Differential expression of type A and type B monoamine oxidase of mouse astrocytes in primary cultures

Astrocyte regulation of synaptic signaling in psychiatric disorders

Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.

Brunner syndrome associated MAOA mutations result in NMDAR hyperfunction and increased network activity in human dopaminergic neurons

Monoamine neurotransmitter abundance affects motor control, emotion, and cognitive function and is regulated by monoamine oxidases. Among these, Monoamine oxidase A (MAOA) catalyzes the degradation of dopamine, norepinephrine, and serotonin into their inactive metabolites. Loss-of-function mutations in the X-linked MAOA gene have been associated with Brunner syndrome, which is characterized by various forms of impulsivity, maladaptive externalizing behavior, and mild intellectual disability. Impaired MAOA activity in individuals with Brunner syndrome results in bioamine aberration, but it is currently unknown how this affects neuronal function, specifically in dopaminergic (DA) neurons. Here we generated human induced pluripotent stem cell (hiPSC)-derived DA neurons from three individuals with Brunner syndrome carrying different mutations and characterized neuronal properties at the single cell and neuronal network level in vitro. DA neurons of Brunner syndrome patients showed reduced synaptic density but exhibited hyperactive network activity. Intrinsic functional properties and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated synaptic transmission were not affected in DA neurons of individuals with Brunner syndrome. Instead, we show that the neuronal network hyperactivity is mediated by upregulation of the GRIN2A and GRIN2B subunits of the N-methyl-d-aspartate receptor (NMDAR), resulting in increased NMDAR-mediated currents. By correcting a MAOA missense mutation with CRISPR/Cas9 genome editing we normalized GRIN2A and GRIN2B expression, NMDAR function and neuronal population activity to control levels. Our data suggest that MAOA mutations in Brunner syndrome increase the activity of dopaminergic neurons through upregulation of NMDAR function, which may contribute to the etiology of Brunner syndrome associated phenotypes.

Preventing neurodegeneration by adrenergic astroglial excitation

Impairment of the main noradrenergic nucleus of the human brain, the locus coeruleus (LC), which has been discovered in 1784, represents one of defining factors of neurodegenerative diseases progression. Projections of LC neurons release noradrenaline/norepinephrine (NA), which stimulates astrocytes, homeostatic neuroglial cells enriched with adrenergic receptors. There is a direct correlation between the reduction in noradrenergic innervations and cognitive decline associated with ageing and neurodegenerative diseases. It is, therefore, hypothesized that the resilience of LC neurons to degeneration influences the neural reserve that in turn determines cognitive decline. Deficits in the noradrenergic innervation of the brain might be reversed or restrained by increasing the activity of existing LC neurons, transplanting noradrenergic neurons, and/or using drugs that mimic the activity of NA on astroglia. Here, these strategies are discussed with the aim to understand how astrocytes integrate neuronal network activity in the brain information processing in health and disease.

Physiology of Astroglia

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

Physiology of Astroglia

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

L-DOPA Uptake in Astrocytic Endfeet Enwrapping Blood Vessels in Rat Brain

Astrocyte endfeet surround brain blood vessels and can play a role in the delivery of therapeutic drugs for Parkinson's disease. However, there is no previous evidence of the presence of LAT transporter for L-DOPA in brain astrocytes except in culture. Using systemic L-DOPA administration and a combination of patch clamp, histochemistry and confocal microscopy we found that L-DOPA is accumulated mainly in astrocyte cell bodies, astrocytic endfeet surrounding blood vessels, and pericytes. In brain slices: (1) astrocytes were exposed to ASP⁺, a fluorescent monoamine analog of MPP⁺; (2) ASP⁺ taken up by astrocytes was colocalized with L-DOPA fluorescence in (3) glial somata and in the endfeet attached to blood vessels; (4) these astrocytes have an electrogenic transporter current elicited by ASP⁺, but intriguingly not by L-DOPA, suggesting a different pathway for monoamines and L-DOPA via astrocytic membrane. (5) The pattern of monoamine oxidase (MAO type B) allocation in pericytes and astrocytic endfeet was similar to that of L-DOPA accumulation. We conclude that astrocytes control L-DOPA uptake and metabolism and, therefore, may play a key role in regulating brain dopamine level during dopamine-associated diseases. These data also suggest that different transporter mechanisms may exist for monoamines and L-DOPA.

Fluoxetine affects GluK2 editing, glutamate-evoked Ca(2+) influx and extracellular signal-regulated kinase phosphorylation in mouse astrocytes

BACKGROUND

We sought to study the effects of chronic exposure to fluoxetine - a selective serotonin reuptake inhibitor (SSRI) and specific 5-HT(2B) receptor agonist in astrocytes - on the expression of kainate receptors (GluK1-5) in cultured astrocytes and in intact brains in mice and on GluK2 editing by adenosine deaminase acting on RNA (ADAR), as well as the ensuing effects of fluoxetine on glutamate-mediated Ca(2+) influx and extracellular signal-regulated kinase (ERK)(1/2) phosphorylation in astrocytes.

