The effect of [Ca2+] and [H+] on the functional recovery of rat brain synaptosomes from anoxic insult in vitro

Effects of external pH variations on brain presynaptic sodium and calcium channels; repercussion on the evoked release of amino acid neurotransmitters

The effects of external pH (pHout) variations on the Na+ and on the Ca2+ dependent fractions of the evoked amino acid neurotransmitter release were separately investigated, using GABA as a model transmitter. In [3H]GABA loaded mouse brain synaptosomes, the external acidification (pHout 6.0) markedly decreased the Na+ dependent fraction of [3H]GABA release evoked by veratridine (10 microM) in the absence of external Ca2+, as well as the Ca2+ dependent fraction of [3H]GABA release evoked by high (20 mM) K+ in the absence of external Na+. The depolarization-induced elevation of [Na(i)] (monitored in synaptosomes loaded with the Na+ indicator dye, SBFI) and the depolarization-induced elevation of [Ca(i)] (monitored in synaptosomes loaded with the Ca2+ indicator dye fura-2) were also markedly decreased at pHout 6. On the contrary, the external alkalinization (pHout 8) facilitated all the above responses. A slight increase of the baseline release of the [3H]GABA was observed when pHout was changed from 7.4 to 8. This effect was only observed in the presence of Ca2+. pHout changes from 7.4 to 6 or to 7 did not modify the baseline release of the transmitter. All the effects of pHout variations on [3H]GABA release were independent on the presence of HCO3-. It is concluded that external H+ regulate amino acid neurotransmitter release by their actions on presynaptic Na+ channels, as well as on presynaptic Ca2+ channels.

Relations between intracellular ions and energy metabolism under acidotic conditions: a study with nigericin in synaptosomes, neurons, and C6 glioma cells

Effects of nigericin were investigated in rat brain synaptosomes, cultured neurons, and C6 glioma cells to characterize the relations among ATP synthesis, [Na+]i, [K+]i, and [Ca2+]i, and pH under conditions when [H+]i is substantially increased and transmembrane electrical potential is decreased. Intracellular acidification and loss of K+ were accompanied by enhanced oxygen consumption and lactate production and a decrease in cellular energy level. Changes in the last three parameters were attenuated by addition of 1 mM ouabain. In synaptosomes treated with nigericin, neither respiration nor glycolysis was affected by 0.3 microM tetrodotoxin, whereas 1 mM amiloride reduced lactate production by 20% but did not influence respiration. In C6 cells, amiloride decreased the nigericin-stimulated rate of lactate generation by about 50%. The enhancement by nigericin of synaptosomal oxygen uptake and glycolytic rate decreased with time. However, there was only a small reduction in respiration and none in glycolysis in C6 cells. Measurements with ion-selective microelectrodes in neurons and C6 cells showed that nigericin also caused a rise in [Ca2+]i and [Na+]i. The increase in [Na+]i in C6 cells was partially reversed by 1 mM amiloride. It is concluded that nigericin-induced loss of K+ and subsequent depolarization lead to an increase in Na+ influx and stimulation of the Na+/K+ pump with a consequent rise in energy utilization; that acidosis inhibits mitochondrial ATP production; that a rise in [H+] does not decrease glycolytic rate when the energy state (a fall in [ATP] and rises in [ADP] and [AMP]) is simultaneously reduced; that a fall in [K+]i depresses both oxidative phosphorylation and glycolysis; and that the nigericin-induced alterations in ion levels and activities of energy-producing pathways can explain some of the deleterious effects of ischemia and hypoxia.

