Distribution of insulin-like growth factor-I receptor mRNA in rat brain. Regulation in the hypothalamo-neurohypophysial system

Higher Prepubertal IGF-1 Concentrations Associate to Earlier Pubertal Tempo in Both Sexes

INTRODUCTION

Pubertal onset is triggered by multiple neuroendocrine interactions. The role of prepubertal IGF-1 in this process has not been explored in both sexes. Our objective was to analyze the association of prepubertal IGF-1 concentration with age at thelarche (B2) and menarche (M) in girls and age at gonadarche (G2) in boys.

METHODS

This is a longitudinal study (n = 1,196 boys and girls) within the Growth and Obesity Chilean Cohort Study (GOCS). At age ≈ 6.7 years, blood sample was taken for IGF-1. Subjects were divided into 4 groups according to the onset age of the pubertal event.

RESULTS

Higher prepubertal IGF-1 levels were observed at earlier ages of B2 (p = 0.003) and M onset (p = 0.041). A taller prepubertal height was observed at younger ages of B2 and M (p=<0.001 and 0.002, respectively). The hazard proportional regression models (HR) showed that with an increase of 1 SD in IGF-1, the HR of presenting B2 at younger ages was 1.25, and this association was maintained when adjusted for confounding variables. Similarly, the HR of presenting M at earlier ages was 1.21. This association was maintained only when adjusting for body mass index but not using further confounders. In boys, prepubertal IGF-1 showed a tendency to be significantly higher in children with earlier G2 and taller height (both p < 0.001). The HR of presenting G2 at younger ages was 1.22, and this association was maintained after adjusting for confounders.

CONCLUSIONS

Higher IGF-1 levels in mid-childhood are associated with earlier puberty onset. The role of IGF-1 in the onset of puberty requires further investigation.

Growth variations with opposite clinical outcomes and the emerging role of IGF-1

Normal growth pattern variations [i.e., constitutional advancement and constitutional delay of growth and puberty (CAGP and CDGP)] are the mirror image of each other and are associated with early puberty (EP) and delayed puberty (DP), respectively. Differences between CAGP and CDGP relate not only to auxological characteristics (height, weight) but also to insulin-like growth factor-1 (IGF-1). IGF-1 levels in CAGP are above average whereas in CDGP they are below average, suggesting a role for IGF-1 in the induction of these growth patterns. Herein, we provide data suggesting that early activation of the growth hormone (GH)/IGF-1 axis induces the growth pattern of CAGP. Moreover, we suggest that IGF-1 is a decisive factor for the release of the gonadotropin-releasing hormone (GnRH) inhibition brake that occurs in prepuberty. It is therefore crucial for puberty onset.

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.

The insulin-like growth factor-1 system in the adult mammalian brain and its implications in central maternal adaptation

Our knowledge on the bioavailability and actions of insulin-like growth factor-1 (IGF-1) has markedly expanded in recent years as novel mechanisms were discovered on IGF binding proteins (IGFBPs) and their ability to release IGF-1. The new discoveries allowed a better understanding of the endogenous physiological actions of IGF-1 and also its applicability in therapeutics. The focus of the present review is to summarize novel findings on the neuronal, neuroendocrine and neuroplastic actions of IGF-1 in the adult brain. As most of the new regulatory mechanisms were described in the periphery, their implications on brain IGF system will also be covered. In addition, novel findings on the effects of IGF-1 on lactation and maternal behavior are described. Based on the enormous neuroplastic changes related to the peripartum period, IGF-1 has great but largely unexplored potential in maternal adaptation of the brain, which is highlighted in the present review.

IGF-1 in the Brain as a Regulator of Reproductive Neuroendocrine Function

Given the close relationship among neuroendocrine systems, it Is likely that there may be common signals that coordinate the acquisition of adult reproductive function with other homeo-static processes. In this review, we focus on central nervous system insulin-like growth factor-1 (IGF-1) as a signal controlling reproductive function, with possible links to somatic growth, particularly during puberty. In vertebrates, the appropriate neurosecretion of the decapeptide gonadotropin-releas-ing hormone (GnRH) plays a critical role in the progression of puberty. Gonadotropin-releasing hormone is released in pulses from neuroterminals in the median eminence (ME), and each GnRH pulse triggers the production of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These pituitary hormones in turn stimulate the synthesis and release of sex steroids by the gonads. Any factor that affects GnRH or gonadotropin pulsatility is important for puberty and reproductive function and, among these factors, the neurotrophic factor IGF-1 is a strong candidate. Although IGF-1 is most commonly studied as the tertiary peripheral hormone in the somatotropic axis via its synthesis in the liver, IGF-1 Is also synthesIzed in the brain, within neurons and glia. In neuroendocrine brain regions, central IGF-1 plays roles in the regulation of neuroendocrine functions, including direct actions on GnRH neurons. Moreover, GnRH neurons themselves co-express IGF-1 and the IGF-1 receptor, and this expression is developmentally regulated. Here, we examine the role of IGF-1 acting in the hypothalamus as a critical link between reproductive and other neuroendocrine functions.