METHODS

We performed reverse transcription-polymerase chain reaction (PCR) to assess mRNA expression. We analyzed RNA editing with amplification refractory mutation system PCR and complementary DNA sequencing. Protein expression and ERK phosphorylation were assessed using Western blots. We studied gene silencing with specific small interfering RNAs (siRNA), and we studied intracellular Ca(2+) using fluorometry.

RESULTS

All GluK subunits were present in the brain in vivo, and GluK2-5 subunits were present in cultured astrocytes. Fluoxetine upregulated GluK2 and ADAR2. Enhanced GluK2 editing by fluoxetine abolished glutamate-mediated increases in intra cellular Ca(2+) and ERK(1/2) phosphorylation. Enhanced editing of GluK2 was prevented by siRNA against the 5-HT(2B) receptor or ADAR2.

LIMITATIONS

Limitations of our study include the use of an in vitro system, but our cultured cells in many respects behave like in vivo astrocytes.

CONCLUSION

Fluoxetine alters astrocytic glutamatergic function.

Ammonia induces the mitochondrial permeability transition in primary cultures of rat astrocytes

Ammonia is a toxin that has been strongly implicated in the pathogenesis of hepatic encephalopathy (HE), and the astrocyte appears to be the principal target of ammonia toxicity. The specific neurochemical mechanisms underlying HE, however, remain elusive. One of the suggested mechanisms for ammonia toxicity is impaired cellular bioenergetics. Because there is evidence that the mitochondrial permeability transition (MPT) is associated with mitochondrial dysfunction, we determined whether the MPT might be involved in the bioenergetic alterations related to ammonia toxicity. Accordingly, we examined the mitochondrial membrane potential (Deltapsi(m)) in cultured astrocytes and neurons using laser-scanning confocal microscopy after loading the cells with the voltage-sensitive dye JC-1. We found that ammonia induced a dissipation of the Deltapsi(m) in a time- and concentration-dependent manner. These findings were supported by flow cytometry using the voltage-sensitive dye tetramethylrhodamine ethyl ester (TMRE). Cyclosporin A, a specific inhibitor of the MPT, completely blocked the ammonia-induced dissipation of the Deltapsi(m). We also found an increase in the mitochondrial permeability to 2-deoxyglucose in astrocytes that had been exposed to 5 mM NH(4)Cl, further supporting the concept that ammonia induces the MPT in these cells. Pretreatment with methionine sulfoximine, an inhibitor of glutamine synthetase, blocked the ammonia-induced collapse of Deltapsi(m), suggesting a role of glutamine in this process. Over a 24-hr period, ammonia had no effect on the Deltapsi(m) in cultured neurons. Collectively, our data indicate that ammonia induces the MPT in cultured astrocytes, which may be a factor in the mitochondrial dysfunction associated with HE and other hyperammonemic states.

Enhanced tolerance of neuroblastoma cells towards the neurotoxin 6-hydroxydopamine following specific cell-cell interaction with primary astrocytes

Dopamine neuroblastoma (SH-SY5Y) cells exhibit a high affinity of adhesion for primary astroglial cells. The homophilic aggregation of SH-SY5Y cells is greatly reduced and the neuroprocesses are enhanced when co-cultured with the astrocytes. However, such affinity was not detected in the mouse when these cells were co-cultured with fibroblast and endothelial cells. SH-SY5Y cells in monoculture are very sensitive towards the neurotoxin 6-hydroxydopamine, but this sensitivity is substantially reduced in co-culture with astrocytes. The acquired cytoprotection of the neuroblastoma cells in co-culture against 6-hydroxydopamine is time dependent following adhesion with the astrocytes. There is no evidence to indicate that the increase in survival of the SH-SY5Y cells against 6-hydroxydopamine is due to inactivation of 6-hydroxydopamine induced by the extracellular factors secreted from the astrocytes, neither is there any indication suggesting the removal of 6-hydroxydopamine by an astrocyte uptake mechanism. The release of trophic factors by the astrocytes does not seem to play a role in the protection of the neuroblastoma cells against 6-hydroxydopamine. The neuroblastoma cells became susceptible to 6-hydroxydopamine in the astrocyte co-cultures when they were physically separated from the astroglial cells by trans-well inserts. Neither non-selective adhesions, such as adhesion with denatured astrocytes or with other types of cells (i.e. endothelial or fibroblast cells), nor adhesion enhanced by chemical agents can increase the cytoprotection of SH-SY5Y against 6-hydroxydopamine. These results suggest that the increase in survival of neuroblastoma cells against 6-hydroxydopamine in the astrocyte co-cultures is probably a result of specific cell-cell adhesion and the subsequent interactions.

