Cellular localization and regulation of glutamine synthetase in primary cultures of brain cells from newborn mice

Spontaneous glutamate release by a "fibrous"-like cerebellar astroglial cell clone

To investigate the role of astrocytes in the metabolism of glutamate, the neurotransmitter of the granule cells of the cerebellar cortex, we have analyzed various parameters related to the synthesis of glutamate in astroglial cell clones that may be the in vitro counterparts of the cerebellar astrocytes. The "fibrous"-like clone spontaneously released large quantities of glutamate, even in the absence of glutamine in the culture medium, but did not release alanine. In contrast, the "Golgi-Bergmann"-like cells released alanine but not glutamate, whereas the "velate-protoplasmic"-like astrocytes released little glutamate and alanine. However, the glutamate oxaloacetate transaminase and glutamate pyruvate transaminase activities of the three astroglial cell lines, measured in the direction of glutamate synthesis, were comparable. In addition, the "velate protoplasmic" and "Golgi-Bergmann" clones did not consume glutamine present at 2 mM in the culture medium. These data suggest that the different types of in vivo cerebellar astrocytes may have distinct roles regarding glutamate-glutamine metabolism.

Regulation of glutamine synthetase synthesis and activity by glucocorticoids and adrenoceptor activation in astroglial cells

Glucocorticoids are known to induce the synthesis and activity of glutamine synthetase (GS; EC 6.3.1.2.) in astroglial cells. In the present paper, noradrenaline (NA), in itself ineffective upon GS regulation, potentiated GS activity in astroglial primary cultures in the presence of the glucocorticoid dexamethasone, the GS activity being further stimulated in the presence of glutamate (glu). Thus, adrenoceptor activation might interact with the glucocorticoid induced GS activity in astroglial primary cultures.

Electrophysiological and biochemical characterization of a continuous human astrocytoma cell line with many properties of well-differentiated astrocytes

Astrocytes comprise about 25% of the cellular volume of the brain, and their main function is to maintain homeostasis of the neuronal environment. These cells are commonly identified on the basis of their membrane electrical properties and the presence of specific proteins. We have characterized the human astrocytoma cell line designated UC-11MG and have shown these cells have many of the traits of differentiated astrocytes. Many of the UC-11MG cells have a large resting membrane potential, averaging -74 mV. The slope of the Em vs log [K]o cuve was 58.5 mV per decade [K]o. The cells were inexcitable when exposed to brief depolarizing current pulses. The astrocytoma traits are virtually identical to those previously reported for normal astrocytes. The astrocytoma cells also express glutamine synthetase activity which is considered specific to astrocytes among brain cells. Previous work had also demonstrated the presence of other astrocyte markers glial fibrillary acidic protein and S-100 protein in the UC-11MG cells. The steady-state ion transport properties of Na+, Cl-, and K+ were also characterized in these cells, and the rates of efflux were found to be similar to those in other astrocytes, with the major difference being the presence of a second kinetic compartment in the UC-11MG cells. From this work, we conclude that the UC-11MG cell line displays prominent features associated with differentiated astrocytes, and may provide an excellent model system for the study of human astrocytes.

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.

Expression of high molecular weight astroglial extracellular proteins is altered by growth environment

Conditioned medium from primary rat cortical glia was analyzed with respect to the composition of the secreted high molecular weight protein species. Developmental characteristics of astroglia are affected by growth in the presence and absence of serum. These growth conditions had a pronounced effect on the extracellular protein profile, cellular morphology, and cell substratum adhesion. Cells cultured in defined serumless medium did not express certain proteins expressed in the presence of serum but rather synthesized proteins specifically stimulated by the defined serumless environment. A morphological change from flat amorphous to a contracted fibrous network having an increased affinity for self-self cellular adhesion rather than adhesion to the surface of the tissue culture dish was also stimulated by the defined serumless medium environment. A comparison of the extracellular proteins secreted by the rat C6 glioma and the rat PC12 cell demonstrated the cell-specific nature of the primary glial proteins.

Regulation of glutamate and GABA transport by adrenoceptors in primary astroglial cell cultures

In astrocytes grown in primary cultures from cerebral cortex of neonatal rats, alpha 1-adrenoceptors regulate the active uptake of glutamate followed by an activation of glutamic oxaloacetate transaminase (GOT; EC 2.6.1.1.) and a slight activation of glutamine synthetase (GS; EC 6.3.1.2.) activity. The beta-adrenoceptors regulate the active uptake of GABA, and this is followed by an activation of gamma-aminobutyric acid alpha-ketoglutarate transaminase (GABA-T; EC 2.6.1.19.). The data suggest that astrocyte adrenoceptors may modulate neurotransmitter induced neuronal excitability.

Thyroid hormone metabolism in neuron-enriched primary cultures of fetal rat brain cells

The metabolism of thyroxine (T4) by cultures of embryonic-rat brain cells grown in a chemically defined medium was studied. Cells in these cultures were predominantly neurons, characterized by the developmental increase of the binding of [3H]flunitrazepam to the high-affinity (0.67 nM) benzodiazepine neuronal receptors. The cultures also contained astrocytes, characterized by immunological studies using an anti-glial fibrillary acidic protein (GFAp) and by the increase in glutamine synthetase (GS). Incubation of the cells, in situ, with 125I-labelled 3,5,3'-triiodothyronine (T3) showed the presence of a single class of high-affinity nuclear receptors for T3 with a maximal binding capacity of 270-470 fmol T3/mg DNA and a Kd of 63 +/- 13 pM. Cells incubated in situ with 50 pM [125I]T4 actively metabolized the hormone. The major metabolite, 3,3',5'-triiodothyronine (rT3) (159 +/- 43 fmol/4 h/mg DNA), was almost completely released into the medium. T3 was a minor metabolite (77 +/- 3 fmol/4 h/mg DNA), 75% of which accumulated in the cells. Of this T3, 35% was bound to the nuclear receptors after 4 h of incubation. In vitro assays showed that the 5'-deiodinase activity increased during culture and the 5-deiodinase decreased slightly. Cytosine-arabinoside (ARAc) treatment of the cultures reduced the DNA content per culture dish, corresponding to a fall in the number of GFAp-positive cells (astrocytes) and to a decrease in GS. A small increase in the number of benzodiazepine sites was observed. ARAc treatment markedly reduced the T3 production (14.5 +/- 0.7 fmol/4 h/mg DNA) and did not change the rT3 production. We suggest that T4 is metabolized to T3 in astrocytes, taken up by neurons and binds to their nuclear receptors.

