Dorsal Cochlear Nucleus Synaptic Reorganization as a Consequence of Noise-Induced Tinnitus: Increases in Somatosensory Influence on Auditory Circuitry

Abstract

Tinnitus is the perception of sound with no corresponding external stimulus. It is estimated that at least 17 million Americans experienced approximately 5 minutes of acute tinnitus in the past year, and at least 10 million Americans experience chronic tinnitus. Tinnitus itself is generally recognized as being a symptom of other conditions or diseases, and there are many methods of tinnitus induction. There are no effective cures for tinnitus, and management therapies, although moderately effective, do not address any pathophysiological changes associated with tinnitus symptoms. A primary focus of tinnitus research has been to elucidate the aberrant anatomy and physiology that underlies tinnitus perception, in the hopes of identifying therapeutic targets. Early research efforts focused on the peripheral auditory system (cochlea, auditory nerve) as likely sources of tinnitus, but increasing evidence suggests that tinnitus is generated and maintained by the central auditory system. The dorsal cochlear nucleus (DCN) likely plays a key role in tinnitus induction and maintenance, and many neural correlates (e.g., hyperactivity) of tinnitus have been observed in the DCN. The DCN is a laminar structure that contains a local circuit, which integrates auditory and non-auditory inputs. This first order central auditory nucleus is the first site of multi-sensory integration in the auditory system. A differential distribution of pre-synaptic markers (i.e., VGlut1, VGlut2, and VGat) has been described for the inputs to this circuit, which allows these excitatory and inhibitory components to be specifically studied. The work presented here utilizes a rat model of sound damage, and seeks to quantify peripheral damage and the corresponding central remodeling of both excitatory and inhibitory synapses in the DCN. We are able to successfully assess tinnitus status of our animals utilizing a behavioral assay for tinnitus (i.e., gap detection), granting us the ability to identify differences in peripheral damage or central remodeling that are unique to either noise-induced hearing loss or tinnitus. Peripheral insult was quantified via cochlear hair cell counts, and auditory brainstem response measures. Central insult and reorganization was quantified, in the dorsal cochlear nucleus, using both pre- and post-synaptic markers for excitatory (VGlut1, VGlut2, and PSD95) and inhibitory (VGat, Gephyrin) synapses. All experiments from baseline tinnitus and auditory assessment through final immunohistochemical labeling of the cochleae and DCN were performed on each animal enrolled in the study, instead of each experimental phase in the sequence involving a unique animal population. Using gap detection data, we sorted animals into two main groups: tinnitus negative (-) and tinnitus positive (+), the latter of which also contained a subgroup of severely impaired (++) animals. Regardless of tinnitus status, all animals had significantly elevated hearing thresholds for frequencies 8 kHz. Similarly, all animals exhibited IHC and OHC loss, although OHC loss was more severe in tinnitus (+) and (++) animals. Central changes in synaptic density in the DCN included an increase in the density of inhibitory synapses in tinnitus (+) and (++) animals. Unexpectedly, no changes were observed in any sound exposed animals in the excitatory granule cell/parallel fiber synapses, auditory nerve synapses, or somatosensory mossy fiber synapses. However, we did observe significant increases in excitatory unipolar brush cell synapses in tinnitus (+) and (++) animals, which suggests a possible increased influence of somatosensory input in the local circuitry of the DCN.