Biomedical subjects

S R Ojeda

Publications and source records attributed to S R Ojeda.

At least 55 records · Page 3Linked to original sources

PubMed1996

Involvement of nerve growth factor in the ovulatory cascade: trkA receptor activation inhibits gap junctional communication between thecal cells.

Activation of trkA, the nerve growth factor (NGF) tyrosine kinase receptor, has been recently implicated in the process of mammalian ovulation. During the hour preceding follicular rupture, a marked increase in trkA and NGF gene expression occurs in thecal-interstitial cells of the ovary. Immunoneutralization of NGF actions or pharmacological blockade of trkA transducing activity inhibits ovulation, suggesting that activation of the NGF-trkA complex in nonneural cells of the periovulatory follicle is a physiological component of the ovulatory cascade. As thecal cells of Graafian follicles are functionally coupled by gap junctions, and the ovulatory rupture requires dissociation of thecal cell-cell communication, we sought to determine whether NGF affects the integrity of this communication. We now report that NGF-induced activation of trkA receptors in isolated ovarian thecal cells disrupts cell to cell communication by affecting the functional integrity of gap junctions. Bovine thecal cells expressing trkA receptors, but not cells lacking the receptors, respond to NGF with a reduction in the transfer of calcein, a fluorescent dye that passes through gap junctions. This effect was associated with a rapid (10-30 min) increase in serine phosphorylation of connexin-43, the main protein constituent of gap junctions in the ovary. The reduction in dye transfer was not observed when the cells were exposed to epidermal growth factor or other neurotrophins, including neurotrophin 3, neurotrophin 4, and brain-derived neurotrophic factor. Thus, cell-specific activation of trkA receptors in periovulatory follicles may provide one of the signals involved in inducing the cellular dissociation of the follicular wall that precedes ovulatory rupture.

Animals↗

PubMed1996

Neural and glial-mediated effects of growth factors acting via tyrosine kinase receptors on luteinizing hormone-releasing hormone neurons.

It is becoming increasingly evident that the secretory activity of LHRH neurons is regulated not only by transsynaptic inputs but also by trophic molecules of glial and neuronal origin. The present experiments were undertaken to gain insights into the potential cell-cell mechanisms by which basic fibroblast growth factor (bFGF) and transforming growth factor-alpha (TGF alpha), two growth factors produced in the hypothalamus, may affect LHRH neuronal function. Northern blot analysis showed that the LHRH-producing cell line GT1-7 contains the messenger RNA (mRNA) encoding the type 1 fibroblast growth factor receptor (FGFR-1) but not that encoding the epidermal growth factor (EGF) receptors, which mediates the biological actions of both TGF alpha and EGF. Ligand-induced receptor phosphorylation experiments demonstrated that GT1-7 cells possess biologically active FGFR-1s but not EGF receptors. Exposure of the cells to bFGF resulted not only in FGFR-1 tyrosine phosphorylation, but also in tyrosine phosphorylation of phospholipase C gamma, one of the initial enzymes in the intracellular signaling cascade initiated by FGFR activation. GT1-7 cells proliferated in response to this activation. Despite the presence of biologically active receptors, bFGF did not significantly stimulate release of the mature LHRH decapeptide. Instead, bFGF increased the steady-state levels of the mRNA encoding the LHRH precursor processing endoprotease PC2, with a time course comparable to that of phorbol esters, suggesting that, as shown in the companion paper, the actions of the growth factor on LHRH neurons involve facilitation of the initial step in LHRH prohormone processing. The increase in PC2 gene expression was not accompanied by changes in LHRH mRNA levels. Unlike these direct actions of bFGF on GT-1 cells, TGF alpha appears to act indirectly via astroglial intermediacy. Exposure of GT1-7 cells to TGF alpha or EGF failed to affect several parameters of cellular activity including LHRH release, LHRH and PC2 mRNA levels, and cell proliferation. In contrast, astrocyte culture medium conditioned by treatment with TGF alpha led to sustained stimulation of LHRH release with no changes in LHRH gene expression and a transient increase in PC2 mRNA levels. Although no definitive evidence for the presence of FGFR-1 in normal LHRH neurons could be obtained by either double immunohistochemistry or double in situ hybridization procedures, fetal LHRH neurons in primary culture responded to bFGF with neurite outgrowth. Thus, normal LHRH neurons may have an FGFR-1 content too low for detection by regular histochemical procedures, and/or detectable expression of the receptor may be confined to a much earlier developmental stage. The mitogenic effect of bFGF on GT1-7 cells supports this possibility and suggests a role for FGF in the cell proliferation events that precede acquisition of the LHRH neuronal phenotype. It appears that once this phenotype is established, bFGF may promote the differentiation of LHRH neurons. The results also suggest that the secretory capacity of LHRH neurons develops under a dual trophic influence, one on peptide processing exerted directly by bFGF on early neurons, and another on LHRH release, exerted by TGF alpha via the intermediacy of astroglial cells.

