探討成年嗅球神經元新生在雌鼠生殖行為中所扮演的角色

Abstract

在成年哺乳類大腦中，位於側腦室旁的側腦室下區與齒狀迴顆粒細胞下區存在有神經幹細胞。在側腦室下區中的神經幹細胞會產生前驅細胞，其會產生神經母細胞。而在此處產生的神經母細胞會沿著rostral migratory stream 移動到嗅球，分化成成熟的神經元，並加入嗅球原有的神經迴路之中。而齒狀迴顆粒細胞下區所產生的神經母細胞，則會往齒狀迴移動，並在成熟後加入神經迴路之中。嗅球分成主嗅球與副嗅球兩個部分。主嗅球主要負責空氣中一般氣味分子的傳遞；而副嗅球主要與費洛蒙訊息的傳遞有關，並會引起動物的先天行為。在先天行為之中，有一個有趣的現象稱為布魯斯效應。布魯斯效應指的是母鼠在懷孕早期因為接觸到其他非伴侶公鼠的費洛蒙後，重新進入動情週期而導致流產。根據實驗室之前的研究發現：公鼠費洛蒙所引起的成年副嗅球神經元新生會因為接觸到不熟悉的公鼠費洛蒙而減少。因此我們假設：成年副嗅球的神經元新生可能與布魯斯效應有關。為了驗證此假說，我們對帶有Nestin-CreER；R26-loxP-STOP-loxP-DTA 的基因轉殖鼠施打tamoxifen，造成側腦室下區的神經幹細胞的死亡，來專一地抑制嗅球的神經元新生。我們發現在施打tamoxifen 八周後，側腦室下區的神經幹細胞、神經母細胞、正在分裂的細胞與嗅球中新生的神經元皆有明顯的下降。因此之後我們選擇在施打完tamoxifen 後六周的母鼠與公鼠交配，並計算懷孕率。我們發現，這些缺少嗅球神經元新生的老鼠無法成功懷孕。我們的研究顯示嗅球神經元新生對於齧齒類生殖行為是必要的。Neural stem cells in the subventricular zone (SVZ) of the lateral ventricles can give rise to neuroblasts that migrate along the rostral migratory stream (RMS) to the olfactory bulb (OB) and differentiate into interneurons. The OB consists of the main (MOB) and accessory olfactory bulb (AOB). MOB is involved in the perception of odorants. On the other hand, AOB responses to pheromonal cues. Pheromonal olfaction is critical for innate olfactory behaviors in rodents. Among them, an interesting phenomenon called the Bruce effect is that pheromones from the urine of males can re-initiate a female’s estrus cycle to cause pregnancy block. However, a female animal can form an “olfactory memory” specifically to the pheromones of its mate, so it will not reenter the estrus cycle after mating and has a miscarriage. Previously, we found that male pheromone-induced adult AOB neurogenesis was attenuated when females were exposed to pheromones from an unfamiliar male. This suggests that decrease of adult AOB neurogenesis contributes to the Bruce effect. To test this hypothesis, we genetically ablated adult neurogenesis by treating tamoxifen to Nestin-CreER; Rosa26-loxP-STOP-loxP-diphtheria toxin fragment A (R26-DTA) mice. In these mice, tamoxifen-induced Cre recombinase results in the expression of DTA in neural stem cells and causes their death. We found that neural stem cells, neuroblasts, proliferating cells in the SVZ and new neurons in the OB were decreased in double transgenic mice eight weeks after tamoxifen treatments. The neuronal density in the OB was decreased eight weeks after tamoxifen treatments, suggesting that adult OB neurogenesis is blocked in these mice. We performed mating experiment six weeks after tamoxifen treatments and found that tamoxifen-treated double transgenic female mice had lower pregnancy rate. This result suggests that adult OB neurogenesis is required for reproduction.