I think I figured out the reason why Keppra works for some people with hyperacusis and reactive tinnitus. Apologies in advance if this has already been said before in this thread.
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If I understood it correctly, in the synapses located at the basolateral membrane of outer hair cells (OHCs),
electrical activity via the activation of voltage-gated L-type calcium channels causes releases of glutamate through vesicles into the boutons of type II afferents. We know that normally,
"glutamatergic synaptic transmission from OHCs [to type II afferent boutons] is weak. Individual OHCs release single vesicles with low probability so that summed excitation from the entire pool [do not know what Fuchs means by entire pool] of presynaptic OHCs is required for action potential initiation. Maximal stimulation [?] of an OHC had a one in four chance of releasing a single vesicle during maximal stimulation. Each vesicle causes a 4-mV depolarization [of the type II afferent] on average, thus requiring 7 or more concurrent vesicles for the type II afferent to reach the 25-mV threshold for action potential initiation."
But here's the curious thing. Paul Fuchs found out that
the number and size of OHC synaptic ribbons increased after mice were exposed to damaging sound. Using a statistical model Paul Fuchs estimated that
a global 20 % increase in OHC ribbon count observed in the noise-exposed mice
is estimated to cause a 99 % increase in action potentials [of type II afferent neurons] in response to maximal stimulation of OHCs across the entire population of 1000 (virtual) type II afferents.
In short, if I understood it correctly,
acoustic stimulation --> L-type calcium channels open --> calcium ions enter the outer hair cell --> OHC depolarizes --> glutamate is sent to type II afferents --> Type II afferents depolarize.
Now, in the case of acoustic damage, because of increased ribbon number and size, the same OHC depolarization makes it more likely for a type II neuron to fire. So that is a possible reason why normally non-painful sounds become painful to people with hearing damage.
Keppra, and also Nifedipine, block the influx of calcium ions through L-type calcium channels. So theoretically it should alleviate hearing disorders where type II neurons are the culprit, as is the case with noxacusis and maybe (reactive) tinnitus as well.