We tested this hypothesis with two experiments
We tested this hypothesis with two experiments. In the first experiment OVX monkeys implanted with an E2 Rimonabant (this results in LH levels similar to the level during the follicular phase) were treated with daily injections of letrozole for 1 week and then EB was injected systemically mimicking the preovulatory E2 increase. Results indicated that while EB injection induced full LH surges in controls, the letrozole treatment greatly attenuated EB-induced LH surges (Fig. 5, Kenealy et al., 2017). Importantly, the latency to the LH surge after EB was not affected by letrozole (Kenealy et al., 2017). These observations suggest that 1) an increase in circulating E2 triggers the LH surge, but 2) an elevated level of circulating E2 is insufficient for sustaining LH surges and E2 synthesized in the hypothalamus and/or pituitary gland is critically important for the EB-induced LH surge. There are abundant aromatase-expressing cells in the hypothalamus and anterior pituitary gland (MacLusky et al., 1986; Roselli et al., 2001; Galmiche et al., 2006; Kadioglu et al., 2008). Thus, the activity of aromatase-synthesizing cells might be influenced by circulating E2. This view is consistent with a report in female baboons showing that aromatase activity in the amygdala and preopotic area assessed by PET scan with 11C-vorozole uptake fluctuates during the menstrual cycle (Pareto et al., 2013). In the second experiment, we examined whether blocking locally synthesized E2 in the hypothalamus by letrozole infusion into the median eminence reduces the EB-induced GnRH surge. Although in the previous study (Kenealy et al., 2013) we showed that neuroestradiol in the hypothalamus is involved in regulation of pulsatile GnRH release in OVX females, we had not examined the involvement of neuroestradiol when there is elevated circulating E2. In this experiment, we used a single high EB injection model in OVX monkeys for the induction of GnRH surges. Results indicate that EB-induced GnRH surges are attenuated within 60 min of the letrozole infusion into the median eminence (Fig. 6A vs. C, Kenealy et al., 2017). Similarly, EB-induced kisspeptin surges are also reduced in the presence of letrozole, when compared to those in vehicle control (Fig. 6B vs. D, Kenealy et al., 2017). We speculate that letrozole infusion reduces the EB-induced E2 release, as shown in the previous study (Kenealy et al., 2013). We now plan to measure E2 release in the median eminence in future experiments. The results from these two experiments provide a new insight into the well-established positive feedback effects of E2. An increase in circulating E2 feeds back to interneurons, such as kisspeptin neurons, that stimulate GnRH neurons resulting in the GnRH surge (Dungan et al., 2007; Clarkson et al., 2008). At the same time, elevated circulating E2 stimulates E2 synthesizing cells in the hypothalamus and locally synthesized neuroestradiol further augments the GnRH surge directly or indirectly through interneurons (Fig. 7B). This view is quite different from the current view in which an increase in circulating E2 solely feeds back to kisspeptin neurons, through which GnRH surges are generated (Fig. 7A). A small increase in circulating progesterone during the late follicular phase in humans or a progesterone increase after E2 priming in OVX monkeys defines the timing of the GnRH/LH surge (Clifton et al., 1975; Terasawa et al., 1982; Hoff et al., 1983). A question arises as to whether GnRH/LH surges augmented by progesterone in the presence of E2, are also accompanied by the release of neuroestradiol. Although additional experiments with direct measurements of E2 in the median eminence are necessary, it is unlikely that progesterone action during the preovulatory phase is affected by letrozole, as letrozole does not influence the latency of the EB-induced LH surge. The stimulatory role of neuroprogesterone, locally synthesized by hypothalamic astrocytes, in the preovulatory LH surge in female rats has been reported in detail (see Micevych and Sinchak, 2011). So far, we have not examined the role of neuroprogesterone in the E2-induced GnRH surge. Nevertheless, it can be expected that the role of neuroprogesterone in the preovulatory GnRH surge in primates would significantly differ from that reported in rodents. This view is based on the difference in the role of progesterone in the preovulatory surge between rodents and primates. For example, whereas in E2-treated OVX female rats progesterone augments the timing and amplitude of the LH surge (DePaolo and Barraclough, 1979), in primates in E2-treated OVX female monkeys progesterone only modifies the timing, but not the overall amplitude, of the LH surge (Terasawa et al., 1980, Terasawa et al., 1984). As a consequence, unlike in rodents, the progesterone-induced LH surge in OVX female monkeys treated with a low dose of E2 (YTerasawa et al., 1980) does not quite resemble a spontaneous preovulatory LH surge or the LH surge induced by a high dose of E2 in OVX female monkeys (Yamaji et al., 1971; Karsch et al., 1973).