Effects of lactation and season on plasma prolactin concentrations and response to bromocriptine during lactation in the Bennett's wallaby (Macropus rufogriseus rufogriseus).

Prolactin concentration was measured in plasma collected each week for 13 months from lactating and non-lactating Bennett's wallabies (Macropus rufogriseus rufogriseus). In non-lactating animals, prolactin concentrations decreased towards the end of the study but such changes did not appear to fit a seasonal pattern. Prolactin concentrations were low during early lactation and at a similar level to non-lactating animals, increased significantly during late pouch life (February-May), and then returned to non-lactating levels at a time coincident with permanent exit of the joey from the pouch. Temporary removal of joeys from their mothers in April was followed by a rapid decline in prolactin concentrations which remained low for 24 h until the joey was returned to its mother, whereupon prolactin concentrations increased significantly within 2 h. The effect of a single injection of bromocriptine (5 mg/kg) on lactation, embryonic diapause and plasma prolactin concentrations was examined at two stages of lactation. In November (lactational diapause), bromocriptine had no effect on prolactin concentrations but two out of four suckling joeys died on days 13 and 14 after treatment, and three out of four females gave birth on days 27, 27 and 28. Bromocriptine treatment in April (seasonal diapause) was followed by a significant reduction in prolactin concentrations and reduced growth rate of joeys belonging to treated females. New births were not observed. In view of the effect of bromocriptine on plasma prolactin concentrations in late lactation and the demonstration that domperidone (a dopamine antagonist) significantly increases plasma prolactin concentrations, it would seem that dopamine can act as a prolactin inhibitory hormone in this as in other mammalian species.