Sinha Anjali K, Lee Choongheon, Holt Joseph C
bioRxiv. 2024 Jan 1:2023.12.30.573731. doi: 10.1101/2023.12.30.573731.
Primary vestibular afferents transmit information from hair cells about head position and movement to the CNS, which is critical for maintaining balance, gaze stability and spatial navigation. The CNS, in turn, modulates hair cells and afferents via the efferent vestibular system (EVS) and its activation of several cholinergic signaling mechanisms. Electrical stimulation of EVS neurons gives rise to three kinetically- and mechanistically-distinct afferent responses including a slow excitation, a fast excitation, and a fast inhibition. EVS-mediated slow excitation is attributed to odd-numbered muscarinic acetylcholine receptors (mAChRs) on the afferent whose activation leads to the closure of a potassium conductance and increased afferent discharge. Likely effector candidates include low-threshold, voltage-gated potassium channels belonging to the KCNQ (Kv7.X) family, which are involved in neuronal excitability across the nervous system and are subject to mAChR modulation. Specifically, KCNQ2/3 heteromeric channels may be the molecular correlates for the M-current, a potassium current that is blocked following the activation of odd-numbered mAChRs. To this end, multiple members of the KCNQ channel family, including KCNQ2 and KCNQ3, are localized to several microdomains within vestibular afferent endings, where they influence afferent excitability and could be targeted by EVS neurons. Additionally, the relative expression of KCNQ subunits appears to vary across the sensory epithelia and among different afferent types. However, it is unclear which KCNQ channel subunits are targeted by mAChR activation and whether that also varies among different afferent classes. Here we show that EVS-mediated slow excitation is blocked and enhanced by the non-selective KCNQ channel blocker XE991 and opener retigabine, respectively. Using KCNQ subunit-selective drugs, we observed that a KCNQ2 blocker blocks the slow response in irregular afferents, while a KCNQ2/3 opener enhances slow responses in regular afferents. The KCNQ2 blockers did not appear to affect resting afferent discharge rates, while KCNQ2/3 or KCNQ2/4 openers decreased afferent excitability. Here, we show pharmacological evidence that KCNQ2/3 subunits are likely targeted by mAChR activation in mammalian vestibular afferents. Additionally, we show that KCNQ3 KO mice have altered resting discharge rate as well as EVS-mediated slow response. These data together suggest that KCNQ channels play a role in slow response and discharge rate of vestibular afferents, which can be modulated by EVS in mammals.
初级前庭传入神经将来自毛细胞的有关头部位置和运动的信息传递至中枢神经系统（CNS），这对于维持平衡、注视稳定和空间导航至关重要。反过来，中枢神经系统通过传出前庭系统（EVS）及其对多种胆碱能信号传导机制的激活来调节毛细胞和传入神经。对EVS神经元进行电刺激会产生三种在动力学和机制上不同的传入反应，包括缓慢兴奋、快速兴奋和快速抑制。EVS介导的缓慢兴奋归因于传入神经上的奇数毒蕈碱型乙酰胆碱受体（mAChRs），其激活导致钾离子电导关闭并增加传入放电。可能的效应候选者包括属于KCNQ（Kv7.X）家族的低阈值电压门控钾通道，它们参与整个神经系统的神经元兴奋性并受mAChR调节。具体而言，KCNQ2/3异源通道可能是M电流的分子相关物，M电流是一种在奇数mAChRs激活后被阻断的钾电流。为此，KCNQ通道家族的多个成员，包括KCNQ2和KCNQ3，定位于前庭传入神经末梢内的几个微区，在那里它们影响传入神经的兴奋性并且可能是EVS神经元的作用靶点。此外，KCNQ亚基的相对表达似乎在感觉上皮细胞之间以及不同传入神经类型之间有所不同。然而，尚不清楚mAChR激活靶向哪些KCNQ通道亚基，以及这在不同传入神经类别中是否也有所不同。在这里，我们表明EVS介导的缓慢兴奋分别被非选择性KCNQ通道阻滞剂XE991和开放剂瑞替加滨阻断和增强。使用KCNQ亚基选择性药物，我们观察到KCNQ2阻滞剂阻断不规则传入神经中的缓慢反应，而KCNQ2/3开放剂增强规则传入神经中的缓慢反应。KCNQ2阻滞剂似乎不影响静息传入放电率，而KCNQ2/3或KCNQ2/4开放剂降低传入神经兴奋性。在这里，我们展示了药理学证据，表明在哺乳动物前庭传入神经中，KCNQ2/3亚基可能是mAChR激活的靶点。此外，我们表明KCNQ3基因敲除小鼠的静息放电率以及EVS介导的缓慢反应发生了改变。这些数据共同表明，KCNQ通道在前庭传入神经的缓慢反应和放电率中起作用，在哺乳动物中可由EVS调节。
