Biomedical subjects

A Reichenbach

Publications and source records attributed to A Reichenbach.

At least 55 records · Page 3Linked to original sources

PubMed1998

The glutathione content of retinal Müller (glial) cells: the effects of aging and of application of free-radical scavengers.

The dependence of intracellular glutathione (GSH), an important radical scavenger, on aging with or without externally applied Ginkgo biloba extract EGb 761, another established radical scavenger, was studied in guinea pig M¿ller (retinal glial) cells by using the fluorescent dye monochlorobimane. The GSH content of freshly dissociated cells from untreated aged animals was significantly lower than that of young controls; most of this reduction was prevented by application of EGb 761. Culturing the cells in amino-acid-free caused a loss of up to 50% of the initial GSH content. When the culture medium contained 100 microM glutamate and 100 microM cystine, ongoing GSH synthesis counteracted the loss of GSH. The rates of net GSH synthesis were equal for the two groups of aged animals but significantly higher for cells from young controls. It is concluded that externally applied radical scavengers may enhance the protective glutathione 'reserve' of M¿ller cells in cases of neuronal degeneration.

Aging↗

PubMed1998

Müller (glial) cell development in vivo and in retinal explant cultures: morphology and electrophysiology, and the effects of elevated ammonia.

Retinal explant cultures have been established as a useful tool to study both the normal development of the mammalian retina and the effects of pathogenic agents. We used such cultures as a model for the (ammonia-induced) hepatic retinopathy, earlier observed in humans with chronical liver failure, and ascribed to a breakdown of Müller (glial) cell function. In the explant cultures, one day exposure to elevated (7 mM) ammonia was sufficient to cause Müller cell reactivity as indicated by increasing immunopositivity for glial fibrillary acidic protein. After 4 days in elevated ammonia, the Müller cells were severely deformed, the layered structure of the retinae became disorganized, and significant neuronal cell death occurred. Using whole-cell voltage-clamp recordings, the expression of K+ channels was compared in Müller cells isolated from retinae of rabbits at postnatal days 9 to 12 and from neonatal explants cultured for 9 to 12 days, respectively. Müller glial cells grown both in vivo and in vitro express the same set of K+ channels in their membranes: (i) inwardly rectifying K+ (K(IR)) channels which were selectively blocked by Ba2+ ions; (ii) large-conductance, Ca2+-activated K+ (BK(Ca)) channels which were blocked by iberiotoxin and were activated by phloretin; and (iii) delayed rectifying voltage-gated K+ channels. The presence of K(IR) channels indicates successful differentiation of the Müller cells grown in vitro, as these channels are not expressed in cells from neonatal animals. Four days of elevated ammonia in the culture medium caused a complete loss of K(IR) channels in Müller cell membranes, and a significant decrease of the membrane potential. The results indicate that in hepatic retinopathy, the well-known morphological and enzymatical alterations of Müller glial cells may be accompanied by changes in their membrane permeability for K+.

4-Aminopyridine↗

PubMed1998

Müller (glial) cells in the teleost retina: consequences of continuous growth.

In the continuously enlarging eye of teleost fishes retinal growth is achieved by the generation of new cells and by stretching the existing tissue. As a consequence of stretching, the density for most neurons decreases except for rod photoreceptors, which are produced by stem cells in the outer nuclear layer (ONL). We investigated retinal Müller glial cells with immunocytochemical markers against vimentin and glutamine synthetase in animals of various sizes. In addition, we used Western immunoblot analysis to investigate the changes in the glia-specific enzyme glutamine synthetase in the enlarging retina. We found that in the cichlid fish Haplochromis burtoni the density of Müller cells decreases from about 14 cells/mm2 to 4 cells/mm2 with increasing body size. Since it is known that the density of rod photoreceptors remains constant, it follows that the neuron to Müller cell ratio increases in the growing eye. In our estimates, this ratio ranges from around 54:1 in small fish to more than 67:1 in larger animals. Quantified Western blots revealed that the amount of glutamine synthetase per retinal area does not change in the growing eye, which means that the amount of this enzyme in each Müller cell must increase. Staining isolated cells and retinal sections from small and large fish with an antibody against glutamine synthetase showed stronger immunoreactivity in larger animals, especially in the areas of the photoreceptor cell bodies and outer limiting membrane and a more extensive branching of Müller cell processes. Thus, Müller cells in growing fish appear to compensate for the increasing metabolic challenge and are able to maintain their function.

