Gary A. Gudelsky

Dopaminergic control of prolactin secretion in the aging male rat

Age-related changes in reproductive processes are well documented3,7,17,18 and, in some cases, are associated with changes in the secretion of pituitary and gonadal hormones 6,9,14,16,19. For example, Meites et al. 9 have shown that in old male rats, serum prolactin concentrations are increased relative to those in young animals. Since there is evidence that dopamine released from tuberoinfundibular neurons into hypophysial portal blood exerts a tonic inhibitory action on the secretion of prolactin 10, it seems reasonable to suspect that the elevated concentration ofprolactin in the serum of old rats is the result of impaired hypothalamic secretion ofdopamine. Therefore, in the present investigation, we have addressed the issue of whether the elevated serum prolactin concentration in old rats is associated with a reduction in the secretion of dopamine into hypophysial portal blood. Adult male rats 2-3 months of age (young animals) and 24-26 months of age (old animals) of the Long-Evans strain were used in these experiments. The rats were maintained in an air-conditioned (22-24 °C) room with controlled lighting (lights on at 03.00 h, lights off at 17.00 h). The animals had free access to food and water. In order to minimize stress-related effects on prolactin secretion at the time the rats were killed, the animals used for the determination of serum prolactin concentrat ons and brain dopamine concentrations were carried by hand on each of the 3 days prior to sacrifice to an adjacent room in which they were to be decapitated 11. At the time of decapitation, the trunk blood was collected, and the brains were excised. The serum from the trunk blood was removed and stored at -20 °C until assayed for prolactin according to the method outlined by the Rat Pituitary Program of the