| In order to study the effect of weak noise on the sound signal extraction of mouse (Mus musculus Km) inferior collicular (IC) neurons from environment, we examined the change in frequency tuning of neuron induced by a weak noise relative to 5 dB below minimum threshold (reMT-5 dB) of tone.12 healthy adult mice (20-25 g, b. wt.) were used for this study. Durations of tone and weak white noise bursts used as sound stimuli were both 40 ms (2-ms rise-decay times), and delivered synchronally at 2 per second in free field.177 neurons were obtained in this experiment, and frequency tuning curves (FTCs) of 76 IC neurons were measured in our study. 51 (67.11% ) neurons were affected by weak noise. Among these neurons, FTCs of most neurons (31/51, 60.78 %) were sharpened. Q10 and Q30 increased from 1.89+1.12 to 2.66+2.13 and 0.86 + 0.47 to 1.17 + 0.71, respectively. On average, mean percent increases in Qio and Q30 were 36.26 + 26.40% (P = 0.0016, n = 31) and 36.26 + 23.56% (P = 0.00012, n - 27), respectively. The inverse-slopes (ISs, IS = AF/AI, kHz/dB) of sharpened FTCs were calculated for comparing the effect of noise on the sharpening high and low frequency tuning of IC neurons (n = 27). Weak noise could basically made IShigh decrease and IS|0W change very little (40.74 %, 11/27). ISlow changed from 0.21 + 0.06 to 0.21 +0.05 kHz (P = 0.832, n = 11), however, IShigh decreased from 0.41+0.24 to 0.24 + 0.16 kHz, and mean percent decrease in IShigh was 41.71 + 15.71 %(P = 0.0061, n = 11). FTCs of 20 neurons were broadened by weak noise. Q,o and Q30 decreased from 2.92 + 1.99 to 2.09+1.39 and 1.32+1.05 to 0.75 + 0.42, respectively. Mean percent decreases in Q10 and Q30 were 26.88+12.77% (P = 0.00057, n = 20) and 29.92 + 27.38% (P = 0.02, n = 20), respectively.To study the mechanism underlying sharpening FTCs by weak noise, 21 of 31 IC neurons accepted bicuculline ionophoresis. It is showed that the sharpened FTCs were broadened during bicuculline application to recorded neurons. And inhibitoryareas of FTC could be abolished partially or completely by bicuculline application. On average, mean percent decreases in Q10 and Q30 were 33.96 + 24.70% (P = 0.00007, n = 21) and 38.01+24.92% (P = 0.00012, n = 21), respectively.From these results, it is tempting to speculate that the response of IC neurons to sound signal could be inhibited or facilitated by the weak noise. Our data revealed for the first time that the weak noise could sharpen frequency tuning and increase the sensitivity of mouse IC neurons to high frequency of sound signal. On the other hand, inhibitory areas of FTC could be abolished partially or completely during bicuculline application. These results showed that GABAergic inhibitory afferent projections to recorded neurons were activated by random resonance caused by weak noise in cochlea, which provided inhibitory input for sharpening frequency tuning and made neuron more precisely analyze sound signal near its BF. Therefore, this experiment presented a possible evidence of cellular level for interpreting why human and animal could extract or capture sound signal of specific frequency from the environment with competing noise. |
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