Research Report5-HT3 receptor expression in the mouse vestibular ganglion
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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