Flavors Enhance Nicotine Vapor Self-administration in Male Mice

Abstract

Introduction: Although the use of combustible cigarettes has decreased in many urban regions of America, the use of electronic nicotine delivery systems (ENDS) has dramatically increased. ENDS, or electronic cigarettes (e-cigarettes), differ from combustible cigarettes given that there are no restrictions on flavorant additives in e-liquids. With 95% of ENDS users vaping flavored e-liquids, it is critical to understand how flavors alter vaping-related behaviors. We have previously shown that menthol and green apple flavors enhance nicotine reward-related behavior in a mouse model and in the present study have investigated how menthol and green apple flavors alter e-Vape self-administration behavior in male mice.

Methods: Adult C57/BL6J male mice were used in vapor-inhalation self-administration assays. Mice were assigned vaping e-liquids (6 mg/mL nicotine with or without menthol or green apple flavor) to escalate on a fixed-ratio 1 (FR1) schedule in daily 3-hour sessions to examine initiation-related behaviors. Following escalation, mice were transitioned to a FR3 and progressive ratio schedules in 3-hour sessions to examine reinforcement-related behaviors.

Results: Here we observed that male mice exhibited increased rates of self-administration escalation on a FR1 schedule when assigned to flavored e-liquids. Upon transition to FR3, mice continued to exhibit enhanced levels of reinforcement with flavored e-liquids. We also observed that mice self-administer zero-nicotine green apple flavored e-liquids.

Conclusions: These data provide additional evidence that ENDS flavors enhance vaping-related initiation and reinforcement-related behavior and promote the need to continue investigating the role ENDS flavors play in vaping-related behaviors.

Implications: There has been much discussion recently regarding the impact of flavors on vaping-related behavior. Our study here shows that flavors significantly enhance the acquisition and reinforcement of vaping-related behavior. This suggests that flavors in electronic nicotine delivery systems significantly increase the risk of addiction-related behaviors among users of vaping products.

Figure 1.

(A) Mice naive to vapor exposure were escalated on designated e-liquids in e-Vape self-administration assays using 10 daily 3-h FR1 sessions (3-s puff with 60-s timeout) (n = 4 mice/group) with the indicated commercial e-liquids. Where not indicated, nicotine is 6 mg/mL. Data are mean (± SEM) e-Vape deliveries for each session. (B) Mean inactive nose-pokes for mice during sessions described in (A).

Figure 2.

(A) Mice on a FR3 schedule were assigned commercial e-liquids during a 3-h e-Vape self-administration session, n = 12 mice/group. (B) Mice were intraperitoneally injected with saline or 2 mg/kg DhβE immediately prior to FR3 self-administration sessions. Two-way ANOVA with post hoc Bonferroni; n = 5 mice. (C) All mice were assigned PGVG on a FR1 schedule to examine extinction-related behavior. Data are mean (± SEM) e-Vape deliveries for each session. Where not indicated, nicotine is 6 mg/mL. *p < .05; **p < .01; ***p < .005. ^####p < .0001; vs. 6 mg/mL nicotine, one-way ANOVA with post hoc Tukey.

Figure 3.

(A) Dose–response of nicotine only (unflavored) at concentrations of 0 (PGVG only), 3, 6, 12, and 18 mg/mL in e-Vape self-administration 3-h FR3 sessions (n = 8 mice). (B) Adult male mice were escalated on “neat” (−)-menthol + nicotine on a FR1 schedule (10 daily 3-h sessions). Data are mean (± SEM) FR1 e-Vape deliveries per session. (C) Mice were placed on a FR3 schedule to examine earned e-Vape deliveries for “neat” nicotine with or without flavors. Where not indicated, nicotine is 6 mg/mL. (A, C) Data are mean (± SEM) FR3 e-Vape deliveries for each condition. *p < .05; **p < .01; ***p < .005; ****p < .001. ^#p < .05; vs. 6 mg/mL nicotine. One-way ANOVA with post hoc Tukey.

Figure 4.

(A) Raster plots that show the number of nose-pokes on the active/inactive sides for one mouse on the first day when the nose-poke assignments were reversed to examine side bias. (B) Mice were place on a PR schedule and assigned nicotine with or without flavors (n = 9). (C) Mice were pre-injected with saline or 2 mg/kg DhβE prior to a FR3 self-administration session (n = 4–6/condition). (B and C) Data are mean (± SEM) e-Vape deliveries per mouse. In all cases, nicotine is 6 mg/mL. *p < .05; **p < .01; ***p < .005; ****p < .001; one-way ANOVA with post hoc Tukey.