Residual inhibition: From the putative mechanisms to potential tinnitus treatment.

Neurons in various sensory systems show some level of spontaneous firing in the absence of sensory stimuli. In the auditory system spontaneous firing has been shown at all levels of the auditory pathway from spiral ganglion neurons in the cochlea to neurons of the auditory cortex. This internal "noise" is normal for the system and it does not interfere with our ability to perceive silence or analyze sound. However, this internal noise can be elevated under pathological conditions, leading to the perception of a phantom sound known as tinnitus. The efforts of many research groups, including our own, led to the development of a mechanistic understanding of this process: After cochlear insult the input to the central auditory system becomes markedly reduced. As a result, the neural activity in the central auditory system is enhanced to compensate for this reduced input. Such hyperactivity is hypothesized to be interpreted by the brain as a presence of sound. This implies that suppression of hyperactivity should reduce/eliminate tinnitus. This review explores research from our laboratory devoted to identifying the mechanism underlying residual inhibition of tinnitus, a brief suppression of tinnitus following a sound stimulus. The key mechanisms that govern neural suppression of spontaneous activity in animals closely resemble clinical psychoacoustic findings of residual inhibition (RI) observed in tinnitus patients. This suppression is mediated by metabotropic glutamate receptors (mGluRs). Lastly, drugs targeting mGluRs suppress spontaneous activity in auditory neurons and reduce/eliminate behavioral signs of tinnitus in mice. Thus, these drugs are therapeutically relevant for tinnitus suppression in humans.