Animals↗

PubMed1997

Heterogenous expression of Ca2+-dependent K+ currents by Müller glial cells.

The flavoid phloretin was used to study large-conductance, Ca2+-activated K+ (BKCa) channel-mediated currents in isolated porcine Müller (retinal glial) cells. In excised membrane patches, application of phloretin (100 microM) increased the open probability of single BKCa channels. In whole-cell records, phloretin increased the amplitude of an outward current which was blockable by iberiotoxin (40 nM) and TEA (1 mM), and, thus, was identified as a BKCa current. Only 50% of the investigated cells expressed phloretin-sensitive BKCa currents. Cells with phloretin-sensitive BKCa currents responded to glutamate (500 microM) with an increase of the amplitude of this current, whereas glutamate had no effects on the K+ currents of cells without BKCa currents. As glutamate is a major neurotransmitter of the retina, and the membrane properties of Müller cells are largely determined by their K+ channels, the observed data may shed new light onto the functions of retinal glia.

Animals↗

PubMed1997

Modification of glutamine synthetase expression by mammalian Müller (glial) cells in retinal organ cultures.

One of the key enzymes in glial-neuronal transmitter recycling is glutamine synthetase (GS). In the retina, GS is exclusively expressed by glial (Müller) cells where it serves to convert neuron-released active transmitter substances (glutamate and GABA) into glutamine. Experiments on avian retinae have shown that GS expression is developmentally regulated by glucocorticoid hormones and, to a lesser extent, by a non-hormonal control mechanism(s). Much less is known about GS regulation in mammalian retinae, although either increases or decreases of GS immunoreactivity have been observed in Müller cells in different forms of retinal pathologies. We studied GS expression in postnatal rabbit retinae both in vivo and explanted as wholemounts in vitro, using immunocytochemistry and Western immunoblotting. GS expression was detectable in vivo from the fourth postnatal day, and increased rapidly within the first weeks of life. Levels were lower in vitro than in vivo by an order of magnitude, and could be significantly stimulated (> 60-110%) in vitro by application of hydrocortisone, conditioned medium from cultured retinal pigment epithelium and glutamate or ammonia, but not GABA. It is concluded that GS expression in mammalian Müller cells is dependent on systemic control by glucocorticoid hormones, as observed in birds, but environmental (activity-dependent) factors may play a more important role in mammals.

Animals↗

PubMed1997

Mammalian retinal glial (Müller) cells express large-conductance Ca(2+)-activated K+ channels that are modulated by Mg2+ and pH and activated by protein kinase A.

The cell-attached and excised patch configurations of the patch clamp technique were used to characterize Ca(2+)-activated maxi-K+ channels in freshly-isolated Müller glial cells. The cells were dissociated from postmortem adult human and porcine retinas. The maxi-K+ channels in Müller cells of both species display a single channel conductance of 175 pS in cell-attached and inside-out patches (125/110 mM K+). The channels are activated by membrane depolarization and by elevation of intracellular Ca2+. In the presence of 10(-5), 10(-4), and 10(-3) M intracellular free Ca2+, the half-activation voltages are +7.2, -26.6, and -47.5 mV, respectively. The half-activation-[Ca2+] at +10 mV is 8.1 microM, and the Hill coefficient of Ca2+ binding is 1.7, Ba2+ exerts a voltage-dependent block of the open-state probability. The maxi-K+ channels of Müller cells are activated by raising of the intracellular pH as well as by Mg2- ions at the cytosolic face of the channels. Phosphorylation of the channel after cytosolic addition of the catalytic subunit of a cAMP dependent protein kinase in the presence of Mg-ATP caused a shift of the activation curve to negative membrane potentials. Between -40 and -80 mV membrane potentials, the open-state probability rose to 190.3% of the control value (100%) after phosphorylation of the channel. Therefore, phosphorylation enhances sensitivity of the channels to Ca2+ and voltage. The maxi-K+ channels may provide a link between second messenger systems and membrane conductance of retinal Müller cells and may have an important function in repolarization of the Müller cell membrane and, therefore, in the maintainance of the retinal spatial K+ buffering mechanisms.

Adenosine Triphosphate↗

PubMed1997

Loss of inwardly rectifying potassium currents by human retinal glial cells in diseases of the eye.

