IGF-1 Influences Gonadotropin-Releasing Hormone Regulation of Puberty.

The pubertal process is initiated as a result of complex neuroendocrine interactions within the preoptic and hypothalamic regions of the brain. These interactions ultimately result in a timely increase in the secretion of gonadotropin-releasing hormone (GnRH). Researchers for years have believed that this increase is due to a diminished inhibitory tone which has applied a prepubertal brake on GnRH secretion, as well as to the gradual development of excitatory inputs driving the increased release of the peptide. Over the years, insulin-like growth factor-1 (IGF-1) has emerged as a prime candidate for playing an important role in the onset of puberty. This review will first present initial research demonstrating that IGF-1 increases in circulation as puberty approaches, is able to induce the release of prepubertal GnRH, and can advance the timing of puberty. More recent findings depict an early action of IGF-1 to activate a pathway that releases the inhibitory brake on prepubertal GnRH secretion provided by dynorphin, as well as demonstrating that IGF-1 can also act later in the process to regulate the synthesis and release of kisspeptin, a potent stimulator of GnRH at puberty.

How alcohol affects insulin-like growth factor-1's influences on the onset of puberty: A critical review.

Alcohol (ALC) is capable of delaying signs associated with pubertal development in laboratory animals, as well as in humans. The normal onset of puberty results from a timely increase in gonadotropin-releasing hormone (GnRH) secretion, which is associated with a gradual decline in prepubertal inhibitory influences, and the establishment of excitatory inputs that increase GnRH release, which together drive pubertal development. In recent years, insulin-like growth factor-1 (IGF-1) has emerged as a pivotal contributor to prepubertal GnRH secretion and pubertal development, whose critical actions are interfered with by ALC abuse. Here we review the neuroendocrine research demonstrating the important role that IGF-1 plays in pubertal development, and describe the detrimental effects and mechanisms of action of ALC on the onset and progression of pubertal maturation.

Alcohol Delays the Onset of Puberty in the Female Rat by Altering Key Hypothalamic Events.

BACKGROUND: Because alcohol (ALC) delays signs of pubertal development, we assessed the time course of events associated with the synthesis of critical hypothalamic peptides that regulate secretion of luteinizing hormone-releasing hormone (LHRH), the peptide that drives the pubertal process. METHODS: Immature female rats were administered either laboratory chow or BioServe isocaloric control or ALC-liquid diets from 27 through 33 days of age. On days 28, 29, 31, and 33, animals were killed by decapitation and tissue blocks containing the medial basal hypothalamus (MBH) and the rostral hypothalamic area (RHA) were isolated and stored frozen until assessed by Western blot analysis. RESULTS: Synthesis of dynorphin (DYN), a prepubertal inhibitor of LHRH secretion, was increased (p < 0.05) in the MBH of ALC-treated animals by day 29. DYN was further elevated (p < 0.01) on day 33 and was associated with an increase (p < 0.01) in DYN receptor expression. ALC did not affect synthesis of neurokinin B (NKB), a prepubertal stimulator of LHRH; however, it did suppress (p < 0.05) NKB receptor expression in the MBH by day 31. The most potent stimulator of prepubertal LHRH secretion, kisspeptin (Kp), was also decreased (p < 0.05) in the MBH as early as day 29, with continued suppression (p < 0.01) through day 33. Similar timely suppressions of mammalian target of rapamycin (mTOR), an immediate upstream regulator of Kp, were also noted. These decreases in mTOR and Kp were consistent with ALC stimulating (p < 0.05) the p-AMP-activated protein kinase/Raptor inhibitory pathway to mTOR on day 29, then later suppressing (p < 0.001) an Akt-mediated induction pathway to mTOR by day 31. In the RHA, ALC affected the pathways regulating Kp in a manner similar to that described in the MBH; however, these effects were not noted until day 33. CONCLUSIONS: ALC acts within the MBH as early as 29 days to induce inhibitor and repressor inputs to LHRH, while depressing stimulatory inputs to the peptide. Collectively, these events lead to delayed signs of pubertal development.

