Activation of serotonergic neurons during salicylate-induced tinnitus.

Abstract

In gerbils in which tinnitus was induced by salicylate, neurons in the dorsal raphe nucleus were activated. Since about half the neurons in this region are serotonergic, this indicates that serotonin (5-HT) might play a role in the mechanisms of central tinnitus. The goal of this study was to determine if serotonergic neurons are activated during salicylate-induced tinnitus. Further, to determine if the same neurons might modulate the cochlea during tinnitus, neuroanatomical tract-tracing with 5-HT immunohistochemistry was used to determine if serotonergic neurons project to the gerbil cochlea. Randomized, prospective, gerbils. Six gerbils were injected with salicylate (saline for controls). Four hours later, the gerbils were euthanized, perfused, and their brains collected for immunohistochemical labeling of 5-HT and c-fos. For the tract-tracing, FluoroGold was injected into the cochleae of 3 gerbils. The gerbils were euthanized and perfused 4-11 days later and the brains immunohistochemically processed for 5-HT. More serotonergic neurons expressed c-fos in the salicylate-injected animals compared to controls. The increase was significant for 3 of the 8 major serotonergic cell groups including B7, B9, and the caudal linear nucleus. Despite robust labeling of olivocochlear and vestibular efferents with FluoroGold, 5-HT-labeled neurons containing FluoroGold were lacking. Salicylate-induced tinnitus activates serotonergic neurons in rostral cell groups. Activation of these neurons is not likely to influence cochlear function directly, but is likely to influence a number of auditory and non-auditory regions known to be involved with tinnitus.