Trophic effects of insulin-like growth factor-I on fetal rat hypothalamic cells in culture

Role of astroglia and insulin-like growth factor-I in gonadal hormone-dependent synaptic plasticity

Gonadal hormones exert a critical influence over the architecture of specific brain areas affecting the formation of neuronal contacts. Cellular mechanisms mediating gonadal hormone actions on synapses have been studied extensively in the rat arcuate nucleus, a hypothalamic center involved in the feed-back regulation of gonadotropins. Gonadal steroids exert organizational and activational effects on arcuate nucleus synaptic connectivity. Perinatal testosterone induces a sexual dimorphic pattern of synaptic contacts. Furthermore, during the preovulatory and ovulatory phases of the estrous cycle there is a transient disconnection of inhibitory synaptic inputs to the somas of arcuate neurons. This synaptic remodeling is induced by estradiol, blocked by progesterone, and begins with the onset of puberty in females. Astroglia appear to play a significant role in the organizational and the activational hormone effects on neuronal connectivity by regulating the amount of neuronal membrane available for the formation of synaptic contacts and by releasing soluble factors, such as insulin-like growth factor I (IGF-I), which promote the differentiation of neural processes. Recent evidence indicates that gonadal steroids and IGF-I may interact in their trophic effects on the neuroendocrine hypothalamus. Estradiol and IGF-I promote the survival and morphological differentiation of rat hypothalamic neurons in primary cultures. The effect of estradiol depends on IGF-I, while the effects of both estradiol and IGF-I depend on estrogen receptors. Furthermore, estrogen activation of astroglia in hypothalamic tissue fragments depends on IGF-I receptors. These findings indicate that IGF-I may mediate some of the developmental and activational effects of gonadal steroids on the brain and suggest that IGF-I may activate the estrogen receptor to induce its neurotrophic effects on hypothalamic cells. In addition, IGF-I levels in the neuroendocrine hypothalamus are regulated by gonadal steroids. IGF-I levels in tanycytes, a specific astroglia cell type present in the arcuate nucleus and median eminence, increase at puberty, are affected by neonatal androgen levels, show sex differences, and fluctuate in accordance to the natural variations in plasma levels of ovarian steroids that are associated with the estrous cycle. These changes appear to be mediated by hormonal regulation of IGF-I uptake from blood or cerebrospinal fluid by tanycytes. These results suggest that tanycytes may be involved in the regulation of neuroendocrine events in adult rats by regulating the availability of IGF-I to hypothalamic neurons. In summary, IGF-I and different forms of neuron-astroglia communication are involved in the effects of estradiol on synaptic plasticity in the hypothalamic arcuate nucleus.

A combination of insulin-like growth factor-I and platelet-derived growth factor enhances myelination but diminishes axonal regeneration into Schwann cell grafts in the adult rat spinal cord

Insulin-like growth factor-I (IGF-I) promotes axonal regeneration in the peripheral nervous system and this effect is enhanced by platelet-derived growth factor (PDGF). We decided, therefore, to study the effects of these factors on axonal regeneration in the adult rat spinal cord. Semipermeable polymer tubes, closed at the distal end, containing Matrigel mixed with cultured rat Schwann cells and IGF-I/PDGF, were placed at the proximal stump of the spinal cord after removal of the thoracic T9-11 segments. Control animals received implants of only Matrigel and Schwann cells or only Matrigel and IGF-I/PDGF. Four weeks after implantation, electron microscopic analysis showed that the addition of IGF-I/PDGF resulted in an increase in the myelinated:unmyelinated fiber ratio from 1:7 to 1:3 at 3 mm in the Schwann cell graft, and that myelin sheath thickness was increased 2-fold. The reduced number of unmyelinated axons was striking in electron micrographs. These results suggested that IGF-I/PDGF enhanced myelin formation of regenerated axons in Schwann cell implants, but there was a 36% decrease in the total number of myelinated axons at the 3 mm level of the graft. This finding and the altered myelinated:unmyelinated fiber ratio revealed that the overall fiber regeneration into Schwann cell implants was diminished up to 63% by IGF-I/PDGF. Histological evaluation revealed that there were more larger cavities in tissue at the proximal spinal cord-graft interface in animals receiving a Schwann cell implant with IGF-I/PDGF. Such cavitation might have contributed to the reduction in axonal ingrowth. In sum, the results indicate that whereas the combination of IGF-I and PDGF enhances myelination of regenerating spinal cord axons entering implants of Matrigel and Schwann cells after midthoracic transection, the overall regeneration of axons into such Schwann cell grafts is diminished.

The effect of insulin-like growth factors on brain myelination and their potential therapeutic application in myelination disorders

Degenerative disorders of the cerebral white matter, leukodystrophies and demyelination diseases, are characterized by the faulty formation or excessive breakdown of myelin. Insulin-like growth factors (IGFs) promote the proliferation of oligodendrocytes as well as their myelin synthesis. IGF-I overexpressing mice show a significant increase in brain weight associated with increased myelin content. In contrast, the brains of IGF-binding protein-1 transgenic mice show a dramatic decrease in myelination. Furthermore, IGFs and IGF-binding proteins are among the factors that are induced by brain injury and have neuroprotective effects. IGFs also induce neurite growth and survival, in particular in glial cells of the peripheral nervous system. In demyelinating diseases, IGF-I may be useful for reducing myelin breakdown and promoting myelin regeneration. These observations may lead to new therapeutic applications for IGFs, for example promoting remyelination or limiting damage following brain injury.

