Dopaminergic and serotonergic mediation of the discriminable effects of ergot alkaloids.

The involvement of dopamine (DA) and serotonin (5-HT) neuronal systems in the discriminative stimulus effects of various ergot derivatives was assessed by administering four ergots to 36 rats which had been trained to discriminate either apomorphine (APO) or d-lysergic acid diethylamide (LSD) from saline. Lergotrile, lisuride and LSD substituted for APO (0.25 mg/kg) while bromocriptine and ergonovine (ergometrine) did not; only lisuride mimicked LSD (0.08 mg/kg). Antagonism tests showed that the DA antagonist haloperidol but not the 5-HT antagonist BC-105 (pizotifen) blocked the APO cue; both the LSD cue and the substitution of LSD for APO were blocked by BC-105 but not by haloperidol. It was concluded that DA receptor activation plays a prominent role in the discriminative stimulus effects of lergotrile and lisuride as well as APO and a secondary role in the LSD cue; 5-HT seems to be of major importance in the mediation of the effects of LSD and, to a lesser extent, lisuride. The functions of the two monoamines in the discriminable effects of bromocriptine and, particularly, ergonovine are less clear.