We compared the inward K+ currents of Müller glial cells from healthy and pathologically changed human retinas. To this purpose, the whole-cell voltage-clamp technique was performed on noncultured Müller cells acutely isolated from human retinas. Cells originated from retinas of four healthy organ donors and of 24 patients suffering from different vitreoretinal and chorioretinal diseases. Müller cells from organ donors displayed inward K+ currents in the whole-cell mode similar to those found in other species. In contrast, this pattern was clearly changed in the Müller cells from patient retinas. In whole-cell recordings many Müller cells had strongly decreased inward K+ current amplitudes or lost these currents completely. Thus, the mean input resistance of Müller cells from patients was significantly increased to 1,129 +/- 812 M omega, compared to 279 +/- 174 M omega in Müller cells from healthy organ donor retinas. Accordingly, since the membrane potential is mainly determined by the K+ inward conductance in healthy Müller cells, a large amount of Müller cells from patient retinas had a membrane potential which was significantly lower than that of Müller cells from control eyes. The mean membrane potentials were -37 +/- 24 mV and -63 +/- 25 mV for patient and donor Müller cells, respectively. The newly described membrane characteristic changes of Müller cells from patient eyes are assumed to interfere severely with normal retinal function: (1) the retinal K+ homeostasis, which is partly regulated by the Müller cell-mediated spatial buffering, should be disturbed, and (2) the diminished membrane potential should influence voltage-dependent transporter systems of the Müller cells, e.g., the Na(+)-dependent glutamate uptake.

Barium↗

PubMed1997

Development of the neonatal rabbit retina in organ culture. 1. Comparison with histogenesis in vivo, and the effect of a gliotoxin (alpha-aminoadipic acid).

Organ cultures from neonatal rabbit retinae grew well over periods of up to 2 weeks in vitro. Proliferation in vitro declined in parallel with the decline seen in vivo, although the rate of proliferation in the explants was slightly reduced. The proliferation of progenitor cells in vitro produced the same cell types produced postnatally in vivo. Postnatally generated cell clones, labeled by means of a retroviral vector, consisted mainly of rods and Müller cells. The layers of the retinae developed as in vivo; an outer plexiform layer occurred after the first 2 days in vitro. Ultrastructurally, ribbon synapses (outer and inner plexiform layer) and conventional synapses (inner plexiform layer) were observed. The photoreceptor cells grew well-developed inner segments and cilia but no mature outer segments. The cultured retinae contained a well-developed, regular lattice of Müller cells expressing vimentin as in vivo. The neuron-to-Müller cell-ratios were essentially the same as in vivo, viz. about 15 to 16 neurons, among them about 10 to 11 (rod) photoreceptor cells per Müller cell. When the glia cell-specific toxin alpha-aminoadipic acid (alpha AAA) was applied, the pattern of vimentin-positive Müller cells became irregular, or even locally missing. In such cases, the tissue became disorganized as indicated by a local disappearance of the regular layering, and development of many rosettes. It is concluded that an intact lattice of Müller cells is necessary for the migration of young neurons, and for correct formation of retinal layers.

2-Aminoadipic Acid↗

PubMed1997

Hepatic retinopathia. Changes in retinal function.

In patients suffering from hepatic failure, the brain is subject to defined morphological and functional changes known as hepatic encephalopathia (HE). The morphological changes are dominated by glial cells (Alzheimer-type II astrocytes). It has recently been possible to demonstrate, that the retinal glia (Müller) cells undergo similar morphological changes. The present study was carried out in order to reveal if these Müller cell changes cause any characteristic functional deficits. We examined 11 patients with different stages of HE due to liver cirrhosis. Six patients were at stage 0 or 1 (group I) and five at stage 2 or 3 (group II). They underwent ophthalmological routine examination, colour vision testing and standard ERG recording. None of the patients reported impaired vision, in daylight or at night. There were no fundus abnormalities except very mild changes of the pigment epithelium and abnormal reflexes of the inner limiting membrane, especially in the higher HE stages. The number of confusions in the colour arrangement test increased with the higher stages of HE, preferably in the tritan axis. The scotopic a- and b-waves of the electroretinogram (ERG) were almost unchanged in group I and significantly decreased and delayed in group II. The photopic ERG b-wave amplitudes were changed in a similar fashion. Oscillatory potentials proved to be most sensitive to hepatotoxic changes. Their latencies were significantly delayed even in group I. Amplitudes were decreased significantly only in group II. Patients suffering from hepatic failure and accompanying HE display functional abnormalities of the retina. These are best demonstrated by the ERG, and correlate well with the degree of HE. A hypothesis is presented that relates the observed functional changes to altered neurotransmitter levels and impaired retinal glial-neuronal interaction, due to Müller cell damage caused by elevated ammonia levels.

