Glucocorticoid hormones inhibit DNA synthesis in glial cells cultured in chemically defined medium

Ligand-based gene expression profiling reveals novel roles of glucocorticoid receptor in cardiac metabolism

Recent studies have documented various roles of adrenal corticosteroid signaling in cardiac physiology and pathophysiology. It is known that glucocorticoids and aldosterone are able to bind glucocorticoid receptor (GR) and mineralocorticoid receptor, and these ligand-receptor interactions are redundant. It, therefore, has been impossible to delineate how these nuclear receptors couple with corticosteroid ligands and differentially regulate gene expression for operation of their distinct functions in the heart. Here, to particularly define the role of GR in cardiac muscle cells, we applied a ligand-based approach involving the GR-specific agonist cortivazol (CVZ) and the GR antagonist RU-486 and performed microarray analysis using rat neonatal cardiomyocytes. We indicated that glucocorticoids appear to be a major determinant of GR-mediated gene expression when compared with aldosterone. Moreover, expression profiles of these genes highlighted numerous roles of glucocorticoids in various aspects of cardiac physiology. At first, we identified that glucocorticoids, via GR, induce mRNA and protein expression of a transcription factor Kruppel-like factor 15 and its downstream target genes, including branched-chain aminotransferase 2, a key enzyme for amino acid catabolism in the muscle. CVZ treatment or overexpression of KLF15 decreased cellular branched-chain amino acid concentrations and introduction of small-interfering RNA against KLF15 cancelled these CVZ actions in cardiomyocytes. Second, glucocorticoid-GR signaling promoted gene expression of the enzymes involved in the prostaglandin biosynthesis, including cyclooxygenase-2 and phospholipase A2 in cardiomyocytes. Together, we may conclude that GR signaling should have distinct roles for maintenance of cardiac function, for example, in amino acid catabolism and prostaglandin biosynthesis in the heart.

Differential promotion of glutamate transporter expression and function by glucocorticoids in astrocytes from various brain regions

Steroids that activate glucocorticoid receptors (GRs) and mineralocorticoid receptors have important regulatory effects on neural development, plasticity, and the body's stress response. Here, we investigated the role of corticosteroids in regulating the expression of the glial glutamate transporters glial glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter (GLAST) in rat primary astrocytes. The synthetic glucocorticoid dexamethasone provoked a marked increase of GLT-1 transcription and protein levels in cortical astrocytes, whereas GLAST expression remained unaffected. Up-regulation of GLT-1 expression was accompanied by an enhanced glutamate uptake, which could be blocked by the specific GLT-1 inhibitor dihydrokainate. The promoting effect of dexamethasone on GLT-1 gene expression and function was abolished by the GR antagonist mifepristone. A predominant role of the GR was further supported by the observation that corticosterone could elevate GLT-1 expression in a dose-dependent manner, whereas aldosterone, the physiological ligand of the mineralocorticoid receptor, exerted only weak effects even when applied at high concentrations. Moreover, we monitored brain region-specific differences, since all corticosteroids used in this study failed to alter the expression of GLT-1 in midbrain and cerebellar glia, although expression levels of both corticosteroid receptor subtypes were similar in all brain regions analyzed. Dexamethasone, however, modestly enhanced GLT-1 expression in cerebellar glia in combination with the DNA methyltransferase inhibitor 5-aza-2-deoxycytidine, suggesting that suppression of GLT-1 expression in cerebellar cultures may at least in part be epigenetically mediated by a DNA methylation-dependent process. Taken together, our data highlight a potential role for glucocorticoids in regulating GLT-1 gene expression during central nervous system development or pathophysiological processes including stress.

