Richard Salvi

The functional neuroanatomy of tinnitus Evidence for limbic system links and neural plasticity

We used PET to map brain regions responding to changes in tinnitus loudness in four patients who could alter tinnitus loudness by performing voluntary oral facial movements (OFMs). Cerebral blood flow was measured in four patients and six controls at rest, during the OFM, and during stimulation with pure tones. OFM-induced loudness changes affected the auditory cortex contralateral to the ear in which tinnitus was perceived, whereas unilateral cochlear stimulation caused bilateral effects, suggesting a retrocochlear origin for their tinnitus. Patients, compared with controls, showed evidence for more widespread activation by the tones and aberrant links between the limbic and auditory systems. These abnormal patterns provide evidence for cortical plasticity that may account for tinnitus and associated symptoms. Although audiologic symptoms and examinations of these patients were typical, the unusual ability to modulate tinnitus loudness with an OFM suggests some caution may be warranted in generalizing these findings.

Auditory plasticity and hyperactivity following cochlear damage.

This paper will review some of the functional changes that occur in the central auditory pathway after the cochlea is damaged by acoustic overstimulation or by carboplatin, an ototoxic drug that selectively destroys inner hair cells (IHCs) in the chinchilla. Acoustic trauma typically impairs the sensitivity and tuning of auditory nerve fibers and reduces the neural output of the cochlea. Surprisingly, our results show that restricted cochlear damage enhances neural activity in the central auditory pathway. Despite a reduction in the auditory-nerve compound action potential (CAP), the local field potential from the inferior colliculus (IC) increases at a faster than normal rate and its maximum amplitude is enhanced at frequencies below the region of hearing loss. To determine if this enhancement was due to loss of sideband inhibition, we recorded from single neurons in the IC and dorsal cochlear nucleus before and after presenting a traumatizing above the units characteristic frequency (CF). Following the exposure, some neurons showed substantial broadening of tuning below CF, less inhibition, and a significant increase in discharge rate, consistent with a model involving loss of sideband inhibition. The central auditory system of the chinchilla can be deprived of some of its cochlear inputs by selectively destroying IHCs with carboplatin. Selective IHC loss reduces the amplitude of the CAP without affecting the threshold and tuning of the remaining auditory nerve fibers. Although the output of the cochlea is reduced in proportion to the amount of IHC loss, the IC response shows only a modest amplitude reduction, and remarkably, the response of the auditory cortex is enhanced. These results suggest that the gain of the central auditory pathway can be up- or down regulated to compensate for the amount of neural activity from the cochlea.

Quantitative measures of hair cell loss in CBA and C57BL/6 mice throughout their life spans

The CBA mouse shows little evidence of hearing loss until late in life, whereas the C57BL/6 strain develops a severe and progressive, high-frequency sensorineural hearing loss beginning around 3-6 months of age. These functional differences have been linked to genetic differences in the amount of hair cell loss as a function of age; however, a precise quantitative description of the sensory cell loss is unavailable. The present study provides mean values of inner hair cell (IHC) and outer hair cell (OHC) loss for CBA and C57BL/6 mice at 1, 3, 8, 18, and 26 months of age. CBA mice showed little evidence of hair cell loss until 18 months of age. At 26 months of age, OHC losses in the apex and base of the cochlea were approximately 65% and 50%, respectively, and IHC losses were approximately 25% and 35%. By contrast, C57BL/6 mice showed approximately a 75% OHC and a 55% IHC loss in the base of the cochlea at 3 months of age. OHC and IHC losses increased rapidly with age along a base-to-apex gradient. By 26 months of age, more than 80% of the OHCs were missing throughout the entire cochlea; however, IHC losses ranged from 100% near the base of the cochlea to approximately 20% in the apex.

Targeted deletion of the cytosolic Cu/Zn-superoxide dismutase gene (Sod1) increases susceptibility to noise-induced hearing loss.

