Transforming growth factor-alpha gene expression in the hypothalamus is developmentally regulated and linked to sexual maturation

Time, telomeres and tumours: is cellular senescence more than an anticancer mechanism?

Normal diploid cells, by definition, have a limited life span: they senesce after a set number of divisions both in vivo and in culture. It has been hypothesized that the molecular mechanism that measures the life span of a cell probably involves the shortening of telomeres that occurs with each round of DNA replication. This loss of telomeres is thought to induce antiproliferative signals that result in the induction of cellular senescence. In this article, Woodring Wright and Jerry Shay present a hypothesis for the mechanisms by which telomere shortening regulates cellular physiology and argue that cellular senescence is not only an anticancer mechanism but is also the cause of many of the degenerative changes of aging.

Activation of erbB-1 signaling in tanycytes of the median eminence stimulates transforming growth factor beta1 release via prostaglandin E2 production and induces cell plasticity

The activation of transforming growth factor alpha (TGFalpha)-erbB-1 and neuregulin-erbB-4 signaling pathways in hypothalamic astrocytes has been shown to play a key role in the process by which the neuroendocrine brain controls luteinizing hormone-releasing hormone (LHRH) secretion. Earlier studies suggested that tanycytes, an ependymoglial cell type of the median eminence, regulate LHRH release during the estrous cycle by undergoing plastic changes that alternatively allow or prevent direct access of the LHRH nerve terminals to the portal vasculature. Neither the molecules responsible for these plastic changes nor the underlying controlling mechanisms have been identified. Here we show that cultured tanycytes express erbB-1 and erbB-2, two of the four members of the erbB receptor family, and respond to TGFalpha with receptor phosphorylation, release of prostaglandin E2 (PGE2), and a PGE2-dependent increase in the release of TGFbeta1, a growth factor previously implicated in the glial control of LHRH secretion. Blockade of either erbB-1 receptor signal transduction or prostaglandin synthesis prevented the stimulatory effect of TGFalpha on both PGE2 and TGFbeta1 release. Time-lapse studies revealed that TGFalpha and TGFbeta1 have dramatically opposite effects on tanycyte plasticity. Whereas TGFalpha promotes tanycytic outgrowth, TGFbeta1 elicits retraction of tanycytic processes. Blockade of metalloproteinase activity abolished the effect of TGFbeta1, suggesting that TGFbeta1 induces tanycytic retraction by facilitating dissolution of the extracellular matrix. Prolonged (>12 hr) exposure of tanycytes to TGFalpha resulted in focal tanycytic retraction, an effect that was abolished by immunoneutralization of TGFbeta1 action, indicating that the retraction was attributable to TGFalpha-induced TGFbeta1 formation. These in vitro results identify tanycytes as targets of TGFalpha action and demonstrate that activation of erbB-1-mediated signaling in these cells results in plastic changes that, involving PGE2 and TGFbeta1 as downstream effectors, mimic the morphological plasticity displayed by tanycytes during the hours encompassing the preovulatory surge of LHRH.

The biology of gonadotropin hormone-releasing hormone: role in the control of tumor growth and progression in humans

It is now well known that different forms of GnRH coexist in the same vertebrate species. In humans, two forms of GnRH have been identified so far. The first form corresponds to the hypophysiotropic decapeptide, and is now called GnRH-I. The second form has been initially identified in the chicken brain, and it is referred to as GnRH-II. GnRH-I binds to and activates specific receptors, belonging to the 7 transmembrane (7TM) domain superfamily, present on pituitary gonadotropes. These receptors (type I GnRH receptors) are coupled to the Gq/11/PLC intracellular signalling pathway. A receptor specific for GnRH-II (type II GnRH receptor) has been identified in non-mammalian vertebrates as well as in primates, but not yet in humans. In the last 10-15 years experimental evidence has been accumulated indicating that GnRH-I is expressed, together with its receptors, in tumors of the reproductive tract (prostate, breast, ovary, and endometrium). In these hormone-related tumors, activation of type I GnRH receptors consistently decreases cell proliferation, mainly by interfering with the mitogenic activity of stimulatory growth factors (e.g., EGF, IGF). Recent data seem to suggest that GnRH-I might also reduce the migratory and invasive capacity of cancer cells, possibly by affecting the expression and/or activity of cell adhesion molecules and of enzymes involved in the remodelling of the extracellular matrix. These observations point to GnRH-I as an autocrine negative regulatory factor on tumor growth progression and metastatization. Extensive research has been performed to clarify the molecular mechanisms underlying the peculiar antitumor activity of GnRH-I. Type I GnRH receptors in hormone-related tumors correspond to those present at the pituitary level in terms of cDNA nucleotide sequence and protein molecular weight, but do not share the same pharmacological profile in terms of binding affinity for the different synthetic GnRH-I analogs. Moreover, the classical intracellular signalling pathway mediating the stimulatory activity of the decapeptide on gonadotropin synthesis and secretion is not involved in its inhibitory activity on hormone-related tumor growth. In these tumors, type I GnRH receptors are coupled to the Gi-cAMP, rather than the Gq/11-PLC, signal transduction pathway. Recently, we have reported that GnRH-I and type I GnRH receptors are expressed also in tumors not related to the reproductive system, such as melanoma. Also in melanoma cells, GnRH-I behaves as a negative regulator of tumor growth and progression. Interestingly, the biochemical and pharmacological profiles of type I GnRH receptors in melanoma seem to correspond to those of the receptors at pituitary level. The data so far reported on the expression and on the possible functions of GnRH-II in humans are still scanty. The decapeptide has been identified, together with a 'putative' type II GnRH receptor, both in the central nervous system and in peripheral structures, such as tissues of the reproductive tract (both normal and tumoral). The specific biological functions of GnRH-II in humans are presently under investigation.

Compromised reproductive function in adult female mice selectively expressing mutant ErbB-1 tyrosine kinase receptors in astroglia

The ErbB-1 tyrosine kinase receptor plays critical roles in regulating physiological functions. This receptor-mediated signaling in astroglia has been implicated in controlling female sexual development via activating neurons that release LH-releasing hormone (LHRH), the neuropeptide required for the secretion of LH. It remains unknown whether astroglial ErbB-1 receptors are necessary for maintaining normal adult reproductive function. Here we provide genetic evidence that astroglia-specific and time-controlled disruption of ErbB-1 receptor signaling by expressing mutant ErbB-1 receptors leads to compromised reproduction due to alteration in LHRH neuron-controlled secretion of LH in adult female mice. Therefore, astroglial ErbB-1 receptors are required for controlling LHRH neuronal function and thus maintaining adult reproduction, suggesting that compromised astroglial ErbB-1 signaling may also contribute to reproductive abnormalities in aging females.

Association of morphological characteristics with precocious puberty and/or gelastic seizures in hypothalamic hamartoma

The pathogenesis of central precocious puberty (PP) and/or gelastic seizures due to a hypothalamic hamartoma (HH) is still under debate. We evaluated the association of clinical symptoms with morphology and localization of the HH in 34 patients. The majority (86.4%) of HHs in patients with isolated PP (n = 22; 68.2% females) revealed a parahypothalamic position without affecting the third ventricle (91%). Half of them were pedunculated, and 40.9% showed a diameter less than 10 mm. In contrast, 11 of 12 patients with seizures, eight of whom were male, presented with a sessile intrahypothalamic hamartoma, 10 of which distorted the third ventricle. Logistic regression analysis revealed an increased relative risk (RR) for epilepsy in males (RR, 4.3; 95% confidence interval, 0.96-19). However, combination of the risk factor gender with intrahypothalamic position (RR, 19; 1.3-285) and distortion of the third ventricle (RR, 10; 0.6-164) reduced the risk associated with male gender to 1.1. The position of a HH and involvement of the third ventricle are likely to be more predictive for clinical characteristics than size and shape. Male gender was associated with an intrahypothalamic HH and epilepsy, suggesting a sexually dimorphic developmental pattern of this heterotopic mass.

Role of glial cells, growth factors and steroid hormones in the control of LHRH-secreting neurons

The mechanisms through which steroid hormones influence the LHRH system are not completely clarified and still represent a crucial and debated field of research in the neuroendocrine control of reproduction. Several data indicate that glial cells influence the activity of hypothalamic LHRH-secreting neurons, via the release of growth factors. It is now well known that glial cells express different kinds of steroid receptors and consequently may be considered as a target for the action of steroid hormones. To this purpose, the possibility that the effects of steroid hormones on LHRH neurons may be mediated by glial elements has been taken in consideration and observations supporting this hypothesis have been reported and discussed here. The results so far obtained strongly suggest that steroid hormones and growth factors, in order to exert their modulatory actions on LHRH dynamic, act in an integrated manner at the level of hypothalamic astrocytes.