Anaerobic glycolysis and postanoxic recovery of respiration of rat cortical synaptosomes are reduced by synaptosomal sodium load

Synaptosomes of rat cerebral cortex were used to study the effect of veratridine-induced Na+ load on postanoxic recovery of respiration and on aerobic and anaerobic ATP turnover, calculated from rates of oxygen consumption and lactate production. Non-stimulated synaptosomes: after onset of anoxia lactate synthesis of synaptosomes rose immediately from 0.8 to 17.7 nmol lactate/min/mg protein indicating an anaerobic ATP turnover of 17.7 nmol ATP/min/mg protein. This value accounts for 80% of ATP synthesized during oxygenated conditions and seems to cover the energetic demand of anoxic synaptosomes. This assumption was supported by linearity of lactate production throughout anoxia (90 min), by unaffected synaptosomal integrity and by complete recovery of postanoxic respiration after 90 min of anoxia. Stimulated synaptosomes: stimulation of oxygenated synaptosomes with 10(-5) mol/l veratridine enhanced ATP turnover 5-fold, due to activation of Na+/K+ ATPase, as a result of veratridine-induced Na+ influx. Consequently, if not limited in capacity, anaerobic ATP synthesis should be enhanced after addition of veratridine during anoxia. However, the opposite effect was observed. Veratridine reduced anaerobic glycolysis in a concentration-dependent manner. This inhibitory effect could be prevented by tetrodotoxin applied 5 min prior to veratridine. Inhibition of anaerobic glycolysis was independent of extrasynaptosomal glucose (1-30 mmol/l) and Ca2+ concentration (Ca(2+)-free and 1.2 mmol/l Ca2+). Veratridine stimulation of anoxic synaptosomes reduced also the recovery of postanoxic respiration. The data indicate that Na+ load inhibits anaerobic ATP synthesis, the only energy source during anaerobic conditions. To our knowledge, inhibition of anaerobic glycolysis due to increased Na+ influx has not been shown so far.

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.

Evidence for the regulatory function of synaptoplasmic acetyl-CoA in acetylcholine synthesis in nerve endings

Isolated synaptosomes maintained a relatively stable level of acetyl-CoA during their incubation in the presence of 30 mM-KCl. Addition of Ca2+ resulted in inhibition of pyruvate oxidation and slight activation of acetylcholine synthesis. The cation decreased acetyl-CoA in intrasynaptosomal mitochondria, but did not alter its content in synaptoplasm. Verapamil did not affect pyruvate oxidation, but decreased acetyl-CoA in synaptoplasm and inhibited acetylcholine synthesis in synaptosomes. It indicates that Ca2+ might regulate acetylcholine synthesis through changes in the direct transfer of acetyl-CoA from mitochondria to synaptoplasm.

Influence of the malate-aspartate shuttle on oxidative metabolism in synaptosomes

beta-Methyleneaspartate, a specific inhibitor of aspartate aminotransferase (EC 2.6.1.1.), was used to investigate the role of the malate-aspartate shuttle in rat brain synaptosomes. Incubation of rat brain cytosol, "free" mitochondria, synaptosol, and synaptic mitochondria, with 2 mM beta-methyleneaspartate resulted in inhibition of aspartate aminotransferase by 69%, 67%, 49%, and 76%, respectively. The reconstituted malate-aspartate shuttle of "free" brain mitochondria was inhibited by a similar degree (53%). As a consequence of the inhibition of the aspartate aminotransferase, and hence the malate-aspartate shuttle, the following changes were observed in synaptosomes: decreased glucose oxidation via the pyruvate dehydrogenase reaction and the tricarboxylic acid cycle; decreased acetylcholine synthesis; and an increase in the cytosolic redox state, as measured by the lactate/pyruvate ratio. The main reason for these changes can be attributed to decreased carbon flow through the tricarboxylic acid cycle (i.e., decreased formation of oxaloacetate), rather than as a direct consequence of changes in the NAD+/NADH ratio. Malate/glutamate oxidation in "free" mitochondria was also decreased in the presence of 2 mM beta-methyleneaspartate. This is probably a result of decreased glutamate transport into mitochondria as a result of low levels of aspartate, which are needed for the exchange with glutamate by the energy-dependent glutamate-aspartate translocator.