Hippocampal Gene Expression Is Highly Responsive to Estradiol Replacement in Middle-Aged Female Rats

In the hippocampus, estrogens are powerful modulators of neurotransmission, synaptic plasticity and neurogenesis. In women, menopause is associated with increased risk of memory disturbances, which can be attenuated by timely estrogen therapy. In animal models of menopause, 17β-estradiol (E2) replacement improves hippocampus-dependent spatial memory. Here, we explored the effect of E2 replacement on hippocampal gene expression in a rat menopause model. Middle-aged ovariectomized female rats were treated continuously for 29 days with E2, and then, the hippocampal transcriptome was investigated with Affymetrix expression arrays. Microarray data were analyzed by Bioconductor packages and web-based softwares, and verified with quantitative PCR. At standard fold change selection criterion, 156 genes responded to E2. All alterations but 4 were transcriptional activation. Robust activation (fold change > 10) occurred in the case of transthyretin, klotho, claudin 2, prolactin receptor, ectodin, coagulation factor V, Igf2, Igfbp2, and sodium/sulfate symporter. Classification of the 156 genes revealed major groups, including signaling (35 genes), metabolism (31 genes), extracellular matrix (17 genes), and transcription (16 genes). We selected 33 genes for further studies, and all changes were confirmed by real-time PCR. The results suggest that E2 promotes retinoid, growth factor, homeoprotein, neurohormone, and neurotransmitter signaling, changes metabolism, extracellular matrix composition, and transcription, and induces protective mechanisms via genomic effects. We propose that these mechanisms contribute to effects of E2 on neurogenesis, neural plasticity, and memory functions. Our findings provide further support for the rationale to develop safe estrogen receptor ligands for the maintenance of cognitive performance in postmenopausal women.

Early growth hormone (GH) treatment promotes relevant motor functional improvement after severe frontal cortex lesion in adult rats

A number of studies, in animals and humans, describe the positive effects of the growth hormone (GH) treatment combined with rehabilitation on brain reparation after brain injury. We examined the effect of GH treatment and rehabilitation in adult rats with severe frontal motor cortex ablation. Thirty-five male rats were trained in the paw-reaching-for-food task and the preferred forelimb was recorded. Under anesthesia, the motor cortex contralateral to the preferred forelimb was aspirated or sham-operated. Animals were then treated with GH (0.15 mg/kg/day, s.c) or vehicle during 5 days, commencing immediately or 6 days post-lesion. Rehabilitation was applied at short- and long-term after GH treatment. Behavioral data were analized by ANOVA following Bonferroni post hoc test. After sacrifice, immunohistochemical detection of glial fibrillary acid protein (GFAP) and nestin were undertaken in the brain of all groups. Animal group treated with GH immediately after the lesion, but not any other group, showed a significant improvement of the motor impairment induced by the motor lesion, and their performances in the motor test were no different from sham-operated controls. GFAP immunolabeling and nestin immunoreactivity were observed in the perilesional area in all injured animals; nestin immunoreactivity was higher in GH-treated injured rats (mainly in animals GH-treated 6 days post-lesion). GFAP immunoreactivity was similar among injured rats. Interestingly, nestin re-expression was detected in the contralateral undamaged motor cortex only in GH-treated injured rats, being higher in animals GH-treated immediately after the lesion than in animals GH-treated 6 days post-lesion. Early GH treatment induces significant recovery of the motor impairment produced by frontal cortical ablation. GH effects include increased neurogenesis for reparation (perilesional area) and for increased brain plasticity (contralateral motor area).

