POSTSYNAPTIC TRANSDUCTION IN THE COCHLEAR NUCLEUS

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This project investigates the membrane properties of neurons from the avian cochlear nucleus, the nucleus magnocellularis (NM). The NM contains the cells which are homologous to the bushy cells of the mammalian anteroventral cochlear nucleus. NM neurons project to nucleus laminaris neurons which underlie the spatial localization of sound by generating action potentials in response to appropriately timed inputs from the ipsilateral and contralateral NM. In order for accurate localization of sound, NM neurons must relay precise timing information about the auditory stimulus. Stimulation of the auditory nerve results in rapid volleys of EPSPs in NM neurons. Since NM neurons fire action potentials in response to AC inputs but not to DC inputs, temporal summation of the EPSPs would not produce firing of action potentials in NM neurons. We have shown that there is a K+ current in NM neurons which causes the large synaptic potentials to decay rapidly with minimal temporal summation. The NM is tonotopically organized: neurons in the posterolateral NM receive auditory nerve inputs activated by the lowest sound frequencies while those in the anteromedial NM receive inputs activated by the highest sound frequencies. An intriguing question is whether there are differences in the membrane properties that permit anteromedial NM neurons to respond best to high frequency stimuli and posterolateral NM neurons to respond best to low frequency stimuli. We have evidence that the membrane properties do indeed vary because action potential threshold is more depolarized in NM neurons from the anterior region of the nucleus than neurons from the posterior region. The aim of this proposal is to determine how transduction properties of NM neurons vary along the tonotopic axis. The hypotheses to be tested are: I. Hypothesis: NM neurons contain several different types of K+ channels and their expression varies along the tonotopic axis. 2. Hypothesis: The density and/or characteristics of Na+ channels vary along the tonotopic axis of NM. 3. Hypothesis: The tonotopic variation of NM neuron membrane properties results in response properties that differ along the tonotopic axis. The electrotonically compact NM neurons are ideal central neurons for voltage-clamp studies. The K+ and Na+ conductances will be isolated in vitro and their contribution to post-synaptic transduction will be mapped with respect to the position of the cells on the tonotopic axis. This study will provide basic information about how the auditory system works. It will also provide a foundation for future investigations about how ion channel expression is regulated over space.