Differential modulation of glutamate metabolizing enzymes in mouse and chick cultured glial cells by insulin

The effect of physiological concentrations of insulin (2 and 20 ng/ml) on glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities were compared in mouse and chick glial cells in culture. Addition of insulin to serum-containing medium increased the level of GS and GDH activities in glial cells prepared from 14-15-day-old embryonic mice. A similar but less pronounced effect was observed with glia derived from newborn mouse brain. In absence of serum, addition of insulin had no effect on the tested enzymes. The effects of insulin on enzymatic activities of glial cells from 14-15-day-old embryonic chick brain hemispheres were, in contrast, quite different. A significant decrease of GS activity was induced by the hormone, only in the absence of serum. Conversely, the presence of serum enhanced an inhibitory effect of insulin toward chick GDH. The different effects of insulin and the different serum dependence observed for the mammalian and the avian model could reflect fundamental chemical differences between both species as indicated by immunoelectrophoretic analysis. However, it can be concluded that insulin may be a physiological factor regulating glial maturation and amino acid neurotransmitter metabolism in the central nervous system.

Cellular location of glutamine synthetase and lactate dehydrogenase in oligodendrocyte-enriched cultures from rat brain

Glial cells were isolated from 1-week-old rat brain and cultured in a serum-free medium supplemented with the hormones insulin, hydrocortisone, and triiodothyronine. After 1 week in culture the cell population consisted mainly of galactocerebroside-positive cells (GC+; oligodendrocytes), the remainder of the cells being positive for glial fibrillary acidic protein (GFAP+; astrocytes). Oligodendrocytes were selectively removed from the cultures by complement-mediated cytolysis. The activities of glutamine synthetase and of various marker enzymes were measured in the nonlysed cells remaining after complement treatment of the cultures and in the culture medium containing proteins of the lysed cells. We found that the cellular activity of glutamine synthetase decreased in parallel with the lysis of GC+ cells and that the activity of glutamine synthetase in the supernatant increased. The activity of glycerol-3-phosphate dehydrogenase, a marker enzyme for oligodendrocytes, was no longer detectable in complement-treated cultures and the activity of glutamine synthetase was markedly lowered, whereas the activity of lactate dehydrogenase was as high as in untreated cultures. The location of glutamine synthetase both in oligodendrocytes and in astrocytes was confirmed by double-label immunocytochemistry with antisera against glutamine synthetase, GC, and GFAP. We conclude that in this culture system glutamine synthetase is expressed in both types of glial cells and that the activity of lactate dehydrogenase is at least one order of magnitude higher in astrocytes than in oligodendrocytes.

Induction of glutamine synthetase in rat astrocytes by co-cultivation with embryonic chick neurons

Co-cultivation of confluent rat astrocyte cultures with embryonic chick neurons resulted in induction of glutamine synthetase activity in the astrocytes. This induction of glutamine synthetase in astrocytes by neurons was independent of induction by hydrocortisone and forskolin, but was dependent on the length of co-cultivation and the number of neurons present in the co-culture. Cycloheximide and actinomycin D inhibited the induction of glutamine synthetase in astrocytes by neurons, whereas cytosine arabinoside had no apparent effect. Results suggest that this induction of glutamine synthetase in astrocytes is mediated by cell contact with neurons and may represent a specific neuronal and glial interaction.

Induction of N-glycosylation activity in cultured embryonic rat brain cells

Developmental changes in protein N-glycosylation activity have been studied using cultures of dissociated fetal rat brain cells as an in vitro model system. These cultures undergo an initial phase of neurite outgrowth and cell proliferation (4-6 days in culture), followed by a period of cellular differentiation. N-Glycosylation activity has been measured by assaying the incorporation of [2-3H]mannose into dolichol-linked oligosaccharides and glycoprotein over a period of 1-25 days in culture. This study revealed a marked induction of N-glycosylation activity beginning at approximately 1 week of culture. [2-3H]Mannose incorporation into the oligosaccharide-lipid intermediate fraction and glycoprotein reached maximal values between 12 and 16 days of culture and declined thereafter. The major dolichol-linked oligosaccharide labeled by the brain cell cultures was shown to be Glc3Man9GlcNAc2 by HPLC analysis. Parallel incorporation studies with [3H]leucine showed that the increase in protein N-glycosylation was relatively higher than a concurrent increase in cellular protein synthesis observed during the induction period. Maximal labeling of glycoprotein corresponded to the period of glial differentiation, as indicated by a sharp rise in the marker enzymes, 2',3'-cyclic nucleotide 3'-phosphohydrolase (an oligodendroglial marker) and glutamine synthetase (an astroglial marker). The results describe a developmental activation of the N-glycosylation pathway and suggest a possible relationship between N-linked glycoprotein assembly and the growth and differentiation of glial cells.

Are astroglial cells involved in morphine tolerance?

Morphine gives rise to a cascade of events in the nervous system affecting, among others, neurotransmitter metabolism. Tolerance develops for various effects shortly after administration of the drug. Also, physical dependence develops and can be demonstrated by precipitation of withdrawal reactions. Biochemical events in nervous tissue have been extensively studied during morphine treatment. This overview will focus upon brain protein metabolism since macromolecular events might be of importance for development of long-term effects, such as tolerance and physical dependence. Both dose- and time-dependent changes in brain protein synthesis and the syntheses of specific proteins have been demonstrated after morphine treatment, although methodological considerations are important. Different experimental models (animal and tissue culture models) are presented. It might be interesting to note that astroglial protein synthesis and the secretion of proteins to the extracellular medium are both changed after morphine treatment, these having been evaluated in astroglial enriched primary cultures and in brain tissue slices. The possibility is suggested that proteins released from astroglial cells participate in the communication with other cells, including via synaptic regions, and that such communication might of significance in modifying the synaptic membranes during morphine intoxication.

