Pulsatile LH Secretion during Sexual Maturation in the Female Sheep: Photoperiodic Regulation in the Presence and Absence of Ovarian Steroid Feedback as Determined in the Same Individual

Abstract

This study in the female lamb determined if photoperiod influences pulsatile LH secretion before puberty. Moreover, we reevaluated the hypothesis that the photoperiod-modulated decrease in responsiveness to ovarian steroid inhibition which results in increased pulsatile LH secretion during sexual maturation reflects an increase in direct central nervous system ‘drive’ of gonadotropin secretion. The experimental approach was to monitor pulsatile LH secretion in the presence and absence of estradiol negative feedback during development in the same individuals. This was accomplished by the periodic replacement and removal of constant-release estradiol capsules every 3 weeks in ovariectomized lambs (OVX) which were raised in photoperiods that delay or permit normal puberty. A new algorithm was used for identification of episodes of LH secretion. In OVX lambs in the permissive sequence of photoperiods (long days of 16L:8D until 18 weeks of age, followed by short days of 8L:16D), LH pulse frequency was low in the presence of estradiol early in life at 9 weeks of age, but increased at later ages. LH pulse frequency in the presence of estradiol feedback was not associated with that in the absence of estradiol replacement. LH pulse frequency was high throughout development in the absence of estradiol and increased further at the time when responsiveness to estradiol negative feedback decreased. In lambs raised in the inhibitory sequence of photoperiods (short days until 18 weeks of age followed by long days), LH pulse frequency in the presence of estradiol remained low throughout the duration of the experiment, but in the absence of estradiol, LH pulse frequency increased with age. In both groups, LH pulse frequency in the absence of estradiol was greater than in the presence of estradiol at all stages of development, but was lower in lambs in inhibitory photoperiod than in those in the permissive photoperiod at 6 weeks of age. Thus, photoperiod can modulate LH secretion even before the age of puberty, as determined in the absence of ovarian steroid feedback. Overall, this study reveals that photoperiod can modulate LH pulse frequency in the absence of estradiol negative feedback before the normal time of puberty, and that changes in photoperiod markedly influence the timing of the developmental decrease in responsiveness to estradiol negative feedback. The decrease in responsiveness to ovarian steroid feedback during the final stages of sexual maturation is not necessarily a consequence of a high hypothalamic drive on gonadotropin secretion because in lambs raised in an inhibitory sequence of photoperiods, the age-related increase in LH pulse frequency in the absence of estradiol is not accompanied by an increase in pulse frequency in the presence of estradiol.