Pharmacological and electrographic properties of epileptiform activity induced by elevated K2+ and lowered Ca2+ and Mg2+ concentration in rat hippocampal slices

We studied some of the physiological and pharmacological properties of an in vitro model of epileptic seizures induced by elevation of [K⁺]₀ (to 8 mM and 10 mM) in combination with lowering of [Mg²⁺]₀ (to 1.4 mM and 1.6 mM) and [Ca²⁺]₀ (to 0.7 mM and 1 mM) in rat hippocampal slices. These concentrations correspond to the ionic constitution of the extracellular microenvironment during seizures in vivo. The resulting activity was rather variable in appearance. In area CA3 recurrent discharges were observed which resulted in seizure-like events with either clonic-like or tonic-cloniclike ictaform events in area CA1. With ion-sensitive electrodes, we measured the field potential and the changes in extracellular ion concentrations which accompany this activity. The recurrent discharges in area CA3 were accompanied by small fluctuations in [K⁺]₀ and [Ca²⁺]₀. The grouped clonic-like discharges in area CA1 were associated with moderate increases in [K⁺]₀ and small decreases in [Ca²⁺]₀ in the order of 2 mM and 0.2 mM, respectively. Large, negative field-potential shifts and increases in [K⁺]₀ to 13 mM, as well as decreases in [Ca²⁺]₀ by up to 0.4 mM, accompanied the tonic phase of ictaform events. The ictaform events were not blocked by D-2-aminophosphonovalerate (2-APV) but were sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) alone and in combination with 2-APV and ketamine. In order to determine the pharmacological characteristics of the ictaform events we bath-applied most clinically employed anticonvulsants (carbamazepine, phenytoin, valproate, phenobarbital, ethosuximide, trimethadione) and some experimental anticonvulsants (losigamone, vinpocetine, and apovincaminic acid). Carbamazepine, phenytoin, valproate, and phenobarbital were effective at clinically relevant doses. The data suggest that the high-K⁺ model of epileptiform activity is a good model of focal convulsant activity.