Role of the dorsal and ventral hippocampus in spatial working memory load capacity.

The Hippocampus (HP) is an anatomic region of the medial temporal lobe involved in long-term spatial memory, which allows for orientation in a new environment using a cognitive map based on external spatial references. Otherwise, it is believed that the hippocampus has limited role in working memory (WM) which is the capacity to temporarily remember a limited amount of information. Nevertheless, recent evidence suggested that WM is affected in patients and monkeys with HP damage in conditions of high memory load. The working memory load is the number of elements maintained in memory for a short time interval. In humans the working memory load is limited to holding seven, plus or minus two digits, of information for a short-retention time interval. This limit is called working memory capacity (WMC). On the contrary, behavioral studies in rodents are still very controversial. First experiments reported that the dorsal HP is involved in spatial learning and memory, whereas the ventral HP mediates non spatial learning but later studies demonstrated that both dorsal and ventral HP are involved in the acquisition of spatial long-term memory in the Morris Water Maze task. The focus of this work is to study the role of the HP in spatial working memory load capacity in CD1 mice, through a brain lesion study. Since anatomic and connection studies suggested that the HP is not a completely homogenous structure along its septotemporal axis, we performed selective dorsal and the ventral HP lesions. Lesioned animals and sham-operated animals have been tested in a working memory capacity version of the 8-arm radial arm maze task, where we modulated the memory load by changing the number of open-baited arms. The results suggest that excitotoxic lesions in both hippocampal subregions cause spatial working memory capacity deficits during the first days of training. Nevertheless, lesioned animals eventually reach the performance of control animals during the last training days of training. This recovery of the performance was concomitant to the development of egocentric strategies, which are known to be HP-independent.. This data provide the first experimental evidence on a specific contribution of the HP in spatial working memory load capacity.