A mouse model for chronic intermittent electronic cigarette exposure exhibits nicotine pharmacokinetics resembling human vapers

Abstract

Electronic cigarettes (E-cig) use is increasing rapidly, particularly among youths. Animal models for E-cig exposure with pharmacokinetics resembling human E-cig users are lacking. We developed an E-cig aerosol exposure system for rodents and a chronic intermittent delivery method that simulates E-cig users who vape episodically during wakefulness and abstain during sleep. Mice were exposed to E-cig in a programmed schedule at very low, low, medium, or high doses defined by duration of each puff, number of puffs per delivery episode and frequency of episodes in the dark phase of a 12/12-h circadian cycle for 9 consecutive days. The plasma nicotine/cotinine levels and their time courses were determined using LC/MS-MS. We assessed the body weight, food intake and locomotor activity of Apolipoprotein E null (ApoE-/-) mice exposed to chronic intermittent E-cig aerosol. Plasma nicotine and cotinine levels were positively correlated with exposure doses. Nicotine and cotinine levels showed a circadian variation as they increased with time up to the maximum nicotine level of 21.8 ± 7.1 ng/mL during the daily intermittent E-cig exposure in the 12-h dark phase and then declined during the light phase when there was no E-cig delivery. Chronic E-cig exposure to ApoE-/- mice decreased body weight, food intake and increased locomotion. Our rodent E-cig exposure system and chronic intermittent exposure method yield clinically relevant nicotine pharmacokinetics associated with behavioral and metabolic changes. The methodologies are essential tools for in vivo studies of the health impacts of E-cig exposure on CNS, cardiovascular, pulmonary, hepatic systems, metabolism and carcinogenesis.

Keywords: Aerosol; Chronic intermittent exposure; E-cigarette; Nicotine; Pharmacokinetics.

Fig. 1:

A. E-cigarette aerosol generation and exposure system for mice. Chamber with mouse without E-cig aerosol (B) and with E-cig aerosol (C). D. Blu-cig E-cigs used in the study.

Fig. 2:

Plasma nicotine (left panel) cotinine (right panel) concentrations (mean ± SE) as a function of E-cig dose. Mice were exposed to different doses of E-cigs as defined in the Methods section during the dark phase of 12/12-h circadian dark/light cycles for 9 days. Mice were sacrificed 30-60 min following the last exposure and plasma nicotine and cotinine were measured (n = 4-5 mice per group, *P < 0.05 vs very low dose with MANOVA analysis).

Fig. 3:

Plasma nicotine and cotinine concentrations (mean ± SE) as a function of time. Mice were exposed to intermittent E-cig aerosol for 0 to 12 h in the dark phase of 12/12-h circadian dark/light cycle (shaded area indicates exposure to E-cigs) for 9 consecutive days. The exposure dose: puff duration = 4 sec; 3 puffs per episode with an inter-puff interval = 30 sec; one episode per 30 min amounting to a total 24 episodes during the 12-h dark phase. Mice were returned to their home cages without E-cig exposure during the 12-h light phase of the dark/light cycles. Blood samples were taken in day 9 by the end of the h time point indicated. Time point 0 is the time right before the first episode of daily 12 h exposure of day 9. It is equivalent to time point 24 h of day 8. Plasma nicotine and cotinine levels were measured (n = 3 - 5 mice per time point).

Fig. 4.

Effects of E-cigs exposure on body weight, food intake and locomotion in Apo E null mice. A) Body weight; data were analyzed using repeated measures ANOVA, P = 0.0046 for Group x time interaction term showing a significant difference in the time trend between the two groups: E-cig exposed vs. saline aerosol exposed as control with an aerosol generation and exposure device. e-cig: E-cig. B) Cumulative food intake; data were analyzed with multiple linear regression, showing a highly significant difference in slope between the two groups, P < 0.00001. C) Locomotor Activity (travel distance) analyzed by Student t test. All values are means ± SE (n = 5 mice per group, *P < 0.05).