Changes Of GABA And Plasticity In Guinea Pig Inferior Colliculus After Noise Exposure

The present study aimed to shed light to the change of GABA and tone coding in guinea pig inferior colliculus (IC) after noise induced cochlea trauma, and to illustrate the mechanism of noise induced drumming in the ear, hyperacusis and loudness recruitment. The study includes three parts.1. The normal electrophysiological characteristic in guinea pig inferior colliculus. In our study, following items were found:1) Most (84.89%) of the frequency response area (FRA) were v-shape, others were non-v-shape.2) Along the dorsoventral axis, our recording demonstrated a systematic shift in the characteristic frequency (CF) as a function of depth as a distinct staircase pattern, the minimum threshold (MT) of CF as a function of CF appeared as a Ushape, and the MT of CF as a function of depth also appeared as a Ushape.3) Rate intensity function (RIF) includes five types:a) u1:the function curve go slowly up to end then down. b) u2:the function curve go slowly up to end. c) u3:the function curve go up quickly to a height then go slowly up or go levelly. d) ud:the function curve go up quickly to the highest point then quickly down. e) Nshape:the function curve is like N or M and W.4) post-stimulus time histogram (PSTH) includes five types:A) inhibitory, the spontaneous spike were inhibited during tone stimulus. B) Transient, response to tone stimulus appeared in the first 25 seconds then less spike followed. C).long-latency, the response pattern is like the transient, but it has a latency of 22 seconds. D). Pauser/buildup, responses with an early peak in activity followed by a brief decrease in activity and a buildup of firing activity to a moderate continuous rate. E), buildup, responses early increased to a plateau.2. Changes of electro-physiology in guinea pig inferior colliculus after noise exposure:1) The models of acoustic trauma were successful.2) The w-shape FRA and n-shape FRA were increased from one day after noise exposure, and until 11～21days after noise exposure, the n-shape-FRA decreased.3) Noise induced hearing loss resulted in a reorganization of the normal tonotopic organization map of the IC. The distinct staircase pattern observed in normal cases was clearly altered and became more or less blurred or significantly distort, a faultage at 4KHz appeared in the staircase pattern.4) The average depth of CF remained in exposed groups increased more than that in the normal.5) The MT of CF decreased in most of the frequency bands in exposed groups.6) The bandwidths at 20 dB above the threshold at CF (BW20dB) decreased in one day and increased from 11～21 days after noise exposure.7) Types of RIF in noise exposed groups were same as it in the controlled group, but the composing ratio of every type was different.8) Types of PSTH in noise exposed groups were same as it in the controlled group, but the composing ratio of every type was different.3) The change of GABA in guinea pig IC after noise exposure. The neurotransmisssion GABA and its receptors GABAA and GABAB decreased significantly after acoustic trauma.According to all above, we draw a conclusion:The mechanism of noise induced drumming in the ear, hyperacusis and loudness recruitment may be related to broader or narrower tuning curve, the changed spike pattern, increased reactive activity of the neuros in inferior colliculus after noise exposure, and the reduction of GABA and its receptors was one of the reasons for the physiological changes in inferior colliculus.