FUNCTIONAL MATURATION OF THE PITUITARY-GONADAL AXIS IN THE NEONATAL FEMALE RAT

The developmental onset of the pituitary feedback response to LH/hCG-s timulated ovarian activity was studied in the neonatal rat. A single i njection of hCG (600 IU/kg BW) was administered to groups of rats betw een 1 and 10 days of age, and the responses, i.e., ovarian estradiol ( E(2)), progesterone (P), and testosterone (T) production as well as se rum LW were monitored 3 days later. The first significant increase in ovarian T and E(2) contents occurred in rats treated on Day 8 of life, and the first significant increase in P content occurred when hCG was administered at the age of 9 days. A significant decrease in serum LH , 3 days after hCG injection, was observed for the first time in anima ls treated on Day 7, but not in those aged 1-6 days. To study whether the appearance of the ovarian response to LH is dependent on FSH, rats received combined treatment with recombinant human (rec)FSH on Days 3 -8 (0.3 IU s.c. twice daily) and a single injection of hCG (600 IU/kg BW s.c.) on Day 6. The elevated FSH levels from Day 3 onward advanced the suppression of the serum LH level after hCG injection, suggesting that enhanced action of FSH promotes the appearance of functional LH r eceptors neonatally. It was concluded that 1) the negative feedback of ovarian activity on pituitary LH secretion is functional in the neona tal rat from Day 10 of life; 2) increased intraovarian T levels reflec t the androgenic dominance of this early feedback action; and 3) eleva ted postnatal levels of FSH may advance the onset of the ovarian respo nse to LH/hCG stimulation.