| Acoustic information is converted into nerve impulse in inner ear and successively processed in the cochlear nucleus(CN),superior olivary complex(SOC),nuclei of lateral lemniscus(NLL),and inferior colliculus(IC)before it is relayed via the thalamus to auditory cortex.The projection of auditory nucleus is complicated,that receive projection from lower brainstem,descending pathway and contralateral nucleus.In previous,lots of discoveries were made about auditory system by the methods of retrograde or anterograde tracing,electrophysiology and anatomy.We know that CN is the first stop of auditory pathway and receive the input of ipsilateral cochlear;SOC is the first nucleuses that receive bilateral information convergent projections;NLL connected brain stem with IC and MGB;IC is an important relay station of auditory pathway as an integrated nucleus with multifunction.However,the lack of technical measures limits the further research.Traditional electrophysiology could not record the activity of multiple cells,anatomy failed to get the function of nerve connection.A technology that specifically marks the activity of group of nerve cells was needed for further auditory study.The expression of c-Fos that widely used in recently years provided a technology that would mark group of nerve cells at the level of single cell,and was considered as the common foundation of neurons in central nervous system.C-Fos is an immediate-early gene that would react first once the cell be activated.It would express to a phosphorylated nuclear protein named Fos rapidly and transiently induced by various sensory stimuli.The expression of c-Fos would be induced by various stimulus like mitogen,anoxia,ischemia,light,sound,mechanical force and pain.The more important is that the expression of c-Fos would change its location,time and expression quantity with the kind of stimuli and the intensity.The expression of c-Fos is considered as the maker of activity of neurons and used to tracing neurological pathways in a particular situation.The earliest time of c-Fos expression used in hearing research was the year of 1991,Ehret and Fischer found the expression of c-Fos in CN and IC after two tones with 20 KHz and 50 KHz.After then,the immunolabelling technique of c-Fos was used in auditory system more and more widely.C-Fos expression was proved suitable for mouse,rat and chinchillas by immunohistochemistry of Fos and situ hybridization of mRNA.The experiment method were also multifarious including sound stimuli,and electrical intracochlear stimulation.Beside that,a lot of experiments involved in the method of cochlear lesion,but took no consideration of the recombination and gain regulation of auditory system after cochlear lesion.In view of that excitatory input enhance the expression of c-Fos and inhibitory input suppresses the expression.Our experiment purpose is to confirm the effect of cochlear lesion to auditory system,and compared the auditory activity of animals received bilateral and unilateral information to analyses the interaction of input of nucleus in auditory pathway,In this paper,we compared the spontaneous expression of c-Fos in CN,SOC,NLL,IC,MGB and AC of normal and unilateral cochlear lesion mouse.We analyze the afferent projections of auditory nucleus by comparing the expression of c-Fos in normal and unilateral cochlear lesion mouse after sound stimuli.Besides that,we also examined the Fos expression of hippocampus,amygdale and prefrontal cortex of mouse in wake state and anesthetized.Noise and tone were used in our experiment as parallel experiment for mutual verification.In the part of auditory experiment,animals were divided into two groups:normal and unilateral cochlear lesion mice.All the subjects underwent a surgery before the sound stimuli,to get their head fixed.Some of they received unilateral cochlea damage after the fixed surgery.Animals were then transferred into a sound-shield room for 7-8 hours to recovery.They were then stimulated with noise or pure tone for 90 min after the adaption for 30 min.The stimuli sound including noise and tone which consists of three randomly bursts of 5 kHz,15 kHz and 45 kHz.In the part of limbic system experiment,half of mice were anesthetized before sound stimuli.Immediately after the end of stimulation,animal received an overdose of pentobarbital sodium and were perfused transcardially with saline and fixative.Remove the brains,and post-fixed at 4? overnight.Coronal sections(40 um)were cut on freezing microtome after immersed in sucrose for cryoprotection,and the Fos protein was visualized by a standard 3-streptavidin-perosidase technique.We found no expression of c-Fos in CN of normal control animals,just a little in LSO and NLL,but sufficient in IC,MGB and AC even more less than IC.This result indicated that higher nucleus in auditory pathway especially IC possess a higer spontaneous activity than low brainstem.Activity of neurons in SOC,NLL,IC,MGB and AC were modulated after unilateral cochlear lesion.Fos marked cells clustered in lesion side of LSO and the contralateral side of NLL,but most be suppressed in the contralateral side of IC.Comparing the sound induced expression of c-Fos in normal and unilateral cochlear lesion mouse,we found only positive cell in contralateral lesion side of CN,it hint that CN neurons receive the input of ipsilatreal auditory nerve;neurons in contralateral lesion side of LSO were activated by sound stimuli,indicated an excited ipsilateral input;positive cells mostly located in contralateral side of NLL,is probably the result of contralateral excited input;IC contralateral side neurons still no positive cells and more sufficient in lesion side,although less than in normal IC;in MGB we found no sound induced expression in MGV and MGD,only little in MGM;sound activated AC neurons specifically but the activity of surface AC in contralateral lesion side were suppressed by cochlear lesion.About the experiment of limbic system,we found that hippocampus and prefrontal cortex would be most vulnerable to anesthesia.Sound activated a lot of neurons in hippocampus of awake animals but little in anesthetized mice.In prefrontal cortex of awake mice,only in regions close to prelimbic cortex c-Fos expression were found,and also no expression in anesthetized animal.Different state induced different cluster of neurons in amygdale,amygdale was traditional divided into three nucleuses:central amygdaloidal nucleus,basolateral nucleus group and corticomedial nucleus.In the state of awake,fos-labeled cells exsited in central amygdaloidal nucleus and basolateral nucleus,but in corticomedial nucleus when anesthetized.Central nervous system would be widely suppressed by anesthesia,the absence of activated neurons in hippocampus and prefrontal cortex maybe indicated that we would loose the ability of learning and clearly episodic memory when deeply anesthetized.The activity of corticomedial nucleus neurons may imply some diferent emotion in narcotism.In conclusion,our study found a lower spontaneous discharge in brainstem and a gain control of nucleus in auditory pathway that related to their afferent message.Comparing the sound induced expression of c-Fos in normal and unilateral cochlear lesion mouse,we confirmed some information of current literature:1.CN neurons received excitatory input from the ipsilateral cochlea and no effective input from contralateral cochlea.2.LSO neurons receive excitatory input from ipsilateral AVCN and inhibitory contralateral input relayed by ipsilateral MNTB.3.IC receives excitatory projection from contralateral CN and contralateral IC.The more important thing is we found that:1.Neurons in DLL received inhibitory ipsilateral input.2.Neurons in the specific sensory relay nuclei of auditory pathway,MGV,could not be activated only by sound stimulation.3.No Fos expression in hippocampus and prefrontal cortex when animal was anesthetized,this result indicated that we will lose the ability of learning and memory.4.The different position of Fos labeled cells in amygdale of anesthetized animals may means different emotion of animal. |
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