Nigrostriatal monoamine oxidase A and B in aging squirrel monkeys and C57BL/6 mice

In this study we assessed the two forms of monoamine oxidase (MAO) in the caudate, putamen, and substantia nigra of young (4-year-old), intermediate-aged (11-year-old), and aged (20-year-old) squirrel monkeys and in the striata of young (2-month-old) and older (10-month-old) C57Bl/6 mice. MAO A and B activities were determined by measuring the rate of oxidation of the specific substrates phenethylamine and serotonin. In squirrel monkey, the vast majority of MAO activity was MAO B with activity of this isoform 10 times greater than of MAO A, while in mice the activity of the two forms was approximately equivalent. Although mice demonstrated nearly twofold selective increases in striatal MAO B between 2 and 10 months of age, neither MAO B nor A showed statistically significant changes with age in squirrel monkeys. These results document the marked differences between nonhuman primates and rodents with respect to the relative activities and the effects of age on MAO A and B, and indicate that increased MAO is not an inevitable feature of aging.

Monoamine oxidase-dependent metabolism of dopamine in the striatum and substantia nigra of L-DOPA-treated monkeys

The effects of monoamine oxidase (MAO) inhibitors on the metabolism of dopamine synthesized from exogenous L-DOPA were investigated in the striatum and substantia nigra of squirrel monkeys. Administration of a single dose of L-DOPA (methyl ester, 40 mg/kg, i.p.) caused a significant increase in the levels of dopamine, 3-4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and in the DOPAC/dopamine ratio in the putamen, caudate and substantia nigra. These changes were more pronounced in the substantia nigra than in the striatum and within the striatum of L-DOPA-treated monkeys, levels of dopamine and its metabolites were higher in the putamen than in the caudate nucleus. When L-DOPA treatment was preceded by the injection of clorgyline or deprenyl at a concentration (1 mg/kg) which selectively inhibited MAO A or MAO B, respectively, striatal dopamine was increased while the striatal DOPAC and HVA levels and DOPAC/dopamine ratio were significantly reduced as compared to the values obtained with 1-DOPA alone. The two MAO inhibitors also counteracted the increase in the DOPAC and HVA levels and DOPAC/dopamine ratio induced by L-DOPA in the substantia nigra. Thus, both MAO A and MAO B contribute to the metabolism of dopamine when higher levels of this neurotransmitter are generated from L-DOPA in the squirrel monkey. The extent of reduction of dopamine catabolism (as assessed by the decrease in DOPAC and HVA levels) in the striatum and substantia nigra was similar with clorgyline and deprenyl even if the ratio MAO A/MAO B was approximately 1 to 10. This indicates that, though catalyzed by both MAO A and MAO B, dopamine deamination following treatment with L-DOPA preferentially involves MAO A.

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

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

The promoter of the human monoamine oxidase A gene

Monoamine oxidase (MAO) A (EC 1.4.3.4) oxidizes norepinephrine and serotonin and is expressed in a cell type-specific manner. Evidence that MAO A deficient males in a large Dutch kindred suffer from mild mental retardation and occasional episodes of impulsive-aggressive behavior makes it important to understand how the human MAO A promoter is regulated. Workers in multiple laboratories have isolated and characterized protein-coding sequences of the human MAO A gene and the DNA region where mRNA synthesis is initiated. After summarizing our published findings concerning where transcription of the human MAO A gene is initiated, I summarize representative results of transient expression assays aimed at assessing whether some potential gene regulatory agents affect the expression of luciferase from MAO A promoter reporter constructs when transfected into a mouse L cell line which expresses MAO A. These studies revealed no specific regulatory effects of serum, dexamethasone or a stable cyclic-AMP analogue on the human MAO A promoter introduced.

Differences in the disposition and toxicity of 1-methyl-4-phenylpyridinium in cultured rat and mouse astrocytes

Species difference in the susceptibility to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was investigated in cultured rat and mouse astrocytes, where 1-methyl-4-phenylpyridinium (MPP+), the toxic mediator of MPTP, is formed. Type A monoamine oxidase (MAO) predominated in both rat and mouse astrocytes, while its activity was slightly higher in mouse cells compared to rat cells; MAO-B activity, on the other hand, was significantly lower in mouse astrocytes than in rat astrocytes. Because both types of MAO have been reported to make similar contributions to MPP+ production in astrocytes, their total activity was examined and results indicated that there was no significant difference between these two species. In addition, MPP+ caused a dose-dependent loss of cell viability as judged by the amount of lactate dehydrogenase released into the incubation medium. The toxicity of MPP+ on astrocytes started to be seen after a 2 day incubation period. Mouse astrocytes were more vulnerable to MPP+ than rat astrocytes. The threshold values for MPP+ toxicity in mouse and rat cultures were 10 microM and 70 microM, respectively. After addition of [3H] MPP+ to the medium, intracellular [3H] MPP+ was found to increase in both cultures. Mouse astrocytes accumulated more MPP+ than rat astrocytes (150 pmol/mg protein vs. 65 pmol/mg protein). When astrocytes were allowed to accumulate [3H] MPP+ and then incubated in fresh medium not containing [3H] MPP+, intracellular levels of [3H] MPP+ in both cells rapidly declined (110 pmol/protein in mouse vs. 40 pmol/mg protein in rat of MPP+ been released).(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of monoamine oxidase activities in synaptic and non-synaptic mitochondria derived from three brain regions: some functional implications

