BDNF and Hippocampal Synaptic Plasticity

Brain-derived neurotrophic factor (BDNF) belongs to a family of small secreted proteins that also include nerve growth factor, neurotrophin 3, and neurotrophin 4. BDNF stands out among all neurotrophins by its high expression levels in the brain and its potent effects at synapses. Several aspects of BDNF biology such as transcription, processing, and secretion are regulated by synaptic activity. Such observations prompted the suggestion that BDNF may regulate activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP), a sustained enhancement of excitatory synaptic efficacy thought to underlie learning and memory. Here, we will review the evidence pointing to a fundamental role of this neurotrophin in LTP, especially within the hippocampus. Prominent questions in the field, including the release and action sites of BDNF during LTP, as well as the signaling and molecular mechanisms involved, will also be addressed. The diverse effects of BDNF at excitatory synapses are determined by the activation of TrkB receptors and downstream signaling pathways, and the functions, typically opposing in nature, of its immature form (proBDNF). The activation of p75^NTR receptors by proBDNF and the implications for long-term depression will also be addressed. Finally, we discuss the synergy between TrkB and glucocorticoid receptor signaling to determine cellular responses to stress.

Adaptive preconditioning in neurological diseases - therapeutic insights from proteostatic perturbations

In neurological disorders, both acute and chronic neural stress can disrupt cellular proteostasis, resulting in the generation of pathological protein. However in most cases, neurons adapt to these proteostatic perturbations by activating a range of cellular protective and repair responses, thus maintaining cell function. These interconnected adaptive mechanisms comprise a 'proteostasis network' and include the unfolded protein response, the ubiquitin proteasome system and autophagy. Interestingly, several recent studies have shown that these adaptive responses can be stimulated by preconditioning treatments, which confer resistance to a subsequent toxic challenge - the phenomenon known as hormesis. In this review we discuss the impact of adaptive stress responses stimulated in diverse human neuropathologies including Parkinson׳s disease, Wolfram syndrome, brain ischemia, and brain cancer. Further, we examine how these responses and the molecular pathways they recruit might be exploited for therapeutic gain. This article is part of a Special Issue entitled SI:ER stress.

Brain ischemia downregulates the neuroprotective GDNF-Ret signaling by a calpain-dependent mechanism in cultured hippocampal neurons

The glial cell line-derived neurotrophic factor (GDNF) has an important role in neuronal survival through binding to the GFRα1 (GDNF family receptor alpha-1) receptor and activation of the receptor tyrosine kinase Ret. Transient brain ischemia alters the expression of the GDNF signaling machinery but whether the GDNF receptor proteins are also affected, and the functional consequences, have not been investigated. We found that excitotoxic stimulation of cultured hippocampal neurons leads to a calpain-dependent downregulation of the long isoform of Ret (Ret51), but no changes were observed for Ret9 or GFRα1 under the same conditions. Cleavage of Ret51 by calpains was selectively mediated by activation of the extrasynaptic pool of N-methyl-d-aspartate receptors and leads to the formation of a stable cleavage product. Calpain-mediated cleavage of Ret51 was also observed in hippocampal neurons subjected to transient oxygen and glucose deprivation (OGD), a model of global brain ischemia, as well as in the ischemic region in the cerebral cortex of mice exposed to transient middle cerebral artery occlusion. Although the reduction of Ret51 protein levels decreased the total GDNF-induced receptor activity (as determined by assessing total phospho-Ret51 protein levels) and their downstream signaling activity, the remaining receptors still showed an increase in phosphorylation after incubation of hippocampal neurons with GDNF. Furthermore, GDNF protected hippocampal neurons when present before, during or after OGD, and the effects under the latter conditions were more significant in neurons transfected with human Ret51. These results indicate that the loss of Ret51 in brain ischemia partially impairs the neuroprotective effects of GDNF.

Proteomics-based technologies in the discovery of biomarkers for multiple sclerosis in the cerebrospinal fluid

Multiple Sclerosis is the most common non-traumatic disorder of the central nervous system and is generally regarded as an immune-mediated disorder that occurs in young adults. Since cerebrospinal fluid is in close contact with the extracellular surface of the brain, it is of great interest to examine possible biomarkers for multiple sclerosis. Proteomic studies of cerebrospinal fluid samples represent an important step towards a better understanding of the disease and may lead to the identification of clinically useful markers. Methodological advances in proteomics allowed the comparison of the protein content in different cerebrospinal fluid samples, using gel or liquid-based approaches coupled with mass spectrometry. In this paper, we discuss the advantages and limitations of the strategies employed and the potential biomarkers for multiple sclerosis identified so far using proteomics-based approaches.

Juice of Bryophyllum pinnatum (Lam.) inhibits oxytocin-induced increase of the intracellular calcium concentration in human myometrial cells

The use of preparations from Bryophyllum pinnatum in tocolysis is supported by both clinical (retrospective comparative studies) and experimental (using uterus strips) evidence. We studied here the effect of B. pinnatum juice on the response of cultured human myometrial cells to stimulation by oxytocin, a hormone known to be involved in the control of uterine contractions by increasing the intracellular free calcium concentration ([Ca2+]i). In this work, [Ca2+]i was measured online during stimulation of human myometrial cells (hTERT-C3 and M11) with oxytocin, which had been pre-incubated in the absence or in the presence of B. pinnatum juice. Since no functional voltage-gated Ca2+ channels could be detected in these myometrial cells, the effect of B. pinnatum juice was as well studied in SH-SY5Y neuroblastoma cells, which are known to have such channels and can be depolarised with KCl. B. pinnatum juice prevented the oxytocin-induced increase in [Ca2+]i in hTERT-C3 human myometrial cells in a dose-dependent manner, achieving a ca. 80% inhibition at a 2% concentration. Comparable results were obtained with M11 human primary myometrial cells. In hTERT-C3 cells, prevention of the oxytocin-induced increase in [Ca2+]i was independent of the extracellular Ca2+ concentration and of voltage-dependent Ca2+-channels. B. pinnatum juice delayed, but did not prevent the depolarization-induced increase in [Ca2+]i in SH-SY5Y cells. Taken together, the data suggest a specific and concentration-dependent effect of B. pinnatum juice on the oxytocin signalling pathway, which seems to corroborate its use in tocolysis. Such a specific mechanism would explain the rare and minor side-effects in tocolysis with B. pinnatum as well as its high therapeutic index.

Excitotoxicity through Ca2+-permeable AMPA receptors requires Ca2+-dependent JNK activation

The GluA4-containing Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (Ca-AMPARs) were previously shown to mediate excitotoxicity through mechanisms involving the activator protein-1 (AP-1), a c-Jun N-terminal kinase (JNK) substrate. To further investigate JNK involvement in excitotoxic pathways coupled to Ca-AMPARs we used HEK293 cells expressing GluA4-containing Ca-AMPARs (HEK-GluA4). Cell death induced by overstimulation of Ca-AMPARs was mediated, at least in part, by JNK. Importantly, JNK activation downstream of these receptors was dependent on the extracellular Ca(2+) concentration. In our quest for a molecular link between Ca-AMPARs and the JNK pathway we found that the JNK interacting protein-1 (JIP-1) interacts with the GluA4 subunit of AMPARs through the N-terminal domain. In vivo, the excitotoxin kainate promoted the association between GluA4 and JIP-1 in the rat hippocampus. Taken together, our results show that the JNK pathway is activated by Ca-AMPARs upon excitotoxic stimulation and suggest that JIP-1 may contribute to the propagation of the excitotoxic signal.

Effect of Skin Sensitizers on Inducible Nitric Oxide Synthase Expression and Nitric Oxide Production in Skin Dendritic Cells: Role of Different Immunosuppressive Drugs

Nitric oxide (NO) is involved in the pathogenesis of acute and chronic inflammatory conditions, namely in allergic contact dermatitis (ACD). However, the mechanism by which NO acts in ACD remains elusive. The present study focuses on the effects of different contact sensitizers (2,4-dinitrofluorbenzene, 1,4-phenylenediamine, nickel sulfate), the inactive analogue of DNFB, 2,4-dichloronitrobenzene, and two irritants (sodium dodecyl sulphate and benzalkonium chloride) on the expression of the inducible isoform of nitric oxide synthase (iNOS) and NO production in skin dendritic cells. It was also studied the role of different immunosuppressive drugs on iNOS expression and NO production. Only nickel sulfate increased the expression of iNOS and NO production being these effects inhibited by dexamathasone. In contrast, cyclosporin A and sirolimus, two other immunosuppressive drugs tested, did not affect iNOS expression triggered by nickel.