Prolonged corticosterone treatment of adult rats inhibits the proliferation of oligodendrocyte progenitors present throughout white and gray matter regions of the brain

It is well established that glucocorticoids inhibit the proliferation of progenitor cells that occurs in the hippocampal dentate gyrus of adult mammals. Active cell proliferation also occurs in the subventricular zone (SVZ) of the lateral ventricle and, to a lesser extent, throughout white and gray matter regions of the adult brain. The aim of the present study was to determine whether extrahippocampal cell proliferation is also affected by glucocorticoids. The cell proliferation marker bromodeoxyuridine (BrdU) was administered to control rats, to adrenalectomized rats, and to rats treated with a daily injection of corticosterone (10 mg/kg) for a period of 15 days. In control and adrenalectomized rats, high to low numerical densities of BrdU-labeled nuclei were detected within the different forebrain regions examined. In rats treated with corticosterone, a dramatic decrease of cell proliferation was detected in the dentate gyrus, but also throughout all white and gray matter regions examined, except for the SVZ of the lateral ventricle. Double-labeling experiments indicated that throughout the different white and gray forebrain regions examined, except for the SVZ, BrdU-labeled nuclei were essentially associated with cells immunostained for the marker of oligodendrocyte progenitors NG2. These data indicate that glucocorticoids inhibit the proliferation of oligodendrocyte precursors located throughout the white and gray matter regions of the adult rat brain. Since the proliferation of oligodendrocyte precursors plays a major role in the processes of remyelination, these data raise the question of possible detrimental effects of therapeutic treatments of CNS trauma based on the administration of glucocorticoids.

Glucocorticoids up-regulate the expression of glial fibrillary acidic protein in the rat suprachiasmatic nucleus

Immunoreactivity against glial fibrillary acidic protein (GFAP) was used as a dynamic index in adrenalectomized rats subjected or not to corticosterone replacement to investigate whether glucocorticoids may interact with astrocytes in the suprachiasmatic nucleus (SCN), the master component of the central circadian clock. GFAP staining in the SCN was significantly higher in rats having received implants that restored physiological plasma levels of corticosterone within diurnal or nocturnal limits than in non-normalized rats. The effects of corticosterone were similar in the parvocellular portion of the paraventricular nucleus but were opposite in the hippocampus, another major site of negative feed-back regulation of the hypothalamic-pituitary-adrenal axis, where a decreased GFAP staining was observed in discrete regions of the dentate gyrus. This indicates that glucocorticoids may positively or negatively regulate GFAP, depending on the target brain structure. In the SCN, that contains only few if any glucocorticoid receptors, indirect mechanisms that may involve serotoninergic neurons are probably responsible for the effects of corticosterone level. It is proposed that the corticosterone-induced increase in GFAP staining in that nucleus accounts for dynamic changes in neurone-astrocyte interactions that might occur in relation with natural fluctuations of glucocorticoids over the 24 h period.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Influence of culture conditions on monoamine oxidase A and B activity in rat astrocytes

Astroglial cells dispersed from newborn rat hemispheres were established in medium supplemented with 20 per cent fetal calf serum (FBS) and then grown to a confluent monolayer in the presence of 10 per cent FBS or charcoal-stripped FBS (CS). Type 1 astrocytes were subcultured and either maintained under the same conditions of the primary cultures or converted to serum-free chemically defined medium (CDM). No differences were found in either MAO A or MAO B activity of astrocytes grown in the presence of FBS or CS after 15 and 21 days in vitro (day 1 and 6 of subculture). In contrast, on day 21 both MAO A and MAO B activities were markedly higher in astrocytes subcultured in CDM compared with cells maintained in serum-supplemented medium. This difference appeared to be due to increased number of enzyme molecules, since kinetic analysis showed an increase in Vmax of both MAO isoenzymes in serum-free medium, but no change in Km. Consistently, the recovery of MAO A and MAO B activity after irreversible enzyme inhibition by clorgyline and deprenyl was faster in CDM than in FBS-supplemented medium, indicating enhanced enzyme synthesis under serum-free condition. Estimates of half-lives for the recovery of MAO A and MAO B activity indicated that, under both culture conditions, type A activity had a higher turnover rate than type B. The effect of CDM on astrocyte MAO does not appear to be due to selection of a subpopulation of cells, but rather linked to a morphological change (differentiation) with increased synthesis of both MAO isoenzymes.

