Studies on the relationship between Ca2+ efflux from mitochondria and the release of amino acid neurotransmitters

The relationship between experimental 2,4-Dinitrophenol administration and neurological oxidative stress: in terms of dose, time and gender differences

Although 2,4-DNP is claimed to promote fast weight reduction, it is also related with an intolerable high risk of serious side effects to various tissues. On the other hand, it is known to have neuroprotective effects. These different effects of 2,4-DNP may be due to the administration conditions. For this reason, in this study, it was aimed for the first time to clarify the oxidative changes that occur in the brain during the use of 2,4-DNP, depending on the dose, time and gender. For this purpose, 60 Wistar rats (30 male, 30 female) were divided into ten groups: control groups, short-term/long-term groups and low dose/high dose groups. Except for the control groups, 2,4-DNP was administered to the other groups by oral gavage. End of the experiment, thiobarbituric acid-reactive substances (TBARs), glutathione (GSH), nitric oxide (NOx) and ascorbic acid (AA) levels were measured in the brain tissues of sacrificed animals. 2,4-DNP administration showed attenuation impact on oxidative stress depending on both dose, time and gender. It can be said that it is more beneficial in terms of neuroprotection, especially in the short-term and male groups. In conclusion, our findings suggest that, depending on the dose, time, and gender, 2,4-DNP may be beneficial in the treatment of neurodegenerative disorders.

Mitochondrial pathophysiology beyond the retinal ganglion cell: occipital GABA is decreased in autosomal dominant optic neuropathy

PURPOSE

It has remained a mystery why some genetic mitochondrial disorders affect predominantly specific cell types such as the retinal ganglion cell. This is particularly intriguing concerning retinal and cortical function since they are tightly linked in health and disease. Autosomal dominant optic neuropathy (ADOA) is a mitochondrial disease that affects the ganglion cell. However, it is unknown whether alterations are also present in the visual cortex, namely in excitation/inhibition balance.

METHODS

In this study, we performed in vivo structural and biochemical proton magnetic resonance imaging in 14 ADOA and 11 age-matched control participants focusing on the visual cortex, with the aim of establishing whether in this genetically determined disease an independent cortical neurochemical phenotype could be established irrespective of a putative structural phenotype. Cortical thickness of anatomically defined visual areas was estimated, and a voxel-based morphometry approach was used to assess occipital volumetric changes in ADOA. Neurochemical measurements were focused on γ-aminobutyric acid (GABA) and glutamate, as indicators of the local excitatory/inhibitory balance.

RESULTS

We found evidence for reduced visual cortical GABA and preserved glutamate concentrations in the absence of cortical or subcortical atrophy. These changes in GABA levels were explained by neither structural nor functional measures of visual loss, suggesting a developmental origin.

CONCLUSIONS

These results suggest that mitochondrial disorders that were previously believed to only affect retinal function may also affect cortical physiology, especially the GABAergic system, suggesting reduced brain inhibition vs. excitation. This GABA phenotype, independent of sensory loss or cortical atrophy and in the presence of preserved glutamate levels, suggests a neurochemical developmental change at the cortical level, leading to a pathophysiological excitation/inhibition imbalance.

Impact of Aging on Calcium Signaling and Membrane Potential in Endothelium of Resistance Arteries: A Role for Mitochondria

