Functional cooperation between the hippocampal subregions and the medial septum in unreinforced and reinforced spatial memory tasks

Abstract

Anatomical connections between the medial septum (MS) and hippocampus (Hipp) via the fimbria-fornix suggest that functional cooperation between these structures may be important for the acquisition and use of spatial reference memories. The present study examined the extent to which this was true for both an unreinforced learning task (object exploration task) and a reinforced learning task (Morris water maze task). In Experiment 1, we compared the performance of MS/Hipp contralateral- and MS/Hipp ipsilateral-lesioned rats. MS/Hipp contralateral-lesioned rats exhibited deficient performance in both the object exploration and Morris water maze tasks. In Experiment 2, we examined the task performance of MS/CA1 contralateral-, MS/CA1 ipsilateral-, MS/CA3 contralateral- and MS/CA3 ipsilateral-lesioned rats. Contralateral MS/CA3 and MS/CA1 lesions were respectively associated with deficient performance at the spatial recognition test and object recognition test in the object exploration task. None of the lesioned rats performed deficiently in the Morris water maze task. These results indicate the importance of spatial reference memory of a functional cooperation between the MS and Hipp as a whole, irrespective of reward contingency. In contrast, functional cooperation between the MS and each of CA1 and CA3 played an important role in the performance of the unreinforced voluntary task, but not in the reinforced task. Further, the functional cooperation of both MS/CA3 and MA/CA1 were important in the spatial reference memory with the unreinforced task.