IONIC MECHANISMS OF FUNCTIONAL MATURATION OF THE BUCCAL STRETCH RECEPTOR

颊侧牵张感受器（buccal stretch receptor，BSR）是一种位于啮齿类动物下颌闭合肌表面的机械感受器。大鼠出生后10天至3周，BSR的阈值牵拉幅度下降，表现出持续放电福尔斯，出生后2至4周，其静态敏感性增加。本研究探讨了钾离子通道的感觉神经末梢的BSR的功能成熟的贡献。2周龄后，在BSR的囊腔中识别出与透明质酸抗体的免疫反应性。当透明质酸酶应用于切除外囊的BSR时，BSR的静态反应消失。这些发现表明，透明质酸在囊腔调节的静态敏感性的成熟的BSR的离子机制。当钾通道阻滞剂对其放电的影响进行了分析，四乙基铵（TEA，0.1-1 mM）抑制的放电频率的所有BSR单位测试的响应范围。在较高浓度（1-10 mM）下，TEA使BSR的响应具有阶段性。4-氨基吡啶（0.1-1 mM）在所有测试单位中诱导自发放电，并以剂量依赖性方式抑制静态敏感性。这些结果表明，BSR神经末梢编码部位的延迟整流电流引起该受体的静态反应，而类似部位的A型电流可能影响其静态敏感性。从1周龄大鼠分离的BSRs的免疫组化显示阳性标记的抗体对Kv1.4亚基的A型钾通道的前终末无髓鞘轴突。3周龄后，Kv1.4仅在轴突的节间段检测到，表明终末前轴突中的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.