Pituitary responsiveness to LH-RH during gestation and puerperium in the rhesus monkey

Pituitary gonadotropin response to luteinizing hormone-releasing hormone (LH-RH; 150 micrograms, administered intravenously) was studied during pregnancy and the early postpartum period in rhesus monkeys. Challenge tests were performed approximately every three weeks during pregnancy and every two weeks after delivery, in breast-feeding and non breast-feeding animals. It was found that the response to the releasing factor was completely abolished as early as day 20 after conception. This absence of pituitary responsiveness to LH-RH continued throughout pregnancy. The return of the response during the puerperium occurred as early as day 15 post-delivery in non-breast-feeding monkeys, but remained abolished up of the response to LH-RH in FSH and LH concentration was observed, in that LH levels increased 70%-90%, whereas FSH levels increased only 20%-30%. Serum prolactin (PRL) levels in breast-feeding animals presented a slight increase the first week post-delivery and thereafter, and decreased gradually until they reached normal levels four weeks post-delivery. In contrast, concentrations of serum PRL in non-breast-feeding animals decreased abruptly to normal levels within the first week post-delivery. These results demonstrate that, due to its similarity to the human, the rhesus monkey is an excellent animal model in which to study pituitary function during pregnancy and the puerperium.