POTASSIUM CURRENTS AND EXCITABILITY IN 2ND-ORDER AUDITORY AND VESTIBULAR NEURONS

Potassium channels are involved in the control of neuronal excitabilit y by fixing the membrane potential, shaping the action potential, and setting firing rates. Recently, attention has been focused on identify ing the factors influencing excitability in second-order auditory and vestibular neurons. Located in the brainstem, second-order auditory an d vestibular neurons are sites for convergence of inputs from first-or der auditory or vestibular ganglionic cells with other sensory systems and also motor areas. Typically, second-order auditory neurons exhibi t two distinct firing patterns in response to depolarization: tonic, w ith a repetitive firing of action potentials, and phasic, characterize d by only one or a few action potentials. In contrast, all mature vest ibular second-order neurons fire tonically on depolarization. Already, certain fundamental roles have emerged for potassium currents in thes e neurons. In mature auditory and vestibular neurons, IK, the delayed rectifier, is required for the fast repolarization of action potential s. In tonically firing auditory neurons, Iq, the transient outward rec tifier, defines the discharge pattern, Ins, a delayed rectifier-like c urrent distinguished by its low threshold of activation, is found in p hasically firing auditory and some developing vestibular neurons where it limits firing to one or a few spikes, and also may contribute to f orming short-duration excitatory postsynaptic potential (EPSPs). Also, I-Ds sets the threshold for action potential generation rather high, which may prevent spontaneous discharge in phasically firing cells. Du ring development, there is a gradual acquisition and loss of some pota ssium conductances, suggesting developmental regulation. As there are similarities in membrane properties of second-order auditory and vesti bular neurons, investigations on firing pattern and its underlying mec hanisms in one system should help to uncover fundamental properties of the other. J.. Neurosci. Res, 53:511-520, 1998. (C) 1998 Wiley-Liss. Inc.