Co-existence between receptors, carriers, and second messengers on astrocytes grown in primary cultures

Estrogen actions on neuroendocrine glia

Astrocytes are the most abundant cells in the central nervous system (CNS). It appears that astrocytes are as diverse as neurons, having different phenotypes in various regions throughout the brain and participating in intercellular communication that involves signaling to neurons. It is not surprising then that astrocytes in the hypothalamus have an active role in the CNS regulation of reproduction. In addition to the traditional mechanism involving ensheathment of neurons and processes, astrocytes may have a critical role in regulating estrogen-positive feedback. Work in our laboratory has focused on the relationship between circulating estradiol and progesterone synthesized de novo in the brain. We have demonstrated that circulating estradiol stimulates the synthesis of progesterone in adult hypothalamic astrocytes, and this neuroprogesterone is critical for initiating the LH surge. Estradiol cell signaling is initiated at the cell membrane and involves the transactivation of metabotropic glutamate receptor type 1a (mGluR1a) leading to the release of intracellular stores of calcium. We used surface biotinylation to demonstrate that estrogen receptor-alpha (ERalpha) is present in the cell membrane and has an extracellular portion. Like other membrane receptors, ERalpha is inserted into the membrane and removed via internalization after agonist stimulation. This trafficking is directly regulated by estradiol, which rapidly and transiently increases the levels of membrane ERalpha, and upon activation, increases internalization that finally leads to ERalpha degradation. This autoregulation temporally limits membrane-initiated estradiol cell signaling. Thus, neuroprogesterone, the necessary signal for the LH surge, is released when circulating levels of estradiol peak on proestrus and activate progesterone receptors whose expression has been induced by the gradual rise of estradiol during follicular development.

Neurosteroids, neuroactive steroids, and symptoms of affective disorders

Neurosteroids (NS) are steroids synthesized by the brain. Neuroactive steroids (NAS) refers to steroids that, independent of their origin, are capable of modifying neural activities. NAS bind and modulate different types of membrane receptors. The gamma amino butyric acid (GABA) and sigma receptor complexes have been the most extensively studied. Oxidized ring A reduced pregnanes, tetrahydroprogesterone (THP), and tetrahydrodeoxycorticosterone (THDOC) bind to the progesterone intracellular receptor (PR), and in this way can also regulate gene expression. Animal experimentation showed that salient symptoms of depression, viz., anxiety, sleep disturbances, and memory and sexual dysfunctions, are modulated by NAS. In turn, psychotropic drugs modulate NS and NAS levels. NS levels as well as NAS plasma concentrations change in patients with depression syndromes, the levels return to normal baseline with recovery, but normalization is not necessary for successful therapy. Results from current studies on the evolution of nervous systems, including evolutionary developmental biology as well as anatomical and physiological findings, almost preclude a categorical classification of the psychiatric ailments the human brain succumbs to. The persistence in maintaining such essentialist classifications may help to explain why up to now the search for biological markers in psychiatry has been an unrewarding effort. It is proposed that it would be more fruitful to focus on relationships between NAS and symptoms of psychiatric disorders, rather than with typologically defined disorders.

Possible involvement of 5α-reduced neurosteroids in adrenergic and serotonergic stimulation of GFAP gene expression in rat C6 glioma cells

Influence of adrenergic and serotonergic stimulation on glial fibrillary acidic protein (GFAP) gene expression in rat C6 glioma cells was first examined as an in vitro model experiment for investigating the neuronal regulation of glial cell differentiation. Stimulation of these cells with isoproterenol and serotonin elevated GFAP mRNA levels followed by an increase in its protein contents, thus suggesting that both adrenergic and serotonergic stimulation might induce the differentiation of the glioma cells. In addition, progesterone and its 5alpha-reduced metabolite dihydroprogesterone also elevated GFAP mRNA levels in rat C6 glioma cells, consistent with their stimulatory actions on GFAP gene expression observed in rat astrocytes. Further studies showed that the elevation of GFAP mRNA levels induced by isoproterenol and serotonin as well as progesterone was abolished by pretreatment of the glioma cells with finasteride, an inhibitor of 5alpha-reduced steroid production. Moreover, the stimulatory actions of isoproterenol and serotonin on GFAP gene expression were inhibited by pretreatment with a GABA(A) receptor antagonist bicuculline and a progesterone receptor antagonist RU486. These findings suggest that both adrenergic and serotonergic stimulation may indirectly activate GFAP gene expression probably through the production of 5alpha-reduced steroid metabolites in rat C6 glioma cells, proposing the possibility that 5alpha-reduced neurosteroids may play a potential role in the neuronal regulation of glial cell differentiation.

