Kindling phenomena induced by the repeated short-term high potassium stimuli in the ventral hippocampus of rats: on-line monitoring of extracellular glutamate overflow

We observed in this study that transient periodic stimuli in response to hi gh potassium (40 mM, 5 min at 40-min intervals, 13-15 stimuli) perfusion in the ventral hippocampus of rats led to the appearance of a kindling-like p henomenon. In this kindling-like phenomenon, we confirmed the augmentation of glutamate release and the prolongation of spike discharge. Changes in th e extracellular glutamate levels before and after the stimuli were monitore d by the application of in vivo microdialysis combined with on-line enzyme fluorometric detection of glutamate. This kindling-like phenomenon was not observed when microdialysis was carried out using a Ca++-free medium. The a ugmentation of glutamate release and the prolongation of spike discharge wi th epileptic convulsions are completely Ca++ dependent. These data show tha t repeated short-term increases in extracellular glutamate levels results i n the enhancement of excitatory neuronal systems, causing an excessive prop agation of seizure activity and culminating in secondary generalized seizur es.