Plasticity at glycinergic synapses in dorsal cochlear nucleus of rats with behavioral evidence of tinnitus

Abstract

Fifteen to 35% of the United States population experiences tinnitus, a subjective "ringing in the ears". Up to 10 percent of those afflicted report severe and disabling symptoms. Tinnitus was induced in rats using unilateral, one-hour, 17 kHz-centered octave-band noise (116 dB SPL) and assessed using a gap-startle method. The dorsal cochlear nucleus (DCN) is thought to undergo plastic changes suggestive of altered inhibitory function during tinnitus development. Exposed rats showed near preexposure ABR thresholds for clicks and all tested frequencies 16 weeks post-exposure. Soundexposed rats showed significantly worse gap detection at 24 and 32 kHz 16 weeks following sound exposure, suggesting the development of chronic, high frequency tinnitus.Message and protein levels of α 1-3 , and β glycine receptor subunits (GlyRs), and the anchoring protein, gephyrin, were measured in DCN fusiform cells 4 months following sound exposure. Rats with evidence of tinnitus showed significant GlyR α 1 protein decreases in the middle and high frequency regions of the DCN while α 1 message levels were paradoxically increased. Gephyrin levels showed significant tinnitus-related increases in sound-exposed rats suggesting intracellular receptor trafficking changes following sound exposure. Consistent with decreased α 1 subunit protein levels, strychnine binding studies showed significant tinnitus-related decreases in the number of GlyR binding sites, supporting tinnitus-related changes in the number and/or composition of GlyRs.Collectively, these findings suggest the development of tinnitus is likely associated with functional GlyR changes in DCN fusiform cells consistent with previously described behavioral and neurophysiologic changes. Tinnitus related GlyR changes could provide a unique receptor target for tinnitus pharmacotherapy or blockade of tinnitus initiation. Keywordshearing loss; sound exposure; fusiform cell; auditory; glycine receptor; gephyrin Correspondence to: Dr. Donald M. Caspary, Phone: 217-545-2195, Fax: 217-545-0145, dcaspary@siumed.edu. Section Editor: Sensory Systems: Dr. Richard Weinberg, University of North Carolina, Department of Cell Biology and Anatomy, CB 7090, Chapel Hill, NC 27599, USA Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2010 December 1. Published in final edited form as:Neuroscience. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptTinnitus is a common audiologic complaint characterized by auditory perception without an external phys...

Mice with behavioral evidence of tinnitus exhibit dorsal cochlear nucleus hyperactivity because of decreased GABAergic inhibition

“…Taken together, these results indicate that DCN circuits in tinnitus mice show evidence of decreased GABAergic inhibition. Although this finding is consistent with previous studies that have suggested reduced inhibition in DCN circuits in tinnitus animal models (11,12,28), our results are unexpected, because previous studies have proposed a reduced glycinergic inhibition as a mediator of tinnitus-induced disinhibition in the DCN (11,12,28).…”

“…Our proposed mechanism is consistent with the effect of several GABA-enhancing drugs that reduce the perceptual loudness of tinnitus in some patients (36). Our results do not exclude the contribution of the previously observed changes in the expression of glycine receptor subunits (28) in determining the expression of other neural correlates of tinnitus such as increased synchrony or altered tonotopy. Additionally, our results cannot exclude changes in intrinsic properties of principal neurons or changes in the excitability of axons.…”

“…Although FA signals are not suitable for cellular resolution, they can reveal specific hypotheses about altered microcircuit and neurotransmitter properties associated with tinnitus that can be assessed with whole-cell recording and more sensitive circuit mapping techniques Our results provide information about the tinnitus-associated changes in the balance of excitation and inhibition in the DCN. Previous studies have suggested that tinnitus-related hyperactivity is mediated by the lack of glycinergic inhibition, which is evidenced by altered in vivo response of DCN principal neurons to increasing levels of sound and changes in the expression of specific glycine receptor subunits (11,12,28). However, no antagonists in animals with behavioral evidence of tinnitus were used in these studies.…”

Enhanced GABAA-Mediated Tonic Inhibition in Auditory Thalamus of Rats with Behavioral Evidence of Tinnitus

“…Forty-five Long-Evans rats were used for electrophysiological recordings in the MGB slice preparation and 13 Fischer Brown Norway (FBN) rats from earlier tinnitus experiments (Wang et al, 2009) were used for MGB in situ hybridization studies. Both strains have similar life expectancy, with young adult animals used in both studies.…”

“…It modulates input/output functions, alters frequency tuning, and sharpens temporal response accuracy while gating neurotransmission to primary auditory cortex (AI; Wang et al, 2008;Edeline, 2011;Richardson et al, 2012). Downregulation of inhibitory function has been suggested to underpin tinnitus pathology in dorsal cochlear nucleus (DCN; Wang et al, 2009;Pilati et al, 2012a;Pilati et al, 2012b), inferior colliculus (IC; Bauer et al, 2008;Dong et al, 2009), and AI (Noreña and Eggermont, 2003;Yang et al, 2007Yang et al, , 2011 where tinnitus is accompanied by increased spontaneous activity, bursting, enhanced sound-evoked responses, and reduction in markers of inhibitory neurotransmission (Eggermont and Roberts, 2004;Middleton et al, 2011). This downregulation of inhibitory function is thought to reflect altered homeostatic plasticity compensating for the loss of peripheral excitatory drive (Noreña, 2011;Richardson et al, 2012).…”