Figure 15.

Mice exhibit memory defects following acute infection with TMEV. Two weeks after infection, mice were tested in the Morris water maze using a competition test strategy to determine whether reference memory was disrupted. In this test, after 1 week of training and acquisition testing, the submerged training platform (small dark gray circles in C) is removed and a visible competition platform is placed in the tank in the opposite quadrant (light gray circles in C). If mice have developed a spatial map of the maze they ignore the visible platform and search in the training quadrant. In contrast, the absence of a spatial memory map results in direct escape to the visible platform. Sham-infected mice (n = 8) exhibited clear evidence of spatial memory formation, with significantly longer escape latency (A) and significantly more time spent in the empty training quadrant (B) than TMEV-infected mice (n = 8). As visual confirmation of this difference, swim paths are shown for three mice in each group (C). The sham-infected animals search the training quadrant before escaping via the visible platform. The TMEV-infected mice swim directly to the visible platform. Likewise, mice were tested at 7 dpi after infection in a scent-based novel object recognition test (D and E). In this test, mice are individually habituated to an environment consisting of a 30 cm × 30 cm acrylic box lined with wood shavings (changed between each mouse to eliminate spurious olfactory cues). After 5 minutes, the mice are briefly removed to a holding cage, and then reintroduced to the testing environment containing two identical scent objects. Testing of numerous objects revealed that apple-, berry-, and cherry-scented tea candles induced considerable interrogation but did not stimulate feeding behavior. During the 10-minute training session mice were exposed to 2 berry-, 2 apple-, or 2 cherry-scented candles (white circles in D), then removed to the holding cage for 5 minutes. The testing phase consisted of reintroducing the mice to the environment containing one candle from the training session and one new candle of a different scent (the black circle in D), randomly assigned. Mice were video taped and the number of interrogations of each object was determined manually during the training and testing sessions. The ratio of interrogations of the novel object to the number of interrogations of the familiar object was taken as a discrimination index. During training, all groups exhibited a discrimination index of 1 (dashed line in E). During testing, uninfected controls (n = 13 mice) exhibited an increase in the discrimination index consistent with interrogating the novel object two times more than the familiar object. In contrast, mice lesioned with kainic acid (n = 4 mice) or mice at 7 dpi (n = 12 mice) exhibited a discrimination index of 1, consistent with an inability to recall the familiar object.