Activity-dependent IGF-1 exocytosis is controlled by the Ca(2+)-sensor synaptotagmin-10

Synaptotagmins Syt1, Syt2, Syt7, and Syt9 act as Ca(2+)-sensors for synaptic and neuroendocrine exocytosis, but the function of other synaptotagmins remains unknown. Here, we show that olfactory bulb neurons secrete IGF-1 by an activity-dependent pathway of exocytosis, and that Syt10 functions as the Ca(2+)-sensor that triggers IGF-1 exocytosis in these neurons. Deletion of Syt10 impaired activity-dependent IGF-1 secretion in olfactory bulb neurons, resulting in smaller neurons and an overall decrease in synapse numbers. Exogenous IGF-1 completely reversed the Syt10 knockout phenotype. Syt10 colocalized with IGF-1 in somatodendritic vesicles of olfactory bulb neurons, and Ca(2+)-binding to Syt10 caused these vesicles to undergo exocytosis, thereby secreting IGF-1. Thus, Syt10 controls a previously unrecognized pathway of Ca(2+)-dependent exocytosis that is spatially and temporally distinct from Ca(2+)-dependent synaptic vesicle exocytosis controlled by Syt1. Our findings thereby reveal that two different synaptotagmins can regulate functionally distinct Ca(2+)-dependent membrane fusion reactions in the same neuron.

Effects of recombinant growth hormone replacement and physical rehabilitation in recovery of gross motor function in children with cerebral palsy

Cerebral palsy is an important health issue that has a strong socioeconomic impact. There is no cure for cerebral palsy, and therapeutic approaches only report small benefits for affected people. In this study we assessed the effects of growth hormone treatment (0.3 μg/kg/day) combined with physical rehabilitation in the recovery of gross motor function in children with growth hormone deficiency and cerebral palsy (four males and six females, mean age 5.63 ± 2.32 years) as compared with that observed in a similar population of cerebral palsy children (five males, five females, mean age 5.9 ± 2.18 years) without growth hormone deficiency treated only with physical rehabilitation for two months. The Gross Motor Function Measure (GMFM-88) and Modified Ashworth Scale were performed before commencing the treatment and after completion thereof. In children with cerebral palsy and growth hormone deficiency, Dimension A (P < 0.02), dimension B (P < 0.02), and dimension C (P < 0.02) of the GMFM-88, and the total score of the test (P < 0.01) significantly improved after the treatment; dimension D and dimension E did not increase, and four of five spastic patients showed a reduction in spasticity. However, in children with cerebral palsy and without growth hormone deficiency, only the total score of the test improved significantly after the treatment period. This indicates that growth hormone replacement therapy was responsible for the large differences observed between both groups in response to physical rehabilitation. We propose that the combined therapy involving growth hormone administration and physical rehabilitation may be a useful therapeutic approach in the recovery of gross motor function in children with growth hormone deficiency and cerebral palsy.

Effects of growth hormone (GH) replacement and cognitive rehabilitation in patients with cognitive disorders after traumatic brain injury

OBJECTIVE

To assess the effects of growth hormone (GH) treatment combined with cognitive rehabilitation in patients with adult growth hormone deficiency (GHD) and cognitive disorders occurring after traumatic brain injury (TBI).

PARTICIPANTS

Nineteen adult patients with TBI: GHD was found in 11 of them.

INTERVENTION

Patients were treated with GH (GHD; sc; 1 mg/day) or vehicle (controls; sc; 1 mg/day); daily cognitive rehabilitation therapy was performed in both groups for 3 months.

MAIN OUTCOME MEASURES

The GHRH-arginine test established GHD. The neuropsychological test WAIS was performed before commencing the treatment and 3 months after commencing it.

RESULTS

Controls achieved significant improvements in digits and in manipulative intelligence quotient (IQ) (p < 0.05 vs. baseline). GHD achieved significant improvements in more cognitive parameters: understanding, digits, numbers and incomplete figures (p < 0.05 vs. baseline) and similarities, vocabulary, verbal IQ, manipulative IQ and total IQ (p < 0.01). GHD reached significantly greater improvements than controls in similarities (p < 0.01) and in vocabulary, verbal IQ and total IQ (p < 0.05).

CONCLUSION

GH administration significantly improved cognitive rehabilitation in GHD patients. Since at the end of treatment period plasma IGF-I levels were similar in both groups it is likely that exogenous GH administration is responsible for the significant differences found.

Growth hormone deficiency and cerebral palsy

Cerebral palsy (CP) is a catastrophic acquired disease, occurring during development of the fetal or infant brain. It mainly affects the motor control centres of the developing brain, but can also affect cognitive functions, and is usually accompanied by a cohort of symptoms including lack of communication, epilepsy, and alterations in behavior. Most children with cerebral palsy exhibit a short stature, progressively declining from birth to puberty. We tested here whether this lack of normal growth might be due to an impaired or deficient growth hormone (GH) secretion. Our study sample comprised 46 CP children, of which 28 were male and 18 were female, aged between 3 and 11 years. Data obtained show that 70% of these children lack normal GH secretion. We conclude that GH replacement therapy should be implemented early for CP children, not only to allow them to achieve a normal height, but also because of the known neurotrophic effects of the hormone, perhaps allowing for the correction of some of the common disabilities experienced by CP children.

