Kainate-induced uptake of calcium by synaptosomes from rat brain

A high-affinity presynaptic kainate-type glutamate receptor facilitates glutamate exocytosis from cerebral cortex nerve terminals (synaptosomes)

Ionotropic glutamate receptor agonists, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate, all facilitated 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes). The non-selective, non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked kainate facilitation of glutamate release. AMPA responses were non-desensitizing and insensitive to the AMPA receptor desensitization inhibitor, cyclothiazide. The AMPA receptor antagonist GYKI 52466 failed to block ionotropic glutamate receptor-mediated facilitation, but the ionotropic glutamate receptor 6 kainate receptor subunit antagonist NS-102 was a potent blocker. Furthermore, kainate and AMPA responses were not additive. Taken together, our results indicate that, in the cerebral cortex, both kainate and AMPA may be facilitating glutamate release through the activation of a high-affinity kainate receptor containing glutamate receptor 6 kainate subunits. Kainate enhanced 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential, indicating that a ligand-gated ion channel that conducts cations may underlie the mechanism by which kainate mediates facilitation of glutamate release. While the facilitatory effect of kainate on glutamate release is consistent with a classical ionotropic action of ionotropic glutamate receptors, our observation that kainate inhibits GABA release suggests that alternative presynaptic mechanisms may operate in cerebrocortical nerve terminals to mediate the ionotropic glutamate receptor modulation of glutamate and GABA release. We conclude that high-affinity kainate-type glutamate autoreceptors represent a positive feed-forward system for potentiating the release of glutamate from cerebrocortical nerve terminals.

Modulation of catecholamine metabolism in synaptosomes by a neuroregulatory factor from mammalian brain

Incubation of synaptosomes from rat brain with bovine brain extract caused inhibition of oxidative deamination of dopamine, decreased formation of 3,4-dihydroxyphenylacetic acid, increased formation of norepinephrine and its N-methyl derivatives and increased release of catecholamines. Omission of Ca2+ from the extrasynaptosomal medium completely blocked the brain extract induced release of [3H]catecholamines and decreased, by about 80%, the effect on changes in catecholamine metabolism. Chromatography of the brain extract on Sephadex-G25 columns resulted in an active compound eluting at the position expected for compounds with molecular weights in the region of 1500 to 2500 Da.

Stereotaxic injection of kainic acid into the striatum of rats induces synthesis of mRNA for heat shock protein 70

Stereotaxic injection of the excitotoxin kainic acid into the striatum of rats has been shown to induce the production of messenger RNA to the 70 kDa heat shock protein. This was evident 2 h after injection and was maximal 24 h after injection of kainic acid. No change in the level of messenger RNA to beta-actin was detected.

Cysteine sulfinate modulated calcium permeability in synaptosomes from rat brain

Cysteine sulfinate (CSA) was shown to increase 45Ca2+ permeability of plasma membrane of synaptosomes isolated from rat brain. The 45Ca2+ enter through a system which has properties different from those of voltage dependent Ca2+ channels or Na+/Ca2+ exchange. The effect of CSA appeared to be mediated by highly specific receptors saturable by ligand. The high concentrations of CSA required (mM) suggest this amino acid is not the physiological agonist for the receptors.

Mechanism of kainate toxicity to cerebellar neurons in vitro is analogous to reperfusion tissue injury

The neuroexcitotoxin kainate has been used as a selective lesioning agent to model the etiology of a number of neurodegenerative disorders. Although excitotoxins cause susceptible neurons to undergo prolonged or repeated depolarization, the proximate metabolic pathology responsible for neuronal necrosis has remained elusive. We report here that kainate-induced death of cerebellar neurons in culture is prevented by inhibiting the enzyme xanthine oxidase, a cellular source of cytotoxic superoxide radicals (O2-.). Moreover, neurons are also protected from excitotoxin-induced death by the addition to the culture medium of either superoxide dismutase or mannitol, which scavenge superoxide and hydroxyl radicals, respectively, or serine protease inhibitor, which forestalls formation of xanthine oxidase. These findings indicate that excitotoxin-induced neuronal degeneration is mediated by superoxide radicals generated by xanthine oxidase, a mechanism partially analogous to that proposed for tissue damage seen upon reperfusion of ischemic tissues.

Calcium and the aging nervous system

Many aspects of calcium homeostasis change with aging. Numerous calcium compartments complicate studies of altered calcium regulation. However, age-related decreases in calcium permeation across membranes and mobilization from organelles may be a common fundamental change. Deficits in ion movements appear to lead to altered coupling of calcium-dependent biochemical and neurophysiological processes and may lead to pathological and behavioral changes. The calcium-associated changes during aging probably do not occur with equal intensity in all cell types or in different parts of the same cell. Thus, cells or compartments with a high proportion of calcium activated processes would be more sensitive to diminished calcium availability. These age-related changes may predispose the brain to the development of age-related neurological disorders. The effects of decreased ion movement may be further aggravated by an age-related decline in other calcium-dependent processes. Depression of some of these calcium-dependent functions appears physiologically significant, since increasing calcium availability ameliorates age-related deficits in neurotransmission and behavior. A better understanding of the interactions between calcium homeostasis and calcium-dependent processes during aging will likely help in the design of more effective therapeutic strategies.

Regulation of calcium uptake in synaptosomes from rat brain by DL-2-amino-5-phosphonovaleric acid

DL-2-Amino-5-phosphonovaleric acid stimulated calcium influx into synaptosomes isolated from rat brain. The increase in Ca2+ permeability of the synaptosomal membranes induced by this amino acid is not markedly dependent on the membrane potential or Na+ concentration. It is postulated that 2-amino-5-phosphonovaleric acid is an agonist for a receptor(s) which regulates intraneuronal free calcium concentrations by modulating a selective calcium channel. The observed stimulation of calcium uptake may provide an assay system for purification of the endogenous ligand for this receptor and for characterization of its physiological role in neuronal function.

Excitatory amino acid receptors and depolarization-induced Ca2+ influx into hippocampal slices

Hippocampal brain slices were incubated with depolarizing agents or excitatory amino acids either alone or in the presence of excitatory amino acid antagonists [omega-phosphonic alpha-aminocarboxylic acids--2-amino-4-phosphonobutyric acid (AP4), 2-amino-5-phosphonovaleric acid (AP5), or 2-amino-7-phosphonoheptanoic acid (AP7)--or gamma-D-glutamylaminomethylsulphonic acid (GAMS)] or a calcium-channel blocker, (S)-1-(3-methoxyphenyl)-3-methylaza-7-cyano-7-(3,4-dimethoxyphenyl )-8-methyl- nonane hydrochloride [(-)-D888]. The uptake of 45Ca2+ and the efflux of glutamate or aspartate induced by veratrine or high K+ was blocked (54-76%) by AP7 (IC50 46-250 microM). AP5 and AP4 were less effective. (-)-D888 (10 microM) caused 100% block of evoked 45Ca2+ uptake. Uptake of 45Ca2+ induced by exogenous glutamate, aspartate, and N-methyl-D-aspartate (NMDA) was also inhibited by AP7, whereas GAMS completely blocked the action of kainate and partially blocked that of glutamate. The action of NMDA in stimulating 45Ca2+ uptake was Mg2+-sensitive, low Mg2+ levels in the incubation medium selectively enhancing the response. It is concluded that Ca2+ uptake evoked by excitatory amino acids is receptor-mediated, and that released excitatory amino acids are responsible for a large part of the action of veratrine and high K+ in stimulating 45Ca2+ uptake.