Fig. 2. Effects of repeated cocaine administration on CB₁ and CB₂ mRNA expression as measured by qRT-PCR.

a Repeated cocaine administration produced robust locomotor sensitization at both 10 and 20 mg/kg doses. b Cocaine, at 20 mg/kg only, significantly increased CB₂ mRNA in the striatum as measured with the mCB₂-KO probe; c cocaine, at 10 or 20 mg/kg, failed to alter CB₂ mRNA expression in the PFC. d Cocaine, at both doses, significantly increased CB₂ mRNA expression in spleen as measured at 24 h after the last cocaine injection. In contrast, repeated cocaine failed to alter CB₁ mRNA expression except that cocaine, at 10 mg/kg, tended to decrease CB₁ mRNA expression in the PFC (c). e Repeated heroin administration also produced robust locomotor sensitization in a dose-dependent manner. f Repeated heroin-treated mice displayed a reduction in CB₂ mRNA expression in the striatum. *P < 0.05, compared to saline group.