Excitotoxicity mediated by Ca2+-permeable GluR4-containing AMPA receptors involves the AP-1 transcription factor

Cells preferentially expressing GluR4-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors are particularly sensitive to excitotoxicity mediated through non-N-methyl-D-aspartate receptors. However, the excitotoxic signalling pathways associated with GluR4-containing AMPA receptors are not known. In this work, we investigated the downstream signals coupled to excitotoxicity mediated by Ca2+-permeable GluR4-containing AMPA receptors, using a HEK 293 cell line constitutively expressing the GluR4flip subunit of AMPA receptors (HEK-GluR4). Glutamate stimulation of GluR4-containing AMPA receptors decreased cell viability, in a calcium-dependent manner, when the receptor desensitisation was prevented with cyclothiazide. The excitotoxic stimulation mediated through GluR4-containing AMPA receptors increased activator protein-1 (AP-1) DNA-binding activity. Inhibition of the AP-1 activity by overexpression of a c-Jun dominant-negative form protected HEK-GluR4 cells against excitotoxic damage. Taken together, the results indicate that overactivation of Ca2+-permeable GluR4-containing AMPA receptors is coupled to a death pathway mediated, at least in part, by the AP-1 transcription factor.

Neuroprotection by BDNF against glutamate-induced apoptotic cell death is mediated by ERK and PI3-kinase pathways

Neurotrophins protect neurons against glutamate excitotoxicity, but the signaling mechanisms have not been fully elucidated. We studied the role of the phosphatidylinositol 3-kinase (PI3-K) and Ras/mitogen-activated protein kinase (MAPK) pathways in the protection of cultured hippocampal neurons from glutamate induced apoptotic cell death, characterized by nuclear condensation and activation of caspase-3-like enzymes. Pre-incubation with the neurotrophin brain-derived neurotrophic factor (BDNF), for 24 h, reduced glutamate-evoked apoptotic morphology and caspase-3-like activity, and transiently increased the activity of the PI3-K and of the Ras/MAPK pathways. Inhibition of the PI3-K and of the Ras/MAPK signaling pathways abrogated the protective effect of BDNF against glutamate-induced neuronal death and similar effects were observed upon inhibition of protein synthesis. Moreover, incubation of hippocampal neurons with BDNF, for 24 h, increased Bcl-2 protein levels. The results indicate that the protective effect of BDNF in hippocampal neurons against glutamate toxicity is mediated by the PI3-K and the Ras/MAPK signaling pathways, and involves a long-term change in protein synthesis.

Contact sensitizers downregulate the expression of the chemokine receptors CCR6 and CXCR4 in a skin dendritic cell line

Chemokines are involved in the control of dendritic cell (DC) trafficking, which is critical for the immune response, namely in allergic contact dermatitis (ACD). In this work, we investigated by flow cytometry the effect of the contact sensitizers 2,4-dinitrofluorobenzene (DNFB), 1,4-phenylenediamine (PPD) and nickel sulfate (NiSO(4)), on the surface expression of the chemokine receptors CCR6 and CXCR4 in DC. As an experimental model of a DC we used a fetal skin-derived dendritic cell line (FSDC), which has morphological, phenotypical and functional characteristics of skin DC. Our results show that all the skin sensitizers studied decreased the membrane expression of the chemokine receptors CCR6 and CXCR4. In contrast, 2,4-dichloronitrobenzene (DCNB), the inactive analogue of DNFB without contact sensitizing properties, was without effect on the surface expression of these receptors. Lipopolysaccharide (LPS), which induces the maturation of DC, also reduced surface CCR6 and CXCR4 expression.

Intracellular lithium and cyclic AMP levels are mutually regulated in neuronal cells

In this work, we studied the effect of intracellular 3',5'-cyclic adenosine monophosphate (cAMP) on Li+ transport in SH-SY5Y cells. The cells were stimulated with forskolin, an adenylate cyclase activator, or with the cAMP analogue, dibutyryl-cAMP. It was observed that under forskolin stimulation both the Li+ influx rate constant and the Li+ accumulation in these cells were increased. Dibutyryl-cAMP also increased Li+ uptake and identical results were obtained with cortical and hippocampal neurons. The inhibitor of the Na+/Ca2+ exchanger, KB-R7943, reduced the influx of Li+ under resting conditions, and completely inhibited the effect of forskolin on the accumulation of the cation. Intracellular Ca2+ chelation, or inhibition of N-type voltage-sensitive Ca2+ channels, or inhibition of cAMP-dependent protein kinase (PKA) also abolished the effect of forskolin on Li+ uptake. The involvement of Ca2+ on forskolin-induced Li+ uptake was confirmed by intracellular free Ca2+ measurements using fluorescence spectroscopy. Exposure of SH-SY5Y cells to 1 mm Li+ for 24 h increased basal cAMP levels, but preincubation with Li+, at the same concentration, decreased cAMP production in response to forskolin. To summarize, these results demonstrate that intracellular cAMP levels regulate the uptake of Li+ in a Ca(2+)-dependent manner, and indicate that Li+ plays an important role in the homeostasis of this second messenger in neuronal cells.

Dexamethasone-induced and estradiol-induced CREB activation and annexin 1 expression in CCRF-CEM lymphoblastic cells: evidence for the involvement of cAMP and p38 MAPK

AIMS

Annexin 1 (ANXA1), a member of the annexin family of calcium-binding and phospholipid-binding proteins, is a key mediator of the anti-inflammatory actions of steroid hormones. We have previously demonstrated that, in the human lymphoblastic CCRF-CEM cell line, both the synthetic glucocorticoid hormone, dexamethasone (Dex), and the estrogen hormone, 17beta-estradiol (E2beta), induce the synthesis of ANXA1, by a mechanism independent of the activation of their nuclear receptors. Recently, it was reported that the gene coding for ANXA1 contains acAMP-responsive element (CRE). In this work, we investigated whether Dex and E2beta were able to induce the activation of CRE binding proteins (CREB) in the CCRF-CEM cells. Moreover, we studied the intracellular signalling pathways involved in CREB activation and ANXA1 synthesis in response to Dex and E2beta; namely, the role of cAMP and the p38 mitogen activated protein kinase (MAPK).

RESULTS

The results show that Dex and E2beta were as effective as the cAMP analogue, dBcAMP, in inducing CREB activation. On the contrary, dBcAMP induced ANXA1 synthesis as effectively as these steroid hormones. Furthermore, the cAMP antagonist, Rp-8-Br-cAMPS, and the specific p38 MAPK inhibitor,SB203580, effectively prevented both Dex-induced, E2beta-induced and dBcAMP-induced CREB activation and ANXA1 synthesis.

CONCLUSIONS

Taken together, our results suggest that,in CCRF-CEM cells, Dex-induced and E2beta-inducedANXA1 expression requires the activation of the transcription factor CREB, which in turn seems to be mediated by cAMP and the p38 MAPK. These findings also suggest that, besides the nuclear steroid hormone receptors, other transcription factors, namely CREB, may play important roles in mediating the anti-inflammatory actions of glucocorticoids and oestrogen hormones.

Dexamethasone prevents interleukin-1beta-induced nuclear factor-kappaB activation by upregulating IkappaB-alpha synthesis, in lymphoblastic cells

AIMS

Glucocorticoids (GCs) exert some of their anti-inflammatory actions by preventing the activation of the transcription factor nuclear factor (NF)-kappaB. The GC-dependent inhibition of NF-kappaB may occur at different levels, but the mechanisms involved are still incompletely understood. In this work, we investigated whether the synthetic GC, dexamethasone (Dex), modulates the activity of NF-kappaB in the lymphoblastic CCRF-CEM cell line. We also evaluated the ability of Dex to prevent the activation of NF-kappaB in response to the potent proinflammatory cytokine, interleukin (IL)-1beta.