The role of the insulin-like growth factors in the central nervous system

Increasing evidence strongly supports a role for insulin-like growth factor-I (IGF-I) in central nervous system (CNS) development. IGF-I, IGF-II, the type IIGF receptor (the cell surface tyrosine kinase receptor that mediates IGF signals), and some IGF binding proteins (IGFBPs; secreted proteins that modulate IGF actions) are expressed in many regions of the CNS beginning in utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. IGF-I expression is predominantly in neurons and, in many brain regions, peaks in a fashion temporally coincident with periods in development when neuron progenitor proliferation and/or neuritic outgrowth occurs. In contrast, IGF-II expression is confined mainly to cells of mesenchymal and neural crest origin. While expression of type I IGF receptors appears ubiquitous, that of IGFBPs is characterized by regional and developmental specificity, and often occurs coordinately with peaks of IGF expression. In vitro IGF-I has been shown to stimulate the proliferation of neuron progenitors and/or the survival of neurons and oligodendrocytes, and in some cultured neurons, to stimulate function. Transgenic (Tg) mice that overexpress IGF-I in the brain exhibit postnatal brain overgrowth without anatomic abnormality (20-85% increases in weight, depending on the magnitude of expression). In contrast, Tg mice that exhibit ectopic brain expression of IGFBP-1, an inhibitor of IGF action when present in molar excess, manifest postnatal brain growth retardation, and mice with ablated IGF-I gene expression, accomplished by homologous recombination, have brains that are 60% of normal size as adults. Taken together, these in vivo studies indicate that IGF-I can influence the development of most, if not all, brain regions, and suggest that the cerebral cortex and cerebellum are especially sensitive to IGF-I actions. IGF-I's growth-promoting in vivo actions result from its capacity to increase neuron number, at least in certain populations, and from its potent stimulation of myelination. These IGF-I actions, taken together with its neuroprotective effects following CNS and peripheral nerve injury, suggest that it may be of therapeutic benefit in a wide variety of disorders affecting the nervous system.

Deficiency of essential neurotrophic factors in conditioned media produced by ethanol-exposed cortical astrocytes

Prior research in this laboratory has shown that in utero ethanol exposure adversely affects the development of serotonergic neurons. The current study investigated the hypothesis that cortical astrocytes produce trophic factors which are essential for the development of the fetal precursors of serotonergic and other raphe neurons (e.g. rhombencephalic neurons), and that ethanol exposure impairs the production of these factors by astrocytes. The results of these experiments demonstrated that cultured cortical astrocytes produce trophic factors which are necessary for the development of rhombencephalic neurons. Conditioned media obtained from control astrocytes promoted both general neuronal development (increased cell number, cell survival, DNA content, protein content, and neurite outgrowth) and serotonergic neuronal development (increased number of serotonin (5-HT) immunopositive cells and [3H]5-HT uptake). However, the conditioned media produced by ethanol-treated astrocytes (ECM) lacked essential neurotrophic factors. Neuronal cultures maintained in ECM had reduced DNA and neuronal survival, and altered neurite outgrowth. 5-HT immunopositive neurons and [3H]5-HT uptake were also decreased in ECM cultures. Thus, the damaging effects of in utero ethanol exposure on developing serotonergic neurons may be due to impaired production of astroglial neurotrophic factors.

Interaction of insulin-like growth factor-I and estradiol signaling pathways on hypothalamic neuronal differentiation

Neurotrophic effects of estradiol and insulin-like growth factor-I were assessed in primary cultures from fetal rat hypothalamus. Cultured neurons were immunostained with an antibody for the microtubule-associated protein-2. While both estradiol and insulin-like growth factor-I increased the number of microtubule-associated protein-2-immunoreactive neurons and the extension of immunoreactive processes, the effect of these two factors was not additive. The estradiol-induced increases in neuronal numbers and extension of neuronal processes were blocked by either the estrogen receptor antagonist ICI 182,780 or by an anti-sense oligonucleotide to the estrogen receptor. Furthermore, incubation of the cultures with an anti-sense oligonucleotide directed against the insulin-like growth factor-I messenger RNA also blocked the effect of estradiol. In turn, the effects of insulin-like growth factor-I were blocked by the estrogen receptor antagonist ICI 182,780 and by the anti-sense oligonucleotide to the estrogen receptor. These findings suggest that estradiol-induced activation of the estrogen receptor in developing hypothalamic cells requires the presence of insulin-like growth factor-I, and that both estradiol and insulin-like growth factor-I use the estrogen receptor as a mediator of their trophic effects on hypothalamic neurons.

The insulin-like growth factor I system in cerebellar degeneration

Brain insulin-like growth factor I (IGF-I) and its related molecules may be involved in neurodegenerative processes in which IGF-I-containing pathways are compromised. Since IGF-I is present in the olivocerebellar circuitry, two types of late-onset cerebellar ataxias (olivopontocerebellar and idiopathic cerebellar cortical atrophy) were chosen to test this hypothesis. The following significant changes in the peripheral IGF-I system of these patients were found: low IGF-I levels, and high IGF-binding protein 1 (BP-1), and BP-3 affinity for IGF-1. Sixty percent of the patients also had significantly low insulin levels. Patients suffering from other neurological diseases with cerebellar dysfunction and ataxia not involving the olivocerebellar pathway also had low IGF-I levels, while IGFBPs and insulin levels were normal. Our data indicate that degeneration of an IGF-I-containing neuronal pathway produces significant changes in the peripheral IGF system. This suggests strongly that the endocrine (bloodborne) and the paracrine/autocrine (brain) IGF systems are linked functionally. We propose that alterations in the blood IGF-I system may constitute a marker of some cerebellar diseases.

Homologous conditioned medium enhances expression of TRH in hypothalamic neurons in primary culture

Primary cultures of hypothalamic cells maintained in the presence of serum were either kept with homologous conditioned medium (CM) (i.e. only half of the medium was removed at each medium change) or without (total medium change). In cultures with homologous CM, TRH levels were increased. The effects of CMs from various intervals of the primary culture were tested. The strongest increases of TRH levels were obtained with CM from cultures enriched with hypothalamic glia.