Glucocorticoids enhance the potency of Schwann cell mitogens

Previous studies have documented that cultured Schwann cells require serum-containing medium to respond maximally to mitogens. We now report that Schwann cells are able to proliferate to a mitogenic response in a serum-free defined medium termed oligodendrocyte defined media (ODM). Glucocorticoids are the essential component of ODM which allow Schwann cell proliferation in the serum-free medium. Charcoal treatment of the fetal calf serum decreases the mitogenic potency of the axolemma-enriched fraction (AEF) by 50%. The addition of 2 microM hydrocortisone to charcoal-treated fetal calf serum restores 75% of the lost mitogenicity. These observations are consistent with the view that glucocorticoids present in fetal calf serum are potent co-mitogens essential for AEF-induced Schwann cell proliferation. The synthetic glucocorticoid, dexamethasone, is a more potent co-mitogen than hydrocortisone, with a maximal effect at concentrations less than 10 nM. In contrast, other steroids including aldosterone, progesterone, testosterone, and 17 beta-estradiol have no effect on enhancing the mitogenic response of Schwann cells to the AEF. The glucocorticoid antagonists RU 486 and dehydroepiandrosterone (DHEA), but not the antiestrogenic compound tamoxifen, block AEF-induced Schwann cell proliferation. These results suggest that glucocorticoid-induced Schwann cell proliferation is mediated through a glucocorticoid receptor (GR) mechanism. We detected immunoreactivity to the GR in the cytoplasm, but not in the nuclei of Schwann cells grown in ODM lacking dexamethasone. The addition of 100 nM dexamethasone to these cultures resulted in immunoreactivity in the nucleus. This data suggests that glucocorticoids working through the GR are potent co-mitogens for Schwann cell proliferation.

Adrenalectomy increases phosphoinositide hydrolysis induced by norepinephrine or excitatory amino acids in rat hippocampal slices

Phosphoinositide hydrolysis induced by norepinephrine, quisqualate, or trans-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD), but not by carbachol, was approximately 50% greater in hippocampal slices from adrenalectomized (14 days) rats compared with controls. These changes appeared to be selective for the hippocampus because no effects of adrenalectomy on phosphoinositide hydrolysis were detected in cortical or striatal slices. The enhanced response to norepinephrine in hippocampal slices after adrenalectomy was observed throughout the effective concentration range of norepinephrine, was not influenced by in vitro addition of corticosterone, was not mimicked or altered by incubation with dibutyryl cyclic adenosine 3',5'-monophosphate (AMP), and did not appear to be due to impaired inhibition of the response to norepinephrine which was elicited by activation of protein kinase C or by inclusion of an inhibitory concentration of quisqualate. These findings indicate that adrenalectomy either removes an inhibitory influence of glucocorticoids on the phosphoinositide system in the hippocampus or that the neurodegeneration of granule cells in the dentate gyrus following adrenalectomy is associated with neurotransmitter-selective increases in phosphoinositide hydrolysis. These data provide further evidence that glucocorticoids modify signal transduction in the brain and extends their known influence to the phosphoinositide second messenger system.

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.

Inhibition of T lymphocyte proliferation by retinal glial Müller cells: reversal of inhibition by glucocorticoids

Study of interactions between retinal glial Müller cells and T lymphocytes have revealed a wide array of reciprocal influences on the functions of these cells. In the present study we show that these interactions can be further modified by corticosteroid hormones. The primary effect of Müller cells on T lymphocytes is an inhibition of the T-cell proliferative response, and it is exerted via a membrane-bound factor. In this report we show that glucocorticoids can reverse the inhibition by suppressing the expression of the Müller cell inhibitory factor. This effect was independent of the action of glucocorticoids on arachidonic acid metabolism.

