Masking Effects Of Forward Masker Sound With Different Durations On Frequency Receptive-field And Minimum Threshold Of Inferior Collicular Neurons Of Mouse

In this study, we used a probe and a masker under different stimulation conditions in order to examine the effect on the IC neurons' receptive field and MT . And we aimed to presume the interaction of excitation and inhibitory input in IC by means of bicuculline (an antagonist for GABA_A) application.16 healthy adult mice (20-25g. b. wt.) were used for this study. We used the masker relative to 5dB above MT, the best frequency of IC neurons, and its duration was 40, 60, 80 and 100 ms in turn. The probe sound duration was 40 ms and the gap between the masker's offset and the probe's onset was fixed.A total of 124 IC neurons were recorded in this experiment, and the frequency receptive fields and MT of 77 IC neurons were measured in our study. After masker application, all frequency receptive fields of neurons reduced and the MT increased obviously (P<0.001). One-way ANOVA test showed that forward masking would increase as the masker duration increased. The inhibitory rates of the frequency receptive field were 23.10±13.60%, 30.97 ± 13.48%, 34.08± 12.86% , 38.77± 16.18% respectively, P<0.01. and the inhibition rates of the MT were 16.76 ± 5.62%, 25.61 ± 8.41%, 28.66 ± 10.66%, 30.90±11.44%. The analysis of the frequency receptive field shows that the inhibition to the high receptive fields is stronger than that of the low ones.To study the mechanism underlying time-dependent masking, 35 of 77 IC neurons accepted bicuculline ionophoresis. Analysis showed that bicuculline produced a great degree of abolishing inhibitory to frequency receptive field and MT during it application to the recorded neurons. The frequency receptive fields of neurons increased obviously (paired t-test, P<0.05). One-way ANOVA test showed that the inhibitory to the frequency receptive field wasn't related to the masker duration after bicuculline application . Four inhibition rates were 18.36±7.52%, 20.98±9.89%, 22.18±11.45%, 21.74±11.34%. And so was thechange of the MT, the four inhibition rates were 18.36±7.52%, 20.98±9.89%, 22.18±11.45%, 21.74±11.34% .From the results above, we could give a hypothesis that it is most likely a result of synaptic integration of excitation and inhibition at the IC neuron. Neurons in the IC of the midbrain receive convergent excitation and inhibition input from both lower and higher auditory nuclei. Interactions of these two opposing inputs shape different response properties of IC neurons. When a masker was prior to a probe , the early arrival of an IPSP elicited by the masker at the recorded neuron will overlap with the late arrival of EPSP elicited by the probe , and the response of the neuron might be inhibited by the masker. The inhibition will increase with increasing masker duration so that the masking is related to the masker duration. We suppose that GABAergic inhibition makes an important contribution to the effect.