Depth distribution and mechanism of changes in extracellular K+ and Ca2+ concentrations in the hippocampus

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Abstract
In the CA1 area of the hippocampus of urethane-anesthetized rats, the greatest .DELTA.[K+]o and .DELTA.[Ca2+]o evoked by repetitive fimbrial-commissural stimulation were always found in the pyramidal cell layer; but there were large increases in [K+]o over a wide range of depth, whereas a major fall in [Ca2+]o was localized almost exclusively to the level of the pyramidal layer. A sustained focal negative potential was also evoked by fimbrial stimulation; it resembled .DELTA.[K+]o in time course and depth distribution, and therefore probably reflected cellular depolarization caused by increased [K+]o. The close correlation between .DELTA.[Ca2+]o and .DELTA.[K+]o, and the appearance of population spikes (especially in bursts of 3-4 spikes) indicate that pyramidal cell firing, and corresponding K-outward and Ca-inward currents are mainly responsible for the accumulation of K+o, and the depletion of Cao2+. In CA3 pyramidal areas, .DELTA.[K+]o and .DELTA.[Ca2+]o were comparable in magnitude and distribution to changes seen in CA1, but they occurred after a longer latency. The major .DELTA.[Ca2+]o had longer duration, consistent with a more prolonged Ca2+ current.