Neural Coding: The enigma of the brain

Information may be coded in neuronal firing patterns in other ways than the instantaneous action-potential frequency; but proving that the brain uses a particular alternative code will not be easy.     

Metabolism of oleoyl-CoA in rat brain synaptosomes

The hydrolysis of acyl-CoA by acyl-CoA hydrolase (EC 3.1.2.2.) in brain synaptosomes was inhibited by calcium. This inhibition was partly due to interaction of Ca²⁺ with the acyl-CoA, which was present in the soluble form, and partly due to complex formation among acyl-CoA, Ca²⁺ and membrane phospholipids. The inhibition of acyl-CoA hydrolase activity, as well as the complex formation. could be reversed if incubation was carried out in the presence of Ca²⁺ chelating agents. Synaptosomes isolated from brain samples after 1 min of postdecapitative treatment showed a decrease in oleoyl-CoA hydrolase activity. The physiological implication of acyl-CoA metabolism in relation to synaptic function is discussed.Abbreviations FFA Free fatty acids - GPC glycerophosphocholines - GPE glycerophosphoethanolamines - GPI glycerophosphoinositols - GPS glycerophosphoserines     

The effect of juglone on cholinergic mechanisms in rat brain synaptosomes

Juglone 5-hydroxy-1,4-napthoquinone exerts three activities on cortical synaptosomal preparations. It inhibits the release of acetylcholine and is an even more potent inhibitor of high-affinity choline transport into synaptosomes. In addition, as has already been shown by others using brain homogenates, juglone inhibits choline acetyltransferase.     

Histochemical localization of acetylcholinesterase in isolated brain synaptosomes

Summary The distribution of acetylcholinesterase in isolated brain synaptosomes of the rat and calf was studied with electron microscopic histochemistry. The enzyme was observed mainly in the membrane of the postsynaptic neurone and to a much lesser extent in the axon membrane.     

Regulation of catecholamine synthesis in rat brain synaptosomes

Abstract— Catecholamine synthesis in synaptosomal preparations of rat striatum, cortex and brain stem was investigated. The striatum had much higher activity than either the cortex or brain stem. Equilibration of labelled tyrosine between tissue and incubation medium was completed within 2 min. The apparent K_m of tyrosine hydroxylase (EC 1.14.3a) and of the overall catecholamine synthetic pathway were both approximately 5 ± 10^?6m for tyrosine. The following amines were found to inhibit striatal dopamine synthesis: dopamine, 25% inhibition at 5 ± 10^?7m ; noradrenaline, 25% inhibition at 5 ± 10^?6m ;and serotonin, 30% inhibition at 10^?5m . The catecholamine-induced inhibition of synthesis was antagonized by pre-incubation with cocaine. Increasing the potassium concentration from 5 to 55 mm caused a release of amines into the medium which was accompanied by a 40% increase in dopamine synthesis, when synthesis was measured during the first 5 min of exposure to elevated potassium. These results indicate that synaptosomal catecholamine synthesis is inhibited by increases in intra-synaptosomal amine levels, and that short-term exposure to depolarizing concentrations of potassium can increase synthesis.     

The effect of thiol reagents on GABA transport in rat brain synaptosomes

The nature of gamma-aminobutyric acid (GABA) transport has been investigated in preparations of rat brain synaptosomes using a number of thiol reagents with varying membrane permeabilities. N-Ethylmaleimide, p-chloromercuribenzoate and p-chloromercuriphenylsulfonate effectively inhibited GABA transport in both directions (i.e., uptake and release) whereas 5,5'-dithiobis-2-nitrobenzoate, mercaptopropionate and N- nitroethylenediamine were much less effective, or ineffective, even at millimolar concentrations. For each of the thiol reagents, the inhibition profile for GABA uptake was approximately the same as that for its release. The effectiveness of the reagents indicates that there is an external, reactable SH-group on the transporter, that the thiol reagent must be somewhat lipophilic for it to react with the SH-group(s), and that the same synaptosomal transport system is responsible for both uptake and release of GABA.     

Interaction of thiamine with rat brain synaptosomes

Thiamine has been shown to be bound specifically by a synaptosomal plasmatic membrane and transported inside to the nervous ending. Apparent K[symbol: see text] and Km for processes of binding and transport have been determined as equal 2.34 +/- 0.55 MKM and 3.92 +/- 1.3 MKM, respectively. The thiamine uptake by the isolated nervous endings (synaptosomes) at its physiological concentration is reduced in presence of metabolic inhibitors and partially depends on Mg2+ and Ca2+ ions, that can testify about the interrelation between endogenic thiamine phosphorilation and its transport through the membrane. Thiamine binding with synaptosomes is inhibited by ouabain and neurotoxins such as, latrotoxin and most significantly--with veratridin; tetrodotoxin fail to be efficient practically. In the conditions of synaptic membranes depolarisation their ability to bind thiamine is reduced and output of already uptaken with synaptosomes thiamine is observed.     

The amoeba enigma

After more than 70 years of intermittent debate over the true relationship between the 'pathogenic' and 'non-pathogenic' forms of Entamoeba histolytica, the application of molecular biology has finally yielded an unambiguous answer: these are not interconvertible phenotypes of the same parasite, a kind of unicellular Jekyll and Hyde, but two quite distinct genetic entities that just happen to look the same. But given the overwhelming evidence now available from gene sequences, pointing to an evolutionary divergence some tens of millions of years ago, why is it that certain eminent workers in the field are still claiming that, at least in vitro, conversion between the two phenotypes can take place? In this article Bill Spice and John Ackers review recent developments in the molecular biology of E. histolytica and assess the continuing controversy over the status of this enigmatic parasite.     

