Hyperexcitability of the Nucleus Accumbens Is Involved in Noise-Induced Hyperacusis

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 ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" id="M1"><a:mi>n</a:mi><a:mo>=</a:mo><a:mn>4</a:mn></a:math> ). Rats without noise exposure were used as the controls ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" id="M2"><c:mi>n</c:mi><c:mo>=</c:mo><c:mn>4</c:mn></c:math> ). 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.