Depolarization increases chloride-dependent glutamate sequestration in synaptic membranes of rat cerebral cortex

On the release of glutamate and aspartate in the basal ganglia of the rat: interactions with monoamines and neuropeptides

Using highly sensitive analytical procedures, glutamate (Glu), aspartate (Asp) and several putative neurotransmitters and metabolites can be monitored simultaneously in the extracellular space of neostriatum, substantia nigra and cerebral cortex of the rat by in vivo microdialysis. Glu and Asp are found at sub-micromolar concentrations in all investigated brain regions. In order to ascertain their neuronal origin, we have extensively studied the sensitivity of extracellular Glu and Asp levels to: (i) K(+)-depolarization, (ii) Na(+)-channel blockade, (iii) removal of extracellular Ca2+, (iv) depletion of presynaptic vesicles, and (v) integrity of neuronal pathways. The relevance of these criteria for several neurotransmitters monitored simultaneously or in parallel experiments has also been examined. The functional interactions among different neuronal pathways in the basal ganglia are studied by using selective pharmacological treatments, administered systemically, or locally via intracerebral injections or the microdialysis perfusion medium. Immunohistochemical evidence for the existence of Glu and/or Asp neuronal pathways in the basal ganglia of the rat is presented, discussing especially new findings indicating the existence of a Glu-independent Asp system, intrinsic to the neostriatum of the rat. The clinical relevance of these interactions is discussed, focusing on the implications for the treatment of neurodegenerative disorders affecting the basal ganglia.

Characterization of L-alpha-aminoadipic acid transport in cultured rat astrocytes

The mechanism of the selective gliotoxicity of L-alpha-aminoadipate (L-alpha AA) is thought to involve its entry into glia as a substrate for glutamate transporters or, alternatively, its ability to inhibit glial glutamate transport. To clarify the properties of L-alpha AA as a transport substrate, we explored the ionic dependence, kinetics and pharmacology of L-[3H] alpha AA uptake in rat cortical astrocytes. We observed two components of saturable L-alpha AA uptake, one Na(+)-dependent and the other Na(+)-independent. These components exhibited the characteristics of system X-AG, the widespread family of Na(+)-cotransporters of aspartate and glutamate, and system x-c, a Cl(-)-dependent glutamate/cystine exchanger, respectively. The K(m) value of Na(+)-dependent L-alpha AA uptake was 629 +/- 42 microM, and Vmax was 62 +/- 4 nmol.min-1.mg-1 protein, which was more than twice the capacity of Na(+)-dependent glutamate uptake. The kinetic parameters of Na(+)-dependent L-alpha AA uptake (K(m) of 20 +/- 2 microM, Vmax of 1.7 +/- 0.4 nmol.min-1.mg-1 protein did not differ from the values for Na(+)-independent glutamate uptake, indicating that L-alpha AA and glutamate are equally good substrates for system x-c.

Pretreatment of rat brain slices with ouabain decreases chloride-dependent L-glutamate transport in synaptic membrane

Possible roles of (Na(+)-K+)-ATPase on regulation of Cl(-)-dependent L-glutamate (L-Glu) transport in rat synaptic membrane were examined. Pretreatment of rat cerebral slices with 20 micrograms/ml veratrine, 56 mM K+ and 20 microM monensin increased Cl(-)-dependent L-[3H]-Glu uptake into membrane vesicles prepared from the slices. Pretreatment with (Na(+)-K+)-ATPase inhibitors, 100 microM ouabain, 100 microM strophanthidin, 100 microM vanadate and K(+)-free medium, decreased the uptake by 25-30%, while 5 microM A23187 and 10 microM ruthenium red had no effect. Ouabain (100 microM) caused the maximal effect in 10-20 min of incubation. The effect of (Na(+)-K+)-ATPase inhibitors was reversible and characterized by a decrease in Vmax of the uptake. Addition of 5 microM ouabain abolished the increases in the uptake induced by either veratrine, 56 mM K+ or monensin. Dose-inhibition curve of ouabain on the increased Cl(-)-dependent L-[3H]-Glu uptake was bi-phasic; ouabain at 0.5-5 microM selectively diminished the stimulatory effect of veratrine and inhibited the basal uptake at higher concentrations. Veratrine had no effect on L-[3H]-Glu uptake in 5 microM strophanthidin containing or K(+)-free medium. These results suggest the involvement of (Na(+)-K+)-ATPase in the veratrine-induced increase in Cl(-)-dependent L-Glu transport in rat brain synaptic membranes.