Adenosinetriphosphatase and nucleotide metabolism in synaptosomes of rat brain

—In the presence of synaptosomes prepared from rat brain, only ATP, dATP and ADP but not dADP were active as substrates of phosphatase (ATP phosphohydrolase; EC 3.6.1 4) in the presence of 150mm-Na⁺ and 20mm-K⁺. An active adenylate kinase (ATP:AMP phosphotransferase; EC 2.7.4.3.) was demonstrated in the synaptosomal fractions by means of paper chromatography, paper electrophoresis and enzymic reactions, so that the high activity with ADP as substrate could represent an activity of an ATPase. Apparently dADP was not a substrate for the kinase; no dATP was formed when dADP was incubated with the synaptosomal fraction in the presence of Na⁺, K⁺ and Mg²⁺. Small amounts of P₁ were liberated with dADP, IDP, GDP or CDP, but not UDP, as substrates, but none was produced in the presence of mononucleotides. The adenine-deoxyribose bond, but not the adenine-ribose bond, was hydrolysed upon the addition of 5% (w/v) TCA to the reaction mixture. The K_M for the hydrolysis of ATP but not ITP, in the presence of Mg²⁺, or of Na⁺, K⁺ and Mg²⁺, was lower for the synaptosomal ATPase than for the microsomal ATPase, and the values for V_max for synaptosomal ATPase were higher. The activation increment was generally higher for the synaptosomal ATPase and no distinct differences in the properties of the enzyme from either particulate fractions were observed. Mg²⁺ could be partially replaced by Mn²⁺ in the synaptosomal ATPase system, but there was little Na⁺-K⁺-activation observed in the presence of the latter. The effects of ouabain and of homogenization under various conditions suggested localization of the K⁺-sensitive site of the ATPase on the surface of the synaptosomal membrane. Activity of the Na⁺-K⁺-Mg²⁺ ATPase increased after freezing and thawing of the sonicated, sucrose or tris-treated preparations but decreased considerably in the synaptosomes treated with 001 m-deoxycholate. Activity of the Mg²⁺ ATPase in the latter preparation showed little change.     

Synthesis of glutamate and aspartate in rat brain synaptosomes

ATP and glutamine are the sources of endogenous ammonia in rat brain synaptosomes. The amount of endogenous ammonia formed from exogenous ATP is not sufficient to assure the maximum rate of aspartate and glutamate accumulation in the synaptosomes utilizing pyruvate + malate. Addition of exogenous NH4+ or depolarization of synaptosome plasma membranes with high K+ concentration led to a twofold increase in the rate of accumulation of these amino acids. This indicates that both exogenous and endogenous NH4+ is involved in the synthesis of aspartate and glutamate in nerve terminals. Accumulation of glutamate was stimulated by aminooxyacetate and inhibited by haloperidol which indicates that NH4+ is bound in the reaction catalysed by glutamate dehydrogenase. Endogenous oxaloacetate derived from pyruvate metabolism was the substrate for synthesis of aspartate. Additive inhibition of aspartate accumulation by fluorocitrate and (-) hydroxyacetate shows that, in addition to the tricarboxylic acid cycle, the reaction catalysed by ATP-citrate lyase serves in the synaptosomes as another source of oxaloacetate.     

Inactivation of depolarization-induced calcium uptake in rat brain synaptosomes

The inactivation of depolarization-induced Ca uptake into rat brain synaptosomes was demonstrated biochemically by comparing⁴⁵Ca fluxes after various intervals of predepolarization achieved by abruptly increasing {K⁺}₀. The chemical composition of the medium was maintained throughout the predepolarization and Ca uptake steps. Under these conditions, inactivation was dependent on depolarization, i.e., basal unstimulated Ca uptake in the presence of 5 mM {K⁺}₀ did not inactivate. Inactivation of stimulated Ca uptake was dependent on the predepolarization interval, moderately dependent on {Ca}₀ and relatively independent of membrane potential, i.e., {K⁺}₀ and ions such as Ni²⁺ and Co²⁺ that blocked Ca uptake. Both cinnarizine and lidoflazine blocked stimulated Ca uptake in a concentration-dependent manner without affecting the % inactivation. Although the amount of stimulated uptake increased greatly between 10 and 30°C, the % inactivation was unaffected by temperature. These findings suggest that inactivation of the presynaptic Ca uptake is an intrinsic property of the channel independent of calcium uptake.     

Developmental regulation of phospholipid methylation in rat brain synaptosomes

Phospholipid methylation was studied in cortical synaptosomes prepared from 7 and 14 day and adult rat brain. Using varying concentrations of [³H] S-adenosylmethionine, Km and Vmax values were determined for the formation of [³H] phosphatidylmonomethylethanolamine (PME), [³H] phosphatidyl-dimethylethanolamine and [³H] phosphatidylcholine (PC). At 25°C, the Km values for the formation of all three products, significantly decreased with development. Increasing the temperature to 37°C increased the Km values in the 14 day and adult but not the 7 day preparation. The Vmax values at 25°C were highest at 7 and 14 days, depending on the product and then decreased in the adult. At 37°C, the Vmax values were highest in the 14 day preparation. The overall results are discussed in terms of the developmentally regulated decrease in both synaptic membrane PC and membrane fluidity.