J Comp Neurol. 2017 Apr 1;525(5):1216-1233. doi: 10.1002/cne.24131.

Nicotinic acetylcholine receptors regulate vestibular afferent gain and activation timing.

Morley BJ1, Lysakowski A2, Vijayakumar S3, Menapace D1, Jones TA3.
1Boys Town National Research Hospital, Omaha, Nebraska, 68131.
2Department of Anatomy and Cell Biology, University of Illinois at Chicago, Chicago, Illinois, 60612.
3Department of Special Education and Communication Disorders, University of Nebraska, Lincoln, Nebraska, 68583.

Abstract

Little is known about the function of the cholinergic efferents innervating peripheral vestibular hair cells. We measured vestibular sensory evoked potentials (VsEPs) in α9 knockout (KO) mice, α10 KO mice, α7 KO mice, α9/10 and α7/9 double KO mice, and wild-type (WT) controls. We also studied the morphology and ultrastructure of efferent terminals on vestibular hair cells in α9, α10, and α9/10 KOs. Both type I and type ll vestibular hair cells express the α9 and α10 subunits. The efferent boutons on vestibular cells in α9, α10, and α9/10 KOs appeared normal, but a quantitative analysis was not performed. Mean VsEP thresholds were significantly elevated in α9 and α9/10 KO animals. Some α9 and α9/10 KO animals, however, had normal or near-normal thresholds, whereas others were greatly affected. Despite individual variability in threshold responses, latencies were consistently shortened. The double α7/9 KO resulted in decreased variance by normalizing waveforms and latencies. The phenotypes of the α7 and α10 single KOs were identical. Both α7 and α10 KO mice evidenced normal thresholds, decreased activation latencies, and larger amplitudes compared with WT mice. The data suggest a complex interaction of nicotinic acetylcholine receptors (nAChRs) in regulating vestibular afferent gain and activation timing. Although the α9/10 heteromeric nAChR is an important component of vestibular efferent activity, other peripheral or central nAChRs involving the α7 subunit or α10 subunit and α9 homomeric receptors are also important. J. Comp. Neurol. 525:1216-1233, 2017. © 2016 Wiley Periodicals, Inc.© 2016 Wiley Periodicals, Inc.

KEYWORDS:

RRID:AB_ 90764; RRID:AB_2079751; RRID:AB_2616561; RRID:IMSR_JAX:000664; RRID:SCK_002865; RRID:SCR_00207; amplitude; efferent; evoked potential; latency; macula; saccule; utricle

PMID: 27718229;  DOI: 10.1002/cne.24131

 

Supplement:

There are two very important issues addressed in our manuscript. We have identified a function of the efferent system innervating the vestibular periphery. Our study found that the efferent system regulates vestibular afferent gain and activation timing through nicotinic acetylcholine receptors (nAChRs). In the intact animal, we found a disinhibition of responses to transient stimuli based on the unusually short onset latencies found in α9 and α9α10 knockout (KO) animals. Yet, this was accompanied by widely varying response sensitivities (thresholds), which in addition may reflect a destabilization of a putative efferent-mediated adjustment of response dynamics across the neuroepithelium. Recent evidence in the intact mammal suggests that such an efferent process may also be mediated in part by calyceal KCNQ channels under the control of mAChRs (Lee et al., 2017). Our studies, together with other recent publications (Holt et al, 2017; Lee et al, 2017; Poppi et al., 2017) have disentangled some of the formerly elusive functions of vestibular efferents.

 

Secondly, our paper highlights the importance of nAChRs in mediating efferent function. We investigated the role of the α7, α9 and α10 nAChR subunits, utilizing single and double constitutive KO animals. An important aspect of this study was that these KOs were backcrossed to C57Bl/6J mice until congenicity (99%+) was achieved, thus there are no confounding aspects of a mixed genetic background. The α9 and α10 subunits are expressed in vestibular hair cells, as they are in cochlear hair cells, and we hypothesized that deletion of those subunits would alter function. Results of previous research suggest that α9 and α10 subunits are assembled and that deletion of α9 would produce the same phenotype as the α9α10 double KO. Our studies instead suggest that both subunits affect afferent gain and timing, and not necessarily as an assembled receptor. The most surprising finding was that the phenotype of the α7 KO was nearly identical to the α10 KO, indicating that the α7 and α9 subunits interact and further suggesting the possibility that the α7 subunit co-assembles with the α10 subunit, producing a novel receptor. There is an emerging literature (e.g., Richter et al., 2017, Zakrzewicz et al., 2017) showing that the α7, α9 and α10 subunits interact in immune cells at canonical and non-canonical receptors. The endogenous agonist for non-canonical α9-containing receptors may be phosphocholine, phosphatidylcholines and common metabolites of phosphatidylcholines (Richter et al., 2017, Zakrzewicz et al., 2017). These data, taken together with our physiological results, point to a complicated molecular and physiological function for these subunits that comprise a separate clade of the vertebrate nAChR tree (Shelukhina et al., 2017).

 

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