Mitochondrial Dysfunction and Decreased Cytochrome c in Cell and Animal Models of Machado-Joseph Disease

Mitochondrial dysfunction has been described in many neurodegenerative disorders; however, there is less information regarding mitochondrial deficits in Machado-Joseph disease (MJD), a polyglutamine (polyQ) disorder caused by CAG repeat expansion in the ATXN3 gene. In the present study, we characterized the changes in mitochondrial function and biogenesis markers in two MJD models, CMVMJD135 (MJD135) transgenic mice at a fully established phenotype stage and tetracycline-regulated PC6-3 Q108 cell line expressing mutant ataxin-3 (mATXN3). We detected mATXN3 in the mitochondrial fractions of PC6-3 Q108 cells, suggesting the interaction of expanded ATXN3 with the organelle. Interestingly, in both the cerebella of the MJD135 mouse model and in PC6-3 Q108 cells, we found decreased mitochondrial respiration, ATP production and mitochondrial membrane potential, strongly suggesting mitochondrial dysfunction in MJD. Also, in PC6-3 Q108 cells, an additional enhanced glycolytic flux was observed. Supporting the functional deficits observed in MJD mitochondria, MJD135 mouse cerebellum and PC6-3 Q108 cells showed reduced cytochrome c mRNA and protein levels. Overall, our findings show compromised mitochondrial function associated with decreased cytochrome c levels in both cell and animal models of MJD.

Mitochondrial and Redox Changes in Periodontitis and Type 2 Diabetes Human Blood Mononuclear Cells

Periodontitis (PDT) and type 2 diabetes (T2D) have demonstrated a bidirectional relationship and imbalanced oxidative stress linked to mitochondrial dysfunction. Therefore, we investigated mitochondrial and redox (de)regulation in peripheral blood mononuclear cells (PBMCs) in comorbid T2D-PDT, compared to PDT, T2D patients, and control individuals. PBMCs were analyzed for mitochondrial respiration, reactive oxygen species, antioxidant proteins, and expression of Nrf2-target genes. PDT and T2D-PDT patients exhibited altered periodontal clinical markers, while T2D and T2D-PDT patients displayed increased blood HbA1c. Decreased oxygen consumption and ATP production were observed in the PDT patient's PBMCs. PDT and T2D-PDT PBMCs also evidenced increased H₂O₂ levels and reduced catalase levels (also detected in T2D patients), whereas a compromised glutathione cycle was observed in T2D-PDT patients. PBMCs from both T2D or T2D-PDT patients showed increased Nrf2 protein levels, enhanced GCL activity and GCL-catalytic subunit protein levels, and maintained GCLc, GST, and HO-1 mRNA levels. In contrast, the expressions of Nrf2-target genes were significantly diminished in the PDT patient's PBMCs. Decreased SOD1 and GST mRNA levels were also observed in CD3+CD8+-lymphocytes derived from PDT and T2D-PDT patients. In conclusion, PBMCs from T2D-PDT patients showed major redox changes, while mononuclear cells from PDT patients showed mitochondrial deregulation and reduced expression of Nrf2-target genes.

Simultaneous Alteration of the Circadian Variation of Memory, Hippocampal Synaptic Plasticity, and Metabolism in a Triple Transgenic Mouse Model of Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by progressive memory deficits accompanied by synaptic and metabolic deficits, namely of mitochondrial function. AD patients also display a disrupted circadian pattern. Thus, we now compared memory performance, synaptic plasticity, and mitochondria function in 24-week-old non-transgenic (non-Tg) and triple transgenic male mice modeling AD (3xTg-AD) at Zeitgeber 04 (ZT-4, inactive phase) and ZT-16 (active phase). Using the Morris water maze test to minimize the influence of circadian-associated locomotor activity, we observed a circadian variation in hippocampus-dependent learning performance in non-Tg mice, which was impaired in 3xTg-AD mice. 3xTg-AD mice also displayed a lack of circadian variation of their performance in the reversal spatial learning task. Additionally, the amplitude of hippocampal long-term potentiation also exhibited a circadian profile in non-Tg mice, which was not observed in 3xTg-AD mice. Moreover, cerebral cortical synaptosomes of non-Tg mice also displayed a circadian variation of FCCP-stimulated oxygen consumption as well as in mitochondrial calcium retention that were blunted in 3xTg-AD mice. In sum, this multidimensional study shows that the ability to maintain a circadian oscillation in brain behavior, synaptic plasticity, and synaptic mitochondria function are simultaneously impaired in 3xTg-AD mice, highlighting the effects of circadian misalignment in AD.

