其他可能的敗者效應生理機制與未來展望

第四章討論

第四節其他可能的敗者效應生理機制與未來展望

雖然敗者效應普遍地在許多物種中被發現，但目前只有非常少數的研究明確且直接地探討經驗效應的生理機制。本研究的結果顯示，

皮質醇以及睪固酮並非K. marmoratus 敗者效應生理機制的主要調控 者，因此，敗者效應應該有其他的生理機制。在硬骨魚中，除了睪固酮之外，最主要之雄性素為睪丸硬甾酮（11–ketotestosterone）（Borg, 1994; Vermeulen et al., 1994）。許多魚類的研究指出睪丸硬甾酮與打鬥行為有密切的交互關係（Oliveira, Almada and Canario, 1996; Hay and Pankhurst, 2005）。而且，在一些慈鯛魚科的研究中發現，當個體參與打鬥時，睪丸硬茲酮分泌量的改變較睪固酮顯著（Oliveira, Almada and Canario, 1996; Hirschenhauser et al., 2004; Desjardins et al., 2006）。另外，雌二醇（estradiol）為睪固酮主要的代謝物，在一些脊椎動物的研究中，發現雌二醇與雌性的攻擊性呈正相關（Woodley and Moore, 1999）。由於睪丸硬甾酮以及雌二醇也是K. marmoratus 體內重要的荷 爾蒙（Minamimoto et al., 2006），因此這兩種荷爾蒙都有可能是 K.

marmoratus 的敗者效應的調控者。另外，勝負經驗會引發荷爾蒙受器

（hormone receptor）數量的變化。在大鼠（Sprague–Dawley rats）的研究中（Marini et al., 2006），強制給予實驗個體一次輸的經驗後，其皮質固酮的分泌量顯著高於沒有經驗的控制組個體，而給予經驗後2 小時，皮質固酮的分泌量即下降並與控制組個體沒有顯著差異。然而，在給予經驗 30 小時後卻發現個體腦內下視丘中糖皮質素受器

（glucocorticoid receptors, GR）的 mRNA 表現量顯著地高於沒有經驗的控制組個體，顯示輸的經驗會導致糖皮質素受器的數量增加。荷爾蒙分子經由結合至受器，經過一連串分子機轉，進而調控個體行為。

許多研究指出，若給予個體荷爾蒙受器之拮抗物將會抑制荷爾蒙對行為的作用（Tokarz, 1987; Tokarz, 1995; DiBattista et al., 2005）。因此荷爾蒙受器數量的變化可能改變荷爾蒙對行為的影響。

也有許多研究指出，神經傳導物質（neurotransmitters）血清素

（serotonin; 5–hydroxytryptamine; 5–HT）與打鬥行為以及社會地位高

度相關（詳見綜論Blanchard, McKittrick and Blanchard, 2001; Nelson and Trainor, 2007 ）。當個體遭遇壓力時，腦內之血清素活性

（serotonergic activity）會增加。在打鬥中落敗的個體可能因承受較大的壓力而使腦中血清素活性顯著地提升（詳見綜論 Blanchard, McKittrick and Blanchard, 2001）。例如，使虹鱒個體在打鬥後持續互

動一天，落敗者腦中的血清素活性顯著地較獲勝者高（Winberg and Lepage, 1998; Øverli, Harris and Winberg, 1999）。另一方面，在脊椎動物的研究中指出，血清素會抑制調控攻擊性的神經迴路，因此，血清素活性增高通常與個體攻擊性下降相關（Summers and Winberg, 2006）。例如，Larson 和 Summers（2001）給予綠變色蜥（green anole, Anolis carolinensis）打鬥獲勝者一星期選擇性血清素再吸收抑制劑

（selective 5–HT reuptake inhibitors, SSRIs），使個體腦中血清素活性提升。讓此個體與先前已打敗過的對手（打鬥能力較差者）再進行打鬥，結果發現個體向對手展示以及攻擊的次數皆顯著地下降，而且在打鬥中落敗的機率降為43%。而同時給予獲勝者與落敗者或是只給予落敗者SSRIs，再與同一對手打鬥則獲勝者仍獲勝，落敗者仍落敗，

維持相同的勝負關係。總合以上所述，在打鬥中落敗可能導致腦中血清素活性增加，並進一步抑制攻擊性。因此，血清素也很有可能是敗者效應的調控者之一。

輸的經驗也可能經由腦內學習與記憶的機制而使個體之後的行為發生改變。個體能從經驗中學習，進而改變之後的行為表現，而其中也包括了打鬥行為（Hsu, Earley and Wolf, 2006a）。例如，一份虹鱒的研究中，利用帕弗洛夫恐懼制約（Pavlovian fear conditioning）理論，

將實驗缸中循環水流停止做為制約式刺激（CS），而與體型較大之個

體打鬥為非制約式刺激（US）。經過 CS–US 配對學習後，若單獨給予個體水流停止的線索，會導致個體表現出屈服行為（Carpenter and Summers, 2009）。雖然許多研究都指出個體可藉由學習改變打鬥行

為，但學習與敗者效應之間的關聯仍舊十分模糊。目前可能解釋敗者效應的學習機制之一為操作制約（operant conditioning）：個體在打鬥中落敗後，可能獲得某形式之正向或負向增強（reinforcement），而在下次遇到競爭對手時降低其攻擊性（Hsu, Earley and Wolf, 2006b）。在

此過程中牽涉到長期記憶的形成：刺激導致基因轉錄與蛋白質的新合成，改變突觸形狀，造成突觸間訊息傳遞速率（synaptic efficacy）增加（Abel and Kandel, 1998）。在哺乳類中，掌控學習與記憶的部位是大腦內的邊緣系統（limbic system）；而魚類則是端腦（telencephalon）。藉由探究這些區域之神經系統變化也許能進一步了解學習機制與敗者效應的關係。

由前述可知，尚有許多因子可能與敗者效應的生理機制的調控相關，這將是未來繼續探討敗者效應之生理機制的方向。此外，由於本研究只探討了敗者效應的荷爾蒙機制，之後應和勝者效應的研究結果整合討論，以進一步比較敗者與勝者效應的生理機制之異同。

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在文檔中探討Kryptolebias marmoratus敗者效應之荷爾蒙機制 (頁 52-89)

第四章 討論

第四節 其他可能的敗者效應生理機制與未來展望

第四章討論

第四節其他可能的敗者效應生理機制與未來展望