Effects of progesterone on concentrations of monoamines in hypothalamic areas and plasma prolactin levels in rats.

The role of progesterone to increase prolactin (PRL) secretion on the first estrous day in pubertal rats was compared with its role in adult cyclic rats. The first estrus was induced by the administration of pregnant mare serum gonadotrophin (5 IU) at 28 days of age. A subcutaneous administration of 2.5 or 7.5 mg of progesterone/100 g body wt significantly increased the concentration of plasma PRL in pubertal rats within 4 hr. The PRL level obtained after progesterone administration was greater than that in similarly treated adult rats. The concentration of dopamine in the arcuate nucleus-median eminence (ARC-ME) in pubertal rats significantly decreased after a lower dosage of progesterone was administered, but no change was found in the preoptic area concentration. In adult estrous rats, the concentration of dopamine in the ARC-ME showed a tendency to decrease after the administration of a larger dose of progesterone (7.5 mg/100 g body wt). No change was observed in the concentrations of indoleamines in the preoptic area and ARC-ME after the administration of progesterone in both pubertal and adult rats. The concentrations of dopamine in the preoptic area and ARC-ME were lower in pubertal rats than in adults. The concentration of 5-hydroxyindole acetic acid and the ratio of 5-hydroxyindole acetic acid to 5-hydroxytryptamine in the ARC-ME were higher in pubertal rats than in adults. These results indicate that progesterone causes a greater increase in tonic PRL secretion in pubertal rats than in adult rats and that a lower hypothalamic dopamine activity and a higher serotonin activity in pubertal rats may account for these differences.