Effect of ergovaline, loline, and dopamine antagonists on rat pituitary cell prolactin release in vitro

Cultured rat pituitary cells were studied to: determine the effects of ergovaline and loline on in vitro prolactin release; delineate the agonistic activity of these alkaloids at the D2 dopamine receptor, using 2 selective D2 dopamine receptor antagonists; and compare the efficacy of 2 dopamine receptor antagonists in reversing effects of the treatments on in vitro prolactin secretion. Ergovaline reduced in vitro prolactin release by at least 40% (P < 0.05) at concentrations of 10(-4),10(-6), and 10(-8) M. However, loline reduced (P < 0.05) prolactin release only at the highest concentration, 10(-4) M. Two standard dopamine agonists, dopamine and alpha-ergocryptine, were used to verify that the inhibitory control mechanisms of in vitro prolactin release were intact. Both compounds reduced prolactin release by at least 40% for concentrations of 10(-4), 10(-6), or 10(-8) M. Selective D2 dopamine receptor antagonists 10(-6) M, domperidone and sulpiride, reversed (P < 0.05) the effect of loline on in vitro prolactin release. However, only domperidone (10(-6) M) was able to reverse (P < 0.05) the effect of ergovaline and only at the lowest ergovaline concentration (10(-8) M). Domperidone was more effective (P < 0.05) in reversing the prolactin-suppressing effect of alpha-ergocryptine than was sulpiride. The dose-response curve for domperidone (cubic fit, P < 0.0001) indicated a threshold concentration (10(-7)M) for reversal of alpha-ergocryptine's (10(-8)M) effect on prolactin release. However, at similar concentration of sulpiride (quadratic fit, P < 0.007), a threshold level was not obtained. These data indicate that ergovaline and loline mayact as D2 dopamine receptor agonists. Additionally, domperidone seems to be a more potent drug for reversal of the alkaloids hypoprolactinenic effect in vitro than does sulpiride.