Insulin-like growth factor I is an afferent trophic signal that modulates calbindin-28kD in adult Purkinje cells

Recent evidence suggests that Purkinje cells are specific targets of insulin-like growth factor I (IGF-I) through their entire life span. During development, Purkinje cell numbers and their calbindin-28kD content increase after IGF-I treatment in culture. In the adult, part of the IGF-I present in the cerebellum is transported from the inferior olive, and modulates Purkinje cell function. We investigated whether IGF-I produced by inferior olive neurons and transported to the contralateral cerebellum through climbing fibers may modulate the levels of calbindin-28kD in the cerebellum of adult animals. Twenty-four hr after injection of an antisense oligonucleotide of IGF-I into the inferior olive, both IGF-I and calbindin-28kD levels in the contralateral cerebellar lobe were significantly reduced, while the number of calbindin-positive Purkinje cells was unchanged. The effect of the antisense on IGF-I levels was fully reversed 3 days after its injection into the inferior olive, with a postinhibitory rebound observed at this time, while calbindin-28kD levels slowly returned to control values. A control oligonucleotide did not produce any change in either IGF-I or calbindin-28kD content in the cerebellum. These results indicate that normal levels of IGF-I in the inferior olive are necessary to maintain appropriate levels of IGF-I in the cerebellum and of calbindin-28kD in the Purkinje cell. These results also extend our previous findings on the existence of an olivo-cerebellar IGF-I-containing pathway with trophic influence on the adult Purkinje cell.

Beneficial effects of insulin-like growth factor-I on wobbler mouse motoneuron disease

Recombinant human insulin-like growth factor-I (IGF-I) is being considered as a possible therapeutic agent for the treatment of motoneuron diseases like amyotrophic lateral sclerosis. The neurological mutant mouse wobbler, carries an autosomal recessive gene (wr) and has been characterized as a model of lower motoneuron disorders with associated muscle atrophy, denervation and reinnervation. The purpose of the present study was to determine the possible beneficial effect of IGF-I administration in this mouse model. Upon diagnosis at 4 weeks of age, affected mice and their control normal littermates received human recombinant IGF-I (1 mg/kg) or vehicle solution, once a day, for 6 weeks. Body weight and grip strength were evaluated periodically during the treatment period. Mean muscle fiber diameter on biceps brachii sections, choline acetyltransferase activity in muscle extracts, and motoneuron numbers in spinal cord sections were determined. IGF-I treated wobbler mice showed a marked weight increase from 3 to 6 weeks of treatment in comparison with placebo treated mutant mice. At the end of the treatment, grip strength, estimated by dynamometer resistance, was 40% higher in IGF-I treated versus placebo treated animals. Mean muscle fiber diameter which is smaller in wobbler mice than in normal mice was increased in IGF-I treated mutants. However, in this study the muscle choline acetyltransferase activity and the number of spinal cord motoneurons were unchanged. Thus, IGF-I administration mainly results in a significant effect on the behavioral and skeletal muscle histochemical parameters of the wobbler mouse mutant.

Insulin-like growth factor-1 mRNA is increased in deafferented hippocampus: spatiotemporal correspondence of a trophic event with axon sprouting

Deafferentation is known to induce axonal sprouting in adult brain, but the signals that direct this response are not understood. To evaluate the possible roles of insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) in central axonal sprouting, the present study used in situ hybridization to evaluate IGF-1 and bFGF mRNA expression in entorhinal deafferented rat hippocampus. Alternate tissue sections were processed for Fink-Heimer impregnation of axonal degeneration, Bandeiraea simplicifolia (BS-1) labeling of microglia, and glial fibrillary acidic protein immunocytochemistry. In control hippocampus, IGF-1 mRNA was localized to a few neurons, with no labeled cells in the dentate gyrus molecular layer; bFGF cRNA hybridization was diffuse in dendritic fields but was dense in CA2 stratum pyramidale. Both mRNA species were increased by deafferentation. The distribution of elevated IGF-1 mRNA corresponded precisely to fields of axonal degeneration and was greatest in the dentate gyrus outer molecular layer and stratum lacunosum moleculare. In these fields, IGF-1 mRNA was elevated by 2 days, reached maximal levels at 4 days, and declined by 10 days postlesion. Double labeling revealed that the majority of IGF-1 cRNA-labeled cells were microglia. In deafferented hippocampus, bFGF mRNA was broadly increased across fields both containing and lacking axonal degeneration. In the dentate, bFGF mRNA levels peaked at 5 days postlesion and remained elevated through 14 days. These results demonstrate that reactive microglia within deafferented hippocampal laminae express IGF-1 mRNA just prior to and during the period of reactive axonal growth and suggest that IGF-1 plays a role in directing the sprouting of spared afferents into these fields.

An immunohistochemical and in situ hybridization study of insulin-like growth factor I within fetal neuron cell cultures

Fetal neuron cell cultures (NCC) from 22 day gestation and 18 day gestation fetal rabbit brain were studied for the presence of insulin-like growth factor I (IGF I). The 22 day gestation NCC were incubated in an IGF I free/insulin free/serum free medium. The 18 day gestation NCC were incubated in: (1) IGF I free/insulin free/serum free medium, (2) IGF I containing medium (100 ng)/serum free medium, and (3) serum containing medium. The 22 day gestation NCC survived in the IGF I free/insulin free/serum free medium. Furthermore, IGF I was detected in the medium by RIA from day one to day ten of incubation. In contrast, the 18 day gestation NCC did not survive in the IGF I free/insulin free/serum medium, but survived in the serum medium. When the 18 day gestation NCC were incubated in the serum free medium containing 100 ng IGF I the cells survived for a period of 2-3 days. Immunoreactive IGF I was found within the 22 day gestation NCC incubated in the IGF I free/insulin free/serum free medium and 18 day gestation NCC in serum medium. Likewise, IGF I mRNA was found only within the 22 day gestation NCC. Internalization studies of IGF I have shown that the peptide was internalized from the medium by the two different gestational age NCC's studied. IGF I receptors were found in both 22 day gestation and 18 day gestation NCC. In conclusion IGF I may promote cell survival in early stages of brain development, and may be of exogenous origin. In contrast the 22 day gestation NCC are capable of producing and secreting IGF I, and indeed appear to respond to this growth factor in an autocrine fashion.

