The term “hidden hearing loss” refers to a form of hearing impairment in which a subject presents with normal otoacoustic emissions and audiometric thresholds but complains of hearing problems such as tinnitus or difficulty understanding speech in noise.1,2 Recent human and animal studies on aging and noise-induced hearing loss suggest that hidden hearing loss may result from ototoxic damage to the inner hair cells (IHC) and/or type I auditory nerve fibers (ANF) that synapse on inner hair cells,3,4 conditions reminiscent of auditory neuropathy.
CARBOPLATIN & CISPLATIN OTOTOXICITY
Cisplatin is an ototoxic drug best known for damaging outer hair cells (OHC) and inner hair cells (IHC); however, platinum-based drugs can also affect vulnerable areas of the central nervous system. Carboplatin is considered less ototoxic than cisplatin.5,6 Nevertheless, when carboplatin is administered to chinchillas, it preferentially destroys the IHC and ANF over the length of the cochlea (Fig. 1A) in contrast to the base-to-apex gradient seen with most ototoxic drugs.7 Carboplatin is neurotoxic and causes rapid excitotoxic swelling of the afferent ANF synapses, followed a few days later by IHC loss.8
Cochlear function. Despite massive IHC and ANF damage, otoacoustic emissions (Fig. 1B) and the cochlear microphonic potential generated by the OHC remain normal, indicating that OHC are functioning normally.7 However, the amplitude of the summating potential generated by the IHC and the amplitude of the compound action potential (Fig. 1C) generated by ANF are reduced in proportion to the amount of IHC loss. Therefore, the summating potential and compound action potential (analogous to ABR wave I) can be used to assess the functional status of IHC and ANF in clinical cases of suspected hidden hearing loss. In carboplatin-treated chinchillas with partial loss of IHC and ANF, single auditory nerve fiber recordings can be used to interrogate the functional capacity of the remaining IHC and ANF.7 When measurements are made from a single auditory nerve fiber connected to a cochlear region where 60 to 90 percent of the IHC/ANF are missing, only a few ANF are still available from which to obtain measurements. Surprisingly, the remaining acoustically responsive ANF behave normally (with low thresholds and sharp tuning) despite massive cochlear damage.
Auditory Perception. How well does a carboplatin-treated chinchilla hear when many IHC/ANF are missing and only a few IHC/ANF are left to detect a tone in quiet? When chinchilla audiograms are measured pre- and post-carboplatin treatment, the hearing thresholds remain normal in animals with up to 80 percent IHC loss (Fig. 1D); however, the thresholds increase precipitously once IHC lesions exceed 85 percent (Fig. 1E).9 Apparently, only a few IHC/ANF are needed to detect a sound in quiet. However, if pure-tone thresholds are measured in broadband noise pre- and post-carboplatin, tone thresholds in noise are elevated significantly in chinchillas with large IHC/ANF lesions even though thresholds in quiet are normal.10
Central Gain Compensation. A carboplatin-induced lesion that destroys half the IHC/ANF would reduce the neural output of the cochlea by roughly 50 percent. If the auditory system behaved as a simple linear system, neural responses recorded at higher auditory centers should be reduced by half, unless the brain possessed a neural amplification system that can boost these weak cochlear signals. To test this hypothesis, electrodes were chronically implanted at the level of the cochlea, inferior colliculus, and auditory cortex to record the local field potentials from these regions pre- and post-carboplatin. When 50 percent of the IHC/ANF were destroyed, the compound action potential from the cochlea was reduced by 50 percent (Fig. 2A, intersection of vertical dashed line with solid red line). However, by the time the neural signal reached the inferior colliculus, the response was reduced by only 20 percent (Fig. 2A, dashed green line), evidence of neural amplification between the cochlea and auditory midbrain.11 Surprisingly, the neural responses from the auditory cortex were even larger than normal (Fig. 2A, dashed blue line) as long as the IHC lesions were not too severe. These results provide evidence of additional amplification between the auditory midbrain and auditory cortex. Thus, the central auditory pathway progressively amplifies weak cochlear signals as they are relayed up the central auditory pathway. Similar central amplification has also been observed in cases of noise- and drug-induced hearing loss.12,13
How is the central auditory pathway able to amplify these weak cochlear signals? The central auditory pathway comprises a complex network of inhibitory and excitatory synapses. The strong excitatory signals relayed from a normal cochlea are normally suppressed by strong GABAergic inhibition similar to negative gain control.14,15 One way to enhance weak cochlear signals from a damaged cochlea is to increase the gain of the central auditory pathway by removing GABAergic inhibition. To test this hypothesis, sound-evoked local field potentials were recoded from the auditory cortex of normal chinchillas and compared with those from another group of carboplatin-treated chinchillas with large IHC/ANF lesions. When GABAergic inhibition was pharmacologically suppressed, the neural responses from the auditory cortex nearly doubled in size, evidence of strong GABAergic inhibition in a normal auditory cortex.16 However, when GABAergic inhibition was pharmacologically suppressed in carboplatin-treated chinchillas, it failed to increase sound-evoked responses in the auditory cortex. These results suggest that GABAergic inhibition had been eliminated in the auditory cortex of carboplatin-treated chinchillas. Because these weak excitatory signals entering the auditory cortex were unopposed by inhibition, they were able to evoke a strong cortical response.
Cisplatin and other platinum-based anticancer drugs are neurotoxic and can damage vulnerable areas of the brain such as the hippocampus.17,18 The hippocampus, which plays an important role in the formation of new memories, is connected with many different parts of the auditory pathway and can be activated by sound.19-21 The hippocampus contains a stem cell niche, and is one of only two areas of the adult brain where new neurons are born (neurogenesis) throughout one's life.22 When hippocampal neurogenesis is suppressed, it becomes much more difficult to form new memories such as recalling where you parked your car after leaving the shopping mall (i.e., spatial navigation).23,24
Anti cancer drugs such as cisplatin suppress cell division, damage hippocampal synapses, and contribute to cognitive dysfunction, a condition referred to as chemobrain.25,26 To determine if cisplatin could suppress hippocampal neurogenesis, we treated rats with cisplatin and measured the number of newborn neurons. Cisplatin suppressed neurogenesis by more than 70 percent at seven days post-treatment and significantly increased the expression of genes that promote cell death in the hippocampus.27,28 Humans and animal studies have shown that loss of sensory information from the vestibular system also reduces neurogenesis and promotes atrophy of the hippocampus.29 These results suggest that the loss of auditory information from a damaged cochlea might also suppress hippocampal neurogenesis. To test this hypothesis, rodents were unilaterally exposed to an intense noise that caused significant unilateral hearing loss and cochlear damage. Importantly, the unilateral hearing loss also suppressed hippocampal neurogenesis by approximately 40 percent30 and impaired performance on a memory spatial navigation task.31,32 Together, these results indicate that platinum-based ototoxic drugs and noise-induced hearing loss can suppress hippocampal neurogenesis, changes that could contribute to cognitive decline among those with hearing loss.33
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