RESULTS

Exposure of the cells to Dex (1 microM) induced the rapid degradation of IkappaB-alpha, leading to the transient translocation of the NF-kappaB family members p65 and p50 from the cytoplasm to the nucleus, as evaluated by western blot. Electrophoretic mobility shift assays revealed that, in the nucleus, these NF-kappaB proteins formed protein-DNA complexes, indicating a transient activation of NF-kappaB. Additionally, Dex also induced de novo synthesis of IkappaB-alpha, following its degradation. Finally, when the cells were exposed to Dex (1 microM) prior to stimulation with IL-1beta (20 ng/ml), Dex was efficient in preventing IL-1beta-induced NF-kappaB activation. The GC antagonist, RU 486 (10 microM), did not prevent any of the effects of Dex reported here.

CONCLUSION

Our results indicate that, in CCRF-CEM cells, Dex prevents NF-kappaB activation, induced by IL-1beta, by a mechanism that involves the upregulation of IkappaB-alpha synthesis, and that depends on the early and transient activation of NF-kappaB.

Dexamethasone induces the secretion of annexin I in immature lymphoblastic cells by a calcium-dependent mechanism

The mechanisms by which glucocorticoids (GC) regulate annexin I (ANXA1) secretion in different cells are still a matter of debate. The aims of this study were to evaluate the ability of dexamethasone (Dex) to induce ANXA1 secretion and to investigate the roles of the intracellular free Ca2+ concentration ([Ca2+]i), and of the GC receptor, on that process. For this purpose, the human immature lymphoblastic CCRF-CEM cell line was used. Treatment of the cells with Dex, for up to 4 h, significantly reduced the intracellular content of ANXA1 and increased the amount of this protein bound to the outer surface of the plasma membrane, whereas exposure of cells to Dex, for 12 h, induced the synthesis of ANXA1. At the same short time periods, Dex also induced a significant increase in the [Ca2+]i. Incubation of the cells with BAPTA-AM (10 microM), a cell-permeant high affinity Ca2+ chelator, completely inhibited Dex-induced ANXA1 secretion. Furthermore, the Ca2+ ionophore, ionomycin, alone induced ANXA1 cleavage, but not its secretion. Additionally, we used brefeldin A to investigate the involvement of the classical endoplasmic reticulum (ER)-Golgi pathway of protein secretion in the release of ANXA1. The GC receptor antagonist, RU486, neither reverted the Dex-dependent ANXA1 secretion nor inhibited the increase of the [Ca2+]i induced by Dex. Together, our results indicate that Dex induces ANXA1 synthesis and secretion in CCRF-CEM cells. ANXA1 secretion in this cell type show the following characteristics: (i) is unlikely to involve the classical ER-Golgi pathway; (ii) requires a Ca(2+)-dependent cleavage of ANXA1; (iii) involves both Ca(2+)-dependent and independent mechanisms; and (iv) is apparently independent of the GC receptor alpha isoform.

Non-specific effects of the MEK inhibitors PD098,059 and U0126 on glutamate release from hippocampal synaptosomes

In order to investigate a role for the extracellular-signal-regulated kinases 1 and 2 (ERK1/2) on hippocampal neurotransmitter release, we studied the effect of commonly used MEK (mitogen-activated protein kinase [MAPK]/ERK kinase) inhibitors, PD098,059 and U0126, on depolarization-induced glutamate release. PD098,059 inhibited glutamate release from hippocampal synaptosomes stimulated with 15 mM KCl in a concentration-dependent manner. At the same range of concentrations, PD098,059 inhibited basal and KCl-stimulated ERK1/2 phosphorylation. U0126, however, did not significantly affect KCl-evoked glutamate release at concentrations shown to inhibit ERK activity. Nonetheless, U0126 unspecifically potentiated depolarization-induced Ca2+-independent glutamate release, which masked a small dose-dependent inhibitory effect on the Ca2+-dependent release. PD098,059 reduced the [Ca2+]i response to KCl by partially inhibiting Ca2+ entry through N- and P-/Q-type voltage-gated Ca2+ channels, whereas U0126 did not affect depolarization-induced Ca2+ influx. To overcome the unspecific effect of PD098,059 on Ca2+ entry, we studied the effect of both MEK inhibitors on glutamate release stimulated by a Ca2+ ionophore. PD098,029 and U0126 showed a small dose-dependent inhibitory effect on ionomycin-induced glutamate release, at concentrations shown to inhibit ionomycin-stimulated ERK phosphorylation. These findings uncover new unspecific actions for both MEK inhibitors and suggest a minor role for ERK in modulating glutamate release in the hippocampus.

Granulocyte-macrophage colony-stimulating factor activates the transcription of nuclear factor kappa B and induces the expression of nitric oxide synthase in a skin dendritic cell line

Nitric oxide (NO) produced by skin dendritic cells and keratinocytes plays an important role in skin physiology, growth and remodelling. Nitric oxide is also involved in skin inflammatory processes and in modulating antigen presentation (either enhancing or suppressing it). In this study, we found that GM-CSF stimulates the expression of the inducible isoform of nitric oxide synthase (iNOS) in a fetal-skin-derived dendritic cell line (FSDC) and, consequently, increases the nitrite production from 11.9 +/- 3.2 micromol/L (basal level) to 26.9 +/- 4.2 micromol/L. Pyrrolidinedithiocarbamate (PDTC) inhibits nitrite production, with a half maximal inhibitory concentration (IC50) of 19.3 micromol/L and the iNOS protein expression in FSDC. In addition, western blot assays revealed that exposure of FSDC to GM-CSF induces the phosphorylation and degradation of the inhibitor of NF-kappaB (IkB), with subsequent translocation of the p50, p52 and RelB subunits of the transcription nuclear factor kappa B (NF-kappaB) from the cytosol to the nucleus. Electrophoretic mobility shift assays (EMSA) showed that FSDC exposure to GM-CSF activates the transcription factor NF-kappaB. Together, these results show that GM-CSF induces iNOS expression in skin dendritic cells by a mechanism involving activation of the NF-kappaB pathway.

17beta-estradiol promotes the synthesis and the secretion of annexin I in the CCRF-CEM human cell line

AIMS

Annexin I (ANXA1), a 37kDa member of the annexin family of Ca2+-binding and phospholipid-binding proteins, is particularly abundant in various populations of peripheral blood leukocytes. Since this protein modulates the anti-inflammatory actions of the steroid hormones, the purpose of this study was to investigate the effects of the female sex steroid hormone, 17beta-estradiol (E2beta), on the synthesis and secretion of ANXA1 in the human CCRF-CEM acute lymphoblastic leukemia cell line.

METHODS

Complementary reverse transcription-polymerase chain reaction and Western blot assays were performed to study the effect of E2beta on the expression of mRNA and protein ANXA1, respectively.

RESULTS AND DISCUSSION

Treatment of CCRF-CEM cells with E2beta, for 30 min, stimulated the synthesis of ANXA1 mRNA molecules, and increased the cellular level of ANXA1 protein. Moreover, when the cells were incubated with E2beta under the same experimental conditions, a significant increase in the amount of ANXA1 secreted from the cells was also detected. ICI 182,780, a selective inhibitor of the intracellular estrogen receptor, had no effect on the E2beta-stimulated expression and externalisation of ANXA1. Taken together, these results indicate that E2beta induces de novo synthesis of ANXA1 and stimulates its secretion in the CCRF-CEM cell line, apparently through a mechanism independent of the intracellular estrogen receptor.

LPS induction of I kappa B-alpha degradation and iNOS expression in a skin dendritic cell line is prevented by the janus kinase 2 inhibitor, Tyrphostin b42

The Janus kinase (JAK) family of protein tyrosine kinases are activated in response to a wide variety of external stimuli. Here we have investigated whether the janus kinase 2 (JAK2) is involved in the induction of nitric oxide synthase type II (iNOS) expression in a mouse fetal skin dendritic cell line (FSDC). In FSDC the expression of iNOS protein and nitric oxide production, in response to the lipopolysaccharide (LPS) stimulus (5 microg/ml), is inhibited by the specific inhibitor of the JAK2, tyrphostin B42 with an half maximal inhibitory concentration (IC(50)) of 9.65 microM. The antioxidant compound pyrrolidinedithiocarbamate (PDTC) inhibits both the nitrite production with an IC(50) of 16.6 microM and the iNOS protein expression in FSDC. In addition, LPS induces the activation of NF-kappa B, and tyrphostin B42 prevents the degradation of the cytosolic factor I kappa B-alpha and blocks the translocation of the NF-kappa B p65 protein subunit into the nucleus. These results indicate that the JAK family of protein kinases and the transcription factor NF-kappa B are involved in the induction of iNOS protein expression in FSDC stimulated with LPS.

