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It is well known that prolactin secretion is inhibited by dopamine acting via the pituitary dopamine D₂ receptor. Dopamine D₁ receptor analogues also affect prolactin levels although the mechanisms and physiological significance are poorly understood. The present study of the ewe was undertaken to characterize the effects of the D₁ receptor agonist SKF 38393 and antagonist SCH 23390 on prolactin in this species and to determine whether the prolactin response to both drugs requires an intact hypothalamo-pituitary axis. Ovariectomized ewes were injected intravenously with vehicle, 0·2, 2 or 20 mg SKF 38393 (D₁ agonist) or SCH 23390 (D₁ antagonist). At the 20 mg dose, plasma prolactin concentrations were significantly (P <0·01) increased by each drug and returned within an hour to control levels. When injected directly into the lateral ventricles of the brain (intracerebroventricular (i.c.v.) injection), a 100-fold lower dose of SKF 38393 (0·2 mg; P <0·05) was sufficient to stimulate prolactin secretion. In contrast, i.c.v injection of SCH 23390 (0·02 and 0·2 mg) had no effect on prolactin levels and at no dose was there evidence for suppression of prolactin levels. These results are in accord with earlier studies in the rat which suggested that the D₁ agonist stimulated prolactin secretion via a direct effect on central dopamine D₁ receptors whereas the

D₁ antagonist interacted with the pituitary dopamine D₂ receptor to increase prolactin secretion. In a further experiment this hypothesis was tested in hypothalamo-pituitary disconnected ewes which were infused with dopamine (0·5 µg/kg per min) for 3 h. After 2 h of the dopamine infusion, animals were challenged with intravenous injections of the vehicle, 20 mg SKF 38393, 20 mg SCH 23390 or 2 mg domperidone (dopamine D₂ antagonist). Infusion of dopamine was followed by a significant (P <0·05) decline in prolactin concentrations so that after 2 h prolactin levels were 40% of the preinfusion value. Following injection of the vehicle, SCH 23390 or SKF 38393, prolactin levels continued to decline for the remainder of the experiment. As expected, injection of domperidone was followed by a significant (P <0·05) increase in prolactin to reach peak levels after 30 min. These results demonstrate that peripheral injections of the dopamine D₁ agonist SKF 38393 or antagonist SCH 23390 increase prolactin secretion in the ewe. The prolactin response to either drug requires an intact hypothalamo-pituitary axis indicating that SKF 38393 and SCH 23390 act at some central site(s) which is linked with hypothalamic secretion of prolactin-releasing or -inhibiting factors.

Journal of Endocrinology (1993) 137, 457–464