Conditions sufficient for nonsynaptic epileptogenesis in the CA1 region of hippocampal slices.
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 Ca(2+) concentration. We show here that nonsynaptic epileptiform activity can be readily induced in vitro in normal (2 mM) Ca(2+) levels. Those conditions sufficient for nonsynaptic epileptogenesis in the CA1 region were determined by pharmacologically mimicking the effects of Ca(2+) reduction in normal Ca(2+) levels. Increasing neuronal excitability, by removing extracellular Mg(2+) 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 K(Ca) antagonists (verruculogen, charybdotoxin, norepinephrine, tetraethylammonium salt, and Ba(2+)) 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 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, 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 Ca(2+) facilitates bursting by increasing neuronal excitability and inhibiting Ca(2+) influx, which might, in turn, enhance a persistent sodium conductance. Furthermore, these data show that nonsynaptic mechanisms can contribute to epileptiform activity in normal Ca(2+) levels.
Bikson
Marom
M
Neural Engineering Center, Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Baraban
Scott C
SC
Durand
Dominique M
DM
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004827:Epilepsy / Q000503:physiopathology*; D005110:Extracellular Space / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000503:physiopathology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000037:antagonists & inhibitors / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D013569:Synapses; D009435:Synaptic Transmission / Q000187:drug effects; D014701:Veratridine / Q000494:pharmacology
D002121:Calcium Channel Blockers; D026902:Potassium Channel Blockers; D024681:Potassium Channels, Calcium-Activated; D015222:Sodium Channels; D002104:Cadmium; D014701:Veratridine; D011188:Potassium; D002118:Calcium