Astrocytes and brain function: implications for reproduction

Recent evidence suggests that astrocytes have important neuroregulatory functions in addition to their classic functions of support and segregation of neurons. These newly revealed functions include regulation of neuron communication, neurosecretion, and synaptic plasticity. Although these actions occur throughout the brain, this review will focus on astrocyte-neuron interactions in the hypothalamus, particularly with respect to their potential contribution to the regulation of gonadotropin-releasing hormone (GnRH) secretion and reproduction. Hypothalamic astrocytes have been documented to release a variety of neuroactive factors, including transforming growth factors-alpha and -beta, insulin-like growth factor-1, prostaglandin E2, and the neurosteroid, 3 alpha-hydroxy-5 alpha-pregnane-20-one. Each of these factors has been shown to stimulate GnRH release, and receptors for each factor have been documented on GnRH neurons. Astrocytes have also been implicated in the regulation of synaptic plasticity in key areas of the hypothalamus that control GnRH release, an effect achieved by extension and retraction of glial processes (i.e., glial ensheathment). Through this mechanism, the number of synapses on GnRH neurons and GnRH regulatory neurons can potentially be modulated, thereby influencing the activation state of GnRH neurons. The steroid hormone 17beta-estradiol, which triggers the GnRH and luteinizing hormone surge, has been shown to induce the astrocyte-regulated changes in hypothalamic synaptic plasticity, as well as enhance formation and release of the astrocyte neuroactive factors, thereby providing another potential mechanistic layer for astrocyte regulation of GnRH release. As a whole, these studies provide new insights into the diversity of astrocytes and their potential role in reproductive neuroendocrine function.

Neuron-to-glia signaling mediated by excitatory amino acid receptors regulates ErbB receptor function in astroglial cells of the neuroendocrine brain

Hypothalamic astroglial erbB tyrosine kinase receptors are required for the timely initiation of mammalian puberty. Ligand-dependent activation of these receptors sets in motion a glia-to-neuron signaling pathway that prompts the secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, from hypothalamic neuroendocrine neurons. The neuronal systems that may regulate this growth factor-mediated back signaling to neuroendocrine neurons have not been identified. Here we demonstrate that hypothalamic astrocytes contain metabotropic receptors of the metabotropic glutamate receptor 5 subtype and the AMPA receptor subunits glutamate receptor 2 (GluR2) and GluR3. As in excitatory synapses, these receptors are in physical association with their respective interacting/clustering proteins Homer and PICK1. In addition, they are associated with erbB-1 and erbB-4 receptors. Concomitant activation of astroglial metabotropic and AMPA receptors results in the recruitment of erbB tyrosine kinase receptors and their respective ligands to the glial cell membrane, transactivation of erbB receptors via a mechanism requiring metalloproteinase activity, and increased erbB receptor gene expression. By facilitating erbB-dependent signaling and promoting erbB receptor gene expression in astrocytes, a neuron-to-glia glutamatergic pathway may represent a basic cell-cell communication mechanism used by the neuroendocrine brain to coordinate the facilitatory transsynaptic and astroglial input to LHRH neurons during sexual development.

Normal female sexual development requires neuregulin-erbB receptor signaling in hypothalamic astrocytes

The initiation of mammalian puberty requires the activation of hypothalamic neurons secreting the neuropeptide luteinizing hormone-releasing hormone (LHRH). It is thought that this activation is caused by changes in trans-synaptic input to LHRH neurons. More recently, it has been postulated that the pubertal increase in LHRH secretion in female animals also requires neuron-glia signaling mediated by growth factors of the epidermal growth factor (EGF) family and their astrocytic erbB receptors. Although it appears clear that functional astrocytic erbB1 receptors are necessary for the timely advent of puberty, the physiological contribution that erbB4 receptors may make to this process has not been established. To address this issue, we generated transgenic mice expressing a dominant-negative erbB4 receptor (DN-erbB4) under the control of the GFAP promoter, which targets transgene expression to astrocytes. DN-erbB4 expression is most abundant in hypothalamic astrocytes, where it blocks the ligand-dependent activation of glial erbB4 and erbB2 receptors, without affecting erbB1 (EGF) receptor signaling. Mice carrying the transgene exhibit delayed sexual maturation and a diminished reproductive capacity in early adulthood. These abnormalities are related to a deficiency in pituitary gonadotropin hormone secretion, caused by impaired release of LHRH, the hypothalamic neuropeptide that controls sexual development. In turn, the reduction in LHRH release is caused by the inability of hypothalamic astrocytes to respond to neuregulin (NRG) with production of prostaglandin E(2), which in wild-type animals mediates the stimulatory effect of astroglial erbB receptor activation on neuronal LHRH release. Thus, neuron-astroglia communication via NRG-erbB4/2 receptor signaling appears to be essential for the timely unfolding of the developmental program by which the brain controls mammalian sexual maturation.

Epidermal growth factor receptor expression is related to post-mitotic events in cerebellar development: regulation by thyroid hormone

It has been established that thyroid hormone and neurotrophic factors both orchestrate developmental events in the brain. However, it is not clear how these two influences are related. In this study, we investigated the effects of thyroid hormone on cerebellar development and the coincident expression of transforming growth factor-alpha (TGF-alpha), a ligand in the epidermal growth factor (EGF) family, and the epidermal growth factor receptor (EGFR). Profiles of thyroid hormone expression were measured in postnatal animals and were found to peak at postnatal day 15 (P15). These levels dropped below detectable levels when mice were made hypothyroid with propylthiouracil (PTU). TGF-alpha and EGFR expression, as determined by RNAse protection assay, was maximal at P6 in normal animals, but remained low in hypothyroid animals, suggesting that thyroid hormone was responsible for their induction. In situ hybridization and immunohistochemical analysis of EGFR expression revealed that this receptor was present on granule cells within the inner zone of the external granule cell layer (EGL), suggesting that EGFR-ligands were not inducing granule cell proliferation. The persistence of EGFR expression on migrating granule cells and subsequent down-regulation of expression in the internal granule cell layer (IGL) implicates a role for EGFR-ligands in differentiation and/or migration. In hypothyroid animals, we observed a delayed progression of granule cell migration, consistent with the persistence of EGFR labeling in the EGL, and in the 'pile-up' of labeled cells at the interface between the molecular layer and the Purkinje cell layer. Taken together, these results implicate thyroid hormone in the coordinated expression of TGF-alpha and EGFR, which are positioned to play a role in post-mitotic developmental events in the cerebellum.

Glia-to-neuron signaling and the neuroendocrine control of female puberty

The sine qua non event of puberty is an increase in pulsatile release of gonadotrophin hormone releasing hormone (GnRH). It is now clear that this increase and, therefore, the initiation of the pubertal process itself, require both changes in transsynaptic communication and the activation of glia-to-neuron signaling pathways. While neurons that utilize excitatory and inhibitory amino acids as transmitters represent major players in the transsynaptic control of puberty, glial cells utilize a combination of trophic factors and small cell-cell signaling molecules to regulate neuronal function and, thus, promote sexual development. A coordinated increase in glutamatergic transmission accompanied by a decrease in inhibitory GABAergic tone appears to initiate the transsynaptic cascade of events leading to the pubertal increase in GnRH release. Glial cells facilitate GnRH secretion via cell-cell signaling loops mainly initiated by members of the EGF and TGF- families of trophic factors, and brought about by either these factors themselves or by chemical messengers released in response to growth factor stimulation. In turn, a neuron-to-glia communication pathway mediated by excitatory amino acids serves to coordinate the simultaneous activation of transsynaptic and glia-to-neuron communication required for the advent of sexual maturity. A different--and perhaps higher--level of control may involve the transcriptional regulation of subordinate genes that, by contributing to neuroendocrine maturation, are required for the initiation of the pubertal process.

