Two distinct types of transient outward currents in area postrema neurons in rat brain slices

We investigated the electrophysiological properties of the area postrema neurons in acutely prepared rat brain slices using the whole-cell patch-clamp technique. Two different types of transient outward potassium current (I_to), fast and slow, were found in the area postrema. Both the decay time constant and rise time were significantly faster in the fast I_to than in the slow I_to. Both current-clamp and voltage-clamp recordings revealed that the activation of fast and slow I_to contributes to generation of the different spiking patterns, late spiking and interrupted spiking, respectively. The activation and inactivation of both I_to were strongly voltage-dependent. Curve fitting by the Boltzmann equation revealed no significant difference in the activation and inactivation curves for each I_to except that the slope factor of inactivation was larger for fast I_to. Both I_to were suppressed dose-dependently by application of 4-aminopyridine. Each spiking pattern was enhanced when cells were held at a more hyperpolarized membrane potential, i.e. a longer latency of the first spike or longer interspike interval between the first and second spikes. The voltage-dependent modulation of the spiking pattern was consistent with the voltage-dependent activation of I_to. The present study shows significant subdivisions of the area postrema neurons distinguished by a difference in the kinetics of I_to and spiking patterns. We discuss the role of I_to as the ionic current underlying neuronal excitability.