Effects Of Long-term Salicylate Administration On Neuroplasticity In The Rat CNS

Objective: The mechanisms and suspected origins of tinnitus have not yet been fully elucidated.We induced tinnitus in rats using a model of long-term salicylate administration,followed by investigate the changes of metabolic activity,ultastructure and plasticity-related proteins in specific regions in rat CNS.The findings of this study will help to elucidate the role of neuroplasticity in these regions and its involvement in tinnitus.Methods: A tinnitus rat model was induced by long-term salicylate administration.Then,the gap prepulse inhibition of acoustic startle(GPIAS)test was used to confirm tinnitus-like behavior.The rats were divided into the acute treatment,chronic treatment,wash-out,and saline groups in every test.Ultrastructural alterations in the synapses of the inferior colliculus(IC),the auditory cortex(AC),the hippocampus(HP)and the cerebellum were assessed by transmission electron microscopy(TEM).Small animal positron emission tomography(micro PET)scans were performed on rats.Comparison of the m RNA and protein expression levels of BDNF,Trk B,and CREB in the auditory cortex(AC)were assessed by means of Real-time PCR and Western-Blotting.Results: 1)Only rats in the chronic salicylate-treatment(14 days)group showed evidence of experiencing tinnitus.2)Quantitatively,the ultrastructure of the IC and AC neurons contained an increased number of synaptic vesicles(P<0.01),with greater postsynaptic densities(PSDs)(IC P<0.01;AC P<0.05),longer synaptic active zones(IC P<0.01;AC P<0.05)and an increased synaptic interface curvature(P<0.05)in the chronic treatment group compared with the saline group.The ultrastructure of the HP neurons contained an increased number of synaptic vesicles(P<0.01)in the chronic salicylate-treatment groups,with increased an synaptic interface curvature(P<0.05)compared with the saline group.More synaptic vesicles were only observed in the AMY(P<0.01)neurons,and no significant ultrastructural changes were observed in the CRB(P>0.05).The ultrastructure of the IC,AC,HP,AMY and CRB neurons showed a remarkably increased number of synaptic vesicles(P<0.01),but no other changes were observed in the acute salicylate-treatment group compared with the saline group.There were no significant differences between the saline and wash-out groups.3)The metabolic activity of the IC,the AC,and the HP were significantly higher in the chronic treatment group compared with saline group.The alterations all returned to baseline 14 days after the cessation of salicylate-treatment(wash-out group),indicating that these changes were reversible.4)The m RNA and protein expression levels of BDNF and protein expression of p-CREB were upregulated with ultrastructural changes in the AC of the chronic treatment group,which returned to normal after 14 days of recovery.There was no difference of m RNA and protein expression of pro BDNF,CREB,and Trk B between all groups.Conclusion: The rats in the chronic salicylate-treatment group were experiencing tinnitus,and tinnitus-like behaviour disappeared 14 days after the cessation of treatment.The rats in the acute treatment group did not experience tinnitus.The TEM results indicate the increased speed and efficacy of chemical synaptic transmission in the three brain regions(IC,AC and HP)during tinnitus.There was only a great amount of neurotransmitter release in the acute treatment group.The micro PET results suggesting relatively increased metabolism in the three brain regions of rats during tinnitus,and decreases to baseline during recovery from tinnitus.The rats in the acute treatment group were in a state of high metabolic CNS activity.Long-term administration of salicylate augmented the expression of BDNF,the phosphorylation of CREB may be involved in this process.These factors play an important role in neuroplasticity and in induced,but reversible,tinnitus.