Impaired blood flow to peripheral tissues during advanced age is associated with endothelial dysfunction and diminished bioavailability of nitric oxide (NO). However, it is unknown whether aging impacts coupling between intracellular calcium ([Ca2+]i) signaling and small- and intermediate K+ channel (SKCa/IKCa) activity during endothelium-derived hyperpolarization (EDH), a signaling pathway integral to dilation of the resistance vasculature. To address the potential impact of aging on EDH, Fura-2 photometry and intracellular recording were applied to evaluate [Ca2+]i and membrane potential of intact endothelial tubes (width, 60 µm; length, 1-3 mm) freshly isolated from superior epigastric arteries of young (4-6 mo) and old (24-26 mo) male C57BL/6 mice. In response to acetylcholine, intracellular release of Ca2+ from the endoplasmic reticulum (ER) was enhanced with aging. Further, treatment with the mitochondrial uncoupler FCCP evoked a significant increase of [Ca2+]i with membrane hyperpolarization in an SKCa/IKCa-dependent manner in the endothelium of old but not young mice. We conclude that the ability of resistance artery endothelium to release Ca2+ from intracellular stores (ie, ER and mitochondria) and hyperpolarize Vm via SKCa/IKCa activation is augmented as compensation for reduced NO bioavailability during advanced age.

A fundamental system of cellular energy homeostasis regulated by PGC-1alpha

Maintenance of ATP levels is a critical feature of all cells. Mitochondria are responsible for most ATP synthesis in eukaryotes. We show here that mammalian cells respond to a partial chemical uncoupling of mitochondrial oxidative phosphorylation with a decrease in ATP levels, which recovers over several hours to control levels. This recovery occurs through an increased expression of the transcriptional coactivator peroxisome proliferator-activated receptor-coactivator 1alpha (PGC-1alpha) and mitochondrial genes. Cells and animals lacking PGC-1alpha lose this compensatory mechanism and cannot defend their ATP levels or increase mitochondrial gene expression in response to reduced oxidative phosphorylation. The induction of PGC-1alpha and its mitochondrial target genes is triggered by a burst of intracellular calcium, which causes an increase in cAMP-response-element-binding protein and transducer of regulated cAMP-response-element-binding proteins actions on the PGC-1alpha promoter. These data illustrate a fundamental transcriptional cycle that provides homeostatic control of cellular ATP. In light of this compensatory system that limits the toxicity of mild uncoupling, the use of chemical uncoupling of mitochondria as a means of treating obesity should be re-evaluated.

The role of mitochondrial porins and the permeability transition pore in learning and synaptic plasticity

Mitochondrial outer membrane permeability is conferred by a family of porin proteins. Mitochondrial porins conduct small molecules and constitute one component of the permeability transition pore that opens in response to apoptotic signals. Because mitochondrial porins have significant roles in diverse cellular processes including regulation of mitochondrial ATP and calcium flux, we sought to determine their importance in learning and synaptic plasticity in mice. We show that fear conditioning and spatial learning are disrupted in porin-deficient mice. Electrophysiological recordings of porin-deficient hippocampal slices reveal deficits in long and short term synaptic plasticity. Inhibition of the mitochondrial permeability transition pore by cyclosporin A in wild-type hippocampal slices reproduces the electrophysiological phenotype of porin-deficient mice. These results demonstrate a dynamic functional role for mitochondrial porins and the permeability transition pore in learning and synaptic plasticity.

Effect of chronic treatment with the GABA transaminase inhibitors gamma-vinyl GABA and ethanolamine O-sulphate on the in vitro GABA release from rat hippocampus

1. The effects of 2, 8 and 21 day oral treatment with the specific gamma-aminobutyric acid transaminase (GABA-T) inhibitors gamma-vinyl GABA (GVG) and ethanolamine O-sulphate (EOS) on brain GABA levels, GABA-T activity, and basal and stimulated GABA release from rat cross-chopped brain hippocampal slices was investigated. 2. Treatment with GABA-T inhibitors lead to a reduction in brain GABA-T activity by 65-80% compared with control values, with a concomitant increase in brain GABA content of 40-100%. 3. Basal hippocampal GABA release was increased to 250-450% of control levels following inhibition of GABA-T activity. No Ca2+ dependence was observed in either control or treated tissues. 4. GVG and EOS administration led to a significant elevation in the potassium stimulated release of GABA from cross-chopped hippocampal slices compared with that of controls. Although stimulated GABA release from control tissues was decreased in the presence of a low Ca2+ medium, GVG and EOS treatment abolished this Ca2+ dependency. 5. GABA compartmentalization, Na+ and Cl- coupled GABA uptake carriers and glial release may provide explanations for the loss of the Ca2+ dependency of stimulated GABA release observed following GVG and EOS treatment. 6. Administration of GABA-T inhibitors led to increases in both basal and stimulated hippocampal GABA release. However, it is not clear which is the most important factor in the anticonvulsant activity of these drugs, the increased GABA content 'leaking' out of neurones and glia leading to widespread inhibition, or the increase in stimulated GABA release which may occur following depolarization caused by an epileptic discharge.