The heterogeneity of monoamine oxidase (MAO; EC 1.4.3.4) activities was studied in two fractions of synaptic mitochondria (SM & SM2) and one fraction of non-synaptic ("free") mitochondria (M) isolated from three rat brain regions (cerebral cortex, striatum, and pons & medulla) by the Lai and Clark (1979, 1989) method in order to elucidate the heterogeneity of MAO at the subcellular and brain regional levels. The activities toward serotonin (MAO-A), benzylamine (MAO-B), and dopamine (MAO-DA) in SM2 from all three regions were different from the corresponding values in SM. In addition, the various MAO activities in SM and SM2 showed heterogeneous distribution with respect to the three brain regions investigated. Both the distribution of MAO-A and MAO-B in non-synaptic mitochondria (M) did not show marked regional differences although MAO-DA in M varied depending on the region. These results clearly demonstrate that the distribution of MAO activities toward different substrates is heterogeneous both at the subcellular and the brain regional levels. The MAO-A:MAO-B ratios in the various mitochondrial fractions also showed trends that are consistent with this hypothesis. Furthermore, in fraction SM of synaptic mitochondria, this ratio was consistently higher than values in the other two mitochondrial fractions (SM2 & M) irrespective of the region from which they were isolated. In view of the functional importance of MAO in the regulation and compartmentation of amine metabolism, the heterogeneity of MAO at subcellular and regional levels may assume pathophysiological importance in neurological diseases (e.g., Parkinsonism) with which altered amine metabolism is associated.

The effects of monoamine oxidase B inhibition on dopamine metabolism in rats with nigro-striatal lesions

The purpose of this study was to examine whether monoamine oxidase type B (MAO-B) has a role in striatal dopamine metabolism in animals with a unilateral lesion of the medial forebrain bundle, and whether 2-phenylethylamine (PE) could have a role in amplification of dopamine (DA) responses in DA depleted striatum. Inhibition of MAO-B did not alter DA metabolism in lesioned striata. PE accumulation decreased with loss of DA as long as there was no DA dysfunction. In lesioned striata with dysfunction of DA transmission at the synaptic level, PE accumulation increased, suggesting a compensatory increase in PE synthesis. This increase in PE levels does not appear to be mediated by an increase in the total striatal aromatic L-amino acid decarboxylase (AADC) activity. We conclude that inhibition of MAO-B has no effect on DA metabolism in the hemi-parkinsonian rat striatum and that PE could be involved in the antiparkinsonian action of MAO-B inhibitors.

Brain regional distributions of monoamine oxidase activities in postnatal development in normal and chronically manganese-treated rats

To investigate the hypothesis that manganese toxicity may affect the development of the monoamine oxidases (MAOs) in brain, the regional distributions of the A (serotonin-oxidizing) and B (benzylamine-oxidizing) forms of MAO were determined in manganese-treated (1 mg or 10 mg of MnCl2.4H2O per ml of drinking water from conception onwards until the rats were used for experiments) and untreated male rats during various stages of postnatal development. The age-related variations in regional MAO-A activities (especially in pons and medulla and in cerebellum) were more marked than those in regional MAO-B activities. The MAO-A:MAO-B activity ratios decreased in all regions during development. Chronic manganese treatment (at the specified doses) did not significantly alter the age-related changes in regional MAO-A and MAO-B activities; nor were the A:B activity ratios affected. The results suggest that there is differential expression of MAO isoforms in various brain regions during postnatal development but the expression is not affected by chronic manganese toxicity.

