The emergence of hyperactivity in the form of elevated spontaneous firing rates after cochlear trauma has been well documented in a number of central auditory structures, including the auditory cortex, inferior colliculus, and dorsal subdivision of the cochlear nucleus. This hyperactivity is of interest as a possible neural substrate of tinnitus. Whether the ventral subdivision of the cochlear nucleus shows hyperactivity has never been investigated despite the fact that, like the dorsal division, it also receives direct input from the damaged cochlea and supplies major ascending inputs to brainstem and midbrain auditory centers. We investigated spontaneous neuronal firing rates in the ventral cochlear nucleus in a guinea pig model of cochlear trauma in which we have shown that hyperactivity consistently develops in the inferior colliculus (Mulders and Robertson, 2009). The mean spontaneous firing rates of ventral cochlear nucleus neurons was significantly elevated compared to sham controls. This hyperactivity was more evident in primary-like and onset categories of neurons. Hyperactivity in the ventral subdivision of cochlear nucleus therefore needs to be considered in relation to neural models of the genesis of tinnitus.

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Spontaneous firing rates of neurons in the central auditory pathway, such as in the inferior colliculus, are known to be increased after cochlear trauma. This so-called hyperactivity is thought to be involved in the generation of tinnitus, a phantom auditory perception. Recent research in an animal model suggests behavioural signs of tinnitus can be significantly reduced by silencing or removal of the paraflocculus (PF) of the cerebellum. The current study investigated the effects of acute PF removal on spontaneous firing rates recorded from single neurons in the right inferior colliculus of guinea pigs with normal hearing (which did not receive acoustic trauma) or with hearing loss caused by acoustic trauma. Spontaneous firing rates were obtained at either 2 or 13 weeks after initial surgery on the left side. In half of the animals in each group the left PF was removed immediately prior to the spontaneous firing rates recordings. In the acoustic trauma groups, spontaneous firing rates in the inferior colliculus were higher when the PF was removed compared to animals with an intact PF. This effect of PF removal was not observed in animals that did not receive acoustic trauma. These results suggest that the PF has a tonic inhibitory effect on hyperactivity in the inferior colliculus in animals with hearing loss, but not on normal spontaneous firing rates in normal hearing animals.