| The auditory system consists of ascending and descending pathways. The descending pathways include descending projections from auditory cortex to subcortical nuclei and descending projections from higher auditory nucleus to the lower auditory nucleus. In past decade, extensive auditory electrophysiology studies focused on the fuction of those descending projections from auditory cortex. In 1996, Yan and Suga first demonstrated a highly specific corticofugal modulation in the auditory midbrain of the mustached bat, such highly specific effects was named as“egocentric selection”. Succedent studies have demonstrated that“egocentric selection”magnism is also fit for corticofugal modulation of auditory thalamus and even for the hair cells in the cochlear. Besides,“egocentric selection”magnism was also reported from study on different species such as mouse, gerbil rat and cat.The cochlear nucleus (CN) of the brainstem is the first neural processing station in the central auditory system and the sole target of the auditory nerve fibers (ANF). The CN is divided into three major subdivisions, the dorsal cochlear nucleus (DCN), the anteroventral cochlear nucleus (AVCN) and the posteroventral cochlear nucleus (PVCN). The three subdivisions are primarily determined by the anatomic features of the CN. Deep layers of the auditory cortex send projects to the granular cell belts around the DCN. Recent studied indicated a highly specific corticofugal modulation of the initial neural processing of sound information from the contralateral and ipsilateral AVCN. However, it is still unclear about the corticofugal modulation of the DCN neuron. Besides, the central nucleus of the inferior colliculus (ICC) also sends large number of the descending projections to the ipsilateral DCN and these projections were considered as topographically arranged. However, the function and significance of these fibers is still unknown.This study focused on the modulation of initial neural processing of sound information on ipsilatral DCN neuron by AI and ICC. Besides, we observed the intracellular response of the AI and DCN neurons using intracellular recording in vivo technich. SD rat with weight ranged 200-250g were used in our experiment. We firstly recorded the excitatory frequency response area of the DCN neurons, and then we studied the corticofugal modulation on the DCN neurons by focal electrical stimulation on AI. We oberved the best frequency(BF), minimum threshold(MT), bandwidth10,30 and 50 dB above the MT( BW10, BW30, BW50),receptive field(RF) and average spike number(ASN) of the DCN neurons before and after focal electrical stimulation on AI. Then we studied the resting membrane potential, action potential and spontaneous periodic firing i.e. up state and down state of the single AI neuron by intracellular recording in vivo. We also compared those up state evoked by acoustic stimulus with spontaneous one. Then we studied the modulation effects on the DCN by focal electrical stimulation on ICC. We observed the BF and MT of the DCN neurons before and after focal electrical stimulation on ICC. Finally, we studied the intracellular response of single DCN neuron to the short and long-duration auditory stimuli.The results were showed as following:1. Excitatory frequency response area of the DCN neuron.The excitatory frequency response area of the DCN neuron can be classified by 4 classes according to their high and low frequency slop.Class I neuron had a slopelow less than 150 and a slophigh more than 250, Class II had a slopelow more than 150 and a slophigh more than 250, Class III had a slopelow less than 150 and a slopehigh less than 250 and Class IV had a slopelow more than 150 and a slopehigh less than 250 or had multi-peaks. The percentage of the each class from 74 neurons recorded in our study is showed as following: Class I: 47.3%(35/74),Class II:14.3%(11/74),ClassIII 27.0%(20/74),Class IV: 11.4 %( 8/74).2. Corticofugal modulation on the ipsilateral DCN neuronsFocal electrical stimulation of the AI can strongly affect the activity of the DCN neuron, when the BF frequency of the stimulated neuron was higher than the recorded neuron, the BF of recorded neuron become higher after electrical stimulated. when the BF frequency of the stimulated neuron was lower than the recorded neuron, the BF of recorded neuron become lower after electrical stimulated. Within the range of -8 to 8 kHz, the larger the difference in BF between AI and DCN neurons, the larger the shifts in BF of DCN neurons. The shift in BFs was linearly(y=-0.4616x+0.1182 ) and significantly (R2=0.8482; n=56; P<0.01) correlated to the BF difference between AI and DCN neurons. Samely, the changes in MTs of DCN neurons were significantly correlated to the difference in MTs between AI and DCN neurons (y=-0.5123x+0.2168; R2=0.4371; P<0.05; n=56). When the MTs of AI neuron were higher than MTs of DCN neurons, on average, the MTs of DCN neurons were increased by 3.2±5.5dB, on the other hand, when the MTs of AI neurons were lower than MTs of DCN neurons on average,the MTs of VCN neurons were decreased by 4.5±5.2dB. The changes of the BW10, 30, 50 and the changes of RF and ASN after electrical stimulation on AI was not linearly correlated to the difference between AI and DCN neurons. Our results indicated that auditory cortex can modulated ipsilateral DCN neurons in a highly specific way.3. Effects of acoustic stimuli on neuronal activity in the auditory cortex of the ratHere, we examined the effects of sound stimuli (noise bursts) on neuronal activity by intracellular recording in vivo from the rat auditory cortex (AC). Noise bursts increased the average time in the up-state by 0.81±0.65s (range, 0.27 - 16 1.74 s) related to a 10 s recording duration. The rise times of the spontaneous up-events averaged 69.41±18.04 ms (range, 40.10- 119.21 ms), while those of the sound-evoked up-events were significantly shorter (p<0.001) averaging only 22.54±8.81 ms (range, 9.31 - 45.74 ms). Sound stimulation did not influence ongoing spontaneous up-events. Our data suggest that a sound stimulus does not interfere with ongoing spontaneous neuronal activity in auditory cortex but can evoke new depolarizations in addition to the spontaneous ones.4. Focal electrical stimulation of the ICC neurons didn't change the activity of the DCN neuron.In this part, the BF and MT of DCN neuron showed no significant changes after focal stimulation of the ICC neuron. Increasing stimulus intensity or changed stimulus mode also didn't change these parameters. Our data indicated the auditory cortex may not modulate the DCN neuron through an indirect way of AI-IC-DCN.5. DCN neurons showed“adaption”to the long-duration acoustic stimulusRepeated short term acoustic stimuli (white-noise burst60-ms duration with 5ms rise/fall time) can evoke DCN neuron's firing of 3-4 action potential. When a long term acoustic stimuli was presented (white-noise 600,500 or 5000-ms duration with 5ms rise/fall time), the DCN neuron showed significant difference of firing rate between the first and the second half of the noise stimulus(p<0.05,n=6)... |
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