探討單次水楊酸誘發耳鳴動物模式中 Fos在中樞聽覺系統表現之差異性

Abstract

耳鳴是在沒有外在聲源刺激下，所產生的一種聽覺感覺，它是最常見的聽覺疾病，因為引起耳鳴的病因眾多，在臨床有很多樣化的表現，以及其潛在的病理神經機制仍然不清楚，所以至今尚無有效的根治方法，而利用水楊酸誘導大鼠耳鳴是目前廣泛使用的耳鳴動物模行之一，已知利用單一高劑量的水楊酸注射可誘發大鼠產生耳鳴，而在中樞聽神經元多處核團之聽神經元有過度活化的情形，但是其中的下行聽覺路徑迴路與其主要核團（如側上橄欖核）是否亦在耳鳴誘發之病理機轉中扮演重要角色則至今不確定。因此本實驗即是以Fos蛋白表現來探討水楊酸注射誘發耳鳴大鼠之上下行中樞聽覺核團神經元神經活性的表現差異。 在本研究總共進行12隻大鼠實驗，經過五個小時在隔音箱中的前適應處理後，對於實驗組大鼠給予腹腔內注射單一劑量250 mg/kg的水楊酸，對照組大鼠則注射等量的生理食鹽水，在注射兩小時候以urethane麻醉大鼠後進行全身性的組織灌流，取出的腦組織在經後固定與脫水後，冷凍切片切成厚度為40 µm的組織切片，再以懸浮染色的方式進行Fos免疫組織染色，之後觀察各腦區包含耳蝸核，上橄欖複合體，下丘核，內側膝狀體和聽覺皮層。 結果分析在注射單一劑量水楊酸後，側上橄欖核及聽覺皮層內表現Fos蛋白的聽神經元數目均顯著增加 (p < 0.05)，但是，在下丘表現量則有些減少，但此一減少並未達到統計意義，內側膝狀體則是無發現有Fos蛋白的聽神經元表現。 本實驗結果證明下行聽覺皮層的調控機轉（如聽覺皮層與側上橄欖和相關的下行調控路徑）在水楊酸誘發耳鳴的過程中扮演重要的角色，但是聽覺皮層與側上橄欖核神經元活性變化的先後時間關係與其詳細調控機轉仍需未來的實驗來進一步的釐清。

Tinnitus is a phantom perception of sound in the absence of external sound stimulation. It is the most prevalent disorder of hearing. Unfortunately, no effective cure is currently available because of its diverse causes and clinical presentations and its underlying neural mechanisms are still poorly understood. SS-induced rat is one of widely used tinnitus animal models. The SS-induced tinnitus was associated with over-activity in the central auditory pathways, e.g., auditory cortex. Whether the lower efferent relays (e.g. lateral superior olivary complex, LSO) are involved remains unclear. In this study, we used Fos-immunohistochemistry to map the over-active neurons at the auditory pathways following a single dose of SS injection. Specifically, we examined the cochlear nucleus (CN), superior olivary complex (SOC), inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC). Experimental rats (n = 12) received single of SS (250 mg/kg, i.p.). Rats were sacrificed at 2 hours after the injection. Standard Fos-immunohistochemical staining procedures were used for processing frozen sections (40 µm in thickness). Control rats received saline injections. Results showed that following the SS injection there was a significant increase (p < 0.05) in the number of Fos-stains at the LSO and AC. In contrast, only small number of IC neurons expressed Fos after SS injections. MGB neurons no expressed Fos after SS injections. Results of an increase of Fos-immunoreactivity at LSO and AC suggested that the pathogenesis of SS-induced tinnitus involves an alteration in the efferent mediated feedback systems such as AC and LSO. However, how AC and LSO neurons may change their activity in a temporal manner and the details of regulation mechanisms during the pathogenesis of SS-induced tinnitus would obviously require a time-lapse study in the future.