Sarah H. Hayes

Behavioral Models of Tinnitus and Hyperacusis in Animals

The phantom perception of tinnitus and reduced sound-level tolerance associated with hyperacusis have a high comorbidity and can be debilitating conditions for which there are no widely accepted treatments. One factor limiting the development of treatments for tinnitus and hyperacusis is the lack of reliable animal behavioral models of these disorders. Therefore, the purpose of this review is to highlight the current animal models of tinnitus and hyperacusis, and to detail the advantages and disadvantages of each paradigm. To date, this is the first review to include models of both tinnitus and hyperacusis.

Expression of doublecortin, a neuronal migration protein, in unipolar brush cells of the vestibulocerebellum and dorsal cochlear nucleus of the adult rat

Doublecortin (DCX) is a microtubule-associated protein that is critical for neuronal migration and the development of the cerebral cortex. In the adult, it is expressed in newborn neurons in the subventricular and subgranular zones, but not in the mature neurons of the cerebral cortex. By contrast, neurogenesis and neuronal migration of cells in the cerebellum continue into early postnatal life; migration of one class of cerebellar interneuron, unipolar brush cells (UBCs), may continue into adulthood. To explore the possibility of continued neuronal migration in the adult cerebellum, closely spaced sections through the brainstem and cerebellum of adult (3-16 months old) Sprague-Dawley rats were immunolabeled for DCX. Neurons immunoreactive (ir) to DCX were present in the granular cell layer of the vestibulocerebellum, most densely in the transition zone (tz), the region between the flocculus (FL) and ventral paraflocculus (PFL), as well as in the dorsal cochlear nucleus (DCN). These DCX-ir cells had the morphological appearance of UBCs with oval somata and a single dendrite ending in a brush. There were many examples of colocalization of DCX with Eps8 or calretinin, UBC markers. We also identified DCX-ir elements along the fourth ventricle and its lateral recess that had labeled somata but lacked the dendritic structure characteristic of UBCs. Labeled UBCs were seen in nearby white matter. These results suggest that there may be continued neurogenesis and/or migration of UBCs in the adult. Another possibility is that UBCs maintain DCX expression even after migration and maturation, reflecting a role of DCX in adult neuronal plasticity in addition to a developmental role in migration.

A novel behavioral assay for the assessment of acute tinnitus in rats optimized for simultaneous recording of oscillatory neural activity

BACKGROUND Human magneto/electrophysiology studies suggest that the phantom sound of tinnitus arises from spontaneous oscillatory neural activity in auditory cortex; however, in animal models, behavioral techniques suitable for testing this hypothesis in combination with electrophysiology recordings have yet to be evaluated. While electrophysiological studies of tinnitus have been reported in passive, awake animals, these studies fail to control for attentional mechanisms likely to play a role in the perception of tinnitus. NEW METHOD A novel appetitive operant conditioning, two-alternative identification task was developed for detecting acute tinnitus in rats. The procedure optimizes conditions for simultaneously recording oscillatory neural activity while controlling for the attentional state of the animal. RESULTS Tinnitus was detected in six of seven rats following systemic injection with sodium salicylate (200mg/kg IP), a known inducer of tinnitus. Analysis of ongoing local field potentials recorded from chronically implanted electrodes in auditory cortex of a rat reporting tinnitus revealed changes in the spectrum of ongoing neural activity. Comparison with existing method(s): Existing tinnitus-detection methods were not explicitly designed for the simultaneous recording of neural activity. The behavioral method reported here is the first to provide the conditions necessary for obtaining these recordings in chronically implanted rats. CONCLUSIONS The behavioral assay presented here will facilitate research into the neural mechanisms of tinnitus by allowing researchers to compare the electrophysiological data in animals with confirmed tinnitus.

Effects of acoustic trauma on the auditory system of the rat: The role of microglia.

