The use of sequential hippocampal-dependent and -non-dependent tasks to study the activation profile of the anterior cingulate cortex during recent and remote memory tests

Abstract

Recent findings suggest that as time passes, cortical networks become recruited for memory storage. In animal models, this has been studied by exposing rodents to one task, allowing them to form a memory representation for the task then waiting different periods of time to determine, either through brain imaging or region-specific inactivation, the location of the memory representation. A number of reports show that 30 days after a memory has been encoded, it comes to be stored in cortical areas such as the anterior cingulate cortex. The present study sought to determine what factors, in addition to the passage of time, would influence whether memory retrieval was associated with cortical activation. To this end, rats were assigned to one of three behavioural groups: (1) Training on one hippocampal-dependent memory task, the water maze (WM); (2) Training on two, different hippocampal-dependent memory tasks, the WM followed by the radial arm maze; (3) Training on one hippocampal-dependent memory task (WM) followed by training on one, non-hippocampal-dependent task, operant conditioning. After training, each group received a recent (2d) or remote (31d) water maze probe test. The group trained on two different hippocampal-dependent tasks and tested 2d later, showed the strongest preference for the platform location during the probe test. This group also displayed a pattern of c-Fos staining in the anterior cingulate cortex similar to the pattern of staining observed in the remotely-tested groups and different from that seen in the other recently-tested groups. These results suggest the formation of multiple hippocampal-dependent memories accelerate the speed at which cortical network recruitment is seen and leads to enhanced behavioural performance in the recent term.

Keywords: Anterior cingulate; Hippocampus; Multiple memories; Radial arm maze; Remote memory; Spatial memory; Water maze.