Regulation of Kisspeptin Synthesis and Release in the Preoptic/Anterior Hypothalamic Region of Prepubertal Female Rats: Actions of IGF-1 and Alcohol.

BACKGROUND: Alcohol (ALC) causes suppressed secretion of prepubertal luteinizing hormone-releasing hormone (LHRH). Insulin-like growth factor-1 (IGF-1) and kisspeptin (Kp) are major regulators of LHRH and are critical for puberty. IGF-1 may be an upstream mediator of Kp in the preoptic area and rostral hypothalamic area (POA/RHA) of the rat brain, a region containing both Kp and LHRH neurons. We investigated the ability of IGF-1 to stimulate prepubertal Kp synthesis and release in POA/RHA, and the potential inhibitory effects of ALC. METHODS: Immature female rats were administered either ALC (3 g/kg) or water via gastric gavage at 0730 hours. At 0900 hours, both groups were subdivided where half received either saline or IGF-1 into the brain third ventricle. A second dose of ALC (2 g/kg) or water was administered at 1130 hours. Rats were killed 6 hours after injection and POA/RHA region collected. RESULTS: IGF-1 stimulated Kp, an action blocked by ALC. Upstream to Kp, IGF-1 receptor (IGF-1R) activation, as demonstrated by the increase in insulin receptor substrate 1, resulted in activation of Akt, tuberous sclerosis 2, ras homologue enriched in brain, and mammalian target of rapamycin (mTOR). ALC blocked the central action of IGF-1 to induce their respective phosphorylation. IGF-1 specificity and ALC specificity for the Akt-activated mTOR pathway were demonstrated by the absence of effects on PRAS40. Furthermore, IGF-1 stimulated Kp release from POA/RHA incubated in vitro. CONCLUSIONS: IGF-1 stimulates prepubertal Kp synthesis and release following activation of a mTOR signaling pathway, and ALC blocks this pathway at the level of IGF-1R.

Differential Effects of Alcohol on Excitatory and Inhibitory Puberty-Related Peptides in the Basal Hypothalamus of the Female Rat.

BACKGROUND: An increase in development of excitatory inputs along with a decline in inhibitory inputs ultimately govern the timely increased secretion of hypothalamic luteinizing hormone-releasing hormone (LHRH) at the time of puberty. As chronic alcohol (ALC) exposure acts at the hypothalamic level to suppress LHRH secretion and delay puberty, we assessed its ability to differentially affect the expression of key puberty-related proteins. METHODS: ALC was administered to female rats from days 27 to 33, at which time animals were killed and tissues collected for protein expression. In the medial basal hypothalamus (MBH), we assessed kisspeptin (Kp) 10, an excitatory peptide critical for prepubertal LHRH secretion, and Lin28b, a peptide with an inhibitory influence on puberty. As a direct mechanism of action of Lin28b was not known, we determined whether its central administration could induce dynorphin (DYN), a peptide that is inhibitory on LHRH secretion. Also, ALC's effect on DYN protein expression was assessed, as well as its effect on DYN release in vitro. RESULTS: ALC markedly suppressed (p < 0.01) the expression of the excitatory Kp protein, while at the same time increased (p < 0.001) the expression of inhibitory Lin28b protein. Subsequently, we showed for the first time that the central administration of Lin28b stimulated (p < 0.01) the synthesis of DYN. Finally, ALC also induced (p < 0.01) the protein expression and stimulated (p < 0.01) the in vitro release of DYN from the MBH. CONCLUSIONS: These results indicate that ALC can simultaneously and differentially alter both excitatory and inhibitory influences governing pubertal development, show for the first time a mechanism of action by which Lin28b exerts its prepubertal inhibitory tone, and further demonstrate the negative influences of ALC on the pubertal process.

Actions and interactions of alcohol and transforming growth factor ß1 on prepubertal hypothalamic gonadotropin-releasing hormone.