Brain growth retardation due to the expression of human insulin like growth factor binding protein-1 in transgenic mice: an in vivo model for the analysis of igf function in the brain

Three lines of transgenic (Tg) mice carrying a fusion gene linking the mouse metallothionein-I promoter to a cDNA encoding human insulin-like growth factor binding protein-1 (hIGFBP-1) were found to express the transgene in brain. As judged by comparing Tg brain weights to those of non-transgenic littermates, adult hemizygotic Tg mice of each line exhibited brain growth retardation (16.2%, 14.4% and 8.1% reductions in weight, respectively in each line). In two lines, total brain DNA and protein content were decreased. Further analysis indicated that the brain growth retardation was manifested in the second week of postnatal life. Given that the insulin-like growth factors (IGFs) stimulate cell proliferation and/or survival in neural cultures and that hIGFBP-1, when present in a molar excess, inhibits IGF interactions with their cell surface receptors, the brain growth retardation in hIGFBP-1 Tg mice likely results from hIGFBP-1 inhibition of IGF-stimulated growth-promoting actions. These hIGFBP-1 Tg mice should prove useful in defining IGF actions during postnatal brain maturation.

The insulin-like growth factor I system in the rat cerebellum: developmental regulation and role in neuronal survival and differentiation

The developmental regulation of insulin-like growth factor I (IGF-I), its receptor, and its binding proteins (IGFBPs) was studied in the rat cerebellum. All the components of the IGF-I system were detectable in the cerebellum at least by embryonic day 19. Levels of IGF-I receptor and its mRNA were highest at perinatal ages and steadily decrease thereafter, although a partial recovery in IGF-I receptor mRNA was found in adults. Levels of IGF-I and its mRNA also peaked at early ages, although immunoreactive IGF-I showed a second peak during adulthood. Finally, levels of IGFBPs were also highest at early postnatal ages and abruptly decreased thereafter to reach lower adult levels. Since highest levels of the different components of the IGF-I system were found at periods of active cellular growth and differentiation we also examined possible trophic effects of IGF-I on developing cerebellar cells in vitro. We found a dose-dependent effect of IGF-I on neuron survival together with a specific increase of the two main neurotransmitters used by cerebellar neurons, GABA and glutamate. Analysis of cerebellar cultures by combined in vitro autoradiography and immunocytochemistry with cell-specific markers indicated that both Purkinje cells (calbindin-positive) and other neurons (neurofilament-positive) contain IGF-I binding sites. These results extend previous observations on a developmental regulation of the IGF-I system in the cerebellum and reinforce the notion of a physiologically relevant trophic role of IGF-I in cerebellar development.

Role for glial cells in regulating the functional expression of neuropeptide Y (NPY) neurons in aggregate cultures derived from dissociated fetal brain cells

A series of studies from our laboratory have established an aggregate culture system of fetal rat brain cells expressing neuropeptide Y (NPY) which can serve as a model to study the role of glia-neuron paracrine interactions in the developmental expression of NPY neurons. In this system, NPY production increases progressively with culture-age and it is induced by forskolin (FOR) and phorbol 12-myristate 13-acetate (PMA). We addressed the following question: Is the functional expression of the NPY neurons impaired in the absence of glial cells (particularly astrocytes) and if so, can secretory products of aggregates composed of the full complement of brain cells (intact aggregates) restore the function of the impaired NPY neurons? Aggregates were generated from 17-day-old fetal rat cortex and maintained in serum-free medium for 13-15 days. Cytosine arabinoside (CA; doses of 0.5-8 microM) was added to the cultures on day 1 and the effectiveness in elimination of glial cells was verified on day 15 by measuring the incorporation of 3H thymidine into DNA and by immunostaining for the astrocyte marker glial fibrillary acidic protein (GFAP). Basal NPY production and FOR (10 microM) + PMA (20 nM) stimulated production of NPY on days 13-15 were taken as functional criteria. FOR + PMA induced approximately 2-fold increase in NPY production in control cultures (no CA). CA inhibited both basal and FOR + PMA induced production of NPY and DNA synthesis in a dose-dependent manner: at 6 microM CA, basal NPY production was reduced by about 50%, FOR + PMA stimulated production of NPY and DNA synthesis were completely inhibited, and astrocytes were essentially eliminated.(ABSTRACT TRUNCATED AT 250 WORDS)

IGF-I and IGF-I24-41 but not IGF-I57-70 affect somatic and neurobehavioral development of newborn male mice

Insulin-like growth factor (IGF-I) is a trophic factor for both neurons and glial cells. Its presence in developing and adult nervous system suggests an important role for this peptide in the development of neural circuitries. Neonatal male mice of the CD-1 outbred strain were injected intracerebroventricularly with either recombinant IGF-I, synthetic IGF-I fragment 24-41 or IGF-I fragment 57-70 on postnatal days (PND) 2, 4, and 7. Physical traits such as body weight gain, body length, and tail length were recorded daily from PND2 to PND13. Sensorimotor development was scored according to a modified Fox's scale. The ultrasonic vocalization pattern on PND8 and homing performance on PND10 were also recorded. Measures for body weight gain and tail length of the pups were significantly increased following treatment with the whole IGF-I peptide. However, neither IGF-I nor the smaller fragments affected mice sensorimotor development. IGF-I and IGF-I24-41 but not IGF-I57-70 increased the rate of ultrasonic calls of the pups measured on PND8. These data provide evidence that IGF-I regulates somatic growth and behavioral development when administered in newborn mice and that different portions of the peptide can exert different effects.

