Effects of intra-hippocampal corticosterone and sleep on consolidation of memories of aversive experience in rats

Abstract

Formation and consolidation of memories for highly stressful (traumatic) events is a complex process that involves interplay between multiple memory systems and has implications for etiology and treatment of stress- and trauma-related disorders. Here we study effects of sleep/wake states and high intra-hippocampal corticosterone on consolidation of aversive contextual memories, as well as consolidation of association between auditory unpaired phasic background cues and fear response in rats. Animals were implanted with EEG and EMG electrodes for sleep assessment and cannulas for intra-hippocampal corticosterone application. They were familiarized to a "safe box" and then trained in a fear conditioning paradigm in a distinct "shock box" with a prominent unpaired phasic background auditory cue. Immediately after conditioning, animals received bilateral intra-hippocampal saline (1 μl) or corticosterone (10 ng in 1 μl) injection and were either allowed to sleep or were kept awake for a following two-hour consolidation period. Memory tests 24 h later revealed that the saline-injected animals that slept during consolidation had significantly stronger fear responses in the shock box compared to the safe box as well as increased fear response in response to the auditory cue. Lack of sleep during the consolidation period in saline injected animals led to generalization of the fear response to the safe context, while association between auditory cue and fear response was preserved. High intra-hippocampal corticosterone levels during memory consolidation led to generalization of fear response to the safe context, regardless of sleep/wake state, while enhancement of response to auditory cue was not observed. Our results show how manipulation of conditions during consolidation can lead to greatly variable memories for an aversive episode with distinct behavioral outcomes. Observed overgeneralization of fear to safe context and altered fear response to background phasic cue has implications for understanding etiology of pathological memory alternations in stress-related conditions e.g., in posttraumatic stress disorder in humans.

Keywords: Corticosterone; Fear conditioning; Hippocampus; Memory consolidation; Sleep.