Rat strain differences in freezing and sleep alterations associated with contextual fear

Abstract

Study Objective: To examine rat strain differences in fear responses and subsequent alterations in sleep associated with contextual fear. Design: Recordings for each strain were obtained of nondisturbed baseline sleep and of sleep after exposure to a novel chamber (handling control). Afterward, the rats were subjected to shock training for 2 sessions (ST1, ST2) and to contextual fear (CF) alone. Percentage time spent in freezing (FT%) was observed during ST1, ST2 and CF exposures. Sleep was recorded for 20 hours (8-hour light and 12-hour dark period) following ST1, ST2 and CF. Setting: NA Subjects: The subjects were 2 inbred rat strains (Fischer 344 [F344] and Lewis [LEW] and one outbred rat strain (Wistar [WST]). Interventions: The rats were surgically implanted with electrodes for recording electroencephalogram and electromyogram for determining arousal state. Measurements and Results: Strain rankings for FT% were F344 = LEW > WST during ST2 and CF. LEW and WST rats exhibited decreased rapid eye movement sleep (REM) after shock training and CF compared with baseline and control; F344 rats did not. F344 and WST rats showed increased dark-period REM after ST1, ST2, and CF compared with baseline but not with control. Conclusions: Shock training and CF induce immediate reductions in REM in rats. However, strain differences in the amount of REM decrease did not simply relate to the strength of fear responses during shock training and CF. We speculate that reported strain differences in stress hormones, particularly prolactin, may contribute to strain differences in fear-induced alterations in REM.