建構單一M1 自主感光視神經細胞之神經迴路及探討其下游之生理功能

Abstract

在哺乳類視網膜中，自主感光視神經細胞傳遞光訊息至腦區，以調控非視覺相關的功能，如生理時鐘和瞳孔反射。目前研究顯示，自主感光視神經細胞會直接投射至生理時鐘的中樞-視叉上核。但現階段對此神經迴路的了解，仍無法解釋光如何造成多樣的生理時鐘變化。這是由於我們的認知侷限於組織層次，究竟單一自主感光視神經細胞如何與視叉上核的細胞連結，光訊息又是如何經由這複雜的神經網路所傳遞和整合，目前仍未知。故本研究欲剖析視網膜和視叉上核間的神經迴路，我們結合藥理與基因轉殖鼠的技術，成功地追蹤單一自主感光視神經細胞，並分析其在視網膜中的樹突型態，與腦中的軸突分佈。我們的研究發現，單一自主感光視神經細胞可同時投射至多個腦區，主要投射目標為–視叉上核(SCN) 和膝狀間區小葉(IGL)。令人驚訝的是，即使異側對同側自主感光視神經細胞的比例為9:1，它們可透過非典型的軸突同時連至兩側的視叉上核，最終使得兩側的視叉上核接收等量的單眼光訊息。此外，每個自主感光視神經細胞會傾向投射至視叉上核中的特定區域，代表可能與單一種視叉上核的細胞連結，也就是光可能透過不同的神經迴路來影響生理時鐘。總結來說，建構單一自主感光視神經細胞至視叉上核的神經連結，是解開此複雜神經迴路突破性的一步，並奠定日後研究光如何調控生理時鐘的基礎。

In the mammalian retina, intrinsically photosensitive retinal ganglion cells (ipRGCs) convey photic inputs to several brain regions for non-image forming functions, such as circadian photoentrainment and pupil light reflex. Recent studies have shown that ipRGCs directly project to the suprachiasmatic nucleus (SCN), the mammalian master clock, through the retinohypothalamic tract (RHT). However, our understandings of retinal -SCN circuit are still limited at the tissue level, which cannot explain the variety of light effects on circadian photo-entrainment. To dissect out the complicated retinal-SCN connectivity by tracing single ipRGC is definitely a key breakthrough for revealing how light information is processed in the SCN. Here we applied a conditional and inducible Cre-LoxP system to randomly label a single ipRGC in mice, and reconstructed their dendritic structure and axonal architecture from the retina to the brain. Our findings reveal that a single ipRGC projects to multiple brain nuclei, especially the predominant targets- the SCN and IGL. Strikingly, we also found that individual ipRGCs, despite with the 9:1 contralateral to ipsilateral ratio, can bilaterally project to both ipsilateral and contralateral sides of the SCN. As a result, the photic inputs from monocular ipRGCs to both sides of the SCN are close to equal. Finally, our results show that individual ipRGCs preferentially target specific regions in the SCN, suggesting that a single ipRGC may form synapses with specific SCN neurons. These distinct neuronal circuits may explain the diverse light responses of SCN in circadian photo-entrainment. Taken together, these detailed morphological features and innervation patterns in a single-cell resolution provide us valuable insight into the complexity of retinal-SCN circuit.