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.

Antidepressant-like behavioral effects of IGF-I produced by enhanced serotonin transmission

Previous research has suggested that mobilization of neurotrophic factors, such as insulin-like growth factor I (IGF-I), can be involved in the effects of antidepressant treatments. The current experiments showed that IGF-I leads to antidepressant-like effects in the modified rat forced swim test when tested 3 days, but not 1 day, after i.c.v. administration. These effects were sustained longer than the antidepressants paroxetine and desipramine. In addition, blockade of the IGF-I receptor with the IGF-I antagonist JB1 30 min before IGF-I administration prevented the antidepressant-like effects of IGF-I. However, when JB1 was administered 3 days after IGF-I administration and 30 min prior to testing, the antidepressant-like effects of IGF-I were still present suggesting that IGF-1 produces a long-term activation of neural systems involved in the antidepressant response. Because the pattern of antidepressant-like effects of IGF-I resembled those of selective serotonin reuptake inhibitors, the role of serotonin in the behavioral effects of IGF-I was studied. Depletion of serotonin, by the tryptophan hydroxylase inhibitor para-chlorophenylalanine, blocked the antidepressant-like effects of IGF-I. Administration of IGF-I increased basal serotonin levels in the ventral hippocampus and altered the effects of acute citalopram. IGF-I administration did not change hippocampal cell proliferation at the 3-day timepoint when behavioral effects were seen. In addition, IGF-I did not alter the expression of mRNA levels of tryptophan hydroxylase or SERT in the brain stem, or [3H] citalopram binding in the hippocampus or cortex. Thus, IGF-I administration initiates a long-lasting cascade of neurochemical effects involving increased serotonin levels that results in antidepressant-like behavioral effects.

Expression of insulin-like growth factor system genes during the early postnatal neurogenesis in the mouse hippocampus

Insulin-like growth factor-1 (IGF-1) is essential to hippocampal neurogenesis and the neuronal response to hypoxia/ischemia injury. IGF (IGF-1 and -2) signaling is mediated primarily by the type 1 IGF receptor (IGF-1R) and modulated by six high-affinity binding proteins (IGFBP) and the type 2 IGF receptor (IGF-2R), collectively termed IGF system proteins. Defining the precise cells that express each is essential to understanding their roles. With the exception of IGFBP-1, we found that mouse hippocampus expresses mRNA for each of these proteins during the first 2 weeks of postnatal life. Compared to postnatal day 14 (P14), mRNA abundance at P5 was higher for IGF-1, IGFBP-2, -3, and -5 (by 71%, 108%, 100%, and 98%, respectively), lower for IGF-2, IGF-2R, and IGFBP-6 (by 65%, 78%, and 44%, respectively), and unchanged for IGF-1R and IGFBP-4. Using laser capture microdissection (LCM), we found that granule neurons and pyramidal neurons exhibited identical patterns of expression of IGF-1, IGF-1R, IGF-2R, IGFBP-2, and -4, but did not express other IGF system genes. We then compared IGF system expression in mature granule neurons and their progenitors. Progenitors exhibited higher mRNA levels of IGF-1 and IGF-1R (by 130% and 86%, respectively), lower levels of IGF-2R (by 72%), and similar levels of IGFBP-4. Our data support a role for IGF in hippocampal neurogenesis and provide evidence that IGF actions are regulated within a defined in vivo milieu.

Peripheral infusion of insulin-like growth factor-I increases the number of newborn oligodendrocytes in the cerebral cortex of adult hypophysectomized rats

We have previously shown that recombinant human (rh) IGF-I induces cell proliferation and neurogenesis in the hippocampus of hypophysectomized rats. In the current investigation, we determined the effects of rhIGF-I on proliferation and differentiation in the cerebral cortex. Adult hypophysectomized rats were injected with bromodeoxyuridine (BrdU) to label newborn cells (once a day for the first 5 d), and rhIGF-I was administered peripherally for 6 or 20 d. In the cerebral cortex, the number of BrdU-labeled cells increased after 20 d but not after 6 d of rhIGF-I infusion. This suggests that rhIGF-I enhances the survival of newborn cells in the cerebral cortex. Using BrdU labeling combined with the oligodendrocyte-specific markers myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase, we demonstrated an increase in oligodendrogenesis in the cerebral cortex. The total amount of myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase was also increased on Western blots of homogenates of the cerebral cortex, confirming the immunohistochemical findings. Also, we observed an increase in the number of capillary-associated BrdU-positive cells, although total capillary area was not increased. rhIGF-I treatment did not affect cortical astrogliogenesis and neurogenesis was not observed. The ability of rhIGF-I to induce cortical oligodendrogenesis may have implications for the regenerative potential of the cortex.

