Fields R D, Guthrie P B, Russell J T, Kater S B, Malhotra B S, Nelson P G
Laboratory of Developmental Neurobiology, National Institute of Health, NICHD, Bethesda, Maryland 20892.
J Neurobiol. 1993 Aug;24(8):1080-98. doi: 10.1002/neu.480240807.
Electrical stimulation causes growth cones of mouse dorsal root ganglion neurons to collapse. During chronic stimulation, however, growth cones resume motility. In addition, these growth cones are now resistant to the collapsing effects of subsequent stimulation, a process we term accommodation. We compared the kinetics of electrically induced Ca2+ transients in naive and accommodated growth cones in order to determine whether the accommodation process results from a change in the Ca2+ transient, or a change in the Ca2+ sensitivity of the growth cones. Three kinetics were determined: (1) the initial increase to peak Ca2+ levels produced by 10 Hz stimulation; (2) recovery from peak Ca2+ levels during stimulus trains lasting 15 min; and (3) clearing of Ca2+ from growth cones after terminating the stimulus. These kinetics were analyzed using single exponential fits to changes in fura-2 fluorescence ratios. The electrically evoked increase in Ca2+ was significantly slower in accommodated growth cones (tau = 6.0 s) compared to naive growth cones (tau = 1.4 s). Despite the slower increase of [Ca2+]i in accommodated growth cones, peak [Ca2+]i was similar to that reached in naive growth cones, and the steady-state Ca2+ level was significantly elevated after chronic stimulation. Thus, accommodated growth cones maintained outgrowth at [Ca2+]i that caused collapse initially. Time course experiments show that accommodation is a slow process (t 1/2 = about 3 h). Accommodation did not induce measurable changes in the rates of Ca2+ homeostasis during or after stimulus trains. The kinetics of Ca2+ recovery during (tau = 90 s) and after 15 min of stimulation (tau = 8.5 s) was not significantly different in accommodated versus naive growth cones. Rates of 45Ca2+ efflux were also similar in both types of growth cones. These results suggest two regulatory processes contributing to growth cone motility during chronic stimulation: (1) recovery of [Ca2+]i to levels permissive to neurite outgrowth, and (2) an increase in the range of optimal [Ca2+]i for growth cone motility. These adaptive responses of mammalian growth cones to chronic stimulation could be involved in the modulation of CNS development by electrical activity of neurons.
电刺激会导致小鼠背根神经节神经元的生长锥塌陷。然而，在慢性刺激期间，生长锥会恢复运动能力。此外，这些生长锥现在对随后刺激的塌陷效应具有抗性，我们将这一过程称为适应。我们比较了未适应和适应后的生长锥中电诱导的Ca2+瞬变动力学，以确定适应过程是由Ca2+瞬变的变化引起的，还是由生长锥对Ca2+的敏感性变化引起的。确定了三种动力学：（1）10Hz刺激产生的Ca2+水平从初始增加到峰值；（2）在持续15分钟的刺激序列中从Ca2+峰值水平恢复；（3）刺激终止后生长锥中Ca2+的清除。使用对fura-2荧光比率变化的单指数拟合来分析这些动力学。与未适应的生长锥（τ = 1.4秒）相比，适应后的生长锥中电诱发的Ca2+增加明显较慢（τ = 6.0秒）。尽管适应后的生长锥中[Ca2+]i的增加较慢，但[Ca2+]i峰值与未适应的生长锥中达到的峰值相似，并且慢性刺激后稳态Ca2+水平显著升高。因此，适应后的生长锥在最初会导致塌陷的[Ca2+]i水平下维持生长。时间进程实验表明，适应是一个缓慢的过程（t1/2约为3小时）。在刺激序列期间或之后，适应并未在Ca2+稳态速率上诱导可测量的变化。适应后的生长锥与未适应的生长锥相比，在刺激期间（τ = 90秒）和刺激15分钟后（τ = 8.5秒）Ca2+恢复的动力学没有显著差异。两种类型的生长锥中45Ca2+外流速率也相似。这些结果表明在慢性刺激期间有两个调节过程有助于生长锥的运动：（1）[Ca2+]i恢复到允许神经突生长的水平，以及（2）生长锥运动的最佳[Ca2+]i范围增加。哺乳动物生长锥对慢性刺激的这些适应性反应可能参与神经元电活动对中枢神经系统发育的调节。
