EFFECT OF INTRAHIPPOCAMPAL DEXAMETHASONE ON THE LEVELS OF AMINO-ACID TRANSMITTERS AND NEURONAL EXCITABILITY

Direct effect of type-II corticosteroid receptor agonist dexamethasone on extracellular amino acid levels and neuronal excitability in the h ippocampus was studied by simultaneous application of in vivo microdia lysis and recording hippocampal evoked responses in freely moving male rats. Microdialysis probes and hippocampal recording electrodes were implanted to the CA1-CA3 regions of dorsal hippocampus. Local dexameth asone infusion via microdialysis resulted in a transient increase in g lutamate level at 30 min, while glutamine decreased by 30-40% througho ut the 180-min sampling period. Taurine increased by 50% and remained elevated up to 180 min. No significant changes were detected in extrac ellular concentration of asparagine, arginine, glycine, threonine, ala nine and serine. In contrast, dexamethasone infusion to the striatum h ad no effect on the extracellular levels of amino acid transmitters. E ffect of dexamethasone injected via microdialysis on the neural activi ty elicited by perforant path stimulation was a decrease in population spikes after 60 min starling dexamethasone infusion. Steroid effect o n neural excitability was reversible. Our data indicate that local inf usion of type-II receptor agonist dexamethasone has a complex effect i n the hippocampus, starts with a change in extracellular glutamate and glutamine concentration and followed by a reduced synaptic excitabili ty.