BACKGROUND: Alcohol (ALC) diminishes gonadotropin-releasing hormone (GnRH) secretion and delays puberty. Glial transforming growth factor ß1 (TGFß1) plays a role in glial-neuronal communications facilitating prepubertal GnRH secretion. We assessed the effects of acute ALC administration on TGFß1-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 TGFß1 and ALC on an adhesion/signaling gene family involved in glial-neuronal communications. METHODS: Prepubertal female rats were administered ALC or water via gastric gavage at 7:30 am. At 9:00 am, saline or TGFß1 (100 ng/3 µl) 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 TGFß1 with or without 50 mM ALC for measurement of GnRH peptide released in vitro. RESULTS: TGFß1 induced GnRH gene expression in the POA, and this effect was blocked by ALC. We also described the presence and responsiveness of the TGFß1 receptor in the POA and showed that acute ALC exposure not only altered the TGFß1-induced increase in TGFß-R1 protein expression but also the activation of receptor-associated proteins, Smad2 and Smad3, key downstream components of the TGFß1 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 TGFß1 and that ALC blocked these effects. Finally, TGFß1 was shown to induce release of the GnRH peptide in vitro, an action that was blocked by ALC. CONCLUSIONS: We have demonstrated glial-derived TGFß1 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 TGFß1. 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.

Alcohol alters insulin-like growth factor-1-induced transforming growth factor ß1 synthesis in the medial basal hypothalamus of the prepubertal female rat.

BACKGROUND: Insulin-like growth factor-1 (IGF-1) and transforming growth factor ß1 (TGFß1) are produced in hypothalamic astrocytes and facilitate luteinizing hormone-releasing hormone (LHRH) secretion. IGF-1 stimulates release by acting directly on the LHRH nerve terminals and both peptides act indirectly through specific plastic changes on glial/tanycyte processes that further support LHRH secretion. Because the relationship between these growth factors in the hypothalamus is not known, we assessed the ability of IGF-1 to induce TGFß1 synthesis and release and the actions of alcohol (ALC) on this mechanism prior to the onset of puberty. METHODS: Hypothalamic astrocytes were exposed to medium only, medium plus IGF-1 (200 ng/ml), or medium plus IGF-1 with 50 mM ALC. After 18 hours, media were collected and assayed for TGFß1. For the in vivo experiment, prepubertal female rats were administered either ALC (3 g/kg) or water via gastric gavage at 07:30 hours and at 11:30 hours. At 09:00 hours, saline or IGF-1 was administered into the third ventricle. Rats were killed at 15:00 hours and the medial basal hypothalamus (MBH) was collected for assessment of TGFß1, IGF-1 receptor (IGF-1R), and Akt. RESULTS: IGF-1 induced TGFß1 release (p < 0.01) from hypothalamic astrocytes in culture, an action blocked by ALC. In vivo, IGF-1 administration caused an increase in TGFß1 protein compared with controls (p < 0.05), an action blocked by ALC as well as a phosphatidylinositol 3 kinase/Akt inhibitor. IGF-1 stimulation also increased both total (p< 0.01) and phosphorylated (p)-IGF-1R (p < 0.05) protein levels, and phosphorylated (p)-Akt levels (p < 0.01), which were also blocked by ALC. CONCLUSIONS: This study shows that ALC blocks IGF-1 actions to stimulate synthesis and release of hypothalamic TGFß1, total and p-IGF-1R, and p-Akt levels further demonstrating the inhibitory actions of ALC on puberty-related events associated with hypothalamic LHRH release.

Hypothalamic actions and interactions of alcohol and IGF-1 on the expression of glial receptor protein tyrosine phosphatase-ß during female pubertal development.