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

Effects of diltiazem on bioenergetics, K+ gradients, and free cytosolic Ca2+ levels in rat brain synaptosomes submitted to energy metabolism inhibition and depolarization

Diltiazem was able to decrease the oxygen consumption rate and lactate production in synaptosomes isolated from rat forebrains, both under control and depolarized (40 microM veratridine) conditions, starting from a concentration of 250 microM. This effect was particularly evident when synaptosomes were depolarized by veratridine. This depolarization-counteracting action was evident also when transplasma membrane K+ diffusion potentials were measured after depolarization induced by veratridine and by rotenone with a glucose shortage. The concentrations of ATP, phosphocreatine, and creatine were less sensitive to diltiazem action. The concentration/response relationships were the same as those found for the oxygen consumption were the same as those found for the oxygen consumption rate, lactate production, and K+ diffusion potentials. The effects of 0.5 mM diltiazem in counteracting inhibition of energy metabolism induced by rotenone without glucose were no longer detectable when either Ca2+ or Na+ was absent from the incubation medium of synaptosomes. Diltiazem at the same concentrations (starting from 250 microM) was able to inhibit both the veratridine-induced and the rotenone-without-glucose-induced increase in intrasynaptosomal free Ca2+ levels evaluated with the fluorescent probe quin2. The results are discussed in view of a possible effect of diltiazem on voltage-dependent Na+ channels and the possibility of utilizing this approach for counteracting neuronal failure due to derangement of energy metabolism or hyperexcitation.

Effect of organic and inorganic calcium channel blockers on gamma-amino-n-butyric acid release induced by monensin and veratrine in the absence of external calcium

The effects of two organic Ca2+ antagonists (verapamil and nitrendipine) and of two inorganic Ca2+ channel blockers (Co2+ and ruthenium red) on the Na+-dependent release of gamma-amino-n-butyric acid (GABA) triggered by veratrine and monensin in the absence of external Ca2+ were studied in mouse brain synaptosomes. Ca2+-independent release of GABA stimulated by the Na+ channel activator veratrine was inhibited with micromolar concentrations of verapamil and nitrendipine. In contrast, GABA release induced by the Na+ ionophore monensin was insensitive to the organic Ca2+ antagonists. Verapamil also failed to modify A23187-stimulated release of GABA in the presence of Ca2+ but inhibited high K+-induced release of the transmitter. Co2+ partially diminished veratrine-induced release but did not change monensin-induced release. Releasing responses to monensin and veratrine were insensitive to ruthenium red, which inhibited the Ca2+-dependent component of GABA release evoked by high K+ depolarization. These data demonstrate that the mechanism of inducing GABA release is different for veratrine and monensin, as evidenced by their differing sensitivities to inhibition by Ca2+ channel antagonists and organic Ca2+ blockers. It is concluded that voltage-sensitive Ca2+ channels of the presynaptic membrane are not involved in the inhibitory action of Ca2+ antagonists on the Na+-dependent, Ca2+-independent mechanism of GABA release.