Nerve growth factor induces elevation of steroid 5alpha-reductase mRNA levels in rat C6 glioma cells through expression of transcription factor Egr-1

Steroid 5alpha-reductase type 1 (5alpha-R), the enzyme converting progesterone and other steroid hormones to their 5alpha-reduced metabolites, has been shown to be localized in both neuronal and glial cells, and this enzyme in glial cells has previously been reported to be activated either by co-culturing with neuronal cells or by adding the conditioned medium of neuronal cells, thus suggesting that neuronal activity may be implicated in the regulation of neurosteroid metabolism in brain. In the present study, to investigate a potential role of neurotrophic factors in the mechanism regulating the production of neuroactive 5alpha-reduced steroid metabolites, the direct action of NGF on 5alpha-R gene expression was examined by measuring the steady-state levels of 5alpha-R mRNA levels in rat C6 glioma cells. Exposure of the glioma cells to NGF increased both 5alpha-R mRNA and its protein levels, and induced the transient elevation of Egr-1 mRNA levels prior to the expression of 5alpha-R mRNA in the cells. Furthermore, NGF failed to induce any significant elevation of 5alpha-R mRNA levels in the cells pretreated with Egr-1 antisense oligodeoxynucleotides. These findings indicate that NGF induces the elevation of 5alpha-R gene expression in the glioma cells through the expression of transcription factor Egr-1, proposing the possibility that NGF, and probably other neurotrophic factors as well, may play a potential role in the regulation of 5alpha-reduced steroid production as one of the factors mediating the intercellular communication between neuronal and glial cells in the brain.

Formation and effects of neuroactive steroids in the central and peripheral nervous system

This chapter summarizes several observations that emphasize the importance of neuroactive steroids in the physiology of the central and peripheral nervous systems. A new, and probably important, concept is emerging: Neuroactive steroids not only modify neuronal physiology but also intervene in the control of glial cell functions. The data presented here underscore that (1) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgens) and nonclassical steroid receptors [gamma-aminobutyric acid type A (GABAA) receptor], (2) in many instances, the actions of hormonal steroids are not due to their native molecular forms but to their 5 alpha- and 3 alpha,5 alpha-reduced metabolites, (3) several neuroactive steroids exert dramatic actions on the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22), and (4) the effects of steroids and of their metabolites might have clinical significance in cases in which the rebuilding of the peripheral myelin is needed (e.g., aging, peripheral injury).

Regional differences in glial cell modulation of synaptic transmission

Metabolic integrity of glial cells in field CA1 of the guinea pig hippocampus is critical to maintenance of synaptic transmission (Keyser and Pellmar [1994] Glia 10:237-243). To determine if this tight glial-neuronal coupling is equally important in other brain regions, we compared the effect of fluoroacetate (FAC), a glial specific metabolic blocker, on synaptic transmission in field CA1 to synaptic transmission in area dentata (DG). FAC was significantly more effective in decreasing synaptic potentials in CA1 than in DG. A similar regional disparity in the FAC-induced decrease in ATP levels was evident. Isocitrate, a glial specific metabolic substrate, prevented the FAC-induced synaptic depression in both CA1 and DG. The results suggest that glia of CA1 and dentate respond differently to metabolic challenge. Modulation of this glial-neuronal coupling could provide a regionally specific mechanism for synaptic plasticity. Additionally, site-specific glial-neuronal interactions can impact on a variety of physiological and pathophysiological conditions.

