Regulation of rat dopamine transporter mRNA and protein by chronic cocaineadministration

This study describes a direct comparison of dopamine transporter (DAT) mRNA and protein, as well as its binding sites, in tissue from the same animals after chronic cocaine administration. Rats were treated twice daily with 2 5 mg/kg cocaine or with saline. After 8 days of cocaine administration, cha nges in DAT mRNA levels in the substantia nigra pars compacta and ventral t egmental area were measured by in situ hybridization, and DAT protein in th e striatum was quantified by immunoblotting. Whereas chronic cocaine treatm ent significantly reduced levels of DAT mRNA in the substantia nigra pars c ompacta and ventral tegmental area as compared with vehicle-treated control s, cocaine treatment did not alter DAT protein levels in the striatum. Furt hermore, the density of DAT binding sites was also measured in the striatum by quantitative autoradiography using two DAT radioligands, 3 beta-(4-[I-1 25] iodophenyl)tropane-2-carboxylic acid methyl ester ([I-125]RTI-55) and [ H-3]propanoyl-3 beta-(4-tolyl)tropane ([H-3]PTT). Similar to the results of immunoblotting of DAT protein, [I-125]RTI-55 and [H-3]PTT binding site lev els also remained unaltered. These results indicate a dissociation in the r egulation of DAT mRNA and its protein levels as a result of cocaine adminis tration in rats. This study also indicates that the DAT ligands [H-3]PTT an d [I-125]RTI-55 provide an accurate assessment of DAT protein levels.