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The author conducted a study to examine the“ pure" working memory ability of Tsukuba High (H) and Low (L) emotional strain rats using an object recognition task in comparison of that of Wistar (W) strain rats similarly examined as a control. The study consisted of two sessions. In the first session, the rats were each subjected to a first trial for exposure to two identical objects (samples) for two minutes, followed (with a delayed time of five minutes) by a second trial in which they were each exposed to two different objects; one the same object as used in the first trial (familiar as the sample) and the other a novel object, for two minutes, to measure the time spent by each rat for spontaneous exploration of both objects in each of the first and second trials. 48 hours after the end of the first session, the second session was performed on the rats in the same manner as in the first session, except using different objects from those used in the first session. The results of the study can be summarized as follows: the H and W rats spent significantly longer time for exploration of the novel objects than the familiar objects, while the L rats showed no such tendency; and the habituation components (h1, h2) and mnemonic components (d2, d3) of the L rats were significantly lower than those of the H and W rats. From the above, it was concluded that the L rats were inferior to the H and W rats, not only in their spatial working memory ability, but also in their non-spatial visual working memory ability.

So far, the spatial learning ability of Tsukuba emotional strain rat was examined using the water maze which needed only reference memory and eight-arm radial maze that needed both working memory and reference memory. As a result, the performances of Tsukuba low emotional strain (L) rat were worse than those of Tsukuba high emotional strain (H) rat only in eight-arm radial maze task, and then the working memory disturbance of the L rat was suggested. The present study inserted a delay problem by eight-arm radial maze learning process, in order to verify more strictly the working memory ability in both H and L rat. Obtained results summarized as follows; although the number of the trials until insertion of the delay problem in L rat were less than those in H rat, the false choice rate in the flrst choice after the flve-minutes delay was higher, and the number of the trials until the learning criterion signiflcantly increased in L rat, and then the working memory disturbance of the L rat became clearer. And it was thought that the cause of the spatial learning disturbance of L rat might be a deflcit of the working memory capacity.

The present study examined the characteristics of the social cognitive behaviors, that is, various active or passive behaviors towards stranger rat (Long-Evans strain rat: L-E), in Tsukuba high(H) and low(L) emotional strain rat. As forms of active behavior, the approaching, sniffi ng, followup behavior and attack behavior towards L-E rats were observed for ten minutes in an adapted square open-fi eld device (80×80×30cm), in which an H or L rat was placed on a diagonal line with respect to an L-E rat. Observations were divided between the fi rst fi ve minute and the second fi ve minutes. Escape and defense behaviors as forms of passive behavior were also observed in the same way. As a result, H rats showed a variety of defense behaviors, as expected. However, a greater number of approaching and attack behaviors was recognized in H rats than in L rats. This tendency was particularly remarkable in H female rats. In contrast, L rats demonstrated many follow-up behaviors, but exhibited only little attack behavior. Therefore, it was suggested that the social cognitive behaviors of the Tsukuba emotional strain rat in a nonresident one to one situation were different from those seen in a resident or group colony.

The present study had a twofold purpose, to examine the relationship between the performance of the spatial learning and the morphological change of the hippocampus of Tsukuba emotional strain rats, and to ascertain how breeding environment condition would affect this relationship. The results of the experiments showed that the number of trials achieved during spatial learning of Tsukuba low emotional strain rats housed under enriched breeding conditions (the RL group) significantly decreased in comparison with that of Tsukuba low emotional strain rats under a standard breeding conditions (the SL group), demonstrating a clear effect of improvement on the spatial learning performance in terms of environmental enrichment. Further, the morphological studies of the hippocampus in the rats were conducted, with the outcome in which the volume ratio of the dorsal hippocampus for the whole hippocampus notably declined in the SL group. It was also revealed that the volume ratio of the dentate gyrus for the dorsal hippocampus in the SL group quite lowered comparing with that in the other group. Moreover, the findings displayed a negative correlation between the number of trials achieved in spatial learning and the volume ratio of the dentate gyrus in the L groups (both the SL and RL groups). Therefore, the neural network including the neurogenesis of the granule cell in the dentate gyrus was considered as one of the factors with reference to the disabilities of spatial learning seen in the SL group as well as the causes of the increase in the volume ratio in the dentate gyrus observed in the RL group.

本実験は,Tsukuba 情動系ラットの中で,特に,Tsukuba 低情動系(L系)ラットの概日リズムの特徴を明らかにするために,12:12時間明暗条件下で1ヶ月間の自発的輪回し行動を2時間毎に連続で観察した.その結果,Tsukuba 高情動系(H系)ラットは,統制群である雑種のWistar 系(W系)ラットの活動パターンとほぼ同様であったのに対して,L系ラットは,H系やW系ラットとは明らかに異なる活動パターンを示した.すなわち,L系ラットは実験開始から徐々に活動量が増加し,活動ピークに達するまでに期間が有意に延長し,最終的な総活動量はH系ラットより約4倍,W系ラットより約3倍に達していた.また,暗期での活動ピークが,W系やH系ラットでは,暗期の各時間帯に分布しているのに対して,L系ラットでは暗期直後の2時間(20時～22時)に活動ピークが集中する傾向が認められた.さらに,ほぼ毎日,暗期に移行する2時間前の明期(18時～20時)から活動が増加することがすべてのL系ラットで観察された.これらの結果から,L系ラットの多動性と同時に特異な概日リズムの証拠が得られた.今後は,これらのL系ラットの概日リズムの特異性を明らかにするために,脳内中枢と言われている視交叉上核の構造的変化と同時に,恒暗条件下での光刺激に対する概日リズムの位相反応などを検討する必要がある.