Boldine Attenuates Synaptic Failure and Mitochondrial Deregulation in Cellular Models of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common cause of senile dementia worldwide, characterized by both cognitive and behavioral deficits. Amyloid beta peptide (Aβ) oligomers (AβO) have been found to be responsible for several pathological mechanisms during the development of AD, including altered cellular homeostasis and synaptic function, inevitably leading to cell death. Such AβO deleterious effects provide a way for identifying new molecules with potential anti-AD properties. Available treatments minimally improve AD symptoms and do not extensively target intracellular pathways affected by AβO. Naturally-derived compounds have been proposed as potential modifiers of Aβ-induced neurodysfunction and cytotoxicity based on their availability and chemical diversity. Thus, the aim of this study was to evaluate boldine, an alkaloid derived from the bark and leaves of the Chilean tree Peumus boldus, and its capacity to block some dysfunctional processes caused by AβO. We examined the protective effect of boldine (1–10 μM) in primary hippocampal neurons and HT22 hippocampal-derived cell line treated with AβO (24–48 h). We found that boldine interacts with Aβ in silico affecting its aggregation and protecting hippocampal neurons from synaptic failure induced by AβO. Boldine also normalized changes in intracellular Ca²⁺ levels associated to mitochondria or endoplasmic reticulum in HT22 cells treated with AβO. In addition, boldine completely rescued the decrease in mitochondrial membrane potential (ΔΨm) and the increase in mitochondrial reactive oxygen species, and attenuated AβO-induced decrease in mitochondrial respiration in HT22 hippocampal cells. We conclude that boldine provides neuroprotection in AD models by both direct interactions with Aβ and by preventing oxidative stress and mitochondrial dysfunction. Additional studies are required to evaluate the effect of boldine on cognitive and behavioral deficits induced by Aβ in vivo.

Characterization of subventricular zone-derived progenitor cells from mild and late symptomatic YAC128 mouse model of Huntington's disease

Huntington's disease (HD) is caused by an expansion of CAG repeats in the HTT gene, leading to expression of mutant huntingtin (mHTT) and selective striatal neuronal loss, frequently associated with mitochondrial dysfunction and decreased support of brain-derived neurotrophic factor (BDNF). New neurons derived from the subventricular zone (SVZ) are apparently not able to rescue HD pathological features. Thus, we analyzed proliferation, migration and differentiation of adult SVZ-derived neural stem/progenitor cells (NSPC) from mild (6month-old (mo)) and late (10mo) symptomatic HD YAC128 mice expressing full-length (FL)-mHTT versus age-matched wild-type (WT) mice. SVZ cells derived from 6mo YAC128 mice exhibited higher migratory capacity and a higher number of MAP2+ and synaptophysin+cells, compared to WT cells; MAP2 labeling was enhanced after exposure to BDNF. However, BDNF-evoked neuronal differentiation was not observed in 10mo YAC128 SVZ-derived cells. Interestingly, 6mo YAC128 SVZ-derived cells showed increased intracellular Ca²⁺ levels in response to KCl, which was potentiated by BDNF, evidencing the presence of differentiated neurons. In contrast, KCl depolarization-induced intracellular Ca²⁺ increase in 10mo YAC128 SVZ-derived cells was shown to be increased only in BDNF-treated YAC128 SVZ-derived cells, suggestive of decreased differentiation capacity. In addition, BDNF-untreated NSPC from 10mo YAC128 mice exhibited lower mitochondrial membrane potential and increased mitochondrial Ca²⁺ accumulation, in relation with NSPC from 6mo YAC128 mice. Data evidence age-dependent reduced migration and decreased acquisition of a neuronal phenotype, accompanied by decreased mitochondrial membrane potential in SVZ-derived cells from YAC128 mice through HD symptomatic phases.

