CB₁-independent inhibition of dopamine transporter activity by cannabinoids in mouse dorsal striatum

Cannabinoid drugs are known to affect dopaminergic neurotransmission in the basal ganglia circuitry. In this study, we used in vitro and in vivo techniques to investigate whether cannabinoid agonists and antagonist could affect dopaminergic transmission in the striatum by acting at the dopamine transporter. Incubation of striatal synaptosomes with the cannabinoid agonists WIN55,212-2 or methanandamide decreased dopamine uptake (IC₅₀ = 2.0 μmol/L and 3.1 μmol/L, respectively). A similar inhibitory effect was observed after application of the inactive WIN55,212-2 isomer, S(-)WIN55,212-3. The CB ₁ antagonist AM251 did not reverse WIN55,212-2 effect but rather mimicked it. WIN55,212-2 and AM251 partially displaced the binding of the cocaine analog [³H]WIN35,428, thus acting as dopamine transporter pseudo-substrates in the high micromolar range. High-speed chronoamperometry measurements showed that WIN55,212-2 (4 mg/kg, i.p.) caused significant release of endogenous dopamine via activation of CB₁ receptors, followed by a reduction of dopamine clearance. This reduction was CB₁-independent, as it was mimicked by S(-)WIN55,212-3. Administration of AM251 (1 and 4 mg/kg, i.p.) increased the signal amplitude and reduced the clearance of dopamine pressure ejected into the striatum. These results indicate that both cannabinoid agonists and antagonists inhibit dopamine transporter activity via molecular targets other than CB₁ receptors.