Oestradiol negative feedback inhibition on LH secretion during lactation is prolonged in adolescent primiparous rhesus monkeys

Abstract

Lactational infertility in rhesus monkeys is significantly prolonged in adolescent primiparous compared with adult multiparous mothers. In order to determine if this longer period of infertility for young mothers is the result of a greater sensitivity to nursing-induced inhibition of LH release either by enhanced oestradiol negative feedback or a direct non-gonadally mediated suppression, the effects of periodic administration of oestradiol on serum LH concentrations in nursing ovariectomized adolescent primiparous (Prp; n = 5) and adult multiparous (Mlt; n = 7) mothers was assessed. Females were treated every 5-6 weeks with a 21-day time-release capsule of oestradiol which produced serum concentrations of approximately 250, 90 and 45 pmol/l by +6, +13 and +20 days after treatment. Thus, the design permitted assessment of LH and prolactin concentrations under a regime of 21 days of decreasing oestradiol levels followed by 2-3 weeks of no oestradiol treatment. Females were studied from week 3 to week 42 post partum and oestradiol treatment occurred during weeks 5, 11, 21, 26, 31, 36 and 41. Behavioural observations indicated that the amount of time mothers nursed their offspring decreased in a similar fashion throughout the lactational period for both Prp and Mlt females. LH concentrations under the 'no oestradiol' conditions progressively increased throughout lactation reaching maximum levels by week 36 post partum in a similar manner for both Prp and Mlt mothers. These data suggest that differences in fertility between adolescent and adult nursing mothers observed previously cannot be attributed to a difference in a direct non-gonadally-mediated affect on LH. With respect to oestradiol negative feedback inhibition of LH, oestradiol treatment effectively suppressed serum LH concentrations at all points during lactation up to week 31, at which time LH concentrations were maximally suppressed in both Prp and Mlt mothers at +6 days after treatment but by day +13 LH values were significantly higher in Mlt females. After oestradiol treatment during week 36, LH values were again maximally suppressed on day +6 in Prp females but not in Mlt females. However, the response of serum LH after oestradiol treatment during week 41 was similar between Prp and Mlt females at all sampling points.(ABSTRACT TRUNCATED AT 400 WORDS)