Functional maturation of the pituitary-gonadal axis in the neonatal female rat.

The development onset of the pituitary feedback response to LH/hCG-stimulated ovarian activity was studied in the neonatal rat. A single injection of hCG (600 IU/kg BW) was administered to groups of rats between 1 and 10 days of age, and the responses, i.e., ovarian estradiol (E2), progesterone (P), and testosterone (T) production as well as serum LH were monitored 3 days later. The first significant increase in ovarian T and E2 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 animals 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 receptors neonatally. It was concluded that 1) the negative feedback of ovarian activity on pituitary LH secretion is functional in the neonatal rat from Day 10 of life; 2) increased intraovarian T levels reflect the androgenic dominance of this early feedback action; and 3) elevated postnatal levels of FSH may advance the onset of the ovarian response to LH/hCG stimulation.