Reactive astrocyte formation in vivo is regulated by noradrenergic axons

Beta adrenergic receptor antagonists greatly reduce reactive astrocyte formation induced by neuronal degeneration. To test the hypothesis that the density of noradrenergic innervation is a factor in the regulation of astrocytosis, we measured glial fibrillary acidic protein (GFAP) optical density after neuronal injury in central nervous system (CNS) regions with permanent noradrenergic sprouting or norepinephrine (NE) depletion. The injury model employs the injection of Ricinus communis lectin into a cranial or peripheral nerve to destroy CNS neurons without the blood-brain barrier disruption and lymphocyte infiltration associated with contusive or surgical lesions. We took advantage of the lack of an NE transporter in the terminals of certain classes of noradrenergic axons to produce noradrenergic sprouting in the trigeminal motor nucleus (MoV) with neonatal 6-hydroxydopamine (6-OHDA) treatment and to produce depletion of NE in the spinal cord dorsal horn with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4) administration. In each of these regions, GFAP optical density in the region of reactive astrocytes on the Ricin lectin-treated side was compared with the untreated contralateral (control) side in animals with NE hyperinnervation or NE depletion. GFAP density was increased about 55% in the injured NE-hyperinnervated MoV and was decreased about 35% in the injured NE-depleted dorsal horn. The degree of reactive astrocyte formation to injury is known to vary in different regions of the CNS, and our results suggest that differences in noradrenergic innervation may contribute to this variation. Along with earlier findings that beta-adrenergic receptor blockade reduces reactive astrocyte formation, these data indicate that the noradrenergic innervation is a factor in the degree of astrocyte reactivity following injury.

Testosterone and progesterone metabolism in the central nervous system: cellular localization and mechanism of control of the enzymes involved

This paper summarizes the most recent data obtained in the authors' laboratory on the metabolism of testosterone and progesterone in neurons and in the glia. 1. The activities of 5 alpha-reductase (the enzyme that converts testosterone into dihydrotestosterone; DHT) and of 3 alpha-hydroxy steroid dehydrogenase (the enzyme that converts DHT into 5 alpha-androstane-3 alpha, 17 beta-diol; 3 alpha-diol) were first evaluated in primary cultures of neurons, oligodendrocytes, and type-1 and type-2 astrocytes, obtained from the fetal or neonatal rat brain. The formation of DHT and 3 alpha-diol was evaluated incubating the different cultures with labeled testosterone or labeled DHT as substrates. The results obtained indicate that the formation of DHT takes place preferentially in neurons; however, also type-2 astrocytes and oligodendrocytes possess considerable 5 alpha-reductase activity. A completely different localization was observed for 3 alpha-hydroxysteroid dehydrogenase; the formation of 3 alpha-diol appears to be prevalently, if not exclusively, present in type-1 astrocytes; 3 alpha-diol is formed in very low yields by neurons, type-2 astrocytes, and oligodendrocytes. Moreover, the results indicate that, in type 1 astrocytes, both 5 alpha-reductase and 3 alpha-HSD are stimulated by coculture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function. 2. Subsequently it was shown that, similarly to what happens when testosterone is used as the substrate, 5 alpha-reductase, which metabolizes progesterone into 5 alpha-pregnane-3,20-dione, (DHP), shows a significantly higher activity in neurons than in glial cells; however, also type-1 and type-2 astrocytes as well as oligodendrocytes possess some ability to 5 alpha-reduce progesterone. On the contrary, 3 alpha-hydroxysteroid dehydrogenase, the enzyme which converts DHP into 5 alpha-pregnane-3 alpha-ol-20-one (THP), appears to be present mainly in type-1 astrocytes; much lower levels of this enzyme are present in neurons and in type-2 astrocytes. At variance with the previous results obtained using androgens as precursors, oligodendrocytes show considerable 3 alpha-hydroxysteroid dehydrogenase activity, even if this is statistically lowe than that present in type-1 astrocytes. The existence of isoenzymatic forms of the enzymes involved in androgen and progesterone metabolism is discussed.

