Neuronal-glial interactions: quantitation of astrocytic influences on development of catecholamine neurons

Potent activation of FGF-2 IRES-dependent mechanism of translation during brain development

Fibroblast growth factor-2 (FGF-2) plays a fundamental role in brain functions. This role may be partly achieved through the control of its expression at the translational level via an internal ribosome entry site (IRES)-dependent mechanism. Transgenic mice expressing a bicistronic mRNA allowed us to study in vivo and ex vivo where this translational mechanism operates. Along brain development, we identified a stringent spatiotemporal regulation of FGF-2 IRES activity showing a peak at post-natal day 7 in most brain regions, which is concomitant with neuronal maturation. At adult age, this activity remained relatively high in forebrain regions. By the enrichment of this activity in forebrain synaptoneurosomes and by the use of primary cultures of cortical neurons or cocultures with astrocytes, we showed that this activity is indeed localized in neurons, is dependent on their maturation, and correlates with endogenous FGF-2 protein expression. In addition, this activity was regulated by astrocyte factors, including FGF-2, and spontaneous electrical activity. Thus, neuronal IRES-driven translation of the FGF-2 mRNA may be involved in synapse formation and maturation.

A semi-automated image analysis method to quantify neurite preference/axon guidance on a patterned substratum

Axon outgrowth and guidance are differentially promoted or inhibited by specific extracellular matrix (ECM) molecules. The effects of these molecules can be examined by culturing neuronal explants on patterned substrata consisting of alternating stripes adsorbed with the molecules of interest. While outgrowth on substrata adsorbed with homogenous molecules can be reliably quantified, current methods of quantifying neurite preference on patterned substrata are subjective, labor intensive, and overall less reliable. Here, we present a quick, semi-automated, lowly subjective macro-based method to quantify the effects of a change in substratum on axon extension and guidance. We plated chick dorsal root ganglion explants on a substratum consisting of alternating stripes of laminin-1 (outgrowth supportive) and chondroitin sulfate proteoglycans (CSPGs, outgrowth inhibitory). We evaluated neurite preference for laminin or CSPG-coated regions by measuring total neurite area, and produced an inhibition index. The quantitative data confirmed previous qualitative data showing that increasing concentrations of CSPGs induced increases in inhibition. The methods presented here: (1) require less stringent image capture criteria; (2) are quicker; (3) are less subjective compared to previously described methods; and (4) are versatile in that they can be used to assay neurite preference for any substratum-bound molecules in living or fixed cultures.

GABA mediates steroid-induced astrocyte differentiation in the neonatal rat hypothalamus

Our previous work has demonstrated that astrocytes in the developing arcuate nucleus of the rat hypothalamus are sexually dimorphic as a result of differential exposure to oestradiol. Moreover, our experiments in neonatal rats suggest an absence of oestrogen receptors (ER) in arcuate nucleus astrocytes in vivo. This, along with the conspicuous lack of evidence in the literature confirming the presence of ER in arcuate nucleus astrocytes of the neonatal rat brain, led us to question the mechanism by which oestrogen induces changes in arcuate nucleus astrocyte morphology. Based on our previous findings that oestradiol increases gamma-aminobutyric acid (GABA) levels in the neonatal rat arcuate, we hypothesize that GABA is released from neighbouring oestrogen-sensitive neurones and alters arcuate nucleus astrocyte morphology. Here, we report that in vivo reduction of GABA synthesis in the neonatal rat brain by antisense oligodeoxynucleotides to glutamic acid decarboxylase prevented gonadal steroid-induced astrocyte differentiation in males and testosterone-treated females. Conversely, activation of GABAA receptors with the agonist muscimol increased astrocyte differentiation in females in the absence of gonadal steroids. Given that GABA is made only in neurones and that its synthesis is increased by oestradiol, we conclude that it acts as a diffusible factor inducing the differentiation of neighbouring astrocytes.