Morphine inhibition of calcium fluxes, neurotransmitter release and protein and lipid phosphorylation in brain slices and synaptosomes

Morphine (1-100 microM) was found to inhibit several concomitant events in brain slices and synaptosomes which are augmented by depolarizing agents. Thus, 45Ca2+ uptake, amino acid neurotransmitter release, increases in 3',5' cyclic AMP levels and 32Pi incorporation to proteins and lipids induced by veratrine (25 microM) and by potassium (56 mM), were each inhibited in a dose related manner. These inhibitory actions of morphine were all prevented by naloxone (1 microM). Evidence is presented that morphine binding to a receptor on the synaptic membrane affects intracellular mechanisms involved in neurotransmitter release possibly via a second messenger system. An enhancing action of GTP on the inhibitory influences of morphine suggests that its actions are mediated at least in part, via a coupling of the receptor to adenyl cyclase in the outer membrane. This is supported by its inhibitory action on the capacity of depolarizing agents to increase cyclic AMP levels.

On the mechanism of enhanced release of [14C]glutamate in hippocampal long-term potentiation

K+-induced release of [14C]glutamate was studied in slices of dentate gyrus prepared from control rats and rats in which long-term potentiation (LTP) had been induced in vivo. At all concentrations of Ca2+ studied, release from potentiated slices was greater than from control slices. In the same preparations both Ruthenium Red and caffeine enhanced basal release but in potentiated tissue the Ruthenium Red-induced release was significantly greater than in control tissue. These results are discussed in the light of our recent finding that enhanced transmitter release is associated with LTP.

Sodium-dependent, calmodulin-dependent transmitter release from synaptosomes

The baseline efflux of gamma-amino[2,3-3H]butyric acid ([3H]GABA) and [3H]dopamine ([3H]DA) from caudate synaptosomes was greatly enhanced by the sodium-ionophore monensin; this stimulatory effect of monensin on transmitter release was markedly inhibited by trifluoperazine (TFP), a potent calmodulin antagonist. TFP also decreased the depolarization-induced, calcium-dependent release of [3H]GABA and this effect was unrelated to the calcium-flux across the plasma membrane since TFP also inhibited the release of GABA elicited by the calcium-ionophore A23187. Our data indicate that transmitter release induced by both increased intraterminal sodium levels and by the calcium entry into the nerve endings during depolarization might be mediated by calmodulin-dependent processes.

Ionophore A23187, verapamil, protonophores, and veratridine influence the release of gamma-aminobutyric acid from synaptosomes by modulation of the plasma membrane potential rather than the cytosolic calcium

The release of GABA induced by veratridine shows no correlation with the synaptosomal Ca content and is therefore not mediated by the release of mitochondrial Ca. Instead, with both Ca-repleted and -depleted synaptosomes, the extent of GABA efflux is correlated with the decrease in plasma membrane potential. The slow release of GABA induced by protonophores and the Ca-dependent release induced by ionophore A23187 are also consequences of the depolarization of the plasma membrane, rather than of elevated cytosolic Ca. Finally, the ability of verapamil to inhibit the release of GABA induced by low veratridine concentrations is due to the ability of the Ca channel inhibitor to antagonize the action of veratridine, rather than to inhibit Ca entry into the synaptosome. It is concluded that it is essential to monitor plasma membrane potentials in experiments in which amino acid efflux from synaptosomes is induced.

Hypothermic effect of 2,4-dinitrophenol infused ICV in the cat

The effects of body temperature and behavior of 2,4-dinitrophenol injected into the cerebral ventricles of the cat was investigated in these experiments. Infused in a volume of 0.1-0.2 ml, 2,4-dinitrophenol produced a dose-dependent fall in body temperature, the duration of which was also dose-dependent. Apart from hypothermia, 2,4-dinitrophenol evoked mydriasis, respiratory irregularities, urination, vomiting, ataxia, muscular weakness, sedation and occasional clonic-tonic convulsions. Of all the autonomic effects, the most consistent was the effect on thermoregulation. The possible mechanisms of action in the brain of 2,4-dinitrophenol on the thermoregulatory mechanisms are discussed.

