Anatomy and surgical approach of rat’s vestibular sensors and nerves
Graphical abstract
Introduction
The vestibular end organ contains five sensors (3 ampullae, 1 utricular macula and one saccular macula) protected by the bony labyrinth which is itself included in the temporal bone. This bone makes it difficult to reach and study each receptor and their specific roles.
Indeed, the classical techniques to stimulate the vestibular sensors fail to be selective for one sensor: stimulation of sensors by rotation always stimulates both labyrinths (the push–pull theory), otolith sensors are always stimulated by gravity, and caloric tests stimulate one lateral canal but also hearing and tactile sensors. The only known method allowing selective stimulation of the vestibular sensors is a surgical approach to each sensor in order to stimulate them electrically. This technique has been successfully used in cats (Fluur, 1959, Suzuki et al., 1969, Fluur and Mellström, 1970a, Fluur and Mellström, 1970b, Goto et al., 2003, Goto et al., 2004), monkeys (Suzuki and Cohen, 1964) and guinea pigs (Curthoys, 1987) to study vestibulo-spinal and vestibulo-ocular function. Recently, the development of a vestibular prosthesis for bilateral vestibular impairment has provided new interest in these techniques, with the development of a new model in chinchilla (Tang et al., 2009, Dai et al., 2011) and the first implantations in humans (van de Berg et al., 2011, Perez Fornos et al., 2014, Guinand et al., 2015). Also other functions of the vestibular system have been recently proven, including vegetative (Yates and Bronstein, 2005), circadian rythms (Fuller and Fuller, 2006, Martin et al., 2015) and numerous cognitive functions ((Lopez, 2013, Smith and Darlington, 2013, Smith and Zheng, 2013, Jamon, 2014) see for review (Hitier et al., 2014, Besnard et al., 2015)).
The best known animal model to study spatial memory is the rat in which it has been discovered that the hippocampal neurons are specialized in spatial orientation (place cells, grid cells and border cells), rewarded by the 2014 Nobel prize in Physiology or Medicine. Studies in rats have also demonstrated the influence of vestibular input on place cells (Sharp et al., 1995, Stackman et al., 2002). But we are still ignorant of the neural networks sustaining these functions and the specific role of each vestibular sensor.
Previous studies in rat have described the surgical approach of the middle ear (Judkins and Li, 1997, Hitier et al., 2010) and the anatomy of the inner ear itself (Curthoys, 1981, Blanks and Torigoe, 1989), but without surgical landmarks to locate the vestibular sensors, or their nerves. This lack of anatomical data in rat makes it difficult to precisely reach each of the sensors without destroying one of them.
Here we propose to establish a selective approach to each vestibular sensor and nerves in rat, which is needed to better understand the central vestibular pathways and distinguish between the roles of different sensors.
Section snippets
Materials and methods
An anatomical study has been performed on 10 male Wistar rats aged 8–12 weeks old, in accordance with the Regulations of the University of Otago Committee on Ethics in the Care and Use of Laboratory Animals and was approved by that Committee (Ethics number 55/12). The anatomy of each vestibular sensor was studied by histological sections, microtomography with 3D reconstruction and surgical dissection.
Bony labyrinth
Microtomography distinguished the bony labyrinth, the membranous labyrinth and the nerves in which myelin is stained by the osmium-tetroxide (Kiernan, 2007).
The 3D reconstruction of the bony labyrinth showed the 3 semicircular canals with – as in other mammals including humans – a common crus between the anterior and posterior canal (Ekdale, 2013). Each of the 3 semicircular canals is roughly perpendicular to each other (Blanks and Torigoe, 1989), as in humans (Della Santina et al., 2005,
Discussion
In this study we described some easy landmarks to locate the five vestibular sensors in rat. We demonstrated that each ampulla’s sensors (anterior, lateral and posterior) lie on a thin bone which is clearly visible through a surgical microscope. Because the anterior and lateral ampullary sensors are located medial to the facial nerve, we need to move or remove the facial nerve and the stapedial artery to reach them. The stapedial artery comes from the internal carotid artery and supplies the
Conclusion
Despite its small size, a surgical approach to each of the vestibular sensors and corresponding nerves is possible in rats with precise surgical landmarks. These results will allow new techniques in rats which remain one of the major models in vestibular science and the neurosciences in general. This also indicates the rat as a possible model to develop a vestibular prosthesis. Such a model would be ideal to study the consequences of chronic electrical vestibular stimulation on cognition.
Acknowledgments
The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under REA grant agreement no. 318980, grant from Region Basse Normandie, CNES, and the Royal Society of New Zealand Marsden Fund (to PFS and YZ).
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