ON THE SIGNAL TRANSDUCTION MECHANISMS AT THE SECONDARY AND THE TERTIARY COCHLEAR NUCLEL

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Avian magno cellular nucleus (NMC) is one of cochlear nuclei which is known to encode the temporal information of sound. The higher auditory nuclei are supposed to detect interaural time difference to locate a sound origin using the temporal information from NMC.In this project, we investigated the membrane properties and ionic currents of the NMC neuron which enable to carry the temporal information of sound using a whole-cell recording form brain stcm slice preparation of a chick embryo. Under current-clamp condition, NMC neuron showed Na-dependent action potentials with a threshold of about-44mV.The neuron generated only a single action potential with a fixed timing at the initial phase of depolarizing current stimuli. This excitable nature was independent from the intensity and the duration of the current stimuli. This characteristic excitability is seemed to be useful to carry the temporal information of the sound. In voltage-clamp condition, the neurone revealed a TTX-sensitive Na current, a low voltage activated (LVA) Ca current and a high voltage (HVA) Ca current. The LVA Ca current and the HVA Ca current were sensitive to Ni (0.1mM) and nifedipime (20 muM) , respectively. The neuron showed at least a TEA-sensitive K.current and a 4-AP-sensitive K current. In the presence of 4-AP (0.5 mM) , the NMC neuron generated multiple action potentials from a lower threshold, in response to long depolarizing current injections. The onset of spikes showed marked fluctuation at smaller current injections. Ni (0.1 mM) prevented effects of 4-AP on the threshold value and the variability of action potential onsets. In voltage clamp experiment, 4-AP uncovered the LVA Ca current at more negative potential than the normal action potential threshold of-44mV.We have conchuded that a 4-AP-sensitive K current overcomes the LVA Ca current to set a higher threshold level to generate a single action potential at the initial phase of synaptic inputs.