Concentrations of physiologically important metal ions in glial cells cultured from chick cerebral cortex

Energy dispersive x-ray fluorescence and atomic absorption spectroscopy were used to determine the concentrations of Mg, Ca, Mn, Fe, Zn, and Cu in primary cultures of astroglial cells from chick embryo cortex in chemically defined serum-free growth medium. The intracellular volume of cultured glia was determined to be 8.34 microliter/mg protein. Intracellular Mn, Fe, Zn, and Cu in these cells were ca. 10-200 microM, or 20-200 times the concentrations in the growth medium. Mg2+ was 7 mM in glial cells, only four-fold higher than in growth medium. Glutamine synthetase (GS), compartmentalized in glia, catalyzes a key step in the metabolism of neurotransmitter L-glutamate as part of the glutamate/glutamine cycle between neurons and glia. Hormones (insulin, hydrocortisone, and cAMP) added to growth medium differentially altered the activity of GS and the intracellular level of Mn(II), but not Mg(II). These findings suggest the possibility that glutamine synthetase activity could be regulated in brain by the intracellular levels of Mn(II) or the ratio of Mn(II)/Mg(II), which may in turn be controlled indirectly by means of transport processes that respond to hormones or secondary metabolic signals.

Effects of medium glutamine, glutamate, and ammonia on glutamine synthetase activity in cultured mouse astroglial cells

Mouse astroglial cells were grown during the last week of culture in either glutamine-free or glutamine-containing medium. The addition of cortisol to the glutamine-containing medium resulted in a doubling of astroglial glutamine synthetase (GS) activity. Withdrawal of glutamine from the medium resulted in a 50% elevation of GS and addition of cortisol to such a medium resulted in a further increase in GS which was not additive to glutamine withdrawal. Both in glutamine-free and glutamine-containing medium, the addition of glutamate resulted in a depression of both basal and cortisol induced GS activity. The simultaneous addition of ammonia plus glutamate to the culture medium ameliorated the glutamate mediated depressive effects on cortisol induced but not basal GS activity. Glutamine withdrawal from the culture medium resulted in an astroglial protein deficit. The addition of ammonia to the medium considerably reduced this deficit and the addition of glutamate completely eliminated this protein deficit.

Peptide uptake by astroglia-rich brain cultures

Uptake of carnosine has been investigated in astroglia-rich primary cultures derived from brains of newborn mice. It could be demonstrated that carnosine is not degraded by these cells but rapidly taken up in an energy- and sodium-dependent process. Uptake and release of carnosine by these cells were found to be mediated by a saturable, high-affinity transport system with apparent kinetic constants of Km = 50 microM and Vmax = 22.7 nmol X h-1 X mg protein-1. Uptake of carnosine is strongly inhibited by other dipeptides as well as by various oligopeptides, e.g., Leu-enkephalin. However, uptake of the radiolabeled tripeptide D-Ala-L-Ala-L-Ala was not observed. Radiolabeled Leu-enkephalin also did not accumulate intracellularly, even if degradation of the peptide was prevented by use of peptidase inhibitors. These results suggest that uptake of carnosine is catalyzed by a dipeptide-specific transport system with broad substrate specificity. With neuronal cells in primary culture, uptake of carnosine or other peptides was not observed.

Synergistic action of thyroid hormone, insulin and hydrocortisone on astrocyte differentiation

We report here on the synergistic regulation of astrocyte development by 3 hormones: thyroid hormone (TH), insulin, and hydrocortisone (HC). Their effect, in a defined serum-free media, on astrocyte morphology, on glia fibrillary acidic protein (GFAP) immunostaining pattern, and on glutamine synthetase (GS) was investigated. TH transformed the flat, polygonal astrocytes into process-bearing cells. This effect was accentuated by insulin, which by itself had no effect on astrocyte morphology. The morphological transformations were accompanied by changes in the pattern of GFAP immunostaining which indicated a more organized and directed cytoskeleton arrangement in the TH-insulin treated cultures. Over 95% of the cells in the culture expressed GFAP. All 3 hormones regulated GS levels. TH increased GS levels by 50% and insulin raised its levels by 3-fold. While having no effect on astrocyte morphology, HC increased GS levels by 3.7-fold in both the hormone-free and insulin-supplemented medium. HC acted synergistically with insulin in its action on GS bringing about a 12-fold increase in the enzyme activity. In contrast, TH did not interact with insulin and was additive with HC in its action on GS. The continuous presence of insulin and TH was required to maintain their morphological and GS effect, suggesting that these hormones might not only be important for astrocyte differentiation, but later on for astrocyte function as well. Since astrocytes interact with and affect neurons and oligodendrocytes, the findings reported here might have bearing on the development and function of these other brain cells as well.

Some metabolic effects of ammonia on astrocytes and neurons in primary cultures

Some metabolic effects on primary cultures of neurons or astrocytes were studied following acute or chronic exposure to pathophysiological concentrations (usually 3 mM) of ammonia. Three parameters were investigated: (1) 14CO2 production from 14C-labeled substrates [glucose, pyruvate, branched-chain amino acids (leucine, valine, isoleucine), and glutamate]; (2) interconversion between glutamate and glutamine; and (3) incorporation of label from labeled branched-chain amino acids into proteins. Neither acute nor chronic exposure to ammonia had any effect on 14CO2 production from [U-14C]glucose in astrocytes and neurons, whereas under certain conditions 14CO2 production from [1-14C]pyruvate in astrocytes was inhibited by ammonia. Production of 14CO2 from [1-14C]branched-chain amino acids was inhibited by acute, but stimulated by chronic, exposure to ammonia (3 mM) in astrocytes, with less effect in neurons. Production of 14CO2 from [1-14C]glutamate in both astrocytes and neurons was inhibited by acute exposure to ammonia. In astrocytes, glutamate levels tended to decrease and glutamine levels tended to increase following acute exposure to ammonia; in neurons, both glutamine and glutamate levels decreased. Protein content (per culture dish) increased in astrocytes but not in neurons, after chronic exposure to ammonia, possibly as a result of enhanced protein synthesis and/or by inhibition of protein degradation.

Thyroid hormone metabolism by glial cells in primary culture

The metabolism of thyroxine (T4) and triiodothyronine (T3) in cultured glial cells was studied in situ. Cultures were prepared from fetal rat brain and grown for the last 4 days in a chemically defined medium (CDM). They contained astrocytes and oligodendrocytes as shown by the enzyme markers, glutamine synthetase and 2',3'-cyclic nucleotide phosphohydrolase. These cells contained high affinity (22-33 pM), limited capacity (120-230 fmol/mg DNA) nuclear receptors for T3. Cells incubated in situ with 50 pM [125I]T4 actively metabolized the hormone. The major iodothyronine produced was T3 (220-570 fmol/4 h/mg DNA). About 70% accumulated in the cells, the remainder was released into the medium. Within the cells, T3 was partly bound to the nuclear receptors (16.5-20 fmol/mg DNA). Reverse T3 (rT3) was a minor metabolite (30-45 fmol/4 h/mg DNA); it was almost completely released into the medium. The half-life of [125I]T3 (50 pM) was found to be about 15 h. These results show that, in situ, glial cell cultures containing astrocytes and oligodendrocytes grown in CDM actively deiodinate T4 to T3 and degrade T3 rather slowly.

