Pituitary response to intravenous hypothalamic releasing peptides in cynomolgus monkeys treated with contraceptive...

The secretory response of the pituitary to an iv bolus dose of hypothalamic releasing peptides (HRP) was evaluated in male and cycling (CYC) or contraceptive-treated female cynomolgus monkeys. Parenteral delivery of levonorgestrel and 17 beta-estradiol by intravaginal ring (CVR) was compared with oral administration (OC) of norgestrel and ethinyl estradiol in the diet. LH secretion was suppressed in the CVR group compared to that in other groups, and the response in males was greater than that in either CYC or OC females (P less than 0.01). Elevated plasma PRL concentrations in the CVR group during the baseline period (P less than 0.05) together with their larger pituitary weights (P less than 0.01) suggested lactotroph hypertrophy or hyperplasia compared to other groups. The plasma GH response was similar in the male, CYC, and CVR groups, but plasma GH levels increased from -15 to 0 min before HRP injection in the OC group (P less than 0.001) and continued to be higher for 15 min after HRP compared to values in the other groups (P less than 0.001), suggesting a treatment effect. Neither plasma TSH nor T4 levels were different among the groups after HRP administration, but T4 was elevated (P less than 0.01) in the OC group due to increased T4-binding globulin. The greater ACTH response 15 min after HRP treatment (P less than 0.05) in the CVR group compared to that in the other groups was associated with greater adrenal weights of the CVR females (P less than 0.05), suggesting chronic tropic stimulation. However the adrenal steroid results did not support this interpretation. We conclude that the differences in pituitary hormone secretion during these studies could be attributed to the nature of the reproductive steroid environment.