Activity-dependent pH shifts and periodic recurrence of spontaneous interictal spikes in a model of focal epileptogenesis

Abstract

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the pidform cortex induced spontaneous interictal spikes (s/Ss) that recurred with high periodicity (8.5 ± 3.1 sec, mean ± SD). Intracellular recordings from principal neurons showed that the early phase of the inter-s/S period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 ± 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-s/S interval. Recordings with ion-sensitive electrodes demonstrated that s/Ss determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-s/S period and returned to control values just before a subsequent sis was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-α-glycyrrethinic acid (20 μM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the s/S. The results reported here suggest that the massive cell discharge associated with an sis induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates s/S periodicity.