IP_3-Activated Ion Channels in Olfactory Receptor Cell Membrane

为了阐明IP_3激活离子通道（IP_3-channels）作为效应离子通道在脊椎动物嗅觉信号转导中的作用，我们用膜片钳技术研究了不同脊椎动物（蛙、大鼠和小鼠）嗅觉受体神经元膜上IP_3-通道的特性。结果表明：（1）IP_3以配体特异性方式直接激活IP_3通道，表明通道分子的磷酸化与其激活无关。(2)IP_3通道与cAMP激活的离子通道（cAMP-channels）对IP_3的反应一样快。用微流灌流室测得的最短潜伏期为IP_3通道24毫秒和cAMP通道5毫秒。(3)大多数IP_3通道在IP_3作用下不发生脱敏。只有少数通道表现出对IP_3的脱敏作用。(4)在Ba^<2+>-外溶液中测定的IP_3-通道的单位电导约为1.25 × 10 ~（-3）。20 pS。(5)IP_3通道不仅分布于索马膜，而且也分布于树突顶膜。单通道记录成功率的估计表明，在顶端树突膜，这被认为是气味的接受部位，通道密度高于索马膜。(6)比较大鼠受体神经元对IP_3和cAMP的全细胞反应的成功率发现，对IP_3有反应的受体神经元的数量较少，这表明利用IP_3通路而不是cAMP通路进行嗅觉传导的受体神经元的数量较少。因此，这些IP_3通道的特性与IP_3通道作为脊椎动物嗅觉传递IP_3通路的效应通道的标准相一致。

To elucidate their role of IP_3-activated ion channels (IP_3-channels) as the effector ion channels in vertebrate olfactory transduction, we have studied the properties of IP_3-channels in the membrane of olfactory receptor neurons from different vertebrate species, frogs, rats and mice, by means of patchclamp techniques. Results are summarised as follows : (1) IP_3-channels were activated directly by IP_3 in a ligand specific manner, indicating that phosphorlylation of the channel molecule is not associated with its activation. (2) IP_3-channels responded to IP_3 as quickly as cAMP-activated ion channels (cAMP-channels) did. The shortest latency measured with a microflow superfusion chamber was 24 msec for IP_3-channels and 5 msec for cAMP-channels. (3) Most of IP_3-channels did not show a desensitization during the application of IP_3. Only a few channels showed a desensitization to IP_3. (4) The unit conductance of IP_3-channels as determined in Ba^<2+>-external solution was ca. 20pS.(5) IP_3-channels were distributed not only in soma mambrane but also in apical dendrite membrane. The estimation by the success rate for recording of single channel activities suggested that the channel density is higher in apical dendrite membrane, which is thought to be the receptive site for odorants, than in soma membrane. (6) The comparison of success rate for recording of whole-cell responses to IP_3 and cAMP in rat receptor neurons showed that a smaller population of individual receptor neurons responded to IP_3, suggesting that a smaller population of individual receptor neurons utilize the IP_3-rather than the cAMP-pathway for olfactory transduction. Consequently, the properties of these IP_3-channels agree well with the criteria for IP_3-channels to be the effector channels for the IP_3-pathway in vertebrate olfactory transduction.