Regulation of AMPA receptors by phosphorylation

The AMPA receptors for glutamate are oligomeric structures that mediate fast excitatory responses in the central nervous system. Phosphorylation of AMPA receptors is an important mechanism for short-term modulation of their function, and is thought to play an important role in synaptic plasticity in different brain regions. Recent studies have shown that phosphorylation of AMPA receptors by cAMP-dependent protein kinase (PKA) and Ca2+- and calmodulin-dependent protein kinase II (CaMKII) potentiates their activity, but phosphorylation of the receptor subunits may also affect their interaction with intracellular proteins, and their expression at the plasma membrane. Phosphorylation of AMPA receptor subunits has also been investigated in relation to processes of synaptic plasticity. This review focuses on recent advances in understanding the molecular mechanisms of regulation of AMPA receptors, and their implications in synaptic plasticity.

Photosensitization of lymphoblastoid cells with phthalocyanines at different saturating incubation times

Photodynamic therapy of cancer is a promising treatment based on the tumor-specific accumulation of photosensitizers followed by irradiation with visible light which induces tumor cell death. The effect of different preincubation times on the photosensitization efficiency of the phthalocyanines AlPc and AlPcS4 was investigated in lymphoblastoid CCRF-CEM cells under conditions that allow maximal uptake of the sensitizers. First, the time course for the uptake of AlPcS4 and AlPc by CCRF-CEM cells and by the pheochromocytoma PC12 cells was compared. The uptake of AlPcS4 by CCRF-CEM cells was not significantly different after 6 h or 24 h incubation, but the photosensitization efficiency of the phthalocyanine was much higher when a 24 h preincubation period was used, with a fluence rate of 5 mW/cm2. However, for a fluence rate of 10 mW/cm2, the photosensitization efficiency of AlPcS4 was almost completely independent of the preincubation time (6 h vs. 24 h) with the phthalocyanine. When the cells were preincubated with 1 micromol/L AlPc for 10 min or 6 h, which allows the same accumulation of sensitizer by the cells, no significant effect of the incubation time on the photodynamic inactivation of CCRF-CEM cells was observed, with fluence rates of 5 mW/cm2 or 10 mW/cm2, for different light doses. Confocal fluorescence microscopy studies did not reveal differences in the localization of the phthalocyanines after maximal uptake was reached. The results show that the preincubation time with AlPcS4, after the maximal uptake is reached, affects cell growth to an extent depending on the fluence rate used, and this effect was not due to a major redistribution of the sensitizer during incubation. However, this was not observed when AlPc was used.

Adenosine A1 receptors inhibit Ca2+ channels coupled to the release of ACh, but not of GABA, in cultured retina cells

We investigated the effect of adenosine A1 receptors on the release of acetylcholine (ACh) and GABA, and on the intracellular calcium concentration ([Ca2+]i) response in cultured chick amacrine-like neurons, stimulated by KCl depolarization. The KCl-induced release of [3H]ACh, but not the release of [14C]GABA, was potentiated when adenosine A1 receptor activation was prevented by perfusing the cells with adenosine deaminase (ADA) or with 1,3-dipropyl-8-cycloentylxanthine (DPCPX). The changes in the [Ca2+]i induced by KCl depolarization, measured in neurite segments of single cultured cells, were also modulated by endogenous adenosine, acting on adenosine A1 receptors. Our results show that adenosine A1 receptors inhibit Ca2+ entry coupled to ACh release, but not to the release of GABA, suggesting that the synaptic vesicles containing each neurotransmitter are located in different zones of the neurites, containing different VSCC and/or different densities of adenosine A1 receptors.

Involvement of JAK2 and MAPK on type II nitric oxide synthase expression in skin-derived dendritic cells

In this report, we demonstrate that a fetal mouse skin-derived dendritic cell line produces nitric oxide (NO) in response to the endotoxin [lipopolysaccharide (LPS)] and to cytokines [tumor necrosis factor-alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF)]. Expression of the inducible isoform of NO synthase (iNOS) was confirmed by immunofluorescence with an antibody against iNOS. The tyrosine kinase inhibitor genistein decreased LPS- and GM-CSF-induced nitrite (NO(-2)) production. The effect of LPS and cytokines on NO(-2) production was inhibited by the Janus kinase 2 (JAK2) inhibitor tyrphostin B42. The p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB-203580 also reduced the NO(-2) production evoked by LPS, TNF-alpha, or GM-CSF, but it was not as effective as tyrphostin B42. Inhibition of MAPK kinase with PD-098059 also slightly reduced the effect of TNF-alpha or GM-CSF on NO(-2) production. Immunocytochemistry studies revealed that the transcription factor nuclear factor-kappaB was translocated from the cytoplasm into the nuclei of fetal skin-derived dendritic cells (FSDC) stimulated with LPS, and this translocation was inhibited by tyrphostin B42. Our results show that JAK2 plays a major role in the induction of iNOS in FSDC.

Characterization of ATP release from cultures enriched in cholinergic amacrine-like neurons

Adenosine triphosphate (ATP) has been proposed to play a role as a neurotransmitter in the retina, but not much attention has been given to the regulation of ATP release from retinal neurons. In this work, we investigated the release of ATP from cultures enriched in amacrine-like neurons. Depolarization of the cells with KCl, or activation of alpha-amino-3-hydroxy- 5-methyl-4-isoxazole-propionate (AMPA) receptors, evoked the release of ATP, as determined by the luciferin/luciferase luminescent method. The ATP release was found to be largely Ca(2+) dependent and sensitive to the botulinum neurotoxin A, which indicates that the ATP released by cultured retinal neurons originated from an exocytotic pool. Nitrendipine and omega-Agatoxin IVA, but not by omega-Conotoxin GVIA, partially blocked the release of ATP, indicating that in these cells, the Ca(2+) influx necessary to trigger the release of ATP occurs in part through the L- and the P/Q types of voltage-sensitive Ca(2+) channels (VSCC), but not through N-type VSCC. The release of ATP increased in the presence of adenosine deaminase, or in the presence of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A(1) receptor antagonist, showing that the release is tonically inhibited by the adenosine A(1) receptors. To our knowledge, this is the first report showing the release of endogenous ATP from a retinal preparation.

Metabotropic glutamate receptors modulate [(3)H]acetylcholine release from cultured amacrine-like neurons

Retinal amacrine cells express metabotropic glutamate receptors (mGluRs), but their physiological role is unknown. We investigated the effect of mGluR on [(3)H]acetylcholine release ([(3)H]ACh) from cultured chick amacrine-like neurons. Activation of group III mGluR with the agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) inhibited [(3)H]ACh release evoked by 25 mM KCl in a dose-dependent manner, and this effect was sensitive to pertussis toxin. In contrast, activation of group I or II mGluR with (S)-3, 5-dihydroxyphenylglycine (DHPG) and (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl)glycine (DCG-IV), respectively, did not affect significantly [(3)H]ACh release. The effect of L-AP4 on [(3)H]ACh release was sensitive to nitrendipine, suggesting that it is, at least in part, due to inhibition of L-type Ca(2+) channels. Activation of group III mGluR also partly inhibited omega-conotoxin GVIA-sensitive Ca(2+) channels, coupled to [(3)H]ACh release. The L-AP4 did not affect the cAMP levels measured in amacrine-like neurons depolarized with 25 mM KCl or stimulated with forskolin, indicating that the effect of group III mGluR on [(3)H]ACh release is not due to inhibition of adenylyl cyclase activity. Inhibition of protein kinase A with KT-5720 was without effect on [(3)H]ACh release evoked by 25 mM KCl, further indicating that the effect of group III mGluR on [(3)H]ACh release cannot be attributed to the inhibition of the kinase. The effect of L-AP4 on [(3)H]ACh release was reversed by DHPG or by DCG-IV, and activation of group II mGluR also partially inhibited cAMP production stimulated by forskolin. Taken together, our results show that the effect of group III mGluR on [(3)H]ACh release may be due to a direct inhibition of L- and N-type Ca(2+) channels and is modulated by group I and group II mGluR.

