Adenohypophysial Dopamine Content during Physiological Changes in Prolactin Secretion*

Studies were undertaken to characterize the relationship between anterior pituitary dopamine (DA) content and changes in PRL secretion that occur during physiological states in which changes in tuberoinfundibular DA neuronal activity have been previously characterized. The results of these studies are summarized as follows. Serum PRL concentrations and the anterior pituitary DA content were not different between male and diestrous female rats, an unexpected result considering that tuberoinfundibular DA neuronal activity in female rats is 2- to 3-fold greater than that in male rats. The surge of PRL secretion during the afternoon of proestrus was accompanied by a significant decrease in the anterior pituitary content of DA. Increases in PRL secretion that occur early in pregnancy and during suckling were accompanied by a decrease in tuberoinfundibular DA neuronal activity and a decrease in the DA content in the anterior pituitary. Restraint stress increased PRL secretion and reduced the anterior pituitary DA content in both male and female rats, although only female rats respond to immobilization with a decrease in tuberoinfundibular DA neuronal activity. In both male and female aged rats, there were increases in serum PRL concentrations and the anterior pituitary DA content, despite the fact that tuberoinfundibular DA neuronal activity is reduced in the aged rat. Dehydration induced by water deprivation was associated with a small increase in serum PRL concentrations and a large increase in anterior pituitary DA content; similar water deprivation regimens have induced no change in indices of tuberoinfundibular DA neuronal activity. These results demonstrate that there is not a simple relationship among physiologically induced changes in PRL secretion, anterior pituitary DA content, and tuberoinfundibular DA neuronal activity. The results also indicate that the content of DA in the anterior pituitary cannot be used as a reliable index of tuberoinfundibular DA neuronal activity.