Fetal dexamethasone exposure impairs cellular development in neonatal rat heart and kidney: effects on DNA and protein in whole tissues

Fetal glucocorticoid exposure causes postnatal growth retardation. To examine the mechanisms underlying effects on specific organ systems, we administered 0.2 or 0.8 mg/kg of dexamethasone to pregnant rats on gestational days 17, 18, and 19 and assessed three biochemical markers of cell development in heart and kidney of the offspring: DNA content per organ as an index of total cell numbers, DNA per g tissue as an index of cell packing density, and protein/DNA ratio as an index of relative cell size. In both tissues, DNA content became markedly subnormal during the first postnatal week, the ontogenetic period of rapid cell division. Partial recovery occurred by the end of the first postnatal month. In the heart, cell packing density was subnormal initially and the cells were significantly enlarged. In contrast, packing density was slightly elevated in the kidney; protein/DNA was increased by the low dose of dexamethasone, but markedly decreased by the high dose. These results suggest that tissue growth impairment caused by prenatal dexamethasone treatment reflects primary deficits in cell proliferation that extend to a variety of different cell types; however, consequent effects on cell packing density and cell size are dose-specific, possibly reflecting actions of glucocorticoids selective for certain cell types or phases of cell development.

Effects of dexamethasone on the differentiation of membrane structure in cultured astrocytes

Astrocytic processes investing vascular structures or forming the surface of mammalian brain have large numbers of orthogonally packed aggregates of intramembrane particles, termed "assemblies." Similar particle aggregates are expressed by astrocytes derived from neonatal rat forebrain in secondary culture, but they are much more uniformly distributed across the membranes of the cultured cells. Dexamethasone, a potent glucocorticoid, affects the differentiation of astrocyte membrane structure in two patterns, depending on the rate of proliferation in the culture. When confluent secondary cultures of astrocytes are exposed to 5 microM dexamethasone, the densities of assemblies increase, and in some cells approach the values present in the glial limitans in vivo. However, when rapidly proliferating astrocytes are exposed to dexamethasone during the first week of secondary culture, most of the astrocytes fail to express any assemblies. The rate of astrocyte proliferation is slowed, and a lower cell density is reached during the first 2 weeks of secondary culture in dexamethasone. The suppression of assemblies is transient: as the cultures approach confluence, the proportion of cells expressing assemblies increases to nearly control levels, and the density of assemblies increases to greater than control values in some astrocytes. Certain of the effects of dexamethasone on cultured astrocytes may have relevance for understanding the mechanism(s) of its action in treating cerebral edema.

Enhanced brain cell proliferation following early adrenalectomy in rats

We have previously demonstrated an increase in adult brain DNA content in rats adrenalectomized on postnatal day 11. The present studies examined cell proliferation in cerebral cortex, cerebellum, hippocampus, and midbrain-diencephalon following adrenalectomy at this age. Compared to sham-operated controls, adrenalectomized animals showed increased [3H]thymidine incorporation into DNA (measured at 1 h following a pulse injection) in all brain regions at 7 and 14 days postsurgery. In some areas, the effect was already present as early as 2 days following adrenalectomy. Chronic replacement with corticosterone prevented this increase in DNA labelling in a dose-dependent manner. When cell proliferation in the cerebral cortex and cerebellum was independently assessed by measuring changes in thymidine kinase activity, enzyme activity was significantly elevated in both areas at 7 and 14 days postsurgery. Finally, histological examination of the cerebellar cortex suggested a delayed disappearance of the external granular layer in several cerebellar lobules of adrenalectomized animals. Overall, these findings indicate that day-11 adrenalectomy leads to a prolonged stimulation of mitotic activity in areas where cell formation at this time is exclusively glial (i.e., cerebral cortex and mid-brain-diencephalon) as well as in areas where postnatal neurogenesis is also occurring (cerebellum and hippocampus). It is hypothesized that this stimulation results from the removal of a tonic inhibitory effect exerted by circulating glucocorticoids in the normal intact animal.

