Mechanisms of onset responses in octopus cells of the cochlear nucleus: implications of a model

The octopus cells of the posteroventral cochlear nucleus receive inputs from auditory-nerve fibers and form one of the major ascending auditory pathways. They respond to acoustic and electrical stimulation transiently and are believed to carry temporal information in the precise timing of their action potentials. The mechanism whereby onset responses are generated is not clear. Proposals aimed at elucidating the mechanism range from neural circuitry and/or inhibition, "depolarization block" (or inactivation of Na+ channels), and the involvement of a 4-aminopyridine (4-AP)-sensitive, low-threshold channel (K(LT)). In the present study, we used a compartment model to investigate possible mechanisms. The model cell contains a soma, an axon, and four passive dendrites. Four kinds of ionic channels were included in the soma compartment: the Hodgkin-Huxley-like Na+ and K+ channels, a 4-AP-sensitive, low-threshold channel, K(LT), and a Cs+-sensitive, hyperpolarization-activated inward rectifier, Ih. DC currents and half-wave-rectified sine waves were used as stimuli. Our results showed that an onset response can be generated in the absence of neuronal circuitry of any form, thus suggesting that the onset response in octopus cells is regulated intrinsically. Among the many factors involved, low-input impedance, partly contributed by Ih, appears to be essential to the basic onset response pattern; also, the K(LT) conductance plays a major role, whereas the inactivation of Na+ channels probably plays only a secondary role. The dynamics of Ih also can modify the response pattern, but due to its slow kinetics, its role is probably limited to longer-term regulation under the conditions simulated in this study.