Discrete expression of TRPV2 within the hypothalamo-neurohypophysial system: Implications for regulatory activity within the hypothalamic-pituitary-adrenal axis

Transient receptor potential channel proteins (TRPs) constitute a steadily growing family of ion channels with a range of purported functions. It has been demonstrated that TRPV2 is activated by moderate thermal stimuli and, in the rat, is expressed in medium to large diameter dorsal root ganglion neurons. In this study, antisera specific for the human TRPV2 homologue were raised and characterized for immunohistochemical use. Subsequently, thorough investigation was made of the localization of this cation channel in the macaque primate brain. TRPV2-immunoreactive material was highly restrictively localized to hypothalamic paraventricular, suprachiasmatic, and supraoptic nuclei. Confocal double- and triple-labeling studies demonstrated that TRPV2 immunoreactivity is preferentially localized to oxytocinergic and vasopressinergic neurons. Few, if any, cells in these regions expressed TRPV2 immunoreactivity in the absence of oxytocin immunoreactivity or vasopressin immunoreactivity. Expression in the paraventricular and supraoptic nuclei suggests that TRPV2 is likely to play a fundamental role in mediating cation transport in neurohypophysial neurons. TRPV2 has been shown to be translocated upon cell activation and neurons expressing TRPV2 immunoreactivity in vivo are among those known to engage in sporadic, intense activity. Taken together, these data suggest that this channel may play a vital role in mediating physiological activities associated with oxytocin and vasopressin release such as parturition, lactation, and diuresis. These data may also implicate the involvement of TRPV2 in disorders of the hypothalamic-pituitary-adrenal axis, including anxiety, depression, hypertension, and preterm labor.

Differential regulation of gonadotropins and glycoprotein hormone alpha-subunit by IGF-I in anterior pituitary cells from male rats

IGF-I has been demonstrated to stimulate basal and GnRH-induced gonadotropin release. IGF-I also elicites alpha-subunit secretion in human pituitary tumor cells. The aims of this study were to evaluate both the effect of IGF-I on gonadotropin LH-beta and FSH-beta mRNA levels and glycoprotein alpha-subunit gene expression in cultured rat anterior pituitary cells. The exposure of pituitary cells to recombinant human IGF-I (rhlGF-I; 2 microg/ml) for 72 h markedly stimulated basal LH and FSH release whereas their mRNA levels remained unmodified. IGF-I elicited a-subunit release from pituitary cells (p < 0.01) and augmented its mRNA levels. Exposure to IGF-I consistently reduced GH release from pituitary cells. This study shows that the gonadotropin-releasing effects of IGF-I are not paralleled by changes in their mRNAs whereas IGF-I stimulates not only alpha-subunit release but also its mRNA levels. This study provides the first observation of alpha-subunit regulation by IGF-I in normal pituitary cells, where a differential regulation between release and synthesis for gonadotropin a-and 1-subunits is also shown.

Depolarization induces insulin-like growth factor binding protein-2 expression in vivo via NMDA receptor stimulation

The effect of depolarization and N-methyl-D-aspartate (NMDA) receptor blockade on insulin-like growth factor-I (IGF-I), IGF binding protein-2 (IGFBP-2) and IGFBP-4 expression was analysed in vivo. Depolarization was induced in adult rat brains by applying 3 M KCl to the exposed cortex for 10 min. A subgroup of animals also received daily injections of MK-801. Four days after KCl exposure, the brains were analysed by in situ hybridization, immunohistochemistry and TUNEL. A significant upregulation of IGFBP-2 mRNA and protein was detected in astrocytes after KCl exposure This upregulation was reduced by MK-801 treatment. No alterations in IGF-I or IGFBP-4 mRNA levels were noted. We did not detect TUNEL positive cells, morphological signs of necrosis or apoptosis, or neuronal loss in the depolarized zone. Taken together, these findings indicate that upregulation of IGFBP-2 by depolarization is mediated by NMDA receptors, and, as no neuronal damage was detected, astrocytic NMDA receptors may be responsible for this upregulation.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

The impact of the GH-IGF-I axis on gonadotropin secretion: inferences from animal models

