Effects of Neurohormones on Glial Cells

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Regulation of apolipoprotein E secretion in rat primary hippocampal astrocyte cultures

Apolipoprotein E isoforms may have differential effects on a number of pathological processes underlying Alzheimer's disease. Recent studies suggest that the amount, rather than the type, of apolipoprotein E may also be an important determinant for Alzheimer's disease. Therefore, understanding the regulated synthesis of apolipoprotein E is important for determining its role in Alzheimer's disease. We show here that in rat primary hippocampal astrocyte cultures, dibutyryl-cAMP increased apolipoprotein E secretion with time in a dose-dependent manner (to 177% at 48 h) and that retinoic acid potentiated this effect (to 298% at 48 h). Dibutyryl-cAMP also gave a rapid, albeit transient, increase of apolipoprotein E mRNA expression (to 200% at 1 h). In contrast, the protein kinase C activator phorbol 12-myristate 13-acetate decreased both apolipoprotein E secretion (to 59% at 48 h) and mRNA expression (to 22% at 1 h). Phorbol 12-myristate 13-acetate also reversed the effects of dibutyryl-cAMP. Apolipoprotein E secretion was also modulated by receptor agonists for the adenylyl cyclase/cAMP pathway. Isoproterenol (50 nM, a beta-adrenoceptor agonist) enhanced, while clonidine (250 nM, an alpha2-adrenoceptor agonist) decreased, secreted apolipoprotein E. We also analysed the effects of agonists for the phospholipase C/protein kinase C pathway. Arterenol (1 microM, an alpha1-adrenoceptor agonist) and serotonin (2.5 microM) enhanced, whereas carbachol (10 microM, an acetylcholine muscarinic receptor agonist) decreased secreted apolipoprotein E. The effects of these non-selective receptor agonists were modest, probably due to effects on different signalling pathways. Arterenol also potentiated the isoproterenol-mediated increase. We also show that phorbol 12-myristate 13-acetate and dibutyryl-cAMP have opposite effects on nerve growth factor, as compared to apolipoprotein E, secretion, suggesting that the results obtained were unlikely to be due to a general effect on protein synthesis. We conclude that astrocyte apolipoprotein E production can be regulated by factors that affect cAMP intracellular concentration or activate protein kinase C. Alterations in these signalling pathways in Alzheimer's disease brain may have consequences for apolipoprotein E secretion in this disorder.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Acutely isolated astrocytes as models to probe astrocyte functions

Neuroscientists have become increasingly aware and accepting of the concept that astrocytes likely have many important functions in the CNS. One limitation in establishing these functions is the usual problem of what constitutes suitable experimental approaches. A major experimental step for functional studies of astrocytes has been the widespread use of primary astrocyte cultures, an approach that Leif Hertz pioneered. However, it is now becoming clear that, building on this work, an experimental paradigm shift is now needed. Namely, to increasingly study preparations corresponding to in situ conditions, such as slices. An alternative experimental system where the cells have some of the technical advantages of primary astrocyte cultures is freshly isolated astrocytes. Recent experiments from our laboratory have shown metabotropic glutamate receptor expression by such cells. Examples are given of how functional receptor studies and channel activity measured by patch clamp electrophysiology can be combined with single cell RT-PCR to define further the receptor or channel type are described to illustrate the uses of such preparations.

Effect of dibutyryl cyclic AMP on the kinetics of myo-inositol transport in cultured astrocytes

Dibutyryl cyclic AMP (dBcAMP) is known to induce maturation and differentiation in astrocytes. As myo-inositol is an important osmoregulator in astrocytes, we examined the effects of maturation and biochemical differentiation on the kinetic properties of myo-inositol transport. Treatment of astrocytes with dBcAMP significantly decreased the Vmax of myo-inositol uptake, but the effect on Km was not significant. The myo-inositol content of astrocytes was significantly decreased in cells treated for 5 days with dBcAMP as compared with untreated controls. Maximum suppression of myo-inositol uptake occurred 7 days after exposure of astrocytes to dBcAMP; this was gradually reversible when dBcAMP was removed from the medium. After exposure to hypertonic medium for 6 h, mRNA expression of the myo-inositol co-transporter was diminished by approximately 36% in astrocytes treated with dBcAMP as compared with untreated cells. It appears that myo-inositol transporters in astrocytes treated with dBcAMP are either decreased in number or inactivated during maturation and differentiation, suggesting that the stage of differentiation and biochemical maturation of astrocytes is an important factor in osmoregulation.

Adenosine receptor mediated stimulation of intracellular calcium in acutely isolated astrocytes

The characteristics of adenosine receptors found in glial fibrillary acid protein (GFAP)-positive astrocytes acutely isolated from the cerebral cortices of 4- to 12-day old rats were examined by evaluating the effects of adenosine and its analogues on intracellular calcium levels. First, these effects were compared with those seen in primary astrocytic cultures, and it was found that acutely isolated astrocytes showed much greater sensitivity to adenosine than their cultured counterparts. Then, the adenosine evoked calcium responses in acutely isolated cells were evaluated under various conditions. The responses to adenosine were not inhibited by papaverine, an uptake blocker, or by removal of extracellular calcium. U73122, a phospholipase C inhibitor, was able to completely inhibit the adenosine response. The receptor inhibitor 3-isobutyl-1-methylxanthine inhibited the calcium response to adenosine, providing evidence that the response is not coupled to the xanthine-insensitive A3 receptor. The stimulatory action of NECA, a non-selective analogue, was blocked neither by the A2A-selective receptor antagonist 8-(3-chlorostyryl) caffeine nor by the A1-selective receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. The A2B receptor antagonist alloxazine, however, was able to completely inhibit the increase in intracellular calcium produced by NECA. Taken together, these data suggest that the adenosine-evoked calcium response in acutely isolated astrocytes is coupled to the A2B receptor.

