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

Abstract

Our recent study indicates that a brief infusion (20 min) of estradiol (E2) benzoate (EB) into the stalk-median eminence (S-ME) stimulates GnRH release with a latency of approximately 10 minutes. In contrast to the effect induced by a brief infusion of EB, it has previously been shown that systemic EB administration suppresses release of GnRH, kisspeptin, and LH with a latency of several hours, which is known as the negative feedback action of E2. We speculated that the differential results by these 2 modes of EB administration are due to the length of E2 exposure. Therefore, in the present study, the effects of EB infusion for periods of 20 minutes, 4 hours, or 7 hours into the S-ME of ovariectomized female monkeys on the release of GnRH and kisspeptin were examined using a microdialysis method. To assess the effects of the EB infusion on LH release, serum samples were also collected. The results show that similar to the results with 20-minute infusion, both 4- and 7-hour infusions of EB consistently stimulated release of GnRH and kisspeptin from the S-ME accompanied by LH release in the general circulation. In contrast, sc injection of EB suppressed all 3 hormones (GnRH, kisspeptin, and LH) measured. It is concluded that regardless of the exposure period, direct E2 action on GnRH and kisspeptin neurons in the S-ME, where their neuroterminals are present, is stimulatory, and the E2-negative feedback effects do not occur at the S-ME level.

Neuroestradiol in regulation of GnRH release

“…The results from a subsequent study indicate that the exposure period does not account for the difference in the 2 modes of EB administration: EB infusion into the median eminence for a prolonged period (4 or 7 hours) keeps stimulating release of GnRH and kisspeptin over several hours (Kenealy et al, 2015), similar to the release observed with a brief (10 or 20 minutes) infusion (Kenealy et al, 2013, Fig. 2).…”

“…Furthermore, a similar brief infusion of EB also stimulated kisspeptin release (see Kenealy et al, 2015). Kisspeptin neuroterminals and cell bodies are also seen in the median eminence (Ramaswamy et al, 2009).…”

“…Likewise, the EB concentration at the median eminence does not likely differentiate the 2 modes of EB administration. Continuous or brief infusion of EB at doses ranging from 1 nM to 1 μM, which translates into one tenth of the concentration (0.1 to 100 nM) in the tissue at the median eminence due to the permeability of microdialysis membrane (Frost et al, 2008), stimulated release of GnRH and kisspeptin, whereas subcutaneous injection of EB at 50 μg/kg, which induces concentrations of 300 to 500 pg/ml or 1-1.5 nM in the circulation (Mizuno and Terasawa, 2005) suppressed both GnRH and kisspeptin release (Kenealy et al, 2015). In contrast, the site of EB exposure is critical for stimulating the release of GnRH and kisspeptin by EB.…”

“…In contrast, the site of EB exposure is critical for stimulating the release of GnRH and kisspeptin by EB. In the EB infusion experiment, the region exposed to EB is restricted within the median eminence, as the tips of microdialysis probes (determined by x-ray ventriculography) are all located within the median eminence (Kenealy et al, 2015), whereas systemically administered EB reaches a large portion of the brain as well as the pituitary gland, as shown by autoradiography (Pfaff et al, 1976). It is speculated that systemically administered EB binds to estrogen receptor alpha (ERα, ESR1) located in infundibular (arcuate) kisspeptin and neurokinin-B neurons (Rance, 2009) resulting in inhibitory action, whereas direct infusion of EB into the GnRH and kisspeptin neuroterminals induces rapid actions mediated through a membrane-initiated mechanism.…”

“…Using an experimental model, it has been shown that systemic injections of EB to OVX monkeys induce GnRH and LH surges, i.e., sustained GnRH and LH elevations for over 36 hours with a latency of ~24 hours (Levine et al, 1985;Xia et al, 1992;Yamaji et al, 1973;Terasawa, 1985, Schultz andTerasawa, 1988). Because our observations show that prolonged infusion of EB into the median eminence keeps GnRH release elevated for several hours (Kenealy et al, 2015) and that the sustained elevation of GnRH release is reminiscent of the preovulatory GnRH surge, we hypothesized that neuroestradiol release is necessary for the estrogen-induced LH/GnRH surge.…”