Reduced gamma range activity at REM sleep onset and termination in fear-conditioned Wistar–Kyoto rats

Recent investigations of rapid eye movement sleep (REMS) continuity have emphasized the importance of transitions both into and out of REMS. We have previously reported that, compared to Wistar rats (WIS), Wistar–Kyoto rats (WKY) responded to fear conditioning (FC) with more fragmented REMS. Gamma oscillations in the electroencephalogram (EEG) are synchronized throughout the brain in periods of focused attention, and such synchronization of cell assemblies in the brain may represent a temporal binding mechanism. Therefore, we examined the effects of FC on EEG gamma range activity (30–50 Hz) at REMS transitions in WKY compared to WIS. Relative power in the gamma range (measured as a percent of total power) at Baseline and upon re-exposure to the fear-inducing conditioning stimulus was measured 35 s before REMS onset to 105 s after REMS onset (ARO) and 85 s before REMS termination (BRT) to 35 s after REMS termination. After baseline recording, rats received 10 tones, each co-terminating with an electric foot shock. On Days 1 and 14 post-conditioning, rats were re-exposed to three tones. Fast-Fourier transforms created power spectral data in the gamma frequency domain. Relative power was extracted from an average of 4–5 REMS transitions. Relative gamma power was always higher in WIS. On Day 14, at 15 s and 25 s ARO, WKY had significant increases in relative gamma power from Baseline. WIS had a significant increase on Day 1 at 25 s ARO. Despite the increases in relative gamma power, WKY never achieved levels attained by WIS. Moreover, at 5 s BRT, only WKY had a significant decrease in relative gamma power from Baseline to Day 14. Gamma range activity may indicate neural activity underlying maintenance of REMS continuity. Low relative gamma power at REMS transitions may be associated with increased REMS fragmentation in WKY after FC.

Research highlights
► Wistar–Kyoto rats (WKY) are stress-sensitive, measured here by REMS fragmentation.
► Relative gamma power may indicate changes in neural activity underlying REMS.
► Fear conditioning increased gamma power at REMS entry in Wistar rats, but not WKY.
► Fear conditioning decreased gamma power at REMS end in WKY, but not Wistar rats.
► WKY never reached the level of gamma power of Wistar rats.
► Low relative gamma power may be associated with REMS fragmentation.