Effects of Kainic Acid on Brain Calcium Fluxes Studied In Vivo and In Vitro

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Abstract
The effect of in vivo administration of kainic acid into the rabbit hippocampus was studied with brain dialysis and subsequent determination of the Ca2+ concentration in the dialysate. When included in the perfusing medium, kainic acid as well as veratridine induced a decrease in extracellular Ca2+. The effect of kainic acid (but not of veratridine) was insensitive to tetrodotoxin. In vitro studies revealed no effect of kainic acid on 45Ca2+ uptake by isolated astrocytes, but showed an enhancement of synaptosomal 45Ca2+ accumulation. This was, however, only 25% of the stimulatory effect of high K+ depolarization. Glutamate activated synaptosomal Ca2+ uptake, whereas dihydrokainate had no effect. The up take evoked by kainate and glutamate was independent of the K+ level in the medium which indicates the involvement of other than voltage-sensitive Ca2+ channels. The results confirm previous findings that kainic acid promotes the uptake of Ca2+ in brain cells. Kainate affects Ca2+ fluxes pre- and postsynaptically. Presynaptic Ca2+ influx may be mediated by chemically gated mechanisms.