The A-current: how ubiquitous a feature of excitable cells is it?

The membranes of most excitable cells contain a distinct set of potassium channels that rapidly open and close following depolarization, giving rise to a transient outward membrane current (I_A). This current seems to play an important role in allowing neurons to encode graded depolarization into spike train information; in action potential repolarization, particularly in cardiac muscle cells; and in several aspects of synaptic transmission. As a modifier of the responsiveness of postsynaptic neurons to synaptic potentials,I_A is a critical determinant of the integrative behavior of certain neural networks. Recent studies indicate that I_A can be physiologically modulated by synaptic influences and during learning. The discovery of Drosophila mutants where I_A is defective or absent provides the hope that this may soon be the first potassium channel amenable to structural analysis by molecular genetic techniques.