Animals↗

PubMed1996

Basic fibroblast growth factor regulates the conversion of pro-luteinizing hormone-releasing hormone (Pro-LHRH) to LHRH in immortalized hypothalamic neurons.

Growth factors are commonly associated with the regulation of cellular proliferation and differentiation. In established cells, growth factors can also serve as trophic agents. Immortalized LHRH neurons contain basic fibroblast growth factor (bFGF) receptors. Although these receptors are coupled to activation of protein kinase C, and phorbol esters are strong activators of protein kinase C-stimulated LHRH release, bFGF did not influence LHRH secretion from these cells. To clarify this discrepancy, the effects of bFGF and phorbol ester on pro-LHRH biosynthesis, protein processing, and secretion were examined in GT1-7 cells. Phorbol ester stimulated LHRH secretion, whereas bFGF either had no effect or stimulated LHRH release depending upon the antiserum used. Pro-LHRH levels in lysate and medium were depressed by phorbol esters; concentrations in bFGF-treated cells were somewhat lower than those in unstimulated controls. HPLC analyses revealed that both agents enhanced the release of LHRH intermediate products into the medium. C-Terminally extended forms of LHRH, especially LHRH-[Gly11], were prominent in medium from bFGF-stimulated neurons. Levels of LHRH were depressed relative to those in the control or phorbol ester groups. These data indicate that phorbol esters control the biosynthesis, secretion, and, to some extent, processing of pro-LHRH. The effects of bFGF are novel because this factor regulates processing of the prohormone so that LHRH-intermediate products are predominantly secreted instead of LHRH. By enhancing the secretion of these intermediates over that of LHRH, bFGF can control the biological activity of the decapeptide and regulate LHRH neuronal function.

Amino Acid Sequence↗

PubMed1996

Insulin-like growth factor I of peripheral origin acts centrally to accelerate the initiation of female puberty.

In several species, including humans, circulating insulin-like growth factor I (IGF-I) levels increase during the onset of puberty, suggesting that this peptide contributes to attaining sexual maturity. Because IGF-I elicits LHRH release from the median eminence (ME) of immature female rats in vitro, we hypothesized that it may represent one of the peripheral signals suspected to link somatic development to the LHRH-releasing system at puberty. We now present evidence in support of this concept. Quantitation of IGF-I messenger RNA (mRNA) levels by ribonuclease protection assay revealed that expression of the IGF-I gene did not change in the medial basal hypothalamus or preoptic area of female rats during peripubertal development. In contrast, the contents of both IGF-Ia and IGF-Ib mRNA, the two alternatively spliced forms of the IGF-I gene, increased significantly in the liver during the early proestrous phase of puberty. This change was followed by an elevation in serum IGF-I levels during the late proestrous phase of puberty along with a concomitant increase is serum gonadotropin levels. The proestrous change in serum IGF-I levels was accompanied by a selective increase in IGF-I receptor (IGF-IR) mRNA in the ME. Small doses of IGF-I (2-200 ng), administered intraventricularly, effectively induced LH release in both juvenile and peripubertal female rats, an increase prevented by prior immunoneutralization of LHRH actions. Importantly, intraventricular injections of IGF-I (20 ng), administered twice daily in the afternoon to immature animals, significantly advanced puberty. Thus, these results suggest that IGF-I of peripheral origin contributes to the initiation of female puberty by stimulating LHRH release from the hypothalamus, an effect that appears to be amplified by the increased synthesis of IGF-I receptors in the ME during first proestrus.

Animals↗

PubMed1995

Expression of neurotrophins and their receptors in the mammalian ovary is developmentally regulated: changes at the time of folliculogenesis.