Orthograde transport and release of insulin-like growth factor I from the inferior olive to the cerebellum

Insulin-like growth factor I (IGF-I) and its receptor are expressed in functionally related areas of the rat brain such as the inferior olive and the cerebellar cortex. A marked decrease of IGF-I levels in cerebellum is found when inferior olive neurons are lesioned. In addition, Purkinje cells in the cerebellar cortex depend on this growth factor to survive and differentiate in vitro. Thus, we consider it possible that IGF-I forms part of a putative trophic circuitry encompassing the inferior olive and the cerebellar cortex and possibly other functionally connected areas. To test this hypothesis we have studied whether IGF-I may be taken up, transported, and released from the inferior olive to the cerebellum. We have found that 125I-IGF-I is taken up by inferior olive neurons in a receptor-mediated process and orthogradely transported to the cerebellum. Thus, radioactivity found in the cerebellar lobe contralateral to the injection site in the inferior olive was immunoprecipitated by an anti-IGF-I antibody, co-eluted with 125I-IGF-I in an HPLC column, and co-migrated with 125I-IGF-I in an SDS-urea polyacrylamide gel electrophoresis. Time-course studies indicated that orthograde axonal transport is relatively rapid since 30 min after the injection, radiolabeled IGF-I was already detected in the contralateral cerebellum. Furthermore, transport of IGF-I from the inferior olive is specific since when 125I-neurotensin was injected in the inferior olive or when 125I-IGF-I was injected in the pontine nucleus, no radioactivity was found in the contralateral cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

A rapid fluorometric assay to measure neuronal survival in vitro

We report the development and characterization of a rapid fluorometric microassay suitable for quantifying neuronal cell survival. The method can be used in two formats: (1) a time course analysis of survival response or (2) as a simple endpoint assay for the assessment of neuronal survival promoted by a variety of reagents. The assay uses calcein AM, a non-fluorescent, electrically neutral, non-polar analogue of fluorescein diacetate, which passively crosses cell membranes and is cleaved to a fluorescent derivative by non-specific intracellular esterases. Once cleaved in viable cells, the resultant fluorescent salts are retained by intact cell membranes. The relative number of viable cells under various conditions can be quantified by measuring the emitted fluorescence. Described herein are the conditions that allow the determination of low viable neuronal cell numbers (10(2)-10(3) cells/cm2).

Long-term depression of glutamate-induced gamma-aminobutyric acid release in cerebellum by insulin-like growth factor I

We tested the possibility that insulin-like growth factor I (IGF-I) acts as a neuromodulator in the adult cerebellar cortex since previous observations indicated that IGF-I is located in the olivo-cerebellar system encompassing the inferior olive and Purkinje cells. We found that conjoint administration of IGF-I and glutamate through a microdialysis probe stereotaxically implanted into the cerebellar cortex and deep cerebellar nuclei greatly depressed the release of gamma-aminobutyric acid (GABA), which normally follows a glutamate pulse. This inhibition was dose-dependent and long-lasting. Moreover, the effect was specific for glutamate since KCl-induced GABA release was not modified by IGF-I. Basic fibroblast growth factor, another growth-related peptide present in the cerebellum, did not alter the response of GABA to glutamate stimulation. In addition, electrical stimulation of the inferior olivary complex significantly raised IGF-I levels in the cerebellar cortex. Interestingly, when the inferior olive was stimulated in conjunction with glutamate administration, GABA release by cerebellar cells in response to subsequent glutamate pulses was depressed in a manner reminiscent of that seen after IGF-I. These findings indicate that IGF-I produces a long-lasting depression of GABA release by Purkinje cells in response to glutamate. IGF-I might be present in climbing fiber terminals and/or cells within the cerebellar cortex and thereby might affect Purkinje cell function. Whether this IGF-I-induced impairment of glutamate stimulation of Purkinje cells underlies functionally plastic processes such as long-term depression is open to question.

Estradiol modulates insulin-like growth factor I receptors and binding proteins in neurons from the hypothalamus

Trophic effects of 17 beta-estradiol (beta E2) on in vitro developing hypothalamic cells have been reported. Insulin-like growth factor I (IGF-I) is also a potent trophic factor for cultured hypothalamic cells. An interaction between sexual steroids and insulin-like growth factors (IGFs) in modulating growth of hypothalamic cells has been suggested. Thus, we tested whether beta E2 modulates the levels of IGF-I, its membrane receptor and its binding proteins in rat hypothalamic cultures. Using both neuron- and glial-enriched cultures obtained from fetal rat hypothalami we found that addition of beta E2 elicited a significant increase in IGF-I receptor levels in neurons, without affecting its affinity. On the other hand, the three different IGF-binding proteins (IGFBPs) found in the conditioned medium of the cultures were differentially modulated by beta E2 in the two types of cells studied. Overall, neuronal cultures produced greater amounts of IGFBPs after treatment with beta E2, with IGFBP2 reaching significantly higher levels. On the contrary, treatment with beta E2 did not significantly alter the amounts of IGFBPs produced by glial cells. Finally, the levels of immunoreactive IGF-I found either in the medium or in cellular extracts in both neuronal and glial cultures were not modified by treatment with beta E2. These results strongly support previous observations of a trophic synergistic interaction between IGFs and beta E2 on hypothalamic cells. Thus, an increase in IGF-I receptors and/or IGFBPs after exposure to beta E2 may result in an enhanced response of hypothalamic neurons to IGF-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of insulin-like growth factor binding protein-4 expression in the developing and adult rat brain: analysis by in situ hybridization

