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In CA3 neurons of disinhibited hippocampal slice cultures the slow afterhyperpolarisation, following spontaneous epileptiform burst events, was confirmed to be Ca(2+) dependent and mediated by K(+) ions. Apamin, a selective blocker of the SK channels responsible for part of the slow afterhyperpolarisation reduced, but did not abolish, the amplitude of the post-burst afterhyperpolarisation. The result was an increased excitability of individual CA3 cells and the whole CA3 network, as measured by burst duration and burst frequency. Increases in excitability could also be achieved by strongly buffering intracellular Ca(2+) or by minimising Ca(2+) influx into the cell, specifically through L-type (but not N-type) voltage operated Ca(2+) channels. Notably the L-type Ca(2+) channel antagonist, nifedipine, was more effective than apamin at reducing the post-burst afterhyperpolarisation. Nifedipine also caused a greater increase in network excitability as determined from measurements of burst duration and frequency from whole cell and extracellular recordings. N-methyl D-aspartate receptor activation contributed to the depolarisations associated with the epileptiform activity but Ca(2+) entry via this route did not contribute to the activation of the post-burst afterhyperpolarisation. We suggest that Ca(2+) entry through L-type channels during an epileptiform event is selectively coupled to both apamin-sensitive and -insensitive Ca(2+) activated K(+) channels. Our findings have implications for how the route of Ca(2+) entry and subsequent Ca(2+) dynamics can influence network excitability during epileptiform discharges.

Type

Journal article

Journal

Neuroscience

Publication Date

2001

Volume

102

Pages

297 - 306

Keywords

Animals, Apamin, Calcium, Calcium Channel Blockers, Calcium Channels, L-Type, Calcium Channels, N-Type, Chelating Agents, Egtazic Acid, Epilepsy, Excitatory Amino Acid Antagonists, Excitatory Postsynaptic Potentials, GABA Antagonists, Hippocampus, In Vitro Techniques, Patch-Clamp Techniques, Potassium Channel Blockers, Potassium Channels, Potassium Channels, Calcium-Activated, Rats, Rats, Wistar, Receptors, N-Methyl-D-Aspartate, Small-Conductance Calcium-Activated Potassium Channels