Effect of glutamate receptor ligands on mitochondrial membrane potential in rat dissociated cerebellar cells

Dactylorhin B reduces toxic effects of β-amyloid fragment (25–35) on neuron cells and isolated rat brain mitochondria

beta-amyloid is strongly implicated in Alzheimer's pathology, and mitochondria play an important role in neurodegenerative disorders. Dactylorhin B [short for bis(4-beta-D-glucopyranosyloxybenzyl)-2-beta-D-glucopyranosyl-2-isobutyltartrate (DHB)] is an active compound isolated from Coeloglossum viride. (L.) Hartm. var. bracteatum (Willd.) and may have neuroprotective effects. In the present study, we investigated the damage of rat brain mitochondrial function induced by beta-amyloid and the protective effect of DHB. Isolated rat brain mitochondria were incubated with amyloid-beta peptide (Abeta)(25-35) alone or together with DHB. reactive oxygen species production, pyruvate dehydrogenase complex activity, alpha-ketoglutarate dehydrogenase complex activity, cytochrome c oxidase activity, mitochondrial swelling, mitochondrial membrane potential and the release of cytochrome c from mitochondria were measured. Data showed that Abeta(25-35) directly disrupted mitochondrial function, inhibited the key enzymes and contributed to apoptosis and the deficiency of energy metabolism. Co-incubation of DHB attenuated Abeta(25-35)-induced pathological changes. The significance of DHB in the management of mitochondria-related disorders is discussed.

Ammonia prevents glutamate-induced but not low K(+)-induced apoptosis in cerebellar neurons in culture

Cultured rat cerebellar granule neurons are widely used as a model system for studying neuronal apoptosis. Either low K(+) (5 mM) or low concentrations of glutamate (1-10 microM) induce apoptosis in cerebellar neurons in culture. However, the molecular mechanism(s) involved remain unclear. We show that long-term treatment with ammonia prevents glutamate-induced but not low K(+)-induced apoptosis in cerebellar neurons, as assessed by measuring DNA fragmentation and activation of caspase 3. Ammonia prevented glutamate-induced increase of intracellular calcium, depolarization of the inner mitochondrial membrane, release of cytochrome c to the cytosol, activation of caspase 3 and fragmentation of DNA. However, ammonia did not prevent low K(+)-induced activation of caspase 3 and fragmentation of DNA. These results indicate that the initial steps involved in the induction of apoptosis by low K(+) or by glutamate are different and that ammonia prevents glutamate-induced apoptosis by reducing glutamate-induced rise of intracellular Ca(2+), thus avoiding the activation of subsequent events of the apoptotic process.

Maturation of vulnerability to excitotoxicity: intracellular mechanisms in cultured postnatal hippocampal neurons

Neuronal vulnerability to excitotoxicity changes dramatically during postnatal maturation. To study the intracellular mechanisms by which maturation alters vulnerability in single neurons, we developed techniques to maintain hippocampal neurons from postnatal rats in vitro. After establishing their neuronal phenotype with immunohistochemistry and electrophysiology, we determined that these neurons exhibit developmentally regulated vulnerability to excitotoxicity. At 5 days in vitro, NMDA-induced cell death at 24 h increased from 3.6% in 3-day-old rats to >90% in rats older than 21 days. Time-lapse imaging of neuronal morphology following NMDA demonstrated increasingly prevalent and severe injury as a function of postnatal age. Neither high- nor low-affinity calcium dyes demonstrated differences in peak NMDA-induced [Ca(2+)](i) increases between neurons from younger and older animals. However, neurons from older animals were uniformly distinguished from those from younger animals by their subsequent loss of [Ca(2+)](i) homeostasis. Because of the role of mitochondrial Ca(2+) buffering in [Ca(2+)](i) homeostasis, we measured NMDA-induced changes in mitochondrial membrane potential (DeltaPsi) as a function of postnatal age. NMDA markedly dissipated DeltaPsi in neurons from mature rats, but minimally in those from younger rats. These data demonstrate that, in cultures of postnatal hippocampal neurons, (a) vulnerability to excitotoxicity increases as a function of the postnatal age of the animal from which they were harvested, and (b) developmental regulation of vulnerability to NMDA occurs at the level of the mitochondrion.

Alteration of mitochondrial calcium homeostasis by ammonia-induced activation of NMDA receptors in rat brain in vivo

The aim of the present work was to assess the effects of activation of NMDA receptors in rat brain in vivo on calcium homeostasis in isolated non-synaptic brain mitochondria. We have shown recently that acute intoxication with large doses of ammonia leads to activation of NMDA receptors in rat brain in vivo. In the present work we injected rats with ammonium acetate to activate NMDA receptors in vivo and isolated non-synaptic mitochondria to assess calcium homeostasis. We also tested whether blocking NMDA receptors with MK-801 prevents effects on calcium homeostasis induced by ammonium injection. It is shown that activation of NMDA receptors in rat brain in vivo leads to a rapid increase in intramitochondrial calcium content followed by a reduction in the calcium capacity and calcium uptake rate in rat brain mitochondria. Activation of NMDA receptors resulted in increased spontaneous calcium efflux from rat brain mitochondria and in a strong inhibition of Na-induced and tert-butylhydroperoxide-induced calcium efflux. All these effects were prevented by previous blocking of NMDA receptors by injection of MK-801. Cyclosporin A did not affect any of the above parameters, indicating that the mitochondrial permeability transition pore does not play a role in calcium efflux under any of the conditions studied. The results reported indicate that ammonia-induced activation of NMDA receptors in rat brain in vivo alters mitochondrial calcium homeostasis at several different steps.

