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

Abstract

Tinnitus has been associated with increased spontaneous and evoked activity, increased neural synchrony, and reorganization of tonotopic maps of auditory nuclei. However, the neurotransmitter systems mediating these changes are poorly understood. Here, we developed an in vitro assay that allows us to evaluate the roles of excitation and inhibition in determining the neural correlates of tinnitus. To measure the magnitude and spatial spread of evoked circuit activity, we used flavoprotein autofluorescence (FA) imaging, a metabolic indicator of neuronal activity. We measured FA responses after electrical stimulation of glutamatergic axons in slices containing the dorsal cochlear nucleus, an auditory brainstem nucleus hypothesized to be crucial in the triggering and modulation of tinnitus. FA imaging in dorsal cochlear nucleus brain slices from mice with behavioral evidence of tinnitus (tinnitus mice) revealed enhanced evoked FA response at the site of stimulation and enhanced spatial propagation of FA response to surrounding sites. Blockers of GABAergic inhibition enhanced FA response to a greater extent in control mice than in tinnitus mice. Blockers of excitation decreased FA response to a similar extent in tinnitus and control mice. These findings indicate that auditory circuits in mice with behavioral evidence of tinnitus respond to stimuli in a more robust and spatially distributed manner because of a decrease in GABAergic inhibition.T innitus, the persistent perception of a subjective sound in the absence of an acoustic stimulus (ringing of the ears), is often a debilitating condition that reduces the quality of life for many of those chronically affected. Estimates of the number of people experiencing tinnitus range from 8% to 20% of the general population (1). Despite the prevalence and growing incidence of tinnitus, the mechanisms underlying the induction and maintenance of tinnitus remain poorly understood.Animal models of tinnitus have contributed significantly to the understanding of the pathophysiology of tinnitus (2-8). An emerging pattern associated with tinnitus pathology indicates that intense noise exposure leads to cochlear damage and hearing loss, which often is not clinically detected. Decreased cochlear input leads to hyperactive, more responsive central auditory circuits, which is evidenced by functional MRI (fMRI) studies in patients with tinnitus and in vivo recordings in animal models of tinnitus (9-13). Increased spontaneous firing rates, increased evoked responses, and reorganization of tonotopic maps are consistent with decreased inhibition (disinhibition) (1, 14). However, direct evidence that disinhibition mediates these changes is still lacking. In addition, the alternative hypothesis that predicts increased excitation as a potential mechanism in mediating these changes has not been tested (15). Addressing these unexamined questions could lead to pharmacological approaches for treating tinnitus patients.We used flavoprotein autofluorescence (FA) imaging in brain slices prepared from...

Is noise-induced cochlear neuropathy key to the generation of hyperacusis or tinnitus?

“…3), or "far" from the startle elicitor, consistent with the timing used in other studies (e.g., Middleton et al 2011) (see Fig. 8, insets).…”

“…Such hyperactivity is measurable within a few hours (Noreña and Eggermont 2003;Salvi et al 2000) and can persist indefinitely, in the absence of further experimental manipulation (Bauer et al 2008;Kaltenbach et al 2000). Neurophysiological studies in vivo and imaging studies in vitro have suggested that decreased inhibition in an early stage of the ascending pathway, for example in dorsal cochlear nucleus (DCN), leads to the "sign change" that underlies the transformation of reduced cochlear nerve output into enhanced central activity (for review, see Roberts et al 2010; see also Middleton et al 2011).…”

Effects of Unilateral Acoustic Trauma on Tinnitus-Related Spontaneous Activity in the Inferior Colliculus

“…Significant changes in gene expression were also observed bilaterally in the ICC. These observations compliment studies that explicitly link hyperactivity and tinnitus behavior to inhibitory changes in the DCN (Middleton et al 2011) andICC (Bauer et al 2000). Consequently, the hyperactivity of ICC neurons may reflect both ascending and local influences that interact with independent time courses.…”

“…According to the hyperactivity model of tinnitus, neurons in the central auditory system respond to the weakened input of a sound-damaged cochlea by down-regulating inhibitory connections (Moller 2007;Middleton et al 2011;Wang et al 2011) and enhancing membrane excitability (Dong et al 2010;Pilati et al 2012;Koehler and Shore 2013;Li et al 2013). This "rebalancing" may raise spontaneous activity (i.e., activity in the absence of sound) to levels that impart a false perception of sound stimulation (Chen and Jastreboff 1995; Kaltenbach and Afman 2000;.…”

Noise-induced hyperactivity in the inferior colliculus: its relationship with hyperactivity in the dorsal cochlear nucleus

“…Such changes have been well characterized at the multiunit and single-unit levels in structures as diverse as the dorsal cochlear nucleus (DCN) (Brozoski et al, 2002;Finlayson and Kaltenbach 2009;Kaltenbach and McCaslin 1996;Kaltenbach et al 2000Kaltenbach et al , 2002Middleton et al 2011;Shore et al 2008), the ventral cochlear nucleus (VCN) (Vogler et al 2011), the inferior colliculus (IC) (Bauer et al 2008;Chen and Jastreboff 1995;Dong et al 2009Dong et al , 2010aDong et al , 2010bJastreboff and Sasaki 1986;Mulders and Robertson 2009;Mulders et al 2010Wallhäusser-Franke et al 2003), and the auditory cortex (Eggermont and Kenmochi 1998;Eggermont 2005, 2006;Ochi and Eggermont 1997;Seki and Eggermont 2003;Wallhäusser-Franke et al 1996), suggesting that tinnitus may be a system-wide pathology. Indeed, functional imaging studies in both animals and human subjects with tinnitus show clear evidence of hyperactivation of centers both within and beyond the auditory system (Brozoski et al 2007;Eichhammer et al 2007;Langguth et al 2006;Lanting et al 2008;Lobarinas et al 2008;Lockwood et al 1998;Melcher et al 2000Melcher et al , 2009Reyes et al 2002).…”