Sex, THC, and hormones: Effects on density and sensitivity of CB1 cannabinoid receptors in rats

Abstract

Background: The recent NIH mandate to consider sex as a biological variable in preclinical research has focused attention on delineation of sex differences in behavior. To investigate mechanisms underlying sex differences in Δ⁹-tetrahydrocannabinol (THC) effects, we examined the effects of sex and gonadal hormones on CB₁ receptors in cerebellum, hippocampus, prefrontal cortex, and striatum.

Methods: Adult Sprague-Dawley rats underwent gonadectomy (GDX) or sham-GDX. Half of the GDX females and males received estradiol or testosterone replacement (GDX+H), respectively. All rats were injected with vehicle or 30 mg/kg THC twice daily for 1 week before brain collection. CP55,940-stimulated [³⁵S]GTPγS and [³H]SR141716A saturation binding assays were performed.

Results: With exception of enhanced receptor activation in the hippocampi of female rats compared to males, vehicle-treated rats exhibited minimal sex differences in CB₁ receptor densities or G-protein coupling. Repeated treatment with THC resulted in pronounced CB₁ receptor desensitization and downregulation in both sexes in all brain regions with a greater magnitude of change in females.

Conclusions: These results suggest that sex differences in the density and G-protein coupling of brain CB₁ receptors may play a limited role in sex differences in acute THC effects not mediated by the hippocampus. In contrast, sex differences after repeated THC were common, with females (intact, GDX, and GDX+H) showing greater downregulation or desensitization in all four brain regions compared to the respective male groups. This result is consistent with a finding that women tend to progress to tolerance and dependence quicker than men after initiation of cannabis use.

Keywords: CB(1) cannabinoid receptor; Gonadal hormones; Sex differences; Tetrahydrocannabinol.

Figure 1.

Effects of sex and hormonal status on [³H]SR141761A saturation binding (B_max and K_d) and CP55,940-stimulated [³⁵S]GTPγS binding (E_max and EC₅₀) in the hippocampus of vehicle- (Veh) or THC-treated adult rats of both sexes. Rats were gonadally intact (i.e., sham gonadectomy) (filled black bars) or had undergone gonadectomy (GDX) with or without hormone replacement (filled grey bars and unfilled bars, respectively). Each point represents the mean (± SEM) of data for 4 rats in each condition. Based upon curves fitted to the binding values, four dependent measures were calculated: B_max (panel A), K_d (panel B), E_max (panel C), and EC₅₀ (panel D). Three-way ANOVA showed significant THC-induced decreases in B_max and E_max for hippocampus (vs. Veh). For 2-way (sex X hormonal status) ANOVAs: $ p<0.05, main effect of sex for designated measure (i.e., different between sexes when collapsed across hormonal condition). # p<0.05, main effect of hormonal condition compared to intact (i.e., hormonal condition produced different results vs intact when collapsed across sex).

Figure 2.

Effects of sex and hormonal status on [³H]SR141761A saturation binding (B_max and K_d) and CP55,940-stimulated [³⁵S]GTPγS binding (E_max and EC₅₀) in the prefrontal cortex of vehicle- (Veh) or THC-treated adult rats of both sexes. Rats were gonadally intact (i.e., sham gonadectomy) (filled black bars) or had undergone gonadectomy (GDX) with or without hormone replacement (filled grey bars and unfilled bars, respectively). Each point represents the mean (± SEM) of data for 4 rats in each condition. Based upon curves fitted to the binding values, four dependent measures were calculated: B_max (panel A), K_d (panel B), E_max (panel C), and EC₅₀ (panel D). Three-way ANOVA showed significant THC-induced decreases in B_max and E_max for prefrontal cortex (vs. Veh). For 2-way (sex X hormonal status) ANOVAs: $ p<0.05, main effect of sex for designated measure (i.e., different between sexes when collapsed across hormonal condition). # p<0.05, main effect of hormonal condition compared to intact (i.e., hormonal condition produced different results vs intact when collapsed across sex).

Figure 3.

Effects of sex and hormonal status on [³H]SR141761A saturation binding (B_max and K_d) and CP55,940-stimulated [³⁵S]GTPγS binding (E_max and EC₅₀) in the cerebellum of vehicle- (Veh) or THC-treated adult rats of both sexes. Rats were gonadally intact (i.e., sham gonadectomy) (filled black bars) or had undergone gonadectomy (GDX) with or without hormone replacement (filled grey bars and unfilled bars, respectively). Each point represents the mean (± SEM) of data for 4 rats in each condition. Based upon curves fitted to the binding values, four dependent measures were calculated: B_max (panel A), K_d (panel B), E_max (panel C), and EC₅₀ (panel D). Three-way ANOVA showed significant THC-induced decreases in B_max and E_max for cerebellum (vs. Veh). For 2-way (sex X hormonal status) ANOVAs: $ p<0.05, main effect of sex for designated measure (i.e., different between sexes when collapsed across hormonal condition). Note: y-axis scale for E_max (panel C) is different for cerebellum than for other 3 brain areas (Figures 2–4).

Figure 4.

Effects of sex and hormonal status on [³H]SR141761A saturation binding (B_max and K_d) and CP55,940-stimulated [³⁵S]GTPγS binding (E_max and EC₅₀) in the striatum of vehicle- (Veh) or THC-treated adult rats of both sexes. Rats were gonadally intact (i.e., sham gonadectomy) (filled black bars) or had undergone gonadectomy (GDX) with or without hormone replacement (filled grey bars and unfilled bars, respectively). Each point represents the mean (± SEM) of data for 4 rats in each condition. Based upon curves fitted to the binding values, four dependent measures were calculated: B_max (panel A), K_d (panel B), E_max (panel C), and EC₅₀ (panel D). Three-way ANOVA showed significant THC-induced decreases in B_max and E_max for striatum (vs. Veh). For 2-way (sex X hormonal status) ANOVAs: $ p<0.05, main effect of sex for designated measure (i.e., different between sexes when collapsed across hormonal condition). # p<0.05, main effect of hormonal condition compared to intact (i.e., hormonal condition produced different results vs intact when collapsed across sex).