The Anterograde And Retrograde Tracingstudy On The Auditory Pathway In Tree Shrew

Objective: To position the auditory pathway in tree shrew and study the location of and the relation among its nuclei by using the cerebral stereotaxic technique combined with the neural tracing technology,so as to provide basis for further studies on function of the auditory pathway in the future,especially for studies on integration of the ascending and descending fibers,auditory plasticity and establishing of acoustic models,etc.Method: 30 healthy adult tree shrews(laboratory tree shrews)were randomly selected and divided into the anterograde tracing group(the AT group),the retrograde tracing group(the RT group)and the auditory cortex tracing group(the CT group),with 10 tree shrews in each group.All the tree shrews had sensitive Preyer reflex when checking the auricle,had unobstructed external auditory canal,intact tympanic membrane and non-infected middle ear as checked by otoscopy,and had normal auditory threshold as checked by ABR test.For tree shrews in the RT group and the CT group,micro-injection pump was used for nerve tracer injection.For tree shrews in the AT group,their acoustic vesicles were opened to expose the tympanic cavity by incision behind the ear.Micro-injection pump(5ul)was used to penetrate from the bottom of the cochlea for tracer injection.The tree shrews were killed by anesthesia after 3 to7 days and their brains were taken after heart perfusion of 4% paraformaldehyde.Nissl’s staining was performed after frozen section and then microscope was used for observation.Results:(1)Labeling by anterograde tracing: after injection of FG and FR into the cochlea on both sides,the cochlear nucleus(CN),the superior olivary nucleus(OS),the nucleus of inferior colliculus(NCI)and the nucleus of medial geniculate body(MGB)on both sides were two-color labeled,showing that there were contralateral projections in the auditory pathway on both sides.(2)Labeling by retrograde tracing: after injection of FG and RB into MGB on both sides,no obvious contralateral projection was observed in the nucleus of medial geniculate body and the nucleus of inferior colliculus on both sides,while the superior olivary nucleus and the cochlear nucleus on both sides were two-color labeled,showing that there were contralateral projections.(3)Labeling by auditory cortex tracing: after injecting FR into the right MGB,a densely stained area can be seen in it and no fluorescence labeled cell can be seen in the MGB on the opposite side;there were fluorescence labeled cells on the same side and in the temporal cortex on the opposite side;meanwhile,there were fluorescence labeled cells in the internal capsule and the optic tract area on both sides.Results showed that MGB on one side could project to the internal capsule and the auditory cortex on both sides.Conclusion:1.Nuclei involved in the auditory pathway in tree shrew are precisely positioned within the brain tissue,including the cochlear nucleus,the superior olivary nucleus,the nucleus of inferior colliculus,the medial geniculate body and the auditory cortex.2.Both the ascending and descending projections of the auditory pathway in tree shrew are bilateral.The auditory pathway and other involved sensory systems interplay and interact with each other,forming the multisensory integration.3.The brain structure of tree shrew has anatomical advantages;the construction pattern and distribution of its brain cells are extremely similar to that of primates,thus using tree shrew for auditory pathway-related studies can not only provide reference for audiology researches relating to tree shrew,but also can lay a foundation for further studies in the future.