Monoamine oxidase-B in astrocytes

In the present report we describe the astrocytic localization and content of monoamine oxidase-B (MAO-B) by means of a 3H-L-deprenyl emulsion autoradiography in primary cultures of rat astrocytes, in cryosectioned astrocytoma surgical specimen, and in cryosections of human spinal cords from patients dying in amyotrophic lateral sclerosis (ALS) and controls. The occurrence of MAO-B enzyme protein depends on the degree of cellular differentiation as demonstrated by studies on astrocytes in primary cultures analyzed at two different stages of maturation. Highly differentiated cells exhibited high relative enzyme concentration whereas glioblasts lacked or showed very low contents of MAO-B enzyme. This was further substantiated by studies performed on human astrocytoma tissue using 3H-L-deprenyl emulsion autoradiography in combination with immunohistochemical detection of glial fibrillary acidic protein (GFAP). Regional increases of MAO-B concentration were found in ALS lumbar sections with quantitative 3H-L-deprenyl autoradiography. On the basis of results obtained from double staining for GFAP and MAO-B, the increase in MAO-B seemed to be due to an increased number of astrocytes as well as an increased content of MAO-B in reactive species of astrocytes. A cell culture model has been used that produces cells with morphology and GFAP-content similar to reactive cells. These astrocytes exhibited high relative content of the MAO-B enzyme protein. In the light of the presented data, taking into account the finding that a subpopulation of reactive cells contained low levels of MAO-B, a heterogeneity among reactive astrocytes was observed.

Uptake of [3H]serotonin and [3H]glutamate by primary astrocyte cultures. I. Effects of different sera and time in culture

Na(+)-dependent, fluoxetine-sensitive high-affinity uptake of serotonin and Na(+)-dependent uptake of glutamate were studied in primary astrocyte cultures from 1-day-old rat neocortex. This uptake was independent of time in culture from 1 to 6 weeks. High-affinity serotonin uptake was decreased when cells were grown in horse serum as compared to fetal bovine serum and was almost absent when cells were grown in chemically defined medium. In contrast, glutamate uptake was unaffected by the composition of the medium in which the cultures were grown. The serum effect on serotonin uptake was not due to the greater level of serotonin in the fetal bovine serum and was only reversed by a change of serum over a time period of days.

2-propyl-1-aminopentane, its deamination by monoamine oxidase and semicarbazide-sensitive amine oxidase, conversion to valproic acid and behavioral effects

2-Propyl-1-aminopentane (2-PAPN), a branched aliphatic amine, was found to be readily deaminated by monoamine oxidase B in the liver of the rat and semicarbazide-sensitive amine oxidase in the aorta of the rat. The deaminated product, 2-propyl-1-pentaldehyde, could be subsequently converted to valproic acid in the presence of aldehyde dehydrogenase and beta-NAD cofactor in vitro as well as in vivo. Valproic acid was identified after derivatization with 4-bromomethyl-6,7-dimethoxycoumarin, followed by HPLC-fluorometric assessment. Absorption and biotransformation of a single intraperitoneal dose of 2-PAPN resulted in the rapid appearance of the drug and its metabolite in the blood and in the brain. The formation of valproic acid from 2-PAPN in vivo, however, was insufficient to facilitate anticonvulsant action. In fact, 2-PAPN itself, at relatively small doses, exhibited distinct tremor effects. Such tremor effects could be prevented by valproic acid. However, 2-PAPN was also found to potentiate the convulsant effect induced by mercaptopropionic acid (MPA) and, in addition, the 2-PAPN-induced tremor could be potentiated by MPA in mice.

Differential effects of angiotensin II and eledoisin on monoamine oxidase A and B activities in rat brain

Intracerebroventricular injections of angiotensin II caused 108, 62, and 54% increases in monoamine oxidase A activities in rat hippocampus, hypothalamus, and striatum, respectively. These activatory effects were abolished by simultaneous injections of eledoisin. No significant changes of monoamine oxidase B activities were found under the same experimental conditions. Neither angiotensin II nor elodoisin changed substrate/inhibitor affinities of both isoenzymes. These data indicate that angiotensin II and tachykinin transmitter systems may exert opposite, long-term regulatory effects on monoaminergic neurons in rat brain.

Immunohistochemical detection of c-fos proteins in cultured human glial cells--induction by cyclic AMP and phorbol ester

The expression of c-fos protein in cultured human glial cells derived from the brain and spinal cord was investigated immunocytochemically. Primary cultures of fetal glial cells were maintained in culture for three weeks and deprived of animal sera for 22 h. The glial cell nature of the cells was ascertained by GFAP-immunoreactivity. Incubations with phorbol dibutyrate, 8-Br-cAMP and sodium nitroprusside representing signal transduction pathways of PKC, PKA and cyclic GMP kinase, respectively, were carried out for 60 and 120 min. The control serum-deprived cultures did not display c-fos protein immunoreactivity (c-fos-IR), whereas phorbol dibutyrate incubation for 120 min induced strong c-fos-IR in the nuclei of both brain and spinal cord derived glial cells. Semiquantitative intensity measurements revealed a slight c-fos-IR induction after 8-Br-cAMP as well, but not after sodium nitroprusside. The observations suggest that c-fos protein is involved in PKC and PKA signal transduction in cultured human glial cells.

