Role of dorsal and ventral hippocampus in Working Memory load capacity

The hippocampus has been traditionally associated to spatial long-term memory (LTM). It is believed that the hippocampus has a limited role in working memory (WM). Nevertheless, recent evidence suggested that it is involved in WM in high memory load (HML) conditions. The WM load is the number of elements retained in memory for a short time interval. This number of elements is limited and it is called working memory capacity (WMC). The aim of this work is to study the role of the hippocampus in WMC in CD1 mice. Anatomic studies suggested, however, that the hippocampus is subdivided into distinct dorsal and ventral portions. To study the role of the dorsal and ventral hippocampus in WMC in CD1 mice we used a neurotoxic selective dorsal and ventral hippocampal lesion approach. We tested control and lesioned mice in a WMC version of the radial maze task using a confinement procedure to force the animals to rely on allocentric spatial information. Both lesioned groups showed impaired spatial WMC. Removal of the confinement procedure favored in control mice the use of a sequential egocentric strategy, which lowered the number of errors by lowering the memory load. Dorsal hippocampus lesioned mice shifted to the sequential strategy as well as control mice, and showed impaired performance only with the highest memory load. In contrast, the ventral lesioned group showed a major deficit in the acquisition of the sequential strategy, and a consequent impaired WM performance. Then, when these same mice have been tested in a WMC task for objects, only the dorsal group showed the impairment. Finally, we tested both control and lesioned mice in a massive protocol of the Morris water maze task, the classical hippocampus - dependent spatial LTM task and both lesioned groups were impaired. These data suggest that both the dorsal and the ventral hippocampus are involved in WMC, as well as in LTM, for spatial information. The ventral hippocampus is more involved in mediating the acquisition of egocentric strategies to solve a spatial task. In contrast, only the dorsal part regulates WMC for objects. Therefore, this study provides an important contribution to the role of the hippocampus subregions along its septo-temporal axis in WMC.