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Essential Role of Somatic Kv2 Channels in High-Frequency Firing in Cartwheel Cells of the Dorsal Cochlear Nucleus

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Essential Role of Somatic Kv2 Channels in High-Frequency Firing in Cartwheel Cells of the Dorsal Cochlear Nucleus Research Article: New Research | Neuronal Excitability Essential Role of Somatic Kv2 Channels in High-Frequency Firing in Cartwheel Cells of the Dorsal Cochlear Nucleus https://doi.org/10.1523/ENEURO.0515-20.2021 Cite as: eNeuro 2021; 10.1523/ENEURO.0515-20.2021 Received: 30 November 2020 Revised: 23 March 2021 Accepted: 24 March 2021 This Early Release article has been peer-reviewed and accepted, but has not been through the composition and copyediting processes. The final version may differ slightly in style or formatting and will contain links to any extended data. Alerts: Sign up at www.eneuro.org/alerts to receive customized email alerts when the fully formatted version of this article is published. Copyright © 2021 Irie This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed. 1 1. Manuscript Title 2 Essential Role of Somatic Kv2 Channels in High-Frequency Firing in Cartwheel Cells of the Dorsal 3 Cochlear Nucleus. 4 2. Abbreviated Title 5 Somatic Kv2-mediated Regulation of Excitability in DCN 6 7 3. Authors 8 Tomohiko Irie 9 10 Affiliation 11 Division of Pharmacology, National Institute of Health Sciences, 3-25-26 Tonomachi, Kawasaki-ku, 12 Kawasaki City, Kanagawa, 210-9501, Japan. 13 14 4. Author Contributions 15 TI Designed Research; TI Performed Research; TI Wrote the paper 16 17 5. Correspondence should be addressed to Tomohiko Irie, Tel.: +81 44 270 6600, E-mail: 18 [email protected] 19 20 6. Number of Figures:7 21 7. Number of Tables: 3 22 8. Number of Multimedia: 0 23 9. Number of words for Abstract: 242 24 10. Number of words for Significance Statement: 119 25 11. Number of words for Introduction: 790 26 12. Number of words for Discussion: 929 27 28 13. Acknowledgements: I thank Dr. Laurence O. Trussell for carefully reading the manuscript and 29 valuable comments, and Editage (www.editage.com) for English language editing. 30 14. Conflict of Interest: Author reports no conflict of interest 31 15. Funding sources: This work was supported by JSPS KAKENHI Grant Number 19K07295. 32 33 34 35 36 1 37 Manuscript Title 38 Essential Role of Somatic Kv2 Channels in High-Frequency Firing in Cartwheel Cells of the Dorsal 39 Cochlear Nucleus. 40 41 Running Title 42 Somatic Kv2-mediated Regulation of Excitability 43 2 44 45 Abstract 46 Among all voltage-gated potassium (Kv) channels, Kv2 channels are the most widely expressed in 47 the mammalian brain. However, studying Kv2 in neurons has been challenging due to a lack of 48 high-selective blockers. Recently, a peptide toxin, guangxitoxin-1E, has been identified as a specific 49 inhibitor of Kv2, thus facilitating the study of Kv2 in neurons. The mammalian dorsal cochlear 50 nucleus (DCN) integrates auditory and somatosensory information. In the DCN, cartwheel inhibitory 51 interneurons receive excitatory synaptic inputs from parallel fibers conveying somatosensory 52 information. The activation of parallel fibers drives action potentials in the cartwheel cells up to 130 53 Hz in vivo, and the excitation of cartwheel cells leads to the strong inhibition of principal cells. 54 Therefore, cartwheel cells play crucial roles in monaural sound localization and cancelling detection 55 of self-generated sounds. However, how Kv2 controls the high-frequency firing in cartwheel cells is 56 unknown. In this study, we performed immunofluorescence labeling with anti-Kv2.1 and anti-Kv2.2 57 antibodies using fixed mouse brainstem slice preparations. The results revealed that Kv2.1 and 58 Kv2.2 were largely present on the cartwheel cell body membrane but not on the axon initial segment 59 nor the proximal dendrite. Whole-cell patch clamp recordings using mouse brainstem slice 60 preparation and guangxitoxin-1E demonstrated that blockade of Kv2 induced failure of parallel 61 fiber-induced action potentials when parallel fibers were stimulated at high frequencies (30-100 Hz). 62 Thus, somatic Kv2 in cartwheel cells regulates the action potentials in a frequency-dependent 63 manner and may play important roles in the DCN function. 64 65 Significance statement 66 The mammalian dorsal cochlear nucleus (DCN) plays a role in the monaural sound localization and 67 cancelling detection of self-generated sounds. In the DCN, cartwheel cells receive excitatory 3 68 synaptic inputs from parallel fibers. Parallel fiber activation can drive action potentials in cartwheel 69 cells at high frequency, but the ionic mechanism of such firing remains unknown. In this study, we 70 found that Kv2.1 and Kv2.2 ion channels were present on the cell bodies of cartwheel cells. 71 Application of a specific blocker of Kv2 induced failure of parallel fiber-induced action potentials 72 only when presynaptic parallel fibers were stimulated at high frequencies. Thus, somatic Kv2 in 73 cartwheel cells regulates action potentials in a frequency-dependent manner and may play important 74 roles in sound processing. 