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

In the CA1 area of the hippocampus of urethane-anaesthetized rats, the greatest delta[K+]omicron and delta[Ca2+]omicron evoked by repetitive fimbrial-commissural stimulation were always found in the pyramidal cell layer; but there were large increases in [K+]omicron over a wide range of depth, whereas a major fall in [Ca2+]omicron 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+]omicron in time course and depth distribution and therefore probably reflected cellular depolarization caused by increased [K+]omicron. The close correlation between delta[Ca2+]omicron and delta[K+]omicron and the appearance of population spikes (especially in bursts of three to four spikes) indicate that pyramidal cell firing and corresponding K-outward and Ca-inward currents are mainly responsible for the accumulation of K+omicron and the depletion of Ca2+omicron. In CA3 pyramidal areas, delta [K+]omicron and delta[Ca2+]omicron were comparable in magnitude and distribution to changes seen in CA1, but they occurred after a longer latency, and the major delta[Ca2+]omicron had a longer duration, consistent with a more prolonged Ca2+ current.