Exposure to loud, prolonged sounds (acoustic trauma, AT) leads to the death of both inner and outer hair cells (IHCs and OHCs), death of neurons of the spiral ganglion and degeneration of the auditory nerve. The auditory nerve (8cn) projects to the three subdivisions of the cochlear nuclei (CN), the dorsal cochlear nucleus (DC) and the anterior (VCA) and posterior (VCP) subdivisions of the ventral cochlear nucleus (VCN). There is both anatomical and physiological evidence for plastic reorganization in the denervated CN after AT. Anatomical findings show axonal sprouting and synaptogenesis; physiologically there is an increase in spontaneous activity suggesting reorganization of circuitry. The mechanisms underlying this plasticity are not understood. Recent data suggest that activated microglia may have a role in facilitating plastic reorganization in addition to removing trauma-induced debris. In order to investigate the roles of activated microglia in the CN subsequent to AT we exposed animals to bilateral noise sufficient to cause massive hair cell death. We studied four groups of animals at different survival times: 30 days, 60 days, 6 months and 9 months. We used silver staining to examine the time course and pattern of auditory nerve degeneration, and immunohistochemistry to label activated microglia in the denervated CN. We found both degenerating auditory nerve fibers and activated microglia in the CN at 30 and 60 days and 6 months after AT. There was close geographic overlap between the degenerating fibers and activated microglia, consistent with a scavenger role for activated microglia. At the longest survival time, there were still silver-stained fibers but very little staining of activated microglia in overlapping regions. There were, however, activated microglia in the surrounding brainstem and cerebellar white matter.

Dissociation of doublecortin expression and neurogenesis in unipolar brush cells in the vestibulocerebellum and dorsal cochlear nucleus of the adult rat.

We have previously shown expression of the protein doublecortin (DCX) in unipolar brush cells (UBCs) in the dorsal cochlear nucleus and vestibulocerebellum of the adult rat. We also saw DCX-immunoreactive elements with the appearance of neuroblasts around the fourth ventricle. Expression of DCX is seen in newborn and migrating neurons and hence considered a correlate of neurogenesis. There were two interpretations of the expression of DCX in UBCs. One possibility is that there might be adult neurogenesis of this cell population. Adult neurogenesis is now well-established, but only for the dentate gyrus of the hippocampus and the subventricular zone. The other possibility is that there is prolonged expression of DCX in adult UBCs that may signal a unique role in plasticity of these neurons. We tested the neurogenesis hypothesis by systemic injections of bromodeoxyuridine (BrdU), a thymidine analog, followed by immunohistochemistry to examine the numbers and locations of dividing cells. We used several different injection paradigms, varying the dose of BrdU, the number of injections and the survival time to assess the possibility of neuronal birth and migration. We saw BrdU-labeled cells in the cerebellum and brainstem; cell division in these regions was confirmed by immunohistochemistry for the protein Ki67. However, neither the numbers nor the distribution of labeled nuclei support the idea of adult neurogenesis and migration of UBCs. The function of DCX expression in UBCs in the adult remains to be understood.

Low-cost blast wave generator for studies of hearing loss and brain injury: Blast wave effects in closed spaces

BACKGROUND Military personnel and civilians living in areas of armed conflict have increased risk of exposure to blast overpressures that can cause significant hearing loss and/or brain injury. The equipment used to simulate comparable blast overpressures in animal models within laboratory settings is typically very large and prohibitively expensive. NEW METHOD To overcome the fiscal and space limitations introduced by previously reported blast wave generators, we developed a compact, low-cost blast wave generator to investigate the effects of blast exposures on the auditory system and brain. RESULTS The blast wave generator was constructed largely from off the shelf components, and reliably produced blasts with peak sound pressures of up to 198dB SPL (159.3kPa) that were qualitatively similar to those produced from muzzle blasts or explosions. Exposure of adult rats to 3 blasts of 188dB peak SPL (50.4kPa) resulted in significant loss of cochlear hair cells, reduced outer hair cell function and a decrease in neurogenesis in the hippocampus. COMPARISON TO EXISTING METHODS Existing blast wave generators are typically large, expensive, and are not commercially available. The blast wave generator reported here provides a low-cost method of generating blast waves in a typical laboratory setting. CONCLUSIONS This compact blast wave generator provides scientists with a low cost device for investigating the biological mechanisms involved in blast wave injury to the rodent cochlea and brain that may model many of the damaging effects sustained by military personnel and civilians exposed to intense blasts.