Recent and remote retrograde memory deficit in rats with medial entorhinal cortex lesions.

The hippocampus is critically involved in the acquisition and retrieval of spatial memories. Even though some memories become independent of the hippocampus over time, expression of spatial memories have consistently been found to permanently depend on the hippocampus. Recent studies have focused on the adjacent medial entorhinal cortex (MEC), as it provides major projections to the hippocampus. These studies have shown that lesions of the MEC disrupt spatial processing in the hippocampus and impair spatial memory acquisition on the watermaze task. MEC lesions acquired after learning the watermaze task also disrupt recently acquired spatial memories. However, the effect of MEC lesions on remotely acquired memories is unknown. The current study examined the effect of MEC lesions on recent and remote memory retrieval using three hippocampus-dependent tasks: the watermaze, trace fear conditioning, and novel object recognition. MEC lesions caused impaired retrieval of recently and remotely acquired memory for the watermaze. Rats with MEC lesions also showed impaired fear memory when exposed to the previously conditioned context or the associated tone, and this reduction was seen both when the lesion occurred soon after trace fear condition and when it occurred a month after conditioning. In contrast, MEC lesions did not disrupt novel object recognition. These findings indicate that even with an intact hippocampus, rats with MEC lesions cannot retrieve recent or remote spatial memories. In addition, the involvement of the MEC in memory extends beyond is role in navigation and place memory.

A role for medial entorhinal cortex in spatial and nonspatial forms of memory in rats.

Many studies have focused on the role of the medial entorhinal cortex (MEC) in spatial memory and spatial processing. However, more recently, studies have suggested that the functions of the MEC may extend beyond the spatial domain and into the temporal aspects of memory processing. The current study examined the effect of MEC lesions on spatial and nonspatial tasks that require rats to learn and remember information about location or stimulus-stimulus associations across short temporal gaps. MEC- and sham-lesioned male rats were tested on a watermaze delayed match to position (DMP) task and trace fear conditioning (TFC). Rats with MEC lesions were impaired at remembering the platform location after both the shortest (1 min) and the longest (6 h) delays on the DMP task, never performing as precisely as sham rats under the easiest condition and performing poorly at the longest delay. On the TFC task, although MEC-lesioned rats were not impaired at remembering the conditioning context, they showed reduced freezing in response to the previously associated tone. These findings suggest that the MEC plays a role in bridging temporal delays during learning and memory that extend beyond its established role in spatial memory processing.