功能性探討自中腦區到未定區之神經輸入

Abstract

在自然界環境中，當動物接收到獎勵相關或是危險刺激時，他們會做出對應的舉 動以獲得獎勵或是迴避危險。有越來越多的研究顯示位於中腦的腹側背蓋區以及 黑質區不僅僅可以調控動機行為之外，也可以調控防禦行為。近幾年的研究也發 現大腦未定區同樣可以調節防禦行為。雖然已知腹側背蓋區以及黑質區有神經投 射到未定區，但是對於這些連結的功能目前還不清楚。因此在本研究中，我想要 研究從中腦到未定區的神經連結，瞭解此神經連結是由哪些種類的神經細胞參與 其中，其可以傳遞哪些資訊，誘發哪些行為，和透過哪些下游腦區來執行功能。 為此，我結合了雙病毒逆行標定方法以及原位雜交(RNAscope)技術來研究投射 到未定區的中腦神經細胞的化學表型，我發現會投射到未定區的腹側背蓋區神經 細胞組成大多是麩胺酸神經元以及少部分的γ-氨基丁酸神經元。此外會投射到未 定區的黑質區神經細胞則是由大部分的γ-氨基丁酸神經元以及少部分的多巴胺神 經元所組成。我進一步的利用小鼠壓力模型與結合神經活動之功能性標定免疫染 色方法，發現投射到未定區的中腦神經細胞在壓力情境下會被活化而參與其中。 接下來我專注於研究黑質區到未定區這條神經迴路，發現投射到未定區的黑質區 神經細胞會受威脅刺激的活化而參與其中。透過利用興奮性光遺傳學以及不同行 為模式組合，我發現當活化黑質區到未定區這條神經迴路時，可以導致嫌惡感並 且促進威脅刺激所引發的防禦行為。最後，我繪製了由黑質區所支配的未定區神 經細胞下游投射圖，發現當黑質區到未定區這條神經迴路被活化時，有幾個與防 禦行為相關的腦區也會被徵召而參與其中。總而來說，我的研究發現了一條新的 調節機制，透過黑質區到未定區這條神經迴路來調控動機與防禦行為。

In nature,when animals perceive reward-related or danger stimuli, they show corresponding responses to retrieve the reward or avoid the threat. Accumulated evidence has shown that the ventral tegmental area (VTA) and substantia nigra (SN) in the midbrain can not only modulate motivational behaviors but also regulate defensive behaviors. Recent studies have indicated that zona incerta (ZI) plays an important role in regulating defensive behaviors. Although both VTA and SN send projections to the ZI, the functional roles of midbrain-to-ZI connections remain elusive. Thus, in the present study, I would like to investigate the midbrain-to-ZI connection and examine what cellular phenotypes are involved in the pathway, what information midbrain-to-ZI input relays, what behavioral outcome the midbrain-to-ZI input results in and what downstream brain region the midbrain-to-ZI input may recruit. To this end, I combined the dual viral retrograde targeting approach with in situ hybridization (RNAscope) technique to study the neurochemical phenotypes of midbrain cells projecting to the ZI. I found that ZI- projecting VTA neurons consisted mostly of glutamatergic and a small population of GABAergic neurons, whereas ZI-projecting SN neurons consisted mostly of GABAergic and some dopaminergic neurons. I further adapted a mouse model of stress and performed neural activity-dependent marker c-Fos immunostaining and found that ZI-projecting midbrain neurons were significantly involved in restraint stress. I focused on SN-to-ZI input and found that ZI-projecting SN neurons were significantly engaged by threat stimulus. By combining an excitatory optogenetic approach with different behavioral tasks, I found that activation of SN-to-ZI pathway was aversive and promoted threat- induced defensive behavior. Lastly, I mapped the downstream projections of SN- innervated ZI cells and found several regions relevant to defensive behavior were functionally recruited when the SN-to-ZI input was activated. Altogether, my study has revealed a novel regulation of substantia nigra onto the ZI in aversion and defensive behavior.