Sexual differentiation of the brain: genes, estrogen, and neurotrophic factors

Based on evidence obtained during the past 50 years, the current hypothesis to explain the sexual dimorphism of structure and function in the brain of vertebrates maintains that these differences are produced by the epigenetic action of gonadal hormones. However, evidence has progressively accumulated suggesting that genetic mechanisms controlling sexual-specific neuronal characteristics precede, or occur in parallel with, hormonal effects. 1. In cultures of hypothalamic neurons taken from gestation day 16 (GD16) embryos, treatment of sexually segregated cultures with estradiol (E2) induces axon growth in neurons from male neurons, but not from female neurons. In these cultures treatment with E2 increased the levels of tyrosine kinase type B (TrkB) and insulin-like growth factor I (IGF-I) receptors in male but not in female neurons. This and other sex differences cannot be explained by differences in hormonal environment, because the donor embryos were obtained when gonadal secretion of steroids is just beginning, before the perinatal surge of testosterone that determines development of the male brain beginning at GD17/18. 2. The response to estrogen is contingent upon coculture with heterotopic glia (mostly astrocytes) from a target region (amygdala) harvested from same-sex fetuses at GD16, whereas in the presence of homotopic glia or in cultures without glia, E2 had no effect. It was concluded that the axogenic effect of E2 depends on interaction between neurons and glia from a target region and that neurons from fetal male donors appear to mature earlier than neurons from females, a differentiated response that takes place prior to divergent exposure to gonadal secretions. 3. The effects of target and nontarget glia-conditioned media (CM) on the E2-induced growth of neuronal processes of hypothalamic neurons obtained from sexually segregated fetal donors were also studied. Estrogen added to media conditioned by target glia modified the number of primary neurites and the growth of axons of hypothalamic neurons of males but not of females. 4. Neither the Type III steroidal receptor blocker tamoxifen nor Type I antiestrogen ICI 182,780 prevented the axogenic effects of the hormone. Estradiol made membrane-impermeable by conjugation to a protein of high molecular weight (E2-BSA) preserved its axogenic capacity, suggesting the possibility of a membrane effect responsible for the action of E2. 5. Western blot analysis of the tyrosine kinase type A (TrkA), type B (TrkB), type C (TrkC), and insulin-like growth factor (IGF-I R) receptors in extracts from homogenates of cultured hypothalamic neurons showed that in cultures of male-derived neurons grown with E2 and CM from target glia, the amounts of TrkB and IGF-I R increased notably. Densitometric quantification showed that these cultures had more TrkB than cultures with CM alone or E2 alone. On the contrary, in cultures of female-derived neurons, the presence of CM alone induced maximal levels of TrkB, which were not further increased by E2; female-derived neurons in all conditions did not contain IGF-I R. Levels of TrkC were not modified by any experimental condition in male- or female-derived cultures and Trk A was not found in the homogenates. These results are compared with similar data from other laboratories and integrated in a model for the confluent interaction of estrogen and neurotrophic factors released by glia that may contribute to the sexual differentiation of the brain.

Oestrogens, via transforming growth factor alpha, modulate basic fibroblast growth factor synthesis in hypothalamic astrocytes: in vitro observations

The data presented here show that, in cultures of type 1 astrocytes obtained from the hypothalamus of neonatal female rat, 17beta-oestradiol is able to increase both the mRNA and the protein levels of basic fibroblast growth factor (bFGF). In particular, after 24 h of exposure to 17beta-oestradiol (10(-9) and 10(-10) m), an increase of messenger levels of bFGF appears in hypothalamic type 1 astrocytes. Similarly, an induction of bFGF protein is also evident at this time of exposure. The effect on the mRNA and protein levels of bFGF is blocked by the presence in the medium of an antibody raised against the transforming growth factor alpha (TGFalpha) receptor. This observation indicates that, TGFalpha, whose synthesis is modulated by oestrogens in hypothalamic astrocytes and which is able to increase, both the mRNA and the protein levels of bFGF in our experimental model, may act as the mediator of the oestrogenic induction of bFGF. Hypothalamic astrocytes, together with hypothalamic neurones synthesizing and secreting luteinizing hormone-releasing hormone (LHRH), form the LHRH network in conjunction with other neuronal systems. Gonadal steroids in general, and oestrogens in particular, play an important role in the control of the activity of this network. In addition, bFGF and TGFalpha, two growth factors released from astrocytes, are able to influence the activity of LHRH neurones. The present observations suggest that oestrogens may also act on LHRH neurones in an indirect fashion (i.e. by modulating the expression of bFGF and TGFalpha in glial cells).

Glial expression of estrogen and androgen receptors after rat brain injury

Estrogens and androgens can protect neurons from death caused by injury to the central nervous system. Astrocytes and microglia are major players in events triggered by neural lesions. To determine whether glia are direct targets of estrogens or androgens after neural insults, steroid receptor expression in glial cells was assessed in two different lesion models. An excitotoxic injury to the hippocampus or a stab wound to the parietal cortex and hippocampus was performed in male rats, and the resultant expression of steroid receptors in glial cells was assessed using double-label immunohistochemistry. Both lesions induced the expression of estrogen receptors (ERs) and androgen receptors (ARs) in glial cells. ERalpha was expressed in astrocytes immunoreactive (ERalpha-ir) for glial fibrillary acidic protein or vimentin. AR immunoreactivity colocalized with microglial markers, such as Griffonia simplicifolia lectin-1 or OX-6. The time course of ER and AR expression in glia was studied in the stab wound model. ERalpha-ir astrocytes and AR-ir microglia were observed 3 days after lesion. The number of ERalpha-ir and AR-ir glial cells reached a maximum 7 days after lesion and returned to low levels by 28 days postinjury. The studies of ERbeta expression in glia were inconclusive; different results were obtained with different antibodies. In sum, these results suggest that reactive astrocytes and reactive microglia are a direct target for estrogens and androgens, respectively.

Growth factors and steroid hormones: a complex interplay in the hypothalamic control of reproductive functions

The mechanisms through which LHRH-secreting neurons are controlled still represent a crucial and debated field of research in the neuroendocrine control of reproduction. In the present review, we have specifically considered two potential signals reaching these hypothalamic neurons: steroid hormones and growth factors. Examples of the relevant physiological role of the interactions between these two families of biologically acting molecules have been provided. In many cases, these interactions occur at the level of hypothalamic astrocytes, which are presently accepted as functional partners of the LHRH-secreting neurons. On the basis of the observations here summarized, we have formulated the hypothesis that a functional co-operation of steroid hormones and growth factors occurring in the hypothalamic astrocytic compartment represents a key factor in the neuroendocrine control of reproductive functions.

Functional integrity of ErbB-4/-2 tyrosine kinase receptor complex in the hypothalamus is required for maintaining normal reproduction in young adult female rats

ErbB-1 tyrosine kinase receptors are necessary for maintaining female reproduction by modulating the release of LH-releasing hormone (LHRH). Changes in ErbB-1 signaling capacity in aging rats are linked to compromised reproduction. The interactive and synergistic nature of different members of ErbB receptors in mediating signal transduction exists in many cellular systems. Particularly, the interactions among ErbB-1 and ErbB-2 or ErbB-4 and ErbB-2 are known to be involved in the stimulation of LHRH secretion during sexual maturation. Thus, ErbB-4/-2 receptors may also play a role in maintaining reproduction during adulthood, and consequently, alteration in ErbB-4/-2 signaling capacity may contribute to compromised reproductive competence during aging. By in situ hybridization histochemistry, ErbB-4/-2 mRNAs were detected in the preoptic area (POA) and arcuate nucleus, which are important areas involved in the control of LHRH neuronal activity. RT-PCR analyses showed that levels of ErbB-4/-2 mRNA increased to a maximal value in the POA of young adult animals before the LH surge. However, no such increase was found in middle-aged female rats. The timing of the decrease in ErbB-4 mRNA in the median eminence-arcuate nucleus of middle-aged rats was delayed compared with that in young adult animals. Disruption of functional ErbB-4/-2 receptor complex by blocking ErbB-2 receptor synthesis in the hypothalamus via an infusion of ErbB-2 antisense oligodeoxynucleotide resulted in an estrous acyclicity in young adult rats. These results indicate that changes in ErbB-4/-2 gene expression and functional integrity of this ErbB-4/-2 receptor complex in the hypothalamus of middle-aged female animals may lead to an altered preovulatory LH release. Thus, the ErbB-4/-2 receptor complex is a physiological component necessary for maintaining female reproduction.

Glial-neuronal-endothelial interactions are involved in the control of GnRH secretion

In recent years compelling evidence has been provided that cell-cell interactions involving non-neuronal cells, such as glial and endothelial cells, are important in regulating the secretion of GnRH, the neuropeptide that controls both sexual development and adult reproductive function. Modification of the anatomical relationship that exist between GnRH nerve endings and glial cell processes in the external zone of the median eminence modulates the access of GnRH nerve terminals to the portal vasculature during the oestrous cycle. The establishment of direct neuro-haemal junctions between GnRH neuroendocrine terminals and the portal vasculature on the day of pro-oestrus may be critical for the transfer of GnRH upon its release into the fenestrated capillaries of the median eminence. Notwithstanding the importance of these plastic rearrangements, glial and endothelial cells also regulate GnRH neuronal function via specific cell-cell signalling molecules. While endothelial cells of the median eminence use nitric oxide to effect this regulatory control, astrocytes employ several growth factors, and in particular those of the EGF family and their erbB receptors to facilitate GnRH release during sexual development. Loss of function of each of these erbB receptors involved in the astroglial control of GnRH secretion leads to delayed sexual development. It is clear that regulation of GnRH secretion by cell-cell communication mechanisms other than transsynaptic inputs is an important component of the central neuroendocrine process controlling mammalian reproduction.

