HYPERPOLARIZATION-ACTIVATED NA+-K+ CURRENT (I-H) IN NEOCORTICAL NEURONS IS BLOCKED BY EXTERNAL PROTEOLYSIS AND INTERNAL TEA

1. After 1 day in culture, neurons derived from the neonatal cerebral cortex of the rat exhibited a slowly activating current gated by hyperpolarizing voltage clamp pulses. The current was blocked by extracellular Cs+ and unaffected by extracellular Ba2+, and was permeable to both Na+ and K+ (P-Na/P-K = 0.29). Its form and pharmacology are consistent with a current termed I-h in other preparations. 2. I-h was absent from cells acutely dissociated from both the neonatal and mature cerebral cortex, despite the use of low enzyme concentrations. The sensitivity of I-h to extracellular proteolysis was demonstrated by superfusing the cells with trypsin (1 mg/ml) while monitoring the presence of I-h in the whole cell mode of recording. I-h was rapidly abolished (t(1/2) similar to 5 min at 22 degrees C) by proteolysis and exhibited no shifts in its range of activation or changes in its activation kinetics during the course of the digestion. 3. Intracellular tetraethylammonium (TEA), at a concentration of <15 mM was shown to block I-h completely, while extracellular TEA had no effect on the current. This suggests that the inner vestibule of I-h may be structurally related to that of potassium channels.