An emerging body of evidence suggests that neurotrophins not only promote neuronal survival and differentiation, but can also target nonneuronal cells for their actions. Neurotrophins initiate their biological effects by binding to cell membrane tyrosine kinase receptors of the trk protooncogene family. In addition, all neurotrophins recognize with similar affinity a different receptor molecule known as p75 nerve growth factor receptor (p75 NGFR) or low affinity NGFR, which appears to interact with the trk receptors to potentiate their response to neurotrophins. The mature mammalian ovary has been shown to synthesize several neurotrophins, including nerve growth factor (NGF), neurotrophin 3 (NT-3), and neurotrophin 4/5 (NT-4/5). The ovary also expresses some of the neurotrophin receptors, including p75 NGFR, trkB [the receptor for NT-4/5 and brain-derived neurotropic factor (BDNF)], and trkA (the NGF receptor). The present experiments were undertaken to determine whether neurotrophins and their receptors are expressed at the time of definitive ovarian histogenesis, and whether any of them exhibit a developmental pattern of expression related to the completion of folliculogenesis. Immunohistochemical identification of p75 NGFR in rat embryonic ovaries revealed that the receptor is predominantly expressed in mesenchymal cells. By gestational day 18, these cells have formed pockets that enclose presumptive pregranulosa cells and groups of oocytes into ovigerous cords. Immediately after birth, the ovigerous cords are subdivided, resulting in the abrupt formation of primordial follicles between 24-48 h after birth. Consistent with these observations, the p75 NGFR messenger RNA (mRNA) content increased after birth and remained elevated at the time of follicular assembly. The NGF and trkA genes showed a different pattern of expression, as the ovarian content of both NGF and trkA mRNA decreased at the time of folliculogenesis. In contrast to the drop in NGF and trkA mRNA expression, NT-4 mRNA levels increased at the time of follicular assembly, coinciding with the abrupt appearance of trkB mRNA. In situ hybridization showed that the increase in NT-4 mRNA expression occurred in a subpopulation of oocytes between 24-48 h after birth, and that the trkB gene became predominantly expressed at this time in epithelial pregranulosa cells. Substantial, but unchanging, levels of NT-3 mRNA and the mRNA encoding trkC, the preferred NT-3 receptor, were detected throughout the perinatal period examined. Very low and invariable levels of BDNF were also detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1995

The primate ovary contains a population of catecholaminergic neuron-like cells expressing nerve growth factor receptors.

The ovary of humans and nonhuman primates is innervated by sympathetic and sensory neurons of the peripheral nervous system. Recent studies demonstrated that the density of the sympathetic innervation to the rhesus monkey ovary is developmentally regulated, with adult density being attained around the time of puberty. In the present study, we used an immunocytochemical approach to obtain insights into the cell-cell signaling mechanisms that may contribute to the functional maintenance of this innervation. Because sympathetic neurons of the peripheral nervous system require target-derived neurotropins for their survival and function, experiments were conducted to determine if one of the receptors recognized by neurotropins is expressed in fibers innervating the primate ovary. A monoclonal antibody to the human low-affinity nerve growth factor (NGF) receptor, termed p75 NGFR because of its molecular weight, demonstrated the presence of this receptor in nerve fibers innervating the ovarian vasculature, interstitial tissue, and developing follicles of the gland. In addition, as shown in rodents, p75 NGFR immunoreactivity was detected in nonneuronal, endocrine cells of the ovary, specifically the thecal cell layer of developing follicles. Unexpectedly, however, the monkey ovary was also found to contain a network of small p75 NGFR immunoreactive cells distributed throughout the ovarian medulla and cortex. These cells, identified as such by confocal microscopy, had a neural-like appearance and displayed both neurofilament and neuron-specific enolase immunoreactivity. They appeared to be densely interconnected and were seen innervating the ovarian vasculature, the thecal cell layer of follicles, and, occasionally, primordial follicles. Double immunohistochemical procedures demonstrated that a subpopulation of these intraovarian, p75 NGFR-bearing neuron-like cells are catecholaminergic, as determined by their immunoreactivity to antibodies to tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis. RNA blot hybridization revealed the presence of p75 NGFR messenger RNA in the monkey ovary, thus demonstrating the ability of the gland to synthesize the receptors. These results demonstrate that the primate ovary contains an intrinsic network of neuron-like cells. Because such a neuronal network has not been detected in rodents or other non-primate species, it would appear that its presence in the primate ovary may have evolutionary significance.

Animals↗

PubMed1995

Neurotrophins and the neuroendocrine brain: different neurotrophins sustain anatomically and functionally segregated subsets of hypothalamic dopaminergic neurons.

Hypothalamic neurons control a variety of important hormonal and behavioral functions. Little is known, however, about the neurotrophic factors that these neurons may require for survival and/or maintenance of their differentiated functions. We conducted experiments to examine this issue, utilizing a combination of immunohistochemical, in situ hybridization and cell culture approaches. We found that the low affinity receptor for nerve growth factor (p75 NGFR) is present in small subsets of hypothalamic peptidergic neurons identified as such by their content of galanin, luteinizing hormone-releasing hormone (LHRH) and vasointestinal peptide (VIP). More prominently, however, examination of hypothalamic dopaminergic (DA) neurons for the presence of p75 NGFR-like immunoreactivity revealed that the receptor was present on tyrosine hydroxylase (TH)-positive neurons of the zona incerta and periventricular region, but not on neuroendocrine DA neurons of the tuberoinfundibular region. In situ hybridization experiments using a p75 NGFR cRNA confirmed this distribution. Regardless of the presence or absence of p75 NGFR, neither DA group expresses trkA mRNA, indicating that these two major hypothalamic subsets of DNA neurons are NGF-insensitive. A substantial fraction of TH mRNA-positive cells in the zona incerta expresses trkB mRNA, which encodes the receptor for brain derived neurotrophic factor (BDNF); in turn BDNF supports the in vitro survival of hypothalamic TH neurons bearing p75-NGFR, suggesting that BDNF is trophic for DNA neurons of the zona incerta. In contrast, tuberoinfundibular DA neurons do not express trkB mRNA, but some have trkC mRNA, which encodes the receptor for neurotrophin-3 (NT-3). The in vitro survival of TH neurons devoid of p75-NGFR is supported by NT-3, implying that NT-3 may be trophic for a subset of tuberoinfundibular DA neurons. These results suggest that, in spite of expressing an identical neurotransmitter phenotype, anatomically and functionally segregated DA neurons of the neurodendocrine brain are sustained by different neurotrophic factors.