Altered gene activity of epidermal growth factor receptor (ErbB-1) in the hypothalamus of aging female rat is linked to abnormal estrous cycles

Activation of the ErbB-1 receptor is necessary for initiating mammalian female puberty by stimulating the release of LH-releasing hormone. It remains unclear whether ErbB-1 is also required in governing reproduction during adulthood and whether altered ErbB-1 signaling is linked to changes in gonadotropin secretion in aging females. The present study examined these issues. RT-PCR was employed to determine changes in ErbB-1 mRNA levels during proestrus in both young adult (YA) and middle-aged (MA) female rats. Before the LH surge, expression levels in the preoptic area of YA rats increased to a maximal value. No such increase in ErbB-1 mRNA was found in MA rats. This difference was confirmed by the analysis of in situ hybridization histochemistry, where a stronger mRNA signal was observed in the preoptic area of YA rats compared with MA females. ErbB-1 protein levels measured by Western blot reflected this difference. A peak level of ErbB-1 mRNA in the median eminence-arcuate nucleus was detected at 0800 h in YA rats, but it was delayed in MA animals. There were intense ErbB-1 mRNA-positive cells in the arcuate nucleus. Pharmacological blockade of ErbB-1 receptor-mediated signal transduction resulted in the disruption of estrous cyclicity in YA rats. These results indicate that ErbB-1 receptors are necessary for maintaining normal estrous cycles. Consequently, age-related alterations in hypothalamic ErbB-1 gene activity may contribute to a delayed preovulatory LH secretion in aging females. Thus, the ErbB-1 signaling system plays an important role in the control of female reproduction during adulthood.

Regulation of daily locomotor activity and sleep by hypothalamic EGF receptor signaling

The circadian clock in the suprachiasmatic nucleus (SCN) is thought to drive daily rhythms of behavior by secreting factors that act locally within the hypothalamus. In a systematic screen, we identified transforming growth factor-alpha (TGF-alpha) as a likely SCN inhibitor of locomotion. TGF-alpha is expressed rhythmically in the SCN, and when infused into the third ventricle it reversibly inhibited locomotor activity and disrupted circadian sleep-wake cycles. These actions are mediated by epidermal growth factor (EGF) receptors on neurons in the hypothalamic subparaventricular zone. Mice with a hypomorphic EGF receptor mutation exhibited excessive daytime locomotor activity and failed to suppress activity when exposed to light. These results implicate EGF receptor signaling in the daily control of locomotor activity, and identify a neural circuit in the hypothalamus that likely mediates the regulation of behavior both by the SCN and the retina.

Regulation of gonadotropin-releasing hormone secretion: insights from GT1 immortal GnRH neurons

The study of the mammalian GnRH system has been greatly advanced by the development of immortalized cell lines. Of particular relevance are the so-called GT1 cells. Not only do they exhibit many of the known physiologic characteristics of GnRH neurons in situ, but in approximately one decade have yielded new insights regarding the intrinsic physiology of individual cells and networks of GnRH neurons, as well as the nature of central and peripheral signals that directly modulate their function. For instance, valuable information has been generated concerning intrinsic properties of the system such as the inherent pulsatile pattern of secretion displayed by networks of GT1 cells. Concepts regarding feedback regulation and autocrine feedback of GnRH neurons have been dramatically expanded. Likewise, the nature of the receptors and of the proximal and distal signal transduction mechanisms involved in the actions of multiple afferent signals has been identified. Understanding this neuronal system allows a better comprehension of the hypothalamic-pituitary-gonadal axis and of the regulatory influences that ultimately control reproductive competence.

Interactions between growth factors and steroids in the control of LHRH-secreting neurons

How the gene expression and the release of luteinizing hormone releasing hormone (LHRH) are controlled in LHRH-secreting neurons is a very crucial and still debated topic of the neuroendocrinology. Several observations present in literature have recently indicated that glial cells may influence the activity of hypothalamic LHRH-secreting neurons, via the release of growth factors. The present review will summarize data obtained in our laboratory indicating that: (a) type 1 astrocytes, a kind of glial cells, are able to release in vitro growth factors belonging to the transforming growth factors beta (TGFbeta) family (i.e. TGFbeta1 and TGFbeta2) which influence the gene expression and the release of the decapeptide in immortalized LHRH-secreting neurons; (b) glial cells are also able to influence the steroid metabolism occurring in these neurons and in some cases this effect is exerted by TGFbeta1; (c) the mRNA levels of TGFbeta1 and of basic fibroblast growth factor (bFGF), another growth factor involved in the control of LHRH-secreting neurons, are modified in the rat hypothalamus during the different phases of the estrous cycle; (d) steroid hormones are able to modulate the gene expression of TGFbeta1 and bFGF both in vivo (i.e. in the whole hypothalamus of ovariectomized rats) and in vitro (cultures of type 1 astrocytes). On the basis of these results a possible functional correlation in the control of LHRH-secreting neurons between growth factors and gonadal steroids will be discussed and proposed.

Administration of transforming growth factor-alpha reduces infarct volume after transient focal cerebral ischemia in the rat

Growth factors promote cell growth and survival and protect the brain from developing injury after ischemia. In this article, the authors examined whether transforming growth factor-alpha (TGF-alpha) was protective in transient focal ischemia and whether alteration of cerebral circulation was involved. Rats received intraventricular TGF-alpha (50 ng, either split into 2 doses given 30 minutes before and 30 minutes after middle cerebral artery occlusion (MCAO), or 1 dose given 30 minutes after MCAO) or vehicle. Rats were subjected to 1-hour intraluminal MCAO and cerebral blood flow was recorded continuously by laser-Doppler flowmetry. Infarct volume was measured 1 and 4 days later. The effects of TGF-alpha on arterial tone were assessed in isolated rabbit basilar and common carotid arteries. Transforming growth factor-alpha before and after ischemia reduced infarct volume by 70% at 1 day and 50% at 4 days. Transforming growth factor-alpha given only after ischemia also did reduce infarct volume by 70% at 1 day and 80% at 4 days. The protective effect was more marked in cortex than in striatum. Transforming growth factor-alpha did not change cortical microvascular perfusion and did not modify arterial passive tone nor agonist-induced active tone. It can be concluded that TGF-alpha reduces infarct volume, even when the factor is exclusively administered at reperfusion, and that this effect is not mediated by changes in microvascular perfusion or cerebral arteries. It is therefore suggested that TGF-alpha has a protective effect against neuronal cell death after transient focal ischemia.

Alterations in hypothalamic insulin-like growth factor-I and its associations with gonadotropin releasing hormone neurones during reproductive development and ageing

Insulin-like growth factor-I (IGF-I) is thought to play a role in the onset of reproductive ability at puberty and the control of reproductive function throughout adult life. It is believed that these effects are mediated at least in part by the activation of gonadotropin releasing hormone (GnRH) neurones by IGF-I, but the interactions of IGF-I with GnRH neurones in vivo are largely unknown. We first examined the anatomical relationship between GnRH and IGF-I cells in neuroendocrine regions. Using double-label immunocytochemistry, we observed that in the preoptic area-anterior hypothalamus (POA-AH), the site of GnRH perikarya, the majority (78%) of GnRH cell bodies expressed IGF-I immunoreactivity. IGF-I immunoreactivity was also high in the median eminence, the site of GnRH release, and GnRH neuroterminals were seen to interweave among IGF-I-immunopositive cells. Due to this substantial overlap of GnRH and IGF-I immunoreactive elements, we then tested the hypothesis that changes in IGF-I may regulate the GnRH system. Animals were examined at the two important reproductive life transitions: puberty and reproductive senescence. IGF-I mRNA levels were measured in POA-AH and medial basal hypothalamus-median eminence (MBH-ME) and effects of IGF-I treatment on GnRH mRNA levels were quantified by RNase protection assay. Although IGF-I treatment did not alter GnRH gene expression, there were significant alterations in hypothalamic IGF-I gene expression at both puberty and reproductive senescence. During puberty, IGF-I mRNA levels in the MBH-ME of rats increased from the juvenile stage (P25) to the day of vaginal opening (P35), and from the day of vaginal opening to young adulthood (P45) in the POA-AH. During reproductive ageing, IGF-I mRNA levels were significantly lower in middle-aged than young rats, particularly in the MBH-ME. At all ages, IGF-I expression was greater in the MBH-ME than in the POA-AH. These experiments demonstrate that: (i) the majority of adult GnRH neurones are immunopositive for the IGF-I protein; (ii) hypothalamic IGF-I levels increase at the onset of reproductive function and decrease at reproductive senescence in a regionally specific manner; and (iii) despite the presence of IGF-I in GnRH perikarya, IGF-I does not affect GnRH gene expression, suggesting that IGF-I may act at the level of GnRH release rather than gene expression.

Hypothalamic transforming growth factor beta1 and basic fibroblast growth factor mRNA expression is modified during the rat oestrous cycle

The present observations show that the mRNA levels of two growth factors, previously described to be involved in the control of neurones synthesizing the luteinizing hormone releasing hormone (LHRH) [i.e. transforming growth factor beta1 (TGFbeta1) and basic fibroblast growth factor (bFGF)], fluctuate in the hypothalamus of adult female rats during the oestrous cycle. In particular, the expression of TGFbeta1-mRNA shows a peak on the morning of the day of proestrus, which precedes the increased secretion of the two gonadotrophins that occurs on that day. In the case of bFGF, the peak is evident in the evening of the same day and is concomitant with that of the gonadotrophins. We evaluated the effects of ovariectomy and of exogenous oestrogens on the mRNA levels of these two growth factors in the hypothalamus. The data indicate that 3 weeks of ovariectomy are not able to change the hypothalamic messenger levels of the two growth factors considered, which remain at the levels found in diestrus 1, and that 17beta-oestradiol is able to induce a significant increase of both TGFbeta1- and of bFGF-mRNA levels in the hypothalamus of the ovariectomized rat. The present in vivo observations support the concept, previously proposed on the basis of in vitro data, that growth factors, such as TGFbeta1 and bFGF, play a role in the hypothalamic control of reproduction, and suggest that the control of LHRH dynamics involves a strict cooperation between gonadal steroids and growth factors.

