Simultaneous administration of TRH and sulpiride caused additive but not synergistic PRL responses in normal subjects

In order to study the mode of action of TRH and sulpiride in man, we administered TRH (500 μg, iv) and sulpiride (DA D₂ receptor antagonist, 100 mg, im) simultaneously to 6 normal females (20-21 yr). Normal females showed significantly greater PRL increments and AUC in response to the combined administration compared to a single administration of each agent (P < 0.05-0.01), while the increment and AUC in response to the combination did not exceed the sum of those responses to a single administration. In contrast, the combined administration of TRH and sulpiride did not elicit an enhanced response of plasma TSH. These results indicate that the sites of action of TRH and sulpiride might be different from each other, and these agents work additively with no interaction in human lactotrophs.