Animals↗

PubMed1994

Expression of epidermal growth factor receptor changes in the hypothalamus during the onset of female puberty.

Recent findings have led to the concept that transforming growth factor alpha (TGF alpha) contributes to the neuroendocrine regulation of female puberty by stimulating the release of luteinizing hormone-releasing hormone (LHRH), the neurohormone controlling sexual development. It was postulated that this effect is mediated by epidermal growth factor receptors (EGFR) and that EGFR may not be located on LHRH neurons, so that TGF alpha-induced LHRH release would require an intermediate cell-to-cell interaction, presumably of glial-neuronal nature. The present study was undertaken to characterize the presence of EGFR in rat hypothalamus and to determine if changes in EGFR gene expression and EGFR protein occur at the time of puberty. RNA blot hybridization demonstrated that the hypothalamus expresses all mRNA species known to encode EGFR. RNase protection assays revealed that alternative splicing of the EGFR primary mRNA transcript occurs in the hypothalamus and produces a predominant transcript encoding the full-length EGFR and a much less abundant, shorter mRNA encoding a truncated, and presumably secreted form of EGFR. EGFR-like immunoreactive material was found in several hypothalamic regions including the organum vasculosum of the lamina terminalis, supraoptic, suprachiasmatic, and paraventricular nuclei, ependymal cells lining the third ventricle, some astrocytes associated with blood vessels, astrocytes of the pial surface, and tanycytes and glial cells of the median eminence (ME). Low levels of EGFR mRNA were detected by hybridization histochemistry in cells of the same areas containing EGFR-like immunoreactivity. Double-immunohistochemistry revealed that even though LHRH neurons are in close proximity to EGFR-positive cells, they do not contain EGFR. In the ME, EGFR-immunonegative LHRH nerve terminals tightly coexist with EGFR-positive cells, presumably tanycytes and glial astrocytes. EGFR mRNA levels measured by quantitative reverse transcription-polymerase chain reaction assay (RT-PCR) in the ME-arcuate nucleus region at the time of puberty decreased in the morning of the first proestrus, i.e., preceding the first preovulatory surge of gonadotropins, and rebounded at the time of the surge. Functional EGFR protein levels, detected by the ability of the receptor to autophosphorylate in response to ligand or divalent antibody-induced activation, changed in a similar manner at the time of puberty. No such changes were observed in the cerebellum, a brain region irrelevant to neuroendocrine reproductive control. These results demonstrate the existence of EGF receptors in the prepubertal female rat hypothalamus and suggest that changes in EGFR gene expression and biologically active EGFR protein contributes to the neuroendocrine process underlying the first preovulatory surge of gonadotropins.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Developmental expression of the genes encoding transforming growth factor alpha and its receptor in the hypothalamus of female rhesus macaques.

Studies in female rats have shown that transforming growth factor alpha (TGF alpha) stimulates release of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual maturation, and that expression of the TGF alpha gene in the hypothalamus increases during both the initiation of normal puberty and after hypothalamic lesions that induce sexual precocity. Since blockade of epidermal growth factor receptors (EGFR), which mediate TGF alpha actions, delayed the normal timing of puberty, it was postulated that TGF alpha/EGFR contributes to the neuroendocrine process that underlies the initiation of normal female puberty. The present study was undertaken to examine the hypothesis that hypothalamic expression of the TGF alpha gene and its receptor changes in relation to the stage of sexual development in nonhuman primates, and to determine whether these changes are accompanied by corresponding alterations in LHRH gene expression. DNA fragments complementary to the coding regions of the rhesus monkey TGF alpha, EGFR and LHRH genes were cloned by reverse transcription-polymerase chain reaction (RT-PCR), sequenced and used to prepare monkey-specific antisense RNA probes. A quantitative RT-PCR was developed in which the cloned sequences were utilized to prepare RNA standards for the quantitation of tissue mRNA levels. Both TGF alpha and EGFR mRNA levels in the medial basal hypothalamus and preoptic area of female monkeys were elevated during neonatal life (1 week to 6 months of age), when FSH secretion is also high, decreased during juvenile development (8-18 months of age), when secretion of both FSH and LH is low, and markedly increased during the expected time of puberty (30-36 months of age). No such changes were observed in either the cerebellum or the cerebral cortex, two brain regions irrelevant to neuroendocrine reproductive control. In contrast to the pronounced alterations in hypothalamic TGF alpha/EGFR gene expression observed during sexual development, LHRH mRNA levels did not vary significantly during this time. Hybridization histochemistry revealed the presence of both TGF alpha and EGFR mRNAs in cells scattered throughout the hypothalamus, but more predominantly in the median eminence, suprachiasmatic nuclei, optic chiasm and cells along the wall of the third ventricle. These results demonstrate that increases in TGF alpha and EGFR gene expression, specific to the neuroendocrine brain, occur during developmental phases in which gonadotropin output is also elevated--most noticeably at the time of puberty.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Immature rat ovaries become revascularized rapidly after autotransplantation and show a gonadotropin-dependent increase in angiogenic factor gene expression.

