Effect of intense noise on neuronal cell production in the adult rat brain
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The hippocampus, a major source of neuronal precursor cells and neurogenesis, plays a vital role in memory function, spatial navigation, learning, and mood. There are various factors which enhance or reduce the rate of neurogenesis. An increase in the rate of neurogenesis can be observed with factors like enriched environment, exercise like running, and seizures while a reduction in rate of neurogenesis can be influenced by factors such as neurodegenerative disorders, chemotherapy, stress, and depression reduce neurogenesis. Tinnitus is a debilitating symptom in which the sufferer continuously hears a ringing noise in his ears. This can be stressful for the individual as it affects concentration, sleep, and other daily activities. High intensity noise, a major cause of hearing loss and tinnitus, and a source of distress, could conceivably suppress neurogenesis. To evaluate this, 6 rats were unilaterally exposed for 2 hours to narrow band noise (centered at 12 kHz) presented at 126 dB SPL. Among these 6 rats, 3 rats developed tinnitus-like behavior while the other 3 noise-exposed rats showed no signs of tinnitus. Control groups consisted of Naïve controls and Sham controls. The Naïve controls (n=3) did not receive any kind of treatment. The Sham Control (n=2) did not undergo any noise exposure but were exposed to the tinnitus screening and isofluorane anesthesia. Ten weeks post-exposure, the inner ear was evaluated for sensory hair cell loss and the hippocampus was evaluated for neurogenesis. All the noise exposed rats showed severe loss of sensory hair cells in the noise exposed ear, but no damage in the unexposed ears. Neurogenesis was evaluated by DCX immunolabeling. Real time polymerase chain reaction, using DCX as the target gene and β-actin as the housekeeping or reference gene, was used to quantify the expression of DCX mRNA. The right brain and the left brain were studied separately as the rats were given noise trauma in the left ear while the right ear was protected using an ear plug. The noise trauma rats and the control rat group were compared using the one-way ANOVA Tukey’s multiple comparison test. The noise trauma rats showed a significant reduction of hippocampal DCX immunolabeling compared to the Sham controls and the Naïve controls. No significant difference in neurogenesis was observed between rats with behavioral evidence of tinnitus versus rats without tinnitus. The right and left hemispheres of brain showed no significant difference which suggests that high intensity noise affects both the hemispheres of brain equally. The real time PCR results showed no significant difference between the noise-exposed rats and the naïve control rats in DCX mRNA expression. The present results indicate that high intensity, unilateral noise exposure significantly down regulates hippocampal neurogenesis.