Metabolism of steroids in pure cultures of neurons and glial cells: role of intracellular signalling

In the brain, the 5 alpha-reductase converting testosterone (T) is present both in neurons and in glial cells, even if it prevails in neurons; the 3 alpha-hydroxysteroid-dehydrogenase (3 alpha-HSD), the enzyme converting dihydrotestosterone (DHT) into 3 alpha-diol, is particularly concentrated in type 1 astrocytes. In glial cells, since the 5 alpha-reductase is activated by a cAMP analogue, PKA seems to be involved in the control of this enzyme, postulating that nervous inputs utilizing cAMP as the second messenger might modify the activity of this enzyme in glial cells. Moreover, the results indicate that, in type 1 astrocytes, both the 5 alpha-reductase and the 3 alpha-HSD are stimulated by the co-culture with neurons and by the addition of neuron-conditioned medium, suggesting that secretory products released by neurons might intervene in the control of glial cell function.

Glutamate-activated ionic currents in cultured astrocytes from trout: evidence for the occurrence of non-N-methyl-D-aspartate receptors

Glutamate-induced currents were recorded from cultured trout astrocytes with the whole-cell variation of the patch-clamp technique. Ninety percent of the tested cells were directly depolarized by the amino acid neurotransmitter in a concentration-dependent manner. The depolarizing effect was due to an inward current that reversed near 0 mV and was accompanied by a noise increase, indicating the opening of an ion channel. Ion substitution experiments revealed that the glutamate-induced current was mainly carried by sodium ions but not chloride or calcium ions. The glutamate-induced response could be mimicked by the neuronal glutamate receptor subtype agonists kainate and quisqualate, while N-methyl-D-aspartate was without detectable effect.

Glial alpha 2-receptors probably inhibit the high-affinity uptake of noradrenaline into astrocytes in the rat brain in vivo

The effect of alpha 2-receptor blockage on the extraneuronal turnover of noradrenaline (NA) has been studied in the intact rat brain. Tropolone and yohimbine, along with reserpine or desmethylimipramine, were given 30 min after intracerebroventricular injection of [7-3H]NA, i.e. after the tracer had been stored or inactivated. Tropolone given alone did not change the fractions of 3H-activity recovered as [3H]NA from hypothalamus, septum, striatum and pons-medulla, but in the presence of yohimbine improved the [3H]NA recovery in all areas except pons-medulla. The maximum effect was seen in the hypothalamus of reserpine-treated rats. Since the alpha 2-autoreceptors were blocked, the increased [3H]NA recovery does not reflect a down-regulated neuronal NA turnover. Instead it seems to show that a fraction greater than normal of neuronally released NA had been taken up into astrocytes and remained unmetabolized if catechol-O-methyltransferase was inactive. It is assumed that yohimbine enabled the protective tropolone effect by blocking astrocytic alpha 2-receptors that otherwise, either by itself or by antagonizing beta-receptor-induced hyperpolarization or cAMP formation, had impaired parameters that stimulate the high-affinity NA Uptake 1 of astrocytes (e.g. membrane potential, Na+,K(+)-ATPase) or control the gap junction permeability in the glial syncytium.

Adenosine A2B receptor signalling is altered by stimulation of bradykinin or interleukin receptors in astroglioma cells

The human astroglioma cell D384 possesses adenosine A2B receptors coupled to the formation of cyclic AMP. These cells also possess bradykinin B2 receptors coupled to phospholipase C and consequent increases in intracellular calcium and protein kinase C. Interleukin 1 beta causes an increase in c-fos, AP-1 transcriptional activity and an increased expression of several genes including NGF, but the initial signalling events are unknown. Bradykinin causes a rapid decrease in A2B receptor mediated cAMP formation, via a mechanism that involves calcium, but not cGMP, and appears to depend upon a direct decrease in adenylyl cyclase. Il-1 beta causes a slowly developing (18-24 h) increase in A2B receptor signalling. The results indicate that adenosine effects in glial cells, believed to be important in neuroprotection, are modified in the short and long-term by inflammatory mediators.

