Animal Behavioral Models of Tinnitus

Pharmacologic treatments in preclinical tinnitus models with special focus on Ginkgo biloba leaf extract EGb 761®

Tinnitus is defined as the perception of sound in the absence of external acoustic stimuli. Frequent comorbidities or associated factors are depression, anxiety, concentration problems, insomnia, resignation, helplessness, headache, bruxism, or social isolation, just to name a few. Although many therapeutic approaches have already been tested with varying success, there still is no cure available for tinnitus. The search for an effective treatment has been hampered by the fact that the mechanisms of tinnitus development are still not fully understood, although several models are available and discussed in this review. Our review will give a brief overview about preclinical models, presenting the heterogeneity of tinnitus sub-types depending on the different inner ear and brain structures involved in tinnitus etiology and pathogenesis. Based on these models we introduce the different target structures and transmitter systems implicated in tinnitus development and provide an extensive overview on preclinical drug-based therapeutic approaches that have been explored in various animal models. As the special extract from Ginkgo biloba leaves EGb 761® has been the most widely tested drug in both non-clinical tinnitus models as well as in clinical trials, a special focus will be given to EGb 761®. The efficacy of terpene lactones, flavone glycosides and proanthocyanidines with their distinct contribution to the overall efficacy profile of the multi-constituent drug EGb 761® will be discussed.

Salicylate-Induced Changes in Hearing Thresholds in Mongolian Gerbils Are Correlated With Tinnitus Frequency but Not With Tinnitus Strength

Tinnitus is an auditory phantom percept without external sound sources. Despite the high prevalence and tinnitus-associated distress of affected patients, the pathophysiology of tinnitus remains largely unknown, making prevention and treatments difficult to develop. In order to elucidate the pathophysiology of tinnitus, animal models are used where tinnitus is induced either permanently by noise trauma or transiently by the application of salicylate. In a model of trauma-induced tinnitus, we have suggested a central origin of tinnitus-related development of neuronal hyperactivity based on stochastic resonance (SR). SR refers to the physiological phenomenon that weak subthreshold signals for given sensors (or synapses) can still be detected and transmitted if appropriate noise is added to the input of the sensor. The main objective of this study was to characterize the neurophysiological and behavioral effects during salicylate-induced tinnitus and compare these to the conditions within the trauma model. Our data show, in line with the pharmacokinetics, that hearing thresholds generally increase 2 h after salicylate injections. This increase was significantly stronger within the region of best hearing compared to other frequencies. Furthermore, animals showed behavioral signs of tinnitus during that time window and frequency range as assessed by gap prepulse inhibition of the acoustic startle reflex (GPIAS). In contrast to animals with noise trauma-induced tinnitus, salicylate-induced tinnitus animals showed no correlation between hearing thresholds and behavioral signs of tinnitus, indicating that the development of tinnitus after salicylate injection is not based on SR as proposed for the trauma model. In other words, salicylate-induced tinnitus and noise trauma-induced tinnitus are not based on the same neurophysiological mechanism.

Improving the Reliability of Tinnitus Screening in Laboratory Animals

Behavioral screening remains a contentious issue for animal studies of tinnitus. Most paradigms base a positive tinnitus test on an animal's natural tendency to respond to the "sound" of tinnitus as if it were an actual sound. As a result, animals with tinnitus are expected to display sound-conditioned behaviors when no sound is present or to miss gaps in background sounds because tinnitus "fills in the gap." Reliable confirmation of the behavioral indications of tinnitus can be problematic because the reinforcement contingencies of conventional discrimination tasks break down an animal's tendency to group tinnitus with sound. When responses in silence are rewarded, animals respond in silence regardless of their tinnitus status. When responses in silence are punished, animals stop responding. This study introduces stimulus classification as an alternative approach to tinnitus screening. Classification procedures train animals to respond to the common perceptual features that define a group of sounds (e.g., high pitch or narrow bandwidth). Our procedure trains animals to drink when they hear tinnitus and to suppress drinking when they hear other sounds. Animals with tinnitus are revealed by their tendency to drink in the presence of unreinforced probe sounds that share the perceptual features of the tinnitus classification. The advantages of this approach are illustrated by taking laboratory rats through a testing sequence that includes classification training, the experimental induction of tinnitus, and postinduction screening. Behavioral indications of tinnitus are interpreted and then verified by simulating a known tinnitus percept with objective sounds.