BACKGROUND: Hypothalamic glial-neuronal communications are important for the activation of luteinizing hormone releasing hormone (LHRH) secretion at the time of puberty. As we have shown that alcohol (ALC) diminishes prepubertal LHRH secretion and delays puberty, we first assessed the effects of short-term ALC administration on the basal expression of a specific gene family involved in glial-neuronal communications. Second, as insulin-like growth factor-1 (IGF-1) is a critical regulator of LHRH secretion and the pubertal process, we then assessed whether IGF-1 could induce the expression of these signaling genes and determine whether ALC can block this affect. METHODS: Immature female rats were fed a liquid diet containing ALC for 6 days beginning when 27 days old. Control animals received either the companion isocaloric liquid diet or rat chow and water. Animals were decapitated on day 33, in the late juvenile stage of development. Medial basal hypothalamic (MBH) tissues were obtained for gene and protein analyses of glial receptor protein tyrosine phosphatase-ß (RPTPß) and the 2 neuronal components, contactin and contactin-associated protein 1 (Caspr1). In the second experiment, IGF-1 was administered into the third ventricle (3V) and the MBH removed 6 hours after peptide delivery, and the above-mentioned 3 genes were analyzed by real-time PCR. To determine whether this action was affected by ALC, immature female rats were administered either ALC (3 g/kg) or water via gastric gavage at 0900 hours. At 1030 hours, the ALC and control groups were subdivided such that half of the animals were injected into the 3V with IGF-1 and the other half with an equal volume of saline. Rats were killed 6 hours after the IGF-1 injection and MBHs collected. RESULTS: Real-time PCR showed that when compared with control animals, ALC caused a marked decrease (p < 0.001) in the basal expression of the RPTPß gene, but did not affect the expression of either contactin or Caspr1. Likewise, analysis by Western blotting demonstrated that ALC caused suppressed (p < 0.001) levels of the RPTPß protein, with the expressions of both contactin and Caspr1 proteins being unaltered. In the second experiment, results showed that only the RPTPß gene was stimulated (p < 0.05) by IGF-1 in the MBH 6 hours after peptide delivery. Assessments revealed that the IGF-1 induced increase (p < 0.01) in the expression of the RPTPß gene was blocked by the presence of ALC. CONCLUSIONS: Prepubertal ALC exposure is capable of interfering with hypothalamic glial-neuronal communications by suppressing the synthesis of the glial product, RPTPß, which is required for binding to the contactin-Caspr1 complex on LHRH neuronal terminals, thus suggesting that this action of ALC contributes to its detrimental effects on the pubertal process.

Insulin-like growth factor-I activates KiSS-1 gene expression in the brain of the prepubertal female rat.

KiSS-1 gene expression has been shown to increase as puberty approaches, and its peptide products, kisspeptins, are involved in LHRH secretion at puberty. Factors contributing to increased KiSS-1 expression, however, have not been identified; thus, the purpose of this study was to assess whether IGF-I could induce transcription of this gene in prepubertal female rats. IGF-I or saline was centrally administered to immature rats that were killed 2, 4, and 6 h later. Real-time PCR revealed that IGF-I induced (P < 0.01) KiSS-1 gene expression at 6 h in a tissue fragment that contained both the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei. Subsequently, the AVPV and ARC nuclei were separated to assess whether region-specific effects could be identified. IGF-I stimulated (P < 0.01) KiSS-1 gene expression in the AVPV nucleus at 6 h after injection, with no change observed in the ARC nucleus. Serum estradiol (E2) levels were not altered at any time point after IGF-I, demonstrating that the increased KiSS-1 expression observed was not caused by an elevation in E2. Additionally, the IGF-I action to induce KiSS-1 gene expression in the AVPV nucleus was further demonstrated when the IGF-I was administered systemically. E2 appears to play an important permissive role because 1-d ovariectomized rats responded to IGF-I with increased (P < 0.01) KiSS-1 expression, whereas, 20 d after ovariectomy, when the E2 levels had fallen below assay sensitivity, the IGF-I was unable to induce KiSS-1 expression. The IGF-I effect was further demonstrated by showing that the IGF-I receptor antagonist, JB-1, blocked the IGF-I-induced increase in KiSS-1 expression. Collectively, these data indicate that IGF-I is an activator of the KiSS-1 gene in the prepubertal female rat.