When the ovaries of 23-day-old juvenile rats are transplanted to an ectopic site, they recover within 1 week the ability to control gonadotropin secretion via steroid negative feedback. Vascular corrosion casting followed by scanning electron microscopy revealed that the transplanted ovary becomes profusely revascularized within 48 h after transplantation. Vascular ingrowth was accompanied by a 40- to 60-fold increase in expression of the genes encoding two angiogenic factors, vascular endothelial growth factor (VEGF) and transforming growth factor-beta 1 (TGF beta 1), as assessed by RNA blot hybridization of the corresponding mRNAs. Although TGF beta 3 mRNA levels also increased, no changes in the levels of mRNAs encoding other putative angiogenic factors, such as TGF alpha, basic fibroblast growth factor, and TGF beta 2, were observed. Hybridization histochemistry demonstrated that in intact ovaries, VEGF mRNA is mainly expressed in granulosa cells of the cumulus oophorus and thecal cells of large antral follicles. Transplantation is followed by an increase in mRNA abundance and a dramatic shift in cellular localization, so that the mRNA becomes predominantly expressed in cells of the outer ovarian cortex. In intact ovaries, low levels of TGF beta 1 mRNA were detected in thecal-interstitial cells; after transplantation, its expression also became more predominant in the ovarian outer cortex, but this change was not as marked as in the case of VEGF. Because ovarian autotransplantation is followed by a rapid increase in serum gonadotropin levels, experiments were conducted to determine the importance of this rise in the activation of VEGF and TGF beta 1 gene expression. After transplantation, some animals were treated with the LHRH antagonist Nal-Glu LHRH (50 micrograms/rat, once a day for 2 days) to prevent the posttransplantation rise in serum gonadotropins. Quantitation of VEGF and TGF beta 1 mRNA by RNase protection assay 48 h later showed that suppression of gonadotropin secretion diminished the increase in both VEGF and TGF beta 1 gene expression. Concomitant treatment with PMSG (8 IU/rat, single injection), which mainly bypasses the suppression of endogenous FSH levels, restored the TGF beta 1 mRNA response, but had no effect on VEGF mRNA. The results suggest that the increase in gonadotropin secretion following ovarian transplantation contributes to revascularization of the graft by up-regulating the gene expression of two major angiogenic factors.

Angiogenesis Inducing Agents↗

PubMed1994

Androgens regulate aromatase cytochrome P450 messenger ribonucleic acid in rat brain.

The conversion of androgens to estrogens by aromatase cytochrome P450 (P450AROM) is an important step in the mechanism of androgen action in the brain. In adult rats, P450AROM activity (AA) is regulated by androgens in the preoptic area and medial basal hypothalamus, but is constitutive in the amygdala. This study was undertaken to determine the distribution of P450AROM messenger RNA (mRNA) and AA in adult rat brain and examine the effects of steroid treatments on their concentrations in various brain regions. AA was determined by a sensitive assay that measures the production of 3H2O during the conversion of [1 beta-3H]androstenedione to estrone. P450AROM mRNA was measured by a ribonuclease protection assay using a RNA probe complementary to the 5'-coding region of rat P450AROM mRNA. The 32P-labeled P450AROM probe protected two mRNA fragments in brain tissues that expressed AA (preoptic area, medial basal hypothalamus, amygdala, and hippocampus). The larger protected RNA fragment was 430 nucleotides (nt) long and corresponded in size to the full-length protected complementary RNA, whereas the shorter protected RNA fragment was 300 nt long. Brain tissues that did not exhibit AA contained either the smaller protected RNA fragment (cingulate and parietal cortex) or no protected RNA (cerebellum). These results suggest that the 430-nt protected RNA fragment represents mRNA that encodes the functional P450AROM enzyme. In agreement with this conclusion, we found that immature rat ovaries that were stimulated with PMSG to synthesize estrogen contained only the 430-nt protected fragment. The levels of the 430-nt protected RNA fragment differed significantly between brain regions (amygdala > > preoptic area > medial basal hypothalamus > or = hippocampus) and were significantly correlated with AA (r = 0.994; P < 0.001). After castration, the concentrations of P450AROM mRNA and AA decreased significantly in the preoptic area and medial basal hypothalamus (P < 0.05), but not in the amygdala. Treatments with testosterone or dihydrotestosterone maintained P450AROM mRNA and AA at levels approximating those found in intact males. Although 17 beta-estradiol treatment increased AA in the preoptic area, it did not affect the P450AROM mRNA content. These results suggest that the increase in AA observed after exposure to androgens results from regulation of the transcription and/or stability of P450AROM mRNA. In contrast, estradiol appears to exert an effect on AA at the posttranscriptional level.(ABSTRACT TRUNCATED AT 400 WORDS)

