Mechanisms regulating hormone release and the duration of dioestrus in the lactating rat

Lactation in rats nursing seven pups was associated with a period of dioestrus lasting for 3 weeks, reduced LH secretion, hyperprolactinaemia and increased serum progesterone levels. Removal of the litter resulted in increased LH secretion, a prompt return of oestrus and termination of the prolactin-dependent luteal phase. Administration of domperidone (2.5 mg/day), a dopamine receptor antagonist, to rats deprived of their litters on day 1 or 9 post partum maintained hyperprolactinaemia and delayed the reappearance of oestrus. Administration of bromocriptine (0.5 mg/day), a dopamine receptor agonist, to lactating rats with suckling pups suppressed prolactin secretion and advanced oestrus, females in the middle of lactation being considerably more sensitive to prolactin suppression than those in the early post-partum period. Cross-fostering experiments revealed that the greater sensitivity to bromocriptine of mothers in late lactation was due to their lactational age rather than to the age of the offspring. Similarly, the length of lactational dioestrus was not affected either by giving newborn pups to females in the middle of lactation or by giving 9-day-old pups to newly parturient females.