IP_3-Activated Ion Channels in Olfactory Receptor Cell Membrane

为了阐明IP_3激活的离子通道作为效应离子通道在脊椎动物嗅觉传导中的作用，我们用膜片钳技术研究了不同脊椎动物嗅觉感受器神经元膜上IP_3通道的特性。结果如下：(1)IP_3以配体特异性的方式直接激活IP_3-通道，表明通道分子的磷酸化与其激活无关。(2)IP_3通道对IP_3的反应速度与cAMP激活的离子通道(cAMP通道)一样快。用微流灌流室测得的最短潜伏期IP_3通道为24毫秒，cAMP通道为5毫秒。(3)在IP_3作用过程中，大部分IP_3通道没有表现出脱敏作用，只有少数通道对IP_3有减敏作用。(4)在外液中测得IP_3通道的单位电导约为20ps。(5)IP_3通道不仅分布于乳房体膜，还分布于顶树突膜。单通道活动记录成功率的估计表明，被认为是气味受体的顶端树突膜的通道密度高于胞体膜。(6)大鼠感受器神经元对IP_3和cAMP全细胞反应的记录成功率比较表明，少数单个感受器神经元对IP_3有反应，提示较少的感受器神经元利用IP_3通路而不是cAMP通路进行嗅觉传导。因此，这些IP_3通道的性质与IP_3通道是脊椎动物嗅觉传导中IP_3通路的效应通道的标准是一致的。

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.