Given the tight, temporal coupling between growth and reproductive development, the idea that a common signal may regulate both adolescent growth and the initiation of puberty has been the focus of much research. Since the rate-limiting step for the onset of puberty is the appropriate hypothalamic secretion of gonadotropin-releasing hormone (GnRH), any factor important for the initiation of puberty must affect GnRH pulsatility. This review examines the hypothesis that GH and/or IGF-I are growth-related signals that regulate the release of GnRH, initiating puberty. By extension, this review also addresses the hypothesis that the GH axis also impacts GnRH and gonadotropin secretion in post-pubertal individuals and, thus, affects the maintenance of fertility in adults. The review examines data from a range of animal models employing a number of different strategies which directly manipulate the activity of either GH or IGF-I. The success of these strategies for producing the desired effects on the GH-IGF-I axis is somewhat variable. Although IGF-I may only play a permissive role in the maintenance of adult fertility, acting at the level of the gonad to increase sensitivity to gonadotropin stimulation, the data indicate that IGF-I is essential for reproductive maturation. However, in addition to its well-documented effects on the gonad, the specific mode of action of IGF-I on the neuroendocrine hypothalamus and GnRH pulsatility remains to be determined. Available evidence suggests that such action by IGF-I may be mediated through neurotransmitter effects on GnRH neurons, changing the availability of metabolic substrates for neuronal activity, or remodeling of synaptic input into GnRH neurons.

Serum growth factors and neuroprotective surveillance: focus on IGF-1

The adult brain requires a constant trophic input for appropriate function. Although the main source of trophic factors for mature neurons is considered to arise locally from glial cells and synaptic partners, recent evidence suggests that hormonal-like influences from distant sources may also be important. These include not only relatively well-characterized steroid hormones that cross the brain barriers, but also blood-borne protein growth factors able to cross the barriers and exert unexpected, albeit specific, trophic actions in diverse brain areas. Insulin-like growth factor I (IGF-I) is until now the serum neurotrophic factor whose actions on the adult brain are best-characterized. This is because IGF-I has been known for many years to be present in serum, whereas the presence in the circulation of other more classical neurotrophic factors has only recently been recognized. Thus, new evidence strongly suggests that IGF-I, and other blood-borne neurotrophic factors such as Fibroblast Growth Factor (FGF-2) or the neurotrophins, exert a tonic trophic input on brain cells, providing a mechanism for what we may refer to as neuroprotective surveillance. Protective surveillance includes "first-line" defense mechanisms ranging from blockade of neuronal death after a wide variety of cellular insults to upregulation of neurogenesis when defenses against neuronal death are overcome. Most importantly, surveillance should also encompass modulation of homeostatic mechanisms to prevent neuronal derangement. These will include modulation of basic cellular processes such as metabolic demands and maintainance of cell-membrane potential as well as more complex processes such as regulation of neuronal plasticity to keep neurons able to respond to constantly changing functional demands.

Evidence that IGF-I acts as an autocrine/paracrine growth factor in the magnocellular neurosecretory system: neuronal synthesis and induction of axonal sprouting

The ability of mature oxytocinergic (OT) and vasopressinergic (VP) neurons of the magnocellular neurosecretory system (MNS) to undergo axonal growth implies that one or more growth factors may be active in the adult MNS, yet little is known regarding their possible identity. One such potential factor is insulin-like growth factor I (IGF-I). We have examined the expression of IGF-I mRNA and IGF-I-immunoreactivity (IGF-I-ir) in the mature MNS and have also determined the in vivo response of OT and VP neurons to hypothalamic implants of IGF-I. In situ hybridization revealed moderate labeling of IGF-I mRNA in both the supraoptic (SON) and the paraventricular (PVN) nuclei of adult male rats. RT-PCR analysis confirmed the presence of authentic IGF-I mRNA in extracts of the basal hypothalamus. Faint IGF-I-ir was detected in scattered magnocellular neurons within both the PVN and the SON of normal rats, but IGF-I-ir was much more intense and the majority of MNS neurons including those in the accessory nuclei were immunoreactive in sections from rats given colchicine, as were some parvocellular neurons in the PVN. Confocal microscopy revealed that IGF-I-ir was present in both OT and VP neurons, but VP neurons contained the most intense IGF-I-ir. Finally, a dramatic growth response of OT but not of VP fibers was observed following implantation of polymer rods containing IGF-I into the hypothalamus. A dense OT fiber plexus grew along the cannula track and OT fibers invaded the leptomeninges ventral to the SON and encircled the rostral cerebral artery. To our knowledge this is the first demonstration of axonal sprouting by mature OT neurons in response to an identified growth factor and the first direct demonstration of sprouting in response to exogenous IGF-I in the adult CNS. These findings suggest that IGF-I is synthesized and transported by adult MNS neurons where it may act as an autocrine and/or paracrine growth factor.

