母體分離壓力 (Maternal separation, MS) 是一種廣泛被利用作為早期生活壓力的動物模式。許多的研究指出其會導致精神生理的功能缺損。此外，與壓力反應有密切關係的下視丘-腦下垂體-腎上腺軸 (hypothalamic-pituitary-adrenal axis, HPA axis) 活性也會受到母體分離調控。腸-腦軸 (Gut-brain axis) 由中樞及腸道神經系統構成，因而做為一個有力支持壓力同時改變腸道環境與大腦功能的途徑。先前的研究指出，腸道中菌種的組成，會受到環境的調控，而其中壓力即是一種重要因素。近年來有許多的研究關注於藉由益生菌改善腸道菌相失衡，進而調節壓力狀態的作用。因此，本篇研究主要的研究目的在於評估益生菌治療對於母體分離的早期生活壓力所產生的有益效應。利用出生早期的Sprague-Dawley (SD) 大鼠，主要探討控制組、母體分離組及益生菌治療組在聽覺恐懼制約學習後，恐懼記憶表現之程度。實驗結果顯示，母體分離的幼鼠 [在出生後第二天至第十四天，每天與母鼠分離三小時] 恐懼記憶增強並能持續表現，同時伴隨比較焦慮的行為表現。而經由母體投予由鼠李糖乳桿菌 (Lactobacillus rhamnosus) 及瑞士乳桿菌 (Lactobacillus helveticus) 組成的市售益生菌-Lacidofil的治療，我們發現益生菌使母體分離之幼鼠腸道菌種組成改變，聽覺恐懼制約學習記憶與焦慮行為獲得改善。此外，我們也發現經歷母體分離幼鼠的HPA axis活性比控制組來的高，說明其壓力感受性可能較好，而益生菌治療能使增加的活性，調降回正常狀態。並且，經過聽覺恐懼制約學習後，獲得一急性壓力刺激，母體分離的幼鼠在基底外側杏仁核 (Basolateral amygdala, BLA) 表現較高的c-fos與腦源性神經滋養因子 (brain-derived neurotrophic factor, BDNF)，反應該組幼鼠學習恐懼時，較多神經細胞被活化，如果透過補充益生菌治療，則消退了母體分離所誘發增強的神經元活性。綜合上述的研究結果，我們認為益生菌的治療的確可以改變腸道環境，進而對早期生活壓力所引發的情緒與記憶障礙，產生有效地改善作用。

Maternal separation (MS) is a kind of early-life stress (ELS) results in psychiatric dysfunction. In addition, MS model leads to behavioral changes due to alteration of hypothalamic-pituitary-adrenal (HPA) axis function. The gut-brain axis consists of the central nervous and the enteric system, connecting the brain with the intestinal functions. Exactly, HPA axis is a potential mechanism involves in regulating the gut-brain axis. Moreover, many researches indicated that gut microbial dysbiosis would change brain homeostasis and plasticity. There has demonstrated that a probiotic formulation, Lacidofil (Lactobacillus rhamnosus and Lactobacillus helveticus) has beneficial impacts on gut and brain activities. Thence, this proposal is to investigate whether microbial composition differently induced by probiotic treatment is associated with ELS-induced revise in emotional development. We found that MS during early life of rats displayed enhancement in long-term fear memory retention and showed anxiety-like behavior. By contrast, Lacidofil treatment significantly reduced persist expression of fear memory in infant rats exposed to MS. Simultaneously, we also found Lacidofil could recover HPA axis activity in infant rats exposed to MS. Altogether, these finding demonstrate that probiotic treatment has a potential role to rescue normal developmental trajectories of fear retention in infant rats suffered from ELS.

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