Oxidative stress involving changes in Nrf2 and ER stress in early stages of Alzheimer's disease

Oxidative stress and endoplasmic reticulum (ER) stress have been associated with Alzheimer's disease (AD) progression. In this study we analyzed whether oxidative stress involving changes in Nrf2 and ER stress may constitute early events in AD pathogenesis by using human peripheral blood cells and an AD transgenic mouse model at different disease stages. Increased oxidative stress and increased phosphorylated Nrf2 (p(Ser40)Nrf2) were observed in human peripheral blood mononuclear cells (PBMCs) isolated from individuals with mild cognitive impairment (MCI). Moreover, we observed impaired ER Ca2+ homeostasis and increased ER stress markers in PBMCs from MCI individuals and mild AD patients. Evidence of early oxidative stress defense mechanisms in AD was substantiated by increased p(Ser40)Nrf2 in 3month-old 3xTg-AD male mice PBMCs, and also with increased nuclear Nrf2 levels in brain cortex. However, SOD1 protein levels were decreased in human MCI PBMCs and in 3xTg-AD mice brain cortex; the latter further correlated with reduced SOD1 mRNA levels. Increased ER stress was also detected in the brain cortex of young female and old male 3xTg-AD mice. We demonstrate oxidative stress and early Nrf2 activation in AD human and mouse models, which fails to regulate some of its targets, leading to repressed expression of antioxidant defenses (e.g., SOD-1), and extending to ER stress. Results suggest markers of prodromal AD linked to oxidative stress associated with Nrf2 activation and ER stress that may be followed in human peripheral blood mononuclear cells.

Impaired Src signaling and post-synaptic actin polymerization in Alzheimer's disease mice hippocampus--linking NMDA receptors and the reelin pathway

Early cognitive deficits in Alzheimer's disease (AD) have been related to deregulation of N-methyl-d-aspartate receptors (NMDARs) and synaptic dysfunction in response to amyloid-beta peptide. NMDAR anchorage to post-synaptic membrane depends in part on Src kinase, which is also implicated in NMDAR activation and actin cytoskeleton stabilization, two processes relevant for normal synaptic function. In this study we analyzed the changes in GluN2B subunit phosphorylation and the levels of proteins involved in Src related signaling pathways linking the Tyr kinase to actin cytoskeleton polymerization, namely reelin, disabled-1 (Dab1) and cortactin, in hippocampal and cortical homogenates obtained from the triple transgenic mouse model of AD (3xTg-AD) that shows progression of pathology as a function of age versus age-matched wild-type mice. Moreover, we evaluated regional post-synaptic actin polymerization using phalloidin labeling in hippocampal slices. Young (3month-old) 3xTg-AD male mice hippocampus exhibited decreased GluN2B Tyr1472 phosphorylation and reduced Src activity. In the cortex, decreased Src activity correlated with reduced levels of reelin and Dab1, implicating changes in the reelin pathway. We also observed diminished phosphorylated Dab1 and cortactin protein levels in the hippocampus and cortex of young 3xTg-AD male mice. Concordantly with the recognized role of these proteins in actin stabilization, we detected a significant decrease in post-synaptic F-actin in 3month-old 3xTg-AD male CA1 and CA3 hippocampal regions. These data suggest deregulated Src-dependent signaling pathways involving GluN2B-composed NMDARs and post-synaptic actin cytoskeleton depolymerization in the hippocampus in early stages of AD.

Dysfunctional synapse in Alzheimer's disease - A focus on NMDA receptors

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly. Alterations capable of causing brain circuitry dysfunctions in AD may take several years to develop. Oligomeric amyloid-beta peptide (Aβ) plays a complex role in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in this devastating disease. Moreover, N-methyl-D-aspartate (NMDA) receptors (NMDARs) activation has been recently implicated in AD-related synaptic dysfunction. Thus, in this review we focus on glutamatergic neurotransmission impairment and the changes in NMDAR regulation in AD, following the description on the role and location of NMDARs at pre- and post-synaptic sites under physiological conditions. In addition, considering that there is currently no effective ways to cure AD or stop its progression, we further discuss the relevance of NMDARs antagonists to prevent AD symptomatology. This review posits additional information on the role played by Aβ in AD and the importance of targeting the tripartite glutamatergic synapse in early asymptomatic and possible reversible stages of the disease through preventive and/or disease-modifying therapeutic strategies. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Glutathione redox cycle dysregulation in Huntington's disease knock-in striatal cells

