Loud noise damages the auditory sensory cells—the cochlear hair cells—which generally results in an elevated auditory threshold detectable on conventional hearing tests. However, loss of hair cells is not the only consequence of noise. Hair cells communicate auditory information to the brain via the spiral ganglion neurons, whose axons form the auditory nerve. Noise also damages the synaptic connections between hair cells and spiral ganglion neurons, disrupting that vital communication and reducing activity in the auditory nerve.1 This damage to synapses, termed synaptopathy, is, in some respects, more insidious than the loss of hair cells. Cochlear synaptopathy can be caused by noise levels lower than those that damage hair cells2, and is therefore likely to be more common. Postmortem observations of human cochleae show intact hair cells with diminished numbers of synapses.3 Because the hair cells remain, auditory thresholds are not significantly elevated and simply measuring thresholds will not reveal any impairment. Consequently, hearing impairment caused by synaptopathy alone is often referred to as hidden hearing loss. The damage is detectable using auditory tests other than measuring threshold. For example, reduced activity in the auditory nerve is detectable by the measure of the auditory brainstem response.4 Hidden hearing loss has been conjectured to be a possible cause of tinnitus and compromised speech comprehension in noisy environments.2
NOISE-INDUCED COCHLEAR SYNAPTOPATHY
Lost hair cells do not regenerate in mammals nor do lost cochlear synapses, although attempts to find means to promote such regeneration are underway in many laboratories. Our laboratory, however, is investigating the prevention of noise-induced cochlear synaptopathy (NICS). To this end, we have investigated the cause of synaptopathy by focusing on the physiology of the cochlear synapse. These synapses use glutamate as the neurotransmitter.5,6 The hair cell, the presynaptic side of the synapse, releases glutamate in response to sound. Glutamate binds to receptors on the spiral ganglion neuron at the postsynaptic side of the synapse.7,8 The glutamate receptors then allow an ionic current to flow into the postsynaptic structure, thereby initiating electrical activity in the auditory nerve. Louder sound results in larger amounts of glutamate released from the hair cell, which increases activity in the auditory nerve to inform the brain about the loudness of the sound. With moderate sound levels, this process of normal synaptic transmission functions without causing any damage. However, very loud sounds can cause excess glutamate release from the hair cell, resulting in greatly increased postsynaptic inward current, including the increased inward flow of calcium ions (Ca2+). Studies of glutamatergic synapses in the brain had already shown that such inward flow of Ca2+ can result in damage to the postsynaptic cell, a phenomenon termed excitotoxicity.9
The possibility that NICS is due to excitotoxicity was investigated by Pujol and colleagues, who showed that direct infusion of glutamate agonists into the cochlea of guinea pigs caused excitotoxic destruction of cochlear synapses resembling damage caused by noise.10,11 They further showed that pharmacological blockade of glutamate receptors prevented NICS.12 These studies established that the proximate cause of NICS is excitotoxicity due to the excessive release of glutamate from hair cells at cochlear synapses. However, blockade of all glutamate receptors is not an optimal means to protect cochlear synapses from noise because such blockade would entirely prevent synaptic transmission and hearing. A more precisely targeted approach is needed, recognizing that glutamate receptors are diverse and not all types of glutamate receptors may be responsible for the damage. An important clue is that excitotoxicity is generally associated with Ca2+ influx and not all glutamate receptors are permeable to Ca2+.
In the brain, excitotoxic Ca2+ influx is mainly via NMDA-type glutamate receptors, but this does not appear to be the case in the cochlea. Glutamate agonists that do not activate NMDA receptors can account for excitotoxicity in the cochlea.10 Rather, these studies implicated AMPA-type glutamate receptors, often referred to as AMPA receptors, which are tetramers made up of combinations of four different subunits, GluA1, GluA2, GluA3, and GluA4, encoded by four different genes, Gria1, Gria2, Gria3, and Gria4, respectively. The subunit composition of individual AMPA receptors depends on which genes are expressed in a cell. For example, in the brain, GluA1 is often a predominant subunit and AMPA receptors will typically have at least one of the subunits being GluA1. In the cochlea, spiral ganglion neurons express little, if any, GluA1 and AMPA receptors at the hair cell synapse that are composed of GluA2, GluA3, and GluA4 subunits.13 What is most relevant to the issue of excitotoxicity is GluA2. AMPA receptors with one or more GluA2 subunits in the tetramer are impermeable to calcium. AMPA receptors lacking GluA2 subunits are calcium permeable and are therefore capable of contributing to excitotoxicity.14,15
Our laboratory took advantage of a paradigm that had been previously developed by Kujawa, Liberman, and their colleagues16 to calibrate noise exposure to mice at a level that caused synaptopathy but that did not damage hair cells. By confining the damage to the postsynaptic elements, it was possible to test compounds that would be specifically protective against synaptopathy. We used IEM-1460, a compound that selectively blocks GluA2-lacking AMPA receptors,17,18 to ask whether these could be the culprits in NICS. We found that when introduced into the cochlea of mice prior to noise and present throughout the noise exposure, IEM-1460 effectively prevented synaptopathy, as demonstrated by the counts of cochlear synapses and the auditory nerve activity in the auditory brainstem response (ABR).19
This result was somewhat unexpected because we know that GluA2 is present at cochlear synapses. This was resolved by observations made by our colleague Mark Rutherford, PhD, who showed that on a nanoscale, the postsynaptic side of the synapse contains patches of AMPA receptors that are deficient in GluA2 and patches in which GluA2 is more abundant.19 Thus, despite the presence of GluA2 at the synapse, GluA2-lacking AMPA receptors can be responsible for excitotoxic trauma to the postsynaptic element. Blocking these receptors with IEM-1460 prevents excitotoxic trauma and so prevents NICS.
The mice appeared to have normal hearing as assessed by ABR measures even while the cochlea was being infused with IEM-1460.19 Presumably, this is because the synapse does have AMPA receptors containing GluA2. These are not blocked by IEM-1460, so they can maintain synaptic transmission even while the GluA2-lacking AMPA receptors are blocked, but the GluA2-containing AMPA receptors do not contribute to excitotoxicity. Thus, the selective blocker of GluA2-lacking AMPA receptors can prevent NICS while not interfering with normal hearing, something not achievable with less selective compounds that block all glutamate receptors or all AMPA-type glutamate receptors.
In principle, such compounds could be used to protect individuals against NICS in loud work environments (e.g., the military) without preventing their ability to hear orders, warnings, or other communications. However, this “chemical earmuff” strategy has only been assessed in mice, and it is yet to be established whether such an approach would be safe and effective in humans.
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