Photolytic manipulation of [Ca²⁺](i) reveals slow kinetics of potassium channels underlying the afterhyperpolarization in hipppocampal pyramidal neurons

The identity of the potassium channel underlying the slow, apamin- insensitive component of the afterhyperpolarization current (S/(AHP)) remains unknown. We studied S/(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 S/(AHP). Loading cells with low concentrations of the calcium chelator EGTA slowed the activation and decay of S/(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 S/(AHP) channels. When [Ca²⁺](i) was decreased rapidly via photolysis of diazo-2, the decay of S/(AHP) was similar to control (1.7 sec). All results could be reproduced by a model potassium channel gated by calcium, suggesting that the channels underlying S/(AHP) have intrinsically slow kinetics because of their high affinity for calcium.