Brain cannabinoid CB₂ receptors modulate cocaine's actions in mice

Abstract

The presence and function of cannabinoid CB(2) receptors in the brain have been the subjects of much debate. We found that systemic, intranasal or intra-accumbens local administration of JWH133, a selective CB(2) receptor agonist, dose-dependently inhibited intravenous cocaine self-administration, cocaine-enhanced locomotion, and cocaine-enhanced accumbens extracellular dopamine in wild-type and CB(1) receptor knockout (CB(1)(-/-), also known as Cnr1(-/-)) mice, but not in CB(2)(-/-) (Cnr2(-/-)) mice. This inhibition was mimicked by GW405833, another CB(2) receptor agonist with a different chemical structure, and was blocked by AM630, a selective CB(2) receptor antagonist. Intra-accumbens administration of JWH133 alone dose-dependently decreased, whereas intra-accumbens administration of AM630 elevated, extracellular dopamine and locomotion in wild-type and CB(1)(-/-) mice, but not in CB(2)(-/-) mice. Intra-accumbens administration of AM630 also blocked the reduction in cocaine self-administration and extracellular dopamine produced by systemic administration of JWH133. These findings suggest that brain CB(2) receptors modulate cocaine's rewarding and locomotor-stimulating effects, likely by a dopamine-dependent mechanism.

Figure 1

Effects of JWH133 on cocaine self-administration. (a) Systemic administration of JWH133 (10, 20 mg/kg, i.p., 30 min prior to testing) inhibits cocaine self-administration under FR1 reinforcement in WT (one-way ANOVA, F_2,16 = 13.09, P < 0.001) and CB₁^−/−(F_2,10 = 5.01, P < 0.05), but not CB₂^−/− (F_2,14=0.56, P = 0.58), mice. (b) Time course of JWH133’s attenuation of cocaine self-administration in WT mice on the test day. (c) Time course of recovery of cocaine self-administration in WT mice after JWH133 administration. (d) In WT mice, JWH133-induced attenuation of cocaine self-administration is prevented by pretreatment with the CB₂ receptor antagonist AM630 (10 mg/kg, i.p., 30 min prior to JWH133), but not by pretreatment with the CB₁ receptor antagonist AM251 (3 mg/kg, i.p.) (F_5,40 = 6.31, P < 0.001). Neither AM630 nor AM251 altered cocaine self-administration in WT mice. Data are means ± s.e.m. * P < 0.05, ** P < 0.01, compared to vehicle (Veh) control groups. ^### P < 0.001, compared to pre-JWH133 (−24 h) condition.

Figure 2

Effects of GW405833 or JWH133 on cocaine self-administration. (a) GW405833 (3, 10 mg/kg, i.p.) dose-dependently inhibited cocaine self-administration under FR1 reinforcement in WT mice (one-way ANOVA, F_2,6 = 20.03, P < 0.01). (b) JWH133 (10, 20 mg/kg) or AM251 (3 mg/kg, i.p.) significantly lowered the cocaine self-administration break-point under PR reinforcement in WT mice (F_3,37 = 13.83, P < 0.001). (c) Intranasal microinjections of JWH133 (50, 100 µg/nostril) dose-dependently inhibited cocaine self-administration under FR1 reinforcement (F_2,18 = 14.34, P < 0.001). (d) Intravenous injection of the same micro-quantity (100, 200 µg) of JWH133 as used intranasally had no effect on cocaine self-administration (F_2,16 = 1.59, P = 0.23). (e) Intra-NAc microinjections of JWH133 (0.3, 1, 3 µg/side) dose-dependently inhibited cocaine self-administration under FR1 reinforcement in WT mice. This inhibition was blocked by intra-NAc co-administration of AM630 (3 µg/side) (F_3,24 = 4.49, P < 0.05). (f) Intra-NAc administration of JWH133 (3 µg/side) had no effect on cocaine self-administration in CB₂^−/− mice (F_1,10 = 2.37, P = 0.15). Data are means ± s.e.m. * P < 0.05, *** P < 0.001, compared to vehicle control group.

