A quantitative genetic analysis of locomotor activity in CXB recombinant inbred mice

Recent studies have identified genes that influence the length of the circa dian period maintained by mice housed under constant lighting conditions. H owever, a less studied circadian activity variable is the amplitude of dail y oscillations in locomotor activity. This parameter reflects spontaneous a ctivity exhibited under standard lighting and housing conditions and, there fore, differs conceptually from assessments of exploratory or open-field ac tivity, voluntary wheel-running, or circadian period during exposure to con stant light or constant darkness conditions. We recently observed a greater daily amplitude of oscillation in spontaneous locomotor activity in C57BL/ 6 mice compared to BALB/cBy mice. To identify genetic loci with potential l inkage to circadian variation in the amount of locomotor activity, we measu red the spontaneous activity of 13 CXB recombinant inbred (RI) strains of m ice. The probability density distributions of locomotor activity phenotypes for the 13 CXB RI strains were consistent with the presence of a low numbe r of major quantitative trait loci affecting this trait. Regions of chromos omes 3, 8, 12, 13, and 19 showed provisional linkage to strain variation in locomotor activity. Probabilities of linkage were not sufficient for decla ration of an activity-related quantitative trait locus but were sufficient to warrant further analysis either with additional RI strains or with F-2 p anels.