短期社會隔離及禁水壓力對成年雄性大鼠海馬細胞增生及焦慮行為有正面影響

Abstract

許多研究顯示長期的社會隔離(social isolation)會對大鼠造成極大的心理壓力，並惡化大鼠對於其他壓力所造成之焦慮反應。但亦有研究指出社會隔離可降低動物在面對新環境時所產生的焦慮感。然而，社會隔離是透過何種機制影響大鼠行為及生理的不同反應，以及當大鼠處於短期的壓力下社會隔離是否會使壓力所產生的反應惡化，目前仍不清楚。過去的研究指出禁水會造成大鼠血液中皮質醇(corticosteroid)濃度之上升，故禁水可視為一種輕微的生理性壓力。本研究之目的在探討處於社會隔離情形下之大鼠在面對短期禁水壓力時，其對於壓力的反應是否會有所改變。 本研究分為兩部份。實驗一將大鼠分為群體組及隔離組，探討大鼠在24小時的禁水壓力下，兩組大鼠對於壓力是否有不同的反應。而上述兩組均以飲水充足供應者作為對照組。禁水處理開始於下午2時，並同時給予所有大鼠劑量為100 mg/kg 的5-bromo- 2 ’-deoxyuridine (BrdU)之腹腔注射處理。且於隔日中午12時再給予一劑BrdU之處理。在實驗開始進行24小時後，即將動物犧牲灌流並以免疫組織化學染色觀察其海馬的齒狀回細胞增生的情形。結果顯示無論與群體禁水組或隔離正常供水組相比，隔離禁水組之大鼠其齒狀回中神經前驅細胞的增生有顯著的增加，顯示禁水且隔離對大鼠在海馬中齒狀回細胞的增生有正面的影響。為進一步分析大鼠的社會行為中肢體的接觸是否為造成群體組與隔離組大鼠齒狀回細胞增生數目差異的主要影響因子，在實驗二的部份，我們設計了特殊的飼養箱可單純阻斷大鼠相互之間肢體的接觸。故將動物分為群體組、隔離組、假隔離組(只阻斷肢體的接觸)三大組，而上述各組均再分為禁水組與正常供水組。實驗時程如同實驗一所描述，觀察各組動物齒狀回細胞的增生情形，並從升高十字迷津(elevated plus-maze)及開放空間行為測試(open field test)等實驗中，分析各組大鼠之焦慮狀況。同時以放射性免疫檢定法分析各組大鼠血清中皮質酮(corticosterone)的濃度。結果顯示隔離禁水組大鼠在海馬中齒狀回細胞增生的數目，明顯較群體禁水組及隔離正常供水組多，但與假隔離禁水組相比則無顯著的差異。而在升高十字迷津測試中也可發現隔離禁水組明顯較群體禁水組、隔離正常供水組及假隔離禁水組進入開放臂(open arm)的次數為多且停留的時間百分比較長。由上述結果可推論，隔離禁水組其焦慮程度明顯較其他組別為低，顯示隔離且禁水可降低大鼠焦慮的程度。此外，從血液內皮質酮的分析中，發現禁水組大鼠皮質酮濃度皆較正常供水組高，顯示禁水會提高皮質酮在大鼠血液中的濃度，但皮質酮濃度在禁水壓力下不同社會狀態相比則沒有差異。 綜合上述結果顯示，大鼠在面對短期禁水壓力時，社會隔離的狀態有助於其海馬中齒狀回的細胞增生數量增加，並減少在升高十字迷津中的焦慮行為。由此可知，社會隔離對生理性禁水壓力所引發的反應有正面的影響。而肢體接觸在社會行為中，對於大鼠社會行為的影響則可能因不同的測試而有不同的影響效果。此外，禁水壓力下社會狀態的不同並不影響大鼠血液中皮質酮濃度的變化。

Many studies have shown that long-term social isolation induces psychological stress in rats and exacerbates negative consequences of stress. But recent studies suggested that social isolation alleviates the anxiety responses to novel environments in rats. However, it remains unclear by what mechanism the social isolation underlies the above functions. The aim of this study is to reveal the effects of short-term physical stressor and social isolation on anxiety-like behavior and cell proliferation in male rats. The physical stressor we used was water deprivation that has been proven to increase corticosteroid in rats in previous studies. Our study was composed of two experiments. In the experiment one, Long-Evans rats were housed either individually or in groups of three, and housing conditions of each group were divided into two subgroups: water deprivation (WD) and water ad libitum (AL). At 14:00 h, animals were randomly divided into four groups as mentioned above, and water deprivation was conducted in the WD animals. At the same time, 5-bromo-2’-deoxyuridine (BrdU) was given at a dose of 100 mg/kg to rats by intraperitoneal injection. On the next day, the rats were given a BrdU injection at 12:00 h and perfused two hours later. The immunohistochemistry of BrdU was conducted to investigate cell proliferation in the granule cell layer (GCL) of the hippocampal dentate gyrus (DG). Our results showed that the isolation-WD rats significantly increased cell proliferation in GCL when compared to both group-WD and isolation-AL rats. In brief, social isolation combining with water deprivation has positive effect on the hippocampal cell proliferation. In the experiment two, we designed a special cage that rats were not able to contact physically with others. Thus, we can reveal the role of physical contact in social behavior. Rats were housed in three different conditions: group housing, isolation or pseudo-isolation. Each condition was divided into two subgroups: WD and AL, and the same schedule as experiment one was conducted. We investigated both cell proliferation in the GCL and anxiety-like behavior in the elevated plus-maze (EPM) and open field. We also collected the blood and analyzed the levels of corticosterone by using radioimmunoassay. Similar to the experiment one, our results showed that isolation-WD rats significantly increased cell proliferation in GCL when compared to group-WD and isolation-AL rats, but there were no differences between pseudo-isolation-WD and isolation-WD. Furthermore, isolation-WD rats showed higher percentage of open arm entries in the EPM test, indicating lowered anxiety-like levels. On the other hand, WD rats showed significantly higher corticosterone levels than AL rats, suggesting that WD caused an increase of corticosterone levels in rats. In summary, short-term WD under social isolation not only facilitates cell proliferation in the GCL but also decreases the anxiety-like behavior in the EPM test. Our results, indicate that when rats are passively under the uncontrollable stress, WD in this study, social isolation has a positive effect on responses induced by physiological stressor in rats. In addition, physical contact may play different roles in social interaction due to different tests. However, the changes of the corticosterone levels under WD do not correlate with the social conditions.