LAYER-SPECIFIC PROPERTIES OF THE TRANSIENT K-CURRENT (I-A) IN PIRIFORM CORTEX

Piriform cortex in the rat is highly susceptible to induction of epile ptiform activity. Experiments in vivo and in vitro indicate that this activity originates in endopiriform nucleus (EN), In slices, EN neuron s are more excitable than layer II (LII) pyramidal cells, with more po sitive resting potentials and lower spike thresholds, We investigated potassium currents in EN and LII to evaluate their contribution to the se differences in excitability, Whole-cell currents were recorded from identified cells in brain slices. A rapidly inactivating outward curr ent (I-A) had distinct properties in LII (I-A,I-LII) versus EN (I-A,I- EN). The peak amplitude of I-A,I-EN was 45% smaller than I-A,I-EN and the kinetics of activation A,LII, and inactivation was significantly s lower for I-A,I-EN. The midpoint of steady-state inactivation was hype rpolarized by 10 mV for I-A,I-EN versus I-A,I-LII whereas activation w as similar in the two cell groups. Other voltage-dependent potassium c urrents were indistinguishable between EN and LII. Simulations using a compartmental model of LII cells argue that different cellular distri butions of I-A channels in EN versus LII cells cannot account for thes e differences. Thus, at least some of the differences are intrinsic to the channels themselves. Current-clamp simulations suggest that the d ifferences between I-A,I-LII and I-A,I-EN can acA,EN count for the obs erved difference in resting potentials between the two cell groups, Si mulations show that this difference in resting potential leads to long er first spike latencies in response to depolarizing stimuli. Thus, th ese differences in the properties of I, could make EN more susceptible to induction and expression of epileptiform activity.