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. Published by Elsevier Ltd on behalf of IBRO. Keywordssalicylate; tinnitus; hyperacusis; auditory cortex; inferior colliculus; GABA A challenging question for tinnitus research is to identify the neural generator(s) in the cochlea and/or central auditory system (CAS) of tinnitus. Although the phantom sound of tinnitus is commonly induced by noise exposure or ototoxic drugs, increasing evidence suggests that noise and drug-induced cochlear damage that reduces the output of the cochlea induces a plethora of functional changes in the CAS. In many cases of cochlear damage, the CAS appears to increase its gain to compensate for the reduced sensorineural input from the cochlea. An excessive increase in central gain may give rise to the phantom sound of tinnitus under quiet conditions, as well as an intolerance to loud sounds (hyperacusis) (Gerken, 1996;Salvi et al., 2000;Eggermont and Roberts, 2004 which has been linked to chronic tinnitus in humans (Goldstein and Shulman, 1996;Nelson and Chen, 2004). Chronic tinnitus and hyperacusis in humans are most often associated with cochlear damage induced by aging, noise or ototoxic drugs. The salicylate-induced pathology provides a reversible and highly reliable method to investigate how the CAS adjusts due to insult as well as the possible neural correlates of tinnitus and hyperacusis.Previous studies have shown that acoustic trauma which damages hair cells in the cochlea causes an enhancement in sound-evoked activity in the cochlear ...

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Although high doses of sodium salicylate impair cochlear function, it paradoxically enhances sound-evoked activity in the auditory cortex (AC) and augments acoustic startle reflex responses, neural and behavioral metrics associated with hyperexcitability and hyperacusis. To explore the neural mechanisms underlying salicylate-induced hyperexcitability and “increased central gain”, we examined the effects of γ-aminobutyric acid (GABA) receptor agonists and antagonists on salicylate-induced hyperexcitability in the AC and startle reflex responses. Consistent with our previous findings, local or systemic application of salicylate significantly increased the amplitude of sound-evoked AC neural activity, but generally reduced spontaneous activity in the AC. Systemic injection of salicylate also significantly increased the acoustic startle reflex. S-baclofen or R-baclofen, GABA-B agonists, which suppressed sound-evoked AC neural firing rate and local field potentials, also suppressed the salicylate-induced enhancement of the AC field potential and the acoustic startle reflex. Local application of vigabatrin, which enhances GABA concentration in the brain, suppressed the salicylate-induced enhancement of AC firing rate. Systemic injection of vigabatrin also reduced the salicylate-induced enhancement of acoustic startle reflex. Collectively, these results suggest that the sound-evoked behavioral and neural hyperactivity induced by salicylate may arise from a salicylate-induced suppression GABAergic inhibition in the AC.

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