Release of glutamate, aspartate, and gamma-aminobutyric acid from isolated nerve terminals

Regulation of the gamma-aminobutyric acidA receptor by gamma-aminobutyric acid levels within the postsynaptic cell

Synaptosomes and synaptoneurosomes were prepared from rat cerebral cortex. Comparison of the amino acid levels in the two types of organelles and of the effects of gabaculine thereon indicated that the neurosome portion of synaptoneurosomes constituted the major influencing component of the organelles. Administration to rats of inhibitors of gamma-aminobutyric acid (GABA) degradation, such as gabaculine and L-cycloserine, resulted in elevated GABA levels in synaptoneurosomes and a decrease in muscimol-stimulated Cl- up-take by the organelles. Addition of gabaculine directly to the incubation medium for the uptake assay had no effect on the Cl- transport. In contrast, administration to rats of isonicotinic acid hydrazide, an inhibitor of GABA synthesis, decreased the GABA level in synaptoneurosomes and increased the muscimol-stimulated Cl- uptake by the organelles. Although the evidence is not unequivocal, it does support the concept of GABA released from nerve endings being taken up by the postsynaptic cell, from where it exerts a regulatory influence on the functioning of the GABA receptor/ion channel complex.

Ischemic damage in hippocampal CA1 is dependent on glutamate release and intact innervation from CA3

The removal of glutamatergic afferents to CA1 by destruction of the CA3 region is known to protect CA1 pyramidal cells against 10 min of transient global ischemia. To investigate further the pathogenetic significance of glutamate, we measured the release of glutamate in intact and CA3-lesioned CA1 hippocampal tissue. In intact CA1 hippocampal tissue, glutamate increased sixfold during ischemia; in the CA3-lesioned CA1 region, however, glutamate only increased 1.4-fold during ischemia. To assess the neurotoxic potential of the ischemia-induced release of glutamate, we injected the same concentration of glutamate into the CA1 region as is released during ischemia in normal, CA3-lesioned, and ischemic CA1 tissue. We found that this particular concentration of glutamate was sufficient to destroy CA1 pyramids in the vicinity of the injection site in intact and CA3-lesioned CA1 tissue when administered during control (non-ischemic) conditions. In contrast, the same amount injected during ischemia in the CA3-lesioned CA1 region destroyed pyramidal cells in a widely distributed zone around the injection site in the CA1 region. It is concluded that the ischemia-induced damage of pyramidal cells in CA1 is dependent on glutamate release and intact innervation from CA3.

Interrelationships between glucose metabolism, energy state, and the cytosolic free calcium concentration in cortical synaptosomes from the guinea pig

The stoichiometries of glycolysis and pyruvate oxidation were determined in cortical synaptosomes under varying rates of ATP consumption. Glycolysis was measured by using D-3-[3H]glucose as a marker and pyruvate oxidation by using D-3,4-[14C]glucose, which has to be metabolized to 1-[14C]pyruvate before being decarboxylated by the pyruvate dehydrogenase complex of intrasynaptosomal mitochondria. Cytosolic free Ca2+ concentration [( Ca2+]c) was determined in parallel and was manipulated by using EGTA in the incubation. The results show that in nonstimulated synaptosomes glycolysis and pyruvate oxidation are tightly coupled and stoichiometric. In the absence of Ca2+, when [Ca2+]c drops from 260 nM to 40 nM, glucose utilization increases, following the increase in energy demand, which has been shown to be due to elevated Na+ cycling. KCl depolarization, veratridine, and a mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, all stimulate glycolysis and pyruvate oxidation stoichiometrically, independently of the presence of external Ca2+. A rise in [Ca2+]c, therefore, is not required to regulate mitochondrial pyruvate metabolism. It is concluded that synaptosomes exhibit a high degree of respiratory control, that they rely on glucose oxidation for their energetics, and that stimulation of energy production can be achieved independently of changes in [Ca2+]c.