Ouabain-induced changes of calcium and potassium in slices of hippocampus of the rat: comparison to hypoxia and effect of R 56865

Sodium-potassium adenosine triphosphatase inhibition enhances membrane accumulation of DiI in rat hippocampus in vivo

Transient brain ischemia induces significant alterations in lipid structures of neuronal membranes, which are believed to result from lipid peroxidation and free radical attack. Such a membrane structural change may serve as an important histological marker of cell injury. In the present study, we examined how the dynamics of DiI/membrane incorporation may reflect early membrane metabolism and dynamic changes following sodium-potassium pump inhibition. Ouabain (1mM) was stereotactically co-administered with either 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate DiI (50 microg/ml) or ethidium homodimer (4 microM) into the granule cell layer of the adult rat hippocampus. Tissue was cryosectioned and examined with epifluorescence microscopy at 1, 2, 3, 4, 6, 8 and 72h post-injection. Alternate sections were stained with thionine or haematoxylin and eosin to evaluate morphological changes. Ouabain-induced pump inhibition resulted in a dramatic increase in DiI fluorescence in granule cell layer neurons as early as 4h post-injection. This increase in DiI incorporation coincided both spatially and temporally with the appearance of reactive changes characterizing early neuronal injury. However, the fluorescence increase was not a result of membrane breakdown because ethidium homodimer, a membrane-impermeable nucleic acid probe used for labeling cells with compromised membranes, when applied in a similar fashion, failed to show any fluorescence changes. The results of this study suggest that pump inhibition results in a specific increase in membrane lipophilicity possibly due to altered lipid structure.

Changes of [3H]MK-801, [3H]muscimol and [3H]flunitrazepam binding in rat brain by the prolonged ventricular infusion of ginsenoside Rc and Rg1

In the present study, we have investigated the effects of centrally administered ginsenoside Rc and Rg1 on the modulation of the NMDA receptor and GABA(A)receptor binding in rat brain. The NMDA receptor binding was analysed by quantitative autoradiography using [(3)H]MK-801 binding, and the GABA(A)receptor bindings were analysed by using [(3)H]muscimol and [(3)H]flunitrazepam binding in rat brain slices. Rats were infused with ginsenoside Rc or Rg1 (10 microg/10 microl h(-1), i.c.v.) for 7 days, through pre-implanted cannula using osmotic minipumps (Alzet, model 2ML). The levels of [(3)H]MK-801 binding were highly decreased in part of the parietal layers of the cortex and cingulated by ginsenoside Rc and Rg1. The levels of [(3)H]muscimol binding were strongly elevated in almost all regions of the frontal cortex by the treatment of ginsenoside Rc but decreased by ginsenoside Rg1. However, the [(3)H]flunitrazepam binding was not modulated by ginsenoside Rc or Rg1 infusion. These results suggest that prolonged infusion of ginsenosides could differentially modulate [(3)H]MK-801 and [(3)H]muscimol binding in a region-specific manner.

Effects of the anion channel blocker DIDS on ouabain- and high K(+)-induced release of amino acids from the rat cerebral cortex

Amino acid release from the rat cerebral cortex was analyzed using an in vivo cortical cup perfusion model. Topical applications of ouabain or high extracellular K(+) were used to mimic two dimensions of ischemic conditions which promote cell swelling and amino acid release. Ouabain (30 microM) induced significant releases of taurine, gamma-aminobutyric acid (GABA), aspartate, glutamate and phosphoethanolamine. The anion channel blocker, 4, 4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS; 1 mM), inhibited ouabain-induced release of all these amino acids except for glutamate. Exposure to high extracellular K(+) (75 mM) induced a delayed rise in the levels of taurine in the superfusates and an immediate increase in GABA levels. There were no significant releases of other amino acids. The release of taurine and GABA was sensitive to the blocking of anion channels with DIDS. Both ouabain- and high K(+)-induced taurine release is likely to be mediated by DIDS sensitive anion channels. The extracellular accumulation of the other amino acids, where insensitive to DIDS, may be mediated by mechanisms other than swelling-induced anion channels.

Intracellular acidification induced by membrane depolarization in rat hippocampal slices: roles of intracellular Ca2+ and glycolysis

To elucidate the mechanism of pHi changes induced by membrane depolarization, the variations in pHi and [Ca2+]i induced by a number of depolarizing agents, including high K+, veratridine, N-methyl-D-aspartate (NMDA) and ouabain, were investigated in rat hippocampal slices by the fluorophotometrical technique using BCECF or fura-2. All of these depolarizing agents elicited a decrease in pHi and an elevation of intracellular calcium ([Ca2+]i) in the CA1 pyramidal cell layer. The increases in [Ca2+]i caused by the depolarizing agents almost completely disappeared in the absence of Ca2+ (0 mM Ca2+ with 1 mM EGTA). In Ca2+ free media, pHi acid shifts produced by high K+, veratridine or NMDA were attenuated by 10-25%, and those produced by ouabain decreased by 50%. Glucose-substitution with equimolar amounts of pyruvate suppressed by two-thirds the pHi acid shifts induced by both high K+ and NMDA. Furthermore, lactate contents were significantly increased in hippocampal slices by exposure to high K+, veratridine or NMDA but not by ouabain. These results suggest that the intracellular acidification produced by these depolarizing agents, with the exception of ouabain, is mainly due to lactate accumulation which may occur as a result of accelerated glycolysis mediated by increased Na+-K+ ATPase activity. A Ca2+-dependent process may also contribute to the intracellular acidification induced by membrane depolarization. Since an increase in H+ concentration can attenuate neuronal activity, glycolytic acid production induced by membrane depolarization may contribute to the mechanism that prevents excessive neuronal excitation.

A comparative study on the effects of tetracyclines and lithium on the cyclic AMP second messenger system in rat brain

1. This study was aimed at investigating the effects of demeclocycline (DMC), minocycline (MC), and lithium (Li) in vitro on cyclic AMP (cAMP) accumulation in rat cerebral cortex stimulated by noradrenaline, forskolin, and ouabain. 2. DMC, MC, and Li dose-dependently reduced noradrenaline-stimulated cAMP formation in cortical slices, but only Li inhibited the cAMP formation induced by forskolin. 3. In contrast to Li, DMC and MC did not affect noradrenaline-stimulated adenylate cyclase activity in cortical membranes. 4. In cortical slices, ouabain stimulated the cAMP production (required the presence of extracellular Ca2+ and was blocked by verapamil). Ouabain-stimulated cAMP accumulation in cortical slices was inhibited by DMC, MC, and Li. 5. DMC and MC do not seem to interact directly with the adenylate cyclase as reported for Li. It is concluded that the tetracyclines, DMC and MC, affect the cAMP signaling system in rat brain by mechanisms that differ from that of Li. The decreased receptor agonist-stimulated cAMP production in cortical slices in the presence of DMC and MC may be due to the Ca(2+)-chelating ability of these tetracyclines.