Serotonin metabolism by monoamine oxidase in rat primary astrocyte cultures

The oxidative deamination of serotonin (5-HT) to 5-hydroxyindoleacetic acid (5-HIAA) by rat primary astrocyte cultures was investigated in intact cells using HPLC. All detectable 5-HIAA accumulated in the extracellular medium, and its rate of production was proportional to the 5-HT concentration over the tested range of 5 x 10(-7) to 10(-4) M. At 5 x 10(-7) M 5-HT, intracellular 5-HT was detectable only in astrocytes treated with monoamine oxidase (MAO) inhibitors. These findings are consistent with the idea that 5-HT taken up into astrocytes is not stored for re-release, but is rapidly metabolized to 5-HIAA, which is then extruded from the cell. At 5 x 10(-7) M 5-HT, 5-HIAA formation in intact cells was blocked 63% by the selective high-affinity 5-HT uptake inhibitor fluoxetine. 5-HT oxidation to 5-HIAA is carried out principally by MAO-A, because clorgyline was more effective at inhibiting the production of 5-HIAA than was pargyline. Radioenzymatic determinations of MAO activity in cell homogenates supported these findings, because under these conditions clorgyline was 1,000-fold more effective than pargyline at inhibiting MAO activity toward 14C-labelled 5-HT. However, the relatively selective MAO-B substrate beta-phenylethylamine (PEA) was also oxidized, showing that these cultures also contained MAO-B activity; the Km values for MAO-A oxidation of 5-HT and MAO-B oxidation of PEA were 135 and 45 microM, and Vmax values were 88 and 91 nmol/mg of total cell protein/h, respectively. Higher concentrations of PEA (greater than 20 microM) were oxidized by both MAO-A and MAO-B isozymes.(ABSTRACT TRUNCATED AT 250 WORDS)

c-fos proto-oncogene expression in astrocytes associated with differentiation or proliferation but not depolarization

Expression of the c-fos proto-oncogene in rat neocortical astrocytes in culture was examined using Northern blotting and immunocytochemistry. Marked induction of c-fos mRNA in astrocytes was observed after treatment with epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), dibutyryl cyclic AMP (db-cAMP), and phorbol diester (TPA; 12-O-tetra-decanoylphorbol 13-acetate), which are known to induce the proliferation or differentiation of astrocytes. Increase of c-fos protein immunoreactivity (IR) was obtained after treatment with fetal calf serum, EGF, bFGF, db-cAMP and TPA. High concentrations of calcium ionophore A23187, which were lethal to cultured astrocytes, also increased c-fos protein-IR. Treatment with lower concentrations of calcium ionophore (which slightly increase Ca2+ uptake), high K+ and nerve growth factor had no detectable effect on c-fos expression. These results show that depolarization does not induce c-fos in astrocytes and suggest that c-fos may play a role in differentiation and proliferation of astrocytes.

Uptake, metabolism, and release of [3H]-histamine by glial cells in primary cultures of chicken cerebral hemispheres

Labelled histamine was taken up into cultured glial cells of chick embryonic brain by a system with high affinity for histamine and diffusion. The active uptake, occurring at low concentrations of the amine, was Na+ dependent and gave an apparent Km of 0.24 microM and a Vmax of 0.31 pmol x mg protein-1 x min-1. The uptake was completely blocked by desmethylimipramine (Ki = 2.5 microM) and partially by the histamine agonists and histamine-N-methyltransferase blockers 4-methylhistamine and 2-methylhistamine (I30 values obtained were 2 microM and 5 microM). Other psychoactive drugs were either ineffective (imipramine) or they showed moderate inhibitory effects (amitriptyline and cocaine). Ouabain (100 microM) inhibited uptake by approximately 50%. Diffusion occurred at high concentrations of the amine, was insensitive to extracellular Na+, and was proportional to histamine concentration up to 1 mM. [3H]-Histamine, taken up into the cells, was metabolized and/or released. The spontaneous efflux of the radioactivity measured after 10 min of exposure to [3H]-histamine (when most of it was still unmetabolized), was moderately Ca++ dependent, accelerated by both reduced concentrations of extracellular Na+ and enhanced concentrations of K+ and inhibited by desmethylimipramine. After prolonged (60 min) incubation, histamine metabolites detected in the cells presented 78% of the chromatogram radioactivity and consisted of N tau-methylhistamine and N tau-methylimidazole acetic acid. These results indicate that at low nM concentrations, histamine is taken up and metabolized by (and released from) glial cells by an Na(+)-dependent system, and the intracellular metabolism seems to serve an increased uptake of the amine.