Synaptic transmission in the hippocampus: critical role for glial cells

The importance of glial cells in controlling the neuronal microenvironment has been increasingly recognized. We now demonstrate that glial cells play an integral role in hippocampal synaptic transmission by using the glial-specific metabolic blocker fluoroacetate (FAC) to selectively inhibit glial cell function. FAC inhibits evoked intracellular postsynaptic potentials (PSPs; IC50 = 39 microM) as well as population PSPs (IC50 = 65 microM) in field CA1 of the guinea pig hippocampal slice. Spontaneous synaptic transmission is concurrently decreased. These effects are time and dose dependent. ATP concentrations in glial but not neuronal elements are also significantly reduced with FAC treatment. Simultaneous application of the metabolic substrate isocitrate with FAC prevents both the reduction in glial ATP concentrations and the decrease in evoked PSPs. Given that isocitrate is selectively taken up by glia, these data further support a glial specific metabolic action of FAC. Additionally, FAC has no postsynaptic effects as peak responses to iontophoretically applied glutamate are unchanged. However, the decay of both iontophoretic and evoked PSPs are prolonged following FAC treatment suggesting inhibition of glutamate uptake may contribute to the FAC-induced depression of synaptic potentials. These results show, for the first time, that glial cells are critical for maintenance of synaptic transmission and suggest a role for glial cells in the modulation of synaptic efficacy.

Kinectin, a major kinesin-binding protein on ER

Previous studies have shown that microtubule-based organelle transport requires a membrane receptor but no kinesin-binding membrane proteins have been isolated. Chick embryo brain microsomes have kinesin bound to their surface, and after detergent solubilization, a matrix with an antibody to the kinesin head domain (SUK-4) (Ingold et al., 1988) bound the solubilized kinesin and retained an equal amount of a microsome protein of 160-kD. Similarly, velocity sedimentation of solubilized membranes showed that kinesin and the 160-kD polypeptide cosedimented at 13S. After alkaline treatment to remove kinesin from the microsomes, the same 160-kD polypeptide doublet bound to a kinesin affinity resin and not to other proteins tested. Biochemical characterization localized this protein to the cytoplasmic face of brain microsomes and indicated that it was an integral membrane protein since it was resistant to alkaline washing. mAbs raised to chick 160-kD protein demonstrated that it was absent in the supernatant and concentrated in the dense microsome fraction. The dense microsome fraction also had the greatest amount of microtubule-dependent motility. With immunofluorescence, the antibodies labeled the ER in chick embryo fibroblasts (similar to the pattern of bound kinesin staining in the same cells) (Hollenbeck, P. J. 1989. J. Cell Biol. 108:2335-2342), astroglia, Schwann cells and dorsal root ganglion cells but staining was much less in the Golgi regions of these cells. Because this protein is a major kinesin-binding protein of motile vesicles and would be expected to bind kinesin to the organelle membrane, we have chosen the name, kinectin, for this protein.

Resting and reactive astrocytes express adrenergic receptors in the adult rat brain

Adrenergic receptor subtypes were localized in situ and in cells isolated from the trigeminal motor nucleus and several other brain regions. To study receptor expression in reactive astrocytes, motor neuron degeneration and a glial reaction were induced in the trigeminal motor nucleus by the injection of the toxic lectin Ricin communis into the trigeminal motor root. Autoradiography following incubation of tissue sections in the alpha 1-ligand 125IBE 2254 (HEAT) or the beta-ligand 125Iodocyanopindolol (ICYP) showed a decrease in alpha 1- and an increase in beta-adrenergic receptor binding in the region of neuronal degeneration and gliosis. Glial hypertrophy, rather than hyperplasia, appears to be mainly responsible for the increased beta-binding, since inhibition of mitosis with cytosine arabinofuranoside only partially blocked elevations of beta-adrenergic receptor binding and GFAP immunolabelling in reactive astrocytes. More direct evidence for the expression of adrenergic receptors in normal and reactive astrocytes was obtained by combined autoradiography and immunohistochemistry of cells dissociated from the cerebral cortex, striatum, cerebellum, and trigeminal motor nucleus of adult rats. More than 88% of GFAP-positive astrocytes showed varying densities of beta-adrenergic receptor binding. In each region, the beta 2-subtype was proportionally greater than the beta 1-subtype. Astrocytes also expressed a significant density of alpha 1-receptors. Trigeminal motor neurons did not show beta-receptor binding, but had a density of alpha 1-receptors tenfold greater than astrocytes. A model for the role of astrocytes in adrenergic receptor-mediated modulation of trigeminal motor neuron excitability is discussed.

