Nonsynaptic epileptogenesis in the mammalian hippocampus in vitro. II. Role of extracellular potassium

1. The role of extracellular K⁺ (K⁺₀) in nonsynaptic epileptogenesis induced in the CA1 area of rat hippocampal slices by lowering [Ca²]₀ was studied with K⁺-selective microelectrodes (KSMs). Extracellular field potentials and [K⁺]₀ were recorded simultaneously with 1-2 KSMs in the CA1 stratum pyramidale. 2. In slices perfused with an oxygenated standard physiological solution (containing 2 mM Ca²⁺), base-line [K⁺]₀ was stable for several hours. The washout of Ca²⁺₀ was accompanied by a gradual tonic rise of [K⁺]₀. Spontaneous and stimulus-evoked maximal seizurelike events (SLEs) appeared when [K⁺]₀ was ~0.5 mM above the initial 5 mM base line. These changes were reversible in normal medium. 3. When K⁺₀ was pressure ejected in the CA1 stratum pyramidale of spontaneously active slices, a local rise in [K⁺]₀ of ~0.5 mM was necessary to trigger a SLE. A similar apparent [K⁺]₀ 'threshold' was associated with SLEs evoked by electrical stimulation. 4. Increasing [K⁺] in the perfusing solution by small increments (1 mM) markedly enhanced SLEs frequency and velocity of spread and decreased the period of absolute refractoriness that succeeded each paroxysm. Similar changes occurred during periods of transient hypoxia. Small [K⁺] decreases in the perfusate had the converse effects. 5. Spontaneous SLEs were associated with phasic increases in [K⁺]₀. In simultaneous [K⁺]₀ recordings from two layers, these transients were largest (up to 3.5 mM above base line) and rose more steeply at the stratum pyramidale. Toward the outer dendritic layers they become smaller, slower in time course, and delayed in onset. We conclude that the main source for these [K⁺]₀ transients are the hippocampal pyramidal cell bodies, which discharge intensely during a SLE, and that excess K⁺₀ is spatially dispersed around the discharge zone of the paroxysm. 6. [K⁺]₀ continued to rise, though at a slower rate, throughout the course of a SLE. Following SLE termination, [K⁺]₀ decayed slowly to base line. 7. The invasion of a CA1 region by a propagating SLE was preceded quite often by a slow rise in [K⁺]₀. A sudden transition to a steeply rising [K⁺]₀ marked the explosive recruitment of this region into the discharge zone of the spreading paroxysm. 8. The total (tonic and phasic) increase in [K⁺]₀ during SLEs did not surpass a maximal level of ~9 mM, which was the ceiling level of [K⁺]₀ in low [Ca²⁺]₀. However, when spreading depression occurred, [K⁺]₀ rose up to 30-40 mM for several minutes. Spreading depression rarely appeared spontaneously despite the recurrence of SLEs, but could be provoked by repetitive electrical stimulation. Sometimes it was triggered by a SLE. 9. We find the data compatible with two hypotheses regarding the role of K⁺₀ in nonsynaptic hippocampus epileptogenesis. First, the regenerative accumulation of K⁺ in the CA1 interstitium may provide the positive feedback underlying the explosive buildup of a SLE. Second, the spatial dispersion of the excess K⁺₀ may support SLE spread through CA1. Possible implications of these findings for hippocampal epileptogenesis in vivo are discussed.