Corelease of two functionally opposite neurotransmitters by retinal amacrine cells: experimental evidence and functional significance

The Dale's law postulates that a neuron releases the same neurotransmitter from all its branches. In the case of multiple neurotransmitters it would require all transmitters to be released from all branches. The retinal cholinergic amacrine cells contain and release gamma-aminobutyric (GABA) and, therefore, if GABA and acetylcholine (ACh) are released at the same sites, this could mean that amacrine cells simultaneously excite and inhibit postsynaptic cells. Conversely, if the two neurotransmitters are released at different synapses, or if their release is regulated in a distinct manner, they may play different physiological roles. Recent studies carried out in cultured cholinergic amacrine-like neurons showed that Ca(2+)-dependent release of ACh and GABA have a different sensitivity to membrane depolarization, to the effect of blockers of voltage gated Ca(2+) channels (VGCC) and to the effect of presynaptic A(1) adenosine receptors. Therefore, it is proposed that in retinal amacrine cells the Ca(2+)-dependent release of ACh and GABA occurs at distinct cellular locations. The possible nature of these release sites and the physiological significance of this model are discussed in this review.

[3H]acetylcholine release from rat amacrine-like neurons is inhibited by adenosine A1 receptor activation

We studied the effect of endogenous adenosine on the release of [3H]acetylcholine ([3H]ACh) in cultures enriched (96.4+/-0.4%) in rat cholinergic amacrine-like neurons, as determined by labeling with an antibody against choline acetyltransferase. A small population of these cells also contained GABA. Using these cultures we observed that both [3H]ACh release, which was largely Ca2+-dependent, and 45Ca2+ influx, evoked by depolarization with 50 mM KCl, were increased when adenosine A1 receptor activation was prevented by removal of endogenous adenosine with adenosine deaminase, or by application of the A1 receptor antagonist DPCPX. Our results indicate that, in cultured rat amacrine-like neurons, the activation of A1 receptors decreases calcium influx and, thereby, inhibits [3H]ACh release.

Kainate-induced retina amacrine-like cell damage is mediated by AMPA receptors

We investigated the effect of domoate, kainate and AMPA on 45Ca2+ uptake and on metabolic activity of cultured chick amacrine-like cells, as measured by reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Domoate and kainate stimulated 45Ca2+ uptake and decreased MTT reduction, in a LY 303070-sensitive manner. AMPA caused a small increase on 45Ca2+ uptake, but it was without effect on MTT reduction. AMPA reduced both the 45Ca2+ entry and neurotoxicity induced by kainate, and cyclothiazide enhanced both the 45Ca2+ entry and neurotoxicity induced by AMPA. The results indicate that the AMPA receptors are the non-NMDA glutamate receptors involved in excitotoxicity.

Modulation of [3H]acetylcholine release from cultured amacrine-like neurons by adenosine A1 receptors

We have investigated the effect of endogenous adenosine on the release of [3H]acetylcholine ([3H]ACh) in cultured chick amacrine-like neurons. The release of [3H]ACh evoked by 50 mM KCl was mostly Ca2+ dependent, and it was increased in the presence of adenosine deaminase and in the presence of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A1 receptor antagonist. The effect of adenosine on [3H]ACh release was sensitive to pertussis toxin (PTX) and was due to a selective inhibition of N-type Ca2+ channels. Ligand binding studies using [3H]DPCPX confirmed the presence of adenosine A1 receptors in the preparation. Using specific inhibitors of the plasma membrane adenosine carriers and of the ectonucleotidases, we found that the extracellular accumulation of adenosine in response to KCl depolarization was due to the release of endogenous adenosine per se and to the extracellular conversion of released nucleotides into adenosine. Activation of adenosine A1 receptors was without effect on the intracellular levels of cyclic AMP under depolarizing conditions, but it inhibited the accumulation of inositol phosphates. Our results indicate that in cultured amacrine-like neurons, the Ca2+-dependent release of [3H]ACh evoked by KCl is under tonic inhibition by adenosine, which activates A1 receptors. The effect of adenosine on the [3H]ACh release may be due to a direct inhibition of N-type Ca2+ channels and/or secondary to the inhibition of phospholipase C and involves the activation of PTX-sensitive G proteins.

Differential acetylcholine and GABA release from cultured chick retina cells

In the present work we investigated the mechanisms controlling the release of acetylcholine (ACh) and of gamma-aminobutyric acid (GABA) from cultures of amacrine-like neurons, containing a subpopulation of cells which are simultaneously GABAergic and cholinergic. We found that 81.2 +/- 2.8% of the cells present in the culture were stained immunocytochemically with an antibody against choline acetyltransferase, and 38.5 +/- 4.8% of the cells were stained with an antibody against GABA. Most of the cells containing GABA (87.0 +/- 2.9%) were cholinergic. The release of acetylcholine and GABA was mostly Ca2+-dependent, although a significant release of [3H]GABA occurred by reversal of its transporter. Potassium evoked the Ca2+-dependent release of [3H]GABA and [3H]acetylcholine, with EC50 of 31.0 +/- 1.0 mm and 21.6 +/- 1.1 mm, respectively. The Ca2+-dependent release of [3H]acetylcholine was significantly inhibited by 1 micrometer tetrodotoxin and by low (30 nm) omega-conotoxin GVIA (omega-CgTx GVIA) concentrations, or by high (300 nm) nitrendipine (Nit) concentrations. On the contrary, the release of [14C]GABA was reduced by 30 nm nitrendipine, or by 500 nm omega-CgTx GVIA, but not by this toxin at 30 nm. The release of either transmitters was unaffected by 200 nm omega-Agatoxin IVA (omega-Aga IVA), a toxin that blocks P/Q-type voltage-sensitive Ca2+ channels (VSCC). The results show that Ca2+-influx through omega-CgTx GVIA-sensitive N-type VSCC and through Nit-sensitive L-type VSCC induce the release of ACh and GABA. However, the significant differences observed regarding the Ca2+ channels involved in the release of each neurotransmitter suggest that in amacrine-like neurons containing simultaneously GABA and acetylcholine the two neurotransmitters may be released in distinct regions of the cells, endowed with different populations of VSCC.

Calcium influx through AMPA receptors and through calcium channels is regulated by protein kinase C in cultured retina amacrine-like cells

The functional modulation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors by protein kinase C (PKC) was investigated in cultures enriched in retinal amacrine-like cells. The kainate-evoked [Ca2+]i increase is due to Ca2+ entry through open AMPA receptor channels, because it was blocked by the active isomer of a 2,3-benzodiazepine (LY 303070), an AMPA receptor antagonist. The AMPA receptor response to kainate was potentiated by phorbol 12-myristate 13-acetate, which specifically stimulates PKC, and it was decreased by bisindolylmaleimide I, a selective inhibitor of PKC, as well as by PKC down-regulation. The results indicate not only that the AMPA receptor activation has a PKC requirement, but also that PKC amplifies maximal receptor activation by 100 microM kainate. The effect of PKC activation or inhibition on voltage-gated Ca2+-channel activity was also investigated. Activation of PKC caused inhibition of Ca2+ channels, and the same effect was produced by inhibition of PKC, whereas the inactive analogue of the phorbol ester did not affect channel activity. Our results show an important role for PKC in regulating the function of both AMPA receptors and Ca2+ channels in cultured retina cells.

Nitric oxide differentially affects the exocytotic and the carrier-mediated release of [3H] gamma-aminobutyric acid in rat hippocampal synaptosomes

We studied the effects of nitric oxide (NO) on the Ca2+-dependent KCl-evoked release of gamma-aminobutyric acid (GABA) by rat hippocampal synaptosomes, measured in the presence of 1-(2-(((diphenyl-methylene)amino)oxy)ethyl)-1,2,5, 6-tetrahydro-3-pyridine-carboxylic acid (NNC-711), which blocks the GABA carrier. Under these conditions, the NO donor, hydroxylamine, up to 1 mM, inhibited the Ca2+-dependent exocytotic GABA release, but did not affect the basal release. However, in the absence of NNC-711, hydroxylamine concentrations higher than 30 microM caused a two-fold increase in the basal release of GABA, and the KCl-evoked release of GABA was higher than in the presence of NNC-711 because both exocytotic and carrier-mediated release occur. Thus, it is expected that when both release mechanisms are operative, NO inhibits the exocytotic release and stimulates the carrier-mediated release, and the overall effect is an increased liberation of the neurotransmitter from the nerve terminals.