Steroid-induced developmental plasticity in hypothalamic astrocytes: implications for synaptic patterning

We have previously demonstrated that astrocytes in the developing arcuate nucleus of the rat hypothalamus exhibit a sexually dimorphic morphology as a result of differential exposure to gonadal steroids. Testosterone via its aromatized byproduct, estrogen, induces arcuate astrocytes to undergo differentiation during the first few days of life. These differentiated astrocytes exhibit a stellate morphology. Coincident with the steroid-induced increase in astrocyte differentiation is a reduction of dendritic spines on arcuate neurons. As a result, the arcuate nucleus of males has fewer axodendritic spine synapses than females and this dimorphism is retained throughout life. In the immediately adjacent ventromedial nucleus, neonatal astrocytes are immature and unresponsive to steroids. Neurons in this region show no change in dendritic spines in the first few days of life but do exhibit increased dendritic branching as a result of testosterone exposure. These findings illustrate the importance of distinct populations of astrocytes in restricted brain regions and their potential importance to the establishment of regionally specific synaptic patterning. Conflicting reports leave the site of steroid-mediated astrocyte responsiveness in the arcuate nucleus unresolved: Are gonadal steroids acting directly on astrocytes or are steroid-concentrating neurons mediating astrocytic responsiveness? In this review, we discuss the current understanding of astrocyte-neuron interactions and the possible mechanisms for steroid-mediated, astrocyte-directed synaptic patterning in the developing hypothalamus.

Alcohol, astroglia, and brain development

Glial cells constitute one of the most common cell types in the brain. They play critical roles in central nervous system (CNS) development. Recent evidence demonstrates that glial cells are profoundly affected by prenatal alcohol exposure, suggesting that alterations in these cells may participate in CNS abnormalities associated with ethanol-induced teratogenesis. In vivo studies show that prenatal exposure to alcohol hampers myelinogenesis and is associated with neuroglial heterotopias and abnormal astrogliogenesis. Studies using primary cultures of rat cortical astrocytes show that ethanol affects DNA, RNA, and protein synthesis, decreases the number of mitotic cells, alters the content and distribution of several cytoskeletal proteins including the astroglial marker, glial fibrillary acidic protein (GFAP), and the levels of plasma-membrane glycoproteins, reduces the capacity of astrocytes to secrete growth factors, and induces oxidative stress. Furthermore, ethanol exposure during early embryogenesis alters the normal development of radial glia cells (the main astrocytic precursors), delays the onset of GFAP expression, and decreases mRNA GFAP levels in fetal and postnatal brains and in radial glia and astrocytes in primary culture. Recent evidence suggests that ethanol interferes with the transcription process of GFAP, thus leading to a reduction in GFAP-gene expression during astrogliogenesis. However, brief exposure of rats to high levels of ethanol during the neonatal period (the period of astrocyte differentiation) causes a transient gliosis, with an increase in GFAP and its mRNA levels. These findings indicate that astroglial cells are an important target of ethanol toxicity during central nervous system (CNS) development.

Use of quantitative ultrastructural immunoperoxidase labeling for analysis of catecholamine neurotoxicity and plasticity

Levels of catecholamines and the synthesizing enzyme, tyrosine hydroxylase (TH) are markedly decreased in the dorsal striatum, caudate-putamen nuclei, following neurotoxic lesions with 6-hydroxy-dopamine (6-OHDA). We examined whether pre-embedding immunoperoxidase labeling of TH could be standardized for quantitatively examining the density and ultrastructure of spared dopaminergic terminals in the striatum of lesioned rats. The peroxidase-antiperoxidase (PAP) method was used to localize rabbit antiserum against TH in caudate-putamen nuclei of adult rats given unilateral nigral injections of either vehicle or 6-hydroxydopamine in the early postnatal period. Experimental differences in fixation and immunocytochemical labeling were minimized by limiting comparisons of immunoreactivity to co-processed sections from the same litters of animals. Imaging software and a Phillips CM-10 electron microscope were used to quantitatively examine immunoreactive profiles in a narrow zone of tissue in contact with the embedding resin. Under these conditions variables attributed to differences in penetration were minimized. There were no significant differences in numbers or mean-cross sectional diameter of immunoreactive terminals in striatum ipsilateral versus contralateral to vehicle injections. Ipsilateral to the 6-OHDA injections, the density (numbers/area) of striatal TH-immunoreactive terminals was reduced by 50-90% in the majority of animals. In the most extensively lesioned rats, the cross-sectional areas of the remaining immunoreactive axons were significantly larger than in the contralateral striatum of the same animal or either hemisphere of vehicle injected controls. These results confirm and extend earlier findings on the plasticity of residual dopaminergic terminals in adult animals after neurotoxic damage. They also establish a quantitative method for ultrastructural analysis of the density of immunoreactivity in thick sections of tissue labeled prior to plastic embedding. The method has broad applicability to quantitative studies of neurotoxicity and plasticity in brain.