We have previously isolated insulin-like growth factor binding protein-4 (IGFBP-4) from media conditioned by a neuronal cell line and have detected IGFBP-4 mRNA in selected regions of the developing and adult rat brain by Northern blot analysis. In this study, the ontogeny and regional distribution of IGFBP-4 expression were determined by in situ hybridization histochemistry. While IGFBP-4 mRNA expression at embryonic day 15 was restricted to choroid plexus primordium and meninges, by embryonic day 20 IGFBP-4 mRNA was also localized in the basal ganglia. In the postnatal rat, at days 1 and 5, IGFBP-4 was also present in the meningeal cell layer surrounding the developing cerebellum and in the hippocampal formation. The distribution of IGFBP-4 mRNA in the adult brain was considerably more widespread. The principal areas where IGFBP-4 mRNA was detected were the cerebral cortex (layers II and IV), olfactory peduncle (anterior olfactory nuclei), limbic system (hippocampus and amygdala), thalamus and basal ganglia, as well as choroid plexus and meninges. The widespread and persistent expression of IGFBP-4 is in marked contrast with IGFBP-2, the other IGFBP in the brain, whose localization by in situ hybridization is reported to be restricted to choroid plexus and meninges. The spatial pattern of IGFBP-4 expression in areas known to either overlap, be adjacent to, or project to regions that express the IGFs or their receptors may reflect a role for IGFBP-4 as a modulator of IGF action in the brain.

Insulin-like growth factor-I (IGF-I) production by astroglial cells: regulation and importance for epidermal growth factor-induced cell replication

The insulin-like growth factors are postulated to play a role during brain development. Because they are believed to act in a paracrine/autocrine manner, the production of insulin-like growth factor-I (IGF-I) by cultured astroglial cells was examined. Quantities of IGF-I in conditioned media were determined by RIA after separation of IGFs from IGF-binding proteins by high-pressure liquid chromatography. Astrocytes from 1-day-old rats and the rat glioma cell line (C6) both secreted 7.5-kDa IGF-I. A peak of immunoreactivity with an apparent mol wt of 12,000 was additionally present in media conditioned by C6 cells. Exposure to epidermal growth factor (EGF) increased media content of immunoreactive IGF-I slightly (60%) in C6 cells but more than 2-fold in normal astrocytes. Fibroblast growth factor also increased the amount of IGF-I contained in media conditioned by normal astrocytes. To determine whether the secreted IGF-I was biologically active, media IGFs were immunoneutralized with a monoclonal antibody (Sm 1.25). In the presence of the antibody, EGF-stimulated astrocyte replication was blocked. These data indicate that IGF-I secretion by rodent astrocytes is stimulated by factors thought to be important for brain growth and development and that the IGFs are likely intimate participants in EGF-induced astrocyte growth.

Expression of insulin-like growth factor I by astrocytes in response to injury

Astrocytes are known to express several growth factors in response to injury and neurological disease. Insulin-like growth factor I (IGF-I) induces astrocytes to divide in vitro and is expressed by developing, but not adult astrocytes both in vivo and in vitro. We tested whether IGF-I is re-expressed by reactive astrocytes in response to injury. We found that astrocytes surrounding the lesioned parenchyma after introduction of a cannula through the cerebral cortex, hippocampus and midbrain contain high levels of immunoreactive IGF-I, as determined by immunocytochemistry using a highly sensitive and specific anti-IGF-I monoclonal antibody. Interestingly, the contralateral hippocampus also contained IGF-I positive astrocytes although in substantial lower numbers. Intact animals showed no detectable IGF-I immunoreactivity in astrocytes. IGF-I was detected at the first time point tested after the lesion was made, 1 week, and for at least 1 month thereafter. Reactive astrocytes expressing high levels of glial fibrillary acidic protein were found in a much wider distribution all along the lesioned area and beyond. We conclude that mechanical injury of the brain induces a specific pattern of expression of IGF-I by a subpopulation of astrocytes. These findings suggest that IGF-I is participating in the response of astrocytes to injury.

Insulin-like growth factor-I counteracts bFGF-induced survival of nitric oxide synthase (NOS)-positive spinal cord neurons after target-lesion in vivo

We have used nitric oxide synthase (NOS) histochemistry as a perikaryal viability marker to trace the retrograde reaction of spinal cord intermediolateral (IML) sympathoadrenal projection (SAP)-neurons to target-removal, i.e., selective adrenomedullectomy and local administration of either insulin-like growth factor-I (IGF-I), basic fibroblast growth factor (bFGF) or a combination of both. Counting of NOS-positive preganglionic spinal cord neurons 4 weeks post surgery indicated that more than 80% of stained neurons were lost from the IML-cell column. This percentage loss corresponds to the numerical loss of NOS-stained SAP-neurons labeled retrogradely with Fast-blue prior to adrenomedullectomy. Basic FGF-supplementation at the site of lesion resulted in maintenance of the majority of NOS-positive IML-neurons, a finding confirmed by the survival rate of Fast-blue prelabeled SAP-neurons. Thus, besides maintenance of the structural integrity of SAP-neurons, bFGF prevents loss of intracellular NOS-activity which may reflect unaltered cell metabolism (and function) of these neurons following target-removal in vivo. By contrast, IGF-I failed to alter the rate of disappearance of NOS-staining and labeling index of neurons within the IML-cell column postlesion, suggesting that IGF-I is not neurotrophic for SAP-neurons by itself. Combined treatment with both factors resulted in a more widespread loss of NOS-stained and Fast-blue-prelabeled SAP-neurons than registered after bFGF-only treatment. No co-trophic effect of bFGF and IGF-I was evident; rather, the pronounced bFGF-induced rescuing effect was significantly suppressed by exogenous IGF-I in vivo, supporting the idea that this or another molecule induced by the treatment enhances rather than prevents retrograde degeneration and neuronal death within the adult lesioned IML-adrenal pathway.