Culture medium components modulate retina cell damage induced by glutamate, kainate or "chemical ischemia"

The aim of this study was to determine whether culture-conditioned medium (CCM) can prevent neuronal damage caused by excitotoxicity or by "chemical ischemia" in cultured chick retina cells. Excitotoxic conditions were obtained by incubating retina cells with glutamate or kainate and "chemical ischemia" was induced by metabolic inhibition. In this case, cultures were briefly exposed to sodium cyanide, to block oxidative phosphorylation and iodoacetic acid, to block glycolysis. The assessment of neuronal injury was made spectrophotometrically by quantification of cellularly reduced MTT. Stimulation of retina cells with glutamate or kainate in serum deprived culture medium (BME-FCS), lead to a decrease in the MTT metabolism that was dependent on the time of exposure to the toxic agents. CCM prevented cell damage, either when present during the stimulation period or during the recovery period. This protection was more prominent in the case of kainate-induced neuronal death. "Chemical ischemia" also lead to a decrease of the MTT metabolism in a time-dependent manner and CCM protected retina cells from "ischemia"-induced lesions when present during the stimulation period and during the recovery period. The protective effect of CCM was partially decreased by the tyrosine kinase inhibitor, genistein, when the cells were stimulated with kainate, but not with glutamate, or when the cells were subjected to "chemical ischemia". CCM protected retina cells against both the acute and the delayed toxicity induced by either glutamate or kainate, or by "chemical ischemia", when present during both the insult and the recovery period. The presence of survival factors in the media may effectively inhibit the cell death signals generated by glutamate receptor activation or by "chemical ischemia".

Glutamate in life and death of retinal amacrine cells

1. Glutamate is the neurotransmitter released by bipolar cells at their synapses with amacrine cells. The amacrine cells express ionotropic (NMDA, AMPA and kainate) and metabotropic (mGluR1, mGluR2, mGluR4 and mGluR7) glutamate receptors and may take up glutamate from the synaptic cleft. 2. Activation of the ionotropic glutamate receptors increases the intracellular free calcium concentration ([Ca2+]i), owing to Ca2+ entry through the receptor-associated channels as well as through voltage-gated Ca2+ channels. The [Ca2+]i response to glutamate may be amplified by Ca2+-induced Ca2+ release from intracellular sources. 3. Activation of NMDA and non-NMDA glutamate receptors stimulates the release of GABA and acetylcholine from amacrine cells. GABA is released by a Ca2+-dependent mechanism and by reversal of the neurotransmitter transporter. 4. Excessive activation of glutamate receptors during ischemia leads to amacrine cell death. An increase in [Ca2+]i due to Ca2+ influx through NMDA and AMPA/kainate receptor channels is related to cell death in studies in vitro. In other studies, it was shown that nitric oxide may also take part in the process of cell damage during ischemia.

'Chemical ischemia' in cultured retina cells: the role of excitatory amino acid receptors and of energy levels on cell death

In this study, we determined whether the retina cell death observed in response to an ischemic-like insult is related to an overactivation of the ionotropic glutamate receptors and/or to a collapse of the energy levels. Cultured chick retina cells were submitted to 'chemical ischemia' by metabolic inhibition with sodium cyanide and iodoacetic acid, which block oxidative phosphorylation and glycolysis, respectively. The assessment of neuronal injury was made spectrophotometrically by quantification of cellularly reduced MTT, which gives information about mitochondrial function, or by staining with fluorescein diacetate (FDA), which correlates with changes in the plasma membrane permeability. 'Chemical ischemia' induced both an acute and a delayed time-dependent degeneration of chick retina cells. We observed that 2 min after the ischemic insult, the levels of ATP were reduced to a minimum. On the other hand, the metabolic inhibition induced the release of aspartate, glutamate and gamma-aminobutyric acid, and the activation of AMPA/kainate receptors during the period of metabolic arrest was partially responsible for the loss of mitochondrial function. However, the NMDA and non-NMDA receptor antagonists (MK-801 and CNQX) did not prevent the plasma membrane damage caused by sodium cyanide and iodoacetic acid. The results show that the collapse of the energy levels, rather than the increase in excitatory amino acids, appears to underlie the observed cell injury, suggesting an important relationship between ischemia-induced depletion of high-energy metabolites and retina cell degeneration.

Oxidative stress affects the selective ion permeability of voltage-sensitive Ca2+ channels in cultured retinal cells

The effect of ascorbate/Fe2+-induced oxidative stress on the intracellular Ca2+ concentration ([Ca2+]i) and on the voltage-sensitive Ca2+ channels (VSCC) of chick retinal cells was evaluated in this study. We also analyzed the effect of oxidation on the intracellular Na+ concentration ([Na+]i) and on the Ca2+-dependent release of [3H])gamma-aminobutric acid (GABA) evoked by 50 mM KCI. The resting [Ca2+]i was not affected by oxidation, but the [Ca2+]i response (delta[Ca2+]i) to K+-depolarization was significantly inhibited under oxidative stress conditions. The Ca2+ influx stimulated by membrane depolarization was mediated by L- and N-type VSCC, and by N-metyl-D-aspartate (NMDA) receptor channel, activated by endogenous glutamate released by glutamatergic cells. In cultured retinal cells L-type channels are the major route of Ca2+ influx during depolarization and the most affected by oxidative stress. The N-type VSCC seem not to be affected by oxidant conditions; they were found to be involved in glutamatergic transmission and only indirectly in the release of [3H]GABA evoked by K+-depolarization. Although the Ca2+-dependent release of [3H]GABA evoked by 50 mM KCl is mediated by Ca2+ entry through L-type Ca2+ channels, it is not affected by pre-incubation with the oxidant pair. The oxidative stress conditions increased the [Na+]i in Ca2+-free medium, by a process dependent of Na+ entry through L-type VSCC. The increased permeability of L-type VSCC to Na+ may increase the Ca2+-independent release of endogenous glutamate which, by activating the NMDA receptors, induces the release of [3H]GABA by reversal of its transporter. The equilibrium between the release of GABA and glutamate may play an in important role in neuroprotection against excitotoxic insults.

Impairment of excitatory amino acid transporter activity by oxidative stress conditions in retinal cells: effect of antioxidants

In the present study we analyzed how oxidative stress conditions induced by ascorbate/ Fe2+ affect the excitatory amino acid (EAA) transport systems in cultured chick retina cells. The uptake of D-[3H]aspartate, which is transported by the same carrier as glutamate, was determined in control cells and in cells subjected to ascorbate/Fe2+. The uptake of this EAA was Na+ dependent and was inhibited by about 40% under oxidative stress conditions. To clarify the molecular mechanisms involved in the inhibition of D-[3H]aspartate uptake by ascorbate/Fe2+, we investigated the effect of vitamin E (Vit E), melatonin, reduced glutathione (GSH), and dithiothreitol (DTT) on the uptake of D-[3H]aspartate and on the extent of lipid peroxidation in control and in peroxidized cells. Preincubation with Vit E (100 microM) abolished lipid peroxidation, but had no significant effect on the inhibition of D-[3H]aspartate uptake evoked by ascorbate/Fe2+. Melatonin was more effective in reducing the formation of TBARS and conjugated dienes than in preventing the D-[3H]aspartate uptake inhibition evoked by the oxidant pair. Conversely, GSH (4 mM) and DTT (4 mM) completely prevented the inhibition of D-[3H]aspartate uptake in cells subjected to oxidative stress, but were without effect on the extent of peroxidation. Free fatty acids, such as arachidonic acid, seem not to be involved in reducing the activity of the D-[3H]aspartate uptake system, whereas the reduction of the Na+ electrochemical gradient that occurs under oxidative stress was in part involved in the reduction of D-[3H]aspartate uptake by the cells. The inhibition of D-[3H]aspartate uptake by ascorbate/Fe2+ persisted for at least 1 h, but could be partially reverted by disulfide reducing agents. It is concluded that oxidative stress causes long-lasting modifications of the glutamate/D-[3H]aspartate transport system (or systems), such as oxidation of protein sulfhydryl (SH) groups, which can be recovered by some antioxidants.