Presynaptic modulation of glutamate and dynorphin release by excitatory amino acids in the guinea-pig hippocampus

Excitatory amino acid agonists and antagonists were evaluated for their ability to affect the concomitant release of endogenous L-glutamate and dynorphin A(1-8)-like immunoreactivity from guinea-pig hippocampal mossy fiber synaptosomes. Previous work in this laboratory demonstrated that L(+)2-amino-4-phosphonobutyrate inhibits the potassium-evoked release of these endogenous neurotransmitters from guinea-pig but not rat hippocampal mossy fiber synaptosomes. Therefore, the present study was conducted to evaluate excitatory amino acid agonists as indices to the functional properties of this L(+)2-amino-4-phosphonobutyrate-sensitive glutamatergic autoreceptor on mossy fiber terminals. Low micromolar concentrations of quisqualate, but not kainate, N-methyl-D-aspartate, nor RS-alpha-amino-3-hydroxy-5-methyl-4-isoazole-propionic acid, significantly inhibited the potassium-evoked release of both L-glutamate and dynorphin A(1-8)-like immunoreactivity. Quisqualate-induced inhibition of L-glutamate release from mossy fiber terminals was antagonized by the non-N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. In contrast, high concentrations of kainate enhanced the potassium-evoked release of L-glutamate and dynorphin A(1-8)-like immunoreactivity, and this potentiation was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Kainate (1 mM) was the only agonist which significantly enhanced the basal release of L-glutamate, whereas the spontaneous efflux of dynorphin A(1-8)-like immunoreactivity was not affected by any of the agonists tested. The results presented in this paper suggest the existence of inhibitory and excitatory presynaptic glutamatergic autoreceptors that act to modulate the release of endogenous L-glutamate- and prodynorphin-derived peptides from guinea-pig hippocampal mossy fiber terminals. These inhibitory and excitatory autoreceptors, which are sensitive to quisqualate/L(+)2-amino-4-phosphonobutyrate or kainate, respectively, may play an important role in regulating synaptic activity at glutamatergic synapses throughout the central nervous system.

Characteristics of Cl(-)-dependent L-[35S]cysteic acid transport into rat brain synaptic membrane vesicles

Uptake of L-[35S]cysteic acid (L-CA) in rat synaptic membrane vesicles was investigated. Preincubation with either 10 mM L-glutamic acid (L-Glu), 25 mM L-CA, 10 mM DL-homocysteic acid, or 25 mM DL-2-amino-4-phosphonobutyrate on membrane vesicles enhanced L-[35S]CA and L-[3H]Glu uptake. Na+ (5 mM) and omission of Cl- from the assay medium decreased L-[35S]CA uptake into both 10 mM L-Glu-loaded and non-loaded membrane vesicles. The anion transport blockers, 4-acetamide-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS) and 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS), inhibited L-[35S]CA uptake in a dose-dependent manner. The maximal uptake rate for L-[35S]CA was decreased by 50 microM SITS, while the apparent Km value of L-CA was not changed. SITS increased the EC50 value of Cl- for L-[35S]CA uptake from 5 mM to 10 mM with reduction of the maximal effect. These results suggested that L-[35S]CA uptake into synaptic membrane vesicles was mediated by a SITS-sensitive hetero-exchange transport with non-labeled substrates.

L-[35S]cysteic acid selectively detects chloride-dependent L-glutamate transporters in synaptic membrane

Na+-independent L-[35S]cysteic acid (CA) accumulation in rat cortical synaptic membrane was examined. In the absence of Cl-, the accumulation was not observed. Addition of Cl- revealed the accumulation in a dose-dependent manner. Br- and NO3- also did. Ca2+-enhanced the Cl- -dependent accumulation, whereas low concentrations of Na+ reduced it. L-[35S]CA accumulation was inhibited by quisqualate, L-glutamate (L-Glu), L-cysteine sulfinate, D,L-homocysteic acid and D,L-2-amino-4-phosphonobutyrate (D,L-APB) potently. L-CA inhibited L-[3H]Glu accumulation in synaptic membrane in the presence of Cl-. The maximal inhibition of L-CA was equal to that of D,L-APB, but L-CA did not inhibit L-[3H]Glu accumulation in the absence of Cl-. These results show that L-[35S]CA selectively detects the Cl- -dependent L-Glu transporters in synaptic membrane.