IONIC MECHANISMS OF FUNCTIONAL MATURATION OF THE BUCCAL STRETCH RECEPTOR

颊牵张感受器(BSR)是一种机械感受器，位于啮齿动物颌骨闭合肌肉表面。在大鼠中，BSR显示持续放电的阈值拉伸幅度从出生后10天下降到3周，其静态敏感性在出生后2周到4周增加。本研究探讨了BSR感觉神经末梢的钾通道对其功能成熟的作用。2周龄后，BSR的囊腔内可见透明质酸抗体的免疫反应。当透明质酸酶应用于切除外囊的BSR时，BSR的静态反应消失。这些发现提示，囊腔中的透明质酸通过离子机制调节BSR静态敏感性的成熟。当分析钾通道阻滞剂对其放电的影响时，四乙基铵(TEA，0.1-1 mM)抑制了所有被测BSR单位在响应范围内的放电频率。在较高浓度(1-10 mM)时，TEA使BSR的反应呈阶段性。4-氨基吡啶(0.1-1 mM)可引起所有受试单位的自发放电，并呈剂量依赖性抑制静态敏感性。这些结果表明，BSR神经末梢编码部位的延迟整流电流诱导了该受体的静态反应，类似部位的A型电流可能影响其静态敏感性。从1周龄大鼠分离的BSR的免疫组织化学显示，抗A型钾通道Kv1.4亚单位的抗体在终末前无髓轴突呈阳性标记。3周龄时，仅在轴突的节间节段检测到Kv1.4，提示终末前轴突的A型钾通道被髓鞘的形成所覆盖。

The buccal stretch receptor (BSR) is a mechanoreceptor that is located on the surface of the jaw closing muscles of rodents. In rats, the threshold stretch amplitude of the BSR to exhibit sustained discharges falls from 10 days to 3 weeks after birth and its static sensitivity increases between 2 and 4 weeks after birth. This investigation examined contribution of potassium channels in the sensory nerve endings of the BSR to its functional maturation. Immunoreactivity with the antibody to hyaluronate was recognized in the capsular space of the BSRs after 2 weeks of age. When hyaluronidase was applied to the BSR whose outer capsule was resected, the static response of the BSR was disappeared. These findings suggest that hyaluronate in the capsular space regulates the maturation of static sensitivity of the BSRs by ionic mechanisms. When effects of potassium channel blockers on their discharges were analyzed, tetraethylammonium (TEA, 0.1-1 mM) depressed the frequency of discharge of all BSR units tested over the response range. At higher concentrations (1-10 mM), TEA made the response of BSRs phasic. 4-aminopyridine (0.1-1 mM) induced spontaneous discharge in all units tested and depressed the static sensitivity in dose-dependent manner. These findings suggest that the delayed rectifier current at the encoding site of nerve endings of the BSR induces the static response of this receptor, and that the A-type current at a similar site may influence its static sensitivity. Immunohistochemistry on BSRs isolated from 1 week old rats revealed positive labeling with an antibody against Kv1.4 subunit of A-type potassium channel in preterminal unmyelinated axons. After 3 weeks of age, Kv1.4 was detected only in internodal segments of the axons, suggesting A-type potassium channels in preterminal axons were covered by formation of myelin sheaths.