Androgens↗

PubMed1994

Overexpression of a human transforming growth factor-alpha (TGF alpha) transgene reveals a dual antagonistic role of TGF alpha in female sexual development.

The importance of transforming growth factor-alpha (TGF alpha) in female reproductive development was assessed using transgenic mice bearing a human TGF alpha complementary DNA under the control of a mouse metallothionein-1 promoter (MT1-hTGF alpha). Examination of the brain and ovaries 5 h after a single sc injection of zinc chloride, administered to activate the MT1-hTGF alpha transgene, revealed that prominent sites of human TGF alpha messenger RNA expression within these tissues were the hypothalamus and ovarian follicles, respectively. In vitro experiments showed that acute transgene activation increased hypothalamic release of LH-releasing hormone. In contrast, the ovarian steroidal response to gonadotropins, examined in vitro, was markedly attenuated. Chronic activation of transgene expression by daily administration of zinc chloride delayed the time of first estrus (an index of peripubertal estrogen secretion), but shortened the interval between first estrus and the onset of estrous cyclicity (an index of reproductive competence). Accumulation of small antral follicles, accompanied by thecal hypertrophy and enhanced androgen production, preceded the acquisition of ovulatory capacity. These changes were accompanied by reduced serum LH levels, suggesting that the relative inability of small antral follicles to develop further in TGF alpha-overexpressing mice is at least in part due to inappropriate gonadotropin support. Serum LH levels in these animals may be reduced by an augmented androgen negative feedback signal. Nontransgenic mouse ovaries, placed under the control of a transgenic hypothalamus by heterologous grafting, rapidly ovulated and initiated estrous cyclicity. In contrast, acquisition of reproductive capacity was severely delayed in nontransgenic mice bearing transgenic ovarian grafts. The results indicate that TGF alpha regulates female reproductive development through two opposing mechanisms: within the brain, it facilitates the neuroendocrine activation of the process; at the ovarian level, modulates the stimulatory effect of gonadotropin hormones on follicular growth and steroidogenesis.

Animals↗

PubMed1994

Region-specific regulation of transforming growth factor alpha (TGF alpha) gene expression in astrocytes of the neuroendocrine brain.

Certain glial cells of the hypothalamus have been implicated in the neuroendocrine control of reproductive development. Hypothalamic astrocytes appear to exert this function via a cell-cell interactive mechanism that involves the production of transforming growth factor alpha (TGF alpha), a polypeptide able to affect both glial and neuronal functions in the CNS. In the hypothalamus, TGF alpha stimulates neuronal secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, via activation of epidermal growth factor receptors (EGFR). Since astrocytes but not LHRH neurons express EGFR, it has been postulated that the stimulatory effect of TGF alpha on LHRH release is not exerted directly on LHRH neurons, but rather via glial intermediacy. The present experiments were undertaken to define whether TGF alpha is able to exert paracrine/autocrine effects on isolated hypothalamic astrocytes, and to determine if estradiol-previously shown to increase TGF alpha mRNA levels in the hypothalamus of immature animals--can act directly on hypothalamic astrocytes to upregulate TGF alpha gene expression. Treatment with either TGF alpha or its structural homolog, epidermal growth factor (EGF), increased TGF alpha mRNA levels within 8 hr of exposure; the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was similarly effective. Blockade of EGFR with either tyrphostin RG-50864, an inhibitor of tyrosine kinase activity, or a monoclonal antibody that prevents ligand binding abolished the upregulatory effect of TGF alpha on TGF alpha mRNA levels. In contrast to hypothalamic astrocytes, cerebellar astrocytes did not respond to either TGF alpha or EGF with changes in TGF alpha mRNA abundance.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Activation of ovarian sympathetic nerves in polycystic ovary syndrome.

