Daló N L, Hackman J C, Storey K, Davidoff R A
Department of Neurology, University of Miami School of Medicine, FL 33101, USA.
J Exp Biol. 1995 Aug;198(Pt 8):1765-74. doi: 10.1242/jeb.198.8.1765.
The effects of sudden cooling of the spinal cord were studied in three species of amphibians--a cold-sensitive tropical toad (Bufo marinus), a cold-resistant, aquatic, hibernating frog (Rana pipiens, northern leopard frog) and a freeze-tolerant frog (Rana sylvatica, wood frog). Ventral root (motoneuron) potentials were recorded from isolated, hemisected spinal cords of each species mounted in a sucrose-gap recording apparatus and superfused with HCO3(-)-buffered Ringer's solution at room temperature (21 degrees C). In the toad, sudden cooling to 6-8 degrees C produced large, sustained motoneuron depolarizations that returned slowly to baseline levels and were accompanied by extensive paroxysmal activity. Larger, but shorter-lasting, motoneuron depolarizations associated with only a limited amount of paroxysmal activity were generated by rapid cooling of the leopard frog spinal cord. Small, brief motoneuron depolarizations followed by a hyperpolarization, or hyperpolarizations not preceded by depolarizations, were seen in cooled wood frog spinal cords. The wood frog displayed a large amount of spontaneous motoneuron activity, but little paroxysmal activity in response to sudden cooling. Following prolonged cooling, rewarming the spinal cords of all three species resulted in motoneuron hyperpolarizations that slowly decayed towards the baseline value. The amplitude of the rewarming-induced response was larger and longer in toad motoneurons than in leopard frog and wood frog motoneurons. At room temperature, a single supramaximal dorsal root stimulus evoked a depolarizing ventral root potential in toad and leopard frog motoneurons that was decreased in amplitude and prolonged when the spinal cords were cooled to 8 degrees C or below. In contrast, at room temperature, the ventral root reflex in the wood frog was followed by a distinct hyperpolarization. Cooling the wood frog spinal cord only slightly reduced the amplitude of the ventral root potential. In contrast, the evoked hyperpolarization was blocked by sudden cooling and also by the addition of dihydro-ouabain to the Ringer's solution. The motoneuron hyperpolarizations induced by sudden cooling in the wood frog were converted to depolarizations when Cl- in the superfusate was replaced with isethionate. The depolarizations elicited by sudden cooling were reduced by the addition of kynurenate in all three species. A dose-response curve generated by short applications of L-glutamate demonstrated that wood frog motoneurons were less sensitive than leopard frog motoneurons to L-glutamate. In summary, three species of amphibians, differing in their adaptations to the temperature of their environments, vary in their responses to sudden reductions in temperature. The relationship of these responses to their environmental adaptations remains to be determined.
在三种两栖动物中研究了脊髓突然降温的影响，这三种两栖动物分别是：对寒冷敏感的热带蟾蜍（海蟾蜍）、耐寒的水生冬眠青蛙（北美豹蛙）和耐冻青蛙（林蛙）。从安装在蔗糖间隙记录装置中的每个物种的离体半切脊髓记录腹根（运动神经元）电位，并在室温（21摄氏度）下用HCO₃⁻缓冲的林格氏液进行灌流。在蟾蜍中，突然降温至6 - 8摄氏度会产生大的、持续的运动神经元去极化，这种去极化缓慢恢复到基线水平，并伴有广泛的阵发性活动。快速冷却豹蛙脊髓会产生较大但持续时间较短的运动神经元去极化，且仅伴有有限的阵发性活动。在冷却的林蛙脊髓中可见小的、短暂的运动神经元去极化，随后是超极化，或者是无前去极化的超极化。林蛙表现出大量的自发运动神经元活动，但对突然降温的阵发性活动很少。长时间冷却后，对所有三个物种的脊髓进行复温会导致运动神经元超极化，这种超极化会缓慢衰减至基线值。复温诱导反应的幅度在蟾蜍运动神经元中比在豹蛙和林蛙运动神经元中更大且持续时间更长。在室温下，单个超强的背根刺激在蟾蜍和豹蛙运动神经元中诱发去极化的腹根电位，当脊髓冷却至8摄氏度或更低时，该电位幅度减小且持续时间延长。相比之下，在室温下，林蛙的腹根反射之后是明显的超极化。稍微冷却林蛙脊髓只会略微降低腹根电位的幅度。相反，诱发的超极化会被突然冷却以及向林格氏液中添加二氢哇巴因所阻断。当灌流液中的Cl⁻被羟乙磺酸盐取代时，林蛙中突然冷却诱导的运动神经元超极化会转变为去极化。在所有三个物种中，添加犬尿烯酸都会降低突然冷却引起的去极化。通过短暂应用L - 谷氨酸产生的剂量反应曲线表明，林蛙运动神经元对L - 谷氨酸的敏感性低于豹蛙运动神经元。总之，三种两栖动物，在对环境温度的适应性方面存在差异，它们对温度突然降低的反应也各不相同。这些反应与其环境适应性的关系仍有待确定。