Astrocytic shape and glial fibrillary acidic protein immunoreactivity are modified by estradiol in primary rat hypothalamic cultures

Primary cultures from fetal rat hypothalamus (embryonic day 15-16) were grown for 9 days in a serum-free medium and then fixed and immunostained for glial fibrillary acidic protein (GFAP). The majority of the GFAP-immunoreactive astrocytes were flat, polygonal, without processes and showed a low intensity immunoreactivity which was restricted to the perinuclear region. Elongated, process-bearing astrocytes, with an intense immunoreactivity in the soma and processes, were also observed in a smaller proportion (30%). Addition of estradiol (10(-12) to 10(-8) M) to the culture medium 24 h before GFAP immunostaining resulted in an increased proportion (59-69%) of process-bearing, intense immunoreactive cells. This effect was blocked by tamoxifen (10(-8) M). The total number of GFAP-immunoreactive astrocytes was not modified by estrogen or tamoxifen. These results indicate that estradiol may modulate the cell shape and the distribution of GFAP in astrocytes in culture and suggest that astrocytes can be a target for sex steroids during development of the central nervous system.

The corticosteroid receptor and the action of various steroids in rainbow trout fibroblasts

Corticosteroid binding sites with the characteristics of steroid receptors were detected with the synthetic corticosteroid, [3H]triamcinolone acetonide (TA), in monolayers of the rainbow trout fibroblast cell line, RTG-2. The sites had low capacity as saturation was achieved at approximately 5 nM. Scatchard plots of the data suggested a single population of high-affinity binding sites. The number of receptors per cell was approximately 20,000; the dissociation constant, 1 nM. Changes in [3H]thymidine incorporation and cellular morphology were monitored as potential corticosteroid-sensitive metabolic responses. Only cortisol and 11-deoxycortisol among 14 naturally occurring steroids and TA, fluocinolone acetonide, dexamethasone, and prednisolone among 6 synthetic corticosteroids inhibited [3H]thymidine incorporation and altered the morphology in RTG-2 cells. Two observations suggested that the corticosteroid receptor mediated these responses. The synthetic steroid, RU 38486, which is an antiglucocorticoid in mammals, did not elicit these responses, had a high affinity for the receptor, and blocked the ability of cortisol and TA to change [3H]thymidine incorporation and cellular morphology. Second, the affinity of various natural steroids for the receptor correlated with their ability to elicit a cellular response. Cortisol, and to lesser extent 11-deoxycortisol, showed strong affinity for the receptor. Cortisone, aldosterone, and the sex steroids had no affinity and did not elicit cellular responses.

Highly basic 30- and 32-kilodalton proteins associated with synapse formation on polylysine-coated beads in enriched neuronal cell cultures

Neuronal proteins involved in axonal outgrowth and synapse formation were examined in an enriched neuronal cell culture system of the cerebellum. In rat cerebellar cell cultures, 98.9% of the cells are neurons and the remaining 1.1% of the cells are flat nonneuronal cells. These enriched neuronal cultures, examined with two-dimensional gel electrophoresis, showed protein patterns similar to those of neonatal cerebellum, but very different patterns from glial enriched cultures. High levels of a neuronal membrane acidic 29-kilodalton (kD) protein were found. It has been shown previously that neuronal cultures incubated with polylysine-coated beads will develop numerous presynaptic elements on the bead surface. We report here that isolation of the beads from enriched neuronal cell cultures incubated with [35S]methionine showed, with two-dimensional nonequilibrium pH gradient gel electrophoresis (2D-NEPHGE), levels of a basic 32-kD protein (pI 8) note detected in cultures alone, and increased levels of a 30-kD protein (pI 10). When culture medium was examined with 2D-NEPHGE, three acidic proteins were identified that were secreted by the cultured neurons. In summary, a neuronal enriched cell culture system was used with isolated polylysine-coated beads to identify basic 30-kD and 32-kD proteins that may be involved in synapse formation.