Adult↗

PubMed1997

Two modes of stimulation by ammonia of taurine release from cultured rabbit Müller cells.

A previous study revealed that a 10-min ('acute') treatment of cultured Müller glia with ammonium ions (further referred to as 'ammonia') at 0.5-5 mM concentration stimulated the release of newly loaded taurine (Tau) by a cAMP-dependent, osmoresistant mechanism. Here we showed that a 24 h treatment of the cells with 1 mM ammonia increased both Tau release and intracellular cAMP content in a degree similar to acute treatment with 5 mM ammonia, and the effects were similarly resistant to an increase of medium tonicity by addition of 50 mM sucrose. A 65 min superfusion of the cells with a guanylate cyclase inhibitor [methylene blue (MB)], a protein kinase inhibitor (H7) and a calcium-free buffer containing 10 mM Mg2+ (OCa-10Mg) also increased Tau release and cAMP level in the cells. Acute treatment with 5 mM ammonia of cells pretreated for 24 h with 1 mM ammonia or for 65 min with MB, H7 or OCa-10Mg produced additional significant stimulation of Tau release, without further increasing the cAMP level in the cells. By contrast, a 10-min treatment with 65 mM KCl, which is a potent, cAMP-independent stimulus of Tau release in untreated Müller glia, produced no further enhancement of Tau release in ammonia-, MB-, H7 or OCa-10Mg-pretreated cells. The results indicate that acute treatment with ammonia, on top of treatments that evoke Tau release associated with an increase of cAMP, produces an extra Tau release that is cAMP-independent. Tau released by this extra ammonia treatment possibly originates from a different pool than Tau liberated by the pretreatments or 65 mM KCl.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1997

Comparative studies on mammalian Müller (retinal glial) cells.

Müller cells from 22 mammalian species were subjected to morphological and electrophysiological studies. In the 'midperiphery' of retinae immunocytochemically labeled for vimentin, estimates of Müller cell densities per unit retinal surface area, and of neuron-to-(Müller) glia indices were performed. Müller cell densities were strikingly similar among the species studied (around 8000-11,000 mm-2) with the extremes of the horse (< or = 5000 mm-2) and the tree shrew (> or = 20,000 mm-2). By contrast, the number of neurons per Müller cell varied widely, being clustered at 6-8 (in retinae with many cones), at about 16, and at up to more than 30 (in strongly rod-dominated retinae). Isolated Müller cell volumes were estimated morphometrically, and cell surface areas were calculated from membrane capacities. Müller cells isolated from thick vascularized retinae (carnivores, rats, mice, ungulates) were longer and thinner, and had smaller volumes but higher surface-to-volume ratios than cells from thin paurangiotic (i.e. with blood vessels only near the optic disc) or avascular retinae (rabbits, guinea pigs, horses, zebras). In whole-cell voltage-clamp studies, Müller cells from all mammals studied displayed two dominant K+ conductances, inwardly rectifying currents and delayed rectifier currents. TTX-sensitive Na+ currents were recorded only in some species. Based on these data, the following hypotheses are presented, (a) neuron-to-(Müller) glia indices are determined by precursor cell proliferation rather than by metabolic demands; (b) Müller cell volumes depend on available space rather than on the number of supported neurons; and (c) it follows that, the specific metabolic activities of Müller cells must differ greatly between species, a difference that may contribute to distinct patterns of retinal vascularization.

Animals↗

PubMed1997

The Müller (glial) cell in normal and diseased retina: a case for single-cell electrophysiology.