A comparison of the developing dopamine neuron phenotype in cultures of embryonic rat mesencephalon and hypothalamus

Development of the dopamine (DA) neuron phenotype was monitored in cultures of embryonic rat mesencephalon (MES) and hypothalamus (HYP) maintained for 1 to 21 days in vitro (DIV) in the absence of glial support cells. Cell counts following immunohistochemistry for tyrosine hydroxylase (TH) demonstrated that the number of DA neurons declined by 85% in MES cultures yet increased 5-fold in cultures of HYP, so that by 21 DIV equal numbers of DA neurons were present in these culture systems. After 21 DIV MES DA neurons exhibited a multipolar morphology, with numerous branching processes. HYP DA neurons were primarily fusiform in shape with fewer processes and process branch points. Double-label immunohistochemistry for TH and microtubule-associated protein 2 identified the majority of TH-positive processes in either culture system as dendrites. Individual MES but not HYP DA neurons were also found to generate axons. Western analysis showed that between 1 and 21 DIV the concentration of TH protein increased 2-fold in MES and 4-fold in HYP cultures. After 21 DIV the concentration of TH protein in MES cultures was twice that found in cultures of HYP. In the period between 1 and 21 DIV levels of tetrahydrobiopterin (BH4) increased by 6-fold in MES and 20-fold in HYP cultures. After 21 DIV BH4 content was 3-fold higher in HYP than in MES cultures. The abundance of the mRNA encoding for GTP cyclohydrolase I, the rate-limiting enzyme in BH4 biosynthesis, was similar in MES and HYP cultures despite this difference in BH4 levels. In contrast, TH mRNA was 4-fold more abundant in MES than in HYP cultures. Treatment of MES cultures with the DA neuron toxin 1-methyl-4-phenylpyridinium decreased DA cell numbers, TH protein content and BH4 levels, demonstrating that BH4 is localized primarily to DA neurons. Similar treatment of HYP cultures did not effect any of these parameters. Steady-state levels of DA and the rate of DA synthesis were both 3-fold higher in MES than in HYP cultures. A 95% decline in BH4 content produced by inhibiting BH4 biosynthesis resulted in 64% and 84% declines in the rate of MES and HYP DA synthesis, respectively. Overall, these observations indicate that, with the exception of the capacity to synthesize DA, DA neurons in MES and HYP cultures share few common properties.

Prenatal exposure to ethanol induces changes in the nerve growth factor and its receptor in proliferating astrocytes in primary culture

We have analyzed the effect of prenatal exposure to alcohol on the binding, internalization and secretion of NGF as well as on the content of the NGF receptor (NGFr) in cortical rat astrocytes in primary culture. Secretion of NGF was approximately 1.8-fold greater in 6-day control astrocytes than in 13-day cells. Intracellular content of NGF was very low. Astrocytes in 6-day cultures from control fetuses expressed a relatively large number of NGFr on the cell surface with a steady-state constant in the low nanomolar range. NGF was internalized by astrocytes at a slow rate. Prenatal exposure to ethanol induces a moderate increase in the number of NGFr on the cell surface as well as an increase in the intracellular pool of both NGF and NGFr which is accompanied by an important reduction in the secretion of this factor. We speculate that this decrease in NGF secretion could alter the neuronal migration pattern during development, resulting in the presence of ectopic neurons in the cortex.

