Changes in Extracellular Ca2+ and K+ Activity Accompanying Hippocampal Discharges

In rats under urethane anaesthesia, changes in the extracellular activities of K+ and Ca2+ (alpha K and alpha Ca) evoked by fimbrial or entorhinal stimulation and recorded in area CA3 with ion-selective microelectrodes are maximal in the pyramidal cell layers. With 10/s stimulation, alpha K increases by 6--9 mM whereas alpha Ca falls by 0.5--1.0 mM. In contrast with the increase in alpha K, which is distributed over a wide range of depth, the reduction in alpha Ca is particularly sharply limited to the level of pyramidal cell bodies. It is regularly associated with a negative aferpotential in the extracellular field, which presumably reflects a large postsynaptic Ca2+ inward current, apparently predominant in the cell bodies. Repetitive stimulation sometimes evokes spreading depressionlike swings in potential, which are seen only near the pyramidal stratum and are accompanied by massive increases in alpha K (to 30--40 mM) and falls in alpha Ca (to < 0.1 mM). A large Ca2+ influx into pyramidal cells may be of significance for "plastic" aspects of hippocampal function and when excessive, as during repetitive convulsive activity, may be responsible for the necrosis of CA3 neurons.