In the retina of most vertebrates there exists only one type of macroglia, the Müller cell. Müller cells express voltage-gated ion channels, neurotransmitter receptors and various uptake carrier systems. These properties enable the Müller cells to control the activity of retinal neurons by regulating the extracellular concentration of neuroactive substances such as K+, GABA and glutamate. We show here how electrophysiological recordings from enzymatically dissociated mammalian Müller cells can be used to study these mechanisms. Müller cells from various species have Na(+)-dependent GABA uptake carriers, but only cells from primates have additional GABA receptors that activate Cl- channels. Application of glutamate analogues causes enhanced membrane currents recorded from Müller cells in situ but not from isolated cells. We show that mammalian Müller cells have no ionotropic glutamate receptors but respond to increased K+ release from glutamate-stimulated retinal neurons. This response is involved in extracellular K+ clearance and is mediated by voltage-gated (inwardly rectifying) K+ channels which are abundantly expressed by healthy Müller cells. In various cases of human retinal pathology, currents through these channels are strongly reduced or even extinguished. Another type of voltage-gated ion channels, observed in Müller cells from many mammalian species, are Na+ channels. In Müller cells from diseased human retinae, voltage-dependent Na+ currents were significantly increased in comparison to cells from control donors. Thus, the expression of glial ion channels seems to be controlled by neuronal signals. This interaction may be involved in the pathogenesis of retinal gliosis which inevitably accompanies any degeneration of retinal neurons. In particular, Müller cell proliferation may be triggered by mechanisms requiring the activation of Ca(2+)-dependent K+ channels. Ca(2+)-dependent K+ currents are easily elicitable in Müller cells from degenerating retinae and can be blocked by 1 mM TEA (tetraethylammonium). In purified Müller cell cultures, the application of 1 mM TEA greatly reduces the proliferative activity of the cells. These data clearly show that Müller cells are altered in cases of neuronal degeneration and may be crucially involved in pathogenetic mechanisms of the retina.

Animals↗

PubMed1997

LiCl-induced malformations of the eyes and the rostral CNS in Xenopus laevis.

Ocular malformations such as synophthalmia and cyclopia occur in all vertebrates including humans. We induced fused eye anlagen by exposure of Xenopus laevis embryos to dissolved LiCl. Three basic forms of malformations were observed in the rostral CNS; these represent major steps in a continuous sequence of ocular abnormalities: (i) "hour-glass eyes", (ii) synophthalmia, and (iii) cyclopia vera. The type of abnormity induced seemed to depend on the time and/or the dosage of LiCl exposure. In all cases studied, the histology of the retinae was normal. We reconstructed olfactory organs, eyes, and rostral portions of the CNS from serial sections of 16 Xenopus larvae. Our results confirm that defective bilateralization affects all parts of the rostral CNS and suggest that the telencephalon displays the weakest, and the eyes have the highest, resistance against midline fusion. Defective bilateralization also involves the pineal anlagen, as duplication or enlargement of the pineal organs occurred in most cases with ocular fusion. We conclude that LiCl-induced interferences with ontogenetic bilateralization of prosencephalic structures are the consequence of an elimination of an, as yet, undefined bilateralization signal from (ventral) midline structures of the neural plate and tube.

Abnormalities, Drug-Induced↗

PubMed1997

Changes in CD44 and ApoE immunoreactivities due to retinal pathology of man and rat.

In cases of retinal light damage, glaucoma, or senile macula degeneration, the loss of retinal neurons is thought to cause alterations of glial cells. We performed immunocytochemical studies on retinae of (i) healthy rats and human donors, (ii) rats exposed to enhanced illumination for 24 months, a procedure which leads to complete loss of photoreceptor cells, (iii) a human donor who had suffered from senile macula (photoreceptor cell) degeneration, and (iv) human donors who had suffered from glaucoma, known to be accompanied by a loss of ganglion cells and other retinal neurons. Furthermore, Müller cells were enzymatically isolated from human glaucomatous retinae. All preparations were subjected to immunocytochemistry for CD44 antigen and Apolipoprotein E (ApoE). In normal rat and human retinae, CD44 immunoreactivity was observed in the microvillous sclerad processes of Müller cells: in human retinae, perivascular (astro-)glial cell processes were also CD44 immunopositive. ApoE immunoreactivity was only found in some perivascular (astro-)glial cell processes of human retinae. Both rat and human Müller cells respond to photoreceptor cell damage by increased, and ectopic, expression of the CD44 antigen. Increased ApoE immunoreactivity was found in Müller cells from degenerative human retinae, but rarely in light-damaged rat retinae. It is concluded that degeneration-related reorganization involves enhanced expression of the glial cell adhesion molecule CD44 as well as elevated activity of the glial lipid transport molecule ApoE.

Aged↗

PubMed1997

Antibody to keyhole limpet hemocyanin labels retinal horizontal cells in some amphibians, but not in others.

Antibody to keyhole limpet hemocyanin (KLH) reacts with putative horizontal cells in anuran amphibians of the superfamily Bufonoidea. The reactive epitope appears to be located on the cell membrane. No KLH-like immunoreactivity was observed in the outer plexiform layer (OPL) of anurans not members of this superfamily, nor in the OPL of urodeles or other vertebrates. Thus KLH-like immunoreactivity in the OPL provides a tool for assessing phylogenetic relationships within anurans.

