Spontaneous activity in the inferior colliculus of CBA/J mice after manipulations that induce tinnitus

Several physiological studies have linked experimentally induced tinnitus to increases in the spontaneous activity of auditory neurons. These results have led to the proposal of hyperactivity models of tinnitus in which elevated neural activity in the absence of auditory stimulation is perceived as phantom sound. Such models are appealing in their simplicity but remain controversial because a generalized elevation of spontaneous rates may not be observed after treatments that induce tinnitus in humans and experimental animals. Our study addressed these issues by characterizing the effects of common methods of tinnitus induction on spontaneous activity in the central nucleus of the inferior colliculus (ICC). The ICC is an interesting structure in tinnitus research because its diverse inputs include putative generator sites in the dorsal cochlear nucleus, as well as brainstem sources that appear to remain normal after tinnitus induction. Groups of CBA/J mice were subjected to one of three induction methods: bilateral or unilateral sound exposure, and acute salicylate intoxication. Relative to normal baselines, bilaterally exposed mice showed increases in the spontaneous rates of neurons with tuning near the exposure frequency. When the sample was separated into physiologically defined response classes, exposure effects were strongest among neurons with broad excitatory bandwidths. By contrast, salicylate decreased the spontaneous rates of low-frequency neurons with transient sound-evoked activity. Our results suggest that the disordered processes of hearing that give rise to tinnitus do not involve a pervasive elevation of spontaneous activity or a single mode of induction.