Regulation of epileptiform discharges in rat neocortex by HCN channels 1 2

25 Hyperpolarization-activated, cyclic-nucleotide gated, non-specific cation (HCN) 26 channels have a well-characterized role in regulation of cellular excitability and network 27 activity. The role of these channels in control of epileptiform discharges is less 28 thoroughly understood. This is especially pertinent given altered HCN channel 29 expression in epilepsy. We hypothesized that inhibition of HCN channels would 30 enhance bicuculline-induced epileptiform discharges. Whole-cell recordings were 31 obtained from layers 2/3 and 5 (L2/3; L5) pyramidal neurons and L1 and L5 GABAergic 32 interneurons. In the presence of bicuculline (10 uM), HCN channel inhibition with 33 ZD7288 (20 uM) significantly increased the magnitude (defined as area) of evoked 34 epileptiform events in both L2/3 and L5 neurons. We recorded activity associated with 35 epileptiform discharges in L1 and L5 interneurons to test the hypothesis that HCN 36 channels regulate excitatory synaptic inputs differently in interneurons versus pyramidal 37 neurons. HCN channel inhibition increased the magnitude of epileptiform events in both 38 L1 and L5 interneurons. The increased magnitude of epileptiform events in both 39 pyramidal cells and interneurons was due to an increase in network activity since 40 holding cells at depolarized potentials under voltage-clamp conditions to minimize HCN 41 channel opening did not prevent enhancement in the presence of ZD7288. In neurons 42 recorded with ZD7288-containing pipettes, bath application of the Ih antagonist still 43 produced increases in epileptiform responses. These results show that epileptiform 44 discharges in disinhibited rat neocortex are modulated by HCN channels. 45 46