The role of adult born neurons of the hippocampus in learning and memory

海馬迴為主要參與學習記憶和認知的腦區，許多認知功能異常與海馬迴有關，例如阿茲海默症、壓力皆會對認知功能造成影響。在成熟哺乳類動物的大腦中發現海馬迴的顆粒細胞下區(Subgranular zone, SGZ)持續有新神經細胞生成。在先前的實驗，我們在海馬迴之顆粒細胞大量死亡的大鼠中成功促進成體神經新生（adult neurogenesis）而且使海馬迴的學習記憶功能恢復，但對於這些新生成神經細胞是否參與學習記憶的進行並不清楚。海馬迴的顆粒細胞需要腎上腺所分泌的皮質酮才能生存，如果將雙側腎上腺移除會造成顆粒細胞大量死亡。因此摘除雙側腎上腺（adrenalectomized,ADX）的老鼠可以當作一個研究海馬迴之顆粒細胞死亡與新生的模型。當ADX三個月後給予sonic hedgehog(shh)和豐富環境後，觀察到有大量的新顆粒細胞生成，而且原本有問題的海馬迴之學習記憶功能也恢復正常。所以接下來我們要探討這些新生成的神經細胞是否併入原本的神經網絡，而且參與學習記憶的過程，利用Arc的組織免疫染色來觀察，Arc屬於當突觸活化時會立即表達的基因（immediate early gene），在進行行為測試後1.5小時可被觀察到。以觀察Arc和BrdU（標定十週前所新生成的細胞）共同染到的狀況，推測新生成神經細胞參與在學習記憶上。從我們的實驗結果顯示給予Shh後待豐富環境的ADX老鼠比待一般鼠籠環境的ADX老鼠在齒狀迴有大量的Arc和BrdU共同被染到，而且在Object-context association行為測試上，待豐富環境的老鼠，表現與正常老鼠一樣，但是待一般鼠籠環境的則與沒給Shh處理的ADX老鼠表現相同，顯示Shh的治療效果必須搭配豐富環境才能顯現出來。從結果推測經過Shh和豐富環境刺激所生成且存活下來的神經細胞會參與在神經網絡的活動中。The hippocampus is a brain region critical to learning and memory and is a frequent target of many neurological diseases such as Alzheimer’s, other forms of dementia, and chronic stress that have dramatic cognitive consequences. The Subgranular zone (SGZ) of the hippocampus is one of the mammalian brain regions where new neurons are generated continuously throughout adult life. Previously, we have successfully promoted adult neurogenesis and demonstrated functional recovery after hippocampal granule cell degeneration in a rat model. This study was undertaken to address the question of whether the adult-born neurons were integrated into a neural network and involved in the process of learning and memory. Corticosterone, secreted by adrenal glands, is required for hippocampal granule cell survival and bilateral removal of adrenal glands lead to granule cell death. Therefore, adrenalectomized (ADX) rats were used to ablate, and regenerate granule cells in the hippocampus. Three months after treatment of ADX animals with sonic hedgehog (shh) and environmental enrichment, significant amount of granule cell regeneration and restoration of brain function was observed. To determine whether the new born neurons were integrated into a neural network and participated in the learning and memory process, immunohistochemistry for Arc, a synaptic activity dependent immediate early gene product was performed after behavior test. Colocalization of Arc and BrdU (a marker for neurons born 10 weeks ago) staining suggests that new neurons which were born during shh treatment were involved in the learning and memory. Colocalization of Arc and BrdU was more abundant in the dentate gyrus of the hippocampus in ADX animals treated with shh and housed in the enriched environment when compared with untreated ADX animals or ADX animals treated with shh but housed in cages. These results suggest that after treatment with shh and environmental enrichment, new born neurons survives for at least 3 months and participates in the activities of neural networks