Cellular distribution of branched-chain amino acid aminotransferase isoenzymes among rat brain glial cells in culture

The first step in the catabolism of branched-chain amino acids (BCAA), reversible transamination, is catalyzed by one of the two isoforms of branched-chain amino acid aminotransferase (BCAT). The mitochondrial isoenzyme (BCATm) is widely distributed among tissues, whereas the cytosolic isoenzyme (BCATc) is restricted to only a few organs. Remarkably, BCATc is the prominent isoenzyme found in brain. The physiological significance of the subcellular compartmentation of BCAT is still not understood. To contribute to the elucidation of the cellular distribution of the two isoenzymes in brain, we used cultured rat glial cells in an immunocytochemical study to determine the pattern of BCAT isoenzyme expression by glial cells. Antiserum against BCATm generated a punctate staining pattern of astroglial cells, confirming the mitochondrial location of this isoenzyme. In contrast, the cytosol of galactocerebroside-expressing oligodendroglial cells and O2A progenitor cells displayed intense staining only for BCATc. In addition, subpopulations of astroglial cells exhibited BCATc immunoreactivity. The presence of BCATm in astrocytes is consistent with the known ability of these cells to oxidize BCAA. Furthermore, our results on BCATc provide support for the hypothesis that BCATs are also involved in nitrogen transfer from astrocytes to neurons.

Development of a culture system for pure rat neurons: advantages of a sandwich technique

Primary cell cultures were derived from the cerebral cortices of embryonic rats (E 17). Survival of the cultures under serum-free conditions was improved by creating a sandwich: a poly-D-lysine-coated coverslip with plated cells was placed upside down in plastic culture dishes. Neurite outgrowth was observed within three hours after plating, and a neuronal network was established after 24 hours. The viability of the neurons gradually decreased. However, the cells could be cultivated for up to 24 days. Under these conditions the contamination with non-neuronal cells was minimized to less than 5%, as evidenced by immunohistochemical methods using the well-established cell marker proteins: neuron-specific enolase (NSE) as neuronal marker, and vimentin and glial fibrillary acidic protein (GFAP) as astroglial markers. Returning the coverslip to a normal open face position led to cell death within 24 hours. In order to investigate the maturation and differentiation of the cultured nerve cells, we looked for synapse formation by staining the synaptic vesicle protein synaptophysin (p38). It could be immunostained after three days in vitro (DIV) only in the neuronal perikarya, in perikarya and axons after six DIV, and in varicosities and contact points between axon terminals and adjacent axons or perikarya after 10-12 DIV. It appears that this simple culture method, which (i) yields highly enriched (> 95%) neuronal cultures with more than 85% cells surviving after five days in vitro, (ii) the absence of non-neuronal cells and (iii) the good maturation/differentiation of the cells, may be useful for the study of the neurochemical, physiological or regulatory mechanisms involved in nerve cell development.

Receptors on astrocytes--what possible functions?

Receptors for transmitters, as varied as those expressed by neurons, have been described on primary astrocyte cultures prepared from new-born rats and mice. A variety of functional effects and considerable cell-to-cell and regional heterogeneity have been observed for such receptors in vitro. The various systems available for studying the presence and properties of receptors on astrocytes in situ, and the results from these studies, are discussed. Much fewer studies using these more difficult systems have been done. So far, some resemblances and differences between in situ and in vitro work have been observed. More of these in situ studies, to supplement the ongoing in vitro work, are needed to enable us to determine unequivocally which receptors are present on astrocytes, and their functions in vivo. If there is cell-to-cell and CNS regional heterogeneity in vivo comparable to that seen in vitro, these analyses will be very complex. To illustrate the importance and variety of receptor-linked functions, a number of suggestions are made in this commentary, based on current proposals for the roles of astrocytes. However, it is argued that we need to have a more complete understanding of astrocyte functions in vivo, before we can really understand the functional significance of astrocyte receptors.

Distribution of acetylcholine receptors in the central nervous system of adult locusts

A polyclonal antibody raised against nicotinic acetylcholine receptor protein from purified locust neuronal membrane was used to analyse the distribution of antigenic sites within the central nervous system of adult Schistocerca gregaria. Light microscopic examination showed that all principal neuropiles in the thoracic ganglia label with the antibody but that the major tracts and commissures do not. Analysis of this pattern of staining in the electron microscope reveals that the receptor is present on specific synaptic and extrajunctional neuronal membranes in the neuropile. Antigenic sites are also evident on the plasma membranes and within the cytoplasm adjacent to Golgi complexes of some neuronal somata, suggesting that these neurones synthesise nicotinic acetylcholine receptors. In addition to neuronal labelling, there is evidence that the receptor is also present on the membranes of three types of glial cells. The implications of this pattern of receptor distribution are discussed.

Glucose, insulin, and insulin-like growth factor I regulate the glycogen content of astroglia-rich primary cultures

The glycogen content of astroglia-rich primary cultures derived from the brains of newborn rats depends on the concentration of glucose in the culture medium. After administration of culture medium lacking glucose, the glycogen content decreases with a half-time of 7 min. Readdition of glucose results in replenishment of the glycogen stores within 2-3 h, but fully only if glucose is present in a concentration of at least 4 mM. Insulin, or the more potent insulin-like growth factor I, increases the content of glycogen approximately 1.7-fold, with the half-maximal effects being attained at concentrations of 10 and 0.5 nM, respectively. These results suggest that (a) glucose or a metabolite of it and (b) insulin-like growth factor I or a closely related peptide, but not insulin, are likely to be physiological regulators of the level of glycogen in astrocytes.

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.

Molecular mechanisms of memory formation

Studies with neonate chicks, trained on a passive avoidance task, suggest that at least two shorter-term memory stages precede long-term, protein synthesis-dependent memory consolidation. Posttetanic neuronal hyperpolarization arising from two distinct mechanisms is postulated to underlie formation of these two early memory stages. Maintenance of the second of these stages may involve a prolonged period of hyperpolarization brought about by phosphorylation of particular proteins. A triggering mechanism for long-term consolidation is postulated to occur at a specific time during the second stage, and may involve reinforcement-contingent release of neuronal noradrenaline stimulating cAMP-dependent intracellular processes. The possibility that astroglia may have a critical role to play in these early stages of memory processing is raised.