Amphibians↗

PubMed1997

Retinal light damage vs. normal aging of rats: altered morphology, intermediate filament expression, and nuclear organization of Müller (glial) cells.

In retinal light damage, degeneration of photoreceptors results in alterations of glial (Müller) cells. In particular, Müller cells show signs of gliosis such as thickening of their stem processes, and expression of glial fibrillary acidic protein (GFAP) which is normally not detectable by immunocytochemistry. We were interested in a quantification of these morphological alterations, and in possible effects of an application of free radical scavengers (Ginkgobiloba extract EGb 761). For this purpose, we studied Müller cells in retinae of albino rats exposed to enhanced illumination for 24 months, a procedure which causes a complete loss of photoreceptor cells. The cells were labeled by (i) bulk filling with the fluorescent dye, Procion yellow, and by (ii) immunocytochemical demonstration of vimentin and GFAP. One group of rats was fed daily with EGb 761 during the last 8 months of life when the remaining photoreceptors (about 50%) died. The retinae were compared with retinae from 3 months-old albino rats, serving as normal young controls, and with retinae from 24 month-old pigment rats, representing normal aging processes. As age-related changes of the ultrastructure of glial cell (astrocytic) nuclei have been described in the literature, the organization of Müller cell nuclei was also studied by an argyrophilic stain, and by electron microscopy. We found that in the thin light-damaged retinae, Müller cells were shorter but thicker than in age-matched control retinae. The volumes of their vitread stem processes were almost unchanged. Müller cells were GFAP-immunoreactive in the light-damaged retinae but not in the controls. The application of EGb 761 prevented the expression by Müller cells of (detectable levels of) GFAP. By contrast, in retinae from EGb 761-treated animals the volumes of the vitread stem processes were significantly increased in comparison to untreated animals. The number of nuclear organization regions was significantly enhanced in Müller cell nuclei from light-damaged untreated albino rats, as compared with the young controls. Application of EGb 761 prevented much of this increase. Thus, exogeneous free radical scavengers do not prevent the occurrence of an reactive hypertrophy but inhibit the expression of "pathological marker molecules", and the (accompanying) signs of enhanced nuclear activity.

Aging↗

PubMed1996

Immunocytochemical demonstration of alpha 2-M-R/LRP on Müller (glial) cells isolated from rabbit and human retina.

The alpha 2-macroglobulin receptor/low-density lipoprotein receptor-related protein (alpha 2-M-R/LRP) is a multifunctional receptor which has been implicated in lipoprotein metabolism, clearance of proteinase-proteinase inhibitor complexes and regulation of growth factor/cytokine metabolism. This receptor is abundantly present in numerous tissues and organs such as liver, lung, placenta and brain. In brain it is expressed in neurones but not in normal macroglia. Using immunocytochemistry and monoclonal antibodies against the large extracellular receptor subunit we have detected alpha 2-M-R/LRP on enzymatically isolated retinal Müller (glial) cells. This receptor may be involved in vital functions of Müller cells.

Animals↗

PubMed1996

Ammonia-induced taurine release from cultured rabbit Müller cells is an osmoresistant process mediated by intracellular accumulation of cyclic AMP.

A previous study demonstrated the release of newly loaded radiolabelled taurine (Tau) from cultured rabbit Müller glia not only following typical cell volume-increasing treatments with high (65 mM) potassium ions or hypotonic media, but also with ammonium chloride (further referred to as ammonia), in a dose-dependent manner, at doses ranging from physiological (0.25 mM) to those accompanying hyperammonemic coma (5 mM) (Faff-Michalak et al., Glia 10:114-120, 1994). Stimulation of Tau release by ammonia, but not by 65 mM potassium, was correlated with a dose-dependent increase of intracellular cAMP levels. The release, as measured at 5 mM ammonia, was abolished by compounds that prevented cAMP increase: an adenylate cyclase inhibitor, miconazole, a protein kinase A inhibitor HA 1004, an anion channel blocker, niflumic acid, and a Tau transport site agonist, beta-alanine. The release by ammonia differed from potassium-induced release in its resistance to 1) increase of medium tonicity by addition of 50 mM sucrose; 2) addition of the anion/cation cotransport blocker, furosemide; and 3) removal of calcium from the superfusion medium. The results suggest that ammonia-induced Tau release is mediated by intracellular accumulation of cAMP and may occur either via an osmoresistant, cAMP-controlled channel or a cAMP-activated Tau transporter. The release observed at the physiological concentration of ammonium chloride suggest a role for ammonia as a signal molecule.

Ammonia↗