Heterogeneity of a cultured neoplastic glial line. Establishment and characterisation of six clones

Six clones have been established from a tumorigenic glial cell line (VMDk P497) originally derived from a spontaneous mouse astrocytoma. The clones express dissimilar morphological, antigenic, kinetic and chromosomal properties, thereby indicating the heterogeneity of the parent culture line.

Specific insulin-mediated regulation of glutamine synthetase in cultured chick astroglial cells

The expression of glutamine synthetase (GS; L-glutamate ammonia ligase; EC 6.3.1.2) in primary cultures of chick astroglial cells and neurons grown in a chemically defined medium, with and without insulin added, was investigated. An inhibitory effect of insulin toward GS activity, and specific to chick astroglial cells, was observed. Neurons in culture were not sensitive to the hormone effect. Modulation of the activating effect of hydrocortisone on glial GS by insulin was also observed. The data suggest that insulin contributes to the regulation of the metabolism of amino acid neurotransmitters via its effect on GS.

Co-cultivation of astroglial enriched cultures from striatum and neuronal containing cultures from substantia nigra

A co-cultivation system was developed with neuron-containing (neuron-specific enolase (NSE) positive) primary cultures from the substantia nigra of 15 to 17-day old embryonic rats which were grown 1 mm apart from astroglial-enriched (glial fibrillary acidic protein (GFAp) positive) primary cultures from the striatum of neonatal rats. The astroglial cells went through a morphological differentiation with extension of processes after co-cultivation with the immunohistochemically-identified neuronal cells. The astroglial-enriched striatum cultures showed a higher active uptake of 3H-L-glutamate after co-cultivation for one week, compared to control cultures from striatum. Vmax (nmol X mg protein-1 X min-1 X was 58.4 +/- 8.3 after co-cultivation and 37.2 +/- 6.3 for control cultures. The glutamine synthetase (GS) activity was slightly increased after co-cultivation. The validity and specificity of the results were ensured. The data suggest that astroglial cells in a primary culture are influenced by co-cultivation with fetal neuron containing cultures resulting in morphological differentiation, and increases in 3H-L-glutamate uptake and GS activity.

Effect of removal of glutamine and addition of dexamethasone on the activities of glutamine synthetase, ornithine decarboxylase and lactate dehydrogenase in primary cultures of forebrain and cerebellar astrocytes

The regulation of glutamine synthetase (GS) and ornithine decarboxylase (ODC) was studied in primary cultures of two types of astrocytes derived from either newborn forebrain or 8-day-old cerebellum of the rat. In the 14-day-old cultures the specific activities of both these enzymes were about twice as great in forebrain astrocytes as in cerebellar astrocytes. Treatment with dexamethasone or removal of glutamine from the culture medium caused a marked increase in the specific activity of GS. The glutamine-mediated relative increase in GS activity was similar in both types of astrocytes. Removal of glutamine caused a transient reduction in ODC activity in the forebrain astrocytes, while in cerebellar astrocytes the activity remained markedly decreased throughout the period of glutamine deprivation. The severe reduction in ODC activity had relatively little effect on the cell numbers of protein content of the astrocyte cultures. The increase in GS activities, involving protein synthesis de novo, caused by removal of glutamine and by addition of dexamethasone, were additive and therefore probably mediated by different mechanisms. The induction of GS after glutamine removal was blocked by cycloheximide but not by alpha-amanitin, suggesting regulation at the post-transcriptional level. In contrast, the dexamethasone-mediated induction of GS appeared to be regulated at the transcriptional level, as it was markedly reduced by alpha-amanitin. None of these conditions had any effect on lactate dehydrogenase activity. Treatment with alpha-amanitin resulted in a complete suppression of the activity of ODC (a protein with a very short half life), in both the control and dexamethasone treated cultures. However, this enzyme activity was reduced only partially in astrocytes cultured in glutamine deficient medium, suggesting that under these experimental conditions the mRNA may be markedly stabilized in astroglial cells.

Brain peptides and glial growth. I. Glia-promoting factors as regulators of gliogenesis in the developing and injured central nervous system

Glia-promoting factors (GPFs) are peptides of the central nervous system which accelerate the growth of specific glial populations in vitro. Although these factors were first discovered in the goldfish visual system (Giulian, D., Y. Tomozawa, H. Hindman, and R. Allen, 1985, Proc. Natl. Acad. Sci. USA., 83:4287-4290), we now report similar peptides are found in mammalian brain. The cerebral cortex of rat contains oligodendroglia-stimulating peptides, GPF1 (15 kD) and GPF3 (6 kD), as well as astroglia-stimulating peptides, GPF2 (9 kD) and GPF4 (3 kD). The concentrations of specific GPFs increase in brain during periods of gliogenesis. For example, GPF1 and GPF3 are found in postnatal rat brain during a peak of oligondendroglial growth while GPF2 and GPF4 are first detected at a time of astroglial proliferation in the embryo. Stab wound injury to the cerebral cortices of rats stimulates astroglial proliferation and induces marked elevations in levels of GPF2 and GPF4. Our findings suggest that two distinct classes of GPFs, those acting upon oligodendroglia and those acting upon astroglia, help to regulate cell growth in the developing and injured central nervous system.

Co-cultivation of astroglial and neuronal primary cultures from rat brain

A technique is described for a two-cell co-cultivation system which permits in vitro evaluation of neuron-glia interactions. Primary astroglial enriched cultures from newborn rat cerebral hemispheres, striatum or cerebral cortex, grown for 3 days, were co-cultivated with primary neuron-containing cultures from 15- to 17-day rat embryo cerebral hemispheres, substantia nigra or brainstem, respectively, grown for 10 days on polylysine-coated surfaces. The neuronal cells were identified morphologically and immunohistochemically by antibodies to neuron-specific enolase. The two cultures were grown together for 7 days, separated by a U-formed 1 mm glass-rod. The results show that neurons exert a morphogenetic effect on astroglial cells in the form of extension of cell processes. The co-culture system allows investigation of potent local humoral interactions between astroglial cells and neurons.