Polycystic ovarian syndrome (PCOS) is one of the most common human ovarian pathologies affecting women of reproductive age. Despite extensive investigation, the etiology of PCOS remains poorly understood. Experimentally, a PCO-like syndrome can be induced in rodents by a single dose of the long-acting estrogen, estradiol valerate (EV). We have used this model to examine the possibility that PCOS is associated with derangement of the sympathetic control of the ovary. The release of newly incorporated norepinephrine (NE) from ovarian nerve terminals in response to transmural stimulation of the gland increased significantly before the formation of cysts (30 days after EV injection) and remained elevated at the time when cysts form (60 days). The increase in evoked NE release was accompanied by an augmented NE content and enhanced incorporation of [3H]NE into ovarian tissue; both of these changes had been initiated by 30 days after EV treatment and became unambiguous at the time of cyst formation. The overall increase in ovarian sympathetic outflow suggested by these alterations in catecholamine homeostasis was accompanied by a thecal cell-interstitial tissue selective down-regulation of beta-adrenergic receptors; the beta-adrenergic receptor concentration in these sympathetically innervated ovarian compartments was significantly lower in PCO than during the estrous phase of the estrous cycle, a time at which the beta-adrenergic receptor concentration reaches its lowest levels in normal cycling ovaries. Tyrosine hydroxylase activity was found to increase only when expressed per mg ovary, but not in absolute terms (i.e. per total ovary), suggesting regulation of enzyme activity by the enhanced catecholamine content. The results demonstrate that an activation of the sympathetic neurons innervating the ovary precedes the development of cysts in EV-induced PCOS and raise the possibility that a derangement of sympathetic inputs to the ovary contributes to the etiology of PCOS.

Animals↗

PubMed1993

Ovarian steroidal response to gonadotropins and beta-adrenergic stimulation is enhanced in polycystic ovary syndrome: role of sympathetic innervation.

Experimental induction of a polycystic ovarian syndrome (PCOS) in rodents by the administration of a single dose of estradiol valerate (EV) results in activation of the peripheral sympathetic neurons that innervate the ovary. This activation is evidenced by an increased capacity of ovarian nerve terminals to incorporate and release norepinephrine (NE), an increase in ovarian NE content, and a decrease in ovarian beta-adrenergic receptor number in the ovarian compartments receiving catecholaminergic innervation. The present experiments were undertaken to examine the functional consequences of this enhanced sympathetic outflow to the ovary. The steroidal responses of the gland to beta-adrenergic receptor stimulation and hCG were examined in vitro 60 days after EV administration, i.e. at the time when follicular cysts are well established. EV-treated rats exhibited a remarkable increase in ovarian progesterone and androgen responses to isoproterenol, a beta-adrenergic receptor agonist, with no changes in estradiol responsiveness. Basal estradiol release was, however, 50-fold higher than the highest levels released from normal ovaries at any phase of the estrous cycle. The ovarian progesterone and androgen responses to hCG were enhanced in EV-treated rats, as were the responses to a combination of isoproterenol and hCG. Transection of the superior ovarian nerve (SON), which carries most of the catecholaminergic fibers innervating endocrine ovarian cells, dramatically reduced the exaggerated responses of all three steroids to both beta-adrenergic and gonadotropin stimulation. SON transection also reduced the elevated levels of ovarian NE resulting from EV treatment and caused up-regulation of beta-adrenoreceptors. Most importantly, SON transection restored estrous cyclicity and ovulatory capacity. The results indicate that the increased output of ovarian steroids in PCOS is at least in part due to an enhanced responsiveness of the gland to both catecholaminergic and gonadotropin stimulation. The ability of SON transection to restore a normal response indicates that the alteration in steroid output results from a deranged activation of selective components of the noradrenergic innervation to the ovary. These findings support the concept that an alteration in the neurogenic control of the ovary contributes to the etiology of PCOS.

Androgens↗

PubMed1993

Hypothalamic lesions that induce female precocious puberty activate glial expression of the epidermal growth factor receptor gene: differential regulation of alternatively spliced transcripts.