Serum and fibroblast growth factor stimulate quiescent astrocytes to re-enter the cell cycle

An in vitro model was used to study the cytokinetics of astroglial cells derived from neonatal rat cerebellum. Confluent monolayers of astrocytes (85% astroglial as assessed by GFAP immunoreactivity) were subcultured at low cell density and after 2-3 days growth were rendered quiescent by shifting them to low serum medium (0.25%) for several days. Cells could be stimulated to re-enter the proliferative compartment by challenging them with high concentrations of fetal bovine serum (5-10% FBS) or fibroblast growth factor (FGF). FGF added alone at a concentration of 25 ng/ml caused quiescent astrocytes to re-enter the cell cycle nearly as effectively as 5-10% serum. Moreover, when FGF (25 ng/ml) was combined with 0.5% serum there was a potentiation of the mitogenic effect seen with FGF alone. This synchronization scheme is an important tool for continuing studies of the growth factor and hormonal requirements for astroglial cell proliferation and differentiation.

Glucocorticoid-induced growth inhibition of human neoplastic salivary gland duct cell line (HSG)

The purpose of this study was to examine the effect of glucocorticoid on human neoplastic salivary duct epithelial cell line (HSG). Dexamethasone was found to inhibit cell growth and to increase cell size and the ratio of protein content to DNA content in a cell. The inhibition of cell growth was dose-dependent; in comparison to the control (33.8 +/- 3.1 h), the population doubling time was 1.57-fold longer in 10(-5) M dexamethasone (P less than 0.01, N-K test). [3H] thymidine incorporation was inhibited in 45.5% of the control at 10(-5) M. Plating efficiency was 20.5 +/- 3.0% in 10(-5) M and 47.0 +/- 4.4% in the absence of dexamethasone. Cell diameters increased 1.29 fold in 10(-5) M dexamethasone in comparison to the control size (16.0 +/- 2.1 micron). The ratio of total protein content of DNA content increased 1.46 fold in 10(-5) M dexamethasone-treated cells on the seventh day of cultivation. Scatchard plot analysis using [6, 7-3H]-triamcinolone revealed that the HSG cells had apparent cytosolic glucocorticoid receptors with an equilibrium dissociation constant (Kd value) of 6.48 nM, whose number of binding sites (NBS) was 57.8 fmol/mg protein.

Estradiol--induced redistribution of glial fibrillary acidic protein immunoreactivity in the rat brain

The immunohistochemical distribution of the glial fibrillary acidic protein (GFAP), a marker of glial filaments, was studied on coronal sections of the globus pallidus, the area CA4 of the hippocampus and the arcuate nucleus of the hypothalamus, 3 estrogen-sensitive areas of the rat brain. The number and the surface density of the GFAP-immunoreactive cells were evaluated in 6 adult ovariectomized rats injected with a single dose (20 mg/kg) of estradiol valerate (OVX + E2 rats) and in 6 ovariectomized littermates injected with vehicle (OVX rats). Two days after the injection, a similar distribution of the GFAP was observed in the arcuate nucleus of OVX + E2 rats when compared to OVX rats, whereas a significantly (P less than 0.001) increased surface density of GFAP immunoreactive material was observed in the globus pallidus and hippocampus of estradiol-treated rats. Since the number of GFAP-positive cells was unchanged by the estradiol injection, the enhanced surface density of GFAP immunoreactive material in the hippocampus and globus pallidus suggest a possible influence of estradiol on GFAP-immunoreactive glial processes.

Presynaptic elements on artificial surfaces. A model for the study of development and regeneration of synapses

Recently a model has been developed to study the synapse formation in which the components of a synapse can be isolated and examined independently. The observation of neurites forming presynaptic elements on polylysine-coated surfaces is a model for which the formation of presynaptic elements can be studied independently of a cellular postsynaptic element. Studies with neurons from both cell cultures and the intact cerebellum have shown that beads coated with poly-basic proteins can serve as a "postsynaptic element." With use of this system, observation have shown that the presynaptic element can form quickly, within 3 h, and contain many of the characteristics of a mature presynaptic element, such as synaptic vesicle antigens. Additional studies have shown that astrocytes appear to be involved in the loss or removal of the presynaptic elements on beads. Thus, synaptogenesis may involve the development of inappropriate synaptic contacts, which are eliminated by astrocytes. The lack of regeneration in the central nervous system (CNS) also may involve the astrocyte's ability to remove immature and/or inappropriate presynaptic elements and growth cones as they attempt to cross the lesion site.