Regulation of extracellular calcium in the hippocampus in vivo during epileptiform activity--role of astrocytes.

Astrocytes have been suggested to regulate the extracellular calcium concentration ([Ca(2+)](o)), but this has not been thoroughly investigated. Adult, male Sprague-Dawley rats were used to record changes in [Ca(2+)](o) in the hippocampus during epileptiform activity. Maximal decreases in [Ca(2+)](o) in CA1 were measured in the pyramidal cell layer during 20 Hz, 20s stimulus trains to the contralateral CA3 region. Maximal decreases in [Ca(2+)](o) in the dentate gyrus were measured when maximal dentate activation had appeared-irrespective of the location, frequency or duration of the stimulation. Maximal decreases were 36% greater in the dentate gyrus than in CA1. During prolonged discharges, [Ca(2+)](o) recovered partially towards the baseline in both hippocampal regions. To investigate the role of astrocytes, local injections of fluorocitrate (FC), a metabolic toxin selectively taken up by astrocytes, were used. FC (0.1, 0.25 or 0.5mM FC), but not vehicle (2 microl), caused a small, but significant decrease in the maximal changes in CA1, but an increase in the dentate gyrus. The results suggest that maximal decreases in [Ca(2+)](o) occur in the hippocampus in response to burst firing of neurons and that astrocytes play a minimal role in the regulation of [Ca(2+)](o) during epileptiform activity.