Biochemistry and genetics of monoamine oxidase

This chapter reviews the two mitochondrial flavin containing isozymes of monoamine oxidase. Section 1, "Biochemistry" discusses assays, substrates and inhibitors, phylogenic and tissue distribution, interactions with lipids, nutritional studies, protein structure, kinetic and chemical mechanistic proposals, and biosynthesis. Section 2, "Inheritance" discusses possible genes involved in expression, genetic studies of platelet MAO-B and fibroblast MAO-A, and chromosomal location. Section 3, "Molecular Genetics" reviews the cloning of their cDNAs, their intra- and interspecies homology and structural inferences made from deduced amino acid sequences. Section 4, "Regulation" gives an overview of levels in development and aging, and effect of drugs. The final section 5, "Role in Human Disease" discusses physiological function and effects of altered levels in humans and animal models including complete absence due to a submicroscopic chromosomal deletion in several human patients.

Histamine inactivation in the brain: aspects of N-methylation

This report deals with molecular and anatomical site of histamine N-methylation assumed to be the exclusive route of HA inactivation. The methyl transfer from the -S-CH3 of S-adenosyl-L-methionine to the ring (tele)-nitrogen of histamine, appears as much more complex than a one-step transformation. It seems that -S-CH3 is transformed before being transferred to the nitrogen of the acceptor probably via methanol (formaldehyde) formation. For localizations of transmethylation of neuronal histamine we assume at least a two-compartment model in which glia participate to a significant extent. The uptake of neuronal HA into glial cells might be the first step of histamine inactivation.

Some pharmacological implications of MAO-mediated deamination of branched aliphatic amines: 2-propyl-1-aminopentane and N-(2-propylpentyl)glycinamide as valproic acid precursors

2-Propyl-1-aminopentane (2-PAP) and N-(2-propylpentyl)glycinamide (PPG) were readily deaminated by rat liver monoamine oxidase B and rat aorta semicarbazide-sensitive amine oxidase. The deaminated product, valproic acid (VPA), was identified by HPLC-fluorometric assessment. Absorption and biotransformation of these compounds and their VPA metabolite into the brain were rapid processes. An investigation was conducted to examine whether these compounds can be used as VPA prodrugs. Both compounds, however, at relatively low doses exhibited distinct tremor effects in mice and rats. They also potentiate the convulsant effect induced by mercaptopropionic acid (MPA).

Ontogenesis of monoamine oxidase-A and -B in the human brain frontal cortex

The effect of several antemortem and postmortem factors (patients' age, sex, postmortem delay, storage time, laterality and brain weight) on both monoamine oxidase-A (MAO-A) and -B (MAO-B) activity was investigated in the frontal cortex of human brains. The MAO-A activity decreases rapidly during the first two years of life and remains constant thereafter. In contrast, the MAO-B activity keeps unchanged during early childhood and raises during advanced age. These findings seem to be consistent with a genetic regulation and a variation in cell type assembling during brain development and aging.

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

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

Monoamine oxidase deficiency in males with an X chromosome deletion

Mapping of the human MAOA gene to chromosomal region Xp21-p11 prompted our study of two affected males in a family previously reported to have Norrie disease resulting from a submicroscopic deletion in this chromosomal region. In this investigation we demonstrate in these cousins deletion of the MAOA gene, undetectable levels of MAO-A and MAO-B activities in their fibroblasts and platelets, respectively, loss of mRNA for MAO-A in fibroblasts, and substantial alterations in urinary catecholamine metabolites. The present study documents that a marked deficiency of MAO activity is compatible with life and that genes for MAO-A and MAO-B are near each other in this Xp chromosomal region. Some of the clinical features of these MAO deletion patients may help to identify X-linked MAO deficiency diseases in humans.

Demonstration of monoamine oxidase-A and -B in the human brainstem by a histochemical technique

The distribution of both monoamine oxidase subtypes, monoamine oxidase-A and -B, is demonstrated in brainstems from 16 humans by use of a histochemical technique. The results presented here, focus primarily upon the aminergic areas of the substantia nigra, the locus coeruleus and the raphe nuclei. While dopaminergic neurons of the substantia nigra revealed no staining for monoamine oxidase, noradrenergic neurons of the locus coeruleus stained positively with the monoamine oxidase-A substrate serotonin, and serotonergic neurons of the raphe nuclei were stained by the monoamine oxidase-B substrate beta-phenylethylamine. In addition, data are presented showing that glial cells stain predominantly for monoamine oxidase-B.

