In VivoGonadotropin-Releasing Hormone Release and Serum Luteinizing Hormone Measurements in Ovariectomized, Estrogen-Treated Rhesus Macaques*

Abstract

The push-pull perfusion technique was used to measure GnRH release in unanesthetized female rhesus macaques (Macaca mulatta) and to examine the dynamic relationship between GnRH release and LH levels during the estrogen-induced LH surge. Each ovariectomized macaque was anesthetized and stereotaxically fitted with a push-pull cannula directed into the median eminence (ME). After at least 1 week of recovery, each animal received an estradiol benzoate (E2B) injection (42 micrograms/kg BW) or an oil (OIL) injection and underwent push-pull perfusion of the ME and blood sampling for at least 5 h between 28 and 56 h postinjection. Continuous 10-min push-pull perfusates were collected and prepared for GnRH RIA. Peripheral venous blood samples were obtained either hourly or every 10 min, and serum LH levels were determined by Leydig cell bioassay. GnRH release was detectable and pulsatile in areas in or adjacent to the ME or arcuate nucleus. In eight OIL monkeys, GnRH pulses were regular (approximately one pulse every 60 min) and of low amplitude (14.7 +/- 12.0 pg), with a mean GnRH release rate of 4.0 +/- 1.7 pg/10 min. In five E2B-treated monkeys, GnRH release during the rising phase of the LH surge occurred as an apparent burst of high amplitude GnRH pulses. The mean GnRH release rate (37.5 +/- 17.9 pg/10 min) and mean GnRH pulse amplitude (170.0 +/- 90.0 pg) during the 5 h before the peak LH level in E2B-treated monkeys were greater than OIL values (P less than 0.025, mean release; P less than 0.05, mean amplitude). Within individual E2B-treated monkeys, hourly mean GnRH release rates were significantly correlated with LH levels during the ascending limb of the LH surge (r = 0.75 +/- 0.11; P less than 0.025). We have concluded that an increase in GnRH neurosecretion occurs in E2B-treated monkeys and that it is associated with generation of the LH surge. On the basis of our observations, we hypothesize that the primate hypothalamus, through changes in GnRH secretion, actively participates in the E2B-induced LH surge.

Actions of estradiol on discrete attributes of the luteinizing hormone pulse signal in man. Studies in postmenopausal women treated with pure estradiol.

“…In particular, the present results are consistent with observations in the spontaneously cycling cow, sheep, and human, i.e., estimated LH pulse frequency increases in the late follicular phase as circulating concentrations of estrogenic and other hormones rise before the LH surge (12,33,34). Moreover, in the estrogen-treated ovariectomized rhesus monkey, a burstlike increase in endogenous GnRH neurosecretion occurs in conjunction with the induced LH surge (35). These experimental findings are consistent with our observations on days 5 and 10 ofaugmented LH pulse frequency in the estrogen-replaced postmenopausal setting.…”

Hypophyseal Portal Blood Collection for Analysis of GnRH Neurosecretion in Shiba Goats.

“…However, the pulsatile pattern of GnRH neurosecretion can not be detected in peripheral blood because of a rapid turnover and dilution of GnRH in the peripheral circulation. The temporal relationship between GnRH pulses and LH pulses has been documented so far in the monkey [15,16], sheep [2][3][4]17], rabbit [18], rat [19,20] and horse [21]. In the present study, we adapted the portal blood sampling technique, which had been originally developed for sheep, for use in goats and confirmed that GnRH pulses were associated with LH pulses in all the ovariectomized goats examined.…”

Microdialysis Measurement of In Vivo Gonadotropin-Releasing Hormone Secretion in the Goat.

“…The GnRH pulses in the perfusate were characterized by their rather blunt pattern of increase, which was similar to those obtained by the pushpull method [4][5][6][7] but in contrast to sharp pulses observed in sequentially collected pituitary portal blood [1-3, 17, 18]. This may arise from the similarity between the principles of the microdialysis and push-pull methods, since both methods provide information on the extracellular GnRH concentration as a summation in a given time.…”

“…In vivo GnRH secretion has been examined by either pituitary portal blood sampling [1][2][3] or the push-pull cannulae method [4][5][6][7]. However, these methods have practical disadvantages arising mainly from technical difficulties which have hindered them from being widely adopted.…”