Haloperidol increases prolactin release and cyclic AMP formation in vitro: inverse agonism at dopamine D2 receptors?

Haloperidol (30 nM, 3 microM) was found to increase prolactin release from GH4C 1 cells transfected with the D2 receptor cDNA (GH4ZR 7) and from wild-type (untransfected) GH 3 cells, but not from wild-type GH4C 1 cells. In addition, haloperidol (3 microM) stimulated cAMP formation in GH 3 cells. It is suggested that haloperidol may act as an inverse agonist rather than as a neutral antagonist at dopaminergic D2 receptors.