Actions and interactions of alcohol and insulin-like growth factor-1 on female pubertal development.

Alcohol (ALC) is a drug that is capable of disrupting reproductive function in adolescent humans, as well as immature rhesus monkeys and rats. Critical to determining the mechanism(s) of the effects of ALC on the pubertal process is to have a better understanding of the important events involved in the initiation of puberty. For years it has been hypothesized that there may be metabolic signals capable of linking somatic growth to the activation of the reproductive system at the time of puberty. In recent years it has been shown that insulin-like growth factor-1 (IGF-1) is one such signal that plays an early role in the pubertal process. In this review, we will describe the actions and interactions of ALC and IGF-1 on molecular and physiological processes associated with pubertal development.

Short-term alcohol administration alters KiSS-1 gene expression in the reproductive hypothalamus of prepubertal female rats.

BACKGROUND: Kisspeptins bind to the G-protein-coupled receptor (GPR54) to activate hypothalamic luteinizing hormone releasing hormone (LHRH) secretion at the time of puberty. Alcohol (ALC) causes depressed prepubertal LHRH release, resulting in depressed luteinizing hormone (LH) secretion and delayed puberty. Because KiSS-1 and GPR54 are important to the onset of puberty, we assessed the effects of chronic ALC administration on basal expression of these puberty-related genes within the reproductive hypothalamus, as well as hormones and transduction signaling pathways contributing to their activity. METHODS: Immature female rats were fed a liquid diet containing ALC for 6 days beginning when 27 days old. Controls received either companion isocaloric liquid diet or rat chow and water. Animals were decapitated on day 33, in the late juvenile stage of development. Blood was collected for the assessment of serum hormone levels. Brain tissues containing the anteroventral periventricular (AVPV) and arcuate (ARC) nuclei were obtained for assessing expression of specific puberty-related genes and proteins. RESULTS: KiSS-1 mRNA levels in the AVPV and ARC nuclei were suppressed (p < 0.001) in the ALC-treated rats. GPR54 gene and protein expressions were both modestly increased (p < 0.05) in AVPV nucleus, but not in ARC nucleus. Alcohol exposure also resulted in suppressed serum levels of insulin-like growth factor-1 (IGF-1), LH, and estradiol (E(2)). As IGF-1, in the presence of E(2), can induce expression of the KiSS-1 gene, we assessed the potential for ALC to alter IGF-1 signaling in the reproductive hypothalamus. IGF-1 receptor gene and protein expressions were not altered. However, protein expression of phosphorylated Akt, a transduction signal used by IGF-1, was suppressed in the AVPV (p < 0.05) and ARC (p < 0.01) nuclei. CONCLUSIONS: Alcohol causes suppressed KiSS-1 gene expression in the reproductive hypothalamus; hence, contributing to this drug's ability to cause suppressed LHRH secretion and disruption of the pubertal process. We suggest that this action, at least in part, is through altered IGF-1 signaling.

Regulation of prepubertal dynorphin secretion in the medial basal hypothalamus of the female rat.