Huntington's disease (HD) is a CAG repeat disorder affecting the HD gene, which encodes for huntingtin (Htt) and is characterized by prominent cell death in the striatum. Oxidative stress was previously implicated in HD neurodegeneration, but the role of the major endogenous antioxidant system, the glutathione redox cycle, has been less studied following expression of full-length mutant Htt (FL-mHtt). Thus, in this work we analyzed the glutathione system in striatal cells derived from HD knock-in mice expressing mutant Htt versus wild-type cells. Mutant cells showed increased intracellular reactive oxygen species (ROS) and caspase-3 activity, which were significantly prevented following treatment with glutathione ethyl ester. Interestingly, mutant cells exhibited an increase in intracellular levels of both reduced and oxidized forms of glutathione, and enhanced activities of glutathione peroxidase (GPx) and glutathione reductase (GRed). Furthermore, glutathione-S-transferase (GST) and γ-glutamyl transpeptidase (γ-GT) activities were also increased in mutant cells. Nevertheless, glutamate-cysteine ligase (GCL) and glutathione synthetase (GS) activities and levels of GCL catalytic subunit were decreased in cells expressing FL-mHtt, highly suggesting decreased de novo synthesis of glutathione. Enhanced intracellular total glutathione, despite decreased synthesis, could be explained by decreased extracellular glutathione in mutant cells. This occurred concomitantly with decreased mRNA expression levels and activity of the multidrug resistance protein 1 (Mrp1), a transport protein that mediates cellular export of glutathione disulfide and glutathione conjugates. Additionally, inhibition of Mrp1 enhanced intracellular GSH in wild-type cells only. These data suggest that FL-mHtt affects the export of glutathione by decreasing the expression of Mrp1. Data further suggest that boosting of GSH-related antioxidant defense mechanisms induced by FL-mHtt is insufficient to counterbalance increased ROS formation and emergent apoptotic features in HD striatal cells.

Endoplasmic reticulum stress occurs downstream of GluN2B subunit of N-methyl-d-aspartate receptor in mature hippocampal cultures treated with amyloid-β oligomers

Alzheimer's disease (AD) is a progressive neurodegenerative disorder affecting both the hippocampus and the cerebral cortex. Reduced synaptic density that occurs early in the disease process seems to be partially due to the overactivation of N-methyl-d-aspartate receptors (NMDARs) leading to excitotoxicity. Recently, we demonstrated that amyloid-beta oligomers (AβO), the species implicated in synaptic loss during the initial disease stages, induce endoplasmic reticulum (ER) stress in cultured neurons. Here, we investigated whether AβO trigger ER stress by an NMDAR-dependent mechanism leading to neuronal dysfunction and analyzed the contribution of GluN2A and GluN2B subunits of this glutamate receptor. Our data revealed that AβO induce ER stress in mature hippocampal cultures, activating ER stress-associated sensors and increasing the levels of the ER chaperone GRP78. We also showed that AβO induce NADPH oxidase (NOX)-mediated superoxide production downstream of GluN2B and impairs ER and cytosolic Ca2+ homeostasis. These events precede changes in cell viability and activation of the ER stress-mediated apoptotic pathway, which was associated with translocation of the transcription factor GADD153 / CHOP to the nucleus and occurred by a caspase-12-independent mechanism. Significantly, ER stress took place after AβO interaction with GluN2B subunits. In addition, AβO-induced ER stress and hippocampal dysfunction were prevented by ifenprodil, an antagonist of GluN2B subunits, while the GluN2A antagonist NVP-AAM077 only slightly attenuated AβO-induced neurotoxicity. Taken together, our results highlight the role of GluN2B subunit of NMDARs on ER stress-mediated hippocampal dysfunction caused by AβO suggesting that it might be a potential therapeutic target during the early stages of AD.

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.

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.

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.

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.