Central peptidergic neurons are hyperactive during collateral sprouting and inhibition of activity suppresses sprouting

Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Cell and molecular biology of the pars tuberalis of the pituitary

The pars tuberalis of the adenohypophysis is mainly composed of a special type of endocrine cells, pars tuberalis-specific cells, lining the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes in morphological appearance, production pattern, and secretory activity during annual cycle show that these cells are highly sensitive to changes in photoperiod. This leads to the hypothesis that the pars tuberalis is involved in the transmission of photoperiodic stimuli to endocrine targets. Several investigations support the theory that pars tuberalis-specific cells are multipotential cells exerting a modulatory influence on the secretory activity of the pars distalis. Specifically, there is accumulating evidence that seasonal modulation of prolactin secretion, independent of hypothalamic input, is due to melatonin-regulated activity of pars tuberalis-specific cells. The exact nature of secretory products and their effects within neuroendocrine regulation, however, remain rather enigmatic. Accordingly, molecular mechanisms regulating gene expression under the influence of photoperiod, respectively, circulating melatonin levels are still incomplete. Recent cloning of melatonin receptor genes and new data on intracellular signal transduction will probably lead to new insights on melatonin action and pars tuberalis-specific cell physiology.

Ethanol blocks the central action of IGF-1 to induce luteinizing hormone secretion in the prepubertal female rat

Insulin-like growth factor-1 (IGF-1) is emerging as a signal of peripheral origin capable of acting centrally to induce luteinizing hormone (LH) secretion and accelerate the initiation of female puberty. Since we have shown previously that ethanol (ETOH) can suppress prepubertal LH release and delay female puberty, we hypothesized that these detrimental effects might be due, at least in part, to the drugs ability to alter the central actions of IGF-1. Thus, we have investigated the effects of ETOH on IGF-1 induced LH release in vivo, and on IGF-1 induced prostaglandin-E2 (PGE2) and LH-releasing hormone (LHRH) release in vitro from rats during the juvenile phase of development as well as from rats during the early stage of first proestrus. For the in vivo experiment three initial jugular blood samples were taken at 10-min. intervals from all rats, then the animals received either a 3g/Kg dose of ETOH or an equal volume of saline by gastric gavage. The rats were subsequently left undisturbed for 90 min. to allow time for ETOH absorption, then a single blood sample was drawn from each rat. Finally, IGF-1 (200 ng/3 microl) was microinjected into the third ventricle of all animals and five more blood samples were withdrawn at 10 min. intervals. We demonstrated that IGF-1 induced the release of LH (p<0.01) in the saline controls from rats in both phases of pubertal development. Conversely, this effect of IGF-1 was blocked by ETOH in both developmental groups. For the in vitro experiment, median eminences (MEs) were dissected, then incubated in the presence or absence of ETOH (50 mM). The amount of PGE2 and LHRH released was measured from the same samples following the addition of IGF-1 (100 ng/ml). As above, similar responses were observed from rats in both developmental phases. IGF-1 stimulated the release of PGE2 (p<0.001) and LHRH (p<0.01) from the incubated nerve terminals in the absence of ETOH; however, these effects were blocked by the presence of ETOH. Thus, these combined in vivo and in vitro results suggest that ETOH can acutely block IGF-1 induced LH release during the juvenile-peripubertal transition period, and that this is a centrally mediated action which is due to the diminished formation of PGE2 resulting in decreased LHRH release.

Increase of insulin-like growth factor (IGF)-1, IGF binding protein-2 and -4 mRNAs following cerebral contusion

The insulin-like growth factor (IGF) system has a role in repair following hypoxic-ischemic injury in many tissues including the brain. To study the involvement of the IGF system following head trauma, we used a rat contusion model, which produces a focal lesion of the cerebral cortex. Molecules in the IGF system were analyzed using in situ hybridization at different times following impact. We observed a dramatic up-regulation of insulin-like growth factor binding protein-2 (IGFBP-2) mRNA in cortical areas adjacent to the injury 24 h after impact, with a peak 10-fold increase engaging most of the ipsilateral cortex 2 and 3 days post-contusion. Seven days after the contusion, IGFBP-2 expression was only moderately up-regulated and again concentrated around the injury. IGFBP-4 mRNA levels increased 4-fold ipsilateral to the site of injury, with retained pattern of cortical expression. IGFBP-3, IGFBP-5 and IGFBP-6 mRNA all displayed distinct expression patterns in the brain but no significant changes were observed following injury. In contrast, IGF-1 mRNA levels were very low prior to contusion, but increased markedly at the site of injury with a peak at day 3. We were unable to detect any changes in the type 1 IGF-receptor or IGF-2 mRNA following contusion. The neuropeptide cholecystokinin (CCK) mRNA was clearly up-regulated following contusion, with an even distribution over the ipsilateral cortex. The expression pattern of molecules in the IGF system post-contusion differs in part to changes observed following hypoxic-ischemia or ischemia alone, perhaps reflecting different regulatory mechanisms depending on the type of injury.

