Intranigral administration of D2 dopamine receptor antisense oligodeoxynucleotides establishes a role for nigrostriatal D2 autoreceptors in the motor actions of cocaine.
Dopamine D2 autoreceptors found on nigrostriatal dopaminergic neurons are thought to inhibit dopamine release, tyrosine hydroxylase activation, and spontaneous firing rate. It is likely that these receptors play an important role in moderating the behavioral response to cocaine, but the lack of potent selective autoreceptor ligands has made it difficult to assess this contribution. We have developed an antisense phosphorothioate oligodeoxynucleotide (ODN) against D2 receptor mRNA, which was used to reduce levels of D2 receptors in vitro and in vivo. Unilateral administration of antisense ODN, via intracerebral cannula, into the substantia nigra of rats for several days caused dramatic contralateral rotational behavior in response to a subcutaneous injection of cocaine. This effect was maximal by 10 min after injection of cocaine and lasted for > 30 min; without cocaine, no spontaneous rotational behavior was noted. In striatal slices, the potency of sulpiride, a D2 antagonist, in enhancing electrically stimulated dopamine release was significantly reduced on the antisense-treated side; this is consistent with a decrease in the striatal D2 autoreceptor population. As measured by quantitative autoradiography, administration of antisense ODN caused a loss of approximately 40% of nigral D2 receptor [125I]iodosulpride binding, compared with the untreated side. In vitro, treatment of WERI-27 retinoblastoma cells with D2 antisense ODN at a concentration of 1 microM reduced D2 receptor levels by 57% after 3 days. The robustness of cocaine-induced rotation and the impaired ability of sulpiride to enhance dopamine release from slices suggest that nigrostriatal D2 autoreceptors play a direct role in reducing the motor response to cocaine administration. Furthermore, the absence of spontaneous rotation in antisense ODN-treated animals suggests that autoreceptor effects are masked by compensatory mechanisms during normal behavior.
Silvia, CP; King, GR; Lee, TH; Xue, ZY; Caron, MG; Ellinwood, EH
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