Reactive oxygen species (ROS) such as superoxide, peroxide and hydroxyl radicals are generated during normal cellular metabolism and are increased in acute injury and in many chronic disease states. When their production is inadequately regulated, ROS accumulate and irreversibly damage cell components, causing impaired cellular function and death. Antioxidant enzymes such as superoxide dismutase (SOD) play a vital role in minimizing ROS levels and ROS-mediated damage. The cytosolic form of Cu/Zn-SOD appears specialized to remove superoxide produced as a result of injury. ‘Knockout’ mice with targeted deletion of Sod1, the gene that codes for Cu/Zn-SOD, develop normally but show enhanced susceptibility to central nervous system injury. Since loud noise is injurious to the cochlea and is associated with elevated cochlear ROS, we hypothesized that Sod1 knockout mice would be more susceptible to noise-induced permanent threshold shifts (PTS) than wild-type and heterozygous control mice. Fifty-nine mice (15 knockout, 29 heterozygous and 15 wild type for Sod1) were exposed to broad-band noise (4.0–45.0 kHz) at 110 dB SPL for 1 h. Hearing sensitivity was evaluated at 5, 10, 20 and 40 kHz using auditory brainstem responses before exposure and 1, 14 and 28 days afterward. Cu/Zn-SOD deficiency led to minor (0–7 dB) threshold elevations prior to noise exposure, and about 10 dB of additional noise-induced PTS at all test frequencies, compared to controls. The distribution of thresholds at 10 and 20 kHz at 28 days following exposure contained three modes, each showing an effect of Cu/Zn-SOD deficiency. Thus another factor, possibly an additional unlinked gene, may account for the majority of the observed PTS. Our results indicate that genes involved in ROS regulation can impact the vulnerability of the cochlea to noise-induced hearing loss.

Salicylate induced tinnitus: behavioral measures and neural activity in auditory cortex of awake rats.

Neurophysiological studies of salicylate-induced tinnitus have generally been carried out under anesthesia, a condition that abolishes the perception of tinnitus and depresses neural activity. To overcome these limitations, measurement of salicylate induced tinnitus were obtained from rats using schedule induced polydipsia avoidance conditioning (SIPAC) and gap pre-pulse inhibition of acoustic startle (GPIAS). Both behavioral measures indicated that tinnitus was present after treatment with 150 and 250 mg/kg of salicylate; measurements with GPIAS indicated that the pitch of the tinnitus was near 16 kHz. Chronically implanted microwire electrode arrays were used to monitor the local field potentials and spontaneous discharge rate from multiunit clusters in the auditory cortex of awake rats before and after treatment with 150 mg/kg of salicylate. The amplitude of the local field potential elicited with 60 dB SPL tone bursts increased significantly 2h after salicylate treatment particularly at 16-20 kHz; frequencies associated with the tinnitus pitch. Field potential amplitudes had largely recovered 1-2 days post-salicylate when behavioral results showed that tinnitus was absent. The mean spontaneous spike recorded from the same multiunit cluster pre- and post-salicylate decreased from 22 spikes/s before treatment to 14 spikes/s 2h post-salicylate and recovered 1 day post-treatment. These preliminary physiology data suggest that salicylate induced tinnitus is associated with sound evoked hyperactivity in auditory cortex and spontaneous hypoactivity.

Age-related hearing loss in C57BL/6J mice is mediated by Bak-dependent mitochondrial apoptosis

Age-related hearing loss (AHL), known as presbycusis, is a universal feature of mammalian aging and is the most common sensory disorder in the elderly population. The molecular mechanisms underlying AHL are unknown, and currently there is no treatment for the disorder. Here we report that C57BL/6J mice with a deletion of the mitochondrial pro-apoptotic gene Bak exhibit reduced age-related apoptotic cell death of spiral ganglion neurons and hair cells in the cochlea, and prevention of AHL. Oxidative stress induces Bak expression in primary cochlear cells, and Bak deficiency prevents apoptotic cell death. Furthermore, a mitochondrially targeted catalase transgene suppresses Bak expression in the cochlea, reduces cochlear cell death, and prevents AHL. Oral supplementation with the mitochondrial antioxidants α-lipoic acid and coenzyme Q10 also suppresses Bak expression in the cochlea, reduces cochlear cell death, and prevents AHL. Thus, induction of a Bak-dependent mitochondrial apoptosis program in response to oxidative stress is a key mechanism of AHL in C57BL/6J mice.

Age-related cochlear hair cell loss is enhanced in mice lacking copper/zinc superoxide dismutase

Age-related hearing loss in humans and many strains of mice is associated with a base-to-apex gradient of cochlear hair cell loss. To determine if copper/zinc superoxide dismutase (Cu/Zn SOD) deficiency influences age-related cochlear pathology, we compared hair cell losses in cochleas obtained from 2-, 7-, and 17- to 19-month-old wild type (WT) mice with normal levels of Cu/Zn SOD and mutant knockout (KO) mice with a targeted deletion of Sod1, the gene that codes for Cu/Zn SOD. WT and KO mice exhibited similar patterns of hair cell loss with age, i.e., a baso-apical progression of hair cell loss, with greater loss of outer hair cells than inner hair cells. Within each age group, the magnitude of loss was much greater in KO mice compared to WT mice. The results indicate that Cu/Zn SOD deficiency potentiates cochlear hair cell degeneration, presumably through metabolic pathways involving the superoxide radical.

