Seasonal and lactational effects on the prolactin response to a dopamine antagonist and TRH in the Bennett's wallaby (Macropus rufogriseus rufogriseus).

In the Bennett's wallaby prolactin is thought to maintain lactational and seasonal quiescence and is essential for early lactation. However, plasma prolactin concentrations determined in daily or weekly samples at these times are unchanged. In the present study, female Bennett's wallabies were blood sampled at 2-hr intervals over a 24-hr period during seasonal quiescence on either natural or artificial photoperiods to determine whether a diurnal rise of prolactin occurs at this time. Prolactin concentrations did not exhibit a diurnal change. Further experiments were aimed at determining whether there was an increase in the prolactin response to a dopamine antagonist or TRH during the transition to seasonal quiescence. Nonlactating and lactating female Bennett's wallabies were treated with saline, 0.5, 1, and 2 mg of the dopamine antagonist domperidone and 100 micrograms TRH in October, December, February, and April. In both groups there was a significant elevation in plasma prolactin concentration in response to domperidone with an increasing response at each successive month. In early (October and December) and peak (April) lactation the prolactin response was greater in lactating animals. There was no significant prolactin response to TRH in lactating animals. In nonlactating wallabies, the prolactin response to TRH was increased in February. At peak lactation (April), the response to 1 mg domperidone was blocked when the dose was administered 2 hr after temporary removal of pouch young (RPY). With either larger doses (20 mg) or a 1-mg dose injected 8 hr after RPY, a significant prolactin response was observed.(ABSTRACT TRUNCATED AT 250 WORDS)