Discriminative stimulus properties of clonidine: substitution by ergot derivatives.

Bromocriptine, lergotrile and lisuride but not apomorphine, ergonovine or lysergic acid diethylamide (LSD) mimicked clonidine in rats trained to discriminate this compound (0.02 mg/kg) from saline. BC 105, ketanserin and haloperidol failed to block the clonidine cue. Yohimbine was an effective antagonist of clonidine but not of the lisuride substitution for clonidine. Since dopamine does not appear to be involved in the stimulus effects of clonidine, the behavioral similarities between clonidine and some ergots may be related to alpha-adrenoceptor stimulation; however, the role of blood pressure changes in the stimulus effects of all of these compounds should not be overlooked.