Coombs J, Thompson S
J Neurosci. 1987 Feb;7(2):443-52. doi: 10.1523/JNEUROSCI.07-02-00443.1987.
Forskolin, a diterpene extracted from Coleus forskolii, stimulates the production of cAMP in a variety of cells and is potentially an important tool for studying the role of cAMP in the modulation of neuronal excitability. We studied the effects of forskolin on neurons of nudibranch molluscs and found that it caused characteristic, reversible changes in the amplitude and waveform of the transient K current, IA, and also activated an inward current similar to the cAMP-dependent inward current previously described in molluscan neurons. Forskolin altered the time course of IA activation and inactivation but did not affect the voltage dependence or the reversal potential of the current. IA normally inactivates exponentially, but in forskolin the time course of inactivation can be fit by the sum of 2 exponentials with an initial rate that is faster than the control and a final rate that is much slower. On depolarization in forskolin, IA begins to activate at the normal rate, but a slower component of activation is also seen. The changes in IA in the nudibranch cells were qualitatively different than the changes caused by forskolin in Aplysia bag cell neurons (Strong, 1984). Experiments were performed to determine whether these effects of forskolin require cAMP. Intracellular injection of cAMP, application of membrane-permeable analogs of cAMP, application of phosphodiesterase inhibitors, and intracellular injection of the active catalytic subunit of cAMP-dependent protein kinase did not affect the amplitude or waveform of IA. Also, the changes in IA that are caused by forskolin were not prevented or reversed by intracellular injection of an inhibitor of cAMP-dependent protein kinase. Cyclic AMP did, however, activate inward current at voltages near the resting potential. We conclude that the changes in IA and the activation of inward current represent separate affects of forskolin. The inward current appears to depend on an increase in intracellular cAMP, while the changes in IA do not. These experiments show that, in addition to activating adenylate cyclase, forskolin may have a separate direct affect on the transient K current.
福斯高林是从毛喉鞘蕊花中提取的一种二萜类化合物，它能刺激多种细胞中cAMP的产生，可能是研究cAMP在调节神经元兴奋性中作用的重要工具。我们研究了福斯高林对裸鳃亚目软体动物神经元的影响，发现它会引起瞬时钾电流IA的幅度和波形发生特征性的、可逆的变化，还会激活一种内向电流，类似于之前在软体动物神经元中描述的cAMP依赖性内向电流。福斯高林改变了IA激活和失活的时间进程，但不影响电流的电压依赖性或反转电位。IA通常呈指数形式失活，但在福斯高林作用下，失活的时间进程可以用两个指数之和来拟合，初始速率比对照快，最终速率则慢得多。在福斯高林存在的情况下进行去极化时，IA开始以正常速率激活，但也会出现较慢的激活成分。裸鳃亚目细胞中IA的变化在性质上与福斯高林对海兔袋状细胞神经元所引起的变化不同（斯特朗，1984年）。进行了实验以确定福斯高林的这些作用是否需要cAMP。细胞内注射cAMP、应用cAMP的膜通透性类似物、应用磷酸二酯酶抑制剂以及细胞内注射cAMP依赖性蛋白激酶的活性催化亚基均未影响IA的幅度或波形。此外，细胞内注射cAMP依赖性蛋白激酶抑制剂并不能阻止或逆转福斯高林所引起的IA变化。然而，cAMP确实在静息电位附近的电压下激活内向电流。我们得出结论，IA的变化和内向电流的激活代表了福斯高林的不同作用。内向电流似乎依赖于细胞内cAMP的增加，而IA的变化则不然。这些实验表明，除了激活腺苷酸环化酶外，福斯高林可能对瞬时钾电流有单独的直接作用。