The onset of puberty is the result of an increase in secretion of hypothalamic gonadotrophin-releasing hormone (GnRH). This action is a result of not only the development of stimulatory inputs to its release, but also the gradual decrease in inhibitory inputs that restrain release of the peptide prior to pubertal onset. Dynorphin (DYN) is one of the inhibitory inputs produced in the medial basal hypothalamus (MBH); however, little is known about what substance(s) control its prepubertal synthesis and release. Because neurokinin B (NKB) increases in the hypothalamus as puberty approaches, we considered it a candidate for such a role. An initial study investigated the acute effects of an NKB agonist, senktide, on the secretion of DYN from MBH tissues incubated in vitro. In other experiments, central injections of senktide were administered to animals for 4 days then MBHs were collected for assessment of DYN synthesis or for the in vitro secretion of both DYN and GnRH. Because insulin-like growth factor (IGF)-1 has been shown to play an important role at puberty, additional animals received central injections of this peptide for 4 days to assess NKB and DYN synthesis or the in vitro secretion of NKB. The results obtained show that senktide administration up-regulates the NKB receptor protein, at the same time as suppressing the DYN and its receptor. Senktide consistently suppressed DYN and elevated GnRH secretion in the same tissue incubates from both the acute and chronic studies. IGF-1 administration caused an increase in NKB protein, at the same time as decreasing DYN protein. Furthermore, the central administration of IGF-1 caused an increase in NKB release, an action blocked by the IGF-1 receptor blocker, JB-1. These results indicate that the IGF-1/NKB pathway contributes to suppressing the DYN inhibitory tone on prepubertal GnRH secretion and thus facilitates the puberty-related increase in the release of GnRH to accelerate the onset of puberty.

The pyrethroid pesticide esfenvalerate suppresses the afternoon rise of luteinizing hormone and delays puberty in female rats.

BACKGROUND: One of the most widely used classes of insecticides is the synthetic pyrethroids. Although pyrethroids are less acutely toxic to humans than to insects, in vitro studies have suggested that pyrethroids may be estrogenic. OBJECTIVES: We assessed pubertal effects by orally administering 0.5, 1.0, and 5.0 mg/kg/day of the type II pyrethroid esfenvalerate (ESF) to female rats beginning on postnatal day (PND) 22 until vaginal opening. ESF administration suppresses serum estradiol and delays pubertal onset. MATERIALS AND METHODS: To assess possible hypothalamic and/or pituitary effects, animals received 0.5 or 1.0 mg/kg ESF or corn oil on PNDs 22-29. On PND30, we drew three blood samples (200 microL) from each rat at 15-min intervals beginning at 1000 hours, and again at 1500 hours. To test hypothalamic responsiveness, after the third afternoon sample, all animals received an intravenous injection of N-methyl-d,l-aspartic acid (NMA; 40 mg/kg), and then we drew two more samples. We performed a second experiment as above except that animals received luteinizing hormone-releasing hormone (LHRH; 25 ng/rat) to test pituitary responsiveness. RESULTS: Basal levels of luteinizing hormone (LH) in the afternoon hours were higher in control animals than in animals treated with 1.0 mg/kg ESF (p < 0.05). Furthermore, NMA- and LHRH-stimulated LH release was similar in control and ESF-treated animals, indicating that both hypothalamic and pituitary responsiveness, respectively, were unaffected. CONCLUSIONS: Although the hypothalamus is able to respond to exogenous stimuli, absence of a normal afternoon rise in LH would indicate a hypothalamic deficit in ESF-treated animals.

Alcohol and Puberty: Mechanisms of Delayed Development

Adolescence represents a vulnerable period for developing youth. Alcohol use and misuse are especially problematic behaviors during this time. Adolescents are more sensitive to alcohol and less tolerant of its detrimental effects than are adults. Research in humans and animals has revealed that early alcohol consumption can result in delayed pubertal development. Animal studies have shown that alcohol detrimentally affects neuroendocrine systems within the hypothalamic region of the brain that are associated with the normal, timely onset of the pubertal process. To effectively restore development and shorten recovery time associated with the adverse effects of alcohol on puberty, researchers must first understand the molecular and physiological mechanisms by which alcohol interferes with critical hypothalamic functions.

Influences of manganese on pubertal development.

The onset of puberty is the result of complex neuroendocrine interactions within hypothalamic region of the brain, as well as from genetic and environmental influences. These interactions ultimately result in the increased synthesis and release of luteinizing hormone-releasing hormone (LHRH). Manganese (Mn) is an essential environmental element known for years to be involved in numerous mammalian physiological processes, including growth and reproductive function. Studies in recent years have shown the ability of Mn to cross the blood-brain barrier and act within the hypothalamus to influence the timing of puberty. This review will depict research showing the molecular and physiological actions of Mn in the control of prepubertal LHRH and discuss the potential for the element to cause either helpful or harmful outcomes on the developmental process depending upon the age and accumulation of Mn within the hypothalamus.

