Mechanisms Underlying The Enhanced Activity Of Primary Auditory Cortex In Sodium Salicylate Induced Chronic Tinnitus Model

BACKGROUND:As one part of the central auditory system, auditory cortex plays a crucial role in sound perception and signal processing and is an important area in the occurrence of auditory disorders, such as tinnitus. Tinnitus is the perception of sound without a corresponding external acoustic stimulus. Since the clinical symptoms of tinnitus is variable and subjective, researchers find it difficult to observe and detect tinnitus in patients. Sodium salicylate(NaSal), the active ingredient of the common drug asprin, has anti-inflammatory and analgesic effects. However, high dose of NaSal can lead to reversible perception of a high-pitch ringing, tinnitus, in humans and animals. Therefore, NaSal is widely applied to induce tinnitus in animal models for the purpose of exploring the biological bases of tinnitus. Studies have shown that NaSal could cause raised activity in the primary auditory cortex. Also, NaSal could result in hyperexcitability in several brain areas in the auditory systems by striking the equilibrium between excitation and inhibition, which is reckoned as one of the possible mechanism of NaSal-induced tinnitus. The present study designed to study mechanisms underneath the increased neural circuit activity of the primary auditory cortex.METHODS:An animal model of chronic salicylate induced tinnitus was developedby intraperitoneal injecting sodium salicylate(NaSal group). A saline group ofanimals(saline group) underwent corresponding treatment. Brain slices were preparedfrom the VGAT-ChR2-EYFP transgenetic mouse. In the transgenic animal, lightsensitive channelrhodopsin-2 is expressed in the GABAergic cells. In the VGAT-ChR2-EYFP transgenetic mice, pyrimidal neurons and GABAergic neurons were identifiedthrough the morphological, electrophisological and optogenetic properties. Whole-cellpatch-clamp technique was adopted to record the intrinsic properties and synaptictransmissions of pyrimidal and GABAergic neurons in the auditory cortex. Specifically,the resting menbrane potential, the membrane input resistance, current or light-evokedfiring, and the spontaneous and miniature excitory postsynaptic currents (sEPSCs andmEPSCs) and the spontaneous and miniature inhibitory postsynaptic currents (sIPSCand mIPSC) were recorded.RESULTS:In layer 2/3 of the auditory cortex slices, (1) pyramidal neurons and GABAergic neurons could be identified through their morphological characteristic and by their distinct electrophysiological properties. (2) The membrane input resistances of pyramidal neurons were significantly higher in the NaSal group than those in the saline group. These results suggest that NaSal changes the intrinsic membrane properties and increase the membrane excitability in pyramidal neurons. (3) In the NaSal group, current-evoked firing rate of pyramidal neurons was higher than the saline group, whereas the firing rate of GABAergic was lower. (4) In the NaSal group, for pyramidal cells, the frequency and amplitude of glutamatergic sEPSCs in pyramidal neurons were increased. Moreover, the frequency of glutamatergic mEPSCs were increased. These results indicate that NaSal enhances the glutamatergic synaptic transmission in glutamatergic neurons through presynaptic mechanism. (5) Decreased frequency and amplitude of sIPSCs are observed in pyramidal neurons in the NaSal group.DISCUSSION:In the present study, in the tinnitus models induced by sodium salicylate, NaSal has effects on the neuronal excitability and synaptic transmissions of the primary auditory cortex. Evidence supporting this conclusion includes:(1) Neuronal excitability of pyramidal neurons was increased in NaSal group, but was depressed in GABAergic neurons; (2) Neuronal inhibitory transmission was decreased in pyramidal neurons in the NaSal group. NaSal could alter the neural excitability of the pyramidal cells and GABAergic cells in the primary auditory cortex in different ways, its effects could portable explain the mechanism of NaSal-induced tinnitus.