tion, and balance (Chiavegatto et al., 2000). Quantitative tests of motor abilities determined at the ages of 8 and 12 months revealed progressive gait disorders in complex-ganglioside knockout mice compared with controls, including reduced length and width of stride, increased hindpaw print length, and marked reduction in rearing. Compared with controls, null mutant mice tended to walk in small labored movements and performed poorly in many tasks that required coordinated movements (Chiavegatto et al., 2000).
Genetically altered animals may differ from wild-type animals in their emotional responses (fear, anxiety, and defensive reactions). Atypical emotional reactions interfere with responses in learning and memory tasks and with assessment of mating, parenting, or aggressive behaviors.
Several assays of anxiety-like behaviors have been developed. The most common are the so-called exploration-based tests (Holmes, 2001). The premise of these tests is that for some species such as rodents, the innate drive to explore a novel place will be inhibited as aversion to the new space increases. A simple version is the open-field test. High levels of exploration of the open, brightly illuminated area of an enclosure are interpreted as low-anxiety behavior (reviewed by Holmes, 2001). Highly anxious mice stay near the wall of the enclosure. Administration and scoring of this test have been automated, and several commercial products for performing the test are available. Defecation constitutes an additional measure of anxiety; high rates of bolus production are correlated with anxiety in wild-type rodents. Treatment with anxiolytics increases the time spent in the “open” portion of the open field and reduces the number of boluses produced (Holmes, 2001). Obviously, gene manipulations that affect metabolism or food intake could affect bolus production and confound the assay of anxiety. Other exploration-based tests of anxiety include the elevated plus maze, the light–dark exploration test, the emergence test, and the free-exploration test (Belzung and Griebel, 2001; Pare et al., 2001).
The elevated plus maze has become the most commonly used screen for novel anxiolytics, as well as a probe for anxiety in transgenic mice (Holmes, 2001). The elevated plus maze is shaped like a plus sign, has two open arms and two enclosed arms, and is usually raised at least 1 meter above the floor (Lister, 1987). The test animal is placed in the open center of the plus maze, and the number of entries into the closed arms is compared with the number of entries into the open arms over some period (commonly 5–15 minutes). High levels of anxiety correlate with more time spent in the enclosed arms.
The light–dark exploration test is based on rodents’ innate preference for small, dark, enclosed spaces over large, light, open spaces and their innate tendency to explore novel environments (Crawley, 1981). Spending more time in the light side of a box than in the dark side indicates low anxiety.