IGF-1 administration to prepubertal female rats can overcome delayed puberty caused by maternal Pb exposure.

Because prepubertal female rats maternally exposed to lead (Pb) exhibit suppressed serum levels of insulin-like growth factor-1 (IGF-1) and delayed puberty, we investigated the ability of centrally administered IGF-1 to stimulate luteinizing hormone (LH) release in vivo and LH-releasing hormone (LHRH) release in vitro from maternally Pb-exposed prepubertal female rats. Additionally, we assessed whether IGF-1 replacement could affect the timing of female puberty. Results demonstrated that IGF-1 stimulated significantly LH release in both control and Pb-exposed animals. When median eminences from control and Pb-exposed females were incubated with rat IGF-1 in vitro, they responded similarly with significant peptide-induced LHRH release. Lastly, we showed IGF-1 replacement reversed the delay in puberty caused by Pb. These results indicate the central LHRH response to IGF-1 is intact and that Pb-induced delayed puberty is due, at least in part, to suppressed circulating IGF-1 available to the hypothalamus.

Manganese acts centrally to activate reproductive hormone secretion and pubertal development in male rats.

Manganese (Mn) is an important element for normal growth and reproduction. Because Mn accumulates in the hypothalamus and is capable of stimulating puberty-related hormones in female rats, we assessed whether this metal could cause similar effects in male rats. We have demonstrated that MnCl2, when administered acutely into the third ventricle of the brain, acts dose dependently to stimulate luteinizing hormone (LH) release. Furthermore, there was a dose dependent stimulation in the secretion of LH-releasing hormone (LHRH) from the medial basal hypothalamus in vitro, and administration of an LHRH receptor antagonist in vivo blocks Mn-induced LH release. To assess potential chronic effects of the metal, male pups were supplemented with 10 or 25 mg MnCl2 per kg by gastric gavage from day 15 until days 48 or 55, at which times developmental signs of spermatogenesis were assessed. Results demonstrate that while significant effects were not observed with the 10 mg/kg dose, the animals receiving the 25 mg/kg dose showed increased LH (p<0.05), FSH (p<0.01) and testosterone (p<0.01) levels at 55 days of age. Furthermore, there was a concomitant increase in both daily sperm production (p<0.05) and efficiency of spermatogenesis (p<0.05), demonstrating a Mn-induced acceleration in spermatogenesis. Our results suggest Mn is a stimulator of prepubertal LHRH/LH secretion and may facilitate the normal onset of male puberty. These data also suggest that the metal may contribute to male precocious pubertal development should an individual be exposed to low but elevated levels of Mn too early in life.

Manganese-Stimulated Kisspeptin Is Mediated by the IGF-1/Akt/Mammalian Target of Rapamycin Pathway in the Prepubertal Female Rat.

Low-dose administration of manganese chloride (MnCl2) causes release of hypothalamic LH-releasing hormone (LHRH) and advances puberty in rat. Recently, this element was shown to up-regulate mammalian target of rapamycin (mTOR), kisspeptin gene (KiSS-1), and LHRH gene expressions in the brain preoptic area (POA)/anteroventral periventricular (AVPV) nucleus. Because these genes are critical for puberty, this study was conducted to identify the upstream mechanism by which Mn activates the mTOR/KiSS-1 pathway. On day 12, immature female rats began receiving a daily supplemental dose of 10 mg/kg of MnCl2 or saline by gavage, and POA/AVPV tissues were collected on day 29 for specific protein assessments. Another experiment assessed in vitro IGF-1 release in response to Mn and assessed signal transduction pathways in the POA/AVPV region after Mn delivery into the third ventricle. Chronic Mn exposure increased (P < .05) basal expressions of mTOR and kisspeptin proteins. Mn increased protein kinase B (Akt) and Ras homolog enriched in brain, both capable of activating mTOR. Central Mn delivery increased expressions of phosphorylated IGF-1 receptor (IGF-1R) (P < .05) and Akt (P < .01) in the POA/AVPV region. The previous central delivery of JB1, an IGF-1R antagonist, blocked Mn-induced expressions of both phosphorylated IGF-1R and Akt. Downstream to Akt, centrally administered Mn increased tuberous sclerosis complex 2 (P < .05), Ras homolog enriched in brain (P < .01), mTOR (P < .05), and kisspeptin (P < .05). Finally, we observed that the early puberty induced by Mn was blocked by the administration of an mTOR inhibitor. These results suggest that Mn acts, at least in part, through the IGF-1/Akt/mTOR pathway to influence prepubertal kisspeptin and LHRH.

