Department of Otolaryngology, Union Hospital of Tongji Medical College, Hua-Zhong University of Science and Technology, Wuhan 430022, People's Republic of China.
Hear Res. 2012 Mar;285(1-2):13-9. doi: 10.1016/j.heares.2012.02.003. Epub 2012 Feb 16.
There are two types of hair cells in the sensory epithelium of vestibular end organ. Type II vestibular hair cell (VHC II) is innervated by the efferent nerve endings, which employ a cholinergic inhibition mediated by SK channels through the activation of α9-containing nAChR. Our previous studies demonstrated that a BK-type cholinergic inhibition was present in guinea pig VHCs II, which may be mediated by an unknown mAChR. In this study, BK channel activities triggered by ACh were studied to determine the mAChR subtype and function. We found the BK channel was insensitive to α9-containing nAChR antagonists and m1, m3, m4 muscarinic antagonists, but potently inhibited by the m2 muscarinic antagonist. Muscarinic agonists could mimic the effect of ACh and be blocked by m2 antagonist. cAMP analog activated the BK current and adenyl cyclase (AC) inhibitor inhibited the ACh response. Inhibitor of Giα subunit failed to affect the BK current, but inhibitor of Giα and Giβγ subunits showed a potent inhibition to these currents. Our findings provide the physiological evidence that mAChRs may locate in guinea pig VHCs II, and m2 mAChRs may play a dominant role in BK-type cholinergic inhibition. The activation of m2 mAChRs may stimulate Giβγ-mediated excitation of AC/cAMP activities and lead to the phosphorylation of Ca(2+) channels, resulting in the influx of Ca(2+) and opening of the BK channel.
前庭感觉上皮中有两种毛细胞。Ⅱ型前庭毛细胞（VHC II）被传出神经末梢支配，传出神经末梢通过激活含有α9 的烟碱型乙酰胆碱受体（nAChR）来实现 SK 通道介导的胆碱能抑制。我们之前的研究表明，在豚鼠 VHC II 中存在 BK 型胆碱能抑制，其可能由未知的 mAChR 介导。在这项研究中，研究了 ACh 触发的 BK 通道活性，以确定 mAChR 亚型和功能。我们发现，BK 通道对含有 α9 的 nAChR 拮抗剂和 m1、m3、m4 毒蕈碱受体拮抗剂不敏感，但被 m2 毒蕈碱受体拮抗剂强烈抑制。毒蕈碱激动剂可模拟 ACh 的作用，并被 m2 拮抗剂阻断。cAMP 类似物激活 BK 电流，而腺苷酸环化酶（AC）抑制剂抑制 ACh 反应。Giα 亚基抑制剂不能影响 BK 电流，但 Giα 和 Giβγ 亚基抑制剂对这些电流具有强烈的抑制作用。我们的研究结果提供了生理证据，表明 mAChR 可能位于豚鼠 VHC II 中，并且 m2 mAChR 在 BK 型胆碱能抑制中可能发挥主导作用。m2 mAChR 的激活可能刺激 Giβγ 介导的 AC/cAMP 活性的兴奋，并导致 Ca（2+）通道的磷酸化，从而导致 Ca（2+）内流和 BK 通道的开放。