Figure 3

Systemic administration of JWH133 (10, 20 mg/kg, i.p., 30 min prior to cocaine) dose-dependently inhibited cocaine-enhanced locomotion in WT (a, two-way ANOVA for repeated measures over time, F_2,16 = 14.45, P < 0.001) and CB₁^−/− (b, F_2,18=12.57, p<0.001), but not in CB₂^−/− (c, F_2,12 = 0.17, P = 0.85), mice. Data are means ± s.e.m. ** P < 0.01, *** P < 0.001, compared to vehicle treatment group.

Figure 4

Effects of systemic or local intra-NAc administration of JWH133 or AM630 on locomotor activity. (a) Systemic administration of JWH133 (10, 20 mg/kg, i.p.) dose-dependently inhibited locomotion in WT (one-way ANOVA, F_2,24 = 8.03, P = 0.002) and CB₁^−/− (F_2,25 = 13.44, P < 0.001) mice, but not in CB₂^−/− (F_2,14 = 3.36, P > 0.05) mice. (b) Intra-NAc microinjections of JWH133 (1, 3 µg/side) significantly inhibited locomotion in WT (F_2,14=4.17, p<0.05) and CB₁^−/− (F_2,12 = 4.91, P < 0.05), but not in CB₂^−/− (F_2,14 = 0.04, P > 0.05), mice (c) Systemic administration of AM630 failed to alter locomotion in any strain of mice. (d) Intra-NAc administration of AM630 (1, 3, 10 µg/side) significantly augmented locomotion in WT (F_3,21 = 4.62, P < 0.05) and CB₁^−/− (F_2,12 = 10.57, P < 0.01), but not in CB₂^−/−(F_2,14 = 0.05, P > 0.05), mice. Data are means ± s.e.m. * P < 0.05, ** P < 0.01, compared to vehicle control group.

Figure 5

Systemic administration of JWH133 (3, 10, 20 mg/kg, i.p.) dose-dependently inhibited basal (a, b, c) or cocaine-enhanced (d, e, f) extracellular NAc DA in WT (a, two-way ANOVA for repeated measures over time, F_3,29 = 25.97, P < 0.001; d, F_2,19 = 4.47, P < 0.05) and CB₁^−/− (b, F_3,28 = 10.07, P < 0.001; e, F_2,16 = 4.78, P < 0.05) mice, but not in CB₂^−/− (c, F_2,23 = 0.10, P > 0.05; f, F_2,22 = 1.53, P > 0.05) mice. AM630 alone (10 mg/kg, i.p.) failed to alter NAc DA in CB₁^−/− mice, while AM630 pretreatment (10 mg/kg, i.p.) prevented JWH133-induced inhibition of NAc DA in CB₁^−/− mice (b). Data are means ± s.e.m. * P < 0.05, ** P < 0.01, *** P < 0.001, compared to pre-drug baseline.^# P < 0.05,^## P < 0.01, compared to vehicle treatment group.

Figure 6

Effects of intranasal or intra-NAc local perfusion of JWH133 or AM630 on extracellular NAc DA. (a) Intranasal administration of JWH133 (50 µg/nostril) significantly lowered extracellular DA in WT and CB₁^−/− mice, but not in CB₂^−/− mice (two-way ANOVA for repeated measures over time, F_2,15 = 10.81, P = 0.001). (b) Intra-NAc local perfusion of JWH133 lowered extracellular DA in WT and CB₁^−/− mice in a dose-dependent manner, while elevating extracellular DA in CB₂^−/− mice (F_2,18=47.00, P < 0.001). (c) Intra-NAc local perfusion of AM630 elevated extracellular DA in WT and CB₁^−/− mice, but not in CB₂^−/− mice (F_2,18 = 12.13, P < 0.001). Further, AM630-enhanced extracellular DA appears more robust in CB₁^−/− mice than in WT mice (F_1,12 = 7.50, P < 0.05). (d) Renormalized data over new baselines 1 h before JWH133 administration from the data in Panel c, illustrating that intra-NAc local perfusion of AM630 blocked JWH133’s action on extracellular DA in WT and CB₁^−/− mice. Data are means ± s.e.m. * P < 0.05, ** P < 0.01, *** P < 0.001, compared to pre-drug baseline.