Release of [3H]GABA evoked by glutamate receptor agonists in cultured chick retina cells: effect of Ca2+

The effect of glutamate receptor agonists (NMDA, kainate, quisqualate and AMPA) on the [Ca2+]i and on [3H]GABA release was studied in cultured chick embryonic retinal cells. The release of [3H]GABA evoked by NMDA, in the absence of Ca2+, was prevented by the NMDA receptor antagonist (+)-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine (MK-801), and that produced by kainate and quisqualate was prevented by 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX). All the agonists tested increased the [Ca2+]i, and when the GABA transporter was blocked by 1-(2-(((diphenyl-methylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3- pyridine-carboxylic acid (NNC-711), NMDA, AMPA or quisqualate, but not kainate, did not induce [3H]GABA release unless Ca2+ (1 mM) was present, showing that exocytotic release of [3H]GABA occurs in retinal cells under these conditions.

Domoic acid induced release of [3H]GABA in cultured chick retina cells

The effect of the neurotoxin domoic acid (DOM), a structural analogue of kainic acid, on the release of [3H]gamma-aminobutyric acid (GABA) and on the [Ca2+]i was studied in cultured chick retina cells. DOM stimulated dose-dependently the release of [3H]GABA with an EC50 of 2.5 microM. In Ca(2+)-containing medium (1 mM), DOM (5 microM) increased the [Ca2+]i by about 190 nM and evoked the release of 11.8 +/- 1.3% of the intracellular [3H]GABA, while in the absence of extracellular Ca2+ DOM induced the release of only 7.9 +/- 1.4% of the accumulated [3H]GABA. The Ca(2+)-independent release of [3H]GABA was blocked by the non-competitive inhibitor of the GABA carrier 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid hydrochloride (NNC-711), but a component of Ca(2+)-dependent release remains. DOM evoked Ca(2+)-independent release of [3H]GABA was significantly depressed in the absence of external Na+ and completely blocked by the non-selective antagonist of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Similarly, CNQX decreased the [Ca2+]i response to DOM, whereas L(+)-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of the metabotropic glutamate receptors, was without effect. MK-801 did not affect the release of [3H]GABA stimulated by DOM. Taken together our results indicate that DOM evokes both Ca(2+)-dependent and Ca(2+)-independent release of [3H]GABA, most likely by activating kainate receptors.

Glutamate increases the [Ca2+]i but stimulates Ca(2+)-independent release of [3H]GABA in cultured chick retina cells

The effect of glutamate on [Ca2+]i and on [3H] gamma-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 microM) increases the [Ca2+]i by Ca2+ influx through Ca2+ channels sensitive to nitrendipine, but not to omega-conotoxin GVIA (omega-Cg Tx) (50%), and by other channels insensitive to either Ca2+ channel blocker. Mobilization of Ca2+ by glutamate required the presence of external Na+, suggesting that Na+ mobilization through the ionotropic glutamate receptors is necessary for the Ca2+ channels to open. The increase in [Ca2+]i was not related to the release of [3H]GABA induced by glutamate, suggesting that the pathway for the entry of Ca2+ triggered by glutamate does not lead to exocytosis. In fact, the glutamate-induced release of [3H]GABA was significantly depressed by Ca(2+)o, but it was dependent on Na(+)o, just as was observed for the [3H]GABA release induced by veratridine (50 microM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [3H]GABA release. Both veratridine- and glutamate-induced [3H]GABA release were inhibited by 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid (NNC-711), a blocker of the GABA carrier. Blockade of the NMDA and non-NMDA glutamate receptors with MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively, almost completely blocked the release of [3H]GABA evoked by glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation of exocytotic release of gamma-aminobutyric acid to Ca2+ entry through Ca2+ channels or by reversal of the Na+/Ca2+ exchanger in synaptosomes

