Sex, species and age: Effects of rodent demographics on the pharmacology of ∆9-tetrahydrocanabinol

Abstract

Cannabis edibles are becoming more common in an increasingly diverse population of users, and the impact of first pass metabolism on cannabis's pharmacological profile across age and sex is not well understood. The present study examined the impact of age, sex and rodent species on the effects of intraperitoneal (i.p.) delta-9-tetrahydrocannabinol (THC) and its primary psychoactive metabolite, 11-OH-THC, in rodent models of psychoactivity and molecular assays of cannabinoid receptor type-1 (CB₁) pharmacology. Like oral THC, i.p. THC also undergoes first pass metabolism. In both species and sexes, 11-OH-THC exhibited marginally higher affinity (~1.5 fold) than THC and both served as partial agonists in [³⁵S]GTPγS binding with equivalent potency; 11-OH-THC exhibited slightly greater efficacy in rat brain tissue. In ICR mice, 11-OH-THC exhibited greater potency than THC in assays of catalepsy (7- to 15-fold) and hypothermia (7- to 31-fold). Further, 11-OH-THC was more potent in THC drug discrimination (7- to 9-fold) in C57Bl/6 J mice, with THC-like discriminative stimulus effects being CB₁-, but not CB₂-, mediated. THC's discriminative stimulus also was stable across age in mice, as its potency did not change over the course of the experiment (~17 months). While sex differences in THC's effects were not revealed in mice, THC was significantly more potent in females Sprague-Dawley rats than in males trained to discriminate THC from vehicle. This study demonstrates a cross-species in the psychoactive effects of i.p. THC across sex that may be related to differential metabolism of THC into its psychoactive metabolite 11-OH-THC, suggesting that species is a crucial design consideration in the preclinical study of phytocannabinoids.

Keywords: 11-OH-THC; Age differences; Cannabinoid; Delta-9-tetrahydrocannabinol; Drug discrimination; Metabolite; Sex differences.

Figure 1.

Effects of THC (filled symbols) and 11-OH-THC (open symbols) on locomotor activity (panel A), ring immobility (panel B), and rectal temperature (panel C) in female (circles) and male (squares) adult ICR mice. Values represent the mean (± SEM) of 6 mice. Asterisks (*) indicate a significant sex X dose interaction, with a significant post hoc difference for the indicated sex and dose (p<0.05) compared to the respective vehicle (at left side of each panel). Pound signs (#) indicate a significant main effect for dose across both sexes (p<0.05), as compared to vehicle (at left side of each panel).

Figure 2.

Dose response effects of THC at the beginning of the study (unfilled symbols) and at the end of the study (filled symbols) in female (circles) and male (squares) adult C57Bl/6J mice trained to discriminate 5.6 mg/kg THC from vehicle. Top panel shows THC effects on percentage of nose-pokes that occurred on the THC-associated aperture and bottom panel shows overall response rate. Control tests with vehicle (V) and the 5.6 mg/kg THC training dose (T) are shown at the left side of the panels. Each point represents the mean (± SEM) of data for 15 female and 20 male C57Bl/6J mice, except for n=13 for %THC aperture responding for 10 mg/kg THC in the second dose-effect curve for females (i.e., data for 2 female mice were excluded due to fewer than 10 overall responses). Dollar sign ($) indicates a significant main effect for sex across all doses (p<0.05) during the first THC dose-effect curve.

Figure 3.

Effects of THC on percentage of responses that occurred on the THC-associated aperture in male (filled squares; panel A) and female (filled circles; panel B) mice trained to discriminate 5.6 mg/kg THC from vehicle in a nose poke procedure. Response rates are also shown for both sexes (panels C and D for male and female mice, respectively). Rimonabant (1 mg/kg) produced rightward shifts in the THC dose-effect curves for %THC-aperture responding in male and mice (panels A and B, respectively), with changes in response rates shown in panel C and D. In contrast, SR144528 (3 mg/kg) did not alter %THC-aperture responding in male (unfilled triangles; panel A) or female (inverted unfilled triangles; panel B) mice, with changes in response rates shown in panel C and D, respectively. Control tests with vehicle (V), 5.6 mg/kg THC (T), and rimonabant (1 mg/kg) and SR144528 (3 mg/kg) alone (SR) were conducted prior to each dose-effect curve, with results shown at the left side of the panels. Each point represents the mean (± SEM) of data for 8 mice, except for %THC aperture responding for 30 mg/kg THC (males, n=1; females, n=4), 10 mg/kg THC (females, n=7), and 30 mg/kg THC + rimonabant (males, n=7). In each case, the n was reduced because data for %THC aperture responding were excluded due to fewer than 10 overall responses. Asterisks (*) indicate a significant sex X dose interaction, with a significant post hoc difference for the indicated sex and dose (p<0.05) compared to the respective vehicle (at left side of each panel). Dollar sign ($) indicates a significant main effect for sex across all doses (p<0.05) for the specific dose-effect curve.

Figure 4.

Effects of 11-OH-THC on percentage of responses that occurred on the THC-associated aperture (top panel) and response rate (bottom panel) in female (filled circles) and male (filled squares) mice trained to discriminate 5.6 mg/kg THC from vehicle in a nose poke procedure. Rimonabant (1 mg/kg) reversed the THC-like effects of 1 mg/kg 11-OH-THC in female (unfilled circles) and male (unfilled squares) mice (top panel), with comparable decreases in response rates shown in the bottom panel. In contrast, SR144528 (3 mg/kg) did not alter %THC-aperture responding for 1 mg/kg 11-OH-THC in female (inverted unfilled triangles) or male (unfilled triangles) mice (top panel), with changes in response rates shown in the bottom panel. Control tests with vehicle (V) and 5.6 mg/kg THC (T) are shown at the left side of the panels. Each point represents the mean (± SEM) of data for 8 mice, except for %THC aperture responding for 3 mg/kg 11-OH-THC (females, n=4) and 1 mg/kg 11-OH-THC + rimonabant (females, n=7). In each case, the n was reduced because data for %THC aperture responding were excluded due to fewer than 10 overall responses. Dollar sign ($) indicates a significant main effect for sex across all doses (p<0.05) for the specific dose-effect curve. At sign (@) indicates a significant main effect of the antagonist across both sexes (p<0.05) compared to the same dose without the antagonist.

Figure 5.

Effects of THC on percentage of responses that occurred on the THC-associated lever (top panel) and response rate (bottom panel) in female (filled circles) and male (filled squares) Sprague-Dawley rats trained to discriminate 1.7 mg/kg THC from vehicle in a lever procedure. Control tests with vehicle (V) and 1.7 mg/kg THC (T) are shown at the left side of the panels. Each point represents the mean (± SEM) of data for 8 rats, except for %THC aperture responding for 3 mg/kg THC (males, n=7). Data for %THC aperture responding at 3 mg/kg THC for 1 male rat were excluded due to fewer than 10 overall responses. Pound sign (#) indicates a significant main effect for dose across both sexes (p<0.05) compared to vehicle (at left side of each panel).