Bilateral Dorsal Cochlear Nucleus Lesions Prevent Acoustic-Trauma Induced Tinnitus in an Animal Model

Animal experiments suggest that chronic tinnitus (“ringing in the ears”) may result from processes that overcompensate for lost afferent input. Abnormally elevated spontaneous neural activity has been found in the dorsal cochlear nucleus (DCN) of animals with psychophysical evidence of tinnitus. However, it has also been reported that DCN ablation fails to reduce established tinnitus. Since other auditory areas have been implicated in tinnitus, the role of the DCN is unresolved. The apparently conflicting electrophysiological and lesion data can be reconciled if the DCN serves as a necessary trigger zone rather than a chronic generator of tinnitus. The present experiment used lesion procedures identical to those that failed to decrease pre-existing tinnitus. The exception was that lesions were done prior to tinnitus induction. Young adult rats were trained and tested using a psychophysical procedure shown to detect tinnitus. Tinnitus was induced by a single unilateral high-level noise exposure. Consistent with the trigger hypothesis, bilateral dorsal DCN lesions made before high-level noise exposure prevented the development of tinnitus. A protective effect stemming from disruption of the afferent pathway could not explain the outcome because unilateral lesions ipsilateral to the noise exposure did not prevent tinnitus and unilateral lesions contralateral to the noise exposure actually exacerbated the tinnitus. The DCN trigger mechanism may involve plastic circuits that, through loss of inhibition, or upregulation of excitation, increase spontaneous neural output to rostral areas such as the inferior colliculus. The increased drive could produce persistent pathological changes in the rostral areas, such as high-frequency bursting and decreased interspike variance, that comprise the chronic tinnitus signal

Local NMDA receptor blockade attenuates chronic tinnitus and associated brain activity in an animal model.

Chronic tinnitus has no broadly effective treatment. Identification of specific markers for tinnitus should facilitate the development of effective therapeutics. Recently it was shown that glutamatergic blockade in the cerebellar paraflocculus, using an antagonist cocktail was successful in reducing chronic tinnitus. The present experiment examined the effect of selective N-methyl d-aspartate (NMDA) receptor blockade on tinnitus and associated spontaneous brain activity in a rat model. The NMDA antagonist, D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.5 mM), was continuously infused for 2 weeks directly to the ipsilateral paraflocculus of rats with tinnitus induced months prior by unilateral noise exposure. Treated rats were compared to untreated normal controls without tinnitus, and to untreated positive controls with tinnitus. D-AP5 significantly decreased tinnitus within three days of beginning treatment, and continued to significantly reduce tinnitus throughout the course of treatment and for 23 days thereafter, at which time testing was halted. At the conclusion of psychophysical testing, neural activity was assessed using manganese enhanced magnetic resonance imaging (MEMRI). In agreement with previous research, untreated animals with chronic tinnitus showed significantly elevated bilateral activity in their paraflocculus and brainstem cochlear nuclei, but not in mid or forebrain structures. In contrast, D-AP5-treated-tinnitus animals showed significantly less bilateral parafloccular and dorsal cochlear nucleus activity, as well as significantly less contralateral ventral cochlear nucleus activity. It was concluded that NMDA-mediated glutamatergic transmission in the paraflocculus appears to be a necessary component of chronic noise-induced tinnitus in a rat model. Additionally, it was confirmed that in this model, elevated spontaneous activity in the cerebellar paraflocculus and auditory brainstem is associated with tinnitus

Psychophysical performance of Exposed (square data points) and Unexposed (round data points) groups.

<p>Prior to D-AP5 treatment, exposed animals were divided into two equal subgroups, one of which was randomly assigned to receive D-AP5 (open, square data points). A downward shift with respect to Unexposed controls was indicative of tinnitus. <b>A</b>. Pre-D-AP5 treatment there was no difference between the two exposed groups (error bars in all panels indicate the standard error of the mean) and both were significantly different than the Unexposed group (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 1). <b>B</b>. After 72 hrs of D-AP5 infusion the treated Exposed group significantly diverged from the untreated Exposed group (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 2), indicating decreased tinnitus. <b>C</b>. The D-AP5 treatment effect persisted for 23 days after discontinuation of drug infusion (statistics summarized in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t001" target="_blank">Table 1</a>, section 3).</p

Representative MEMRI scans of an exposed (tinnitus) untreated rat (left column), an unexposed (no-tinnitus) untreated rat (center column), and an exposed treated rat (right column).

<p>Panel rows are indexed with respect to the auditory nerve (dark arrows in row 0). The row labels indicate distance in mm rostral (positive values) or caudal (negative values) to the auditory nerve. The dorsal cochlear nucleus (dark arrows) appears in row −0.6 and the paraflocculus (white arrows) in row −0.9. The scale bar (top right panel) is 2 mm, R and L indicate the right and left hemispheres, respectively.</p

Typical AOI boundaries used in MEMRI quantification.

<p>A. Paraflocculus (PFL) blue, dorsal cochlear nucleus (DCN) green, posterior ventral cochlear nucleus, red. The 2 mm scale bar applies to all panels. B. Anterior cochlear nucleus (AVCN). C. Inferior colliculus (IC). D. Medial geniculate body (MGB).</p

Post D-AP5 spontaneous neural activity indicated by manganese enhanced magnetic resonance imaging (MEMRI).

<p>Depicted are normalized data (AOI / adjacent muscle area). Elevation was evident in the ipsilateral paraflocculus (PFL) and dorsal cochlear nucleus (DCN) of Exposed untreated, but not in Exposed D-AP5 treated animals, in comparison to Unexposed controls (top panel). Contralateral elevation (bottom panel) was evident in the PFL, DCN, and the anterior ventral cochlear nuclei (AVCN). Posterior ventral cochlear nucleus (PVCN), inferior colliculus (IC), medial geniculate body (MGB). Error bars indicate mean deviation (*** p<0.0001; * p<0.05); statistical summary in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077674#pone-0077674-t002" target="_blank">Table 2</a>.</p

Anova summary of psychophysical data (20 kHz diagnostic stimulus).

<p>Anova summary of psychophysical data (20 kHz diagnostic stimulus).</p

Pre-imaging, immediate post-psychophysics, auditory brainstem response (ABR) hearing thresholds for Exposed (see text) and Unexposed groups.

<p>Error bars indicate the standard error of the mean.</p

Anova summary of MEMRI data.

<p>Anova summary of MEMRI data.</p