The minimum volume of depolarized neural tissue required for triggering cortical spreading depression in rat

Summary Cortical spreading depression (CSD) was evoked in anaesthetized rats by intracortical microinjection of 3.4·10^–8 mol KCl (the single injection threshold T₁). With two simultaneous injections at 1 mm tip separation 59% T₁ had to be applied to each point to elicit CSD. For interfocal distances 2, 3 and 4 mm the double injection thresholds T_1,2 were 65%, 74% and 97% of T₁ respectively. The spatial summation effect was still significant at 3 mm and undetectable at 4 mm tip separation. Recording electrodes placed 1–3 mm from the point of injection detected local slow potential changes which attained with subthreshold KCl injections 16% of the maximum CSD negativity at the 1.5 mm distance. The threshold amount of KCl required to trigger CSD at different intervals (30–480 sec) after initial injection of 0.8 T₁, exponentially increased with time from 28.2% t₁ at 30 sec to 88.9% at 480 sec with the slope 16.8% T₁ for a twofold increase of the interinjection interval. Quantitative analysis of results based on equations describing diffusion from an instantaneous point source indicates that the critical volume of depolarization is reached 61 sec after injection. According to the value of the diffusion coefficient of potassium the critical K+ concentration lies between 45 and 12 mequ/l and is reached at a distance of 600–950 from the injection.Visiting scientist from Department of Physiology, Gifu University, School of Medicine, Gifu City, Japan.