Ungless Mark A, Gasull Xavier, Walters Edgar T
Department of Integrative Biology and Pharmacology, University of Texas-Houston Medical School, Houston, Texas 77030, USA.
J Neurophysiol. 2002 May;87(5):2408-20. doi: 10.1152/jn.2002.87.5.2408.
In many neurons, axotomy triggers long-lasting alterations in excitability as well as regenerative growth. We have investigated mechanisms contributing to the expression of axotomy-induced, long-term hyperexcitability (LTH) of mechanosensory neurons in Aplysia californica. Electrophysiological tests were applied to pleural sensory neurons 5-10 days after unilateral crush of pedal nerves. Two-electrode current-clamp experiments revealed that compared with uninjured sensory neurons on the contralateral side of the body, axotomized sensory neurons consistently displayed alterations of passive membrane properties: notably, increases in input resistance (R(in)), membrane time constant (tau), and apparent input capacitance. In some cells, axotomy also depolarized the resting membrane potential (RMP). Axotomized sensory neurons showed a lower incidence of voltage relaxation ("sag") during prolonged hyperpolarizing pulses and greater depolarizations during long (2 s) but not brief (20 ms) pulses. In addition to a reduction in spike accommodation, axotomized sensory neurons displayed a dramatic decrease in current (rheobase) required to reach spike threshold during long depolarizations. The increase in tau was associated with prolongation of responses to brief current pulses and with a large increase in the latency to spike at rheobase. Two-electrode voltage-clamp revealed an axotomy-induced decrease in a current with two components: a leakage current component and a slowly activating, noninactivating outward current component. Neither component was blocked by agents known to block other K(+) currents in these neurons. In contrast to the instantaneous leakage current seen with hyperpolarizing and depolarizing steps, the late component of the axotomy-sensitive outward current showed a relatively steep voltage dependence with pulses to V(m) > -40 mV. These features match those of the S-type ("serotonin-sensitive") K(+) current, I(K,S). The close resemblance of I(K,S) to a background current mediated by TREK-1 (KCNK2) channels in mammals, raises interesting questions about alterations of this family of channels during axotomy-induced LTH in both Aplysia and mammals. The increase in apparent C(in) may be a consequence of the extensive sprouting that has been observed in axotomized sensory neurons near their somata, and the decrease in I(K,S) probably helps to compensate for the decrease in excitability that would otherwise occur as new growth causes both cell volume and C(in) to increase. In peripheral regions of the sensory neuron, a decrease in I(K,S) might enhance the safety factor for conduction across regenerating segments that are highly susceptible to conduction block.
在许多神经元中，轴突切断会引发兴奋性以及再生生长的长期改变。我们研究了加州海兔机械感觉神经元轴突切断诱导的长期兴奋性增高（LTH）表达的相关机制。在单侧挤压足神经5 - 10天后，对胸膜感觉神经元进行电生理测试。双电极电流钳实验显示，与身体对侧未损伤的感觉神经元相比，轴突切断的感觉神经元始终表现出被动膜特性的改变：显著的是，输入电阻（R(in)）、膜时间常数（tau）和表观输入电容增加。在一些细胞中，轴突切断还使静息膜电位（RMP）去极化。轴突切断的感觉神经元在长时间超极化脉冲期间电压松弛（“下垂”）的发生率较低，在长（2秒）而非短（20毫秒）脉冲期间去极化程度更大。除了峰电位适应减少外，轴突切断的感觉神经元在长去极化期间达到峰电位阈值所需的电流（基强度）显著降低。tau的增加与对短电流脉冲的反应延长以及基强度下峰电位潜伏期的大幅增加有关。双电极电压钳显示轴突切断诱导一种具有两个成分的电流减少：一个漏电流成分和一个缓慢激活且非失活的外向电流成分。已知在这些神经元中阻断其他钾离子电流的药物均不能阻断这两个成分。与超极化和去极化步骤时的瞬时漏电流不同，轴突切断敏感外向电流的晚期成分在向V(m) > -40 mV的脉冲时表现出相对陡峭的电压依赖性。这些特征与S型（“5-羟色胺敏感”）钾离子电流I(K,S)的特征相符。I(K,S)与哺乳动物中由TREK-1（KCNK2）通道介导的背景电流非常相似，这引发了关于在加州海兔和哺乳动物中轴突切断诱导的LTH过程中该通道家族改变的有趣问题。表观C(in)的增加可能是轴突切断的感觉神经元在其胞体附近广泛发芽的结果，而I(K,S)的减少可能有助于补偿由于新生长导致细胞体积和C(in)增加而可能发生的兴奋性降低。在感觉神经元的外周区域，I(K,S)的减少可能会提高跨高度易发生传导阻滞的再生节段传导的安全系数。
