Abstract
Reduced tolerance to sound stimuli (hyperacusis) is commonly seen in tinnitus patients. Dysfunction of limbic systems, such as the nucleus accumbens (NAc), may be involved in emotional reactions to the sound stimuli in tinnitus patients. To study the functional changes in the NAc in hyperacusis, we have examined the neural activity changes of the NAc using c-Fos staining in an animal model of hyperacusis. The c-Fos staining was also examined in the medial geniculate nucleus (MGN), a central auditory pathway which has neural projections to the NAc. Postnatal rats (14 days) were exposed to loud noise (115 dB SPL, 4 hours for two consecutive days) to induce hyperacusis (n = 4). Rats without noise exposure were used as the controls (n = 4). After P35, rats in both groups were put in a behavioral training for sound detection. After they were trained to detect sound stimuli, their reaction time to noise bursts centered at 2 kHz (40-110 dB SPL) was measured. Rats in the noise group showed a significantly shorter reaction time than those in the control group to the noise bursts at high intensities, suggesting the noise exposure induced hyperacusis behavior. The c-Fos expressions in the NAc and the MGNs of the noise group were significantly higher than those of the control group. Our results suggested that early-age noise exposure caused hyperactivity in the NAc and the MGNs which may induce the loudness increase in these rats.
Highlights
Tinnitus is a phantom sound perception which occurs when there is no external sound in the surrounding environment
We tested the effects of early-age noise exposure on sound reaction time and c-Fos expression in the nucleus accumbens (NAc) and the medial geniculate nucleus (MGN) in rats
To detect the neural activity changes of the limbic systems that may contribute to the sound perception changes, the c-Fos stainings in the NAc and the MGN have been evaluated. c-Fos is a well-established marker to identify activated neurons in the autonomous or central nervous systems after multiple stimuli [27,28,29]
Summary
Tinnitus is a phantom sound perception which occurs when there is no external sound in the surrounding environment. Patients who suffer from tinnitus and hyperacusis often share the limbic-associated psychological profiles with an increased tendency to anxiety, fatigue, and depression [5,6,7,8,9]. These anxiety disorders can exacerbate the severity of their tinnitus and hyperacusis symptoms [10]. Neural Plasticity understand whether the functional changes of the limbic system are involved in the sound loudness changes, in this study, we used c-Fos immunostaining to detect the neural activity in the NAc and the MGNs in the rats with behavioral evidence of hyperacusis
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