Differential Sensitivity of A(2A) and Especially D (2) 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 A2A and D2 immunoreactivities in previously characterized A2A-D2 CHO cell lines. Receptor double immunofluorescence staining was performed with D2 and A2A 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 A2A 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 A2A-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 A2A-D2 trafficking induced by allosteric actions of cocaine at D2 receptors may contribute to the alterations of D2 signaling found in cocaine abusers.