Gender, neuroendocrine-immune interactions and neuron-glial plasticity. Role of luteinizing hormone-releasing hormone (LHRH)

Signals generated by the hypothalamic-pitutary-gonadal (HPG) axis powerfully modulate immune system function. This article summarizes some aspects of the impact of gender in neuroendocrine immunomodulation. Emphasis is given to the astroglial cell compartment, defined as a key actor in neuroendocrine immune communications. In the brain, the principal hormones of the HPG axis directly interact with astroglial cells. Thus, luteinizing hormone releasing hormone, LHRH, influences hypothalamic astrocyte development and growth, and hypothalamic astrocytes direct LHRH neuron differentiation. Hormonally induced changes in neuron-glial plasticity may dictate major changes in CNS output, and thus actively participate in sex dimorphic immune responses. The impact of gender in neuroimmunomodulation is further underlined by the sex dimorphism in the expression of genes encoding for neuroendocrine hormones and their receptors within the thymus, and by the potent modulation exerted by circulating sex steroids during development and immunization. The central role of glucocorticoids in the interactive communication between neuroendocrine and immune systems, and the impact of gender on hypothalamic-pituitary-adrenocortical (HPA) axis modulation is underscored in transgenic mice expressing a glucocorticoid receptor antisense RNA.

Insulin-like growth factor I receptors and estrogen receptors colocalize in female rat brain

Several findings indicate that there is a close interaction between estrogen and insulin-like growth factor I in different brain regions. In adult brain, both estrogen and insulin-like growth factor I have co-ordinated effects in the regulation of neuroendocrine events, synaptic plasticity and neural response to injury. In this study we have qualitatively assessed whether estrogen receptors and insulin-like growth factor I receptor are colocalized in the same cells in the preoptic area, hypothalamus, hippocampus, cerebral cortex and cerebellum of female rat brain using confocal microscopy. Immunoreactivity for estrogen receptors alpha and beta was colocalized with immunoreactivity for insulin-like growth factor I receptor in many neurons from the preoptic area, hypothalamus, hippocampus and cerebral cortex. Furthermore, estrogen receptor beta and insulin-like growth factor I receptor immunoreactivities were colocalized in the Purkinje cells of the cerebellum. Colocalization of estrogen receptor beta and insulin-like growth factor I receptor was also detected in cells with the morphology of astrocytes in all regions assessed. The co-expression of estrogen receptors and insulin-like growth factor I receptor in the same neurons may allow a cross-coupling of their signaling pathways. Furthermore, the colocalization of immunoreactivity for estrogen receptor beta and insulin-like growth factor I receptor in glial cells suggests that glia may also play a role in the interactions of insulin-like growth factor I and estrogen in the rat brain. In conclusion, the co-expression of estrogen receptors and insulin-like growth factor I receptors in the same neural cells suggests that the co-ordinated actions of estrogen and insulin-like growth factor I in the brain may be integrated at the cellular level.

TTF-1, a homeodomain gene required for diencephalic morphogenesis, is postnatally expressed in the neuroendocrine brain in a developmentally regulated and cell-specific fashion

TTF-1 is a member of the Nkx family of homeodomain genes required for morphogenesis of the hypothalamus. Whether TTF-1, or other Nkx genes, contributes to regulating differentiated hypothalamic functions is not known. We now report that postnatal hypothalamic TTF-1 expression is developmentally regulated and associated with the neuroendocrine process of female sexual development. Lesions of the hypothalamus that cause sexual precocity transiently activate neuronal TTF-1 expression near the lesion site. In intact animals, hypothalamic TTF-1 mRNA content also increases transiently, preceding the initiation of puberty. Postnatal expression of the TTF-1 gene was limited to subsets of hypothalamic neurons, including LHRH neurons, which control sexual maturation, and preproenkephalinergic neurons of the lateroventromedial nucleus of the basal hypothalamus, which restrain sexual maturation and facilitate reproductive behavior. TTF-1 mRNA was also detected in astrocytes of the median eminence and ependymal/subependymal cells of the third ventricle, where it colocalized with erbB-2, a receptor involved in facilitating sexual development. TTF-1 binds to and transactivates the erbB-2 and LHRH promoters, but represses transcription of the preproenkephalin gene. The singular increase in hypothalamic TTF-1 gene expression that precedes the initiation of puberty, its highly specific pattern of cellular expression, and its transcriptional actions on genes directly involved in neuroendocrine reproductive regulation suggest that TTF-1 may represent one of the controlling factors that set in motion early events underlying the central activation of mammalian puberty.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Central administration of an antisense oligodeoxynucleotide against type I pituitary adenylate cyclase-activating polypeptide receptor suppresses synthetic activities of LHRH-LH axis during the pubertal process

Central administration of an antisense oligodeoxynucleotide against type I pituitary adenylate cyclase-activating polypeptide receptor suppresses synthetic activities of LHRH-LH axis during the pubertal process In the present study, we determined the expression of pituitary adenylate cyclase-activating polypeptide (PACAP) and PACAP receptor type I (PAC1) genes during juvenile development and the pubertal process. Female rats were assigned--based on uterine weights, the presence and abundance of uterine fluid, and their vaginal patency--to one of the following: anestrus (AE), early proestrus (EP), late proestrus (LP) or first estrus (E). The hypothalami from 22-, 24- and 26-day-old animals and from those in the peripubertal phases of AE, EP, LP and E were collected, and the content of PACAP and PAC1 mRNA was assessed. These levels were found to decrease in EP and LP. To determine the effect of PACAP on prepubertal luteinizing hormone-releasing hormone (LHRH) and LH synthesis through PAC1, a PAC1 antisense oligodeoxynucleotide (ODN) was i.c.v.-administered, and mRNA levels of LHRH, LH beta, and LHRH receptor (LHRH-R) were determined. Prepubertal increases in LHRH, LH beta, and LHRH-R mRNA levels were markedly suppressed, and the onset of puberty was delayed by the i.c.v. injection of the antisense PAC1 ODN. These data suggest that PACAP may play a role in the regulation of hypothalamic LHRH neurons, through which it regulates synthetic machinery of pituitary LH, during the pubertal process.

Basic fibroblast growth factor (bFGF) acts on both neurons and glia to mediate the neurotrophic effects of astrocytes on LHRH neurons in culture

Luteinizing hormone-releasing hormone (LHRH) neurons play a pivotal role in the neuroendocrine control of mammalian reproduction. Astrocytes were shown to be involved in the regulation of LHRH neuronal function, but little is known about the contribution of astroglial-derived factors in the regulation of LHRH neuron development. In order to gain insight into the mechanisms regulating the development of these cells, at morphological and biochemical levels we characterized the neurotrophic effects exerted by young astrocytes (maintained in culture for 8 days in vitro) and old astrocytes (maintained 26 days) on the differentiation, proliferation, and phenotypic expression of immortalized hypothalamic LHRH (GT(1-1)) neurons in vitro. Culturing GT(1-1) cells in the presence of young glia for different time intervals caused a marked acceleration in the acquisition of their neuronal phenotype. At all times examined, GT(1-1) cells cocultured with young glia exhibited a significantly greater extension of processes/cell, larger number of processes/cell and greater surface area of growth cones than GT(1-1) cells grown over nonglial adhesive substrates (polylysine). By contrast, when GT(1-1) neurons were cocultured with old glia, the length of neuronal processes and the growth cone surface area were significantly lower than in control GT(1-1) neurons cultured in the absence of glia. At 3 days in vitro (DIV), GT(1-1) neurons cocultured with young glia exhibited a 50% lower incorporation of [(3)H]thymidine than GT(1-1) neurons cultured without glia. By contrast, in the presence of old glia [(3)H]thymidine incorporation was significantly higher in cells cocultured with glia than in GT(1-1) neurons cultured alone. Localization of the proliferating cells by dual immunohistochemical staining revealed that the incorporation of bromodeoxiuridine (BrdU) was restricted to nuclei of GT(1-1) neurons when these were cocultured with young glia, but associated with both neurons and astrocytes in the presence of old glia. At the functional level, coculture of GT(1-1) neurons with young glia increased the spontaneous release of LHRH as compared to GT(1-1) neurons grown in the absence of glia. By contrast, in the presence of old glia LHRH release in the medium was significantly lower than in controls. Conditioned medium of young glia (ACM-Y) induced significant neurotrophic and functional effects on GT(1-1) cells, but these effects were 50% less potent than the coculture itself. Heat denaturation of ACM-Y totally abolished its neurotrophic and functional properties, indicating that they involved a peptide factor. Suppression of bFGF activity in ACM-Y reduced its neurotrophic activity by approximately 40%, but did not affect its LHRH release-promoting effects. By contrast, neutralization of endogenous bFGF activity in GT(1-1) neurons cocultured with young glia counteracted both neurotrophic and functional effects of young glia. Treatment of old glia with bFGF rescued its neurotrophic and functional effects on GT(1-1) cells. Moreover, the ACM of aged bFGF-treated old glia was the most powerful neurotrophic stimulus for GT(1-1) neurons. These results suggest that: 1) soluble peptidic factors, including bFGF, and mechanism(s) requiring coculture are responsible for the highly potent neurotrophic and functional effects of young glia; 2) the inhibitory effects of old glia on neurite outgrowth and LHRH release are mediated in part by soluble inhibitory molecules and in part by factors requiring coculture with old glia; 3) old glia may revert to a growth-supporting state when treated with bFGF and this functional shift involves a diffusible molecule with potent neurotrophic and functional effects on immortalized LHRH neurons. (c) 2000 Wiley-Liss, Inc.

