Antagonism of the discriminative stimulus properties of cocaine with the combination of a dopamine Dl

Antagonism of the discriminative stimulus properties of 10 mglkg cocaine was studied in rats by use of the dopamine Dl antagonist SCH 23390 and the Dl antagonist haloperidol. Whereas SCH 23390 and haloperidol were by themselves unable to antagonize the cueing properties of cocaine, the combination of both dopamine antagonists resulted in a complete blockade of the cocaine cue. In the presence of a fixed dose of 0.01 and 0.04 mglkg haloperidol, the EDso's (it is the effective dose in 50% of the animals) of SCH 23390 for cocaine antagonism were 0.043 and 0.01 2 mgkg, respectively. Similarly, the EDso's of haloperidol in combination with 0.01 and 0.04 mg/kg SCH 23390 were 0.021 and 0.024 mgkg. The combined treatment of haloperidol and SCH 23390 resulted in strong response-rate reductions. At all combination regimens resulting in a complete blockade of the cocaine cue, response rate was reduced to less than 20% of the control values. These results indicate that the cueing properties of cocaine are both dopamine Dland Dl-mediated and that a combined antagonism of both receptor subtypes can lead to a complete antagonism of the cueing properties of cocaine which is associated with severe attenuation of response rate.