探討新生期投予dexamethasone對成年期大白鼠憂鬱行為的影響

Abstract

人工合成糖皮質激素dexamethasone ( DEX ) 在臨床上常用於治療早產兒因肺部發育不良而引發的呼吸窘迫症現象，可提高早產兒的存活率，但近期研究發現在新生期接受過高劑量DEX投藥可能對其腦部發育與認知功能產生不良的影響。杏仁核( amygdala )調控情緒，更是調控恐懼記憶的形成與削減的重要核區。過去尚未有研究者針對新生期投予DEX對杏仁核功能的影響進行持續的追蹤。因此，本研究利用Wistar大鼠，模擬臨床上投藥的方式，於出生後第一至第三天，以皮下注射的方式，投予遞減劑量 (tapering dosage)的DEX (0.5 mg/kg, 0.3 mg/kg and 0.1 mg/kg)，待動物成年後評估其對杏仁核功能的影響。研究結果發現，新生時期投予DEX的成年大鼠在面臨急性壓力時於強迫游泳行為(forced swim test, FST )模式中不游動的時間明顯高於控制組老鼠，表現出類憂鬱行為的反應。此外，胞外電生理記錄結果發現，以高頻電刺激誘發的LTP訊號，新生期DEX組大鼠在視丘投射至側杏仁核的神經訊號值明顯高於控制組大鼠，杏仁核MAPK磷酸化程度亦顯著的高於控制組動物，且在血清中的皮質固醇激素(corticosterone)濃度也高於控制組動物，顯示新生期DEX的投予對動物的反應具有長期影響。透過MAPK 抑制劑PD98059的投予能有效降低DEX組動物不游動的時間，利用伴隨人工腦脊髓液的灌流，PD98059能使神經路徑過度活化的現象回復，顯示 MAPK磷酸化的程度在調控動物面臨急性壓力時的生理反應扮演重要角色。此外，投予低劑量利尿劑bumetanide能改變細胞內離子濃度變化，調節神經細胞活性，亦可達到回復效果。 新生期投予DEX對於動物條件化恐懼的建立形成障礙，此障礙能透過D-cycloserine的投予增加NMDA受器的活性而得到改善。本研究結果亦指出於新生鼠接受DEX注射前給予乙醯化酶抑制劑trichostatin A (TSA)能有效避免DEX所造成的負面影響，顯示DEX的投予可能改變部份基因的表現，對動物情緒反應造成持續性的長期影響。本研究結果有助於瞭解新生期DEX的投予對動物杏仁核神經投射路徑與分子活性的影響，並對新生期不良經驗導致的憂鬱症致病成因提供研究的參考。

Synthetic glucocorticoid dexamethasone (DEX) is frequently used as a therapeutic agent to lessen the morbidity of chronic lung disease in premature infants. Previous studies suggested that neonatal DEX treatment altered brain development and cognitive function. It has been recognized that the amygdala is involved in emotional processes and also a critical site of neuronal plasticity for fear conditioning. Little is known about the possible long-term adverse effect of neonatal DEX treatment on amygdala function. The present study was aimed to evaluate the possible effect of neonatal DEX treatment on the synaptic function of amygdala in adult rats. Newborn Wistar rats were subjected to subcutaneous injection of tapering dose of DEX (0.5 mg/kg, 0.3 mg/kg and 0.1 mg/kg) from postnatal day 1 to 3, PN1~PN3. The animals were then studied by forced swim test (FST) and electrophysiological recording at the age of 8 weeks. Results showed neonatal DEX treatment increased depression-like behavior in adulthood which had been determined by FST. After acute stress evoking, the percentage of time spent in free floating was significantly increased in the DEX treated group as compared with the control animals. Furthermore, neonatal DEX treatment elevated long-term potentiation (LTP) response and the phosphorylation level of MAPK in lateral nucleus of amygdala (LA). Compared with control group, corticosterone (CORT) in plasma was significantly increased in DEX treated group after acute stress pre-exposed FST without influencing amygdaloid total amount of glucocorticoid receptor (GR). Intracerebroventricular infusion of MAPK inhibitor, PD98059 exhibited certain reverse effects, including the reduction of depression-like behavior and the restoration of LTP to normal range, implying that the behavior disorder could be reduced through a modification of the ionic conduction of the neuron cell. Pre-treatment of the potent inhibitor of histone deacetylase (HDAC) trichostatin A (TSA) prevented the adverse effects caused by neonatal DEX treatment. This finding implied that the defective effects of neonatal DEX treatment might be mediated by the epigenetic regulatory processes. In conclusion, our results suggested that neonatal DEX treatment may disrupt amygdala development and gene expression which then let to an resulted in adverse consequence in the adult age, such as the enhancement of the susceptibility for depression disorder in later life.