Primary cultures from defined brain areas. III. Effects of seeding time on [3H]L-glutamate transport and glutamine synthetase activity

Primary astroglial-enriched cultures from various brain regions were studied with respect to age at seeding and time in culture and its effect on the [3H]L-glutamate transport capacity and glutamine synthetase (G.S.) activity. Three phylogenetically different rat brain areas were used: cerebral cortex, striatum and brainstem. There was a high-affinity and high-capacity [3H]glutamate uptake during development in all cultures studied with Km in the microM range and Vmax in the nmol range. Vmax was most prominent in cultures seeded from newborn rat cerebral cortex and striatum when grown for 2 weeks and in brainstem cultures seeded from 17-day-old rat embryos when grown for 4 weeks. There were no significant differences in Vmax comparing the cultures from the various brain regions, that is when seeded from 17-day-old embryos and grown for 2 or 3 weeks in culture. The G.S. activity was determined under similar culture conditions as above. The highest enzyme activities were found in cultures from cerebral cortex and striatum seeded from newborn and 7-day-old rats and grown for 2 or 3 weeks. G.S. activity increased during postnatal maturation in cerebral cortex, striatum and brainstem, the highest activity values being found in cerebral cortex and striatum. Vmax for glutamate uptake and G.S. activity showed many similarities in the respective cultures during cultivation. Both parameters were affected by age at seeding and time in culture. The differences between the cultures from the various brain regions, with lower values in cultures from brainstem, indicated a heterogeneity among astroglial cells in the brain regions studied.

The hormonal regulation of gene expression of glial markers: glutamine synthetase and glycerol phosphate dehydrogenase in primary cultures of rat brain and in C6 cell line

Increases in the mRNA levels of two neuroglial markers, glutamine synthetase (EC 6.3.1.2; GS) and glycerolphosphate dehydrogenase (EC 1.1.1.8; GPDH), were observed in hydrocortisone-treated cultures of astrocytes and oligodendrocytes, respectively, and in C6 cells by Northern blot analysis and in situ hybridization. In vitro transcription assays demonstrated increased GS transcription in isolated nuclei from hydrocortisone (HC)-treated primary cultures of astrocytes and C6 cells, relative to untreated cells. This increased transcription is reflected in increased GS mRNA levels in the cytoplasm and increased levels of GS protein synthesis. Sodium butyrate (NaB) blocked the glucocorticoid-mediated increase in GS transcription in the primary cultures of astrocytes but not in C6 cells. From our earlier observations (Kumar et al: J Neurochem 43:1455-1463, 1984) we found NaB in combination with HC to increase the levels of GS mRNA and GS protein synthesis (Weingarten et al: FEBS Lett 126:289-291, 1981). We now report that NaB, alone or in combination with HC, does not increase the rate of transcription, suggesting that NaB plays a role in post-transcriptional regulation of GS in C6. In addition, we report the presence of two distinct sizes of GS mRNA, 2.9 and 1.8 kb, in the primary cultures of astrocytes and C6 cells.

Selective increase of R-I subunit of cyclic AMP-dependent protein kinase in glia-rich primary cultures upon treatment with dibutyryl cyclic AMP

Levels of cyclic GMP-dependent protein kinase and of the subunits (R-I, R-II and C) of cyclic AMP-dependent protein kinase were determined in two types of neural primary cell cultures that were either treated or not treated with dibutyryl cyclic AMP. Astroglia-rich cell cultures from newborn rat brain responded to exposure to dibutyryl cyclic AMP by a 2-3-fold increase in the level of R-I subunit, as demonstrated by two radioimmunological procedures, while the levels of the other subunits (R-II and C) and of cyclic GMP-dependent protein kinase remained unaffected. In contrast, neuron-rich cell cultures from embryonic rat brain did not display such a change in the level of R-I subunit. Thus, the elevation in the level of R-I elicited by dibutyryl cyclic AMP in normal non-malignant neural cells in culture was restricted to glial rather than neuronal cells.

Muscarinic receptor subclasses in the chick embryo retina: influence of corticosterone treatment

The present study was performed on retinas of chick embryos receiving at day 8 of incubation an intracerebral injection of 0.02 microgram of corticosterone. We had previously shown with the use of [3H]quinuclidinylbenzilate [( 3H]QNB) that such treatment induced the appearance of two muscarinic binding sites in the treated retinas, whereas only one was detectable in the controls. In the present study we investigated muscarinic cholinergic receptor subclasses with agonist and antagonist binding. Agonist binding was studied by varying the concentrations of carbachol and acetylcholine (10(-9) M-10(-5) M) in the presence of a constant concentration (0.2 nM) of [3H]QNB. Two subpopulations of receptors were revealed, a high- and a low-affinity receptor, in both treated and control retinas. However, in the hormone-treated retinas, the two subpopulations significantly differed from the controls in their affinity and in their relative percentage among the total receptor population. Moreover, using pirenzepine, an antagonist known to have the capacity to distinguish between muscarinic cholinergic subclasses, two receptor subpopulations were found to be present in the hormone-treated retinas but a single one in the controls. It is suggested that hormone treatment can either induce the appearance of a new subclass of muscarinic cholinergic receptors or favor the maturation of a population of retinal cells having these receptors. Pirenzepine binding in retinas from intact embryos of 7, 9, and 11 days of incubation revealed one receptor subpopulation. Thus, these findings are more consistent with the hypothesis that corticosterone effects the target cells, either inducing changes in muscarinic receptor and/or modifying the receptor environment.

Selective culture of neurons from rat cerebral cortex: morphological characterization, glutamate uptake and related enzymes during maturation in various culture media

Dissociated cerebral cortex of fetal rat was grown in a serum-free, chemically defined medium (CDM) (containing insulin, progesterone, estradiol, transferrin, putrescine, selenium and 15 mM KCl) and compared with cultures grown in a medium containing 20% fetal calf serum (SCM). Neurons survived well using either medium, but in the serum-free medium the cellular population was exclusively neuronal (at 96%), while glial cells began to proliferate after one week in the SCM. The various cellular morphologies are described in the present report and the presence of immunological markers characteristic of neurons was investigated. Autoradiographic experiments have been performed after incubation with various putative neurotransmitters and we have shown the presence of a strikingly high proportion of glutamatergic neurons in these cultures. Glutamate high affinity uptake was also greatly increased in neuronal cultures maintained in a CDM compared to a SCM, especially in young cultures. The development of different enzymes involved in the metabolism of glutamate was also studied; the specific activity of glutaminase increased in culture and was found to be higher in a CDM than in a SCM, while the inverse was true for glutamine synthetase. The relative proportion of both enzymes in neurons compared to glial cells was opposite, as neuronal cultures had higher levels of glutaminase and glial cultures were enriched in glutamine synthetase activity. It seems that the proportion of glutamate-neurons increases when cultured in a CDM compared to a SCM and we suggest that this culture procedure may provide a purely neuronal population enriched in mature glutamatergic neurons. It may thus be useful for future in vitro studies on glutamate and GABA metabolism in neurons.

