The combined effect of quipazine and haloperidol on plasma prolactin release.

The possible dopaminergic properties of the central serotonergic agonist quipazine was examined using plasma prolactin release as a model to determine whether quipazine exerts an agonist effect on only serotonin receptors or on dopamine receptors as well. Drugs acting on two different receptors have only an additive effect on the same parameter while different drugs acting on the same receptor exert a synergistic effect. Therefore we determined whether combining quipazine with haloperidol, which has no effect on serotonin receptors, would produce a synergistic or only an additive effect on prolactin release, relative to the release induced by quipazine alone. A distinct synergism was produced, suggesting that quipazine acts on both DA and 5-HT receptors. Moreover, the model used may be useful for distinguishing whether a given drug acts on one or two receptors concurrently.