Analysis of potential shifts associated with recurrent spreading depression and prolonged unstable spreading depression induced by microdialysis of elevated K in hippocampus of anesthetized rats

The potential shifts (ΔV_o) associated with spreading depression (SD) were analysed with the help of multiple extracellular recording and ion-selective microelectrodes in the CA1 region of the dorsal hippocampus of anesthetized rats. Recurrent waves of SD were induced by perfusing high K⁺ solution through microdialysis probes. SD-related ΔV_o had a composite wave shape, consisting of an early, rapidly shifting part (phase I) followed by a slower shift to a second negative maximum (phase II). ΔV_o shifts in stratum radiatum usually started earlier, always lasted longer and had lartger amplitude than those recorded in stratum pyramidale. The ΔV_o responses in stratum radiatum had an inverted saddle shape created by a transient relatively positive “hump” interposed between phases I and II. During this “hump”, the potentials in the two layers transiently approached one another. During continuous high K⁺ dialysis, successive ΔV_o waves episodes evolved according to a consistent pattern: while phase I remained unchanged, phase II increased in amplitude and duration with each episode. Eventually, a depressed state developed which lasted for many minutes, termed here prolonged unstable spreading depression. During phase I, ΔV_o and extracellular K (K⁺]_o) changes were correlated. During phase II, K⁺]_o decreased even as ΔV_o continued to increase. During SD, Ca²⁺]_o decreased to <0.01 mM. During phases I and II, both Ca²⁺]_o and Na⁺]_o remained low. the recoverries of Ca²⁺]_o and Na⁺]_o had an initial fast and a later much slower phase and took several minutes longer than the recoveries of K⁺]_o and ΔV_o. Depth profiles of ΔV_o and ΔK⁺]_o revealed strikingly steep gradients early and late during a wave; but voltage and ion gradients were not precisely correlated either in time or in space. We conclude that ΔV_o of phases I and II are generated by different processes. Membrane ion currents cannot fully explain the ΔV_o responses. The possible contributions by ion diffusion and by active ion transport are discussed. The extremely low level to which Ca²⁺]_o sinks during SD, and its two-phase recovery, indicate intracellular sequestration or binding of substantial amounts of Ca²⁺ ions. The residual deficit of Ca²⁺_o following recovery of SP shifts may account for the persistent depression of synaptic transmission after repolarization of neurons.