75 76 Keyword 77 Kv2 channels; guangxitoxin-1E; sustained firing; cartwheel cells; dorsal cochlear nucleus 78 4 79 80 Introduction 81 Neurons express a wide variety of voltage-gated potassium (Kv) channels that feature different 82 voltage dependence and kinetics, thereby endowing neurons with a wide range of firing patterns 83 (Hille, 2001; Vacher et al., 2008; Johnston et al., 2010). The Kv channel family can be divided into 84 several subfamilies based on nucleotide sequence similarity and function. In the mammalian brain, 85 messenger RNA expression studies and antibody-based protein detection indicate that Kv2 channels 86 are the most widely expressed voltage-gated K channels in terms of tissue distribution (Trimmer, 87 1991; Hwang et al., 1993; Mandikian et al., 2014). Kv2 channels consist of Kv2.1 and Kv2.2, and 88 are present not only in cortical, hippocampal, and -motoneurons but also in retinal bipolar cells, 89 cardiac myocytes, vascular and gastrointestinal smooth muscle, and pancreatic beta cells (Johnson et 90 al., 2019). Physiologically, Kv2 channels produce delayed-rectifier currents, and Kv2.1 channels 91 have a high activation threshold of –15 mV (Johnston et al., 2010; Johnson et al., 2019). Indeed, 92 delayed-rectifier currents in various neurons are generally due to Kv2 channels (Murakoshi and 93 Trimmer, 1999; Malin and Nerbonne, 2002; Guan et al., 2007; Johnston et al., 2008; Tong et al., 94 2013). However, the study of Kv2-mediated current in neurons has been challenging due to the 95 limited availability of high-selective blockers. In previous studies, Kv2 channels were blocked by an 96 anti-Kv2 antibody, gene expression of dominant-negative Kv2 channels, or non-specific Kv2 97 channel blockers, or were eliminated by the knock-out mice. In 2006, a peptide toxin, 98 guangxitoxin-1E (GxTX), extracted from the venom of the Chinese tarantula Plesiophrictus 99 guangxiensis, has been identified as a potent and specific inhibitor of Kv2.1 and Kv2.2 channels, 100 with a half-blocking concentration of 2 – 5 nM (Herrington et al., 2006; Herrington, 2007). Using 101 this toxin, the physiological roles of Kv2 channels have been explored in the superior cervical 102 ganglion neurons, CA1 pyramidal neurons, and the entorhinal cortex layer II stellate cells (Liu and 5 103 Bean, 2014; Kimm et al., 2015; Honigsperger et al., 2017). Interestingly, Kv2.1 expression in some 104 types of cerebral neurons coincides with the presence of clustered ryanodine receptors in the cell 105 body, suggesting that Kv2.1 expression can be observed in neurons expressing clustered ryanodine 106 receptors in other brain regions such as the brainstem (Mandikian et al., 2014). 107 The mammalian brainstem contains several auditory nuclei, and among them, the dorsal cochlear 108 nucleus (DCN) integrates auditory inputs and multimodal ones, including somatosensory, vestibular, 109 and higher-level auditory information (Oertel and Young, 2004). Auditory inputs are conveyed by 110 auditory nerve fibers and multimodal inputs by parallel fibers, which are axons of excitatory granule 111 cells in the cochlear nuclei. The DCN plays several crucial roles in the monaural sound localization 112 and cancelling detection of self-generated sounds (e.g., sound induced by licking behavior) (May, 113 2000; Singla et al., 2017). Singla et al. (2017) reported that during licking behavior in mice, 114 complex-spiking single units (i.e., firing brief high-frequency bursts of spikes) were activated in the 115 DCN; these complex-spiking neurons likely correspond to cartwheel inhibitory interneurons (Manis 116 et al., 1994; Davis and Young, 1997; Tzounopoulos et al., 2004; Roberts et al., 2008; Ma and 117 Brenowitz, 2012). Cartwheel cells are medium-sized GABAergic/glycinergic neurons which fire 118 mixtures of simple and complex action potentials (Manis et al., 1994; Golding and Oertel, 1996; 119 Kim and Trussell, 2007; Roberts et al., 2008; Roberts and Trussell, 2010). Moreover, cartwheel cells 120 have profuse spiny dendrites that receive excitatory synaptic inputs from parallel fibers (Wouterlood 121 and Mugnaini, 1984). The activation of parallel fibers drives action potentials in cartwheel cells up 122 to 130 Hz in vivo, and the excitation of cartwheel cells leads to the strong inhibition of principal cells 123 (Davis and Young, 1997; Roberts and Trussell, 2010). Recently, it has been shown that cartwheel 124 cells express clustered ryanodine receptors abundantly at putative subsurface cisterns immediately 125 beneath the somatic membrane (Yang et al., 2016; Irie and Trussell, 2017). Considering that 126 clustered ryanodine receptor-expressing neurons have Kv2.1 channels (Mandikian et al., 2014), it is 6 127 possible that cartwheel cells also express Kv2.1 channels. However, where Kv2 channels are 128 expressed in cartwheel cells in terms of subcellular localization and how Kv2 channels control 129 high-frequency firing are unknown.
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