The specific inhibitor of the gamma-aminobutyric acid (GABA) carrier, NNC-711, (1-[(2-diphenylmethylene)amino]oxyethyl)- 1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride, blocks the Ca(2+)-independent release of [3H]GABA from rat brain synaptosomes induced by 50 mM K+ depolarization. Thus, in the presence of this inhibitor, it was possible to study the Ca(2+)-dependent release of [3H]GABA in the total absence of carrier-mediated release. Reversal of the Na+/Ca2+ exchanger was used to increase the intracellular free Ca2+ concentration ([Ca2+]i) to test whether an increase in [Ca2+]i alone is sufficient to induce exocytosis in the absence of depolarization. We found that the [Ca2+]i may rise to values above 400 nM, as a result of Na+/Ca2+ exchange, without inducing release of [3H]GABA, but subsequent K+ depolarization immediately induced [3H]GABA release. Thus, a rise of only a few nanomolar Ca2+ in the cytoplasm induced by 50 mM K+ depolarization, after loading the synaptosomes with Ca2+ by Na+/Ca2+ exchange, induced exocytotic [3H]GABA release, whereas the rise in cytoplasmic [Ca2+] caused by reversal of the Na+/Ca2+ exchanger was insufficient to induce exocytosis, although the value for [Ca2+]i attained was higher than that required for exocytosis induced by K+ depolarization. The voltage-dependent Ca2+ entry due to K+ depolarization, after maximal Ca2+ loading of the synaptosomes by Na+/Ca2+ exchange, and the consequent [3H]GABA release could be blocked by 50 microM verapamil. Although preloading the synaptosomes with Ca2+ by Na+/Ca2+ exchange did not cause [3H]GABA release under any conditions studied, the rise in cytoplasmic [Ca2+] due to Na+/Ca2+ exchange increased the sensitivity to external Ca2+ of the exocytotic release of [3H]GABA induced by subsequent K+ depolarization. Thus, our results show that the vesicular release of [3H]GABA is rather insensitive to bulk cytoplasmic [Ca2+] and are compatible with the view that GABA exocytosis is triggered very effectively by Ca2+ entry through Ca2+ channels near the active zones.

Ca(2+)-dependent release of [3H]GABA in cultured chick retina cells

Depolarization by K+ (50 mM) of cultured chick retina cells released 1.14 +/- 0.28% of the accumulated [3H] gamma-aminobutyric acid (GABA) in the absence of Ca2+, but when 1.0 mM Ca2+ was present, the internal free calcium ion concentration [Ca2+]i rose by about 750 nM and the [3H]GABA release about doubled to a value of 2.22 +/- 0.2% of the total [3H]GABA. Nitrendipine (0.1 microM), a blocker of the L-type Ca2+ channels, blocked the [Ca2+]i response to K+ depolarization by about 65%, and the omega-Conotoxin GVIA (omega-CgTx) (0.5 microM), a blocker of the N-type of Ca2+ channels, inhibited by 27% the [Ca2+]i rise due to K+ depolarization. Parallel experiments showed that nitrendipine inhibits [3H]GABA release to the level observed in the absence of Ca2+, whereas omega-CgTx did not inhibit significantly the release of [3H]GABA. The results also show that the release of [3H]GABA due to K(+)-depolarization in the absence of Ca2+ can be totally blocked by 1-(2-(((Diphenylmethylene) amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711), an inhibitor of the GABA carrier. However, in the presence of Ca2+, NNC-711 blocks the release only by about 66%, corresponding to the Ca(2+)-independent release. Thus, it is concluded that [3H]GABA is released in chick retina cells by the exocytotic mechanism, which is Ca(2+)-dependent, and by reversal of the carrier, which is Ca(2+)-independent, in much the same way as has been found for other GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptosomal [Ca2+]i as influenced by Na+/Ca2+ exchange and K+ depolarization

The modulation of the intrasynaptosomal concentration of Ca2+, [Ca2+]i, by Na+/Ca2+ exchange was studied using Indo-1 fluorescence. The electrochemical gradient of Na+ was manipulated by substituting Li+ or choline for Na+ in the external medium and, then, the influx of 45Ca2+ and the [Ca2+]i were measured. It was found that the increase in [Ca2+]i induced by K+ depolarization is lower if the value of [Ca2+]i has been previously raised by Na+/Ca2+ exchange, suggesting that Ca2+ entering by Na+/Ca2+ exchange reduces the Ca2+ entering by voltage-dependent calcium channels. Our results show that a value of [Ca2+]i of about 650 nM induced by Na+/Ca2+ exchange reduces by 50% the Ca2+ entering due to K+ depolarization and no Ca2+ enters through the channels if the [Ca2+]i is previously raised above about 800 nM. Furthermore, predepolarization of the synaptosomes in a Ca-free medium also inhibits by at least 40% the [Ca2+]i rise through Ca2+ channels. Thus, the results suggest that both predepolarization and [Ca2+]i rise due to Na+/Ca2+ exchange decrease the Ca2+ entering by voltage-sensitive Ca2+ channels. The Ca2+ entering by Na+/Ca2+ exchange might contribute to the regulation of neurotransmitter release. Our results also show that the presence of Li+ in the external medium decreases the buffering capacity of synaptosomes, probably by releasing Ca2+ from mitochondria by Li+/Ca2+ exchange.

