Reduced-volume cues effectively support fear conditioning despite sleep deprivation

Sleep deprivation impairs contextual but not cued learned fear, and it has been suggested that this pattern reflects an insensitivity of the amygdala to sleep loss. The lack of effect of sleep deprivation on cued conditioning, however, might simply be due to the strong attention drawn by the typically loud cue tone. We reduced tone volume from our standard 80 dB to either 70 or 60 dB, to test if reduced cue volume allowed effects of sleep deprivation to be detected. Using the platform-over-water method, male C57BL/6 mice were sleep-deprived for 24 h: control mice were moved to novel cages for 24 h. Mice then underwent fear conditioning with a standard "delay" protocol, and were tested for contextual and cued learning the next day. A control group received no footshock during conditioning. In the cue test, and for both cue volumes, SD had no effect on freezing to the tone, which was very robust in conditioned mice regardless of sleep treatment. As expected, freezing to the tone in the no-shock groups was essentially absent. Also, freezing prior to the tone was low in all mice. At the lowest volume, the tone was only similar to 10 dB above background noise. 24 h sleep deprivation, however, blocked contextual fear in the same mice. These results support a pattern of sleep deprivation sparing amygdaloid function but impairing hippocampal function. (C) 2008 Elsevier Inc. All rights reserved.