Elsevier

Brain Research

Volume 1557, 4 April 2014, Pages 74-82
Brain Research

Research Report
5-HT3 receptor expression in the mouse vestibular ganglion

https://doi.org/10.1016/j.brainres.2014.02.016Get rights and content

Highlights

  • 5-HT3A and 5-HT3B receptors are expressed in the mouse vestibular ganglion (VG).

  • 5-HT3 expression is limited to the VG in the peripheral vestibular system.

  • 5-HT3A receptors are mainly expressed in medium or large sized VG neurons.

  • The presence of 5-HT3 receptors in the VG was also confirmed by physiological assay.

Abstract

The 5-hydroxytryptamine type 3 (5-HT3) receptor is a ligand-gated ion channel and a member of the Cys-loop family of receptors. Previous studies have shown 5-HT3 receptor expression in various neural cells of the central and peripheral nervous systems. Although the function and distribution of the 5-HT3 receptor has been well established, its role in the inner ear is still poorly understood. Moreover, no study has yet determined its localization and function in the peripheral vestibular nervous system. In the present study, we reveal mRNA expression of both 5-HT3A and 5-HT3B receptor subunits in the mouse vestibular ganglion (VG) by RT-PCR and in situ hybridization (ISH). We also show by ISH that 5-HT3 receptor mRNA is only expressed in the VG (superior and inferior division) in the peripheral vestibular nervous system. Moreover, we performed Ca2+ imaging to determine whether functional 5-HT3 receptors are present in the mouse VG, using a selective 5-HT3 receptor agonist, SR57227A. In wild mice, 32% of VG neurons responded to the agonist, whereas there was no response in 5-HT3A receptor knockout mice. These results indicate that VG cells express functional 5-HT3 receptor channels and might play a modulatory role in the peripheral vestibular nervous system.

Introduction

In contrast to the other 5-hydroxytryptamine (5-HT) receptor subfamilies, that are all G protein coupled receptors, the 5-HT type 3 (5-HT3) receptor is the only ligand-gated cation channel. It is a member of the Cys-loop family of receptors, which also includes the glycine, GABAA and nicotinic acetylcholine receptors, and plays major roles in fast synaptic transmission (Derkach et al., 1989, Lester et al., 2004, Maricq et al., 1991, Yakel and Jackson, 1988). In rodents, the 5-HT3 receptor consists of two subunits (A and B); however, the human genome contains five genes encoding different 5-HT3 subunits (A, B, C, D and E). The 5-HT3A and 5-HT3B receptor subunits are mainly involved in formation of functional receptors. Both 5-HT3A homomeric and 5-HT3AB heteromeric receptors can form functional receptors, whereas 5-HT3B homomeric receptors are not functional. 5-HT3AB heteromeric receptors have different pharmacological and biophysical properties from 5-HT3A homomeric receptors, exhibiting large single-channel conductance, low permeability to calcium ions, and a linear current-voltage relationship (Davies et al., 1999, Niesler et al., 2003, Niesler et al., 2007).

Previous studies have shown 5-HT3 receptor expression in various neurons of the central nervous systems (CNS), and its involvement in anxiolytic action and cognitive functions, such as learning, attention, memory and fear extinction (Bhatnagar et al., 2004, Harrell and Allan, 2003, Kelley et al., 2003; Kondo at el., in press; Miquel et al., 2002, Morales et al., 1996, Morales et al., 1998, Tecott et al., 1993, Thompson and Lummis, 2007). In the peripheral nervous system, 5-HT3 receptor expression has been found in dorsal root, nodose, superior cervical, trigeminal, and vagal nerve ganglia (Hoyer et al., 1989, Rosenberg et al., 1997, Morales et al., 2001, Morales and Wang, 2002), with functional involvement in the vomiting reflex, tissue injury-induced pain, and hyperalgesia (Galligan, 2002, Liang et al., 2011, Minami et al., 2003, Sommer, 2004, Zeitz et al., 2002). Generally, although the function and distribution of 5-HT3 receptors is well established, its role in the inner ear is still not understood. To our knowledge, no study has been done on the expression and function of 5-HT3 receptor in the peripheral vestibular nervous system. A few studies have reported the expression of 5-HT receptors in the inner ear. Expression of 5-HT receptors, including 5-HT1A, 5-HT1B, 5-HT2B, 5-HT2C, 5-HT3, 5-HT5B and 5-HT6 receptor subunits, have been shown in mouse cochlear tissues by Reverse-transcription polymerase chain reaction (RT-PCR) (Oh et al., 1999). In addition, the distribution of 5-HT1B and 5-HT1D receptor subunits in rat and monkey inner ear has been shown by immunohistochemistry, and suggests that the receptors contribute to comorbidity of headache and vertigo in migraine (Ahn and Balaban, 2010). Nevertheless, the role of 5-HT receptors is still unclear in the inner ear.

Interestingly, a recent clinical study reported the preventive effect of ondansetron, a 5-HT3 receptor antagonist, on the vestibular deficit in acute-phase vestibular neuritis (Venail et al., 2012). Although the basis of this mechanism is unknown, it has been speculated that the 5-HT3 receptor plays a functional role in the vestibular system. Therefore, to address this question, we investigate here the localization of the 5-HT3 receptor in the mouse peripheral vestibular nervous system.

Section snippets

Reverse transcription-polymerase chain reaction (RT-PCR)

We performed RT-PCR analysis on adult mouse whole inner ear, VG, and whole cochlea tissues (Fig. 1). First, we observed that both 5-HT3A (HTR3A) and 5-HT3B receptor (HTR3B) transcripts were found in the whole inner ear tissue (Fig. 1A). Next, we closely examined 5-HT3 receptor expression in each inner ear division. HTR3A and HTR3B transcripts were both detected in the VG, while only 5-HT3A receptors were detected in the cochlea tissue (Fig. 1B, upper panel). The trigeminal ganglion (TG) was

Discussion

The 5-HT3 receptor is a ligand-gated ion channel, and plays a major role in fast synaptic transmission. Although the function and distribution of the 5-HT3 receptor is generally well established, few studies have examined its role and focal localization in the inner ear. A previous RT-PCR study found 5-HT3 receptor mRNA expression in the mammalian cochlea and its subdivisions, and showed that it is only expressed in the SG (Oh et al., 1999). However, neither that study nor any others have

Animals

All animal experiments were conducted in accordance with the institutional guidelines set by the Osaka University School of Medicine Animal Care and Use Committee. Every effort was made to minimize animal suffering and reduce the number of animals used. The animals were purchased from Japan SLC Inc. (Hamamatsu, Japan). We established 5-HT3A receptor KO (5-HT3A receptor −/−) mice from a breeding pair obtained from the Jackson Laboratories (B6.129×1-Htr3atm1Jul/J; Zeitz et al., 2002), and

Role of authors

All authors had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: A.H, A.U, H.I, S.N, S.O, S.S, T.I, T.K, Y.I, and Y.T. Acquisition of data: Y.T. Analysis and interpretation of data: M.K, S.S, T.K, T.Y, Y.I, Y.N, and Y.T. Drafting of the manuscript: Y.T. Critical revision of the manuscript for important intellectual content: S.S and Y.I. Statistical analysis: Y.T. Obtained funding: S.S

Acknowledgments

We thank Dr. Kentaro Takezawa for helpful advice and support.

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