痕跡恐懼制約（trace fear conditioning）為一種行為典範，在此關聯性學習當中，制約刺激（如：聲音）與非制約刺激（如：足部電擊）配對之間存在一段時間間隔，並且海馬迴（hippocampus, HPC）與內側前額葉皮質（medial prefrontal cortex, mPFC）皆參與於此學習之習得階段。視丘中線連結核（nucleus reuniens, Re）與菱形核（rhomboid nucleus, Rh）（合稱ReRh）與海馬迴及內側前額葉皮質有雙向投射關係。我們先前的實驗結果發現在習得階段抑制連結核與菱形核會妨害痕跡恐懼制約的學習。然而，若是在習得及提取時皆抑制了此腦區，則在測試時，動物除了會對於聲音表現出增高的恐懼之外，也會對於新環境表現出類化恐懼。在此篇研究中，我們目標透過增加環境暴露時間以及減少訓練試驗刺激配對次數以降低環境恐懼，探究連結核與菱形核對於調控提示聲引發之恐懼之特定重要性，我們利用藥理學抑制的方法，再次確認在習得階段抑制連結核與菱形核會妨害痕跡恐懼制約的學習，然而，此現象只在較強的訓練下出現。另外，若是在所有行為實驗過程皆抑制了連結核與菱形核，動物會表現出不容易消除的高度環境恐懼，並會類化至其他環境，且會在出現聲音後表現再次升高的恐懼反應。由於海馬迴相關迴路對於痕跡恐懼的學習和表現是重要的，於是我們檢驗當連結核與菱形核功能不全時，動物是否透過非海馬迴相關迴路學習到恐懼，若假設為真，則對海馬迴操弄不會影響此學習表現。實驗指出阻斷腹側海馬迴中的麩胺酸鹽受體會妨害痕跡恐懼的學習及表現，與之前的文獻一致。然而，連結核與菱形核損傷的組別同樣也受麩胺酸鹽受體阻斷的影響，顯示在連結核與菱形核損傷情況下，痕跡恐懼的學習及表現仍須依賴海馬迴。總結而言，我們的結果發現連結核與菱形核對於痕跡恐懼制約精確習得及表現是非常重要的。

The reuniens (Re) and rhomboid (Rh) nuclei (ReRh) of the midline thalamus interconnect the hippocampus (HPC) and the medial prefrontal cortex (mPFC). The HPC and mPFC are both involved in the acquisition of trace fear conditioning, in which a conditioned stimulus (tone) and an aversive unconditioned stimulus (footshock) are paired but separated in time with a trace interval. Earlier, we demonstrated that ReRh inactivation during trace conditioning impaired the acquisition of cued fear. In contrast, ReRh inactivation during both conditioning and test resulted in heightened fear to tones during retrieval. Because there was a generalized contextual fear on top of heightened fear to tones in the latter experiment, here we aimed to examine the specific importance of the functional ReRh in cued fear by decreasing the contextual fear through introducing prolonged contextual exposure and reducing the number of training trials. We pharmacologically inactivated the ReRh with muscimol (or saline as controls) before each experimental session. We confirmed that ReRh inactivation before trace fear conditioning impaired the acquisition of cued fear. However, the effect only emerged when a strong training procedure was implemented. Moreover, without the functional ReRh throughout the entire behavioral sessions, these animals showed heightened contextual fear that did not extinguish with the passage of time, which generalized to the other context, and fear to tones reprised when the tones were presented. Because the HPC-dependent circuitry is important for precise control of trace fear learning and expression, we next examined whether the fear acquired without the functional ReRh is mediated through extra-HPC system and would therefore become insensitive to manipulations of the HPC. Consistent with previous literatures, blockade of the NMDA receptors in the ventral HPC (vHPC) impaired the acquisition and expression of trace fear. However, rats with permanent ReRh lesions still showed similar impairment as their sham controls, suggesting that without the ReRh, acquisition and expression of the trace fear still relied on functional HPC. Together, our results suggested that functional ReRh are important for precise control of fear to context and tones acquired under trace procedure.

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