Behavioral and biochemical changes monitored in two inbred strains of mice during exploration of an unfamiliar environment

Mice of the C57BL/6 (C57) and DBA/2 (DBA) strains were introduced individually in an unfamiliar environment (a large cage where food, water and sawdust had been removed). Over a 90-min period of observation, both strains presented a time-dependent decrease of locomotion and leaning and an increase of grooming. C57 mice were characterized by more cage cover climbing than DBA mice during the first 15-min stay in the new cage and by a significant decrease of this behavior after 90 min. During the first 60 min in the new environment, the DBA mice were less active than C57 mice, and both strains presented a significant increase of immobility after 90 min of test. After 30 min in the test situation, C57 presented a larger increase of plasma corticosterone levels than DBA mice. The plasma corticosterone levels were back to control values after 60 min of test in mice of the C57 strain and after 90 min in the DBA strain. Finally, both C57 and DBA mice presented a significant increase of homovanillic acid concentrations in the nucleus accumbens, but not in the striatum at 30, 60 and 90 min of testing. These results are discussed in terms of the possible involvement of mesolimbic dopaminergic system in mouse behavioral responses to an unfamiliar environment and of possible habituation to the stressful properties of this experience.