Estradiol and progesterone regulate the expression of insulin-like growth factor-I receptor and insulin-like growth factor binding protein-2 in the hypothalamus of adult female rats

Gonadal hormones interact with insulin-like growthfactor-I (IGF-I) to regulate synaptic plasticity during the estrous cycle in the rat mediobasal hypothalamus. It has been proposed that tanycytes, specialized glial cells lining the ventral region of the third ventricle, may regulate the availability of IGF-I to hypothalamic neurons. IGF-I levels in tanycytes fluctuate during the estrous cycle. Furthermore, estrogen administration to ovariectomized rats increases IGF-I levels in tanycytes, while progesterone, injected simultaneously with estrogen, blocks the estrogen-induced increase of IGF-I levels in tanycytes. To test whether hormonal regulation of IGF-I receptor (IGF-IR) and IGF binding protein-2 (IGFBP-2) may be involved in the accumulation of IGF-I in tanycytes, we assessed the effect of ovarian hormones on the levels of these molecules in the mediobasal hypothalamus of adult female rats. Ovariectomized animals were treated with either oil, estrogen, progesterone, or estrogen and progesterone simultaneously and then killed 6 or 24 h later. Some neurons, some astrocytes, and many tanycytes in the mediobasal hypothalamus were found by confocal microscopy to be immunoreactive for IGF-IR. IGFBP-2 immunoreactivity was restricted almost exclusively to tanycytes and ependymal cells and was colocalized with IGF-IR immunoreactivity in tanycytes. By electron microscope immunocytochemistry using colloidal gold labeling, IGF-IR and IGFBP-2 immunoreactivities were observed in the microvilli of tanycytes in the lumen of the third ventricle. IGF-IR and IGFBP-2 immunoreactive levels on the apical surface of tanycytes were significantly decreased by the administration of progesterone, either alone or in the presence of estradiol. IGF-IR levels in the mediobasal hypothalamus, measured by Western blotting, were not significantly affected by the separate administration of estradiol or progesterone to ovariectomized rats. However, the simultaneous administration of both hormones resulted in a marked decrease in IGF-IR protein levels. Estradiol administration to ovariectomized rats increased IGFBP-2 immunoreactive levels in the hypothalamus. While progesterone did not significantly affect IGFBP-2 expression, the simultaneous injection of estradiol and progesterone resulted in a marked decrease in IGFBP-2 protein levels. The effect of estradiol on IGFBP-2 was observed both in protein and mRNA levels, suggesting a transcriptional regulation. However, the simultaneous administration of progesterone and estradiol had different effects on IGF-IR protein and IGF-IR mRNA levels, as well as on IGFBP-2 protein and IGFBP-2 mRNA levels, suggesting a postranscriptional action. These findings indicate that estradiol and progesterone regulate the expression of IGF-IR and IGFBP-2 in the mediobasal hypothalamus of adult female rats. Regulation of the hypothalamic IGF-I system by ovarian hormones may be physiologically relevant for neuroendocrine regulation and for synaptic plasticity during the estrous cycle. These results do not support the hypothesis that estrogen-induced accumulation of IGF-I by tanycytes is mediated by the hormonal regulation of IGF-IR. However, estrogen-induced up-regulation of IGFBP-2 and progesterone-induced down-regulation of IGF-IR and IGFBP-2 levels in the apical plasma membrane of tanycytes may be involved in the fluctuation of IGF-I levels in the mediobasal hypothalamus during the estrous cycle.

Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid activity in the female rat. A focus on the EDSTAC recommendations

In 1996, the US Environmental Protection Agency was given a mandate by Congress to develop a screening program that would evaluate whether variously identified compounds could affect human health by mimicking or interfering with normal endocrine regulatory functions. Toward this end, the Agency chartered the Endocrine Disruptor Screening and Testing Advisory Committee in October of that year that would serve to recommend a series of in vitro and in vivo protocols designed to provide a comprehensive assessment of a chemical's potential endocrine-disrupting activity. A number of these protocols have undergone subsequent modification by EPA, and this review focuses specifically on the revised in vivo screening procedure recommended under the title Research Protocol for Assessment of Pubertal Development and Thyroid Function in Juvenile Female Rats. Background literature has been provided that summarizes what is currently known about pubertal development in the female rat and the influence of various forms of pharmaceutical and toxicological insult on this process and on thyroid activity. Finally, a section is included that discusses technical issues that should be considered if the specified pubertal endpoints are to be measured and successfully evaluated.

Neuregulins signaling via a glial erbB-2-erbB-4 receptor complex contribute to the neuroendocrine control of mammalian sexual development

Activation of erbB-1 receptors by glial TGFalpha has been shown to be a component of the developmental program by which the neuroendocrine brain controls mammalian sexual development. The participation of other members of the erbB family may be required, however, for full signaling capacity. Here, we show that activation of astrocytic erbB-2/erbB-4 receptors plays a significant role in the process by which the hypothalamus controls the advent of mammalian sexual maturation. Hypothalamic astrocytes express both the erbB-2 and erbB-4 genes, but no erbB-3, and respond to neuregulins (NRGs) by releasing prostaglandin E(2) (PGE(2)), which acts on neurosecretory neurons to stimulate secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development. The actions of TGFalpha and NRGs in glia are synergistic and involve recruitment of erbB-2 as a coreceptor, via erbB-1 and erbB-4, respectively. Hypothalamic expression of both erbB-2 and erbB-4 increases first in a gonad-independent manner before the onset of puberty, and then, at the time of puberty, in a sex steroid-dependent manner. Disruption of erbB-2 synthesis in hypothalamic astrocytes by treatment with an antisense oligodeoxynucleotide inhibited the astrocytic response to NRGs and, to a lesser extent, that to TGFalpha and blocked the erbB-dependent, glia-mediated, stimulation of LHRH release. Intracerebral administration of the oligodeoxynucleotide to developing animals delayed the initiation of puberty. Thus, activation of the erbB-2-erbB-4 receptor complex appears to be a critical component of the signaling process by which astrocytes facilitate the acquisition of female reproductive capacity in mammals.

Definitive evidence for the existence of morphological plasticity in the external zone of the median eminence during the rat estrous cycle: implication of neuro-glio-endothelial interactions in gonadotropin-releasing hormone release

Despite intense investigation, the demonstration of morphological plasticity in the external zone of the median eminence concerning the gonadotropin-releasing hormone system has never been reported. In this study, we investigate whether dynamic transformations of the gonadotropin-releasing hormone nerve terminals and/or tanycytes in the external zone of the median eminence of the hypothalamus occurred during the rat estrous cycle, by following individual gonadotropin-releasing hormone-immunoreactive nerve terminals on serial ultrathin sections observed by electron microscopy. Female rats were killed at 16.00 diestrus II (n = 3), i.e. when estrogen levels are basal and gonadotropin-releasing hormone release is low, and at 16.00 proestrus (n = 4), i.e. when estrogen levels peak and the preovulatory gonadotropin-releasing hormone surge occurs. Our results show that, in the median eminence obtained from proestrus rats, 12+/-2% of the gonadotropin-releasing hormone nerve terminals were observed to make physical contact with the parenchymatous basal lamina, i.e. the pericapillary space. In the median eminence obtained from diestrus II rats, no contacts were observed. On proestrus, numerous physical contacts between gonadotropin-releasing hormone nerve terminals and the basal lamina occurred by evagination of the basal lamina and/or by emerging processes from gonadotropin-releasing hormone nerve terminals. The quantification of the evagination of the basal lamina revealed that the basal lamina was at least twofold more tortuous in appearance during proestrus. These results demonstrate for the first time the existence of dynamic plastic changes in the external zone of the median eminence, allowing gonadotropin-releasing hormone nerve terminals to contact the pericapillary space on the day of proestrus, thus facilitating the release of the neurohormone into the pituitary portal blood.

