Synaptic Regulation of the Slow Ca2+-Activated K+Current in Hippocampal CA1 Pyramidal Neurons: Implication in Epileptogenesis

The slow Ca2+-activated K+current (sIAHP) plays a critical role in regulating neuronal excitability, but its modulation during abnormal bursting activity, as in epilepsy, is unknown. Because synaptic transmission is enhanced during epilepsy, we investigated the synaptically mediated regulation of the sIAHPand its control of neuronal excitability during epileptiform activity induced by 4-aminopyridine (4AP) or 4AP+Mg2+-free treatment in rat hippocampal slices. We used electrophysiological and photometric Ca2+techniques to analyze the sIAHPmodifications that parallel epileptiform activity. Epileptiform activity was characterized by slow, repetitive, spontaneous depolarizations and action potential bursts and was associated with increased frequency and amplitude of spontaneous excitatory postsynaptic currents and a reduced sIAHP.The metabotropic glutamate receptor (mGluR) antagonist (S)-α-methyl-4-carboxyphenylglycine did not modify synaptic activity enhancement but did prevent sIAHPinhibition and epileptiform discharges. The mGluR-dependent regulation of the sIAHPwas not caused by modulated intracellular Ca2+signaling. Histamine, isoproterenol, and (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid reduced the sIAHPbut did not increase synaptic activity and failed to evoke epileptiform activity. We conclude that 4AP or 4AP+Mg-free–induced enhancement of synaptic activity reduced the sIAHPvia activation of postsynaptic group I/II mGluRs. The increased excitability caused by the lack of negative feedback provided by the sIAHPcontributes to epileptiform activity, which requires the cooperative action of increased synaptic activity.