Intracellular signalling systems controlling the 5 alpha-reductase in glial cell cultures

Glial cells are able to metabolize testosterone into DHT through the action of the enzyme 5 alpha-reductase. DHT may be further processed to 3 alpha-diol by the 3 alpha-hydroxysteroid-dehydrogenase. The aim of this study was to analyze if a modulation of two second messenger systems might be able to modify the 5 alpha-reductase and the 3 alpha-hydroxysteroid-dehydrogenase activities present in glial cells. To this purpose, the formation of DHT has been measured in rat glial cell cultures after different time of exposure to TPA, 4 alpha-Ph, an active and an inactive phorbol ester respectively, and 8-Br-cAMP. The results obtained indicate that the formation of DHT is not modified by the addition of phorbol esters. On the contrary, a statistically significant increase of 5 alpha-reductase activity, over control levels, has been observed after 6, 12, and 24 h of incubation with 8-Br-cAMP (10(-3) M). The effect of the cAMP analogue appears to be specific for the 5 alpha-reductase, since the 3 alpha-hydroxysteroid-dehydrogenase did not show any variation after exposure to the drug. In conclusion, the present data suggest that proteinkinase A (PKA) might be involved in the control of the 5 alpha-reductase in glial cells. It is postulated that nervous inputs utilizing cAMP as the second messenger might modify the activity of this enzyme in glial cells.

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.

Adenosine receptor-induced cAMP changes in D384 astrocytoma cells and the effect of bradykinin thereon

In human D384 astrocytoma cells, cyclic AMP accumulation can be conveniently studied after labelling of the adenosine triphosphate pool (15 fmol cell-1) with [3H]adenine. In this study, adenosine had a biphasic effect on cyclic AMP accumulation, which was scarcely altered by blocking adenosine uptake and metabolism. Low concentrations of adenosine led to an inhibition of cyclic AMP accumulation, and higher concentrations led to stimulation. No effect of adenosine on cyclic AMP was observed unless phosphodiesterase was inhibited by rolipram. The A1 receptor antagonist DPCPX attenuated the inhibitory phase of adenosine response, and enhanced the cyclic AMP accumulation induced by adenosine analogues. The cyclic AMP accumulation was stimulated by NECA greater than ADO greater than CGS 21680 greater than CV 1808 greater than CPA greater than or equal to CHA, indicating mediation by A2 receptors. The stimulatory effect of NECA was much more effectively blocked by the combined A1 and A2 receptor antagonist CGS 15943 (KB 4 nmol l-1) than by the A1 antagonist DPCPX (KB 110 nmol l-1). Treatment of the cells with pertussis toxin (0.2 microgram ml-1 for 2.5 h) potentiated the cyclic AMP response to adenosine analogues significantly. The cyclic AMP response to NECA was enhanced by the protein kinase C activator phorbol dibutyrate even after pertussis toxin treatment. By contrast, nanomolar concentrations of bradykinin, which increases Ca(2+)-levels and protein kinase C activity in D384 cells, reduced NECA-induced cyclic AMP accumulation in control and pertussis toxin-treated cells. Thus, D384 cells possess both A1 and A2 adenosine receptors influencing cyclic AMP in opposite directions.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of energy metabolism by alpha-adrenergic agonists in primary cultures of astrocytes

Noradrenaline effects on glucose oxidation were studied in primary cultures of astrocytes. CO2 formation from labeled glucose was enhanced in the presence of noradrenaline. The stimulatory effect by noradrenaline was exerted both on lactate formation (approximately 20%) and on tricarboxylic acid activity (CO2 production from glutamate) (approximately 40%). The effect was, at least partly, exerted on the alpha-ketoglutarate dehydrogenase step. The EC50 value for noradrenaline on lactate formation was significantly lower (60 nM) than that on oxidative metabolism (1,900 nM). Studies with specific adrenergic agonists and antagonists showed that various receptor subtypes are involved. Thus, the effect on lactate formation was mediated exclusively by stimulation of an alpha 1 receptor whereas oxidative metabolism was enhanced by both alpha 1 and alpha 2 receptor stimulation. No effects were exerted by beta receptor agonists or antagonists.