Localization of [125I]IGF-I binding on the ovine pars tuberalis

In the sheep, it has been shown that the pars tuberalis of the pituitary may mediate the photoperiodic control of seasonal changes in prolactin secretion. High concentrations of melatonin receptors are present on the ovine pars tuberalis and melatonin is known to inhibit forskolin-stimulated cyclic AMP production in this tissue. Other hormonal inputs to the ovine pars tuberalis have not yet been identified. In the rat mRNA for the IGF-I receptor has been identified in the pars tuberalis using in situ hybridization. In order to define whether IGF-I may influence the function of the ovine pars tuberalis the presence of receptors for IGF-I has been investigated. Using in vitro autoradiography specific [125I]IGF-I binding was found in high concentrations over the ovine pars tuberalis particularly associated with certain of the capillaries. Homogenate receptor assays showed saturable specific binding of [125I]IGF-I with a mean dissociation constant (Kd) of 0.5 +/- 0.1 nM (n = 4). Competition studies revealed a rank order of potency of IGF-I > IGF-II > > > insulin, in displacing [125I]IGF-I binding, indicative of a mixed population of IGF-I and IGF-II/mannose-6-phosphate receptors and insulin-like growth factor binding proteins (IGFBPs). Cross-linking of [125I]IGF-I to pars tuberalis membrane homogenates and analysis by SDS-PAGE under reducing conditions confirmed the presence of both IGF-I receptors and binding proteins. Autophosphorylation of a 97 kDa substrate, compatible with the beta-sub-unit of the IGF-I receptor, was increased in the presence of IGF-I, indicating the existence of functional IGF-I receptors on the ovine pars tuberalis. In contrast in the rat [125I]IGF-I binding was restricted to the median eminence region of the brain and was not detectable over the pars tuberalis.

A DNA methylation site in the male-specific P450 (Cyp 2d-9) promoter and binding of the heteromeric transcription factor GABP

The Cyp 2d-9 gene encodes the male-specific steroid 16 alpha-hydroxylase in mouse liver and shares a conserved regulatory element (-100TTCCGGGC-93) with another male-specific Slp promoter. As shown with the Slp promoter (N. Yokomori, R. Moore, and M. Negishi, Proc. Natl. Acad. Sci. USA 92:1302-1306, 1995), the male-preferential demethylation also occurs at CpG/-97 in the Cyp 2d-9 promoter. The transcription factor which specifically binds to the demethylated element has been purified. The peptide sequences reveal that the factor consists of GABP alpha and GABP beta 1 with Ets and Notch motifs, respectively. Both DNase I footprinting and gel shift assays indicate that the bacterially expressed glutathione S-transferase-GABP fusion proteins bind to the regulatory element only when CpG/-97 is demethylated. Moreover, Cyp 2d-9 promoter is trans-activated by coexpression of GABP proteins in HepG2 cells. Given the additional results that CpG/-50 of the female-specific steroid 15 alpha-hydroxylase (Cyp 2a-4) promoter is preferentially demethylated in the females, the sex-specific expressions of the P450 genes correlate very well with DNA demethylation. We also conclude that GABP is a methylation-sensitive transcription factor and is a potential transcription activator of the male-specific Cyp 2d-9 promoter.

Sexually dimorphic DNA demethylation in the promoter of the Slp (sex-limited protein) gene in mouse liver

Mouse Slp, a duplicate of the fourth complement component (C4) gene, exhibits EDTA-independent complement activity with a hepatic expression that is male specific. To provide an underlying mechanism for the male-specific expression, we have analyzed the promoter activity of the various 5'-flanking sequences and CpG demethylation of the Slp gene. Transient transfections using HepG2 cells indicate that the element TTCCGGGC (nt -124 to -117) regulates the promoter activity. Moreover, CpG at position -121 of this regulatory element is demethylated to a much higher degree in males than in females. This sexually dimorphic DNA demethylation is consistent with the male-specific expression of the Slp gene in DBA/2 males. The regulatory element binds to the different TTCCGGGC-specific nuclear proteins depending on the methylation of the CpG site. In contrast, the corresponding CpG at position -119 of the C4 gene, which is expressed in both males and females, is demethylated at equal and high levels in both sexes. We therefore propose that the DNA demethylation and methylation-sensitive transcription factors may be a part of the regulatory mechanism for the male-specific expression of the Slp gene.