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Glial localization of adenylate-cyclase-coupled beta-adrenoceptors in rat forebrain slices

Fluorocitrate (FC), a selective inhibitor of glial cell respiration, was used to estimate the extent to which glial cells contain adenylate cyclase-coupled beta-adrenoceptors in rat brain slices. The drug blocked 75-95% of the elevation of cyclic AMP caused by the beta-agonist, isoproterenol, in the 4 forebrain regions sampled (frontal and parietal cortex, caudate nucleus, olfactory tubercle). Intracellular recording of neurons in the treated slices confirmed that they were unaffected by FC. Treatment with the neurotoxin, kainic acid, eliminated all electrophysiological activity but did not affect the cAMP response. The results indicate that glial cells contain the preponderance of adenylate-cyclase-coupled beta-adrenoceptors in slices of the rat forebrain and may constitute an important target of the central noradrenergic system in vivo.

Characterization of opioid receptors in cultured neurons

The appearance of mu-, delta-, and kappa-opioid receptors was examined in primary cultures of embryonic rat brain. Membranes prepared from striatal, hippocampal, and hypothalamic neurons grown in dissociated cell culture each exhibited high-affinity opioid binding sites as determined by equilibrium binding of the universal opioid ligand (-)-[3H]bremazocine. The highest density of binding sites (per mg of protein) was found in membranes prepared from cultured striatal neurons (Bmax = 210 +/- 40 fmol/mg protein); this density is approximately two-thirds that of adult striatal membranes. By contrast, membranes of cultured cerebellar neurons and cultured astrocytes were devoid of opioid binding sites. The opioid receptor types expressed in cultured striatal neurons were characterized by equilibrium binding of highly selective radioligands. Scatchard analysis of binding of the mu-specific ligand [3H]D-Ala2,N-Me-Phe4,Gly-ol5-enkephalin to embryonic striatal cell membranes revealed an apparent single class of sites with an affinity (KD) of 0.4 +/- 0.1 nM and a density (Bmax) of 160 +/- 20 fmol/mg of protein. Specific binding of (-)-[3H]bremazocine under conditions in which mu- and delta-receptor binding was suppressed (kappa-receptor labeling conditions) occurred to an apparent single class of sites (KD = 2 +/- 1 nM; Bmax = 40 +/- 15 fmol/mg of protein). There was no detectable binding of the selective delta-ligand [3H]D-Pen2,D-Pen5-enkephalin. Thus, cultured striatal neurons expressed mu- and kappa-receptor sites at densities comparable to those found in vivo for embryonic rat brain, but not delta-receptors.

Purification of glycogen phosphorylase from bovine brain and immunocytochemical examination of rat glial primary cultures using monoclonal antibodies raised against this enzyme

The physiological function in brain of glycogen and the enzyme catalyzing the rate-limiting step in glycogenolysis, glycogen phosphorylase (EC 2.4.1.1), is unknown. As a first step toward elucidating such a function, we have purified bovine brain glycogen phosphorylase isozyme BB 1,700-fold to a specific activity of 24 units/mg protein. When analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent silver staining, a single major protein band corresponding to an apparent molecular mass of 97 kDa was observed. Mouse monoclonal antibodies raised against the enzyme were purified and shown to be monospecific as indicated by immunoblotting. Immunocytochemical examination of astroglia-rich primary cultures of rat brain cells revealed a colocalization of glycogen phosphorylase with the astroglial marker glial fibrillary acidic protein in many cells. The staining for the enzyme appeared at two levels of intensity. There were other cells in the culture showing no specific staining under the experimental conditions employed. Neurons in neuron-rich primary cultures did not show positive staining. The data suggest that glycogen phosphorylase may be predominantly an astroglial enzyme and that astroglia cells play an important role in the energy metabolism of the brain.

Visualisation of specific binding sites for atrial natriuretic peptide on non-neuronal cells of cultured rat sympathetic ganglia

The distribution of atrial natriuretic peptide binding sites on cells in dissociated culture preparations of neonatal rat superior cervical ganglia and in explant cultures of rat thoracic sympathetic chain ganglia has been studied. The autoradiographic visualisation of atrial natriuretic peptide binding sites has been combined with the use of specific immunocytochemical markers for glial cells (antiserum to S-100 protein), fibroblasts (antiserum to fibronectin) and neurones (antiserum to protein gene product 9.5) in order to achieve unambiguous identification of the cell types in culture. Specific binding sites for rat 125I-atrial natriuretic peptide(1-28) were observed over subpopulations of fibronectin-like-immunoreactive fibroblasts and S-100-like-immunoreactive glia in the dissociated superior cervical ganglion cultures. However, only a subpopulation of fibronectin-like-immunoreactive fibroblasts possessed atrial natriuretic peptide binding sites in the explant culture preparations. No atrial natriuretic peptide-like-immunoreactive cells were present in either culture. The distribution of autoradiographic grains over individual cell surfaces in culture was uniform, but there were distinct differences in the density of labelling of single cells of the same type. This apparent variation in the number of binding sites on glial cells and fibroblasts in culture did not seem to be related to the morphology of the cells or the surrounding cell types. No sympathetic neurones were labelled with autoradiographic grains in either the dissociated or explant culture preparations. However, the presence of atrial natriuretic peptide binding sites on non-neuronal cells of sympathetic ganglia in culture may be linked to the relationship between atrial natriuretic peptide and the sympathetic nervous system.

Is Alzheimer's disease an anterograde degeneration, originating in the brainstem, and disrupting metabolic and functional interactions between neurons and glial cells?

