The Effects Of Postnatal Middle-level Noise Exposure On The Stimulus-specific Adaption Of Neurons In The Rat Auditory Cortex

In natural environments,human and animals can detect and discriminate novel stimuli or deviate stimuli from monotonous background stimuli.This is important for animals and human to respond quickly to the changes in environments.In sensory systems,many neurons often exhibit stimulus-specific adaptation(SSA),i.e.,they respond weakly to frequently occurring repetitive stimuli but respond strongly to the same stimuli when presented rarely.The SSA of neurons has been demonstrated in the auditory system from the midbrain to the primary auditory cortex(AI),and has been proposed to be one of the basis for novelty detection or deviance detection.These studies were done in animals reared in normal environment.Previous studies have shown that postnatal noise exposure at young age impairs cortical processing of sound frequency,level,spatial,and temporal information.However,it is not understood whether postnatal noise exposure affects cortical processing of novel sound frequency detection.It is also not known whether noise exposure at different postnatal periods has differential impacts on the SSA of neurons in AI.To answer these questions,we studied the SSA in sound frequency in AI neurons in three groups of rats:1)normal control group,the rats were reared in normal environments without noise exposure;2)young noise exposure group,rats were reared in 70 dB continuous white noise environment during postnatal day P10 to P56,and in normal environments during other time periods;3)adult noise exposure group,rats were reared in 70 dB continuous white noise environment during P57 to P103,and in normal environments during other time periods.We did single-cell recording in the rat AI to record the responses of AI neurons to novel sound stimulus sequence.In the novel sound stimulus paradigm,the two sound stimuli with different frequencies were presented at various probability.The stimuli that presented in lower probability was defined as novel stimuli or rare stimuli,and the stimuli presented in higher probability were defined as standard stimuli or common stimuli.All the recordings were done in adulthood in control rats,and two weeks after ending the noise exposure in the two noise exposure group.We analyzed the responses of AI neurons to the sound stimuli presented in common and to the same stimuli presented in rare,and we calculated the indices for SSA including frequency-specific SSA index and the neuron-specific SSA index.The higher the indices are,the stronger the ability of the SSA of AI neurons are.We found that,compared with the SSA indices of Al neurons determined in control group,in the young noise exposure group,the frequency-specific SSA index and the neuron-specific SSA index of AI neurons were significantly decreased,and the proportion of neurons exhibiting SSA was significantly reduced.However,we did not found significant differences in the frequency-specific SSA index and in the neuron-specific SSA index of AI neurons between control group and the adult noise exposure group.Moreover,the proportion of neurons exhibiting SSA were not significantly different between the control group and the adult noise exposure group.These results demonstrated that middle-level noise exposure at young age(before adulthood)impaired the ability of SSA of AI neurons in adulthood,and might decrease the ability of AI neurons to detect novel sound frequency.However,the same noise exposure during adulthood did not have significant effects on the ability of SSA of AI neurons.The results in the present study provide new experimental evidences for the functional impairment in the adult central auditory system by postnatal noise exposure during the development of auditory function at young age.