Pharmacological Characterization Of Suppression Of Rebound Depolarization In Neurons Of Mouse Medial Geniculate Body By Sodium Salicylate

BACKGROUND: Sodium salicylate (NaSal) is the active metabolite of aspirin,which is widely used as an antipyretic,analgesic and anti-inflammatory drug. NaSal in high doses can reversibly induce tinnitus in humans and animals, therefore, NaSal is commonly used to induce tinnitus in animal models for studying the neural mechanisms underlying tinnitus, an auditory phantom sensation in the absence of external sound. Previous studies have shown that neural activity of brain regions,including the medial geniculate body (MGB) that plays an important gating role on the auditory signal processing and transmission, greatly increased in animal models of tinnitus induced by NaSal. On the other hand, rebound depolarization (RD) as an important characteristic of the MGB can regulate the gate activity. The aims of the present study were to investigate the effects of MGB on RD of the MGB neurons in different concentrations for understanding the neural mechanisms underlying NaSal-induced tinnitus.METHODS: In this study, the RD of the MGB neurons in adult and VGAT-ChR2-EYFP transgenic mice was recorded by using brain slice and whole-cell patch-clamp techniques. The transgenic mice have expressed light-sensitive channelorhodopsin-2 protein in all GABAergic neurons. GABAergic neurons and pyramidal neurons in the MGB were identified based on cellular morphology,electrophysiological characteristics and optogenetic techniques. The resting membrane potential, membrane input resistance, latency, amplitude and area of the RD in MGB neurons were recorded with perfusion of different concentrations of NaSal.RESULTS: In the MGB slices of adult VGAT-ChR2-EYFP transgenic mice, I found that (1) the pyramidal neurons and GABAergic neurons could be identified based on their different cellular morphology, electrophysiological characteristics and optogenetic features; (2) 1.4 mM, 0.8 mM, 0.5 mM and 0.3 mM NaSal were able to reversibly hyperpolarize the resting membrane potential of MGB neurons. 1.4 mM NaSal, but not NaSal at 0.8 mM, 0.5 mM and 0.3 mM, significantly and reversibly reduced the membrane input resistance, and suppressed the generation of RD; (3)With partial stimulation of hyperpolarizing current, 0.8 mM, 0.5 mM, 0.3 mM NaSal reduced the latency, amplitude and area of RD; (4) CGP55845, the blocker of GABAB receptor, and could obviously block the suppressing effect of NaSal on RD when co-applied with 1.4 mM NaSal; (5) When co-applied with baclofen, an agonist of GABAB receptor, 0.3 mM NaSal could also suppress RD; (6) When co-applied with Tertinpin-Q, the blocker of the inward rectifier potassium channel, 1.4 mM NaSal and baclofen still suppressed RD.DISCUSSION: In the present study, I show that NaSal at different concentrations can hyperpolarize the resting membrane potentials of MGB neurons. Furthermore,the degree of hyperpolarization increases with the increase in NaSal concentrations.NaSal at 1.4 mM can reversibly suppress the generation of RD. NaSal at low concentrations does not suppress the generation of RD but actually alters characteristics of RD, suggesting that the effect of NaSal at different concentrations on the auditory processing information are also different. This study indicates that GABAB receptors participate in the suppressing of NaSal (1.4mM) on RD. However,GABAB-activated downstream ion channels are different from G protein-gated inward rectifier potassium channels in young rats. There may be some other ion channels that coupled to G proteins and regulate the resting membrane potentials. The current findings not only reveal that the effect of NaSal at different concentrations on MGB neurons' intrinsic characteristics are significantly different, but also imply that NaSal may affect auditory information processing and transmission through altering the intrinsic properties of MGB neurons, thus contributing to the occurrence of tinnitus.