Search for Neural Substrates Mediating Inhibitory Effects of Oestrogen on Pulsatile Luteinising Hormone‐Releasing Hormone Release In Vivo in Ovariectomized Female Rhesus Monkeys (Macaca mulatta)

Abstract

Neural substrates mediating the negative feedback effects of oestrogen on luteinising hormone-releasing hormone (LHRH) release were studied using the in vivo push-pull perfusion method in female rhesus monkeys. Twelve long-term ovariectomized female monkeys were implanted with Silastic capsules containing 17beta-oestradiol 2 weeks before the experiments and, on the day of the experiment, oestradiol benzoate (EB, 50 microg/kg) or oil was subcutaneously injected. Push-pull perfusate samples from the stalk-median eminence were collected in 10-min fractions from 4 h before to 18-20 h after EB or oil injection. LHRH and neuropeptide Y (NPY) levels in the same perfusates were measured by radioimmunoassay, and glutamate and GABA in the same perfusates were assessed by high-performance liquid chromatography (HPLC). The results indicate that EB significantly suppressed LHRH release (P < 0.005) starting within 2 h after EB, and continued for 18 h or until the experiment was terminated. Pulse analysis suggested that oestrogen suppressed the pulse amplitude, but not pulse frequency, of LHRH release. By contrast, EB did not alter any parameters (mean release, pulse amplitude or frequency) of pulsatile NPY release throughout the experiment. HPLC analysis further suggested that neither glutamate nor GABA levels in the stalk-median eminence were changed with oestrogen-induced LHRH suppression. Oil treatment did not alter LHRH, NPY, GABA and glutamate levels. It is concluded that oestrogen induces suppression of pulsatile LHRH release within 2 h, but the inhibitory effect of oestrogen on LHRH release does not appear to be mediated by NPY, GABAergic, or glutamatergic neurones.

Neuroendocrine Control of Gonadotropin Secretion

“…In contrast, a similar treatment inhibited GnRH concentrations by about 50% in samples collected from the median eminence of OVX monkeys pretreated with an E 2 implant that produced follicular-phase concentrations of this steroid (Figure 33.6, study b). 188 It should also be noted that all three of these studies used acute injections of estradiol benzoate that produced supraphysiological levels of E 2 , so the relevance of the data to the normal negative feedback actions of E 2 is unclear.…”

“…Variabilities in data from sheep (SED) are presented as the standard error of mean of difference among groups from analysis of variance. Data from two different studies (labeled a 187 and b 188 ) in rhesus monkeys are presented to illustrate variability in the effects of E 2 . Data from rats compare mean GnRH values at 1300 h from OVX and intact ("low E" on diestrus and "high E" on proestrus) animals (bars on left) 189 and from OVX and OVX + E rats (bars on right).…”

Prolonged Infusion of Estradiol Benzoate Into the Stalk Median Eminence Stimulates Release of GnRH and Kisspeptin in Ovariectomized Female Rhesus Macaques

“…This treatment was similar to that used previously (2). Dialysates (40 L) from experiment 3 were divided into 2 (20 L) samples for measurement of GnRH and kisspeptin peptide levels.…”

“…A wealth of data indicates that circulating gonadal steroids reach the preoptic area-hypothalamus and anterior pituitary and stimulate or inhibit GnRH release as well as gonadotropin release. Specifically, an increase in estradiol (E 2 ) inhibits GnRH/LH release with a latency of several hours and stimulates GnRH/LH release with a latency of approximately 24 hours (1,2). Moreover, it has been well documented that the negative feedback actions of E 2 occur at the medial basal hypothalamus (MBH), ie, in the arcuate nucleus (ARC) (3)(4)(5)(6)(7).…”

“…It is possible that the length of EB exposure of GnRH neurons is a determining factor, because the rapid EB action was examined with a 20-minute infusion of EB to the S-ME (14), whereas elevated levels of EB remain for several hours after systemic injection (2). Alternatively, the site of EB exposure within the hypothalamus may be a determinant, because direct EB infusion into the S-ME may be limited to the action in the GnRH neurons and their neuroterminals, whereas EB injected systemically diffuses extensively throughout the hypothalamus, including the ARC and S-ME (15), where interneurons mediate EB actions.…”