雌性素對年輕雄鼠空間學習記憶與制約學習的影響

Abstract

學習與記憶是個體生存的重要能力。過去有關卵巢激素對認知影響的研究多以雌鼠作為實驗的對象；至於其對雄鼠的學習記憶究竟有何影響，迄今並不清楚。本實驗以鼠齡11-13週之雄鼠為研究對象，探討雌性素與助孕酮對大鼠(1)空間學習與記憶及(2)味覺嫌惡制約(conditioned taste aversion)能力的影響。在完成行為測試後，犧牲動物，進行Golgi染色以觀察海馬回錐狀細胞的樹突棘突密度的變化。同時使用高壓液相層析法(HPLC)分析不同腦區內神經傳導素的濃度。大鼠在睪丸切除手術後第3週，給予高劑量(120 μg)或低劑量(30 μg)雌性素或助孕酮(500 μg)之皮下注射處理者為實驗組，而給予等體積橄欖油處理者則作為對照組。以Morris水迷宮作業測試其空間學習記憶的行為表現。結果發現，經過為期4日之測試訓練後，各組皆隨訓練日數的增加而減少其抵達平台之時間。接受高劑量或低劑量雌性素處理之組別，於測試初期均較對照組需較多之時間抵達平台；但僅接受高劑量雌性素處理之大鼠於測試之第3、4日，仍需較多之時間抵達平台。而合併給予助孕酮處理的大鼠與對照組相比，兩組學習行為的表現並無顯著之差異。神經傳導素的分析方面，高劑量雌性素組海馬回中的Epinephrine、5-HT增加，但5-HIAA減少，雌性素-助孕酮組則DOPAC增加。高劑量雌性素組杏仁核中的NE增加但DA減少，而雌性素-助孕酮組則DOPAC減少。低劑量雌性素使大鼠海馬回中的5-HIAA及杏仁核中的DA減少，但杏仁核中的5-HT及5-HIAA增加；合併助孕酮處理之後，則使大鼠杏仁核之NE、DOPAC及5-HIAA增加。低劑量雌性素則使大鼠尾狀核-被殼中的DOPAC濃度減少。第二部份的實驗係將動物睪丸切除後第19日開始每日禁水23.5小時，持續1週。於術後第26日進行第一次制約，並於制約前48小時開始每日給予皮下注射雌性素(30 μg)或等體積之橄欖油(對照組)，部份動物於制約前4 小時再予助孕酮(500 μg)之皮下注射，爾後持續相同之處理至實驗結束。制約時在給予糖水後立即進行鋰鹽或等體積之生理食鹽水注射，並持續測試3天，以(糖水攝取的總量) / (全部攝取液體的總量)作為嫌惡指標，以評估學習記憶。並以高壓液相層析法分析其島狀皮質(insular cortex)、下視丘外側區與依核中神經傳導素的含量。結果發現，雌性素-鋰鹽組與雌性素-助孕酮-鋰鹽組大鼠較橄欖油-鋰鹽組大鼠之嫌惡指標為低，顯示雌性素或雌性素加助孕酮會增強其對糖水的嫌惡，有加強聯結學習的作用。此外亦發現大鼠對糖水嫌惡之主因為鋰鹽本身的給予而非雌性素的作用。神經傳導素的分析的結果顯示，雌性素-助孕酮-鋰鹽組大鼠島狀皮質內的NE增加。在下視丘外側區中，雌性素-鋰鹽組大鼠之Epinephrine減少，且橄欖油-食鹽水組、雌性素-助孕酮-生理食鹽水組大鼠及雌性素-鋰鹽組大鼠之NE均較雌性素-生理食鹽水組大鼠為低。依核中，雌性素-鋰鹽組與雌性素-生理食鹽水組大鼠之DA增加，雌性素-鋰鹽組大鼠之DOPAC減少。我們的實驗結果顯示(1)雌性素會減弱雄鼠對空間記憶的學習，但卻增進其味覺嫌惡制約的學習(2)助孕酮能拮抗雌性素減弱雄鼠空間記憶學習的作用，但在味覺嫌惡制約的學習上其影響較不明顯。

Learning and memory is critical for the survival of organisms during development. Female rodents have been used for a long time as subjects for investigating the relationship between ovarian hormones and cognitive performance. However, it remains unclear what role of ovarian hormones may play in the process of learning and memory in male rats. The aim of this study is to reveal the influence of estradiol (E2) and progesterone (P) on male rats in different mnemonic tasks. In part one, the investigation was designed to examine if different doses of E2 have the opposite effects on spatial learning tasks in young male rats. Long-Evans hooded rats were gonadectomized and pre-trained in a water maze with a visible platform. Following pre-training, high dose (120 μg/kg body weight, SC) or low dose E2 (30μg/kg body weight, SC) or oil was administered every single day since 48 h prior to testing and P (500 μg, SC) 4 h prior to testing. Rats were tested in a water maze with a submerged platform for 15 trials with randomed start locations for 4 days.The E2 treatment in both doses showed significantly impairments in the beginning of the testing days but only the high dose E2 treated group showed longer latency to the platform on the third and forth testing days. Interestingly, none of the E2 plus P treated animals showed significantly impaired performance relative to the control groups. In the high dose E2 treated group, levels of the epinephrine and 5-HT were elevated but 5-HIAA was decreased in hippocampus, the level of NE was elevated but DA decreased in the amygdala. In high dose E2-P treated group, DOPAC was elevated in hippocampus but decreased in amygdala. In the low dose E2 treated group, level of 5-HIAA in the hippocampus and DA in the amygdala were decreased, however, levels of 5-HT and 5-HIAA were increased in the amygdalar region. In the low dose E2-P treated group, levels of NE, DOPAC and 5-HIAA were elevated in the amygdala. In part two, the experiment was mainly designed to determine if similar procedures and treatments of steroidal hormones would have the same effect on the acquisition of conditioned taste aversion in young male rats. Sixty gonadectomized male rats received multiple doses of E2 (30μg/kg body weight, SC) or oil and P (500 μg, SC) before and during acquisition of LiCl-induced conditioned taste aversion. During acquisition, 30-min access to novel saccharin solution was given to rats and injected with either 0.15M LiCl or normal saline (4 ml/kg body weight). The results showed that E2 and E2-P treated groups enhance the acquisition of conditioned taste aversion compared to the oil-treated one. Besides, the well-established suppressive intake effect of E2 was reflected in reduction of both the total fluid intake and the body weight. However, significant aversion toward sucrose solution was mainly due to the effect of sucrose-LiCl conditioning reflected in results of comparison between LiCl and saline as an unconditioned stimulus. In the granular insular cortex, level of NE was elevated in E2-P-LiCl treated group. In the lateral hypothalamus, level of epinephrine was increased in E2-LiCl treated group and levels of NE in E2-saline treated group was higher than oil-saline treated group, E2-P-saline treated group and E2-LiCl treated group. Taken together, similar procedures and treatments of ovarian hormones had different effects with regard to different types of learning and memory in male rats.