Pituitary response to a dopamine antagonist at different times of the day in normal women.

In order to determine whether or not pituitary responsiveness to the dopaminergic antagonist clebopride changes during the nyctohemeral cycle, 10 healthy women with regular cycles were given 1 mg of clebopride orally at 09.00 h and 24.00 h with at least a 5 day interval between each test. In addition, 5 of the women were given a placebo instead of clebopride at midnight to evaluate the spontaneous hormonal changes. During the 24.00 h test the women had significantly higher (P less than 0.05) mean TSH basal levels. Serum prolactin (Prl) increased significantly (P less than 0.001) after clebopride administration while these changes did not occur when placebo was used instead of clebopride at midnight. The Prl response to clebopride was qualitatively similar at 09.00 h and at 24.00 h. Clebopride given at midnight induced a significant increase (P less than 0.05) in serum TSH while this change did not occur when the drug was given at 09.00 h or when placebo was given at midnight. The administration of clebopride resulted in no discernible alternations in serum LH, FSH or GH in either the 09.00 h or the 24.00 h tests. Thus, Prl responses to clebopride were similar in the morning and at midnight, TSH significantly increased after clebopride at midnight whereas this did not occur when the drug was given in the morning, and no significant changes were induced in LH, FSH or GH at the times studied.