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

Abstract

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, [Ca 2+] o decreased to <0.01 mM. During phases I and II, both [Ca 2+] o and [Na +] o remained low. the recoverries of [Ca 2+] 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 Δ[K +] 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 [Ca 2+] o sinks during SD, and its two-phase recovery, indicate intracellular sequestration or binding of substantial amounts of Ca 2+ ions. The residual deficit of [Ca 2+ o following recovery of SP shifts may account for the persistent depression of synaptic transmission after repolarization of neurons.