Imagine a busy city street, filled with all sorts of sounds: cars honking, people talking, music blaring. Now imagine if the volume suddenly increased to an unbearable level, like a huge explosion going off right next to you! Well, it turns out that loud noise can actually have a similar effect on the cells in our auditory cortex. A recent study found that exposure to loud noise changed the firing frequency of specific types of neurons in the mouse auditory cortex. These neurons, called layer 5 pyramidal neurons and Martinotti cells, play a crucial role in processing sound information. The researchers discovered that after exposure to loud noise, the firing patterns of these cells were altered. Some decreased their firing frequency, while others increased it. This disruption in firing patterns could potentially contribute to conditions like tinnitus, where individuals perceive ringing or buzzing noises in their ears even when there is no external sound. To learn more about this fascinating research and its implications for understanding how our brains process and respond to sound, check out the full article!

IntroductionLoud noise-exposure can generate noise-induced tinnitus in both humans and animals. Imaging and in vivo studies show that noise exposure affects the auditory cortex; however, cellular mechanisms of tinnitus generation are unclear.MethodsHere we compare membrane properties of layer 5 (L5) pyramidal cells (PCs) and Martinotti cells expressing the cholinergic receptor nicotinic alpha 2 subunit gene (Chrna2) of the primary auditory cortex (A1) from control and noise-exposed (4–18 kHz, 90 dB, 1.5 h, followed by 1.5 h silence) 5–8 week old mice. PCs were furthermore classified in type A or type B based on electrophysiological membrane properties, and a logistic regression model predicting that afterhyperpolarization (AHP) and afterdepolarization (ADP) are sufficient to predict cell type, and these features are preserved after noise trauma.ResultsOne week after a loud noise-exposure no passive membrane properties of type A or B PCs were altered but principal component analysis showed greater separation between type A PCs from control and noise-exposed mice. When comparing individual firing properties, noise exposure differentially affected type A and B PC firing frequency in response to depolarizing current steps. Specifically, type A PCs decreased initial firing frequency in response to +200 pA steps (p = 0.020) as well as decreased steady state firing frequency (p = 0.050) while type B PCs, on the contrary, significantly increased steady state firing frequency (p = 0.048) in response to a + 150 pA step 1 week after noise exposure. In addition, L5 Martinotti cells showed a more hyperpolarized resting membrane potential (p = 0.04), higher rheobase (p = 0.008) and an increased initial (p = 8.5 × 10–5) and steady state firing frequency (p = 6.3 × 10–5) in slices from noise-exposed mice compared to control.DiscussionThese results show that loud noise can cause distinct effects on type A and B L5 PCs and inhibitory Martinotti cells of the primary auditory cortex 1 week following noise exposure. As the L5 comprises PCs that send feedback to other areas, loud noise exposure appears to alter levels of activity of the descending and contralateral auditory system.

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