Pituitary LH and FSH responses 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 A.M.-P.M. variations in pituitary responsiveness were observed at any age studied. The increased pituitary response found in infantile rats did not appear to b e 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 tered. Treatment with 5a-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 signals of ovarian origin. DHT and P appear to be the steroids responsible for such a stimulatory action.
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