Effects of ethanol on cultured fetal astroglia

This study investigated the effects of a 4-day ethanol exposure on cultured rhombencephalic astroglia. The contents of astroglial protein and DNA, and astroglial uptake of serotonin (5-HT) were determined. Fetal rhombencephalic astroglia were examined because of this laboratory's evidence that in utero ethanol exposure markedly impairs the development of serotonergic neurons, which are located in this fetal brain area, and because of the recently demonstrated importance of local support glia in neuronal development. The results of these experiments demonstrated that protein was significantly reduced in astroglia cultured in ethanol at either 150 or 300 mg/dl. In addition, these astroglia exhibited decreased [3H]5-HT uptake per well. However, no significant ethanol-associated differences were detected when [3H]5-HT uptake was expressed per mg protein rather than per well. In contrast to the effects of a 4-day ethanol exposure, the acute ethanol exposure did not significantly alter astroglial uptake of [3H]5-HT/well. In addition, the 4-day exposure to 50 to 300 mg/dl of ethanol did not significantly alter astroglial DNA content. In summary, it appears that a 4-day exposure of cultured fetal rhombencephalic astroglia to 150 to 300 mg/dl of ethanol reduces astroglial protein content and astroglial 5-HT uptake. A reduction in total astroglial proteins, potentially including those that act as essential growth factors, could contribute to some of the ethanol-associated alterations in central nervous system development.

Trophic actions of transforming growth factor alpha on mesencephalic dopaminergic neurons developing in culture

Transforming growth factor alpha messenger RNA and protein levels are highest in the striatum, the target area of mesencephalic dopaminergic neurons of the substantia nigra, suggesting a role as a target-derived neurotrophic factor for these cells. To test this hypothesis, we characterized the actions of transforming growth factor alpha on fetal rat dopaminergic neurons in culture. Transforming growth factor alpha promoted dopamine uptake in a dose- and time-dependent manner. Administration of transforming growth factor alpha at the time of plating for 2 h produced a significant increase in dopamine uptake after five days of growth in vitro. As cultures aged they became less responsive to transforming growth factor alpha, such that longer times of exposure were required to elicit a similar, but weaker, response. Dopaminergic cell survival was selectively promoted by transforming growth factor alpha, since there was an increase in the number of tyrosine hydroxylase-immunostained cells without a parallel increase in the total number of neuron-specific enolase-immunopositive cells. Neurite length, branch number and soma area of tyrosine hydroxylase-immunopositive cells also were enhanced by transforming growth factor alpha treatment. Increases in each of the dopaminergic parameters due to transforming growth factor alpha were accompanied by a rise in glial cell number, making it possible that these effects were mediated by this cell population. The neurotrophin antagonist, K252b, failed to inhibit the transforming growth factor alpha-induced increase in dopamine uptake, indicating that transforming growth factor alpha's effects were not mediated by neurotrophin mechanisms. The actions of transforming growth factor alpha on the differentiation of dopaminergic neurons only partially overlapped with those of epidermal growth factor. Thus, while transforming growth factor alpha and epidermal growth factor are believed to share the same receptor they differentially affect dopaminergic cell development in vitro. These results indicate that transforming growth factor alpha is a trophic factor for mesencephalic cells in culture and suggests that transforming growth factor alpha plays a physiological role in the development of these cells in vivo.

Identification of a collagen potentiated neurite promoting factor isolated from C6 glioma cells

The C6 glial cell line has been used as a model cell system for the investigation of new glial produced neurotrophic and neurotropic molecules. By using the C6 cell line grown in a defined medium on collagen, this laboratory has isolated a distinct neurite promoting factor (NPF) that is potentiated by the presence of collagen (CPNPF). We have observed that C6 cells cultured in a defined medium on collagen (rat type-I) slowed their growth rate and expressed an astrocytic- or oligodendrocytic-like morphology. CPNPF, at this state of purity, appears to be a distinct NPF which induces neurite outgrowth (neurites of 1 or more somal diameters) in PC12 cells. These neurite promotion effects, however, appear to support the neuron morphology for only a short period (4 days) of time without the presence of neurotrophic factor (NTF). The neurite promoting activity is ineffective in inducing neurite outgrowth using mouse neuroblastoma cells (neuro-2a). CPNPF appears to be a heat stable protein whose activity does not depend on the presence of intact collagen, heparin sulfate proteoglycan (HSPG), or chondroitin sulfate proteoglycan (CSPG). Exposure to dissociative conditions results in a loss of neurite promoting activity. CPNPF is not a glycoprotein that contains an accessible alpha-D-mannopyranosyl, alpha-D-glucopyranosyl, or a sterically related residue (hydroxyl groups in the C-3,4, and 5 positions). Although these residues are not present on all glycoproteins, it does indicate that CPNPF is most likely not a glycoprotein. CPNPF activity is not blocked by neutralizing antibodies directed toward NGF, beta-FGF, IL-1 beta, IL-6, TGF-beta 2, TGF-beta 1.2, TGF-beta 3, TGF-beta 5, or EGF. CPNPF appears to either be oligomeric protein or a complex of proteins. On the basis of indirect evidence, it does not appear to be glial derived protease nexin-I. The alteration in morphology of the C6 glial cell line by serum-free conditions in the presence of collagen may have induced the production of a potentially new NPF not seen by previous investigators.

