Activation of a Calcium-Activated Cation Current During Epileptiform Discharges and Its Possible Role in Sustaining Seizure-Like Events in Neocortical Slices

Abstract

Schiller, Yitzhak. Activation of a calcium-activated cation current during epileptiform discharges and its possible role in sustaining seizure-like events in neocortical slices. J Neurophysiol 92: 862-872, 2004; 10.1152/jn.00972.2003. Epileptic seizures are composed of recurrent bursts of intense firing separated by periods of electrical quiescence. The mechanisms responsible for sustaining seizures and generating recurrent bursts are yet unclear. Using whole cell voltage recordings combined with intracellular calcium fluorescence imaging from bicuculline (BCC)-treated neocortical brain slices, I showed isolated paroxysmal depolarization shift (PDS) discharges were followed by a sustained afterdepolarization waveform (SADW) with an average peak amplitude of 3.3 Ϯ 0.9 mV and average half-width of 6.2 Ϯ 0.6 s. The SADW was mediated by the calcium-activated nonspecific cation current (I can ) as it had a reversal potential of -33.1 Ϯ 6.8 mV, was unaffected by changing the intracellular chloride concentrations, was markedly diminished by buffering [Ca 2ϩ ] i with intracellular bis-(o-aminophenoxy)-N,N,NЈ,NЈ-tetraacetic acid (BAPTA), and was reversibly abolished by the I can blocker flufenamic acid (FFA). The Ca 2ϩ influx responsible for activation of I can was mediated by both N-methyl-D-aspartate-receptor channels, voltagegated calcium channels and, to a lesser extent, internal calcium stores. In addition to isolated PDS discharges, BCC-treated brain slices also produced seizure-like events, which were accompanied by a prolonged depolarizing waveform underlying individual ictal bursts. The similarities between the initial part of this waveform and the SADW and the fact it was markedly reduced by buffering [Ca 2ϩ ] i with BAPTA strongly suggested it was mediated, at least in part, by I can . Addition of FFA reversibly eliminated recurrent bursting, and transformed seizure-like events into isolated PDS responses. These results indicated I can was activated during epileptiform discharges and probably participated in sustaining seizure-like events.

Sodium and Calcium Current-Mediated Pacemaker Neurons and Respiratory Rhythm Generation

“…TRPM4-or TRPM5-like ion channels engender I CAN and are blocked by 30 -100 M FFA (Teulon, 2000;Launay et al, 2002;Montell et al, 2002;Guinamard et al, 2004;Moran et al, 2004). We abolished bursting with 10 -200 M FFA, which is consistent with a role for TRPM4/5-mediated I CAN (Schiller, 2004). FFA also blocks gap junctions (Srinivas and Spray, 2003;Ye et al, 2003), enhances K ϩ currents (Stumpff et al, 2001), with EC 50 Ͻ 50 m so that doses Ͼ100 M could have side effects that impact respiratory network activity .…”

Cellular Mechanisms Underlying Antiepileptic Effects of Low- and High-Frequency Electrical Stimulation in Acute Epilepsy in Neocortical Brain Slices In Vitro

“…Extracellular recordings, which were performed with glass pipettes (0.1-0.5 MOhm) filled with the extracellular solution, were amplified 1,000-fold and filtered with a low-pass filter of 3 KHz and high-pass filter of 1 Hz. Intracellular whole cell recordings were performed from the soma of layer 5 and layer 2/3 pyramidal neurons as previously described (Schiller 2002;Schiller et al 2000). The slice was bathed in artificial cerebrospinal fluid (ACSF) that contained (in mM) 125 NaCl, 25 NaHCO 3 , 25 glucose, 4 KCl, 1.25 NaH 2 PO 4 , 1.5 CaCl 2 , and 1 MgCl 2 , 0.01 BCC, or 0 MgCl 2 ; pH 7.4.…”

“…Figures 1 and 2 show two typical examples where the slice was stimulated at 1 Hz. During the control prestimulation period, the slice spontaneously produced both interictal-like discharges and seizure-like events (Hablitz 1987;Schiller 2002. At the onset of the 1-Hz electrical stimulation, PDS discharges were evoked.…”

“…Neocortical brain slices treated with bicuculline (BCC) or magnesium-free solutions serve as acute models of neocortical epilepsy in vitro, because they produce both interictal-like depolarizing paroxysmal shift (PDS) discharges and seizurelike events (Avoli et al 1991;Hablitz 1987;Schiller 2002Valenzuela and Benardo 1995). In this study, we used extracellular field potential measurements and whole cell intracellular voltage recordings from neocortical brain slices treated with either BCC or magnesium-free solution to investigate the antiepileptic effects of two stimulation paradigms: sustained low-frequency stimulation (0.1-5 Hz for 5 min or longer), which is best suited for prolonged open-loop stimulation, and short trains of high-frequency stimulation (25-200 Hz for 1-5 s), which is best suited for closed-loop stimulation.…”

Endogenous Alkaline Transients Boost Postsynaptic NMDA Receptor Responses in Hippocampal CA1 Pyramidal Neurons

“…With the L-type channel antagonist D-600 in the patch pipette (Schiller, 2004), there was no effect on the baseline ADP, and the addition of XCA still caused a significant curtailment of the ADP in seven cells. The half-width measured 971 Ϯ 44 ms before versus 854 Ϯ 38 ms after addition of XCA ( p Ͻ 0.001).…”