Abstract

Pituitary LH and FSH repsonses to synthetic LHRH as estimated by increases in plasma FSH and LH 15 and 45 min following its iv injection were enhanced during the first 2 weeks of life, reaching a maximum around day 10-15 and declining thereafter. No AM.-PM. variations in pituitary responsiveness were observed at any age studied. The increased pituitary response found in infantile rats did not appear to be caused by a slower rate of disappearance of LHRH in blood of the younger animals. Ovariectomy-adrenalectomy. (Ovx-Adrx) or Ovx at day 10, but not Adrx alone, resulted in elevated LH and FSH levels 5 days later and almost complete obliteration of the FSH response to LHRH. The LH response was not altered. Treatment with 5alha-dihydrotestosterone (DHT) but not with estradiol benzoate (EB) or testosterone propionate (TP) suppressed the post-Ovx-Adrx rise in plasma LH and FSH. Progesterone (P) potentiated the effect of DHT. Restoration of basal plasma LH and FSH levels (by DHT and/or P) restored FSH responsiveness to exogenous LHRH. EB and TP were ineffective. The LH response was slightly depressed by EB + DHT. It is concluded that the elevated plasma FSH levels in the infantile female rat may be due at least in part to a high degree of pituitary responsiveness to LHRH and/or FSH-RF brought about by steroidal signal of ovarian origin. DHT and P appear to be the steroids responsible for such a stimulatory action.