An exploratory factor analysis of the Tail Suspension Test in 12 inbred strains of mice and an F2 intercross.

To explore the genetic dimensions of the stress response in rodents, we tested 12 inbred strains of mice and an F2 intercross (n=745) on the Tail Suspension Test (TST) and the Tail Suspension-Induced Hyperthermia (TSIH) paradigm. These selected 12 strains provide a representative sampling of the genetic heterogeneity of mousedom. An F2 intercross was derived from NMRI and 129S6 strains, which differ in their responses on the TST. Both inbred strains and F2 mice underwent a standardized protocol of automated TST with two sessions: (1) baseline and (2) imipramine TST. The duration of immobility and the body temperature after TST were recorded. The inbred strains were also tested in the Light-Dark Transition (LDT) test and in the Open Field Test (OFT), measuring the distance traveled, vertical movements, and center time as independent variables. The F2 mice were measured for core temperature after TST (TSIH). High intercorrelations among strain means were found for the LDT and OFT measures. Principal components analysis extracted four factors: "exploratory fear," body weight, imipramine response on immobility, and "stress reactivity." These dimensions were largely confirmed in the F2 population with one additional factor: imipramine response on TSIH. The results support a distinction between "stress reactivity" as measured by the TST and "exploratory fear" behavior as measured by the LDT and OFT.