In mice: Effect of testThe open-field behavior of two inbred strains of mice (BALBjc! and C57BLj6!) C57BL/6J mice (a pigmented strain) have higher activity and lower defecation scores than BALB/cJ mice (albino) when tested in the open field under conditions of moderately bright illumination (approximately 40 ft-c). In segregating F2, F3, and F4 generations, pigmented animals have also been found to have relatively higher activity and lower defecation scores than albinos, indicating that approximately 20% of the difference between the original inbred strains in both open-field activity and defecation was due to a single-gene difference at the albino locus (DeFries et al, 1966). These differences persisted when animals of an F5 generation were tested under white light, but largely disappeared when tested under red light; therefore, it was suggested that albino mice are more photophobic than pigmented animals.If differences in open-field behavior are due in part to a visually mediated fear reaction which is a function of illumination, then members of both inbred strains should manifest increased "emotionality" (higher defecation and lower activity) as level of test illumination is increased. However, if albinos are more photophobic than pigmented animals, members of the BALB/cJ strain should be more affected than those of the C57BL/6J strain by changes in test illumination, at least at the lower levels. In order to investigate these possibilities, the open-field behavior of these two inbred strains of mice was observed under five levels of illumination (0 to 480 ft-c) in approximately equal log steps.
MethodMembers of two inbred strains of mice (BALB/cJ and C57BL/6J) were mated within strains at 70± 10 days of age and the resulting inbred offspring were tested at 40± 2 days of age in a square (24 x 24 x 8 in.) open field. The floor and sides of the field were of white, painted Plexiglas. Two sets of five light sources were beamed through holes and red
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