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 piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 Ϯ 3.1 sec, mean Ϯ SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS 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 Ϫ32.1 Ϯ 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-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K ϩ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS 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 ϩ 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 NH 4 Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-␣-glycyrrethinic acid (20 M) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. 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 sIS periodicity.

Role of gap junctions in epilepsy

“…In a 4-AP model of epilepsy, carbenoxolone suppresses spontaneous large inhibitory postsynaptic currents and ectopic spikes [101] , and reduces the frequency and cumulative duration of spontaneous ictal-like activity, while the Cx36-selective blocker quinidine diminishes this activity [102] . Carbenoxolone [80,81] , octanol and 18α-glycyrrhetinic acid [103] also depress the spontaneous epileptiform discharges induced by bicuculline. In a Co 2+ epilepsy model, Mylvaganam et al demonstrated that octanol diminishes ictal and interictal discharges [79] .…”

Neuronal hyperactivity causes Na+/H+ exchanger-induced extracellular acidification at active synapses

“…Several studies have addressed the question of how neuronal activity elicits changes in intracellular and extracellular H + concentration, sometimes yielding contrasting results (Caldwell et al, 2013;Chen and Chesler, 2015;Cichy et al, 2015;de Curtis et al, 1998;DeVries, 2001;Lu et al, 2012;Palmer et al, 2003;Zhang et al, 2010;Zhao et al, 2011). We studied the effects of sustained heightened neural activity on the extracellular H + concentration by using in vitro models of epileptic-like network states characterized by synchronized neuronal discharges.…”

Kinetics of Activity-Evoked pH Transients and Extracellular pH Buffering in Rat Hippocampal Slices

“…Seizure activity is of course defined by sustained synchronous discharge and has been shown to give rise to large summated alkaline shifts on the order of 0.10 -0.20 unit pH (de Curtis et al 1998). Synchronous discharge, however, is also a fundamental property of normal brain function.…”