Lack of persistent changes in the dopaminergic system of rats withdrawn from methamphetamine self-administration

A continuing challenge for studies in the neurobiology of drug abuse is to identify and characterize long-lived neuroadaptations that can trigger craving and relapse. We previously reported that rats that had actively self-administered methamphetamine for 5 weeks and were then withdrawn from methamphetamine for 24 h showed marked decreases in somatodendritic dopamine D(2) autoreceptor levels in the ventral tegmental area and median and dorsal part of the substantia nigra zona compacta with a corresponding down-regulation of dopamine D(1) receptors in the shell of the nucleus accumbens. The purpose of the present study was to determine whether neuroadaptive changes in dopamine receptors or transporters in the brains of rats withdrawn for 24 h from chronic methamphetamine self-administration are persistent changes that can be demonstrated long after withdrawal. A "yoked" procedure was used in which rats were tested simultaneously in groups of three, with only one rat actively self-administering methamphetamine while the other two received yoked injections of either methamphetamine or saline. In vitro quantitative autoradiography was used to determine densities of dopamine uptake sites and dopamine D(1) and D(2) receptors in different brain regions following 7- and 30-day periods of withdrawal from chronic methamphetamine self-administration. No changes in dopamine transporter and dopamine receptor numbers were detected in any brain region examined in rats self-administering methamphetamine compared with littermates receiving yoked infusions of either methamphetamine or saline. Thus, neuroadaptive changes in densities of dopamine receptors or transporters in certain brain areas may contribute to the reinforcing effects of methamphetamine during the acquisition and maintenance phases of self-administration, but do not appear to contribute to the long-lasting neuroadaptive effects of chronic methamphetamine self-administration, which may trigger craving and relapse.