Selective damage in striatum and hippocampus with in vitro anoxia

An in vitro model of anoxia-induced brain damage was utilized to help elucidate the biochemical basis of cell damage due to reduced oxygen availability. Previous studies suggest that anoxia-induced damage may vary presynaptically, post-synaptically or in the cell body. Thus, the consequences of an anoxic treatment incubation were examined with hippocampal slices, which contain cholinergic nerve terminals but not cell bodies, and with slices from whole striatum or its subregions, which contain both cholinergic cell bodies and nerve terminals. Slices were preincubated with either oxygen or nitrogen (treatment incubation) and the persistent effects of this treatment on [14C]acetylcholine and 14CO2 production from [U-14C]glucose were assessed in a subsequent incubation under optimal conditions (test incubation). An anoxic treatment incubation reduced the subsequent test incubation production of CO2 about 40% in the hippocampus and striatum. The anoxic treatment incubation diminished ACh production by 46% in the striatum, but only minimally affected that in the hippocampus. Anoxic treatment incubations of synaptosomes did not alter test-incubation ACh synthesis or CO2 production. Omission of calcium from the anoxic treatment incubation increased striatal ACh synthesis by 88% and CO2 production in both regions. These results suggest that anoxia produces persistent changes in postsynaptic processes or cell bodies (in this model cholinergic ones) that differ from those in nerve terminals and that calcium is important in the production of these deficits.

An in vitro model of anoxic-induced damage in mouse brain

An in vitro model of anoxic-induced brain damage was developed to help elucidate the biochemical basis of cell damage due to reduced oxygen availability. Mouse forebrain slices were preincubated under various conditions (treatment incubation). The effects of this treatment incubation on [14C]acetylcholine (ACh) and 14CO2 production from [U-14C]glucose were subsequently assessed in an incubation under optimal conditions (test incubation). A variety of treatment incubation conditions decreased 14CO2 and 14C-ACh production in the test incubation in parallel (r = 0.932). For example, treatment incubations with no oxygen and high K+ reduced test incubation ACh (-63.2%) and CO2 (-67.3%) production. An anoxic-induced increase in calcium-45 uptake and the amelioration of anoxic induced changes by the calcium antagonist verapamil or by the omission of calcium from the treatment incubation suggest that altered calcium homeostasis was important in the production of the anoxic-induced deficits. These results provide in vitro evidence that anoxic induced increases in calcium may be pathophysiologically important and that reducing calcium entry postsynaptically may alleviate anoxic-induced changes. This model may prove useful in elucidating the molecular basis of these changes.

Divalent cation modulation of the ionic permeability of the synaptosomal plasma membrane

Synaptosomes from guinea-pig cerebral cortex reveal two distinct Na+ permeabilities when divalent cations are removed from the incubation. In the presence of Mg2+, Ca2+ chelation by EGTA causes a partial activation of a voltage-dependent tetrodotoxin-sensitive pathway, manifested as a ouabain-sensitive respiratory increase, a partial depolarization of the plasma membrane, and a lowered gradient of gamma-amino[14C]butyrate. In addition there is a hyperpolarization of the mitochondrial membrane potential. When Mg2+ is omitted from the incubation, Ca2+ chelation induces a substantially larger permeability which is only partially sensitive to tetrodotoxin. The tetrodotoxin-insensitive component is not associated with a non-specific permeabilization of the plasma membrane and may be reversed by either Mg2+ or Ca2+.

