MECHANISMS OF ONSET RESPONSES IN OCTOPUS CELLS OF THE COCHLEAR NUCLEUS - IMPLICATIONS OF A MODEL

The octopus cells of the posteroventral cochlear nucleus receive input s from auditory-nerve fibers and form one of the major ascending audit ory pathways. They respond to acoustic and electrical stimulation tran siently and are believed to carry temporal information in the precise timing of their action potentials. The mechanism whereby onset respons es are generated is not clear. Proposals aimed at elucidating the mech anism 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 pr esent study, we used a compartment model to investigate possible mecha nisms. The model cell contains a soma, an axon, and four passive dendr ites. Four kinds of ionic channels were included in the soma compartme nt: the Hodgkin-Huxley-like Na+ and K+ channels, a 4-AP-sensitive, low -threshold channel, K-LT, and a Cs+-sensitive, hyperpolarization-activ ated inward rectifier, I-h. DC currents and half-wave-rectified sinewa ves were used as stimuli. Our results showed that an onset response ca n be generated in the absence of neuronal circuitry of any form, thus suggesting that the onset response in octopus cells is regulated intri nsically. Among the many factors involved, low-input impedance, partly contributed by Ih, appears to be essential to the basic onset respons e 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 I-h 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.