The sensitivities of mammalian myelinated axons to potassium channel blockers was studied over the course of development using in vitro sucrose gap and intra-axonal recording techniques. 2. Application of 4-aminopyridine (4-AP; 1.0 mM) to young nerves led to a delay in return to base line of the sciatic nerve compound action potential and to a postspike positivity (indicative of hyperpolarization) lasting for tens of milliseconds. These effects were very much attenuated during the course of maturation. 3. Tetraethylammonium chloride (TEA; 10 mM) application alone had little effect on the waveform of the compound action potential at any age. However, the 4-AP-induced postspike positivity was blocked by TEA, Ba/+, and Cs+. This block was observed in Ca2+-free electrolyte solutions containing EGTA (1.0 mM). 4. Immature sciatic nerves (approximately 3 wk postnatal) were incubated in a potassium-free electrolyte solution containing 120 mM CsCl for up to 1 h in an attempt to replace internal potassium with cesium. When the nerves were tested in the sucrose gap chamber using solutions containing 3.0 mM CsCl substituted for KCl, the compound action potential was broadened and a prolonged depolarization appeared, but there was no postspike positivity; the CsCl effect was similar to the combined effects of 4-AP and TEA. 5. Intra-axonal recordings were obtained to study the effects of 4-AP and TEA on individual axons. In the presence of 4-AP a single stimulus led to a burst of action potentials followed by a pronounced afterhyperpolarization (AHP) in sensory fibers. The AHP was blocked by TEA. In motor fibers 4-AP application resulted in action potential broadening with no AHP. 6. Repetitive stimulation (200-500 Hz; 100 ms) was followed by a pronounced AHP in both sensory and motor fibers at all ages studied. This activity-elicited AHP was sensitive to TEA at all ages. 7. The results indicate that 4-AP and TEA sensitivity change over the course of development in rat sciatic nerve. The effects of 4-AP are much more pronounced in immature nerves than in mature nerves, suggesting that 4-AP-sensitive channels become masked as they are covered by myelin during maturation. However, the TEA-sensitive channels, demonstrable after repetitive firing, remain accessible to TEA after myelination. These channels therefore may have a nodal representation.
摘要
利用体外蔗糖间隙和轴突内记录技术，研究了哺乳动物有髓轴突在发育过程中对钾通道阻滞剂的敏感性。2. 将4-氨基吡啶（4-AP；1.0 mM）应用于幼嫩神经，导致坐骨神经复合动作电位恢复到基线的延迟，并出现持续数十毫秒的锋后正电位（提示超极化）。在成熟过程中，这些效应大大减弱。3. 单独应用氯化四乙铵（TEA；10 mM）在任何年龄对复合动作电位的波形几乎没有影响。然而，4-AP诱导的锋后正电位被TEA、Ba²⁺和Cs⁺阻断。在含有EGTA（1.0 mM）的无钙电解质溶液中观察到这种阻断。4. 将未成熟的坐骨神经（出生后约3周）在含有120 mM CsCl的无钾电解质溶液中孵育长达1小时，试图用铯替代细胞内钾。当在蔗糖间隙室中使用含有3.0 mM CsCl替代KCl的溶液对神经进行测试时，复合动作电位变宽并出现延长的去极化，但没有锋后正电位；CsCl的作用类似于4-AP和TEA的联合作用。5. 进行轴突内记录以研究4-AP和TEA对单个轴突的影响。在存在4-AP的情况下，单个刺激导致感觉纤维中动作电位的爆发，随后是明显的超极化后电位（AHP）。AHP被TEA阻断。在运动纤维中，应用4-AP导致动作电位变宽且无AHP。6. 在所有研究的年龄组中，重复刺激（200 - 500 Hz；100 ms）后，感觉和运动纤维中均出现明显的AHP。这种活动诱发的AHP在所有年龄对TEA均敏感。7. 结果表明，大鼠坐骨神经在发育过程中4-AP和TEA敏感性发生变化。4-AP的作用在未成熟神经中比在成熟神经中更明显，这表明4-AP敏感通道在成熟过程中被髓鞘覆盖时变得被掩盖。然而，在重复发放后可证明的TEA敏感通道，在髓鞘形成后仍可被TEA作用。因此，这些通道可能有结区的表现。