Interactions between type 1 astrocytes and LHRH-secreting neurons (GT1-1 cells): modification of steroid metabolism and possible role of TGFbeta1

The hypothesis that type 1 astrocytes (A1) might modify the activities of the enzymes 5alpha-reductase (5alpha-R) and 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) present in the GT1-1 cells has been tested. The data obtained indicate that, utilizing a co-culture technique, A1 are able to: (1) decrease the formation of dihydrotestosterone (DHT) from testosterone (T); (2) increase the formation of dihydroprogesterone (DHP) from progesterone (P); (3) decrease the conversion of DHP into tetrahydroprogesterone (THP) in GT1-1 cells. Moreover, GT1-1 cells are able to increase the formation of DHP in A1; that of DHT was unchanged. The present data might suggest the possible existence of a third isoform of the enzyme 5alpha-R; details on this hypothesis are provided in the text. Interestingly, the inhibitory effect exerted by A1 on the formation of DHT in GT1-1 cells can be mimicked by transforming growth factor beta1 (TGFbeta1). Since TGFbeta1 had been previously shown to be directly involved in the stimulatory control of LHRH secretion by GT1-1 cells, acting both on LHRH release [R.C. Melcangi, M. Galbiati, E. Messi, F. Piva, L. Martini, M. Motta, Type 1 astrocytes influence luteinizing hormone-releasing hormone release from the hypothalamic cell line GT1-1: is transforming growth factor-beta the principle involved? Endocrinology 136 (1995) 679-686.] and gene expression [M. Galbiati, M. Zanisi, E. Messi, I. Cavarretta, L. Martini, R.C. Melcangi, Transforming growth factor-beta and astrocytic conditioned medium influence LHRH gene expression in the hypothalamic cell line GT1, Endocrinology 137 (1996) 5605-5609], the present data also show that TGFbeta1 might intervene in modulating feedback signals reaching hypothalamic LHRH producing neurons. The present findings underline once more the importance of the physiological cross-talk between A1 and neurons.

Roles of transforming growth factor-alpha and related molecules in the nervous system

The epidermal growth factor (EGF) family of polypeptides is regulators for tissue development and repair, and is characterized by the fact that their mature forms are proteolytically derived from their integral membrane precursors. This article reviews roles of the prominent members of the EGF family (EGF, transforming growth factor-alpha [TGF-alpha] and heparin-binding EGF [HB-EGF]) and the related neuregulin family in the nerve system. These polypeptides, produced by neurons and glial cells, play an important role in the development of the nervous system, stimulating proliferation, migration, and differentiation of neuronal, glial, and Schwann precursor cells. These peptides are also neurotrophic, enhancing survival and inhibiting apoptosis of post-mitotic neurons, probably acting directly through receptors on neurons, or indirectly via stimulating glial proliferation and glial synthesis of other molecules such as neurotrophic factors. TGF-alpha, EGF, and neuregulins are involved in mediating glial-neuronal and axonal-glial interactions, regulating nerve injury responses, and participating in injury-associated astrocytic gliosis, brain tumors, and other disorders of the nerve system. Although the collective roles of the EGF family (as well as those of the neuregulins) are shown to be essential for the nervous system, redundancy may exist among members of the EGF family.

Glial-neuronal interactions in the neuroendocrine control of mammalian puberty: facilitatory effects of gonadal steroids

It is now clear that astroglial cells actively contribute to both the generation and flow of information within the central nervous system. In the hypothalamus, astrocytes regulate the secretory activity of neuroendocrine neurons. A small subset of these neurons secrete luteinizing hormone-releasing hormone (LHRH), a neuropeptide essential for sexual development and adult reproductive function. Astrocytes stimulate LHRH secretion via cell-cell signaling mechanisms involving growth factors recognized by receptors with either serine/threonine or tyrosine kinase activity. Two members of the epidermal growth factor (EGF) family and their respective tyrosine kinase receptors appear to play key roles in this regulatory process. Transforming growth factor-alpha (TGFalpha) and its distant congeners, the neuregulins (NRGs), are produced in hypothalamic astrocytes. They stimulate LHRH secretion indirectly, via activation of erbB-1/erbB-2 and erbB-4/erbB-2 receptor complexes also located on astrocytes. Activation of these receptors leads to release of prostaglandin E(2) (PGE(2)), which then binds to specific receptors on LHRH neurons to elicit LHRH secretion. Gonadal steroids facilitate this glia-to-neuron communication process by acting at three different steps along the signaling pathway. They (a) increase astrocytic gene expression of at least one of the EGF-related ligands (TGFalpha), (b) increase expression of at least two of the receptors (erbB-4 and erbB-2), and (c) enhance the LHRH response to PGE(2) by up-regulating in LHRH neurons the expression of specific PGE(2) receptor isoforms. Focal overexpression of TGFalpha in either the median eminence or preoptic area of the hypothalamus accelerates puberty. Conversely, blockade of either TGFalpha or NRG hypothalamic actions delays the process. Thus, both TGFalpha and NRGs appear to be physiological components of the central neuroendocrine mechanism controlling the initiation of female puberty. By facilitating growth factor signaling pathways in the hypothalamus, ovarian steroids accelerate the pace and progression of the pubertal process.

Discrete localizations of hedgehog signalling components in the developing and adult rat nervous system

Sonic hedgehog (Shh), a morphogen molecule implicated in embryonic tissue patterning, displays inductive, proliferative, neurotrophic and neuroprotective activities on various neural cells. Shh might exert its biological functions through binding to patched (Ptc) associated with smoothened (Smo), leading to downstream activation of target genes such as the transcription factor Gli1. We have performed a detailed localization of cells expressing transcripts of Shh, Ptc, Smo and Gli1 in brain and spinal cord of the adult rat as well as in the developing cerebellum. In the adult, Shh-positive cells were mainly observed in forebrain structures, in the Purkinje cells of the cerebellum and in motor neurons. Ptc-positive cells were frequently observed in brain areas devoid of any Shh transcripts, except in the median eminence or the facial nucleus, suggesting local Shh signalling. Interestingly, Smo transcripts were predominantly present within circumventricular organs, in granular cells of the dentate gyrus and in neurons of the reticular thalamic nucleus. The presence of Shh, Ptc and Smo transcripts in hypothalamic areas may indicate a role of Shh signalling in the modulation of neuroendocrine functions. The expression pattern of these three genes as well as of Gli1, and their developmental regulation in the cerebellum, suggest a possible role for Hedgehog signalling in the control of various cell populations within the cerebellum, particularly in granule cell proliferation and/or differentiation that might be impaired in proliferative states such as medulloblastomas.

The Oct-2 POU domain gene in the neuroendocrine brain: a transcriptional regulator of mammalian puberty

POU homeodomain genes are transcriptional regulators that control development of the mammalian forebrain. Although they are mostly active during embryonic life, some of them remain expressed in the postnatal hypothalamus, suggesting their involvement in regulating differentiated functions of the neuroendocrine brain. We show here that Oct-2, a POU domain gene originally described in cells of the immune system, is one of the controlling components of the cell-cell signaling process underlying the hypothalamic regulation of female puberty. Lesions of the anterior hypothalamus cause sexual precocity and recapitulate some of the events leading to the normal initiation of puberty. Prominent among these events is an increased astrocytic expression of the gene encoding transforming growth factor-alpha (TGF alpha), a tropic polypeptide involved in the stimulatory control of LHRH secretion. The present study shows that such lesions result in the rapid and selective increase in Oct-2 transcripts in TGF alpha-containing astrocytes surrounding the lesion site. In both lesion-induced and normal puberty, there is a preferential increase in hypothalamic expression of the Oct-2a and Oct-2c alternatively spliced messenger RNA forms of the Oct-2 gene, with an increase in 2a messenger RNA levels preceding that in 2c and antedating the peripubertal activation of gonadal steroid secretion. Both Oct-2a and 2c trans-activate the TGF alpha gene via recognition motifs contained in the TGF alpha gene promoter. Inhibition of Oct-2 synthesis reduces TGF alpha expression in astroglial cells and delays the initiation of puberty. These results suggest that the Oct-2 gene is one of the upstream components of the glia to neuron signaling process that controls the onset of female puberty in mammals.

Hormonal regulation of glutamate receptor gene expression in the anteroventral periventricular nucleus of the hypothalamus

Glutamate plays an important role in mediating the positive feedback effects of ovarian steroids on gonadotropin secretion, and the preoptic region of the hypothalamus is a likely site of action of glutamate. The anteroventral periventricular nucleus (AVPV) of the preoptic region is an essential part of neural pathways mediating hormonal feedback on gonadotropin secretion, and it appears to provide direct inputs to gonadotropin releasing hormone (GnRH)-containing neurons. Immunohistochemistry and in situ hybridization were used in this study to define the distribution and hormonal regulation of glutamate receptor subtypes in the AVPV of juvenile female rats. Neurons that express the NMDAR1 receptor subtype are abundant in the AVPV, as are cells that express AMPA receptor subtypes (GluR1, GluR2, and GluR3 but not GluR4), and the AVPV appears to contain a dense plexus of NMDAR1-immunoreactive presynaptic terminals. However, AVPV neurons do not seem to express detectable levels of kainate receptor (GluR5, GluR6, and GluR7) or metabotropic receptor (mGluR1-6) subtypes. Treatment of ovariectomized juvenile rats with estradiol induced expression of GluR1 mRNA but did not alter levels of GluR2 or GluR3 mRNA. Treatment of estrogen-primed ovariectomized juvenile rats with progesterone caused an initial increase in GluR1 mRNA expression, followed by a small decrease 24 hr after treatment. In contrast, estrogen appears to suppress levels of NMDAR1 mRNA in the AVPV, which remained unchanged after progesterone treatment. Thus, one mechanism whereby ovarian steroids may provide positive feedback to GnRH neurons is by altering the sensitivity of AVPV neurons to glutamatergic activation.

