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

Abstract

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.

Keywords: Medial entorhinal cortex; Memory; Rat; Spatial; Temporal; Trace fear conditioning.

Figure 1.. Extent of MEC lesions versus sham tissue

A. Photographs through the rat medial entorhinal cortex (MEC) at three sagittal levels (lateral to medial, centered around the target location of 4.6 mm lateral from the midline) for rats with sham or MEC lesions. The black arrows indicate the dorsal and ventral borders of the MEC. Scale bars below each sham tissue section indicate 1 mm.

Figure 2.. MEC lesioned rats show a delay-dependent impairment on the DMP task.

Swim path distances for Trial 2 across different delays between Trial 1 and Trial 2. MEC lesioned rats were impaired at finding the previously located platform when the delay between Trials 1 and 2 was long (6 hrs). MEC lesioned rats also failed to show superior memory at the shortest delay of 1 min between Trials 1 and 2, as shown by the SHAM rats. Error bars indicate SEM. Asterisks indicate difference from SHAM group (* p < 0.05).

Figure 3.. MEC lesions impair fear memory for temporally discontiguous tone, but not TFC context.

Mean percent freezing to the previously conditioned context (A) or to the associated tone (B) during 8-minute retention tests for 5 discontiguous tone-shock pairs. Although rats with MEC lesions showed intact fear memory when exposed to the previously conditioned context, they showed impaired fear memory when exposed to the associated tone in a new context. Error bars indicate SEM. Asterisks indicate difference from sham group (* p < 0.05).