Glutamine synthetase and energy metabolism enzymes in cultured chick glial cells: modulation by dibutyryl cyclic AMP, hydrocortisone, and trypsinization

Modifications induced by dibutyryl cyclic AMP (diBcAMP) and hydrocortisone in the energy metabolism of chick astroblasts in culture have been investigated. DiBcAMP does not modify the levels of enolase, malate dehydrogenase (MDH), total lactate dehydrogenase (LDH) and glutamine synthetase (GS) activities in these cultured glial cells. However, these cells can be sensitized to the nucleotide analog by trypsinization before seeding. The phenomenon affects specifically GS activity and the synthesis, with an inhibitory effect, of the H subunit of LDH. Addition of hydrocortisone to the culture medium stimulates MDH and GS activities of the cells; trypsinization accentuates the stimulatory effect on GS. This hormone also modifies the synthesis of H and M subunits of LDH in a positive and negative way respectively. The phenomenon is increased by trypsin treatment. The present studies indicate clearly that hydrocortisone generates in cultured chick glial cells metabolic modifications qualitatively different from those obtained by diBcAMP. It is suggested that trypsin treatment, by altering some protein constituents of the cell surface, modifies the adhesiveness of different cell types present in the cell suspension after dissociation of the brain and thus leads to select, in culture, a specific astroglial subpopulation.

Observations on cell growth and regulation of glutamine synthetase by dexamethasone in primary cultures of forebrain and cerebellar astrocytes

Cell growth, development of glutamine synthetase and its regulation by glucocorticoids, were studied in primary cultures of two types of astrocytes derived from rat brain, one from newborn forebrain and another from either newborn or 8-day-old cerebellum. Cell number per dish increased linearly following an initial decrease due to removal of non-astrocytic cells, and after the second week reached a stationary phase in both types of cultures at more or less the same time, although the cell number in cerebellar cultures was about 35% lower than in forebrain cultures of the same age. At all ages, irrespective of cell density, the cerebellar astrocytes were larger in size than the forebrain astroglial cells. The developmental curves for glutamine synthetase activity were similar in vitro and in vivo; however, the increase in enzyme activity in vitro was significantly greater than in vivo and this difference was more marked in forebrain than in cerebellar cultures. Throughout the period studied the specific activity of glutamine synthetase was significantly higher in forebrain astrocytes than in cerebellar astroglial cells. Treatment with dexamethasone caused a marked increase in the specific activity of glutamine synthetase. However, in agreement with our previous in vivo findings, the steroid induction in forebrain astrocytes was significantly less than that in cerebellar astrocytes. In culture, both types of astrocytes remain responsive to the hormone for longer than in vivo. The differences in the biochemical properties of the forebrain and cerebellar astrocytes seem to be intrinsic, and not related to the cell density or to the purity of the cultures.

Cellular composition of primary cultures from cerebral cortex, striatum, hippocampus, brainstem and cerebellum

Primary cultures from newborn rat cerebral cortex, striatum, hippocampus, brainstem and cerebellum were grown for 14 days. There was a linear relationship between the amount of material seeded and the protein content of the respective culture. The amount of tissue material seeded was selected so that the different cultures reached confluence at 6-7 days and contained similar amounts of protein when 7 and 14 days old. The cellular content was evaluated by astroglial markers, such as the glial fibrillary acidic protein (GFAp; alpha-albumin) and the S-100 protein, and by markers for other cells expected to be in the cultures (14-3-2 protein, macrophage acidic protein (MAP), alkaline phosphatase, myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP]. Astroglial-like cells represented 60-70% of the cells present in the different cultures. Quantitation of GFAp (alpha-albumin) showed similar amounts to be present in cultures from cerebral cortex, hippocampus and striatum; however, on lower levels expressed in soluble proteins than in the corresponding brain regions of adult rats. Brainstem of adult rat contained large amounts of GFAp (alpha-albumin), while low levels were found in brainstem culture. Also, phagocytic cells (macrophages), endothelial-like cells, mesenchymal-like cells, ependymal-like cells and oligoblasts were found. Neither mature neurons, nor oligodendroglial cells were observed. It is concluded that although there might be some differences in the degree of maturation or in the cellular composition of the various cultures, they could serve as a good model system for studying the characteristics of astroglial cells from various brain regions.

Relation of cholesterol to astrocytic differentiation in C-6 glial cells

The relation of cellular cholesterol content to a biochemical expression of astrocytic differentiation was investigated in cultured C-6 glial cells. The astrocytic marker, glutamine synthetase, was studied. Cellular sterol content was perturbed with compactin, a specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase and, thereby, cholesterol biosynthesis. Depletion of cellular sterol resulted in 72 h in a more than twofold increase in glutamine synthetase activity. Production of various degrees of sterol depletion with different concentrations of compactin demonstrated a striking inverse relationship between glutamine synthetase activity and the cellular sterol/phospholipid molar ratio. That the effect of compactin, in fact, is mediated by depletion of sterol was shown further by prevention of the compactin-induced increase in synthetase activity by simultaneous addition of exogenous cholesterol. Moreover, addition of cholesterol alone to the culture medium led to both a decrease in glutamine synthetase activity and an increase in the sterol/phospholipid molar ratio. The possibility that the compactin-induced increase in glutamine synthetase activity is caused by an increase in synthesis of the enzyme was suggested by prevention of the increase by cycloheximide. The data suggest that astrocytic differentiation is stimulated by a decrease in cellular sterol content. When considered with our previous observation that oligodendroglial differentiation is inhibited by such a decrease, the findings suggest that cellular sterol content is a critical determinant of the direction of glial differentiation, i.e., whether along astrocytic or oligodendroglial lines.

