Channels underlying the slow afterhyperpolarization in hippocampal pyramidal neurons: neurotransmitters modulate the open probability.

The slow afterhyperpolarization in hippocampal pyramidal neurons is mediated by a calcium-activated potassium current (IAHP) and is a target for variety of different neurotransmitters. The characteristics of the channels underlying IAHP and how they are modulated by neurotransmitters are, however, unknown. In this study, we have examined the properties of the channels underlying IAHP using fluctuation analysis of the macroscopic current. Our results indicate that this channel has a unitary conductance of 2-5 pS and a mean open time of about 2 ms. When the peak amplitude of IAHP was maximal, these channels have an open probability of 0.4. Noradrenaline and carbachol reduced IAHP amplitude by lowering open channel probability. These result indicate that a novel calcium-activated potassium channel underlies IAHP. This channel is modulated in a similar fashion by two different transmitter systems that utilize distinct protein kinases.