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Glutamate receptor-driven gene expression in learning

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The role of protein biosynthesis and ene expression in learning has been well documented. Similarly, the role of activation of glutamate receptors in neuronal plasticity have been shown repeatedly. In our studies we found that L-glutamate, acting through different kinds of its receptors may stimulate expression of c-fos and other genes encoding components of transcroption factors both in vivo and in vitro. We have also documented elevated expression of c-fos after induction of long lasting long term potentiation and variaous forms of behavioral training. In this paper these data are reviewed and a hypothesis, suggesting that neuronal nuclei may act as information integration device in memory formation is proposed.
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Ischemic neuronal injury is supposed to be caused in part by the extracellular accumulation of excitatory amino acids (EAA). Neurotransmitter and metabolic EAA can be released from synaptic vesicles and cytoplasm of neurones and glial cells. In this study the release of the glutamate analogue [3H]D-aspartate (3[H]D-ASP), loaded into 500 mugm slices of rat hippocampus, was investigated. The efflux of the label was measured during anoxic - aglycemic ("ischemic") and normoxic K+ depolarization. To identify the pools from which [3H]D-ASP is released we have estimated its calcium dependence and the effects of inhibitors of: (1) Na+ - dependent transporter of amino acids (100 mugM L-trans-pyrrolidine-2,4-dicarboxylic acid /L-trans-PDC/), (2) sodium channel (1 mugM tetrodotoxin TTX), and (3) anion channel (1mM furosemide). [3H]D-ASP released upon normoxic depolarization was 40% inhibited by TTX,nearly 40% by L-trans-PDC and over 50% by furosemide. The "ischemic" release was in 40% calcium dependent, completely TTX independent and in approximately 50% blocked by furosemide treatment. Our data suggest that EAA accumulated in the synaptic cleft during ischemia are mainly released from the cytosolic compartment by mechanisms wich are connected with the ischemic increase of extracellular potassium concentration.
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