Cellular heterogeneity in primary cultures of brain cells revealed by immunocytochemical localization of glutamine synthetase

The co-localization of glutamine synthetase, glial fibrillary acidic protein, galactocerebroside and fibronectin was investigated by immunofluorescence double staining in primary cultures of dissociated brain cells from newborn mice. In cultures, grown in serum-free medium, containing dibutyryl-cAMP, glutamine synthetase was found in about half of the glial fibrillary acidic protein containing astroblasts. After addition of dexamethasone to the cultures, glutamine synthetase appeared also in another cell type, in which no glial fibrillary acidic protein, but fibronectin was detectable. This demonstrates that these cells, which are present in substantial amounts, are not of glial nature. In cultures treated with dibutyryl-cAMP no other cell type was found positive for fibronectin. For cultures grown in serum-containing medium lacking dibutyryl-cAMP, evidence was obtained that glutamine synthetase was induced by dexamethasone only in part of all cells staining for fibronectin. This suggests the presence of two different populations of fibronectin-positive cells. Oligodendrocytes, revealed by staining for galactocerebroside, never contained detectable amounts of glutamine synthetase, irrespective of the presence of dexamethasone. This also holds for cultures grown in serum-free medium containing dibutyryl-cAMP. However, under these conditions, oligodendroblasts are seen only very rarely. Our results demonstrate an unexpected heterogeneity in the cellular composition of primary cultures from newborn mouse brain.

Interrelationship between differentiation and malignancy-associated properties in glioma

The phenotypic expression of cells derived from human anaplastic astrocytomas, rat glioma, normal human adult and foetal brain tissue have been examined for differentiated and malignancy-associated properties. Glial fibrillary acidic protein (GFAP), high affinity glutamate and gamma-amino butyric acid (GABA) uptake and glutamine synthetase were used as indicators of astroglial differentiation. Plasminogen activator and tumour angiogenesis factor were the malignancy-associated markers. The normal adult brain-derived lines showed some differentiated astroglial features and expressed low levels of the malignancy-associated properties. The foetal cultures contained highly differentiated astroglia while the glioma lines showed considerable phenotypic heterogeneity from highly differentiated to undifferentiated. The least differentiated glioma cells exhibited the highest plasminogen activator activities. The density-dependent control of phenotypic expression was also investigated. High affinity GABA uptake, and GFAP in rat C6 glioma cultures, increased with increasing monolayer cell density, events probably mediated by an increase in the formation of cell-cell contacts at confluence. Plasminogen activator activity decreased with increasing cell density.

Cellular composition of a cerebral hemisphere primary culture

In this overview attention is given to available markers and methods for characterizing cell elements in a culture system. Primary cultures from newborn rat cerebral hemispheres were grown for 14 days. The population of cells was dominated by astrocytic glial cells (60-70%), but cells with properties of macrophages, endothelial-like cells, mesenchymal-like cells, ependymal-like cells, and oligoblasts were also found. Neither mature neurons nor oligodendroglial cells were observed. The enrichment in astroglial-like cells makes the cultures a satisfactory astroglial-cell model, at least for some purposes.

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.

Changes in medium radioactivity and composition accompany high-affinity uptake of glutamate and aspartate by mouse brain slices

In measurements of high affinity transport in tissue slices, the incubation medium is often treated as an "infinitely large pool". External substrate concentrations, even at the micromolar level, are assumed to be constant and metabolic interactions between tissue and medium are neglected. In the present report we describe experiments in which glutamic and aspartic acid uptake by mouse brain slices were studied using techniques that could test these assumptions. Cerebral hemispheres were cut into 0.1 mm sections and about 90 mg of tissue incubated in 10 ml of oxygenated medium. After 45 minutes of equilibration, radioactive substrates were added and the concentrations and specific activities of the amino acids and their metabolites in the medium were determined. During the first 10 min following substrate addition, rapid decreases in glutamic and aspartic acid concentrations in the medium were accompanied by large decreases in specific activity caused by the continuous release of these amino acids from the tissue. In addition, extensive conversion of both substrates to glutamine and the preferential accumulation of this metabolite, in the medium, was found. These results demonstrate that metabolism and release occur simultaneously with uptake during transport experiments in vitro and that these processes can take place in specific tissue compartments. It is therefore necessary to measure the tissue and medium concentration levels of amino acids along with their radioactivity in such experiments, since all three processes (transport, metabolism, and compartmentation) are interrelated in the clearance of amino acids from the incubation medium and probably from the extracellular spaces in vivo as well.

Enzyme activities of monoamine oxidase, catechol-O-methyltransferase and gamma-aminobutyric acid transaminase in primary astroglial cultures and adult rat brain from different brain regions

The activities of monoamine oxidase (MAO), catechol-O-methyltransferase (COMT) and gamma-aminobutyric acid transaminase (GABA-T) were measured in primary cultures from newborn rat cultivated from 6 different brain regions. These primary cultures contained mostly astroglial cells, evaluated by the presence of the glial fibrillary acidic protein (GFAp, alpha-albumin) and the S-100 protein. The enzyme activities in the corresponding brain area from adult rat were also quantified. MAO activities were on the same level in 14-day old cultures and in adult rat brain homogenates, with significantly lower values in brain stem as compared to the other brain regions examined. COMT activities were on a higher level in the cultures than in adult rat brain homogenates. Astroglial cells from hippocampus were found to have the highest and those from brain stem the lowest COMT-activities. GABA-T activities were lower in the cultures than in adult rat homogenates. No significant differences were seen in the various astroglial cultures. Accumulation of [3H]dopamine and [3H]gamma-aminobutyric acid (GABA) visualized by autoradiography showed only a slight uptake of dopamine in comparison with the uptake of GABA. It is concluded that astroglial cells in culture have enzymatic properties similar to those of astroglial cells in different brain regions of adult rat brain. Studies are in progress to evaluate if the regional heterogeneity observed among cultivated astroglial cells is affected by in vivo differentiation until cultivation and/or time in culture.

