Changes in extracellular Ca²⁺ and K⁺ activity accompanying hippocampal discharges

Abstract

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.

The Impact of Astrocytic Gap Junctional Coupling on Potassium Buffering in the Hippocampus

“…4 E). Furthermore, at maximal stimulation intensity, [K ϩ ] o did not exceed 12 mM in wt mice (13 of 13 slices), a value described previously as the "ceiling level" in the hippocampus (Benninger et al, 1980;Krnjevic et al, 1980 initially decayed very rapidly, followed by a prolonged phase of slower decay, which was often associated with an undershoot (Heinemann and Lux, 1975). To only consider the fast decay phase, we initially determined the time at which [K ϩ ] o had decayed to 1/e of the maximal value (t 1/e ), providing a first descriptive parameter for the decay process.…”

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

“…Increases in [K + ] o are not unexpected during hypersynchronous activity such as that associated with an interictal discharge that is contributed by glutamatergic conductances leading to sustained action potential firing, and they have been well characterized both in in vivo (Heinemann et al, 1977;Krnjevic et al, 1980) and in vitro preparations Avoli et al, 1990). What is surprising in the 4AP model is that similar [K + ] o elevations can be observed concomitant to the glutamatergic-independent interictal discharges generated by neuronal networks during application of ionotropic glutamatergic receptor antagonists (Fig.…”

GABA Transaminase Inhibition Induces Spontaneous and Enhances Depolarization-Evoked GABA Efflux via Reversal of the GABA Transporter

“…Probably for this reason, reversal of the GABA transporter has been considered to be important only during pathological conditions such as ischemia (Nicholls and Attwell, 1990;Levi and Raiteri, 1993). However, carrier-mediated GABA release can occur in response to presynaptic stimulation (Schwartz, 1987;Bernath and Zigmond, 1988) and also can occur in response to an elevation in [K ϩ ] o from 3 to 12 mM (Gaspary et al, 1998), a level of [K ϩ ] o that is reached in vivo during neuronal firing (Krnjevic et al, 1980;Somjen and Giacchino, 1985) and during seizures (Fisher et al, 1976).…”