Effects of the training dose on generalization of morphine stimulus to clonidine.

The relationship between the stimulus properties of morphine and clonidine was tested in rats trained to discriminate morphine sulfate (4, 2 or 1 mg/kg) from saline in a two-lever food-rewarded task. The response trained in the low dose group generalized to low doses of clonidine (0.125 to 0.5 mg/kg) whereas the response trained with the high dose of morphine generalized only to higher doses of clonidine (0.625 to 1.0 mg/kg). Naloxone blocked the generalization in the low dose group but only partially blocked it in the high dose group. Yohimbine blocked the generalization to clonidine in the high morphine dose group and reversed the response rate suppressant effect of clonidine in all groups.