視交叉上核神經迴路調節哺乳動物生理時鐘之光校正

Abstract

生理時鐘是一種進化上保守的機制，通過在特定時間喚醒動物來增強動物的適應性。在哺乳動物中，視交叉上核（SCN）作為中央晝夜振盪器。生理時鐘由時鐘基因驅動，如Cry和Per，這些基因對維持晝夜節律周期至關重要。光是晝夜節律同步的最重要因素，這個過程稱為光同步。不同時間的光刺激在相位反應曲線（PRC）上引起不同的相位的變化，分別會使得動物清醒時間在死區不變（CT8）、延遲區延遲（CT16）和提前區提前（CT22）。但是目前生理時鐘的光同步機制的當前理解是不完整的。過去的研究指出在所有時間點接受光刺激時，即使在行為死區（CT8），分子時鐘基因Per1仍會被誘導 (Meijer et al., 1998; Shigeyoshi et al., 1997)。最近的研究表明，CT16光和CT22光在齧齒動物SCN中誘導不同cFos表達模式 (Duy et al., 2020)。此外，我們的團隊通過活體內視鏡鈣離子影像揭示了死區（CT8）、延遲區（CT16）和提前區（CT22）期間SCN的不同光響應模式（Yeh, 2024）。因此，我們假設不同時間的SCN神經迴路分別控制相位延遲或提前。 使用TRAP2小鼠和化學遺傳學調控延遲區（CT16）和提前區（CT22）標記的SCN光反應神經元，我們的數據表明，提前區(CT22)光標記的神經迴路與典型的光誘導PRC反應模式一致。值得注意的是，在所有時間點延遲區（CT16）光標記的神經元會導致顯著的相位延遲，甚至在死區（CT8）和提前區（CT22）也如此。相對地，在CT16抑制CT16標記的神經元會減弱光誘導的延遲效應。然而，在CT22抑制CT16捕獲的神經元會提前內在時鐘。對於標記的神經元鑑定，我們發現標記的細胞類型比例與整個SCN中的細胞類型分布比例相似，表明晝夜節律光同步不是由單一細胞類型主導的，而是由多樣的SCN神經網絡主導的。總之，我們的研究結果表明，PRC的延遲部分可以成功地分離出來，挑戰了關於SCN如何整合和響應光刺激的既定範式，並推導出解釋哺乳動物晝夜節律光同步的可能的雙峰迴路。

The circadian clock is an evolutionarily conserved mechanism that enhances animal fitness by waking animals at specific times. The suprachiasmatic nucleus (SCN) is the mammalian central circadian oscillator. The circadian rhythm is driven by clock genes such as Cry and Per, which are crucial for maintaining the circadian cycle. Light is the most important factor for circadian entrainment, known as photoentrainment. Light stimuli at different times cause phase shifts on the phase response curve (PRC), resulting in no change in waking time in the dead zone (CT8), phase delay in the delay zone (CT16), and phase advance in the advance zone (CT22). However, the current understanding of the mechanism of circadian photoentrainment is incomplete. Previous studies indicated that the clock gene Per1 is induced at all times of light stimulation, even in the behavioral dead zone (CT8) (Meijer et al., 1998; Shigeyoshi et al., 1997). Recent studies have shown that light at CT16 and CT22 induces different cFos expression patterns in the SCN of rodents (Duy et al., 2020). Additionally, our team revealed different SCN light-responsive patterns during the dead zone (CT8), delay zone (CT16), and advance zone (CT22) using in vivo calcium imaging (Yeh, 2024). Therefore, we hypothesize that SCN neural circuits control phase delay or advance at different times. Using TRAP2 mice and chemogenetic modulation of light-responsive SCN neurons labeled in the delay zone (CT16) and advance zone (CT22), our data show that the neural circuitry labeled by light at CT22 aligns with the typical pattern of light-induced PRC. Notably, neurons labeled by light at CT16 cause significant phase delay at all times, including the dead zone (CT8) and the advance zone (CT22). Conversely, inhibiting CT16-labeled neurons at CT16 reduces the light-induced delay effect. However, inhibiting CT16-labeled neurons at CT22 advances the internal clock. For the identification of labeled neurons, we found that the proportion of labeled cell types is similar to the distribution of cell types in the entire SCN, indicating that circadian photoentrainment is not dominated by a single cell type but by diverse SCN neural networks. In conclusion, our findings indicate that the delay portion of the PRC can be successfully isolated, challenging established paradigms regarding how the SCN integrates and responds to light stimuli, and deriving a possible bimodal circuitry to explain mammalian circadian photoentrainment.