passage 4.mp3 Passage 4 Mice and rats have long been used in medical research, but now scientists have trained rats to 1 ______ _____, which seemed to help the rats 2 ______. The research suggests that rat brains may be more 3 ______ and 4______ than once thought. The researchers examined levels of two hormones in rats—one that causes stress and another that 5 ______ it. They found that the rats that drove themselves showed higher levels of the 6 ______ hormone. The research may open new areas of non-drug treatments of people suffering from 7 ______ _____ condition.