Effects of astrocytes, insulin and insulin-like growth factor I on the survival of motoneurons in vitro

We isolated motoneurons from E15 dissociated mouse spinal cord by density centrifugation and planted them onto poly-ornithine-coated coverslips in a growth medium (DMEM/F12) supplemented with progesterone, transferrin, selenium, horse serum and muscle extract. Under these conditions only 28% of the motoneurons survived for 8 days. When living astrocytes on a separate coverslip were introduced into dishes containing motoneurons, there was a two-fold increase in neuronal survival. The addition of insulin and insulin-like growth factor I (IGF-I) to such cultures alone or together, still further increased motoneuron survival, but this did not happen in the absence of astrocytes. We conclude that (a) astrocytes exert a trophic role in the survival of spinal motoneurons, (b) the effect does not require physical contact of the cells, and (c) insulin and IGF-1 have neurotrophic activity for motoneurons, an effect possibly mediated by living astrocytes.

Presence of IGF-I in human midgut carcinoid tumours--an autocrine regulator of carcinoid tumour growth?

The presence of IGF-I and IGF-I receptors in human midgut carcinoid tumours has been investigated. Using immunocytochemistry, IGF-I-positive tumour cells were demonstrated in 11/11 tumour cases studied. Labelling of consecutive sections with antibodies against IGF-I and proliferating cell nuclear antigen (PCNA)/cyclin demonstrated a co-distribution of the 2 antigens in carcinoid tumours. Extracts of tumour tissues were subjected to radioimmunoassay and shown to contain significant amounts of IGF-I. Reverse-phase HPLC of tumour extracts demonstrated a major IGF-I-immunoreactive component eluting in the position of rhIGF-I, but also 2 other more hydrophobic forms. Conditioned serum-free media from primary cultures of carcinoid tumors contained detectable amounts of IGF-I, indicating a spontaneous release of IGF-I from tumour cells into the culture medium. Levels of IGF-I in media were reduced (19%) after incubation of cultures with a somatostatin analogue for 4 days. IGF-I receptors were observed on tumour cells in 4/10 tumours by immunocytochemistry. Tumour cells with immunoreactive IGF-I receptors could be stimulated to enhanced growth, measured as an increase in DNA contents, by exogenous administration of IGF-I every 3-4 days for 2 weeks. The results show that cultured human midgut carcinoid tumours secrete IGF-I and that some of the tumours also have IGF-I receptors. We therefore suggest that IGF-I may act as an autocrine or paracrine regulator of carcinoid tumour-cell growth.

Estradiol promotes cell shape changes and glial fibrillary acidic protein redistribution in hypothalamic astrocytes in vitro: a neuronal-mediated effect

We have previously shown that in hypothalamic mixed neuronal-glial cultures both astrocytic shape and distribution of glial fibrillary acidic protein (GFAP) are modified by estradiol. In the present study, we have investigated whether or not the presence of neurons is necessary for these hormonal effects. In mixed neuronal-glial hypothalamic cultures the proportion of process-bearing GFAP-immunoreactive cells was significantly increased after treatment for 30 min with 10(-12) M 17 beta estradiol. This effect was present for at least 1 day and was reverted by incubating the cells in estradiol-free medium. Estradiol incubation resulted in a progressive differentiation of GFAP-immunoreactive cells from a flattened epithelioid morphology to bipolar, radial, and stellate shapes. This effect was not observed in pure hypothalamic glial cultures. Furthermore, incubation of hypothalamic glial cells with medium conditioned by estradiol-treated mixed hypothalamic cultures did not affect the shape of GFAP-immunoreactive astrocytes. In contrast, addition of hypothalamic neurons, but not cerebellar neurons or fibroblasts, to established hypothalamic glial cultures affected the development of estradiol sensitivity in astrocytes. These results indicate that estradiol induction of shape changes in hypothalamic astrocytes is not only dependent on the presence of hypothalamic neurons, but that physical contact between astrocytes and neurons is necessary for the manifestation of the effect of this hormone.

Climbing fiber deafferentation reduces insulin-like growth factor I (IGF-I) content in cerebellum

The presence of insulin-like growth factor I (IGF-I) and its mRNA in adult rat cerebellum has recently been documented. Previous immunocytochemical studies showed prominent IGF-I-like staining in fibers around Purkinje cell somas. To determine the origin of this IGF-I input to the Purkinje cell we destroyed the inferior olivary complex by either 3-acetylpiridine administration or electrolytical lesions. In both types of lesions we found a similar significant depletion of IGF-I levels in cerebellum (40-50% of controls). No changes were found in cerebellar IGF-I receptors. These results suggest that almost half of the IGF-I content in cerebellum is provided by climbing fiber afferents arising from the inferior olivary complex.

