Layer-Specific Properties of the Transient K Current (IA) in Piriform Cortex

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Abstract
Piriform cortex in the rat is highly susceptible to induction of epileptiform activity. Experiments in vivo and in vitro indicate that this activity originates in endopiriform nucleus (EN). In slices, EN neurons are more excitable than layer II (LII) pyramidal cells, with more positive resting potentials and lower spike thresholds. We investigated potassium currents in EN and LII to evaluate their contribution to these differences in excitability. Whole-cell currents were recorded from identified cells in brain slices. A rapidly inactivating outward current (IA) had distinct properties in LII (IA,LII) versus EN (IA,EN). The peak amplitude ofIA,EN was 45% smaller thanIA,LII, and the kinetics of activation and inactivation was significantly slower forIA,EN. The midpoint of steady-state inactivation was hyperpolarized by 10 mV forIA,EN versusIA,LII, whereas activation was similar in the two cell groups. Other voltage-dependent potassium currents were indistinguishable between EN and LII. Simulations using a compartmental model of LII cells argue that different cellular distributions ofIA channels in EN versus LII cells cannot account for these differences. Thus, at least some of the differences are intrinsic to the channels themselves. Current-clamp simulations suggest that the differences between IA,LIIand IA,EN can account for the observed difference in resting potentials between the two cell groups. Simulations show that this difference in resting potential leads to longer first spike latencies in response to depolarizing stimuli. Thus, these differences in the properties of IAcould make EN more susceptible to induction and expression of epileptiform activity.