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

鸟类的磁胞核（NMC）是耳蜗核中编码声音时间信息的核之一。较高的听核被认为是利用NMC的时间信息来检测耳际时间差来定位声音的来源。在这个项目中，我们研究了NMC神经元的膜特性和离子电流，使其能够携带声音的时间信息，使用全细胞记录从鸡胚胎的大脑stcm切片制备。在电流箝位条件下，NMC神经元表现出na依赖性动作电位，阈值约为- 44mv。在去极化电流刺激的初始阶段，神经元只产生一个固定时间的单一动作电位。这种兴奋性与当前刺激的强度和持续时间无关。这种特征的兴奋性似乎有助于传递声音的时间信息。在电压箝位条件下，神经元显示出ttx敏感的Na电流、低电压激活的（LVA） Ca电流和高电压（HVA） Ca电流。LVA Ca电流和HVA Ca电流分别对Ni （0.1mM）和硝苯地啶（20 muM）敏感。该神经元至少表现出tea敏感的K电流和4- ap敏感的K电流。在4-AP （0.5 mM）存在的情况下，NMC神经元从较低的阈值产生多个动作电位，以响应长时间的去极化电流注射。在小电流注入时，尖峰的出现显示出明显的波动。Ni （0.1 mM）阻止了4-AP对动作电位起作用的阈值和变异性的影响。在电压钳形实验中，4-AP在比正常动作电位阈值44mv更负的电位下发现了LVA Ca电流。我们已经得出结论，4- ap敏感的K电流克服了LVA - Ca电流，在突触输入的初始阶段设置了更高的阈值水平，从而产生单个动作电位。

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.