Opposing roles of CB1 and CB2 cannabinoid receptors in the stimulant and rewarding effects of cocaine

Abstract

Background and purpose: The endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG) bind to CB₁ and CB₂ cannabinoid receptors in the brain and modulate the mesolimbic dopaminergic pathway. This neurocircuitry is engaged by psychostimulant drugs, including cocaine. Although CB₁ receptor antagonism and CB₂ receptor activation are known to inhibit certain effects of cocaine, they have been investigated separately. Here, we tested the hypothesis that there is a reciprocal interaction between CB₁ receptor blockade and CB₂ receptor activation in modulating behavioural responses to cocaine.

Experimental approach: Male Swiss mice received i.p. injections of cannabinoid-related drugs followed by cocaine, and were then tested for cocaine-induced hyperlocomotion, c-Fos expression in the nucleus accumbens and conditioned place preference. Levels of endocannabinoids after cocaine injections were also analysed.

Key results: The CB₁ receptor antagonist, rimonabant, and the CB₂ receptor agonist, JWH133, prevented cocaine-induced hyperlocomotion. The same results were obtained by combining sub-effective doses of both compounds. The CB₂ receptor antagonist, AM630, reversed the inhibitory effects of rimonabant in cocaine-induced hyperlocomotion and c-Fos expression in the nucleus accumbens. Selective inhibitors of anandamide and 2-AG hydrolysis (URB597 and JZL184, respectively) failed to modify this response. However, JZL184 prevented cocaine-induced hyperlocomotion when given after a sub-effective dose of rimonabant. Cocaine did not change brain endocannabinoid levels. Finally, CB₂ receptor blockade reversed the inhibitory effect of rimonabant in the acquisition of cocaine-induced conditioned place preference.

Conclusion and implications: The present data support the hypothesis that CB₁ and CB₂ receptors work in concert with opposing functions to modulate certain addiction-related effects of cocaine.

Linked articles: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Figure 1

Roles of CB₁ and CB₂ receptors in cocaine‐induced hyperlocomotion. (A) The CB₁ receptor antagonist, rimonabant (Rim; 10 mg·kg⁻¹), prevented the hyperlocomotion induced by cocaine 20 mg·kg⁻¹ (n = 8, 10, 8, 8, 12). (B) The CB₂ receptor agonist, JWH133 (JWH; 20 mg·kg⁻¹), prevented cocaine‐induced hyperlocomotion (n = 7, 7, 7, 8, 8). (C) A combination of sub‐effective doses of rimonabant (3 mg·kg⁻¹) and JWH133 (10 mg·kg⁻¹) prevented cocaine hyperlocomotion (n = 7, 8, 7, 7, 8). (D) The CB₂ receptor antagonist, AM630 (AM; 10 mg·kg⁻¹), reversed the inhibitory effect of rimonabant (10 mg·kg⁻¹) on cocaine‐induced hyperlocomotion (n = 8, 10, 8, 8, 12). (E) Rimonabant (10 mg·kg⁻¹), but not AM630 (10 mg·kg⁻¹), prevented ACEA (5 mg·kg⁻¹)‐induced hypolocomotion (n = 7,6,6,7). (F) Rimonabant (10 mg·kg⁻¹), AM630 (10 mg·kg⁻¹) and JWH133 (20 mg·kg⁻¹) did not interfere with basal locomotor activity as compared to the vehicle (Veh; cremophor–ethanol–saline, 1:1:18); n = 6. Data shown are individual values with means ± SEM; n as indicated. *P < 0.05, significantly different from vehicle‐vehicle group; ^# P < 0.05, significantly different from vehicle‐cocaine or vehicle‐ACEA groups; ANOVA followed by Newman–Keuls test.