A novel hypothesis is suggested for the pathogenesis of Alzheimer's disease, i.e. that a degeneration of adrenergic neurons in locus coeruleus and/or of serotonergic neurons in the raphe nuclei leads to impairment in metabolic and functional interactions between neurons and astrocytes (in the cerebral cortex and hippocampus as well as in nucleus basalis magnocellularis), and that a resulting deficient supply of substrates and failing energy metabolism in both neurons and astrocytes causes neuronal cell death in these areas and thus interference with additional transmitter systems. The hypothesis is based on (1) the topographical distribution of ascending pathways from locus coeruleus and the raphe nuclei; (2) the peculiar termination of many of these fibres in varicosities, from which released transmitter molecules reach their targets by diffusion, rather than in genuine synapses, suggesting a partly non-neuronal target; (3) the effects of locus coeruleus lesions in experimental animals; (4) the emergence of new knowledge in cellular neurobiology, indicating profound metabolic and functional interactions between neurons and astrocytes; and (5) the effects of adrenergic and serotonergic agonists upon metabolism and function in rodent astrocytes and neurons. These compounds influence energy metabolism, membrane transport of potassium and production of growth factors in astrocytes, and glutamate release from glutamatergic neurons. They thus influence essential metabolic interactions between neurons and astrocytes, as well as neuronal-astrocytic interactions in potassium homeostasis at the cellular level. Obviously, neither the individual findings alone, nor their combination into a conceptual framework, prove the correctness of the hypothesis. However, they do provide a basis for further experimental work, using postmortem brain tissue from Alzheimer's patients and lesion studies in rodents, which can confirm or refute the hypothesis.

Are glial cells targets of the central noradrenergic system? A review of the evidence

It has been suggested by a number of investigators that glial cells as well as neurons are targets of the central noradrenergic system. This important hypothesis, however, has not been presented previously in a systematic and unified manner. The present review was therefore undertaken to accomplish this. The evidence supporting noradrenergic action on glia consists primarily of findings that beta-adrenoceptors, norepinephrine (NE)-stimulated cyclic AMP (cAMP) responses and glycogen are localized preferentially in glial cells and that beta-receptor density and glycogen hydrolysis are under the control of neuronally released NE. While there is some disagreement as to the extent to which beta-receptors are preferentially localized in glia, there is a consensus that most glycogen in the forebrain is localized in this cellular compartment. The presumed function of the noradrenergic action on glia appears to be the release of glucose for production of energy, the synthesis of neurotrophic factors such as nerve growth factor, and the release of substances which may affect local neurotransmission including taurine, cAMP and its metabolites. These glial responses may be intimately related to the electrophysiological actions of NE on neurons.

Hypoxia-induced dysfunctions and injury of astrocytes in primary cell cultures

The effects of severe hypoxia were studied in a primary culture of astrocytes prepared from newborn rat cerebral cortex. Hypoxia was created by placing cultures in an airtight chamber that was flushed with 95% N2/5% CO2 for 15 min before being sealed. The hypoxic environment was maintained constant for up to 24 h. During the first 12 h of hypoxia, astrocytes showed no morphological changes by phase-contrast microscopy. After 18 h of hypoxia, some astrocytes in culture became swollen and started to detach from the culture dish. All cells in the culture were destroyed after 24 h of hypoxia. The lactate dehydrogenase level in the culture medium increased more than tenfold between 12 and 24 h of hypoxia. Glutamate uptake was inhibited 80% by similar hypoxic conditions. The cell volume of astrocytes, as measured by 3-O-methyl-[14C]-D-glucose uptake, was increased. These observations suggested cell membrane dysfunction. The malondialdehyde level of hypoxic cultures increased two-fold after 24 h of hypoxia. Verapamil (0.5 mM), furosemide (1 mM), indomethacin (1 mM), MgCl2 (10 mM), and mannitol (10 mM) reduced but never completely abolished the release of lactate dehydrogenase from hypoxic astrocytes. These data suggest multifactorial causes for severe injury in hypoxic astrocytes.

Elevation by atrial natriuretic factors of cyclic GMP levels in astroglia-rich cultures from murine brain

Atrial natriuretic factors, peptide hormones originally found in the heart, slowly but strongly elevate the level of cyclic GMP in primary astrocyte-rich cultures derived from brains of newborn rats or mice but not in neuron-rich cultures prepared from embryonic rat brain. In the absence of a phosphodiesterase inhibitor, a plateau level of cyclic GMP is obtained within 10 min. In the presence of the inhibitor 3-isobutyl-1-methylxanthine, the concentration of cyclic GMP continues to rise, even after 30 min. The elevation of the level of cyclic GMP in response to atrial natriuretic factor is much more pronounced in the rat cultures than the mouse cultures. Even at peptide concentrations of 1 microM, plateaus of the concentration-response curves are not yet reached. The potencies of the active peptides vary over a range of approximately 1.5 orders of magnitude, with atriopeptins II and III and auriculin A being the most potent ones. These results suggest (a) that atrial natriuretic factors may regulate functions of glial cells, most likely of astrocytes, in brain and (b) that such cultures may be useful tools in defining such astroglial functions.

Regulation by dibutyryl cyclic AMP of carnosine synthesis in astroglia-rich primary cultures kept in serum-free medium

The synthesis of carnosine (beta-Ala-His) by astroglia-rich primary cultures was much higher if the cells were cultivated in Ham's nutrient mixture F-12 than if they were grown in Dulbecco's modified Eagle's medium. Carnosine synthesis was not affected by the presence of insulin, transferrin, phorbol myristate acetate, or dexamethasone. However, dibutyryl cyclic AMP and other agents that can, directly or indirectly, activate cyclic AMP-dependent protein kinases strongly lower the rate of carnosine synthesis. The depression of carnosine synthesis was dependent on the concentration of dibutyryl cyclic AMP. The effect was maximal (approximately 80% inhibition) in cultures preincubated with 1 mM dibutyryl cyclic AMP for 4 days. The adenylate cyclase activator forskolin, the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, and 8-bromo-cyclic AMP caused the same depression as dibutyryl cyclic AMP, whereas neither butyrate nor dibutyryl cyclic GMP elicited any effect.

