Flavor-specific enhancement of electronic cigarette liquid consumption and preference in mice

Abstract

Background: The use of electronic cigarettes has increased over the past decade. To determine how the abuse liability of electronic cigarette liquids (e-liquids) differs from nicotine alone, and to determine the impact of flavor, we compared nicotine-containing fruit- and tobacco-flavored e-liquids, and their nicotine-free versions, to nicotine alone in mouse models of oral consumption, reward and aversion.

Methods: Adult male C57BL/6 J mice voluntarily consumed oral nicotine, equivalent nicotine concentrations of fruit- and tobacco-flavored e-liquid, and equivalent dilutions of the nicotine-free versions in 2-bottle choice tests. Conditioned place preference and place aversion were assessed with peripherally administered e-liquids or nicotine. Serum nicotine and cotinine levels were measured after subcutaneous injections of e-liquid or nicotine.

Results: Mice showed higher consumption and preference for the fruit-flavored e-liquid compared with nicotine alone. This increase was not due to the flavor itself as consumption of the nicotine-free fruit-flavored e-liquid was not elevated until the highest concentration tested. The increased consumption and preference were not observed with the tobacco-flavored e-liquid. The conditioned place preference, place aversion and nicotine pharmacokinetics of the fruit-flavored e-liquid were not significantly different from nicotine alone.

Conclusions: Our data suggest that fruit, but not tobacco flavor, increased the oral consumption of e-liquid compared with nicotine alone. Moreover, this enhancement was not due to increased consumption of the flavor itself, altered rewarding or aversive properties after peripheral administration, or altered pharmacokinetics. This flavor-specific enhancement suggests that some flavors may lead to higher nicotine intake and increased use of e-liquids compared with nicotine alone.

Keywords: Electronic cigarette; aversion; consumption; mice; nicotine; preference.

Fig. 1.. Consumption and preference of nicotine-containing fruit-flavored e-liquid, nicotine-free fruit-flavored e-liquid and nicotine alone in 2-bottle choice tests.

(A) The average consumption and (B) preference for nicotine alone, nicotine-containing fruit-flavored e-liquid at equivalent nicotine concentrations, and nicotine-free fruit-flavored e-liquid at equivalent dilutions. The consumption for nicotine and nicotine-containing e-liquids are in mg/kg/day, and in hypothetical mg/kg/day for the nicotine-free e-liquid. *P<0.05, **P<0.01, ***P<0.001 and ***P<0.0001 for all comparisons. *indicates comparisons between nicotine-containing fruit-flavored e-liquid and nicotine alone, ^ϕindicates comparisons between nicotine-free fruit-flavored e-liquid and nicotine alone, and îndicates comparisons between nicotine-containing fruit-flavored e-liquid and the nicotine-free version. Mean ± SEM, n=15 for nicotine alone, n=12 for nicotine-containing fruit-flavored e-liquid, n=13 for the nicotine-free fruit-flavored e-liquid.

Fig. 2.. Consumption and preference of nicotine-containing tobacco-flavored e-liquid, nicotine-free tobacco-flavored e-liquid and nicotine alone in 2-bottle choice tests.

(A) The average consumption and (B) preference for nicotine alone, nicotine-containing tobacco-flavored e-liquid at equivalent nicotine concentrations, and nicotine-free tobacco-flavored e-liquid at equivalent dilutions. The consumption for nicotine and nicotine-containing e-liquids are in mg/kg/day, and in hypothetical mg/kg/day for the nicotine-free e-liquid. *P<0.05, **P<0.01, ***P<0.001 and ***P<0.0001 for all comparisons. ^ϕindicates comparisons between nicotine-free tobacco-flavored e-liquid and nicotine alone, and îndicates comparisons between nicotine-containing tobacco-flavored e-liquid and the nicotine-free version. Mean ± SEM, n=15 for nicotine alone, n=14 for nicotine-containing tobacco-flavored e-liquid, n=14 for the nicotine-free tobacco-flavored e-liquid.

Fig. 3.. Total fluid consumption.

(A) The total fluid consumption in the 2-bottle choice over the 5-week experiment did not differ between the nicotine-containing fruit-flavored e-liquid, nicotine-free fruit-flavored e-liquid and nicotine alone groups at any week. (B) The total fluid consumption of the nicotine-free tobacco-flavored e-liquid group was significantly different from the nicotine group. Mean ± SEM, n=15 for nicotine alone, n=12 for nicotine-containing fruit-flavored e-liquid, n=13 for the nicotine-free fruit-flavored e-liquid, n=14 for nicotine-containing tobacco-flavored e-liquid, n=14 for the nicotine-free tobacco-flavored e-liquid.

Fig. 4.. Nicotine-containing fruit-flavored e-liquid does not differ from nicotine alone in conditioned place preference or conditioned place aversion assays.

(A) The conditioning index after CPP with 0.5 mg/kg nicotine or nicotine-containing fruit-flavored e-liquid at an equivalent nicotine concentration. n=17 for nicotine, n=15 for e-liquid groups. (B) The conditioning index after saline or nicotine-free fruit-flavored e-liquid at an equivalent dilution to 0.5 mg/kg nicotine. n=16 for saline, n=14 for nicotine-free e-liquid groups. (C) The conditioning index after CPA with 2.0 mg/kg nicotine or nicotine-containing fruit-flavored e-liquid at an equivalent nicotine concentration. n=12 for nicotine and e-liquid groups. *P<0.05, **P<0.01 for a one-sample t-test between the conditioning index and a hypothetical index of zero.

Fig. 5.. Nicotine and cotinine clearance.

(A) The average serum nicotine and (B) serum cotinine levels after injection of 2.5 mg/kg s.c. of nicotine alone, nicotine-containing fruit-flavored e-liquid and nicotine-containing tobacco-flavored e-liquid. *P<0.05, **P<0.01, ***P<0.001 and ***P<0.0001 for all comparisons. *indicates comparison between nicotine-containing tobacco-flavored e-liquid and nicotine alone, and îndicates comparison between nicotine-containing tobacco-flavored e-liquid and nicotine-containing fruit-flavored e-liquid.