Characterization of Inhibitory GABA-A Receptor Activation during Spreading Depolarization in Brain Slice

Spreading depolarization (SD) is a slowly propagating wave of near complete depolarizations of neurons and glia. Previous studies have reported large GABA releases during SD, but there is limited understanding of how GABA release and receptor activation are regulated and influence the propagating SD wavefront, as well as an excitatory phase immediately following the passage of SD. The present study characterized GABA-A type receptor (GABA(A)R) currents during SD generated by KCl microinjection in acute hippocampal slices from adult mice. Spontaneous GABA(A)R- mediated currents (sIPSCs) were initially enhanced, and were followed by a large outward current at the wavefront. sIPSC were then transiently supressed during the late SD phase, resulting in a significant reduction of the sIPSC/sEPSC ratio. The large outward current generated during SD was eliminated by the GABA(A)R antagonist gabazine, but the channel potentiator/agonist propofol failed to potentiate the current, likely because of a ceiling effect. Extracellular Cl- decreases recorded during SD were reduced by the antagonist but were not increased by the potentiator. Together with effects of GABA(A)R modulators on SD propagation rate, these results demonstrate a significant inhibitory role of the initial GABA(A)R activation and suggest that intracellular Cl- loading is insufficient to generate excitatory GABA(A)R responses during SD propagation. These results provide a mechanistic explanation for facilitating effects of GABA(A)R antagonists, and the lack of inhibitory effect of GABA(A)R potentiators on SD propagation. In addition, selective suppression of GABA transmission in the late SD period and the lack of effect of GABA(A) modulators on the duration of SD suggests that GABA modulation may not be effective approach to protect neurons during the vulnerable phase of SD.