Histamine H1-receptors mediate phosphoinositide hydrolysis in astrocyte-enriched primary cultures

Astrocyte-enriched primary cultures of newborn rat brain hemispheres, prelabeled with [3H]inositol, accumulated [3H]inositol phosphate but not [3H]inositol bis- and tris-phosphate, after exposure to histamine for 60 min in the presence of 10 mM LiCl. The response to histamine was not a function of contaminating meningeal fibroblasts since no accumulation of [3H]inositol phosphate was elicited by histamine in meningeal cultures. The stimulation of phosphoinositide hydrolysis by histamine in astrocytes was dose-dependent (EC50 = 1.7 microM, maximal effect = 345% over basal levels) and was mimicked by several H1-receptor agonists. The use of selective receptor antagonists confirmed that the histamine response was the result of activation of H1-receptors. The histamine-induced [3H]inositol phosphate accumulation was completely abolished by omission of Ca2+ from the incubation medium. Astrocyte membranes specifically bound the radiolabeled H1-antagonist, [3H]mepyramine with an affinity (Kd = 5.9 nM) and a density of binding sites (Bmax = 113 fmol/mg protein) similar to rat brain. These results demonstrate the presence of functional histamine H1-receptors in rat brain astrocytes and suggest a role for histamine as a neuromodulator of astrocyte function.

Protein kinase C in primary astrocyte cultures: cytoplasmic localization and translocation by a phorbol ester

The distribution of calcium-activated, phospholipid-dependent protein kinase (protein kinase C) in supernatant and particulate fractions of primary cultures of rat astrocytes and its translocation by a phorbol ester were studied. We observed that 91% of protein kinase C activity in astrocytes was in the supernatant fraction, as measured by lysine-rich histone phosphorylation assay. Attempts to uncover latent activity in the particulate fraction were unsuccessful. Approximately 75% of the supernatant protein kinase C activity could be translocated to the particulate fraction by prior treatment (30-60 min) of the cultures with 100 nM 12-O-tetradecanoyl-phorbol 13-acetate (TPA), but not with 4 alpha-phorbol, an inactive phorbol ester. Investigation of endogenous substrates for protein kinase C showed that TPA treatment brought about an increase in phosphorylation in membrane proteins and a decrease in phosphorylation of supernatant proteins. These findings indicate that the distribution of protein kinase C in astrocytes differs substantially from that in whole brain tissue, where approximately two-thirds of the protein kinase C activity is associated with the particulate fraction. Because protein kinase C is concentrated in the cytosol of astrocytes and most of this activity can be translocated to membranes, astrocytes may be particularly well-suited to respond to signals that activate phosphoinositide-linked receptors in brain.

Location of angiotensin II binding sites on neuronal and glial cells of cultured mouse spinal cord: an autoradiographic study

Cultures of mouse spinal cord were used to visualize binding sites for [125I]angiotensin II (AII) by autoradiography. Visualization by light microscopy shows that neurones, but also glial cells possess angiotensin II binding sites which are located both on soma and processes. These findings open a new field of investigation for the understanding of the physiological significance of AII in the CNS.

Protein phosphorylation in primary astrocyte cultures treated with and without dibutyryl cyclic AMP

Protein phosphorylation was investigated in primary rat astrocyte cultures treated with and without dibutyryl cyclic AMP. Astrocytes maintained in dibutyryl cyclic AMP for several weeks displayed increased phosphate incorporation in 5 protein bands (55, 52, 45, 43 and 28 kDa) while incorporation in one band (42 kDa) was decreased. Phosphate incorporation in several other protein bands was unchanged. Calcium-dependent phosphate incorporation was also altered by prior exposure of the cells to dibutyryl cyclic AMP: addition of calcium to broken cell preparations resulted in increased incorporation in 75, 53 and 52 kDa while decreased incorporation occurred in 100 kDa. These differences in protein phosphorylation may be related to the previously reported biochemical and morphological changes brought about by dibutyryl cyclic AMP and may provide insights into the mechanisms of reactive gliosis.

A Model for Cellular Neurotoxicology

A model system for cellular neurotoxicology involving the use of primary cultures from various brain regions, is presented. Morphine affected protein synthesis in astroglial enriched cultures dose-dependently showing increases and decreases with time. A differential sensitivity to morphine was considered probably to be due to the cellular heterogeneity and specialisation found even in small brain regions.

Beta-adrenergic receptors of cerebellar astrocytes in culture: intact cells versus membrane preparation

Experiments were carried out to assess: the influence of culture conditions on the expression of beta-adrenergic receptors in intact glial cells from the central nervous system; and the extent to which quantitation of receptor sites in membrane preparations reflects the receptor population of the whole cells they are derived from. Cerebellar astrocytes were chosen for this study since essentially one receptor subtype, the beta 2 one, is present in adult cerebellum. Intact, attached cerebellar astrocytes exhibit only one class of binding sites for the beta-adrenergic antagonist, [3H]CGP 12177. Replating of the astrocytes after a few days of culture in vitro induces an up-regulation of the receptors. This effect is particularly important when astrocytes are maintained for 6 days in the presence of horse serum, a condition that favors cellular differentiation. Only 30-50% of the beta-adrenergic receptors of the intact cells can be detected on membrane preparations. When membranes are prepared from astrocytes grown either in the presence of horse serum or under chemically controlled medium (i.e. under differentiation promoting conditions) two classes of binding sites for [125I](-)-iodocyanopindolol are revealed. Several hypotheses, mainly related to the morphology of the cells, may provide an explanation for such differences. Studies of the pharmacological specificity of receptors of membrane fractions show that cerebellar astrocytes cultured in vitro exhibit both beta 1 and beta 2 receptor subtypes. The beta 1 subtype receptors are slightly more abundant when astrocytes are grown in fetal calf serum (FCS), a condition under which they exhibit a polygonal, poorly differentiated morphology. When culture conditions favor cellular differentiation, more receptors of the beta 2 subtype are seen, which can be related to what is observed in the adult in vivo where the astrocytes exhibit a differentiated morphology.

