Microglia changes in rat dorsal cochlear nucleus correlate to behavioural tinnitus evidence

Tinnitus is a phantom auditory perception (buzzing or ringing) that can become chronical, affecting quality of life similarly to chronic pain. Although several risk factors have been identified, its etiogenesis has not been clarified yet. Dorsal cochlear nuclei (DCN) show changes in plasticity that are thought to be necessary for tinnitus onset but not after its chronicization (Brozoski et al. 2011). One of the major problems in animal tinnitus models is the unreliability of tinnitus induction protocols: a large fraction of treated animals, in fact, do not develop the symptom (Koehler and Shore 2013), and tinnitus is also often associated with deafness and/or hyperacusia, which complicate interpretation of results. After noise trauma, neural plasticity in DCN fusiform cells appears associated to actual tinnitus presence (Koehler and Shore 2013). We investigated the possible involvement of microglia in tinnitus-associated aberrant DCN plasticity. In our experiments we induced tinnitus with unilateral cochlear destruction, noise trauma or salicylate and observed Iba-1 immunofluorescence in DCN slices by confocal microscopy. Although all treatments were able to induce tinnitus (tested as in Turner 2006), DCN microglial responses of tinnitus-positive animals were different: after salicyilate treatment, microglia significantly increased in density (from 117.3+47.2, n=15 to 215.3+54.8 cell/mm2, n=18), but did not show activation signs, whereas after cochlear destruction, microglia showed a larger increase in density (maximal in the ipsilateral nucleus after 5 days, 466,4+185,7; n=14) and clear signs of activation. After noise trauma, microglia became less uniformly distributed, showing a cluster in the DCN region corresponding to noise trauma frequencies (n=1). Microglia activation after cochlear destruction was necessary for tinnitus onset: in animals treated with minocycline after surgery, neither microgliosis (after 5 days, ipsilateral DCN density was 144.1+33.4, n=5) nor behavioural signs of tinnitus were observed. On the other hand, salicylate-induced tinnitus was not dependent on microglia activation, since behavioural signs of tinnitus were observed after treatment with salicylate and minocycline (n=2). Our results suggest that DCN microglia follows different functional pathways upon different tinnitus-inducing treatment. A similar or even larger heterogeneity in human tinnitus population would agree with the wide variability of different treatment efficacy.