Expression of inherent neuronal shape characteristics after transient sensitivity to epigenetic factors

We investigated effects of different substrates and culture media on the early morphological differentiation of rat neocortical neurons in culture. In particular, we examined the effects of homotypic astrocytes, the adhesive glycoprotein laminin and the polycationic substrate poly-L-lysine, as well as diffusible astrocyte-derived conditioned medium factors and serum on (1) soma area, (2) total neuron area and (3) primary neurite number. To assess variations in morphological reactions of neurons with a defined neurotransmitter phenotype, we analyzed the differentiation of GABAergic neurons. The morphology of young neocortical neurons was dramatically affected by both substrate and culture medium. Replacement of the astrocytic monolayer or the astrocyte-conditioned medium by other substrates or non-conditioned medium, respectively, was accompanied by (1) spreading and flattening of neuronal somata, (2) a marked decrease in total neuron area and (3) an increase in the number of primary neurites. The various morphological parameters studied exhibited different sensitivities to changes of these external factors. Moreover, the influences of epigenetic factors on the generation of primary neurites depended on the transmitter phenotype of the neuron. The induced morphological alterations were transient. At the end of the first week in culture, the surviving neurons underwent substantial remodeling of their morphology leading to an expression of in vivo shape characteristics. These observations suggest that despite an early, transient sensitivity to environmental influences, the neuronal differentiation with respect to the morphological parameters studied in culture is to a large degree determined by intrinsic factors.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Dissociated cell culture of rat cerebral cortical neurons in serum-free, conditioned media: GABA-immunopositive neurons

The gamma-aminobutyric acid (GABA)ergic properties of embryonic (E15d) rat cortical neurons were studied in dissociated serum-free culture by immunohistochemical methods. GABA-like immunoreactivity was found in a subpopulation of neurons from the first day onwards. The number of GABA-positive neurons reached mature values (10.5-12.6%) within the first week, while their morphological differentiation was not found to be fully completed until the 11th day of culture and was characterized by several discrete developmental stages. First, GABA-positive neurons gained their mature complement of neurites at 3 days in vitro (DIV). Three days later somal maturation became evident, followed at least by the maturation of the neuritic arbor. Double-labelling studies revealed the coexpression of GABA and tyrosine hydroxylase within the same cells. The similarities of relative number, morphology, time course of development and biochemistry of cultured GABAergic neurons compared with those in situ suggest that the applied culture system is a useful model to investigate several aspects of GABAergic neurotransmission at the cellular level.

Ascorbic acid in mesencephalic cultures: effects on dopaminergic neuron development

Ascorbic acid exists in high intracellular concentrations in fetal rat brain. In mesencephalic cultures the cellular ascorbic acid content drops sharply to undetectable levels when no ascorbic acid is added to the medium, thus creating a model of scorbutic neuronal tissue and affording the study of ascorbic acid's effects on mesencephalic cell development and function. Cultures treated with 0.2 mM ascorbic acid were compared with controls (scorbutic cultures) by using morphological and biochemical indices. Ascorbic acid cultures at 7 and 14 days in vitro showed a marked increase in glial proliferation on glial fibrillary acidic protein staining and increased neurite growth and number on tyrosine hydroxylase staining. Significantly higher dopamine uptake and levels of dopamine and 3,4-dihydroxyphenylacetic acid were also observed after 7 and 14 days of ascorbic acid treatment. The capacity to accumulate ascorbic acid and the ability to retain the intracellular ascorbic acid developed gradually as the cultures matured. Ascorbic acid reached the embryonal levels by day 14 in vitro. We conclude that although neuronal cultures can survive and grow in the absence of detectable levels of ascorbic acid, its presence exerts a broad effect on dopamine neuron morphology and biochemical functioning either directly or through increased glial proliferation, or possibly both.

