Hyperactivity Induced By Vapor Inhalation of Nicotine in Male and Female Rats

Abstract

Rationale: Preclinical models of electronic nicotine delivery system (ENDS; "e-cigarette") use have been rare, so there is an urgent need to develop experimental approaches to evaluate their effects.

Objective: To contrast the impact of inhaled nicotine across sex.

Methods: Male and female Wistar rats were exposed to vapor from a propylene glycol vehicle (PG), nicotine (NIC; 1-30 mg/mL in PG), or were injected with NIC (0.1-0.8 mg/kg, s.c.), and then assessed for changes in temperature and activity. The antagonist mecamylamine (2 mg/kg) was administered prior to NIC to verify pharmacological specificity. Plasma levels of nicotine and cotinine were determined after inhalation and injection.

Results: Activity increased in females for ~60 minutes after nicotine inhalation, and this was blocked by mecamylamine. A similar magnitude of hyperlocomotion was observed after s.c. administration. Body temperature was reduced after nicotine inhalation by female rats but mecamylamine increased this hypothermia. Increased locomotor activity was observed in male rats if inhalation was extended to 40 minutes or when multiple inhalation epochs were used per session. The temperature of male rats was not altered by nicotine. Plasma nicotine concentrations were slightly lower in male rats than in female rats after 30-minute nicotine vapor inhalation and slightly higher after nicotine injection (1.0 mg/kg, s.c.).

Conclusions: Nicotine inhalation increases locomotor activity in male and female rats to a similar or greater extent than by subcutaneous injection. Sex differences were observed, which may be related to lower nicotine plasma levels, lower baseline activity and/or a higher vehicle response in males.

Figure 1.

Mean (±SEM for N=7 male; N=8 female) A, B) activity rate and C, D) temperature after inhalation of the PG vehicle or nicotine (NIC; 30 mg/mL; 30 min). Open symbols indicate a significant difference from both the PG and the within-treatment baseline at a given time-point and shaded symbols indicate a significant difference from the baseline only. A significant difference from PG inhalation conditions is indicated by *.

Figure 2.

Mean (±SEM; N=7 male) A) Activity rate and B) temperature after inhalation of the PG vehicle or nicotine (NIC; 30 mg/mL; 15, 30, 40 min). Open symbols indicate a significant difference from both the within-treatment baseline and the vehicle at a given time-point and shaded symbols indicate a significant difference from the baseline only. A significant difference from the PG inhalation condition is indicated by *.

Figure 3.

Mean (N=8; ±SEM) plasma nicotine and cotinine after A) inhalation of nicotine (30 mg/mL) vapor for 30 minutes or B) nicotine injection (1.0 mg/kg, s.c.) in age matched (30 weeks) male and female Wistar rats. *=significant sex difference within time and analyte; #=significant difference across time within analyte and sex; &=significant analyte difference.

Figure 4.

Mean (±SEM; N=8 female) A, B) activity rate and C, D) temperature after inhalation of the PG vehicle or nicotine (A, C; NIC 10, 20, 30 mg/mL; B, D; NIC 1, 5, 10 mg/ml; 15 min). Open symbols indicate a significant difference from both the within-treatment baseline and the vehicle at a given time-point, while shaded symbols indicate a significant difference from the baseline only. A significant difference from PG inhalation condition is indicated by *.

Figure 5.

Mean (±SEM; N=8 female) A, B) activity rate and C, D) temperature after inhalation of the PG vehicle or nicotine (NIC; 30 mg/mL; 15 min) twice per session, with Saline or Mecamylamine pre-treatment. All eight treatment conditions were counterbalanced in one experiment and are presented by the initial vapor condition for clarity. Shaded symbols indicate a significant difference from the baseline or 120-minute timepoint, within treatment condition. For a given time-point, a significant difference from both mecamylamine pre-treatment conditions is indicated by *, a difference from all other conditions with # and a difference between Saline and Mecamylamine pre-treatments for the same vapor sequence with $.

Figure 6.

Mean (±SEM; N=8 male) A) activity rate and B) temperature after inhalation of the PG vehicle or nicotine (NIC; 30 mg/mL; 15 min) twice per session. Shaded symbols indicate a significant difference from the baseline or 120-minute timepoint, within treatment condition. For a given time-point, a significant difference from both PG inhalation conditions is indicated by *, a significant difference between PG-NIC and PG-PG for sessions with &, and between PG-NIC and NIC-NIC and sessions with #.

Figure 7:

Mean (±SEM; N=8 male) activity rate after inhalation of the PG vehicle for 15 minutes and then either PG or nicotine (NIC; 30 mg/mL; 15 min) for 30 minutes one hour later. The four conditions were analyzed together but are presented in pairs for clarity. The session time is relative to the initial saline/mecamylamine injection. Arrows indicate timing of pre-session injection of Saline or Mecamylamine and the bars indicate the inhalation intervals. Shaded symbols indicate a significant difference from the 60-minute timepoint, within treatment condition. For a given time-point, a significant difference between inhalation conditions is indicated by *.

Figure 8.

Mean (±SEM; N=8 female) A, B) activity rate and C, D) temperature after injection of the vehicle or nicotine (NIC; 0.1, 0.2, 0.4 and 0.8 mg/kg). Open symbols indicate a significant difference from both the Saline condition at a given time-point and the within-treatment baseline, while shaded symbols indicate a significant difference from the baseline, within treatment condition, only. A significant difference of NIC (0.8 mg/kg) from saline is indicated by * and a significant difference of NIC (0.4 mg/kg) from saline is indicated by ^#. A significant difference from mecamylamine across NIC condition is indicated with &.

Figure 9.

Mean (±SEM; N=8 male) A, B) activity rate and C, D) temperature after injection of A, C) nicotine (NIC; 0.0, 0.1, 0.2, 0.4 and 0.8 mg/kg, s.c.) or B, D) methamphetamine (0.0, 1.0 mg/kg, s.c.). The inset in A depicts the entire session activity average. Open symbols indicate a significant difference from both the baseline, within treatment condition, and saline at the respective timepoint. A significant difference from saline is indicated by * and from the 0.1 dose with # in the inset.

Figure 10.

Mean (±SEM) plasma nicotine and cotinine in male Wistar rats after A) injection of nicotine (0.2–0.8 mg/kg, s.c.) 15 minutes before blood collection and B) vapor inhalation of nicotine (10 and 30 mg/ml in the PG vehicle) for 30 minutes. A significant difference from all other s.c. doses within analyte is indicated with #, a difference from the 0.2 mg/kg s.c. dose with *, and a significant difference from the 10 mg/ml inhalation condition with %. A significant difference between nicotine and cotinine concentrations is indicated with &.