The lack of extracellular Na+ exacerbates Ca2+-dependent damage of cultured cerebellar granule cells

'Mild Uncoupling' does not decrease mitochondrial superoxide levels in cultured cerebellar granule neurons but decreases spare respiratory capacity and increases toxicity to glutamate and oxidative stress

Cultured rat cerebellar granule neurons were incubated with low nanomolar concentrations of the protonophore carbonylcyanide-p-trifluoromethoxyphenyl hydrazone (FCCP) to test the hypothesis that 'mild uncoupling' could be neuroprotective by decreasing oxidative stress. To quantify the uncoupling, respiration and mitochondrial membrane potential (Deltapsi(m)) were determined in parallel as a function of FCCP concentration. Deltapsi(m) dropped by less than 10 mV before respiratory control was lost. Conditions for the valid estimation of matrix superoxide levels were determined from the rate of oxidation of the matrix-targeted fluorescent probe MitoSOX. No significant change in the level of matrix superoxide could be detected on addition of FCCP while respiratory control was retained, although cytoplasmic superoxide levels measured by dihydroethidium oxidation increased. 'Mild uncoupling' by 30 nmol/L FCCP did not alleviate neuronal dysregulation induced by glutathione depletion and significantly enhanced that due to menadione-induced oxidative stress. Low protonophore concentrations enhanced N-methyl-d-aspartate receptor-induced delayed calcium deregulation consistent with a decrease in the spare respiratory capacity available to match the bioenergetic demand of chronic receptor activation. It is concluded that the 'mild uncoupling' hypothesis is not supported by this model.

Role of mitochondria in the mechanisms of glutamate toxicity

Current data on glutamate-induced functional and morphological changes in mitochondria correlating with or being a result of their membrane potential changes are reviewed. The important role of Ca2+, Na+, and H+ in the potentiation of such changes is considered. It is assumed that glutamate-induced loss of mitochondrial potential is mediated by Ca2+ overload resulting in the induction of nonspecific permeability of the inner mitochondrial membrane.

Ultrastructural clues for glutamate-induced necrosis in parietal and cerebellar neurons

Glutamate excitotoxicity has been postulated to underlie the neuronal death that occurs after ischemia. The most sensitive tissues to ischemic injury are hippocampus and cerebellum, whereas cerebrum is more resistant. We studied the glutamate-induced ultrastructural alterations in rat parietal and cerebellar neurons comparatively. We observed that glutamate (45 min, 10-7 m) causes considerable nuclear, mitochondrial and cytoplasmic changes in both the neuron types. Mitochondrial and nuclear changes were particularly more severe in cerebellar granular, than the ones in parietal neurons. It has been concluded that glutamate induces necrotic changes in both parietal and cerebellar neurons. But cerebellar cortex was found to be more sensitive to glutamate excitotoxicity than cerebral cortex. We suggest that mitochondrial damage is, probably, an important factor in neuron necrosis, which is mediated by glutamate excitotoxicity.

Optical scatter imaging detects mitochondrial swelling in living tissue slices

Mitochondrial swelling is observed in neuronal injury and is a key event in many pathways to cell death. Currently, there is no technique for directly measuring mitochondrial size changes within living tissue slices with a field of view of several millimeters. In this paper, we test our hypothesis that Mie light-scatter theory can be used to study mitochondrial swelling in living tissue sections. Using a unique dual-angle scatter ratio (DASR) optical imaging system previously demonstrated to be sensitive to latex particle size changes and N-methyl-D-aspartate (NMDA) treatment of hippocampal slices, we studied mitochondrial swelling induced by 500 microM NMDA treatment of hippocampal slices. We observed a strong (R(2) = 0.73) and significant (P < 0.000005) correlation between the electron microscopy-determined diameters of swollen, intact mitochondria and the DASR imaging. We examined the robustness of the technique by evaluating the correlation between the dual-angle scatter ratio and the diameter of the dendrites, observed to swell, in NMDA-treated slices and found no correlation (R(2) = 0.06). The advantage of DASR imaging over electron microscopy or other methods of studying mitochondrial swelling is the sensitivity of DASR imaging to mitochondrial swelling over a large field of view (>9 mm(2)) in an intact tissue slice. This novel technique may allow for the study of regional changes in mitochondrial swelling and recovery as sequential events within a single specimen. This technique will eventually be useful in studying the efficacy of stroke and other disease therapies targeting mitochondrial swelling.

Optical light scatter imaging of cellular and sub-cellular morphology changes in stressed rat hippocampal slices

Optical imaging, such as transmission imaging, is used to study brain tissue injury. Transmission imaging detects cellular swelling via an increase in light transmitted by tissue slices due to a decrease in scattering particle concentration. Transmission imaging cannot distinguish sub-cellular particle size changes from cellular swelling or shrinkage. We present an optical imaging method, based on Mie scatter theory, to detect changes in sub-cellular particle size and concentration. The system uses a modified inverted microscope and a 16-bit cooled CCD camera to image tissue light scatter at two angles. Dual-angle scatter ratio imaging successfully discriminated latex microsphere suspensions of differing sizes (0.6, 0.8, 1 and 2 microm) and concentrations. We applied scatter imaging to hippocampal slices treated with 100 microM N-methyl-D-aspartate (NMDA) to model excitotoxic injury or -40 mOsm hypotonic perfusion solution to cause edema injury. We detected light scatter decreases similar to transmission imaging in the CA1 region of the hippocampus for both treatments. Using our system, we could distinguish between NMDA and hypotonic treatments on the basis of statistically significant (P<0.0003) differences in the scatter ratio measured in CA1. Scatter imaging should be useful in studying tissue injuries or activity resulting in brain tissue swelling as well as morphological changes in sub-cellular organelles such as mitochondrial swelling.

Mitochondrial membrane potential and neuronal glutamate excitotoxicity: mortality and millivolts

In the past few years it has become apparent that mitochondria have an essential role in the life and death of neuronal and non-neuronal cells. The central mitochondrial bioenergetic parameter is the protonmotive force, Deltap. Much research has focused on the monitoring of the major component of Deltap, the mitochondrial membrane potential Deltapsim, in intact neurones exposed to excitotoxic stimuli, in the hope of establishing the causal relationships between cell death and mitochondrial dysfunction. Several fluorescent techniques have been used, and this article discusses their merits and pitfalls.

Short-term block of Na+/K+-ATPase in neuro-glial cell cultures of cerebellum induces glutamate dependent damage of granule cells

Granule cells in a dissociated neuro-glial cell culture of cerebellum when exposed to ouabain (10(-3) M) for 25 min apparently swell, increase their [Ca2+]i with obvious depolarization of the mitochondrial membrane. In 3 h after ouabain was omitted from the solution, 62 +/- 3% of granule cells had pycnotic nuclei. The supplement of a solution with competitive specific antagonist of NMDA receptors, L-2-amino-7-phosphonoheptanoate (10(-4) M, APH) together with ouabain prevented cells from swelling, mitochondrial deenergization, neuronal death and increase of [Ca2+]i. These data suggest that cellular Na+/K+-ATPase inactivation in neuro-glial cell cultures of cerebellum leads to glutamate (Glu) accumulation, hyperstimulation of glutamate receptors, higher Ca2+ and Na+ influxes into the cells through the channels activated by Glu. This process leads to cell swelling, mitochondrial deenergization and death of granule cells. Possibly, the decrease of Na+/K+-ATPase activity in brain cells can lead to the onset of at least some chronic neurological disorders.