Time-limited involvement of dorsal hippocampus in unimodal discriminative contextual conditioning

Converging evidence examining the effects of post-training manipulations of the hippocampus suggests that the hippocampus may play a time-limited role in the maintenance of a variety of forms of memory. In particular, either lesions or inactivation of the dorsal hippocampus results in many cases in a time-limited retrograde impairment in nondiscriminative contextual conditioning paradigms. However, the extent to which hippocampal manipulations result in a time-limited retrograde amnesia for a variety of forms of learning has recently been called into question (reviewed in Sutherland, Sparks, & Lehmann (2010)). The present study examined the effect of inactivation of the dorsal hippocampus either 7, 28, or 42 days following training in an explicitly nonspatial, discriminative contextual conditioning paradigm (Otto and Poon, 2006 and Parsons and Otto, 2008). Inactivation of the dorsal hippocampus resulted in a significant deficit in the expression of contextual conditioning at 7 and 28 days, but not 42 days, following training. Importantly, inactivation of the hippocampus did not affect either baseline freezing levels or conditioning to an explicit CS. Together with previous data exploring hippocampal contributions to discriminative unimodal contextual conditioning, these data suggest that the hippocampus may play a particularly prominent role in the temporary maintenance of memory in discriminative contextual paradigms.

Research highlights
► The role of the hippocampus in the temporary storage or recall of previously-acquired memories is currently the source of considerable debate.
► The present study examined the effect of dorsal hippocampal inactivation either 7, 28, or 42 days following acquisition of an olfactory discriminative contextual fear conditioning paradigm.
► A retrograde gradient was observed such that inactivation 7 or 28, but not 42 days following conditioning resulted in a robust impairment in memory expression.
► No effect of hippocampal inactivation was observed on conditioning to an explicit auditory CS or on several baseline measures of fear.
► These data suggest that the hippocampus plays a prominent but temporary role in the maintenance of discriminative contextual information prior to its ultimate consolidation in extra-hippocampal areas.