Figure 2

Effect of inhibitors of endocannabinoid hydrolysis, alone or in combination with a subthreshold dose of the CB₁ receptor antagonist, rimonabant, on cocaine‐induced hyperlocomotion. (A) URB597 (URB; 0.1, 0.3 and 1.0 mg·kg⁻¹), the anandamide hydrolysis inhibitor, did not change cocaine effect (n = 7, 6, 8, 6, 8). (B) A similar pattern was observed after treatment with the 2‐AG hydrolysis inhibitor, JZL184 (JZL; 1.0, 3.0 and 10 mg·kg⁻¹) (n = 6, 7, 6, 7, 7). (C) Combined treatment with a subthreshold dose of rimonabant (Rim; 3 mg·kg⁻¹) and URB597(1 mg·kg⁻¹) did not change cocaine‐induced hyperlocomotion (n = 7, 7, 6, 6, 7). (D) Combined treatment with a subthreshold dose of rimonabant (3 mg·kg⁻¹), and JZL184 (10 mg·kg⁻¹), prevented cocaine‐induced hyperlocomotion (n = 8,8,10,8,9). Data shown are individual values with means ± SEM; n as indicated. *P < 0.05, significantly different from vehicle‐vehicle; ^# P < 0.05, significantly different from vehicle‐cocaine group; ANOVA followed by Newman–Keuls test.

Figure 3

Opposing roles for CB₁ and CB₂ receptors on c‐Fos expression induced by cocaine. (A) Rimonabant (Rim; 10 mg·kg⁻¹) prevented the increase in c‐Fos expression induced by cocaine (20 mg·kg⁻¹) on the shell region of the nucleus accumbens, an effect reversed by previous treatment with AM630 (AM; 10 mg·kg⁻¹) (n = 6/group). (B) The number of c‐Fos cells in the shell region of the nucleus accumbens correlated with the total distance moved in the open field. The best fit and confidence intervals are represented by continuous and dashed lines respectively. r = 0.6019, P < 0.05; n = 24. (C) Rimonabant (10 mg·kg⁻¹) prevented cocaine‐induced increased in c‐Fos expression on the core portion of nucleus accumbens, an effect reversed by previous treatment with AM630 (n = 6/group). (D) The number of c‐Fos cells in the core regions of the nucleus accumbens correlated with the total distance moved in the open field The best fit and confidence intervals are represented by continuous and dashed lines respectively. r = 0.5742, P < 0.05 (n = 24). *P < 0.05, significantly different from vehicle‐vehicle; ^# P < 0.05, significantly different from vehicle‐cocaine group; ANOVA followed by Newman–Keuls test.

Figure 4

Representative photomicrographs of coronal sections (40 μm) of the nucleus accumbens showing c‐Fos immunoreactivity. For each treatment, (vehicle,Veh; rimonabant, Rim; cocaine, Coca) the total area of this structure is presented at a 10× magnification, whereas the shell and core subregions are presented at a 63× magnification. c‐Fos positive cells are identified as dark dots. Scale bars: 100 μm.

Figure 5

Effect of cocaine on endocannabinoid levels. Treatment with cocaine (20 mg·kg⁻¹) did not change the levels of anandamide in the (A) striatum, (B) hippocampus or (C) prefrontal cortex. Data shown are individual values with means ± SEM; n = 5. A similar lack of effect was observed for 2‐AG levels in these structures (D, E, and F). Data shown are individual values with means ± SEM; n = 5. Student's t‐test.

Figure 6

Effect of CB₁ and CB₂ receptor antagonism on cocaine‐induced CPP. (A) Rimonabant (Rim; 10 mg·kg⁻¹), prevented cocaine‐induced place preference, an effect reversed by previous treatment with AM‐630 (10 mg·kg⁻¹). Data shown are individual values with means ± SEM; n = 8,8,7. *P < 0.05, significantly different from vehicle‐vehicle‐cocaine group; ^# P < 0.05, significantly different from vehicle‐Rim‐cocaine group; ANOVA followed by Newman–Keuls test. (B) CB₁ and CB₂ receptor blockade did not induce either preference or aversion. Data shown are individual values with means ± SEM; n = 8, 7, 6; ANOVA followed by Newman–Keuls test.