探討生理時鐘中區精胺酸血管加壓素的神經連結迴路

Abstract

生理時鐘調控我們的行為週期和身體的機制，例如代謝作用和內分泌系統的調節。我們的日常作息，代謝作用以及睡眠週期都是按照24小時為一周期。腦部的下視丘的上視神經交叉核（suprachiasmatic nuclei, SCN）是哺乳動物的生理時鐘的主要調控中心。為了與外界的光暗週期同步，光的訊息會通過視網膜上的內發性感光視網膜神經節細胞（intrinsically photosensitive retinal ganglion cells, ipRGCs）傳遞到上視神經交叉核。在收到光的刺激後，ipRGCs會釋放麩氨酸（glutamate）到上視神經交叉核以傳遞訊息。ipRGCs和上視神經交叉核的連接也透過追踪特定基因表現的研究中得以證實。上視神經交叉核的構造可分為腹外側（ventral lateral）和背外側（dorsal medial），並由多種神經細胞所構成。在眾多神經細胞中，對於生理時鐘的形成有著重要影響的是表現血管活性腸肽（vascular intestinal peptide，VIP）和精胺酸血管加壓素（arginine vasopressin peptide, AVP）。腹外側主要由表現VIP的神經細胞組成，而背外側由表現AVP 的神經細胞所分佈。在過去的研究中，視神經交叉核的神經同步以及訊息的整合是由於表現VIP的神經細胞接受ipRGCs所傳遞的光訊號。之後生理時鐘的訊息會由表現AVP 的神經傳到其他腦區。然而，單一ipRGCs追踪法讓我們發現ipRGCs並非如一般認知僅限傳遞到表現VIP的神經細胞，實際上ipRGCs會分佈到整個上視神經交叉核。現如今的研究中，上視神經交叉上視神經交叉核的神經細胞之間如何運作產生生理時鐘仍然處於探討的階段。於是我們使用鈣離子影像記錄方式，發現上視神經交叉核中表現AVP的神經細胞在河魨毒素（tetrodotoxin，TTX）下（動作電位在其他神經中受阻截）依然會受到麩氨酸，N-甲基-D-天門冬胺酸受體（N-methyl-D-aspartate receptor，NMDA）和代谢型谷氨酸（metabotropic glutamate，mGlu）的促效劑所活化。因此我們推測ipRGCs的訊息會被上視神經交叉核中表現AVP的神經細胞所接受。為了證實這項猜想，我們把表現AVP的神經細胞攜帶感知鈣離子濃度的熒光蛋白---GCaMP7f，並利用光調控的方式激發ipRGCs，並觀察神經細胞的變化。實驗結果顯示AVP 的神經細胞會接受來自ipRGC的訊息並表現出多種鈣離子濃度變化的反應，因此AVP的神經細胞可能會透過ipRGC受到外界的刺激進而影響生理時鐘。

Circadian rhythms modulate our daily activity patterns and other body mechanisms, such as metabolic and neuroendocrine rhythms. It controls our daily behavior, metabolism, and sleep-wake cycle for a period of about 24 hours. Suprachiasmatic nucleus (SCN) in the hypothalamus is known as the central clock that controls circadian rhythms in mammals. The external light signal could entrain SCN through intrinsically photosensitive retinal ganglion cells (ipRGCs) by releasing glutamate. The ipRGC is a type of neurons in the retinal of mammalian eyes. The innervation of ipRGCs to SCN has been revealed by using genetic tracing techniques and contributes to the synchronization of the circadian rhythm. SCN is a heterogenous structure that contains different neurons, while the networking of circadian rhythm is prevalently related to AVP and VIP neurons. The shell region of SCN mainly consists of AVP neurons, while VIP neurons are major in the core region. In previous studies, VIP neurons are suggested as light signal-receiving neurons from ipRGCs, and AVP neurons are the primary output neurons for the SCN. However, in single ipRGC tracing study suggested that ipRGCs innervation is not limited to VIP neurons specifically but throughout the whole SCN. In addition, the mechanisms of communication within SCN neurons are still unrevealed in recent studies. By using in vitro calcium imaging, here we showed that glutamate, NMDA, and metabotropic glutamate agonists activated AVP neurons in SCN under the application of TTX to block multi-synaptic activation. To know whether ipRGC and AVP neurons in SCN are a monosynaptic pathway, we use a calcium sensor, GCaMP7f, to image the neural activity in SCN and stimulated by optogenetic simultaneously. The results show that AVP neurons receive signals from ipRGCs and exhibit different responses. Our results suggested that AVP neurons may also receive direct ipRGC input from the retina for circadian clock regulation.