Transforming growth factor-alpha acting at the epidermal growth factor receptor reduces infarct volume after permanent middle cerebral artery occlusion in rats

Transforming growth factor-alpha (TGF-alpha) is a ligand for the epidermal growth factor (EGF) receptor (EGFR), and is more abundant than EGF in the brain. The authors studied whether administration of exogenous TGF-alpha into the brain can protect neurons against ischemia in a model of permanent middle cerebral artery (MCA) occlusion in the rat, and whether any effect of TGF-alpha was mediated by EGFR by administering 4,5-dianilinophthalimide (DAPH), a protein-tyrosine kinase inhibitor with high selectivity for EGFR. Rats received either TGF-alpha (10 or 25 ng), DAPH (100 ng), DAPH plus TGF-alpha (25 ng), or vehicle in the ipsilateral first ventricle. Drugs were administered twice: 30 minutes before and 30 minutes after MCA occlusion, and infarct volume was evaluated 24 hours later. Transforming growth factor-alpha at the dose of 25 ng caused a statistically significant reduction of infarct volume (60%) in relation to ischemic rats administered vehicle. This reduction was no longer seen when TGF-alpha was administered in combination with DAPH. The present results show that TGF-alpha can protect neurons from ischemic damage, and that this effect is mediated by EGFR. It is suggested that activation of EGFR-mediated intracellular signalling pathways contributes to the survival of neural cells susceptible to ischemic injury.

Noradrenergic neurotoxin suppresses gonadotropin-releasing hormone (GnRH) and GnRH receptor gene expression in ovariectomized and steroid-treated rats

The present study was designed to investigate whether noradrenergic neurotransmission regulates the gene expression of gonadotropin-releasing hormone (GnRH) in the preoptic area and GnRH receptor in the pituitary. To this end, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4, 50 mg/kg), an intraperitoneal (i.p.) injection of selective noradrenergic neurotoxin, was administered 1 h before progesterone (1 mg) treatment in ovariectomized and estradiol-treated prepubertal rats. Treatment with DSP4 effectively blocked the progesterone-induced increase in hypothalamic noradrenaline content, but not dopamine content, indicating that DSP4 selectively inhibits noradrenergic neurotransmission. DSP4 significantly blocked progesterone-induced increase in serum luteinizing hormone (LH) concentrations as well as GnRH release from hypothalamic fragments incubated in vitro. DSP4 concomitantly down-regulated GnRH mRNA levels in the preoptic area, as determined by competitive reverse transcription-polymerase chain reaction. DSP4 also clearly down-regulated progesterone-induced GnRH receptor mRNA levels in the pituitary, whereas it failed to alter LHbeta mRNA levels. In summary, blockade of noradrenergic neurotransmission with DSP4 resulted in profound reductions of hypothalamic GnRH and pituitary GnRH receptor gene expression.

Epidermal growth factor tyrosine kinase receptors and the neuroendocrine control of mammalian puberty

In recent years evidence has begun to accumulate indicating that the central control of mammalian puberty requires not only changes in transsynaptic communication, but also the participation of glial cells. Neurons and astrocytes control the pubertal process by regulating the secretory activity of those neurons that produce luteinizing hormone-releasing hormone (LHRH), the neuropeptide that governs sexual development. LHRH, in turn, directs sexual development by stimulating the secretion of pituitary gonadotropins. Astrocytes affect LHRH neuronal function via cell-cell signaling mechanisms involving several growth factors acting via receptors endowed with tyrosine kinase activity. We have identified two members of the epidermal growth factor/transforming growth factor alpha (EGF/TGFalpha) family and their respective receptors as key players in the glial-neuronal interactive process that regulates LHRH secretion. Our results indicate that TGFalpha and its distant congener neuregulin (NRG) are produced in hypothalamic astrocytes and stimulate LHRH release indirectly via activation of their respective receptors, located--surprisingly--not on LHRH neurons, but on astrocytes. Activation of EGF receptors by TGFalpha, and/or the erbB2/erbB4 receptor complex by NRG, leads to glial release of prostaglandin (PG) E2, which then acts directly on LHRH neurons to stimulate LHRH release. That a central blockade of TGFalpha or NRG action delays puberty, and focal overexpression of TGFalpha advances it, leads to the conclusion that both TGFalpha and NRG are physiological components of the central mechanism controlling the initiation of female puberty.

Luteinizing hormone-releasing hormone is a primary signaling molecule in the neuroimmune network

The brain-pituitary-reproductive axis and the brain thymus-lymphoid axis are linked by an array of internal mechanisms of communication that use similar signals (neurotransmitters, peptides, growth factors, hormones) acting on similar recognition targets. Moreover, such communication networks form the basis and control each step and every level of reproductive physiology. This presentation highlights the extent to which endocrine, neural, glial, or immunologically competent cells may achieve their specific functions using common mechanisms, but employing them to different degrees. In particular, this work will focus on LHRH, the chief hormone orchestrating reproductive events. Within the thymus LHRH plays a unique role of immunomodulator, contributing to the sex-dependent changes in immune responsiveness during the estrous-menstrual cycle as well as pregnancy. From the recent cloning and sequencing of lymphocyte LHRH, the expression of LHRH receptor mRNA in lymphocyte, the transduction mechanisms involved, and the steroidogenic sensitivity of the intralymphocyte LHRH system. It would appear that this peptide may act as an immunological response modifier in the brain-pituitary-lymphoid-gonadal axis. The interplay between neuronal, endocrine, and immune compartments is further emphasized in the study of LHRH-astroglial interactions. Astrocytes are able to manufacture a wide variety of signaling agents and can secrete immunoregulatory molecules that influence immune cells, as well as the glial cells themselves. Astroglia and the immortalized hypothalamic LHRH (GT1-1) neurons communicate with an array of mechanisms, via soluble mediators as well as cell-to-cell contacts. Manipulation of astroglial-derived cytokines and nitric oxide (NO) in GT1-1 neuron-astroglia cocultures, underscores a potential cross-talk between different intra/inter-cellular mediators in the dynamic control of LHRH release. Further studies aimed to disclose at a biochemical and a molecular level such bidirectional, informative network will give us new insights into more general issues concerned with the malfunction of the neuroendocrine-immune axis.

Semi-quantitative ultrastructural analysis of the localization and neuropeptide content of gonadotropin releasing hormone nerve terminals in the median eminence throughout the estrous cycle of the rat

The ultrastructural appearance of gonadotropin releasing hormone-immunoreactive elements was studied in the external zone of the median eminence of adult female Wistar rats. On the one hand, the purpose of the study was to determine the distribution of gonadotropin releasing hormone terminals towards the parenchymatous basal lamina at the level of hypothalamo-hypophyseal portal vessels, throughout the estrous cycle. On the other hand, we have semi-quantified the gonadotropin releasing hormone content in nerve terminals or preterminals during this physiological condition. A morphometric study was coupled to a colloidal 15 mn gold postembedding immunocytochemistry procedure. Animals were killed at 09.00 on diestrus II, 0.900, 10.00, 13.00, 17.00 and 18.00 on proestrus and 09.00 on estrus (n = 4-8 rats/group). A preliminary light microscopic study was carried out to identify an antero-posterior part of median eminence strongly immunostained by anti-gonadotropin releasing hormone antibodies but which was, in addition, easily spotted. This last condition was necessary to make a good comparison between each animal. Contacts between gonadotropin releasing hormone nerve terminals and the basal lamina were observed only the day of proestrus. Such contacts, however, were rare and in the great majority of cases, gonadotropin releasing hormone terminals are separated from basal lamina by tanycytic end feet. The morphometric analysis showed no significant variation in average distance between gonadotropin releasing hormone terminals and capillaries throughout the estrous cycle. Consequently, it did not appear that a large neuroglial plasticity exists during the estrous cycle. However, the observation of contacts only on proestrus together with some ultrastructural images evoke the possibility of a slight plasticity. The semi-quantitative results show that the content of gonadotropin releasing hormone in the nerve endings presented two peaks on proestrus: one at 09.00 (23 +/- 5 particles/micrograms2, P < 0.03) before the onset of luteinizing hormone surge, and the second at 18.00 (16 +/- 2 particles/micrograms2, P < 0.01) concomitantly with the luteinizing hormone surge, when compared to baseline values on proestrus 10.00 (8 +/- particles/micrograms2).