Autoradiography of high affinity uptake of catecholamines by primary astrocyte cultures

Uptake of D.L-[3H]norepinephrine ([3H]NE) and [3H]dopamine ([3H]DA) by primary astrocyte cultures prepared from neonatal rat brains, which are greater than or equal to 95% glial fibrillary acidic protein (GFAP(+)), was studied by measuring accumulation of tritium label, and localizing such uptake at the cellular level by autoradiography. Uptake of [3H]NE was 95% Na+ dependent at 10(-7) M and 80% Na+ dependent at 7.5 X 10(-7) M [3H]NE. Uptake of [3H]DA at 7.5 X 10(-7) M was 58% Na+ dependent, but total uptake of [3H]DA was greater than uptake of [3H]NE. Autoradiography of cells incubated with 7.5 X 10(-7) M [3H]NE or [3H]DA showed that a high proportion of all the cells in these cultures had a grain density which was clearly above background. When Na+ was omitted from the medium, the temperature was lowered to 4 degrees C, or 10(-7) M desmethylimipramine or 10(-7) M amitryptyline were present, cellular grain density after exposure to both [3H]NE and [3H]DA was greatly reduced, to close to background levels. It also appeared necessary to have inhibitors of both monoamine oxidase (pargyline) and catecholamine-O-methyltransferase (tropolone) present to see clear cellular localization for [3H]DA. In the case of [3H]NE the presence of tropolone alone was adequate to observe cellular localization. These results confirm our previous findings of the existence of a high affinity uptake process for catecholamines in primary astrocyte cultures based on uptake properties, and in the present study also localizes such uptake to the major, astrocytic cell type.

Occurrence of phenolsulfotransferase in primary glial culture cells of rat

Phenolsulfotransferase (PST) activity towards phenol and monoamines was determined in rat brain and in primary cultures of rat astrocytes. The pH requirement, Km values and the proportion of PST activity with respect to phenol and dopamine as substrates were similar between PST from the glial cells and the rat cortex. The enzyme activity increased with age in the brain of older animals, and also with increasing incubation time in the primary culture of astroglia. The specific PST activity of the astroglia appeared to be higher than that of the brain enzyme. In glial cultures treated with 0.25 mM dibutyryl cyclic AMP in the same culture conditions, PST activity is suppressed to about 25% of its untreated counterpart, even though dibutyryl cyclic AMP at concentrations of ImM only slightly inhibited PST activity in vitro.

High-affinity uptake of serotonin into immunocytochemically identified astrocytes

Primary cultures of astrocytes from neonatal rat brain were incubated with tritiated serotonin. After fixation they were stained by immunofluorescence for the astrocyte-specific marker glial fibrillary acidic protein and processed for autoradiography. Silver grain density was increased over cells positive for glial fibrillary acidic protein and was reduced to background levels when sodium was omitted from the medium or the specific inhibitors of serotonin uptake fluoxetine and chlorimipramine were present. The results indicate that mammalian astrocytes can take up serotonin by a sodium-dependent, high-affinity system previously thought to be the exclusive property of serotonergic nerve endings.

Expression of A and B types of monoamine oxidase in differentiated neuroblastoma hybrid cells

The total activities of monoamine oxidase (MAO) and the ratio of type B/type A activities were determined in mouse neuroblastoma N1E-115 cells, and in NX31T and NG108-15 hybrid cells derived from mouse neuroblastoma X rat sympathetic ganglion hybrid or mouse neuroblastoma X rat glioma hybrid cells. N1E-115 and NX31T cells possessed type A activities exclusively, although NG108-15 cells showed both type A (65-90%) and type B (10-35%) MAO activities. The activity of type A MAO in NX31T and N1E-115 cells was relatively constant during culturing periods in the presence or absence of dibutyryl cyclic AMP (Bt2cAMP), whereas total MAO activity and the ratio of type B MAO/type A MAO in NG108-15 cells increased as a function of culture periods. Prostaglandin E1 (PGE1) and theophylline, the best known combination to increase intracellular cyclic AMP content of NG108-15 cells, caused similar increases of MAO and of the type B/type A ratio in NG108-15 cells. The results suggest that MAO activity and expression of MAO B activity are regulated in NG108-15 cells in a cyclic AMP-dependent manner.

Monoamine oxidase in young and adult rat brain capillaries

Capillaries were isolated from young and adult rat brain and the mitochondrial type A and B monoamine oxidase activities in these endothelial cells were determined. Type A activity was observed to be predominant in both young and adult rats and was increased with maturation while type B activity remained unchanged. The predominance of type A MAO in the rat brain capillaries is consistent with that of the rat aorta and heart tissue.

Type A and B monoamine oxidase in glial cells in long-term culture

Monoamine oxidase (MAO) in astroglial primary cultures increased during cell development. The enzyme activity towards type B substrate was further enhanced in cultures treated with 0.25 mM dibutyl cyclic AMP (dBcAMP), although dBcAMP (1 mM) does not effect MAO activity in vitro. It has been estimated that the dBcAMP treated cultures contain 70% type A and 30% type B MAO, while in the untreated one, there was virtually only type A MAO. Following the application of Percoll density gradient centrifugation, MAO associated with different subcellular fractions was obtained. The dBcAMP treated fractions were slightly heavier than the untreated ones.