Compartmentation and regulation of acetylcholine synthesis at the synapse

Acetylcholine and choline release was measured by using an automated and modified version of the chemiluminescence technique of Israel & Lesbats [(1981) Neurochem. Int. 3, 81-90]. A comparison of acetylcholine and choline release from synaptosomes demonstrated that acetylcholine release was K+-stimulated and inhibited by the Ca2+ ionophore A23187 and cyanide. Choline release, however, did not vary markedly under different conditions, suggesting that it is not associated with acetylcholine release at the nerve ending. Total acetylcholine synthesis in synaptosomal preparations was measured concurrently with the incorporation of [14C]acetyl and [3H]choline moieties by using the chemiluminescence method. Under sub-optimal glucose concentrations or in the absence of treatment of the synaptosomes with the acetylcholinesterase inhibitor phospholine, the incorporation of radioactivity exceeded total synthesis, indicating that cycling between acetylcholine and its precursors may occur. After treatment with phospholine, acetyl-group incorporation from D-[U-14C]glucose occurred without dilution of the precursor at optimal (1.0 mM) and low (0.1 mM) glucose concentrations; however, at very low (0.01 mM) glucose concentrations, dilution by a small endogenous pool occurred. [14C]Acetyl incorporation into acetylcholine was compared with various metabolic parameters. A closer correlation was observed between [14C]acetyl-group incorporation into acetylcholine and the calculated acetyl-carrier efflux from the mitochondria than with the calculated pyruvate-dehydrogenase-complex flux. The results are discussed with respect to the regulation of acetylcholine concentrations at the synapse and the mechanism whereby turnover occurs.

Subsynaptosomal calcium distribution during hypoxia and 3,4-diaminopyridine treatment

Previous results demonstrate that hypoxia (low oxygen) diminishes calcium uptake by synaptosomes. The present studies examined the effects of low oxygen on calcium homeostasis in the digitonin-resistant (mitochondrial) and the digitonin-labile (nonmitochondrial) compartments of intact synaptosomes and their relation to altered membrane potentials. A 10-min hypoxic incubation in low-potassium media reduced total (-38.3%), mitochondrial (-43.3%), and nonmitochondrial (-27.8%) calcium uptake. In high-potassium media, low oxygen reduced mitochondrial (-41.2%) and total (-34.4%) uptake whereas nonmitochondrial (+ 6%) calcium uptake was essentially unaffected. A temporal analysis of nonmitochondrial calcium uptake revealed an initial depression (0-5 min) followed by a stimulation (5-10 min). Hypoxic-induced alterations in the subsynaptosomal distribution of calcium resembled those produced by uncouplers [FCCP (carbonylcyanide-p-trifluoromethoxyphenylhydrazone) or rotenone plus oligomycin]. 3,4-Diaminopyridine partially ameliorated the hypoxic- and FCCP-induced decreases in synaptosomal calcium uptake. Low oxygen reduced the total synaptosomal membrane potential (i.e., plasma plus mitochondrial membrane potential) as measured by an increased efflux of tetraphenylphosphonium ion. This hypoxic-induced efflux of tetraphenylphosphonium was slowed by pretreatment with 3,4-diaminopyridine. Thus, both drug and membrane potential studies suggest that hypoxic-induced alterations in the subcellular distribution of calcium may be due to an uncoupling mechanism and a collapse of the synaptosomal mitochondrial membrane potential.

Neurotransmitter amino acids in the CNS. II. Some changes in amino acid levels in rat brain synaptosomes during and after in vitro anoxia and simulated ischemia

The effects of in vitro anoxia and membrane depolarization by veratridine on the uptake and release of amino acids were investigated in suspensions of synaptosomes isolated from the forebrains of rats. It was observed that GABA, aspartate and glutamate were released from synaptosomes in anaerobic conditions and upon addition of veratridine in a time-dependent manner. The release of the two latter amino acids was faster and more pronounced than that of GABA. The other amino acids were not affected in any systematic way by either condition. Re-introduction of oxygen or addition of tetrodotoxin to veratridine-treated synaptosomes resulted in the re-uptake of GABA, aspartate and glutamate, which was much faster and more complete for GABA than for the acidic amino acids, especially at acid pH values. The amounts of aspartate and glutamate in the incubation mixture remained constant during all the manipulations whereas that of GABA increased by about 30% during anaerobiosis, in agreement with the results obtained during in vivo ischemia. It is postulated that synaptosomes which utilize glutamate and aspartate as neurotransmitters are more damaged by anoxia and depolarization with veratridine than the population which utilizes GABA. These observations may explain reports that those neurons which are thought to receive major glutamatergic input are particularly sensitive to the lack of oxygen.