by Will Boggs, MD

NEW YORK (Reuters Health) – Dorsal cochlear nucleus responses to trigeminal stimulation increase after noise-induced hearing loss, suggesting that somatosensory neurons may play a role in the pathogenesis of tinnitus, according to the findings of a guinea pig study.

"Previous studies have linked hyperactivity in the cochlear nucleus (i.e., increased spontaneous firing rates) to tinnitus measured behaviorally," Dr. Susan E. Shore from University of Michigan Medical School, Ann Arbor, told Reuters Health. "Our study suggests that the neurons that step up their activity in the cochlear nucleus are those that are activated by somatosensory inputs (especially those from the head and neck region)."

Dr. Shore and associates investigated the possibility that reduction of VIIIth nerve input to the cochlear nucleus results in trigeminal system compensation for the loss of auditory inputs by measuring responses of dorsal cochlear nucleus neurons to trigeminal and bimodal (trigeminal plus acoustic) stimulation in normal and noise-damaged guinea pigs.

Animals with noise-induced hearing loss had significantly lower thresholds, shorter latencies and durations, and increased amplitudes of response to trigeminal stimulation than did normal animals, the authors report in the January issue of the European Journal of Neuroscience.

Compared with normal animals, noise-damaged animals showed a greater proportion of inhibitory and a lesser proportion of excitatory responses to stimulation.

The number of cells showing bimodal integration and the degree of bimodal integration were enhanced after noise damage, the investigators say, and this integration was entirely suppressive.

"The correlation of increased spontaneous rates with the presence of tinnitus in animals, along with an increased prevalence of somatosensory influences in patients with tinnitus and the findings of the present study suggest that the somatosensory system may play a role in the generation of tinnitus," the authors conclude.

"It is important to consider the somatosensory system in diagnosing the patient’s type of tinnitus," Dr. Shore said. "There are no current standards for doing this, but some physicians in Europe are working on developing these types of tests."

"We have a series of studies ongoing in animals exploring the manner in which somatosensory stimulation alters the firing patterns of neurons in the cochlear nucleus," she continued. "We are planning to investigate differences in these modulation patterns in animals with behavioral evidence of tinnitus."

"Increasing awareness of factors contributing to tinnitus is important in developing alternative methods of treatment," Dr. Shore concluded.

Eur J Neurosci 2008;27:155-168.

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