Calcium-activated, phospholipid-dependent protein kinase and protein substrates in primary cultures of astrocytes

Phosphoinositide-linked transmembrane signaling in the brain involves calcium-activated, phospholipid-dependent protein kinase (protein kinase C), but little is known about the glial contribution to this system. We observed that phosphorylation of several proteins in a cytosol fraction of rat astrocytes in primary culture was increased by the addition of calcium and phosphatidylserine. These agents also stimulated phosphate incorporation into lysine-rich histone, a substrate for protein kinase C. Addition of diacylglycerol, an activator of protein kinase C, further increased histone phosphorylation, whereas polymyxin B, an inhibitor of protein kinase C, blocked the stimulatory effect of calcium and phosphatidylserine. Based on enzyme units per mg protein, the activity of protein kinase C in astrocytes appears similar to that in whole brain cytosol. These results indicate that astrocytes display protein kinase C activity and suggest that the glial enzyme may be an important component of the receptor-linked phosphoinositide response system in the brain.

Atrial natriuretic hormones raise the level of cyclic GMP in neural cell lines

Atriopeptin III and related atrial natriuretic peptide hormones strongly elevate the level of cyclic GMP in three neural tumor cell lines. At peptide concentrations of 1 microM clear-cut plateaus of the dose-response curves are not yet reached. Atriopeptin III increases the intracellular concentration of cyclic GMP to a maximum in the course of 30-40 min. The effect of atriopeptin III on the cellular cyclic GMP level is independent of the concentration of extracellular Ca2+ and is not affected by the Ca2+ ionophore A23187. These results suggest (1) that atrial natriuretic hormones may play an important role in the nervous system, and (2) that cultured neural cells may be useful tools in the elucidation of the mechanisms of action of these hormones.

Activation of beta-adrenergic receptors stimulates release of an inhibitory transmitter from astrocytes

Activation of beta-adrenergic receptors on astrocytes in primary cell culture results in the release of taurine, an inhibitory transmitter. Taurine release occurs via a cyclic AMP-mediated intracellular pathway, because (a) taurine release and intracellular cyclic AMP accumulation have similar pharmacologies and time courses of activation and (b) N6,O2'-dibutyryl cyclic AMP stimulates release with a time course similar to that observed with the beta-adrenergic agonist isoproterenol. These results describe a previously unrecognized physiological function of astrocytes in the CNS-receptor-mediated release of the neuroactive amino acid taurine. This observation indicates that astrocytes may function as local regulators of neuronal activity.

Immunocytochemical characterization of neuron-rich primary cultures of embryonic rat brain cells by established neuronal and glial markers and by monospecific antisera against cyclic nucleotide-dependent protein kinases and the synaptic vesicle protein synapsin I

Primary cell cultures derived from embryonic rat brain were characterized by immunocytochemical methods using established cell markers and monospecific antisera against cyclic nucleotide-dependent protein kinases and the synaptic vesicle protein, synapsin I. The cultures contained predominantly neurons, few astroglial cells and no oligodendroglial cells, based on immunocytochemical studies of the distribution of neuron-specific enolase, glial fibrillary acidic protein, myelin basic protein and galactocerebroside. Subsequently, the immunocytochemical localization of synapsin I, the cyclic GMP-dependent protein kinase and the various subunits of cyclic AMP-dependent protein kinase was determined. Synapsin I, a substrate for both the cyclic AMP- and Ca2+/calmodulin-dependent protein kinases, appeared particularly useful as a specific neuronal marker in primary cultures. Both immunocytochemical and immunoblotting techniques readily detected synapsin I in neuron-rich embryonic brain cultures, but indicated that synapsin I was absent from glia-rich primary cultures of newborn rat brain cells which lacked neurons. The intracellular localization of synapsin I in neurons changed markedly during the time of cell culture. In the first 10 days of cell culture, synapsin I appeared to be confined to neuronal cell bodies, whereas later it shifted to a patchy distribution in neuronal processes, perhaps indicating the transport of synapsin I in synaptic vesicles from the compartment of protein synthesis to its final synaptic location. Within neuron-rich embryonic cultures, the regulatory subunit (R-II) and the catalytic subunit (C) of cyclic AMP-dependent protein kinase appeared to be highly concentrated in neurons examined immunocytochemically. However, biochemical experiments demonstrated that R-II and C were also present in non-neuronal cell types of brain cell primary cultures. Cyclic GMP-dependent protein kinase, a marker protein for cerebellar Purkinje cells and for smooth muscle cells, was not detected immunocytochemically in neuron-rich cultures of embryonic brain cells, suggesting that Purkinje cells and smooth muscle cells were either absent from or not sufficiently developed in these cultures.

Oxidative metabolism in cultured astroglial cells from rat brain

Growth, morphology, glutamine synthetase activity, cytochrome C oxidase activity and respiratory activity of rat brain cultures enriched in astrocytes were studied during four weeks in culture. Two different polarographic methods were used for measurement of respiratory activity: one newly developed perfusion method leaving the cellular monolayer morphologically intact and still attached to the culture dish, and one traditional stirring method involving the removal of cells from the culture vessel. Regardless of the method used, a stable respiratory activity was registrated throughout the four weeks of culturing. Also the cytochrome C oxidase activity remained unchanged. In perfusion all absolute values for respiration were found to be higher than those obtained with the stirring method. The use of the stirring technique resulted in a doubling of oxygen consumption upon succinate addition. No such effect was seen in perfusion. It can thus be concluded that the removal of cultured astrocytic cells from their substratum alters their respiratory activity and their response to added substrates.

Octopamine reduces potassium permeability of the glia that form the insect blood-brain barrier

Octopamine caused only a slight reduction in the potential across the perineurial glia of the cockroach, had no effect upon sodium-induced changes in potential, but did reduce potassium-induced changes (at 10(-7) M and above). The effect of 10(-7) M octopamine was accompanied by a rise in resistance, was mimicked by 10(-7) M synephrine and blocked by 10(-6) M phentolamine. Transperineurial potassium permeability was reduced by 10(-6) M octopamine. It is concluded that octopamine receptors mediate a reduction in potassium conductance of the basolateral membrane of these glia, and a reduction in the net potassium permeability of the barrier.

