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Volume 16, Number 12, Issue of June 15, 1996 pp. 3862-3876
Copyright ©1996 Society for Neuroscience

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

Matthew I. Banks¹, Lewis B. Haberly^{2, 3}, and Meyer B. Jackson^{1, 3}

Departments of ¹ Physiology and ² Anatomy, and ³ Neuroscience Training Program, University of Wisconsin Medical School, Madison, Wisconsin 53706

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 (I_A) had distinct properties in LII (I_A,LII) versus EN (I_A,EN). The peak amplitude of I_A,EN was 45% smaller than I_A,LII, and the kinetics of activation and inactivation was significantly slower for I_A,EN. The midpoint of steady-state inactivation was hyperpolarized by 10 mV for I_A,EN versus I_A,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 of I_A 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 I_A,LII and I_A,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 I_A could make EN more susceptible to induction and expression of epileptiform activity.

Key words: potassium channels; piriform cortex; epilepsy; compartmental models; voltage clamp; membrane excitability

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