221 96 96 3 3 R. Torp B. Arvin E. Le Peillet A. G. Chapman O. P. Ottersen B. S. Meldrum Department of Anatomy Institute of Basic Medical Sciences, University of Oslo Blindem, N-0317 P.O. Box 1105 Oslo Norway Department of Neurology Institute of Psychiatry De Crespigny Park SE5 8AF London UK Abstract The redistribution of neurotransmitter amino acids resulting from 20 min of ischaemia was studied in the rat hippocampus by quantitative, electron microscopic immunocytochemistry and by in vivo microdialysis. Changes in the distribution of glutamate, glutamine, aspartate and GABA in various cell compartments of CA1 were analysed immediately after ischaemia or after 60 min of reperfusion, by incubating ultrathin sections with antisera raised against protein glutaraldehyde conjugates of the respective amino acids and subsequently with a secondary antibody coupled to colloidal gold particles. Transverse microdialysis probes coupled with HPLC and implanted in the same animals were used to determine the extracellular concentration of amino acids in the left hippocampus and to apply a drug (BW 1003C87) believed to modify the extracellular release of amino acids induced by ischaemia. Forebrain ischaemia was induced by temporary occlusion of the common carotid arteries in rats with permanently occluded vertebral arteries. The extracellular concentrations of glutamate, aspartate and GABA increased markedly during ischaemia, but returned rapidly to normal during reperfusion. BW 1003C87 (250 μM, in the dialysis fluid) did not modify the increase in extracellular concentration of amino acids during ischaemia. Glutamate-like immunoreactivity was reduced in pyramidal cell somata both immediately after ischaemia and after 60 min of reperfusion. This reduction appeared to be somewhat less pronounced for cells in the left hemisphere (perfused with BW 1003C87) than in the contralateral hemisphere. Ischaemia caused no consistent changes in terminals. The ratio between the intracellular levels of glutamate and glutamine was assessed by double-labelling immunocytochemistry, using two different gold particle sizes. The glutamate-glutamine ratio in glial cells was greatly increased after ischaemia, but recovered to a normal level within 1 h of reperfusion. Aspartate-like immunoreactivity was substantially reduced in pyramidal cell somata both immediately and 60 min after ischaemia, while profiles that were immunopositive for GABA in control brains showed increased GABA immunolabelling. These results suggest that postsynaptic neuronal elements as well as glial cells contribute to the extracellular overflow of excitatory amino acids during an ischaemic event: post-synaptic elements by leaking or releasing glutamate and aspartate, and glial cells by losing their ability to convert glutamate to glutamine effectively. The temporal association between the changes in the glial contents of glutamate and glutamine, and the extracellular amino acid fluctuations recorded by microdialysis in the same animals, underline the strategic role of glia in regulating the extracellular level of glutamate.
Experimental Brain Research – Springer Journals
Published: Nov 1, 1993
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