Regulation of carrier-mediated and exocytotic release of [3H]GABA in rat brain synaptosomes

In this study we investigated the role of external monovalent cations, and of intracellular Ca2+ concentration ([Ca2+]i) in polarized and depolarized rat cerebral cortex synaptosomes on the release of [3H]-gamma-aminobutyric acid (3H-GABA). We found that potassium-depolarization, in the absence of Ca2+, of synaptosomes loaded with 3H-GABA releases 7.4 +/- 2.1% of the accumulated neurotransmitter, provided that the external medium contains Na+, and an additional 19.0 +/- 2.5% is released upon adding 1.0 mM CaCl2 to the exterior. The Ca(2+)-independent release component does not occur in a choline medium and it is only 3.4 +/- 0.8% of the 3H-GABA accumulated in a Li+ medium, but both ions support the Ca(2+)-dependent release of 3H-GABA (13.4 +/- 0.6% in choline and 15.4 +/- 1.5% in Li+), which suggests that the exocytotic release is independent of the external monovalent cation present, whereas the carrier-mediated release specifically requires Na+ outside. Furthermore, previous release of the cytosolic 3H-GABA due to predepolarization in the absence of Ca2+ does not influence the amount of 3H-GABA subsequently released by exocytosis due to Ca2+ addition (19.1 +/- 2.5% or 19.1 +/- 1.1%, respectively). In choline or Li+ medium, the value of the [Ca2+]i is raised by Na+/Ca2+ exchange to 663 +/- 75 nM or 782 +/- 54 nM, respectively, within three minutes after adding 1.0 mM Ca2+, in the absence of depolarization, and parallel release experiments show no release of 3H-GABA in the choline medium, but a substantial release (7.1 +/- 2.1%) of 3H-GABA occurs in the Li+ medium without depolarization. Subsequent K(+)-depolarization shows normal Ca(2+)-dependent release of 3H-GABA in the choline medium (14.1 +/- 2.0%) but only 8.6 +/- 1.1% release in the Li+ medium, which suggests that raising the [Ca2+]i by Na+/Ca2+ exchange, without depolarization, supports some exocytotic release in Li+, but not in choline media. The role of [Ca2+]i and of membrane depolarization in the release process is discussed on the basis of the results obtained and other relevant observations which suggest that both Ca2+ and depolarization are essential for optimal exocytotic release of GABA.

Calcium stores in electropermeabilized HL-60 cells before and after differentiation

Non-induced HL-60 cells (N-IND) and HL-60 cells induced to differentiate with 2 microM retinoic acid (IND) were electropermeabilized with electrical discharges, and the intracellular Ca2+ stores were measured in each type of cell. Both N-IND and IND cells accumulate Ca2+ in the presence of ATP after electropermeabilization. The Ca2+ is stored in at least two different compartments; accumulation in one of the compartments is inhibited by oligomycin and CCCP, and it is not releasable by Ins(1,4,5)P3. The maximal accumulation of Ca2+ by the Ins(1,4,5)P3 sensitive pool is about 0.3 nmol/10(6) cells and 0.9 nmol/10(6) cells for the N-IND and for the IND cells, respectively, and the half-maximal value occurs at a free Ca2+ concentration of 0.23 microM and 0.63 microM, respectively. The oligomycin + CCCP sensitive pool hardly accumulates any Ca2+ at this level of free Ca2+, but at higher free [Ca2+] (greater than microM) its maximal capacity is 80-100-fold higher than the Ins(1,4,5)P3-sensitive pool (about 17-18 nmol/10(6) cells). It is concluded that at physiological free Ca2+ concentrations, the non-mitochondrial Ca2+ pool is regulating the intracellular free Ca2+ in N-IND and IND HL-60 cells, and that this Ca2+ pool can be mobilized by Ins(1,4,5)P3. Furthermore, the capacity of this pool increases about 3-fold when the cells are induced to differentiate with retinoic acid.