Differential Sensitivity of A2A and Especially D2 Receptor Trafficking to Cocaine Compared with Lipid Rafts in Cotransfected CHO Cell Lines. Novel Actions of Cocaine Independent of the DA Transporter

The effects of low and high concentrations of cocaine have been studied in vitro on the trafficking of plasma membrane A_2A and D₂ immunoreactivities in previously characterized A_2A-D₂ CHO cell lines. Receptor double immunofluorescence staining was performed with D₂ and A_2A antibodies, planar lipid rafts immunolabeling with biotinylated cholera toxin subunit B and membrane invaginations with an anti-caveolin-1 antibody. A computer-assisted image analysis demonstrated a substantial and highly significant rise of membrane-associated D₂ immunoreactivity (IR) after 8 h of exposure to a low concentration of cocaine (150 nM). At this low concentration of cocaine, there was also an increase of membrane associated A_2A immunoreactivity but smaller and less significant. However, this increase became considerably larger and highly significant at 150 μM at which concentration the rise of D₂ immunoreactivity had begun to disappear. It may be suggested that an allosteric action of cocaine at 150 nM on the D₂ receptors may primarily increase the insertion of D₂ monomers, homomers and also of a subpopulation of A_2A-D₂ heteromers from the cytoplasm into the plasma membrane due to the conformational change induced by cocaine in the D₂ receptor. The planar lipid rafts and the caveolae are only affected by the higher concentrations of cocaine. It is proposed that changes in D₂ and A_2A-D₂ trafficking induced by allosteric actions of cocaine at D₂ receptors may contribute to the alterations of D₂ signaling found in cocaine abusers.