Cell lines (VMDk) derived from a spontaneous murine astrocytoma. Morphological and immunocytochemical characterization

Three cell lines (VMDk) derived from a spontaneous, murine astrocytoma, which produce tumours when injected either subcutaneously or intracranially into syngeneic mice, have been examined in vitro. Ultrastructurally, the cells show astrocytic features but each line differs in its degree of differentiation. Treatment with both dexamethasone and dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) increases intracytoplasmic differentiation and causes surface structural changes. The addition of dbcAMP also induces a statistically significant increase in the length and number of cell processes. All three cell lines express the astrocyte-specific markers, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS), by indirect immunofluorescence, with two lines showing an increased propensity to stain for GFAP following dbcAMP treatment. The cell surface antigen fibronectin is also detected in all lines. Thus these VMDK cell lines exhibit both the morphological and antigenic characteristics of astrocytes and respond to dexamethasone and dbcAMP and may be used to provide a suitable in vivo-in vitro model system for the study of astrocytoma.

Epidermal growth factor (EGF) stimulation of cultured brain cells. I. Enhancement of the developmental increase in glial enzymatic activity

Serum-free aggregating cell cultures of fetal rat telencephalon grown in the presence of 3 ng/ml (5 X 10(-10) M) epidermal growth factor (EGF) until day 12 showed 2- to 3-fold increased activities in the two glial enzymes, glutamine synthetase (GLU-S) and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase). This effect was concentration-dependent, with maximal stimulation in cultures treated daily with 3 ng/ml EGF. Addition of EGF during the first 10 culture days was sufficient to produce a maximal stimulation of both GLU-S and CNPase on day 19, whereas treatments starting on day 12 were ineffective. The stimulation of GLU-S preceded that of CNPase. The EGF-induced increase in GLU-S activity was not directly dependent on the presence of insulin, triiodothyronine, or hydrocortisone in the medium, whereas insulin was required for the stimulation of CNPase. A single dose of 5 ng/ml EGF on day 2 caused a slight but significant decrease in DNA synthesis after day 6. The present results indicate that in serum-free aggregating cell cultures of fetal rat telencephalon EGF partially inhibits DNA synthesis, and stimulates an early step in glial differentiation.

Trypsinization of chick glial cells before seeding: effects on energy metabolism enzymes and glutamine synthetase

In order to test the possible involvement of surface proteins on some metabolical aspects of chick glial cell differentiation in culture, perturbations were induced on the glial cell surface membrane by limited trypsinization before seeding. The developmental changes of enzymes involved in the energy metabolism of the cell: malate dehydrogenase (MDH), glutamate dehydrogenase (GDH), hexokinase (HK), lactate dehydrogenase (LDH), enolase as well as glutamine synthetase (GS) were determined in trypsin treated cells and controls. The total protein and DNA content per dish was higher in treated cells than in controls, however the protein ratio towards DNA remained unchanged. The levels of GS, GDH, LDH, and enolase activities were significantly enhanced after trypsin treatment of the cells compared to controls. The enhanced value of total LDH activity is essentially the result of the increase of M subunit containing isoenzymes. Considering that a higher level of GS activity characterizes some maturation of the glial cells (as observed during the maturation of the chick brain) it is apparent that modifications of cell surface located factors, by trypsin treatment, induce differentiation phenomena at the functional state of the glial cells in culture. This may indicate that interactions located at the cell surface are involved in the modulation of key enzymes of the energy metabolism pathway.

Regulation of in vivo glutamine synthetase activity by glucocorticoids in the developing rat brain

Induction of glutamine synthetase in vivo by glucocorticoids was studied in different brain regions of the rat during development. Corticosterone treatment resulted in an age-dependent increase in glutamine synthetase activity. In 11-day-old rats, in comparison with controls, the increase was about 80% in the cerebellum, 50% in the olfactory bulbs and 20% in the forebrain. During development the effect diminished markedly and at day 20 a marked increase was detectable only in the cerebellum, suggesting that the elevation of glutamine synthetase was dependent on the maturational state of the region at the time of hormone treatment. The increase in enzyme activity was dose-dependent, and was also observed after treatment with dexamethasone but not with testosterone, estradiol or progesterone. Pre-treatment with the protein synthesis inhibitor anisomycin blocked the hormonal response. Also, immunochemical detection of glutamine synthetase in brain homogenates fractionated by SDS-polyacrylamide gel electrophoresis and transferred to diazobenzyloxymethyl paper showed that the increase in enzyme activity was due to induction of protein synthesis. This may be the first report showing a regulation by glucocorticoids of mammalian brain glutamine synthetase in vivo, and it is in good agreement with previous findings with chick retina in vivo and various avian and mammalian cells in vitro.

Glucocorticoid regulation of synaptic development

The effect of glucocorticoid hormones on the developmental step in which a presynaptic neuron acquires the ability to transmit excitatory information across a synapse was explored using a retina muscle cell culture system. Cholinergic neurons dissociated from the perinatal rat retina form functional synapses in culture with rat striated muscle cells. Early in the functional maturation of these retina muscle synapses, there is a period in which release of acetylcholine occurs spontaneously, but cannot be evoked. This stage is followed by the emergence of neurotransmitter release that is stimulus-evoked and dependent on extracellular calcium. Here, it is reported that glucocorticoid hormones accelerate this developmental sequence. Experimental findings indicate that this hormonal effect occurs at physiological concentrations, involves glucocorticoid receptors, acts at the transcriptional level and requires protein synthesis. A hypothesis is that glucocorticoids regulate the development of mechanisms which couple neuronal depolarization with release of neurotransmitter. The acceleration of the functional maturation of cholinergic retinal neurons also can occur in utero if pregnant rats are injected with a synthetic glucocorticoid or stressed by cold exposure. Thus, alterations in the time-course of synaptic maturation are not restricted to manipulation of culture conditions. The results presented here indicate that glucocorticoid hormones can regulate the timing of the developmental step in which cholinergic neurons of the rat retina become capable of releasing acetylcholine in response to excitatory stimulation.

Tetrodotoxin-sensitive ion channels characterized in glial and neuronal cells from rat brain

Ion channels were studied in primary neuronal and in primary glial cultures from rat brain by measuring the uptake of guanidinium, an ion that can permeate the Na+ channel. Neuronal cells exhibit a veratridine-stimulated (EC50 30 microM) guanidinium uptake, which is blocked by tetrodotoxin (IC50 30nM). This demonstrates the presence of a voltage-dependent Na+ channel. In glial cells veratridine + scorpion toxin, but not veratridine or scorpion toxin alone can stimulate a tetrodotoxin-sensitive ion uptake, thus indicating a 'silent' Na+ channel in the glial cells. Phentolamine, propranolol and various local anesthetic drugs (e.g. tetracaine, dibucaine) blocked the two different kinds of Na+ channels in the two cell populations investigated.

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.