Activity of ionotropic glutamate receptors in retinal cells: effect of ascorbate/Fe(2+)-induced oxidative stress

The effect of oxidative stress induced by the oxidant pair ascorbate/Fe2+ on the activity of ionotropic glutamate receptors was studied in cultured chick retina cells. The release of [3H]GABA and the increase of the intracellular free Na+ concentration ([Na+]i), evoked by glutamate receptor agonists, were used as functional assays for the activity of the receptors. The results show that the maximal release of [3H]GABA evoked by kainate (KA; approximately 20% of the total) or AMPA (approximately 11% of the total) was not different in control and peroxidized cells, whereas the EC50 values determined for peroxidized cells (33.6 +/- 1.7 and 8.0 +/- 2.0 microM for KA and AMPA, respectively) were significantly lower than those determined under control conditions (54.1 +/- 6.6 and 13.0 +/- 2.2 microM for KA and AMPA, respectively). The maximal release of [3H]GABA evoked by NMDA under K+ depolarization was significantly higher in peroxidized cells (7.5 +/- 0.5% of the total) as compared with control cells (4.0 +/- 0.2% of the total), and the effect of oxidative stress was significantly reduced by a phospholipase A2 inhibitor or by fatty acid-free bovine serum albumin. The change in the intracellular [Na+]i evoked by saturating concentrations of NMDA under depolarizing conditions was significantly higher in peroxidized cells (8.9 +/- 0.6 mM) than in control cells (5.9 +/- 1.0 mM). KA, used at a subsaturating concentration (35 microM), evoked significantly greater increases of the [Na+]i in peroxidized cells (11.8 +/- 1.7 mM) than in control cells (7.1 +/- 0.8 mM). A saturating concentration (150 microM) of this agonist triggered similar increases of the [Na+]i in control and peroxidized cells. Accordingly, the maximal number of binding sites for (+)-5-[3H]methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10- imine 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate ([3H]MK-801) was increased after peroxidation, whereas the maximal number of binding sites for [3H]KA was not affected by oxidative stress. These data suggest that under oxidative stress the activity of the ionotropic glutamate receptors is increased, with the NMDA receptor being the most affected by peroxidation.

Intracellular free Na+ concentration increases in cultured retinal cells under oxidative stress conditions

The effect of oxidative stress, induced by ascorbate/Fe2+, on the intracellular free Na+ concentration ([Na+]i) of cultured chick retina cells was determined using the fluorescent indicator Na(+)-binding benzofuran isophthalate (SBFI). The resting[Na+]i of retina cells submitted to oxidative stress (15.5 +/- 1.9 mM) was significantly higher than that of control cells (8.9 +/- 0.8 mM). KCl (50 mM) depolarization induced a sustained [Na+]i increase (delta[Na+]i), which was significantly higher in peroxidized cells (8.1 +/- 0.7 mM) than in control cells (4.9 +/- 0.9 mM). The glutamate receptor antagonists, MK-801 and CNQX, reduced more significantly the initial delta[Na+]i induced by K(+)-depolarization under oxidative stress conditions (65% of inhibition), than in control cells (20% of inhibition). Moreover, in the presence of MK-801 and CNQX the increase in the [Na+]i, which was similar in control and peroxidized cells, was followed by a decrease towards a plateau. The Na+ channel blocker, tetrodotoxin (TTX), also reduced the sustained increase of the [Na+]i evoked by 50 mM KCl in both experimental conditions. However, TTX and glutamate receptor antagonists tested together failed to abolish the delta[Na+]i upon K(+)-depolarization, indicating that TTX-resistant Na+ channels were involved in the Na+ influx. The entry of Na+ through these channels contributed mainly to the early phase of the [Na+]i rise upon K(+)-depolarization, whereas the glutamate receptors seem to contribute more significantly to the [Na+]i response for stimulations longer than 30-50 s. The results suggest that an excessive activation of glutamate receptors increases the influx of Na+ and the resting [Na+]i under oxidative stress conditions.

On-line detection of glutamate release from cultured chick retinospheroids

A continuous fluorometric assay was adapted to measure the release of endogenous glutamate from cultured chick retinospheroids. The results obtained with this technique are compared with the release of [3H]D-aspartate from monolayer cultures of chick retina cells. It is shown that although excitatory amino acids may be released in a Ca(2+)-dependent manner, most of the neurotransmitter release from cultured retina cells occurs by reversal of the glutamate transporter. The presence of extracellular Ca2+ may actually inhibit glutamate release by the cells present in the retinospheroids, or the [3H]D-aspartate release by cells in monolayers, when veratridine is the depolarizing agent.

Glutamate release evoked by glutamate receptor agonists in cultured chick retina cells: modulation by arachidonic acid

We studied the effect of ionotropic glutamate receptor agonists on the release of endogenous glutamate or of [3H]D-aspartate from reaggregate cultures (retinospheroids) or from monolayer cultures of chick retinal cells, respectively. Kainate increased the fluorescence ratio of the Na+ indicator SBFI and stimulated a dose-dependent release of glutamate in low (0.1 mM) Ca2+ medium, as measured using a fluorometric assay. Under the same experimental conditions, the release evoked by N-methyl-D-aspartate (NMDA; 400 microM) was about half of that evoked by the same kainate concentration; alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA; 400 microM) did not trigger a significant response. In the presence of 1 mM CaCl2, all of the agonists increased the [Ca2+]i, as determined with the fluorescence dye Indo-1, but the glutamate release evoked by NMDA and kainate was significantly lower than that measured in 0.1 mM CaCl2 medium. Inhibition by Ca2+ of the kainate-stimulated release of glutamate was partially reversed by the phospholipase A2 inhibitor oleiloxyethyl phosphorylcholine (OPC), suggesting that the effect was mediated by the release of arachidonic acid, which inhibits the glutamate carrier. Accordingly, kainate, NMDA, and AMPA stimulated a Ca(2+)-dependent release of [3H]arachidonic acid, and the direct addition of the exogenous fatty acid to the medium decreased the release of glutamate evoked by kainate in low (0.1 mM) CaCl2 medium. In monolayer cultures, we showed that NMDA, kainate, and AMPA also stimulated the release of [3H]D-aspartate, but in this case release in the presence of 1 mM CaCl2 was significantly higher than that evoked in media with no added Ca2+. The ranking order of efficacy for stimulation of Ca(2+)-dependent release of [3H]D-aspartate was NMDA > > kainate > AMPA.

Ca2+ influx through glutamate receptor-associated channels in retina cells correlates with neuronal cell death

We studied the effect of glutamate, N-methyl-D-aspartate (NMDA), kainate or K+ depolarization, on neurotoxicity in cultured chick retinal cells, under conditions in which we could discriminate between Ca2+ entering through ionotropic glutamate receptors and voltage-sensitive Ca2+ channels (VSCCs). When neurons were challenged with NMDA, kainate or glutamate, in Na(+)-containing medium, a decrease in cell survival was observed, whereas K+ depolarization did not affect the viability of the cells. The Mg2+ ion completely prevented the toxic effect mediated by the NMDA receptor, and had a small but significant protective effect at the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptor-induced cell death. We observed that, in a Na(+)-free N-methyl-D-glucamine (NMG) medium, to avoid the activation of VSCCs indirectly by the glutamate receptor agonists, stimulation of the glutamate receptors causes Ca2+ influx only through NMDA and AMPA/kainate receptor-associated channels, and that Ca2+ entry correlates well with subsequent cell death. These results show that the activation of NMDA or AMPA/kainate receptors can cause excitotoxicity in retinal neurons by mechanisms not involving Na+ influx, but rather depending on the permeation of Ca2+ through glutamate receptor-associated channels. For small Ca2+ loads the entry of Ca2+ through the NMDA receptor-associated channel was more efficient in triggering cell death than the influx of Ca2+ through the AMPA/kainate receptor.

[Ca2+]i regulation by glutamate receptor agonists in cultured chick retina cells

The effect of glutamate receptor agonists on the intracellular free calcium concentration ([Ca2+]i), measured with Indo-1, was studied in populations of cultured chick embryonic retina cells. The agonists of the ionotropic glutamate receptors, N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) increased the [Ca2+]i through a composite effect, comprising Ca2+ permeating the receptor-associated channels, and Ca2+ entering through voltage-gated Ca2+ channels. Furthermore, the [Ca2+]i responses to NMDA and AMPA also involved Ca2+ release from intracellular stores, which could not be mobilized by stimulation of the metabotropic receptor.

