Luteinizing Hormone-Releasing Hormone Release during the Rat Estrous Cycle and after Ovariectomy, as Estimated with Push-Pull Cannulae*

Abstract

A push-pull perfusion (PPP) system was used to carry out the first examination of LHRH release from the mediobasal hypothalami (MBH) of conscious, freely moving rats during stages of the estrous (E) cycle and after ovariectomy (Ovx). Female rats received push-pull cannula (PPC) implants into the MBH and then were allowed to recover for 2–10 weeks before PPP experiments. During that time, E cycles were determined by daily inspection of vaginal smears. After exhibiting two consecutive E cycles, rats were fitted with indwelling jugular catheters between 0830–1030 h and subjected to PPP of the MBH and hourly bleeding for more than 6 h. LHRH and LH levels were determined by RIA in perfusates and plasma, respectively. PPP and bleeding sessions were performed on the afternoon of proestrus (Pro; n – 10), diestrous day I (DI; n – 5), diestrous day II (DII; n – 5), estrus (E; n – 5), or more than 28 days after Ovx (n – 5). LHRH output was detectable in at least some samples in all rats whose PPC tips resided within 0.5 mm of the rostrolateral median eminence. Basal LHRH output (<0.2 to 1.2 pg.12 min) appeared to be pulsatile in all groups. During Pro, higher LHRH levels permitted pulse frequency determinations (average Pro interpulse interval, 48.0 min). Overall LHRH output during Pro was elevated (P < 0.01) compared to that in all other groups and was distinctly biphasic; a putative priming pulse (P < 0.05) occurred 2–3 h before the occurrence of a larger main peak (1.6–7.0 pg-12 min) at approximately 1600–1730 h (lights on from 0500–1900 h). Proestrous (Pro) LH levels in rats bearing PPC implants were only 10–30% of those in intact rats regardless of PPP. Nonetheless, these rats did exhibit temporally normal Pro LH surges concident with LHRH release. In DI and DII rats, LHRH pulse amplitude increased moderately for a brief period in the late afternoon (P < 0.01 only when data was normalized to the largest peak). LHRH output in E rats was low and mostly undetectable. Pulse amplitude in ovariectomized (Ovx) rats remained constant and low throughout the afternoon, while LH levels were elevated to typical post-Ovx values. We conclude from this study that (1) a biphasic LHRH surge occurs on the afternoon of Pro which may act to prime and then stimulate pituitary gonadotrophes, (2) small but significant increases in LHRH pulse amplitude occur between 1500–1900 h in DI and DII rats, but not in Ovx or E rats, and (3) LH, but not LHRH, release is increased in Ovx rats, suggesting that the negative feedback effects of ovarian steroids operate primarily at the level of the pituitary gland.