Role of presynaptic dopamine receptors in regulation of the glutamatergic neurotransmission in rat neostriatum

In experiments with the use of a push-pull cannula and simultaneous recording of electrical activity at the site of perfusion, the release of L-[3H]glutamic acid from rat neostriatum induced by K+-depolarization (60 mM K+ in perfusate) has been shown to be inhibited by replacing Ca2+ in the perfusion medium by Co2+. In contrast, release of L-[3H]glutamate induced by electrical stimulation of frontal cortex is enhanced by replacement of these cations. Application of dopamine (10(-5)-10(-3) M). apomorphine (10(-4) M) or beta-phenylethylamine (10(-3) M) as well as stimulation of the substantia nigra enhanced the basal release of L-[3H]glutamate. Haloperidol (10(-4) M) completely abolished the effects of apomorphine and beta-phenylethylamine, and partially abolished the effect of dopamine. The enhancement induced by apomorphine is strongly dependent on the presence of Na+ in the perfusion medium. On the other hand, apomorphine (10(-4) M) and beta-phenylethylamine (10(-3) M) inhibited the release of glutamate induced by electrical stimulation of the frontal cortex and that by K+-depolarization (the latter was shown for apomorphine). This inhibition is also abolished by haloperidol. A possible functional role of endogenous dopamine in the regulation of glutamatergic neurotransmission in rat neostriatum is discussed.

Synaptosomal calcium metabolism during hypoxia and 3,4-diaminopyridine treatment

A decline in the calcium-dependent release of neurotransmitters appears to underlie the decreased neuronal function that accompanies reduced oxygen tensions (hypoxia). To determine if alterations in calcium uptake are primary to these changes, synaptosomal calcium uptake was measured in the presence of 100%, 2.5%, or 0% oxygen. Calcium uptake declined 60.2 +/- 0.1 and 82.4 +/- 2.5% with 2.5% and 0% when compared with 100% oxygen, respectively. 3,4-Diaminopyridine stimulated calcium uptake by synaptosomes when they were incubated in low-potassium media. It also diminished the hypoxic-induced decline in calcium uptake to 30.6 +/- 3.1 and 33.5 +/- 3.1% with 2.5% and 0% oxygen, respectively. External binding to the synaptosomal plasma membrane declined to 29.2 +/- 0.3 or 11.8 +/- 0.9% when the oxygen tension was reduced to 2.5% or 0% oxygen. 3,4-Diaminopyridine increased this superficial binding from 111.7 +/- 0.3 to 86.5 +/- 0.9 or 23.4 +/- 0.9% with 100%, 2.5%, or 0% oxygen when compared with 100% oxygen without 3,4-diaminopyridine, respectively. Thus, the decline in neuronal processing that accompanies acute hypoxia may be due to altered calcium homeostasis, which diminishes neurotransmitter release.

Further studies on the role of calcium in the regulation of glutamate decarboxylase activity in brain slices

[3H]GABA synthesis in brain slices was used as a model to study the role of Ca2+ in the regulation of GAD activity. Experimental conditions were chosen to increase and decrease the flux of Ca2+ and to promote the increase in free intracellular Ca2+. The blockade of electron transport and the inhibition of oxidative phosphorylation in the slices inhibited [3H]GABA synthesis. High K+ depolarization stimulated [3H]GABA synthesis and this effect was not blocked by lidocaine, trifluoperazine, or verapamil, but the stimulation was blocked by the intracellular Ca2+ antagonist TMB-8. The data do not differentiate between the relative contributions of extra- and intracellular Ca2+ but reflect that GAD activity is modulated by a dynamic balance between these two compartments as well as between stored and free Ca2+ within the cells.

