Lasting effects of repeated ∆9 -tetrahydrocannabinol vapour inhalation during adolescence in male and female rats

Abstract

Background and purpose: Adolescents are regularly exposed to ∆⁹ -tetrahydrocannabinol (THC) via smoking and, more recently, vaping cannabis extracts. Growing legalization of cannabis for medical and recreational purposes, combined with decreasing perceptions of harm, makes it increasingly important to determine the consequences of frequent adolescent exposure for motivated behaviour and lasting tolerance in response to THC.

Experimental approaches: Male and female rats inhaled THC vapour, or that from the propylene glycol (PG) vehicle, twice daily for 30 min from postnatal day (PND) 35-39 and PND 42-46 using an e-cigarette system. Thermoregulatory responses to vapour inhalation were assessed by radio-telemetry during adolescence and from PND 86-94. Chow intake was assessed in adulthood. Blood samples were obtained from additional adolescent groups following initial THC inhalation and after 4 days of twice daily exposure. Additional groups exposed repeatedly to THC or PG during adolescence were evaluated for intravenous self-administration of oxycodone as adults.

Key results: Female, not male, adolescents developed tolerance to the hypothermic effects of THC inhalation in the first week of repeated exposure despite similar plasma THC levels. Each sex exhibited tolerance to THC hypothermia in adulthood after repeated adolescent THC. However, enhanced potency was found in females. Repeated THC male rats consumed more food than their PG-treated control group, without significant bodyweight differences. Adolescent THC did not alter oxycodone self-administration in either sex but increased fentanyl self-administration in females.

Conclusions and implications: Repeated THC vapour inhalation in adolescent rats has lasting consequences observable in adulthood.

Figure 1

Mean (N = 8 per group; ±SEM) body temperature responses after inhalation of PG (on PND30) or THC (100 mg·ml⁻¹; on PND31) vapour for 30 min in the subgroups eventually assigned to the repeated PG or repeated THC (a) female and (b) male groups. Open symbols indicate a significant difference from both vehicle at a given time point and the within‐treatment baseline (Base), while shaded symbols indicate a significant difference from the baseline only

Figure 2

Mean (N = 8 per sex; ±SEM) body temperature recorded for the first vapour inhalation session of each day of the repeated treatment weeks is shown for the repeated THC groups; Week 1 = PND 35–39; Week 2 = PND 42–46. The PND 30 PG inhalation data are shown in upper and lower panels for comparison. The pre‐inhalation baseline temperature is indicated by “Base.” ^& P <.05, significantly different from all other days at a given time after the start of inhalation; ^§a P <.05, significantly different from all days except Day 2; ^# P <.05, significantly different from PG, Days 4 and 5; ^@ P <.05, significantly different from PG, Days 7 and 10; ^{^} P <.05, significantly different from PG, Days 6, 8, and 9; ^* P <.05, significantly different from PG.

Figure 3

Mean (N = 8 per sex; ±SEM) body temperature 60 min after the start of inhalation for all inhalation days during adolescence. ^* P <.05, significant difference from the PG value, within group; ^# P <.05, significant sex difference on a given day.

Figure 4

Mean (±SEM) body temperature of female and male rat cohorts (N = 8 per group) exposed to repeated PG or THC during adolescence and challenged in acute sessions with vapour from PG or varying concentrations of THC (25–200 mg·ml⁻¹) from PND 85–94. Open symbols indicate a significant difference from the pre‐inhalation baseline (Base) and the corresponding time after PG inhalation, and grey symbols represent a difference from the baseline only. ^{^} P <.05, significantly different from the PG condition; ^# P <.05, significantly different from the 25 and 50 mg·ml⁻¹ condition; ^§ P <.05, significantly different from the 100 mg·ml⁻¹ condition; ^% P <.05, significantly different from the 50 mg·ml⁻¹ condition, ^& P <.05, significantly different from the 25 mg·ml⁻¹ condition; ^* P <.05, significantly different between treatment groups at given dose, within sex.

Figure 5

(a,c) Mean (±SEM) plasma THC levels from female and male rats (N = 8 per group) exposed to (a) THC (100 mg·ml⁻¹) vapour on PND 31 (acute), 36–39 (chronic) or (c) THC (50–200 mg·ml⁻¹) on PND 86, 100, and 107. Corresponding bodyweights are presented for the adolescent (b) and adult (d) age intervals. ^* P <.05, significant sex difference for a given dose or day;, ^& P <.05, significant difference from the acute condition across groups; ^# P <.05, significant difference from the 50 mg·ml⁻¹ condition.

Figure 6

Mean (±SEM) chow intake and bodyweight of female and male rat cohorts exposed to repeated PG or THC during adolescence (N = 8 per group except N = 7 female/THC) and assessed during adulthood. ^* P <.05, significant difference between treatment groups, within sex.

Figure 7

(a) Mean infusions and (b) percent drug‐appropriate lever responding (Lvr discrimination) for male rats trained to self‐administer oxycodone (0.15 mg·kg⁻¹ per infusion) within 8‐hr‐extended access sessions, starting on PND112. (c) Mean (THC cohort, N = 11; PG cohort, N = 12; ±SEM) infusions during self‐administration under an FR1 schedule and (d) breakpoint values during self‐administration under a PR schedule following acute injection of THC (0.006–0.15 mg·kg⁻¹, i.p.). ^& P <.05, significantly different from the first session; ^* P <.05, significantly different from 0.006 mg·kg⁻¹ per infusion; ^# P <.05, significantly different from the 0.06 mg·kg⁻¹ per infusion.

Figure 8

(a) Mean (N = 8; ±SEM) infusions and (b) percent drug‐appropriate lever responding (Lvr discrimination) for female rats trained to self‐administer oxycodone (0.15 mg·kg⁻¹ per infusion) within 8‐hr‐extended access sessions, starting on PND112. (c) Mean (THC cohort, N = 7; PG cohort, N = 8; ±SEM) infusions during self‐administration under an FR1 schedule and (d) breakpoint values during self‐administration under a PR schedule following acute injection of THC (0.006–0.15 mg·kg⁻¹, i.p.). ^& P <.05, significantly different from the first session; ^* P <.05, significantly different from 0.006 mg·kg⁻¹ per infusion; ^# P <.05, significantly different from the 0.06 mg·kg⁻¹ per infusion.

Figure 9

Mean (adolescent repeated THC cohort, N = 6; adolescent repeated PG cohort, N = 8; ±SEM) infusions of fentanyl obtained by female rats during self‐administration under an FR1 schedule. ^* P <.05, significant main effect of group (across doses); ^# P <.05, significant group difference at a specific dose.

Figure 10

(a) Mean (±SEM) infusions and (b) lever discrimination during acquisition for the female (N = 15) and male (N = 24) animals collapsed across adolescent treatment groups. (c) Mean (±SEM) infusions obtained during the FR dose substitution and (d) breakpoints reached in the PR dose substitution procedures are also depicted. ^* P <.05, significant difference between sexes; ^& P <.05, a significant change from the first session within group; ^# P <.05, significant difference between doses, collapsed across sex.