| Background and purposesTinnitus is a symptom of otology without any outside sound source or electrical stimulation of the ear while the sound produced by feeling. It is a high incidence of disease, and with the increase in cardiovascular diseases , aging of the population and industry, environmental noise, the incidence of tinnitus increases year by year,so tinnitus seriously affect people's daily work and live. However, many of the outcome of clinical treatment of patients with tinnitus is still not so good nowadays ,which mainly due to the formation mechanism of tinnitus has not yet completely clear. In 1988, according to principles of Pavlov reflex, Jastreboff have confirmed that animals injected with sodium salicylate does feel tinnitus by observing behavior in rats, and established the first classic animal model of acute salicylate for tinnitus through inhibition of drinking water. Currently sodium salicylate has become the most widely used drugs to establish model of tinnitus. Tinnitus often associate with hearing loss, especially the high-frequency hearing loss, trauma and exposure to strong sound are the two major factors to lead to tinnitus, exposure to strong sound can cause hearing loss which lead to changes in neural plasticity. A large number of studies have shown that neural plasticity can lead to tinnitus. Synaptophysin is a calcium-binding glycoprotein with the relative molecular weight of 38 KD, existing in almost all the presynaptic vesicles, it is an index to measure the synaptic number and density. The expression of synaptophysin increase significantly in the synaptic reconstruction process.Therefore, synaptophysin is closely related to neural plasticity , and it can be used as an indicator of central nervous activity. In this study, we established animal model of tinnitus using sodium salicylate as drugs, and detected the expression of synaptophysin of rat cochlear nucleus, superior olivary nucleus, inferior colliculus and auditory cortex by immunohistochemical techniques to study the relationship of the plasticity changes of the auditory pathway with the pathogenesis of tinnitus .Materials and methods1,Established rat model of tinnitus.Establishment of tinnitus model referenced to the "suppression of drinking water" of Jia Minghui. Choosed 36 male Wistar rats with 2 to 3 months of age, health, sensitive in preyer reflex, with no signs of middle ear infection, auditory brainstem response test (ABR) response threshold in both ears 20 ~ 30dBSPL, weighing 200 ~ 250g(the Experimental Animal Center of Slater in Shanghai supply),all the experimental animals without exposed to intense noise or ototoxic drug use record. After a week of adaptive feeding , the animals were divided into 6 groups randomly(draw by lot with paper bags),named groupâ… A,â… B,â… C,â…¡A,â…¡B,â…¡C severally(n=6 in each group) .The rats were divided into groupâ… andâ…¡according to the length of time of injecting drugs,that the groupâ… with injection time of 7 days and the groupâ…¡with injection time of 14 days(No injection in blank group); groupâ… andâ…¡were divided into three groups(A, B, C )respectively, group A representatives for sodium salicylate group, group B representatives for physiological saline group and group C is the representation for the blank contrast. Speaking in definitly,from establishing a conditioned response to the end of the experiment,groupsâ… A received a intraperitoneal injection (i.p.) dose of 350 mg sodium salicylate per kg bodyweight dissolved in physiological saline 3 hours before training,while groupâ… B was intraperitoneally injected the same volume of physiological saline at the same time,and groupâ… C was kept in a quiet environment, not limited to diet, without any injection and conditioning training. Divided groupâ…¡to A ,B and C as before.2,Took the ABR test for the rats at different time periods during making the rat model of tinnitus, detectting the response threshold and every latency; 3,Used immunohistochemical techniques to study the changes in the expression of synaptophysin of the four nuclei (cochlear nucleus, superior olivary nucleus, auditory cortex, hypothalamus) in the auditory pathway of tinnitus rat.Result1. The conditioned reflex time of groupâ… A and groupâ…¡A was significantly longer than other groups of rats, and there was a significant difference (P <0.01).2. ABR waveform was stable in normal rats, but ABR waveform for rats with repeated injections of sodium salicylate was poorly differentiated, each amplitude decreased ,ABR thresholds increased, II-wave latency was prolonged. The difference between experimental group with blank contrast group was significant (P <0.01),and the difference between early tinnitus group (â… A ) with continuous tinnitus group (â…¡A) was also (P <0.05).3. Comparing early tinnitus group (â… A) and continuous tinnitus group (â…¡A)with B, C group, the expression of synaptophysin in cochlear nucleus, superior olivary nucleus, auditory cortex, hypothalamus was significantly increased (P <0.01) , and the expression of synaptophysin inâ…¡A group was higher thanâ… A group in each auditory nuclei (P<0.01).Conclusions1.The behavioral experiment confirms that the rats experienced tinnitus after salicylate injection.2.The ABR test provide objective audiologic evidence for the toxic effect on rat after high-dose salicylate injection.3.The alteration of the synaptophysin in the auditory nucleus supports neuronal plasticity in the auditory center and further favor for the hypothesis that the central nervous system is involved in tinnitus origin.
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