Adenohypophysial Dopamine Content and Prolactin Secretion in the Aged Male and Female Rat*

Abstract

In male rats the concentrations of dopamine (DA) in the adenohypophysis and of PRL in serum increase progressively with age. The increase in the DA content is not a consequence of reduced metabolism of DA, since the activities of the enzymes that deaminate (monoamine oxidase) and 0- methylate (catechol-O-methyltransferase) this amine are not reduced in the anterior pituitary of aged male rats; indeed, both type A and B monoamine oxidase activities are increased in the pituitaries of aged rats. The increased DA present in the aged adenohypophysis is not in a static pool but, as in young rats, is rapidly decreased by pharmacological treatments that reduce the activity of tuberoinfundibular DA neurons (7-butyrolactone), reduce the synthesis of DA in these neurons (a-methyltyrosine), or block DA receptors (haloperidol) in the adenohypophysis. All of these treatments increased serum PRL levels and reduced DA concentrations in the adenohypophysis of both young and aged rats. Similarly, in female rats, where age-related differences in BW and body composition are minimized, serum PRL concentrations and DA concentrations in the adenohypophysis of both young and aged rats changed in the same way after the administration of a DA agonist (apomorphine) or a DA antagonist (haloperidol). Apomorphine reversed the a-methyltyrosine-induced increase in serum PRL concentrations and the decrease in anterior pituitary DA content in both young and aged rats, although the latter animals appear to be more responsive to this drug. Similarly, aged rats were more sensitive to the increase in serum PRL concentrations and the decrease in anterior pituitary DA content caused by haloperidol. The actions of acute and chronically administered haloperidol are reversible, with the reduced DA content of the adenohypophysis and the elevated serum concentrations of PRL returning to respective pretreatment levels in both young and aged rats once treatment stops. The time course for recovery of serum PRL levels and anterior pituitary DA contents after both the acute and chronic haloperidol treatment is of longer duration in the aged rat, which is consistent with a decreased rate of clearance of haloperidol in these animals. Twelve daily injections of bromocriptine, a DA agonist, reduced serum PRL and anterior pituitary DA concentrations in both young and aged rats, and these effects persisted for up to 7 days after the injections were stopped. Thus, unlike recovery from chronic haloperidol treatment, the bromocriptineinduced decrease in serum PRL and anterior pituitary DA concentrations did not return to the previous high pretreatment levels in the aged rat. This suggests that bromocriptine changes the incorporation or storage of DA in the aged pituitary. (Endocrinology116: 1316-1323,1985)