Baseline heart rates of test/subject mice during different phases of the conditioning protocol.

Baseline heart rates of test/subject mice were calculated from a 90-s period that occurred immediately before the beginning of each habituation, conditioning/pre-exposure and testing period. Each of these periods began 20 min after a mouse was placed in the conditioning apparatus. There were no within-strain differences in baseline heart rate across different phases of the conditioning protocol. A previously described strain difference in baseline heart rate [42] was found during all phases of the experiments (P<0.01). Data are presented as the mean±s.e.m.