α5* nAChR for ethanol (EtOH)-mediated behaviors using male α5+/+ and α5-/- transgenic mice. We characterized the effect of hypnotic doses of EtOH and investigated drinking behavior using an adapted drinking-in-the-dark (DID) paradigm that has been shown to induce high EtOH consumption in mice.

Results: We found the α5 subunit to be important in mediating the sedative effects of EtOH. The α5-/- mice showed slower recovery from EtOH-induced sleep, as measured by loss of righting reflex. Additionally, the α5-/- mice showed enhanced impairment to EtOH-induced ataxia. We found the initial sensitivity to EtOH and EtOH metabolism to be similar in both α5+/+ and α5-/- mice. Hence, the enhanced sedation is likely due to a difference in the acute tolerance of EtOH in α5-/- mice. However, the α5 subunit did not play a role in EtOH consumption for EtOH concentrations ranging from 5 to 30% using the DID paradigm. Additionally, varenicline was effective in reducing EtOH intake in α5-/- mice.

Conclusions: Together, our data suggest that the α5 nAChR subunit is important for the sedative effects of EtOH but does not play a role in EtOH consumption in male mice. Varenicline can be a treatment option even when there is loss of function of the α5 nAChR subunit.

Figure 1. The α5 subunit plays an important role for sedative dose of ethanol and influences ethanol-induced ataxia

(A) The loss of righting reflex (LORR) duration for 3.2g/kg (i.p.) ethanol was shorter for α5+/+ (black) compared to α5−/− (gray) mice. (B) On the accelerating rotarod, α5−/−(gray) had an increased latency to fall compared to α5+/+ (black) mice for 2 g/kg (i.p.) ethanol dose. In A, n=8–13 animals, B, n=11 animals. The values are expressed as duration (mins) ± SEM (A) and duration (secs) ± SEM (B) (two-tailed unpaired t-test (A) and two-way ANOVA (B) followed by Newman-Keuls test, *p<0.05).

Figure 2. The initial sensitivity to ethanol and ethanol metabolism is similar for α5+/+ and α5−/− mice

(A) The threshold dose (ED₅₀) of ethanol required to induce LORR was similar for α5+/+ and α5−/− mice. (B) The blood ethanol concentrations were not different between α5+/+ and α5−/− mice at any of the time points measured. In A, n=6 animals per genotype, B, n=3 animals per genotype for each time point. The values are expressed as alcohol dose (g/kg) ± SEM (A) and blood ethanol concentrations (mg/dl) ± SEM (two-tailed unpaired t-test (A) and two-way ANOVA (B)).

Figure 3. The α5 nAChR does not play a role in ethanol consumption in mice using the drinking-in-the-dark paradigm

(A) Both the α5+/+ (black) and the α5−/− (gray) mice show similar 20% ethanol consumption during the 30 days of exposure. (B) The average ethanol intake across 30 exposures for α5+/+ and α5−/− mice is similar. (C) α5+/+ and α5−/− mice shown similar ethanol intake at 5%, 10% and 30% ethanol concentrations. In A&B, n=7–11 animals and in C, n=5–6 animals. The values are expressed as mean alcohol intake (g/kg/2 hrs) ± SEM (two-way ANOVA repeated measures (A) and two-way ANOVA (C) followed by Newman-Keuls test and two-tailed unpaired t-test (B)).

Figure 4. Varenicline is effective in selectively reducing ethanol consumption in α5+/+ and α5−/− mice without affecting water or sucrose consumption

Varenicline (2 mg/kg, i.p.) treatment decreased voluntary ethanol consumption in both α5+/+ (A) and α5−/− (B) mice 2 hrs after the onset of drinking using the two bottle choice DID paradigm. Varenicline was selective in decreasing ethanol consumption with no effect on water consumption (C&D) and sucrose consumption (E&F) in both α5+/+ and α5−/−mice. n=9–13 animals. The values are expressed as mean ethanol or sucrose intake (g/kg) ± SEM (A,B,E,F) or water intake (ml/100g) ± SEM (C,D) (repeated measures ANOVA followed by Newman-Keuls post hoc test). *p<0.05, **p<0.01, compared with vehicle.