Cells from embryonic rat striatum cocultured with mesencephalic glia express dopaminergic phenotypes

To study region-specific transmitter phenotype expression, cells of embryonic day 14 (E14) rat neostriatum (ganglionic eminence plus cortical plate) or of the substantia nigra (ventral mesencephalon) were cultured on glial cells either from substantia nigra or neostriatum (E21). By antityrosine hydroxylase immunocytochemistry, immunoblotting of tyrosine hydroxylase protein and quantitation of dopamine and its metabolites by HPLC, dopaminergic cells were revealed in nigral and neostriatal cultures plated on nigral glial cells. No dopaminergic cells were found among neostriatal neurons plated on neostriatal glial cells. It is concluded that glia from substantia nigra but not glia from neostriatum is capable of inducing development or promoting survival of dopaminergic cells.

Thrombin indirectly affects cholinergic cell expression in primary septal cell cultures in a manner distinct from nerve growth factor

The effects of thrombin were examined in primary cultures of dissociated medial septal cells from fetal (embryonic day 17) rat brains. Seven days of continuous exposure of these cultures to thrombin produced a dose-dependent increase in the activity of the enzyme choline acetyltransferase (EC 2.3.1.6) and no change in the number of acetylcholinesterase (EC 3.1.1.7)-positive cells. Maximal induction of choline acetyltransferase activity occurred around 1-2 nM thrombin and was first detected after five days of treatment. In addition, thrombin promoted neuronal cell aggregation, proliferation of the astroglia, and changes in astroglial cell morphology. Neuronal aggregation was first noted after 24 h of treatment, while the proliferative response of the astroglia was first apparent after four days of treatment, slightly prior to the increase in choline acetyltransferase enzymatic activity. In order to see if the induction of the enzyme choline acetyltransferase was dependent upon the astroglial cell response, we included the anti-mitotic agent 5-fluoro-2'-deoxyuridine, to find that astrocyte proliferation, as well as thrombin-induced increase in choline acetyltransferase, were both abolished. In contrast, the aggregation of neurons was not affected. Finally, thrombin-induced changes in choline acetyltransferase could not be antagonized by immunoneutralizing anti-nerve growth factor antibodies and when thrombin was added simultaneously with 100 ng/ml 2.5-S nerve growth factor, the increase in choline acetyltransferase activity was additive. In conclusion, it appears that thrombin affects cholinergic septal neurons indirectly via the responsive astrocytes in a manner distinct from nerve growth factor.

Ontogeny of the serotonergic system in the rat: serotonin as a developmental signal

The serotonergic system is an early forming component of the CNS circuitry, beginning its development on gestational days 11-12 in the rat. Owing to its early presence in the embryonic nervous system, 5-HT has been proposed to act as a developmental signal for receptive cells. In vivo and in vitro evidence that 5-HT can influence both biochemical and morphological differentiation of raphe neurons and receptive target cells suggests that this neurotransmitter may have an organizing function in the developing nervous system which involves effects on neurite outgrowth and other aspects of neuronal differentiation, including synaptogenesis. Such functions may be mediated by a variety of 5-HT receptors located on both neuronal and non-neuronal cells. The apparent function of 5-HT as a differentiation signal in the developing nervous system raises important issues regarding the use of psychoactive serotonergic drugs by pregnant women, since these drugs may act as neural teratogens in the unborn child.

Neuronal-glial interactions: complexity of neurite outgrowth correlates with substrate adhesivity of serotonergic neurons

To study the interactions between neurons of known transmitter phenotype and non-neuronal cells of glial or fibroblastic origin, serotonergic (5-HT) neurons were tested for their strength of adhesion and neurite outgrowth patterns on substrates of astrocytes or fibroblasts using a cell adhesion assay for transmitter-identified neurons, and morphometry of immunocytochemically stained neurons in dissociated cell cultures. Both the strength of adhesion and the rate and complexity of neurite outgrowth by 5-HT neurons were significantly greater on substrates of astrocytes compared to fibroblasts. These results provide evidence that 5-HT neurons can interact selectively with glia via cell surface determinants, and that this process may be important for the development of complex (dendrite-like) neuritic arbors. The methods developed in this study will be useful for future studies of interactions between transmitter-identified neurons and glial cells during ontogeny of the embryonic brain.