Manganese protects against the effects of alcohol on hypothalamic puberty-related hormones.

AIMS: Since manganese (Mn) is capable of stimulating the hypothalamic-pituitary unit and advancing female puberty, we assessed the possibility that this element might overcome some of the detrimental effects of prepubertal alcohol (ALC) exposure on the hypothalamic control of pituitary function. MAIN METHODS: Rats received either saline or Mn (10mg/kg) daily by gastric gavage from day 12 to day 31. After weaning, all rats were provided Lab Chow diet ad libitum until day 27 when they began receiving either the Bio Serv control or ALC diet regime. On day 31, the medial basal hypothalamus (MBH) was collected to assess luteinizing hormone-releasing hormone (LHRH) and cyclooxygenase 2 (COX2) protein levels. Release of prostaglandin-E2 (PGE2), LHRH and serum luteinizing hormone (LH) were also assessed. Other animals were not terminated on day 31, but remained in study to assess timing of puberty. KEY FINDINGS: Short-term ALC exposure caused elevated hypothalamic LHRH content, suggesting an inhibition in peptide release, resulting in a decrease in LH. Both actions of ALC were reversed by Mn supplementation. COX2 synthesis, as well as PGE2 and LHRH release were suppressed by ALC exposure, but Mn supplementation caused an increase in COX2 synthesis and subsequent PGE2 and LHRH release in the presence of ALC. Mn supplementation also ameliorated the action of ALC to delay puberty. SIGNIFICANCE: These results suggest that low level Mn supplementation acts to protect the hypothalamus from some of the detrimental effects of ALC on puberty-related hormones.

Manganese acts centrally to stimulate luteinizing hormone secretion: a potential influence on female pubertal development.

Manganese (Mn), an essential element considered important for normal growth and reproduction, has been shown in adults to be detrimental to reproductive function when elevated. Because Mn can cross the blood-brain barrier and accumulate in the hypothalamus, and because it has been suggested that infants and children are potentially more sensitive to Mn than adults, we wanted to determine the effects of Mn exposure on puberty-related hormones and the onset of female puberty. We demonstrated that MnCl(2) when administered acutely into the third ventricle of the brain acts dose-dependently to stimulate luteinizing hormone (LH) release in prepubertal female rats. Incubation of hypothalami in vitro showed that this effect was due to a Mn-induced stimulation of luteinizing hormone releasing hormone (LHRH). Further demonstration that this is a hypothalamic site of action was shown by in vivo blockade of LHRH receptors and lack of a direct pituitary action of Mn to stimulate LH in vitro. To assess potential short-term effects, animals were supplemented with MnCl(2) (10 mg/kg) by gastric gavage from day 12 until day 29, or, in other animals, until vaginal opening (VO). Mn caused elevated serum levels of LH, follicle stimulating hormone, and estradiol, and it initiated a moderate but significant advancement in age at VO. Our results are the first to show that Mn can stimulate specific puberty-related hormones and suggest that it may facilitate the normal onset of puberty. They also suggest that Mn may contribute to precocious puberty if an individual is exposed to elevated levels of Mn too early in development.