Developmental changes in luteinizing hormone secretion in the female guinea pig. I. Effects of ovariectomy, estrogen, and luteinizing hormone-releasing hormone.

Negative feedback regulation of LH release was studied by ovariectomy and estrogen treatment in female guinea pigs of four different ages (days 10, 30, 50, and 150). The average age at first ovulation in guinea pigs in our colony was 60.1 +/- 2.2 days (n = 33). Bilateral ovariectomy (OVX) on day 30, 50, or 150 of age resulted in a sharp increase in serum LH concentrations within 3 days, and this rise continued for 9 days, when LH values reached a plateau. In contrast, when OVX was performed on day 10 of age, only a slight elevation in LH levels was observed during the first 16 days after surgery. Twenty days after OVX (day 30 of age), LH concentrations started to increase rapidly and reached the maximum value on day 38 of age; this latter response was similar to that observed immediately after OVX in animals in the three older age groups. Serum LH concentrations after LHRH administration (2 micrograms/kg BW) were similar in ovariectomized females on days 23 and 43 of age, suggesting that the postcastration increase in LH secretion after day 26 of age was not due to a change in pituitary sensitivity. Daily injections of estradiol benzoate at doses of 0.5 and 2.0 micrograms/kg BW from 15-20 days after OVX suppressed LH levels in all four age groups. The present findings indicate that an increase in the capacity for LH release occurs around day 30 in the female guinea pig. Since this increased LH release is not due to an apparent change in pituitary sensitivity to LHRH, the endogenous release of LHRH may be inhibited before day 30 of age. These results suggest that an alteration in the regulation of LH release by the brain may occur before the onset of puberty in female guinea pigs.