Abstracts

Rationale: Hyperpolarization activated non-selective cation (HCN) channels, expressed primarily within the distal dendrites of pyramidal neurons, have been implicated in cellular mechanisms contributing to epilepsy. Within distal dendrites, HCN channels serve to normalize the time-courses of distal excitatory inputs. Upon membrane hyperpolarization, the non-inactivating cationic current (Ih) slightly depolarizes the resting membrane potential while decreasing the input resistance. Ih inhibition can enhance summation from distal excitatory inputs and increases the spiking frequency of cells in response to depolarizing current injections. Decreased HCN channels and Ih are observed in a variety of epilepsy models. Our goal was to characterize the effect of HCN channel inhibition on evoked epileptiform activity in neocortical pyramidal cells and GABAergic interneurons. Methods: Acute neocortical slices (300 m) were cut from 20-28 day old rats and kept in standard ACSF at room temperature. Individual slices were transferred to a recording chamber perfused with oxygenated ACSF (3mL/min) at 32o C. Whole cell recordings were obtained from neocortical pyramidal cells and interneurons. Epileptiform activity was evoked in the presence of bicuculine (10 M) using a bipolar stimulating electrode. Cells were identified visually, by their spiking properties and distance from the pial surface. Biocytin was included in the recording pipette for subsequent visualization. ZD-7288 (10-20 M) was used in inhibit HCN channels. Results: Intracortical stimulation evoked a long lasting (200-300 ms) depolarization (20-30 mV) and persistent spiking in layer V pyramidal neurons. Evoked epileptic activity in layer 1 interneurons was of similar duration (200-300 ms) but was lower in amplitude (10-20 mV) with significantly less spiking. Interneurons exhibited a late inhibitory response immediately following evoked epileptic activity. Bath application of ZD-7288 increased the duration of epileptic activity in layer V pyramidal neurons and increased the number of spikes (N = 7). Epileptic activity in layer I interneurons also exhibited significantly increased duration. Small increases in amplitude following ZD-7288 application. Furthermore, the late inhibitory response was changed to a late depolarized response in layer I cells. Conclusions: The direct effects of HCN channel inhibition on epileptiform activity have received little attention. Our results indicate that HCN channels normally constrain polysynaptic epileptiform responses within the neocortex. Results indicate a differential effect of Ih inhibition on GABAergic interneurons versus pyramidal neurons. Pyramidal neurons exhibit increased spiking while interneurons only exhibit a prolonged depolarization. These changes could have important implications for regulation of network excitability. NS22373