Continuous infusion of cyclosporin A postinjury significantly ameliorates cortical damage following traumatic brain injury

Traumatic brain injury (TBI) results in the rapid necrosis of cortical tissue at the site of injury. In the ensuing hours and days, secondary injury exacerbates the original damage resulting in significant neurological dysfunction. Recent reports from our lab demonstrate that a bolus injection of the immunosuppressant cyclosporin A (CsA) is neuroprotective following TBI. CsA transiently inhibits the opening of the mitochondrial permeability transition pore and maintains calcium homeostasis in isolated mitochondria. The present study utilized a unilateral controlled cortical impact model of TBI to assess whether the neuroprotective effects of CsA could be extended by chronic infusion. Adult rats were subjected to a moderate (2 mm) cortical deformation and the extent of cortical damage was assessed using modern stereological techniques. Animals were administrated a 20 mg/kg intraperitoneal bolus of CsA or vehicle 15 min postinjury and osmotic minipumps were implanted subcutaneously to deliver CsA (4.5 or 10 mg/kg/day) or vehicle. All animals receiving CsA demonstrated a significant reduction in lesion volume, with the highest dose offering the most neuroprotection (74% reduction in lesion volume). These results extend our previous findings and demonstrate that chronic infusion of CsA is neuroprotective following TBI. These findings also suggest that the mechanisms responsible for tissue necrosis following TBI are amenable to manipulation.

Cyclosporin A attenuates acute mitochondrial dysfunction following traumatic brain injury

Experimental traumatic brain injury (TBI) results in a rapid and significant necrosis of cortical tissue at the site of injury. In the ensuring hours and days, secondary injury exacerbates the primary damage, resulting in significant neurological dysfunction. Recent reports from our lab and others have demonstrated that the immunosuppressant cyclosporin A (CsA) is neuroprotective following TBI. The opening of the mitochondrial permeability transition pore (MPTP) is inhibited by CsA, thereby maintaining the mitochondrial membrane potential and calcium homeostasis in isolated mitochondrial. In the present study we utilized a unilateral controlled cortical impact model of TBI to assess mitochondrial dysfunction in both isolated mitochondria and synaptosomes to elucidate the neuroprotective role of CsA. The results demonstrate that administration of CsA 15 min postinjury significantly attenuates mitochondrial dysfunction as measured using several biochemical assays of mitochondria integrity and energetics. Following TBI, mitochondria isolated from the injured cortex of animals treated with CsA demonstrate a significant increase in mitochondria membrane potential and are resistant to the induction of mitochondrial permeability transition compared to vehicle-treated animals. Similarly, synaptosomes isolated from CsA-treated animals demonstrate a significant increase in mitochondria membrane potential, accompanied by lower levels of intramitochondrial Ca2+ and reactive oxygen species production than seen in vehicle-treated animals. These results suggest that the neuroprotective properties of CsA are mediated through modulation of the MPTP and maintenance of mitochondria homeostasis. Amelioration of cortical damage with CsA indicates that pharmacological therapies can be devised which will significantly alter neurological outcome after injury.

In vitro and in vivo protective effect of orphenadrine on glutamate neurotoxicity

The anticholinergic drug orphenadrine is used in the treatment of Parkinson's disease. In this study we evaluate the neuroprotective effects of orphenadrine on excitotoxicity in vivo and in vitro. Orphenadrine prevented the mitochondrial and the cytoplasmic membrane potential decrease evoked by NMDA (100 microM) in rat dissociated cerebellar granule cells showing an IC50 value of 11.6 +/- 4.7 microM (mean +/- SEM, n = 5) and 13.5 +/- 2.3 microM (n = 3), respectively. Orphenadrine was able to protect cerebellar granule cell cultures from glutamate-induced neurotoxicity. Kainic acid (KA, 10 mg/kg)-induced excitotoxicity was evaluated in vivo using the microglial marker peripheral-type benzodiazepine receptor (PBR) and heat shock protein 72 (HSP72) expression in the hippocampus. The Bmax of PBR for control tissues was 589.1 +/- 40.0 fmol/mg protein (n = 4), increasing to 1692.5 +/- 51.6 fmol/mg protein (n = 5) after the KA treatment. Pretreatment with orphenadrine (10 mg/kg) blocked the KA-induced increase in PBR density. As expected, KA-administration induced the expression of HSP72 that was blocked in the orphenadrine + KA-treated rats. We demonstrate that orphenadrine, interacting at the NMDA receptor, is able to prevent the neurotoxicity mediated by activation at glutamate ionotropic receptors.

