Study objectives: This study assessed the influence of contextual fear on sleep in inbred and hybrid mouse strains.
Design: Uninterrupted baseline recordings of sleep were obtained for 3 days in 3 mouse strains. After baseline sleep recording sessions, fear-conditioned mice were presented 15 shock presentations on 4 consecutive days. Control mice were subjected to the same procedures except that they never received shock. Sleep was examined after shock training and after exposure to the shock context alone.
Settings: N/A.
Participants: The subjects were mice of 2 inbred (C57BL/6J [B6], N=15; BALB/cJ [C], N=19) strains and 1 hybrid (CB6F1/J [CB6: C x B6], N=17) strain.
Interventions: Electroencephalograms (EEG) and activity were recorded by telemetry, and behavioral states were visually scored based on EEG and motor activity records.
Measurements and results: Shock training selectively reduced rapid eye movement (REM) sleep in mice compared to time-matched baseline recordings. The reduction of REM sleep was significantly greater in the reactive C and the F1 hybrid strain compared to the less reactive B6 strain. Posttraining exposure to the context alone reduced REM sleep in much the same manner as exposure to the footshock, with the CB6 hybrid mice exhibiting greater alterations in sleep and a greater reduction of REM sleep. In contrast, mice that did not receive shock training showed increased REM sleep after being returned to the chamber.
Conclusion: Aversive events and the fearful contexts that become associated with them can alter sleep in much the same way. Sleep disruptions after an aversive event or its reminders vary with genetic background.