Glutamate receptor agonists evoked Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate from cultured chick retina cells

We studied the release of [3H]D-aspartate evoked by glutamate receptor agonists from monolayer cultures of chick retina cells, and found that activation of the glutamate receptors can evoke both Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate. In Ca(2+)-free (no added Ca2+) Na+ medium, the agonists of the glutamate receptors induced the release of [3H]D-aspartate with the following rank order of potency: kainate > alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) approximately N-methyl-D-aspartate (NMDA). In media containing 1 mM CaCl2 the release of [3H]D-aspartate evoked by NMDA, kainate and AMPA was increased by about 112 percent, 20 percent and 39 percent, respectively, as compared to the release evoked by the same agonists in Ca(2+)-free medium. NMDA was the most potent agonist in stimulating the Ca(2+)-dependent release of [3H]D-aspartate, possibly by exocytosis, and AMPA was as potent as kainate. The Ca(2+)-dependent release of [3H]D-aspartate evoked by kainate was dependent on the influx of Ca2+ through the receptor associated channel, as well as through the N-(omega-Conotoxin GVIA-sensitive) and L- (nitrendipine-sensitive) type voltage-sensitive Ca2+ channels (VSCC). The exocytotic release of [3H]D-aspartate evoked by AMPA relied exclusively on Ca2+ entry through the L-type VSCC, whereas the effect of NMDA was partially mediated by the influx of Ca2+ through the receptor-associated channel, but not through L- or N-type VSCC. Thus, activation of these different glutamate receptors under physiological conditions is expected to cause the release of cytosolic and vesicular glutamate, and the routes of Ca2+ entry modulating vesicular release may be selectively recruited.

Voltage-sensitive Ca2+ channels in rat striatal synaptosomes: role on the [Ca2+]i responses to membrane depolarization

The fluorescent Ca2+ indicator Indo-1 was used to study the effect of depolarization evoked by KCl or 4-aminopyridine (4-AP) on the intracellular free calcium concentration responses (delta[Ca2+]i) in rat striatal synaptosomes. Depolarization of the synaptosomes with [KCl] > 7.5 mM induced a rapid increase of the [Ca2+]i followed by a decay towards a plateau. The size of the [Ca2+]i response varied sigmoidally with the synaptosomal membrane potential, with a transition potential of -27.3 mV. Depolarization with 4-AP evoked a dose-dependent sustained increase of the [Ca2+]i. Nitrendipine, omega-Conotoxin GVIA (omega-CgTx) and omega-Agatoxin IVA (omega-Aga IVA) were used to evaluate the relative role of L-, N-, P- and possibly Q-type voltage-sensitive Ca2+ channels (VSCCs) on the [Ca2+]i changes evoked by each of the two depolarizing agents. Nitrendipine caused only about 10% inhibition of the effect of either agent on the [Ca2+]i, suggesting that the L-type VSCCs have a modest contribution. The omega-CgTx decreased the response to KCl and 4-AP by 15 and 30%, respectively, but the latter effect may be partially due to a non-specific effect on Na+ channels. The omega-Aga IVA reduced the response to 4-AP by 26.5%, and this effect was additive to that of omega-CgTx, further suggesting that the striatal nerve terminals possess P- and/or Q-type, in addition to N-type Ca2+ channels. Neomycin (0.35 mM), tentatively used as an antagonist of the P-type channels, had a potent effect, decreasing the response to K(+)-depolarization and to 4-AP by, respectively, 32.5 and 48.5%. It is suggested that at the concentration used the antibiotic also partially blocks VSCCs which do not belong to the L-, N-, P- or Q-type VSCCs. We conclude that striatal nerve endings are equipped with at least four to five pharmacologically distinct classes of VSCCs, which are sensitive to well known antagonists of the L-, N-, P-, and Q-type VSCCs.

Involvement of class A calcium channels in the KCl induced Ca2+ influx in hippocampal synaptosomes

Using Ca2+ channel toxins, we determined the types of voltage-sensitive calcium channels activated by two levels of KCl depolarization in hippocampal synaptosomes. The increase in the intracellular Ca2+ concentration ([Ca2+]i) induced by 30 mM KCl was equally sensitive to either omega-agatoxin IVA (omega-Aga IVA) or to omega-conotoxin MVIIC (omega-CgTx MVIIC), and the inhibition produced by these two peptides was not additive. The present results indicate that omega-Aga IVA and omega-CgTx MVIIC do not distinguish between two different VSCC in hippocampal synaptosomes and that they both inhibit a channel with the alpha 1A subunit which is present in the rat hippocampus.

Modulation of N-methyl-D-aspartate receptor activity by oxidative stress conditions in chick retinal cells

The effect of oxidative stress, induced by ascorbate (1.5 mM)/Fe2+ (7.5 microM), on the cellular responses to N-methyl-D-aspartate (NMDA) receptor activation was evaluated by measuring the release of [3H]GABA induced by NMDA from cultured retina cells. In retina cells submitted to oxidative stress the [3H]GABA release evoked by NMDA, in a medium containing physiological concentrations of Mg2+ (1.6 mM) and K+ (4 mM), was significantly higher than in control cells. The [3H]GABA release evoked by NMDA was potentiated by glycine and was abolished by MK-801, suggesting that the [3H]GABA release was due to NMDA receptor activation. The increased effect of NMDA in peroxidized cells was significantly reduced by TTX, suggesting that the higher cellular responses to the activation of NMDA receptors are due to a hyperexcitability of retina cells submitted to oxidative stress. No significant differences were found between the average resting membrane potential of control and peroxidized cells. However, membrane potential is more tightly regulated by K(+)-channels sensitive to 4-aminopyridine (100 microM), alpha-dendrotoxin (100 nM) and gamma-dendrotoxin (100 nM) under oxidative stress.

Relation of [Ca2+]i to dopamine release in striatal synaptosomes: role of Ca2+ channels

We compared the effects of KCl and 4-aminopyridine (4-AP) stimulation on the coupling of Ca2+ channel activation to [3H]dopamine ([3H]DA) release in rat striatal synaptosomes and used specific Ca2+ channel blockers to discriminate between the different VSCC's activated by the two stimulatory agents. We found that whereas [3H]DA release is strictly Ca(2+)-dependent in the case of KCl depolarization, 4-AP, at concentrations above 100 microM, progressively causes a large Ca(2+)-independent release of [3H]DA. Thus, at 1 to 3 mM 4-AP, as much as 80-95% of the [3H]DA release is Ca(2+)-independent and can be partially blocked by nomifensine, indicating that some [3H]DA release is occurring through reversal of the DA carrier. Therefore, in the studies relating [Ca2+]i to [3H]DA release we selected 4-AP concentrations lower than 100 microM and corrected for the Ca(2+)-independent release. Under these conditions, we determined that: (1) Ca2+ entry through N-type VSCC's is involved in [3H]DA release both in the case of KCl depolarization (35% inhibition by omega-CgTx) and in 4-AP stimulation (23% inhibition by omega-CgTx); (2) Ca2+ entering through P-type and/or Q-type VSCC's is also involved in [3H]DA release due to 4-AP stimulation (26% inhibition by 200 nM omega-Aga IVA); (3) Neomycin (0.35 mM) inhibited the [3H]DA release due to 4-AP stimulation by about 20% and decreased the KCl induced [3H]DA release by 55%; the effects of neomycin (0.35 mM) and omega-CgTx were additive in both cases, indicating that, at this concentration, the antibiotic does not affect significantly N-type Ca2+ channels; (4) When applied together, omega-CgTx and omega-Aga IVA inhibited the 4-AP stimulated [3H]DA release by about 40-50%, suggesting that the remaining large fraction of the VSCC's activated by 4-AP stimulation are non-N, non-P VSCC's and are coupled to Ca(2+)-dependent [3H]DA release; (5) The contribution of L-type VSCC's is uncertain, since there seemed to be a small contribution in the case of KCl depolarization, but not in the case of 4-AP stimulation. On the whole, the results suggest that the release of [3H]DA in the rat striatal nerve terminals depends on Ca2+ entry through N-, P-, possibly Q-, and other non-N-, non-P-type VSCC's when either KCl or 4-AP stimulation is utilized.