Effects of U-83836E on glutamate-induced neurotoxicity in dissociated rat cerebellar granule cells

The effects of the lazaroid compound U-83836E on the glutamate-induced production of reactive oxygen species (ROS) were studied in dissociated rat cerebellar granule cells by flow cytometry. U-83836E completely inhibited ROS production with an estimated IC50 value of 21.7 +/- 9.1 nM. However, U-83836E did not inhibit the glutamate-evoked decrease in mitochondrial membrane potential (MMP). Nevertheless, U-83836E (10 nM to 10 microM) prevented cell death induced by 10 mM of glutamate. At concentrations above 10 microM, U-83836E by itself showed slight cytotoxicity, which was significant at a 100 microM concentration. U-83836E (25 to 200 microM) also increased the cytosolic calcium levels in a concentration-dependent manner. Our results indicate that the cytotoxic effects found at micromolar concentrations of U-83836E could be explained by an increase in [Ca2+]i. Finally, since U-83836E did not prevent the MMP decrease evoked by glutamate, it is suggested that antioxidant pharmacotherapy would not be sufficient to block the neurotoxic effects of glutamate.

Effect of 1-methyl-4-phenylpyridinium (MPP+) on mitochondrial membrane potential in cerebellar neurons: interaction with the NMDA receptor

The effect of MPP+, a dopaminergic neurotoxin, in mitochondrial membrane potential was investigated in dissociated cerebellar granule cells using rhodamine 123 and flow cytometry. MPP+ (1 mM) decreased the mitochondrial membrane potential by 30%. Antagonists of the NMDA receptor complex, such as MK-801 (IC50 value of 20.92 +/- 0.02 nM), 5,7-dichlorokynurenic acid (IC50 value of 6.46 +/- 1.06 microM) and D-AP5 (IC50 value of 8.29 +/- 0.63 microM), inhibited the action of MPP+. Neither NBQX, nor riluzole, nor desipramine modified the action of MPP+. Dibucaine restored the basal values of mitochondrial membrane potential altered by MPP+. Since, in the presence of NMDA, MPP+ antagonized the effect of this total agonist, it can be concluded that, in this preparation, MPP+ interacts with the NMDA receptor complex as a partial agonist. This interaction could be the result of an allosteric modulation of the NMDA receptor complex by MPP+. The decrease of mitochondrial membrane potential induced by MPP+ is antagonized by dibucaine, suggesting that this effect is mediated by an activation of phospholipase A2.

Modulation of neuronal mitochondrial membrane potential by the NMDA receptor: role of arachidonic acid

Activation of NMDA receptors in dissociated cerebellar granule cells reduced mitochondrial membrane potential (MMP), as measured by rhodamine 123 fluorescence in a flow cytometer. This effect was inhibited by several NMDA-receptor antagonists with the following rank order of potency: MK-801 > PCP > TCP > dextrorphan > dichlorokynurenic acid > D-AP5 > dextromethorphan. Neither spermine nor arcaine modified the NMDA-induced reduction in MMP, whereas ifenprodil and eliprodil inhibited this response in the micromolar range. The mechanism responsible for the alteration of MMP mediated by NMDA was studied. Mepacrine and dibucaine prevented the MMP reduction induced by NMDA, as did W13 (calmodulin antagonist). In contrast, this effect was not blocked by cyclooxygenase or lipooxygenase inhibitors, H7 (a protein kinase C inhibitor) or nitroarginine (nitric oxide synthase inhibitor). These data suggest a direct interaction between NMDA-receptor activation and arachidonic acid formation, and indicate that NMDA receptor-mediated effect on MMP could involve arachidonic acid.

Determination of nitric oxide generation in mammalian neurons using dichlorofluorescin diacetate and flow cytometry

A method for the rapid detection of intracellular nitric oxide (NO) generation in dissociated cerebellar granule cells using dichlorofluorescin (DCFH) and flow cytometry was developed. DCFH can be oxidized specifically by NO and this was assessed by 1) the use of SIN-1 (10 nM-100 microM), an NO donor, that induced a concentration-dependent increase in dichlorofluorescein (DCF) fluorescence and 2) the use of hemoglobin (10 microM), an NO-scavenger, that totally inhibited the increase of fluorescence induced by SIN-1 (10 microM). This assay was used to determine the ability to kainate to stimulate NO production in dissociated cerebellar granule cells. Kainate (1 microM-10 mM) induced an increase in DCF fluorescence that was partially reduced by NG-nitro-L-arginine (1 nM-10 microM), a nitric oxide synthase inhibitor (61.9% +/- 9.1), or hemoglobin (10 microM) (55.0% +/- 4.1). The method described allows evaluation of the oxidation of DCFH to produce DCF as a parameter for measuring intracellular NO generation. The extent of DCFH oxidation by NO and ROS can be determined by using NO scavengers or NO synthase inhibitors.