Enhanced evoked response amplitudes in the inferior colliculus of the chinchilla following acoustic trauma

Evoked response amplitude-level functions were measured from electrodes in the inferior colliculus of the chinchilla before and after exposure to a 2 kHz pure tone of 105 dB SPL. The exposure produced approximately 20-30 dB of permanent threshold shift from 2 to 8 kHz, but little or no hearing loss at higher or lower frequencies. Generally less than 60% of the outer hair cells were missing in the region of hearing loss. The amplitude-level functions measured at 4 and 8 kHz generally showed a loss in sensitivity at low sound levels, a reduction in the maximum amplitude and sometimes steeper than normal slopes. The amplitude-level functions measured at 2 kHz also showed a loss in sensitivity; however, the maximum amplitude was often greater than normal. Even though there was no loss in sensitivity at 0.5 kHz, the amplitude-level function was steeper than normal and the maximum amplitude of the evoked response was almost always substantially larger than normal. The enhancement of the evoked response amplitude from the inferior colliculus does not appear to originate in the cochlea, but may reflect a reorganization of neural activity in the central auditory pathway.

GAD levels and muscimol binding in rat inferior colliculus following acoustic trauma

Pharmacological studies of the inferior colliculus (IC) suggest that the inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in shaping responses to simple and complex acoustic stimuli. Several models of auditory dysfunction, including age-related hearing loss, tinnitus, and peripheral deafferentation, suggest an alteration of normal GABA neurotransmission in central auditory pathways. The present study attempts to further characterize noise-induced changes in GABA markers in the IC. Four groups (unexposed control, 0 h post-exposure, 42 h post-exposure, and 30 days post-exposure) of 3-month-old male Fischer 344 rats were exposed to a high intensity sound (12 kHz, 106 dB) for 10 h. Observed hair cell damage was primarily confined to the basal half of the cochlea. There was a significant decrease in glutamic acid decarboxylase (GAD(65)) immunoreactivity in the IC membrane fraction compared to controls (P<0.05) at 0 h (-41%) and 42 h (-28%) post-exposure, with complete recovery by 30 days post-exposure (P>0.98). Observed decreases in cytosolic levels of GAD(65) were not significant. Quantitative muscimol receptor binding revealed a significant increase (+20%) in IC 30 days after sound exposure (P<0.05). These data suggest that changes in GABA neurotransmission occur in the IC of animals exposed to intense sound. Additional studies are needed to determine whether these changes are a result of protective/compensatory mechanisms or merely peripheral differentiation, as well as whether these changes preserve or diminish central auditory system function.

Salicylate increases the gain of the central auditory system

High doses of salicylate, the anti-inflammatory component of aspirin, induce transient tinnitus and hearing loss. Systemic injection of 250 mg/kg of salicylate, a dose that reliably induces tinnitus in rats, significantly reduced the sound evoked output of the rat cochlea. Paradoxically, salicylate significantly increased the amplitude of the sound-evoked field potential from the auditory cortex (AC) of conscious rats, but not the inferior colliculus (IC). When rats were anesthetized with isoflurane, which increases GABA-mediated inhibition, the salicylate-induced AC amplitude enhancement was abolished, whereas ketamine, which blocks N-methyl-d-aspartate receptors, further increased the salicylate-induced AC amplitude enhancement. Direct application of salicylate to the cochlea, however, reduced the response amplitude of the cochlea, IC and AC, suggesting the AC amplitude enhancement induced by systemic injection of salicylate does not originate from the cochlea. To identify a behavioral correlate of the salicylate-induced AC enhancement, the acoustic startle response was measured before and after salicylate treatment. Salicylate significantly increased the amplitude of the startle response. Collectively, these results suggest that high doses of salicylate increase the gain of the central auditory system, presumably by down-regulating GABA-mediated inhibition, leading to an exaggerated acoustic startle response. The enhanced startle response may be the behavioral correlate of hyperacusis that often accompanies tinnitus and hearing loss.