2.4. Spatial learning and memory test with the morris water maze (MWM)

The Morris Water Maze was conducted in a black circular pool (180 cm diameter, 50 cm high), filled with water (21–23 ℃), mounted with four different visual cues at four directions. The water surface was divided into 4 quadrants by the two vertically crossed lines. The 4 quadrants were defined as southeast (SE), northeast (NE), northwest (NW) and southwest (SW) quadrants, receptively. A platform 10 cm in diameter was placed 2 cm below the water surface 45 cm away from the center of the circle pool in the NE quadrant. The latency to escape onto the hidden platform was recorded by a computerized video imaging analysis system (Morris Water maze Tracking System 100, Chengdu Taimeng software Co. Ltd.). The whole test consist two portions, the training time and the probe time. The day before the first training, each rat was allowed to swim freely for 120 sec and then taken back to its home-cage. During the training time, all rats received three trials per day for five consecutive days, and the starting quadrant was varied randomly over the trials. For each trial, the time of swimming and climbing the platform was counted after the rat was put into the pool. And then the rats were allowed to stay on the platform for 30 sec. Any rat that failed to reach the platform within 120 sec was guided to find the platform and allowed to stay on it for 3 sec. On the 6th day, after the last training for 24 hr, the probe test was conducted without the hidden platform in the pool. Each rat was placed in the water at the farthest point from the former location of the platform and was allowed to swim for 120 sec. The number of platform-crossing times was recorded as the target cross number.