The Journal of Neurophysiology Vol. 86 No. 5 November 2001, pp. 2299-2311
Copyright ©2001 by the American Physiological Society

Potassium Currents in Octopus Cells of the Mammalian Cochlear Nucleus

Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706

Bal, Ramazan and Donata Oertel. Potassium Currents in Octopus Cells of the Mammalian Cochlear Nucleus. J. Neurophysiol. 86: 2299-2311, 2001. Octopus cells in the posteroventral cochlear nucleus (PVCN) of mammals are biophysically specialized to detect coincident firing in the population of auditory nerve fibers that provide their synaptic input and to convey its occurrence with temporal precision. The precision in the timing of action potentials depends on the low input resistance (~6 M) of octopus cells at the resting potential that makes voltage changes rapid ( ~ 200 µs). It is the activation of voltage-dependent conductances that endows octopus cells with low input resistances and prevents repetitive firing in response to depolarization. These conductances have been examined under whole cell voltage clamp. The present study reveals the properties of two conductances that mediate currents whose reversal at or near the equilibrium potential for K⁺ over a wide range of extracellular K⁺ concentrations identifies them as K⁺ currents. One rapidly inactivating conductance, g_KL, had a threshold of activation at 70 mV, rose steeply as a function of depolarization with half-maximal activation at 45 ± 6 mV (mean ± SD), and was fully activated at 0 mV. The low-threshold K⁺ current (I_KL) was largely blocked by -dendrotoxin (-DTX) and partially blocked by DTX-K and tityustoxin, indicating that this current was mediated through potassium channels of the Kv1 (also known as shaker or KCNA) family. The maximum low-threshold K⁺ conductance (g_KL) was large, 514 ± 135 nS. Blocking I_KL with -DTX revealed a second K⁺ current with a higher threshold (I_KH) that was largely blocked by 20 mM tetraethylammonium (TEA). The more slowly inactivating conductance, g_KH, had a threshold for activation at 40 mV, reached half-maximal activation at 16 ± 5 mV, and was fully activated at +30 mV. The maximum high-threshold conductance, g_KH, was on average 116 ± 27 nS. The present experiments show that it is not the biophysical and pharmacological properties but the magnitude of the K⁺ conductances that make octopus cells unusual. At the resting potential, 62 mV, g_KL contributes ~42 nS to the resting conductance and mediates a resting K⁺ current of 1 nA. The resting outward K⁺ current is balanced by an inward current through the hyperpolarization-activated conductance, g_h, that has been described previously.