Nitriphenol compound induces Ca-dependent exocytotic secretion of catecholamines by a direct effect on the plasma membranes of the adrenal medullary cells

2,4-Dinitrophenol induced Ca-dependent secretion of adrenaline from the perfused adrenals of the rabbits. Secretion of adrenaline was accompanied by that of dopamine-beta-hydroxylase showing the exocytotic nature, 2,4,5-Trinitrophenol, which is devoid of uncoupling activity, and other nitrophenol compounds were also effective in inducing secretion, suggesting that secretion by nitrophenol compound is due to increased Ca entry through its direct effect on the plasma membrane.

Effect of phospholipase A2 on calcium transport in brain synaptosomes

Pretreatment of isolated brain synaptic endings with commercial phospholipase A2 isolated from venom of Apis mellifera, followed by a BSA washing, selectively inhibited the depolarization-dependent portion of 45Ca uptake. Phospholipase A2 initially caused an increase of synaptosome respiration and subsequently inhibited their oxygen uptake, but this effect was completely abolished in BSA-containing media. The classical uncoupler of oxidative phosphorylation, DNP, produced release of 45Ca or [3H]GABA from superfused synaptosomes previously loaded with radioactive calcium or GABA. The treatment of synaptosomes with phospholipase A2 had no effect on the spontaneous efflux of 45Ca or [3H]GABA. However, depolarization-dependent release of [3H]GABA from synaptosomes treated with phospholipase A2 was significantly inhibited. We suggest that the inhibition of depolarization-dependent influx of 45Ca into synaptosomes treated with phospholipase A2 may be attributed to the lesion of the specific function of plasma membrane rather than to the impairment of the calcium-sequestrating function of intraterminal structures.

The effects of morphine and methionine-enkephalin on the release of purines from cerebral cortex slices of rats and mice

1 Slices of cerebral cortex from Wistar rats, TO mice or C57 mice were preincubated with [3H]-adenosine, and labelled purines were subsequently releases by electrical stimulation or by perfusing with ouabain, 100 micro M. 2 Electrically-evoked purine release was substantially reduced when the Ca2+ concentration in the medium was lowered from 2.4 to 0.1 mM. In both rats and mice, the electrically-evoked release was increased by morphine and methionine-enkephalin (Met-enkephalin), 10 micro M, and in rats and TO mice by morphine 1 micro M, both drug effects being prevented by naloxone. 3 Purine release evoked by ouabain was also increased by morphine 1 and 10 micro M, though not by Met-enkephalin, from slices of rat cortex. Ouabain-induced release from TO mice was reduced by morphine, and from C57 mice was unchanged. 4 The enhancement by morphine of electrically-evoked purine release may indicate that purines mediate some effects of morphine in the CNS.

Sodium-dependent efflux of [3H]GABA from synaptosomes probably related to mitochondrial calcium mobilization

It has been suggested that mitochondria might modify transmitter release through the control of intracellular Ca2+ levels. Treatments known to inhibit Ca2+ retention by mitochondria lead to an increased transmitter liberation in the absence of external Ca2+, both at the frog neuromuscular junction and from isolated nerve endings. Sodium ions stimulate Ca2+ efflux from mitochondria isolated from excitable tissues. In the present study, the effect of increasing internal Na+ levels on [3H]gamma-aminobutyric acid ([3H]GABA) release from isolated nerve endings is reported. Results show that the efflux of [3H]GABA from prelabeled synaptosomes is stimulated by ouabain, veratrine, gramicidin D, and K+-free medium, which increase the internal sodium concentration. This effect was not observed when Na+ was omitted from the incubation medium and it was independent of external Ca2+, the experiments having been performed in a Ca2+-free, EGTA-containing medium. Since preincubation of synaptosomes with 2,4-diaminobutyric acid did not prevent the stimulatory effect of increased internal Na+ levels on [3H]GABA efflux, it appears to be unrelated to an enhanced activity of the outward carrier-mediated GABA transport. These results suggest that the augmented release of [3H]GABA may be due to an increased Ca2+ efflux from mitochondria elicited by the accumulation of Na+ at the nerve endings.