Ontogeny of insulin-like growth factor I, its receptor, and its binding proteins in the rat hypothalamus

A role for the insulin-like growth factors (IGFs) in brain growth and differentiation has recently been suggested. In previous studies on fetal hypothalamic cells we found a trophic influence of IGF-I on in vitro survival and differentiation of both neurons and glia. We have now investigated the expression of IGF-I, its receptor and its binding proteins in the rat hypothalamus to determine whether endogenous IGF-I might serve as a trophic factor during development of this brain area. Both IGF-I receptors and IGF-I binding proteins showed marked developmental stage-dependent variations. Thus, IGF-I receptors as measured by both binding and cross-linking techniques, were highest during fetal life and steadily decreased thereafter to reach low adult levels. Changes in receptor numbers rather than in its affinity constant accounted for the differences seen in binding activity during development. In addition, we found 3 different IGF-I binding proteins (IGFBPs) of apparent Mr of 24, 29 and 32 kDa respectively, whose levels also showed a specific developmental pattern. Highest levels of the 29 and 32 kDA IGFBPs were found in fetal and early postnatal life, whereas levels of the 24 kDa form were highest in young adults. Changes in the concentration of IGFBPs rather than in their affinities for IGF-I accounted for the different binding capacities found. Using a specific IGF-I radioimmunoassay we found that IGF-I-like immunoreactivity (IGF-I-li) levels had no direct correlation with developmental stage. IGF-I-li levels oscillated with no apparent trend throughout development of the hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of insulin-like growth factor I (IGF-I)-like immunoreactivity in the developing and adult rat brain

The cellular distribution of insulin-like growth factor I (IGF-I) immunoreactivity was examined in the rat brain from embryonic day 15 to maturity. IGF-I immunoreactivity was found in the perikarya of neurons distributed along the entire extension of the neuronal tube in all the embryonic ages studied (E15, E17, E19 and E21). In E21 animals, the majority of immunoreactive neurons was located in the olfactory bulb, cerebral cortex, hippocampus, striatum, diencephalon, mesencephalic colliculi, trigeminal nuclei, trigeminal ganglion and in motoneurons of the brainstem. In 10- and 20-day-old rats, in addition to the above areas, IGF-I immunoreactivity was also observed in capillary walls, ependymal cells, choroid plexus, glial cells and most fiber paths. In postnatal ages, immunoreactivity in neuronal somas was mainly restricted to the cell nuclei. However, IGF-I immunoreactivity in the neuronal cytoplasm was observed in 20-day-old rats treated with colchicine while fiber paths and neuronal cell nuclei were negative in these animals. In the telencephalon of 20-day-old rats injected with colchicine, the most intense immunoreactive neurons were observed in the olfactory bulb, cerebral cortex, tenia tecta, hippocampus, islands of Calleja, septal nuclei, striatum, endopyriform nucleus and amygdala. Most diencephalic nuclei, the substantia nigra, the mesencephalic colliculi, Purkinje cells in the cerebellar cortex and several nuclei in mesencephalon, pons and medulla oblongata were also immunoreactive. In adult rats injected with colchicine, IGF-I immunoreactivity was located in the same areas as in 20-day-old rats. The number of immunoreactive cells and the intensity of the staining was reduced in adult rats as compared to that found in young postnatal animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of insulin-like growth factor-1 mRNA in murine central nervous system during postnatal development

Insulin-like growth factor-1 (IGF-1) is believed to play a role in the regulation of brain growth. The identity of cells responsible for its synthesis in the immature brain, however, has not been established. To identify potential sites of IGF-1 synthesis, in situ hybridization has been utilized to localize IGF-1 mRNA in the murine brain during the first postnatal month. Although IGF-1 mRNA was detected in all regions of the neonatal brain, there was considerable regional variation in the level of expression. Neurons were the principle sites IGF-1 mRNA expression and expression was typically restricted to one or two neuronal cell types within each region. In the cerebellar cortex, for example, only Purkinje cells hybridized to the IGF-1 probe. In contrast to gray matter, IGF-1 labeled cells were rarely found in presumptive white matter tracts of the forebrain. The hybridization signal was most prominent in regions where neurogenesis persisted after birth, including the cerebellum, olfactory bulb, and hippocampal complex. The timing of IGF-1 mRNA expression appeared to be temporally related to local neuronal proliferation. The number of labeled cells and intensity of hybridization signal was greatest during the first 2 postnatal weeks, a period of rapid neuronal proliferation in these regions. At the end of the first month, when neurogenesis had essentially ceased, IGF-1 signal strength had declined to background levels. The temporal and spatial pattern IGF-1 mRNA expression in the immature CNS was consistent with a role for locally produced IGF-1 in the regulation of brain development.

Further evidence of the presence of rat embryonic hypothalamic factors that induce the differentiation of gonadotrophic hormone-releasing hormone-containing secretory neurons

We examined the presence of factor(s) in the embryonic medial basal hypothalamus (MBH) that may influence nasal placode (NAP)-derived luteinizing hormone-releasing hormone (LHRH) neurons in determining their secretory phenotype. In this study, we performed organotypic culture and transplantation of the NAP from 12.5-day-old embryos of rats and vomeronasal organ (VNO) from 14.5-day-old embryos. Surgical operations, however, were performed on 16.5-day-old embryos. The NAP and VNO were cultured singly or with the MBH obtained from the embryos of the same age and, further, in a medium with a nerve growth factor or fibroblast growth factors. Although LHRH neurons were derived from the NAP and VNO in all the cultures, judging from numbers and cellular morphologies, the MBH was most effective. The VNO was transplanted into the third ventricle of adult female rats singly or with the cerebral cortex, the mesencephalon-myelencephalon complex, or the MBH from 14.5-day-old embryos. All the grafts gave rise to LHRH neurons, but the number of the neurons was far greater in the grafts cotransplanted with the MBH, in which the neurons projected long processes to blood capillaries and formed neurovascular complexes, the feature of which may suggest the occurrence of the secretory activity in the fibers. The animals were examined 5 days after the surgical operations. In rhinoectomized embryos, LHRH neurons were distributed throughout the brain in the same pattern as found in intact rats, showing normal cellular morphology. In the encephalectomized rats, immunoreactive LHRH cells were present only in the terminal ganglia. These findings indicate that the embryonal MBH has a factor (s) that is essential to the development of secretory LHRH neurons.