FUNCTIONAL AND IONIC PROPERTIES OF A SLOW AFTERHYPERPOLARIZATION IN FERRET PERIGENICULATE NEURONS IN-VITRO

Intracellular recordings from spontaneously spindling GABAergic neuron s of the ferret perigeniculate nucleus in vitro revealed a fast afterh yperpolarization after each action potential, a medium-duration afterh yperpolarization after each low-threshold Ca2+ spike, and a slow after hyperpolarization after the cessation of spindle waves. The slow after hyperpolarization was associated with an increase in membrane conducta nce, and the reversal potential was sensitive to extracellular [K+](o) , indicating that it is mediated at least in part by the activation of a K+ conductance. However, the block of Ca2+ channels did not block t he slow afterhyperpolarization, whereas the block of Na+ channels did block this event, even after the generation of repetitive Ca2+ spikes, indicating that it is mediated by a Na+-activated K+ current. Applica tion of apamin reduced the afterhyperpolarization and enhanced a plate au potential after each low-threshold Ca2+ spike. This plateau potenti al could result in a prolonged depolarization of perigeniculate neuron s, even before the application of apamin, resulting in the generation of tonic discharge. The plateau potential was blocked by the local app lication of tetrodotoxin, indicating that it is mediated by a persiste nt Na+ current. The activation and interaction of these slowly develop ing and persistent currents contributes significantly to low-frequency components of spindle wave generation. In particular, we suggest that the activation of the slow afterhyperpolarization may contribute to t he generation of the spindle wave refractory period in vitro.