Desensitization of beta-receptors on primary astrocyte cultures by norepinephrine but not by tricyclic antidepressants

Primary astrocyte cultures from neonatal rat cerebral hemispheres were treated chronically for up to 3 weeks with the tricyclic antidepressants amitryptyline (AMT) or desipramine (DMI), or acutely with AMT and DMI added at the same time as the agonist, norepinephrine (NE). AMT and DMI were added at concentrations from 10(-9) to 10(-5) M. Both types of treatment did not decrease the increase in cyclic AMP (cAMP) content of these cells in response to a 10 min exposure to 10(-5) M NE. Chronic exposure to the antidepressants also did not affect stimulation of cAMP by isoproterenol (iso) in both rat and mouse primary astrocyte cultures. In contrast to the lack of effect of the tricyclic antidepressants pretreatment of the cultures with 10(-5) M NE resulted in total inhibition of the cAMP response after 2 h, with a 50% decrease occurring in about 45 min. This is similar to the agonist-induced desensitization of the beta-receptor-adenylate cyclase system seen in many other cells. This effect could, in part, be a direct response to increased intracellular cAMP since pretreatment with 0.25 and 1.0 mM N6-2'-O-dibutyryl cAMP (DBcAMP) also resulted in total inhibition of the cAMP response after 4 h. Receptor labelling experiments using [125I]cyanopindolol showed no decreases in apparent binding sites up to 3 h after exposure to 10(-5) M norepinephrine, suggesting that the rapid desensitization of the cAMP response was primarily due to an uncoupling of the receptor from the adenyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic quantitation of beta-adrenergic receptors on neural cells in primary cultures. II. Comparison of receptors on various types of immunocytochemically identified cells

We have developed a microcomputer-based video method to quantify neurotransmitter receptors on single, immunocytochemically labeled cultured cells. This method has been applied to determine whether beta-adrenergic receptors are more numerous on neurons, astroglia, oligodendroglia or fibroblasts in primary neural cell cultures, and to assess the heterogeneity of receptor expression within a single cell type. Dissociated cells from perinatal rat cerebral cortex were grown in very sparse cultures on polylysine-coated glass slides. The cultured cells were fixed and permeated, then stained with fluorescently labeled immunocytochemical markers for astroglia (glial fibrillary acidic protein), fibroblasts (fibronectin), oligodendroglia (galactocerebroside) or neurons (A2B5). beta-Adrenergic receptors were labeled with [125I]pindolol or [125I]cyanopindolol, and dry-mount autoradiography was carried out on the fixed cells. Cells were identified according to their morphology and cell-type specific staining, then autoradiographic grains associated with the defined cells were visualized by reflected polarized light microscopy and counted with a microcomputer-based video digitizing system. Using this technique, we have determined that fibroblasts have less than 15% of the number of beta-adrenergic receptors expressed by polygonal astroglia, whereas oligodendroglia and neurons had no detectable binding of 125I-labelled ligands. This suggests that in these mixed neural cell cultures, the great majority of beta-adrenergic receptors are associated with astroglia. Furthermore, we determined that process-bearing astroglia have less than 5% of the number of beta-adrenergic receptors expressed by polygonal astroglia. Since process-bearing astroglia are thought to be derived from polygonal astroglia, these results suggest that the beta-adrenergic receptor is lost from this population of astroglia during development.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular Ca2+ elevation induced by a neurotransmitter in a glial cell clone

By fluorometry using a Ca2+-indicator Quin 2, we found an elevation of intracellular Ca2+ concentration ([Ca2+]i) in response to an application of serotonin in a rat clonal glial cell (C6BU-1). The [Ca2+]i rise depended on the dose of applied serotonin and the level of environmental Ca2+. The possibility was suggested that neuron-glia interactions might be controlled by a receptor-coupled [Ca2+]i-regulation system.

Neuroblastoma membranes inhibit isoproterenol-stimulated rise of cAMP in glioma cells

C6 glioma cells respond to beta-adrenergic agonists (isoproterenol) with a transient rise in intracellular cyclic adenosine monophosphate level. This beta-responsiveness of C6 cells is inhibited by the presence of a plasma membrane fraction, which has a five- to six-fold purification of membrane markers, showed a greater inhibition of beta-responsiveness in C6 cells than any other subcellular fractions of B104 cells. The inhibitory effector(s) is apparently associated with integral membrane structure(s) since ionic extraction and treatment with chelating agents did not remove the effect from the particulate membrane fraction. The effector is probably proteinaceous in nature as judged by its susceptibility to inactivation by heat and protease treatment. The data indicate that neither adenylate cyclase nor phosphodiesterase enzyme is likely to be directly involved in mediating the beta-nonresponsiveness of C6 cells.

Cell culture as models for studying neural functions

Two cell culture systems were used for studies of neural functions in vitro. A neuronal hybrid cell line (neuroblastoma x glioma hybrid cells) and primary glial-rich cultures of newborn murine brain. The level of cyclic AMP in both systems is regulated by two groups of hormones, those that stimulate and those that inhibit formation of cyclic AMP. Among the inhibitory hormones active on the hybrid cells are opioids. Therefore the cells are being used in the elucidation of action of opioids. The list of stimulating and inhibitory hormones regulating the primary glial-rich cultures includes several peptide hormones such as the gastrointestinal peptides secretin and vasoactive intestinal peptide, the calcaemic hormones parathyrin and calcitonin, adrenocorticotropin and melanotropins, and somatostatin. Noradrenaline (via alpha- and beta-adrenergic receptors) and adenosine (via A1 and A2 receptors) inhibit and stimulate cyclic AMP synthesis in the primary glial-rich cultures. Bradykinin slowly hyperpolarizes the hybrid cells and elicits formation of cyclic GMP. Both responses desensitize rapidly. Substance P increases the permeability of hybrid cells for Na+, as measured by using 14C-guanidinium as substitute for Na+. Hybrid cells actively accumulate taurine, an amino acid that appears to fulfill important functions in the nervous system. The transport of taurine across the plasma membrane is highly specific for and strictly dependent on Na+. The pumped station hypothesis of taurine action in the nervous system views taurine gradient plus taurine carrier as a transport system for the elimination of sodium from neurons during phases of high neuronal activity.

