Photolytic Manipulation of [Ca²⁺]_iReveals Slow Kinetics of Potassium Channels Underlying the Afterhyperpolarization in Hipppocampal Pyramidal Neurons

Abstract

The identity of the potassium channel underlying the slow, apamin-insensitive component of the afterhyperpolarization current (sI_AHP) remains unknown. We studied sI_AHP in CA1 pyramidal neurons using simultaneous whole-cell recording, calcium fluorescence imaging, and flash photolysis of caged compounds. Intracellular calcium concentration ([Ca²⁺]_i) peaked earlier and decayed more rapidly than sI_AHP. Loading cells with low concentrations of the calcium chelator EGTA slowed the activation and decay of sI_AHP. In the presence of EGTA, intracellular calcium decayed with two time constants. When [Ca²⁺]_i was increased rapidly after photolysis of DM-Nitrophen, both apamin-sensitive and apamin-insensitive outward currents were activated. The apamin-sensitive current activated rapidly (<20 msec), whereas the apamin-insensitive current activated more slowly (180 msec). The apamin-insensitive current was reduced by application of serotonin and carbachol, confirming that it was caused by sI_AHP channels. When [Ca²⁺]_i was decreased rapidly via photolysis of diazo-2, the decay of sI_AHPwas similar to control (1.7 sec). All results could be reproduced by a model potassium channel gated by calcium, suggesting that the channels underlying sI_AHP have intrinsically slow kinetics because of their high affinity for calcium.