Injury of the nervous system triggers a complex series of repair mechanisms that include production of neurotrophic and mitogenic factors by cells neighboring the injured area. While trauma of most parts of the brain results in loss of function, lesions of certain regions of the female hypothalamus enhance the secretory activity of a group of specialized neurons that produce luteinizing hormone-releasing hormone (LHRH), the neuropeptide that controls sexual development. The increased output of LHRH causes sexual precocity by prematurely activating the neuroendocrine reproductive axis. Recent studies have implicated transforming growth factor alpha (TGF alpha) produced by reactive astrocytes in the process by which lesions hasten sexual maturation, and have suggested that the stimulatory actions of TGF alpha on LHRH neurons require the intermediacy of epidermal growth factor receptors (EGFRs). In the present study, we examined the changes in EGFR gene expression following lesions of the preoptic-anterior hypothalamic area (POA-AHA) of immature female rats, identified the cell types where EGFR synthesis increases, and assessed the biochemical activity of the newly formed EGFR protein. RNase protection assays demonstrated that the lesion significantly increased the levels of a predominant mRNA transcript encoding the full-length, membrane-spanning EGFR, but did not affect those of a much less abundant, alternatively spliced mRNA that encodes a truncated, presumably secreted form of EGFR. Following lesions, antibody-induced EGFR kinase activity increased twofold. Antibodies directed against a peptide sequence contained within the carboxy terminus of EGFR showed intense EGFR immunoreactivity in cells surrounding the lesion site; double immunohistochemistry identified these cells as astrocytes since EGFR immunoreactivity was colocalized with that of glial fibrillary acidic protein, an astrocytic marker. That these changes result from an increase in EGFR gene expression was indicated by the elevated levels of EGFR mRNA detected by in situ hybridization in cells of the same area. Although POA-AHA lesions did not result in appearance of EGFR in LHRH neurons themselves, EGFR-positive cells and processes were seen in close proximity to LHRH neurons and their nerve terminals, particularly in the area surrounding the lesion. Since TGF alpha gene expression is also increased in reactive astrocytes of POA-AHA lesions and blockade of EGFR prevented the advancing effect of the lesion on puberty (Junier et al., 1991b), the present results support the concept that, in lesioned animals, TGF alpha stimulates LHRH secretion indirectly via a paracrine mechanism that involves its interaction with EGFRs located on astroglial cells.

Animals↗

PubMed1992

Ovarian innervation develops before initiation of folliculogenesis in the rat.

Sympathetic neurotransmitters have been shown to be present in the ovary of the rat during early postnatal development and to affect steroidogenesis before the ovary becomes responsive to gonadotropins, and before the first primordial follicles are formed. This study was undertaken to determine if development of the ovarian innervation is an event that antedates the initiation of folliculogenesis in the rat, Rattus norvegicus. Serial sections of postnatal ovaries revealed a negligible frequency of follicles 24 h after birth (about 1 primordial follicle per ovary). Twelve hours later there were about 500 follicles per ovary, a number that more than doubled to about 1300 during the subsequent 12 h, indicating that an explosive period of follicular differentiation occurs between the end of postnatal days 1 and 2. Electron microscopy demonstrated that before birth the ovaries are already innervated by fibers containing clear and dense-core vesicles. Immunohistochemistry performed on either fetal (day 19) or newborn (less than 15h after birth) ovaries showed the presence of catecholaminergic nerves, identified by their content of immunoreactive tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. While some of these fibers innervate blood vessels, others are associated with primordial ovarian cells, thereby suggesting their participation in non-vascular functions. Since prefollicular ovaries are insensitive to gonadotropins, the results suggest that the developing ovary becomes subjected to direct neurogenic influences before it acquires responsiveness to gonadotropins.

Adrenergic Fibers↗

PubMed1992

Brain aromatase cytochrome P-450 messenger RNA levels and enzyme activity during prenatal and perinatal development in the rat.

Aromatase cytochrome P-450 (P-450AROM) enzyme activity catalyzes the conversion of androgens to estrogens in specific brain areas. During development local estrogen formation is thought to influence the sexual differentiation of neural structures (i.e. increase neurite growth and establish neural circuitry) and modulate reproductive functions. This study was undertaken to investigate the ontogeny of the (P-450AROM) enzyme and its messenger RNA (mRNA) in medial basal hypothalamic (MBH) and preoptic area (POA) tissue during late fetal and perinatal development of the rat. Aromatase activity in the MBH-POA was negligible before gestational day (GD) 16 (< 0.1 pmol/h/mg protein), increased over 10-fold at GD 17 and continued to increase (over 5-fold) to peak levels at GD 19 (> 5.0 pmol/h/mg protein), and then declined to low levels at GD 22 and 2 days post-birth (approximately 1 pmol/h/mg protein). The profile of P-450AROM mRNA in the MBH-POA tissue was characterized by a predominant 2.7 kilobase (kb) mRNA species, similar in size to the largest functional P-450AROM mRNA observed in adult rat ovarian tissue. At GD 15, the P-450AROM mRNA was undetectable; low but detectable levels were seen at GD 17, the abundance increased at later time points and remained at peak levels on GDs 18 through 20, decreased slightly by GD 22, and then declined further by 2 days post-birth. The developmental increase in P-450AROM mRNA levels correlated with the ascending pattern of enzyme activity before GD 19, but the marked decrease in enzyme activity seen after GD 19 was not accompanied by a corresponding decline in mRNA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