Melanin: the organizing molecule

The hypothesis is advanced that (neuro)melanin (in conjunction with other pigment molecules such as the isopentenoids) functions as the major organizational molecule in living systems. Melanin is depicted as an organizational "trigger" capable of using established properties such as photon-(electron)-phonon conversions, free radical-redox mechanisms, ion exchange mechanisms, and semiconductive switching capabilities to direct energy to strategic molecular systems and sensitive hierarchies of protein enzyme cascades. Melanin is held capable of regulating a wide range of molecular interactions and metabolic processes primarily through its effective control of diverse covalent modifications. To support the hypothesis, established and proposed properties of melanin are reviewed (including the possibility that (neuro)melanin is capable of self-synthesis). Two "melanocentric systems"--key molecular systems in which melanin plays a central if not controlling role--are examined: 1) the melanin-purine-pteridine (covalent modification) system and 2) the APUD (or diffuse neuroendocrine) system. Melanin's role in embryological organization and tissue repair/regeneration via sustained or direct current is considered in addition to its possible control of the major homeostatic regulatory systems--autonomic, neuroendocrine, and immunological.

Corticotropin peptides and melanotropins elevate the level of adenosine 3':5'-cyclic monophosphate in cultured murine brain cells

Cell cultures derived from mouse and rat brain and consisting mainly of astroblasts are known to respond to several hormones by increasing or decreasing their intracellular concentration of cyclic AMP. In the present study these cultures were analyzed for their susceptibility to various additional hormonal and other neuroactive compounds. Only the peptides of the corticotropin (ACTH)/melanotropin (MSH) family were found active. Their potency for elevating the intracellular level of cyclic AMP decreases in the sequence (values for the half-maximally stimulating concentrations, EC50, in parentheses) ACTH-(1-24) (10 m) greater than alpha-,beta-MSH (30 nm) greater than ACTH (greater than or equal to 100 nm) gamma-MSH, ACTH-(1-10), -(4-10), -(4-11) (greater than or equal to 0.5 microM). The lack of additivity of the maximal effects of the peptides suggests that they all act at the same receptor. The stimulation exerted by these peptides is partially suppressed by hormones known to inhibit cyclic AMP formation in that culture, i.e., noradrenaline (acting via an alpha-adrenergic receptor), adenosine (acting via an A1 receptor), and somatostatin. It is concluded that the receptors for the ACTH/MSH peptides and the inhibitory hormones are located on the same cells, presumably the astroblasts. The maximal response to ACTH and alpha- and beta-MSH depends strongly on the age of culture. The results are discussed in view of the facts that (1) peptides of the ACTH/MSH family affect behavior and learning in animals, and (2) ACTH and alpha-MSH occur in brain.

Ionic mechanism of a hyperpolarizing 5-hydroxytryptamine effect on leech neuropile glial cells

The ionic mechanism of a membrane effect of 5-hydroxytryptamine (5-HT) on neuropile glial (NG) cells in ganglia of the medicinal leech was investigated with conventional single-barrelled microelectrodes. Control experiments were made with double-barrelled ion-selective microelectrodes. 5-Hydroxytryptamine hyperpolarized the NG-cell membrane and increased the conductance considerably. Methysergide, a potent 5-HT antagonist, blocked the 5-HT-induced hyperpolarization completely. When leech ganglia were superfused with physiological bathing media free of 5-HT, the NG-cell membrane conductance returned to the original value, but the membrane potential recovered only partially from the hyperpolarization in most experiments. In glial membranes artificially depolarized by means of constant-current injection, the amplitude of the 5-HT response increased. The amplitude decreased with membrane hyperpolarization and reversed at - 73 mV, close to the potassium equilibrium potential. The reversal potential changed by 52 mV when the extracellular potassium concentration was altered by a factor of 10. We conclude that 5-HT increases the potassium conductance of NG-cell membranes.

Bradykinin induces hyperpolarizations in rat glioma cells and in neuroblastoma X glioma hybrid cells

The effect of the nonapeptide bradykinin on the membrane potential of permanent cell lines from neural origin was studied. A hyperpolarizing response of 10-30 s duration was produced when bradykinin was iontophoretically applied onto polyploid rat glioma cells (clone C6-4-2). Starting from the resting membrane potential the peak value of the hyperpolarizing response was reached within 0.5-1.5 s. Then the potential returned more slowly to the original value. The hyperpolarization was associated with an approximately 50% decrease in membrane resistance. Neither Na+ nor Cl- seemed to be important for the hyperpolarizing response, since bradykinin elicited similar hyperpolarizations in cells exposed to media in which Na+ or Cl- were replaced by choline or isethionate, respectively. Ca2+ fluxes are unlikely to be involved, since the addition of D600 did not affect the hyperpolarizations induced by bradykinin. However, a 10-fold increase in the concentration of K+ in the medium reduced the amplitude of the hyperpolarization by 40 mV. Thus, the hyperpolarization induced by bradykinin is associated with decrease in membrane resistance which is likely to be caused by an increased K+-conductance. The glioma cells showed a desensitization upon repeated application of bradykinin. However, the sensitivity of the cells to bradykinin was restored after 3-8 min of incubation in the absence of bradykinin. Since an antagonist of bradykinin is not known, the specificity of the action of bradykinin is difficult to assess. Nevertheless, the hyperpolarizing response to bradykinin appears to be specific insofar as other peptides, i.e. lutoliberin, thyroliberin, neurotensin, substance P and apamin, exerted no effect on the membrane potential of the glioma cells. Bradykinin-elicited hyperpolarizations with characteristics similar to those described above could also be demonstrated in neuroblastoma X glioma hybrid cells, but not in multinucleated fibroblast cells.