Potassium currents in octopus cells of the mammalian cochlear nucleus

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 o ccurrence with temporal precision. The precision in the timing of action po tentials depends on the low input resistance (similar to6 M Omega) of octop us cells at the resting potential that makes voltage changes rapid (tau sim ilar to 200 mus). It is the activation of voltage-dependent conductances th at 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 o f two conductances that mediate currents whose reversal at or near the equi librium potential for K+ over a wide range of extracellular K+ concentratio ns 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 la rgely blocked by alpha -dendrotoxin (alpha -DTX) and partially blocked by D TX-K and tityus-toxin, indicating that this current was mediated through po tassium channels of the Kv1 (also known as shaker or KCNA) family. The maxi mum low-threshold K+ conductance (g(KL)) was large, 514 +/- 135 nS. Blockin g I-KL with alpha -DTX revealed a second K+ current with a higher threshold (I-KH) that was largely blocked by 20 mM tetraethylammonium (TEA). The mor e slowly inactivating conductance, g KH, had a threshold for activation at -40 mV, reached half-maximal activation at -16 +/-5 mV, and was fully activ ated at +30 mV. The maximum high-threshold conductance, g(KH), was on avera ge 116 +/- 27 nS. The present experiments show that it is not the biophysic al and pharmacological properties but the magnitude of the K+ conductances that make octopus cells unusual. At the resting potential, -62 mV, g(KL) co ntributes similar to 42 nS to the resting conductance and mediates a restin g K+ current of 1 nA. The resting outward K+ current is balanced by an inwa rd current through the hyperpolarization-activated conductance, g(h), that has been described previously.