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Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats.

ABSTRACT: Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female rats primed with E2 and progesterone or vehicle. In young females, blockade of IGF-I receptors attenuated the steroid hormone-induced LH surge, reduced the percent of GnRH neurons expressing c-fos on the day of the LH surge, and decreased the total number of neurons expressing c-fos in the preoptic area. Middle-aged females had fewer GnRH neurons expressing c-fos during the LH surge than young females, and the LH surge amplitude was attenuated. Infusion of an IGF-I dose previously shown to increase LH surge amplitude did not increase the percent of GnRH neurons expressing c-fos in middle-aged females. Brain IGF-I receptor blockade did not modify E2 induction of progestin receptor-immunoreactive neurons in the preoptic area, arcuate, or ventromedial hypothalamus of young rats. These findings indicate that brain IGF-I receptors are required for E2 activation of GnRH neurons in young rats and for robust GnRH release from axon terminals in middle-aged females. IGF-I likely exerts its effects by actions on E2-sensitive neurons that are upstream of GnRH neurons and terminals.

SUBMITTER: Sun Y

PROVIDER: S-EPMC3199007 | biostudies-literature | 2011 Nov

REPOSITORIES: biostudies-literature

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Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats.

Sun Yan Y Todd Brigitte J BJ Thornton Kimberly K Etgen Anne M AM Neal-Perry Genevieve G

Endocrinology 20110913 11

Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female ra ...[more]

PMID: 21914776

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Project description:BackgroundLuteinizing hormone secreted by the anterior pituitary gland regulates gonadal function. Luteinizing hormone secretion is regulated both by alterations in gonadotrope responsiveness to hypothalamic gonadotropin releasing hormone and by alterations in gonadotropin releasing hormone secretion. The mechanisms that determine gonadotrope responsiveness are unknown but may involve regulators of G protein signaling (RGSs). These proteins act by antagonizing or abbreviating interaction of Galpha proteins with effectors such as phospholipase Cbeta. Previously, we reported that gonadotropin releasing hormone-stimulated second messenger inositol trisphosphate production was inhibited when RGS3 and gonadotropin releasing hormone receptor cDNAs were co-transfected into the COS cell line. Here, we present evidence for RGS3 inhibition of gonadotropin releasing hormone-induced luteinizing hormone secretion from cultured rat pituitary cells.ResultsA truncated version of RGS3 (RGS3T = RGS3 314-519) inhibited gonadotropin releasing hormone-stimulated inositol trisphosphate production more potently than did RSG3 in gonadotropin releasing hormone receptor-bearing COS cells. An RSG3/glutathione-S-transferase fusion protein bound more 35S-Gqalpha than any other member of the G protein family tested. Adenoviral-mediated RGS3 gene transfer in pituitary gonadotropes inhibited gonadotropin releasing hormone-stimulated luteinizing hormone secretion in a dose-related fashion. Adeno-RGS3 also inhibited gonadotropin releasing hormone stimulated 3H-inositol phosphate accumulation, consistent with a molecular site of action at the Gqalpha protein.ConclusionsRGS3 inhibits gonadotropin releasing hormone-stimulated second messenger production (inositol trisphosphate) as well as luteinizing hormone secretion from rat pituitary gonadotropes apparently by binding and suppressing the transduction properties of Gqalpha protein function. A version of RGS3 that is amino-terminally truncated is even more potent than intact RGS3 at inhibiting gonadotropin releasing hormone-stimulated inositol trisphosphate production.

Project description:Alcohol (ALC) diminishes gonadotropin-releasing hormone (GnRH) secretion and delays puberty. Glial transforming growth factor ?1 (TGF?1) plays a role in glial-neuronal communications facilitating prepubertal GnRH secretion. We assessed the effects of acute ALC administration on TGF?1-induced GnRH gene expression in the brain preoptic area (POA) and release of the peptide from the medial basal hypothalamus (MBH). Furthermore, we assessed actions and interactions of TGF?1 and ALC on an adhesion/signaling gene family involved in glial-neuronal communications.Prepubertal female rats were administered ALC or water via gastric gavage at 7:30 am. At 9:00 am, saline or TGF?1 (100 ng/3 ?l) was administered into the third ventricle. At 3:00 pm, the POA was removed and frozen for gene expression analysis and repeated for protein assessments. In another experiment, the MBH was removed from ALC-free rats. After equilibration, tissues were incubated in Locke's medium only or medium containing TGF?1 with or without 50 mM ALC for measurement of GnRH peptide released in vitro.TGF?1 induced GnRH gene expression in the POA, and this effect was blocked by ALC. We also described the presence and responsiveness of the TGF?1 receptor in the POA and showed that acute ALC exposure not only altered the TGF?1-induced increase in TGF?-R1 protein expression but also the activation of receptor-associated proteins, Smad2 and Smad3, key downstream components of the TGF?1 signaling pathway. Assessment of an adhesion/signaling family consisting of glial receptor protein tyrosine phosphatase beta and neuronal contactin-associated protein-1 (Caspr1) and contactin showed that the neuronal components were induced by TGF?1 and that ALC blocked these effects. Finally, TGF?1 was shown to induce release of the GnRH peptide in vitro, an action that was blocked by ALC.We have demonstrated glial-derived TGF?1 induces GnRH gene expression in the POA and stimulates release of the peptide from the MBH, actions necessary for driving the pubertal process. Importantly, ALC acted at both brain regions to block stimulatory effects of TGF?1. Furthermore, ALC altered neuronal components of an adhesion/signaling family previously shown to be expressed on GnRH neurons and implicated in glial-GnRH neuronal communications. These results further demonstrate detrimental effects of ALC at puberty.

Dataset Information

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats.

Publications

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats.

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