Aspects of learned fear related to the hippocampus are sleep-dependent.

Reduced sleep interferes with contextual but not cued learned fear, and it was suggested that this selectivity reflects underlying neural substrates. The apparent lack of contextual fear in sleep-deprived animals, however, could be secondary to hyperactivity. Also, changing the parameters of cued conditioning can change the neural pathways involved, such that some types of cued fear might be sensitive to sleep loss. To address these issues, we measured fear expressed with conditioned defecation as well as behavior, and used a trace cued learning paradigm. Using the platform-over-water method, male Sprague-Dawley rats were continuously sleep-deprived for 3 days, or for 20 h/day for 3 days. Animals then underwent fear conditioning, and were tested for learning the next day. Sleep-deprived or -restricted animals showed a lack of contextual fear at testing, as conditioned freezing and defecation were minimal. Sleep deprivation also blocked cued fear after trace conditioning. Therefore, reduced sleep impairs contextual learning, and impairs cued learning only when the hippocampus is involved. The data support a model in which sleep loss interferes with hippocampal function while sparing amygdala function.