Acute Influences of Bisphenol A Exposure on Hypothalamic Release of Gonadotropin-Releasing Hormone and Kisspeptin in Female Rhesus Monkeys.

Bisphenol A (BPA) is an industrial compound with pervasive distribution in the environments of industrialized countries. The U.S. Centers for Disease Control recently found that greater than 90% of Americans carry detectable levels of BPA, raising concern over the direct influences of this compound on human physiology. Epidemiologic evidence links elevated BPA serum concentrations to human reproductive dysfunction, although controlled studies on the acute effect of BPA exposure on reproductive function are limited, particularly in primates. We evaluated the effect of direct BPA exposure on female primate hypothalamic peptide release. Specifically, using a microdialysis method, we examined the effects of BPA (0.1, 1, and 10nM) directly infused to the stalk-median eminence on the release of GnRH and kisspeptin (KP) in mid to late pubertal ovarian intact female rhesus monkeys. We found that the highest level of BPA exposure (10nM) suppressed both GnRH and KP release, whereas BPA at lower concentrations (0.1 and 1nM) had no apparent effects. In addition, we measured BPA in plasma and hypothalamic dialysates after an iv bolus injection of BPA (100 µg/kg). We found a relatively stable distribution of BPA between the blood and brain (plasma:brain ≅ 5:1) persists across a wide range of blood BPA concentrations (1-620 ng/mL). Findings of this study suggest that persistent, high-level exposures to BPA could impair female reproductive function by directly influencing hypothalamic neuroendocrine function.

Neuroestradiol in the hypothalamus contributes to the regulation of gonadotropin releasing hormone release.

Release of gonadotropin releasing hormone (GnRH) from the medial basal hypothalamus (MBH)/median eminence region (S-ME) is essential for normal reproductive function. GnRH release is profoundly regulated by the negative and positive feedback effects of ovarian estradiol (E2). Here we report that neuroestradiol, released in the S-ME, also directly influences GnRH release in ovariectomized female monkeys, in which the ovarian source of E2 is removed. We found that (1) brief infusion of E2 benzoate (EB) to the S-ME rapidly stimulated release of GnRH and E2 in the S-ME of ovariectomized monkeys, (2) electrical stimulation of the MBH resulted in GnRH release as well as E2 release, and (3) direct infusion of an aromatase inhibitor to the S-ME suppressed spontaneous GnRH release as well as the EB-induced release of GnRH and E2. These findings reveal the importance of neuroestradiol as a neurotransmitter in regulation of GnRH release. How circulating ovarian E2 interacts with hypothalamic neuroestrogens in the control of GnRH release remains to be investigated.

Tonic control of kisspeptin release in prepubertal monkeys: implications to the mechanism of puberty onset.

Previously we have shown that a reduction in γ-amino butyric acid (GABA) inhibition is critical for the mechanism initiating puberty onset because chronic infusion of the GABA(A) receptor antagonist, bicuculline, significantly increased GnRH release and accelerated the timing of menarche and first ovulation in female rhesus monkeys. Because previous studies in our laboratory indicate that in prepubertal female monkeys, kisspeptin release in the medial basal hypothalamus is low, whereas kisspeptin-10 can stimulate GnRH release, we hypothesized that a low level of kisspeptin release prior to puberty onset is due to tonic GABA inhibition. To test this hypothesis we examined the effects of bicuculline infusion on kisspeptin release using a microdialysis method. We found that bicuculline at 1 µM dramatically stimulates kisspeptin release in the medial basal hypothalamus of prepubertal monkeys but had little effect on kisspeptin release in midpubertal monkeys. We further examined whether bicuculline-induced GnRH release is blocked by the presence of the kisspeptin antagonist, peptide 234. We found that inhibition of kisspeptin signaling blocked the bicuculline-induced stimulation of GnRH release, suggesting that kisspeptin neurons may relay inhibitory GABA signals to GnRH neurons. This implies that a reduction in tonic GABA inhibition of GnRH release is, at least in part, mediated through kisspeptin neurons.

Involvement of G protein-coupled receptor 30 (GPR30) in rapid action of estrogen in primate LHRH neurons.

Previously, we have reported that 17beta-estradiol (E(2)) induces an increase in firing activity of primate LH-releasing hormone (LHRH) neurons. The present study investigates whether E(2) alters LHRH release as well as the pattern of intracellular calcium ([Ca(2+)](i)) oscillations and whether G protein-coupled receptor 30 (GPR30) plays a role in mediating the rapid E(2) action in primate LHRH neurons. Results are summarized: 1) E(2), the nuclear membrane-impermeable estrogen, estrogen-dendrimer conjugate, and the plasma membrane-impermeable estrogen, E(2)-BSA conjugate, all stimulated LHRH release within 10 min of exposure; 2) whereas the estrogen receptor antagonist, ICI 182,780, did not block the E(2)-induced LHRH release, E(2) application to cells treated with pertussis toxin failed to induce LHRH release; 3) GPR30 mRNA was expressed in olfactory placode cultures, and GPR30 protein was expressed in a subset of LHRH neurons; 4) pertussis toxin treatment blocked the E(2)-induced increase in [Ca(2+)](i) oscillations; 5) knockdown of GPR30 in primate LHRH neurons by transfection with small interfering RNA (siRNA) for GPR30 completely abrogated the E(2)-induced changes in [Ca(2+)](i) oscillations, whereas transfection with control siRNA did not; 6) the estrogen-dendrimer conjugate-induced increase in [Ca(2+)](i) oscillations also did not occur in LHRH neurons transfected with GPR30 siRNA; and 7) G1, a GPR30 agonist, resulted in changes in [Ca(2+)](i) oscillations, similar to those observed with E(2). Collectively, E(2) induces a rapid excitatory effect on primate LHRH neurons, and this rapid action of E(2) appears to be mediated, in part, through GPR30.