Conditions Sufficient for Nonsynaptic Epileptogenesis in the CA1 Region of Hippocampal Slices

Abstract

Nonsynaptic mechanisms exert a powerful influence on seizure threshold. It is well-established that nonsynaptic epileptiform activity can be induced in hippocampal slices by reducing extracellular Ca2+ concentration. We show here that nonsynaptic epileptiform activity can be readily induced in vitro in normal (2 mM) Ca2+ levels. Those conditions sufficient for nonsynaptic epileptogenesis in the CA1 region were determined by pharmacologically mimicking the effects of Ca2+ reduction in normal Ca2+ levels. Increasing neuronal excitability, by removing extracellular Mg2+ and increasing extracellular K+ (6–15 mM), induced epileptiform activity that was suppressed by postsynaptic receptor antagonists [d-(−)-2-amino-5-phosphonopentanoic acid, picrotoxin, and 6,7-dinitroquinoxaline-2,3-dione] and was therefore synaptic in nature. Similarly, epileptiform activity induced when neuronal excitability was increased in the presence of KCaantagonists (verruculogen, charybdotoxin, norepinephrine, tetraethylammonium salt, and Ba2+) was found to be synaptic in nature. Decreases in osmolarity also failed to induce nonsynaptic epileptiform activity in the CA1 region. However, increasing neuronal excitability (by removing extracellular Mg2+ and increasing extracellular K+) in the presence of Cd2+, a nonselective Ca2+channel antagonist, or veratridine, a persistent sodium conductance enhancer, induced spontaneous nonsynaptic epileptiform activity in vitro. Both novel models were characterized using intracellular and ion-selective electrodes. The results of this study suggest that reducing extracellular Ca2+ facilitates bursting by increasing neuronal excitability and inhibiting Ca2+ influx, which might, in turn, enhance a persistent sodium conductance. Furthermore, these data show that nonsynaptic mechanisms can contribute to epileptiform activity in normal Ca2+ levels.

Cellular and network mechanisms of electrographic seizures

“…In recordings from granule cells of the dentate gyrus in hippocampus at different levels of ionic concentrations in vitro, a simultaneous increase in [K + ] o and decrease in [Ca 2+ ] o caused cellular bursts to appear at K + /Ca 2+ concentrations that were previously recorded in vivo before the onset of synchronized reverberatory seizure activity (Pan and Stringer 1997). Spontaneous nonsynaptic epileptiform activity was found in hippocampal slices after increasing neuronal excitability (by removing extracellular Mg 2+ and increasing extracellular K + ) in the presence of Cd 2+ , a nonselective Ca 2+ channel antagonist, or veratridine, a persistent sodium conductance enhancer (Bikson et al 2002). In recordings from rat hippocampal slices with high [K + ] o , population bursts in CA1 were generated locally by intrinsically bursting pyramidal cells, which recruited and synchronized other neurons (Jensen and Yaari 1997).…”

Potassium diffusive coupling in neural networks

“…Traditionally, non-synaptic bursting was thought only to occur in the low-Ca 2þ and zero-Ca 2þ models. However, experimental data show that it is also possible to generate non-synaptic activity associated with increases in potassium concentration in the presence of normal synaptic transmission (Bikson et al 2002). Therefore, besides synaptic transmission, potassium diffusion and its subsequent coupling effect can work together to modulate synchronization in neural networks.…”

Synchrony Levels During Evoked Seizure-Like Bursts in Mouse Neocortical Slices

“…We selected the neocortex because it pertains to the seizures most commonly seen in the clinical setting of pediatric epilepsy. Other researchers have successfully used cortical and hippocampal slice preparations in the study of epilepsy (e.g., Avoli et al 1987;Bikson et al 2002;Connors 1984;Connors and Amitai 1993;Dickson and Alonso 1997;Gutnick et al 1982;Hablitz 1987;Netoff and Schiff 2002;Nishimura et al 1996;Schwartzkroin 1994) and other state-dependent processes, such as sleep-and wakefulness (Sanchez-Vives and McCormick 2000). The neocortical slice enabled us to perform simultaneous extra-and intracellular recordings in a controlled environment, thus allowing us to examine to what extent a cell is synchronized with its environment-"the network."…”