The formation of compensatory contextual fear memory in the absence of dorsal hippocampus does not change sleep architecture

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Although the dorsal hippocampus (DH) plays an essential role in the consolidation of contextual fear-conditioned (CxFC) memory, this consolidation may also occur in the absence of DH. It is, however, not known if the development of a compensatory circuit for CxFC memory is time-dependent. The DH-dependent contextual fear memory influences sleep architecture, but whether the compensatory fear memory can influence sleep, is not known. Here, we have studied (a) the temporal progression of compensatory contextual fear memory in the absence of DH and (b) the influence of compensatory contextual fear memory on sleep architecture. Rats were surgically prepared for chronic polysomnographic recordings and drug injections in the DH. They were divided into four groups: DH-non-lesioned and fear-conditioned, DH-non-lesioned and non-fear-conditioned, DH-lesioned and fear-conditioned and DH-lesioned and non-fear-conditioned groups. The DH was lesioned with ibotenic acid. The animals were conditioned to contextual fear twice: 1st training on Day 5 and testing on Day 6; 2nd training on Day 10 and testing on Day 11. The DH-lesioned and fear-conditioned animals did not exhibit freezing response during the first testing but showed a robust freezing response when re-trained after a gap of three days. In addition, wakefulness and NREM sleep amount did not change, but REM sleep significantly decreased in the DH-dependent CxFC memory group. Interestingly, REM sleep did not decrease in the DH-independent CxFC memory group. Our findings suggest that the development of compensatory CxFC memory is a time-dependent process and the compensatory CxFC memory may not influence sleep architecture.