Comparison of medial and lateral septal neuron activity during performance of spatial tasks in rats

The septal complex, having close and reciprocal connections with the hippocampus, is known to play an important role in learning and memory. Anatomically, the septal complex is divided into the medial and lateral areas (MS and LS). In the present study, in order to elucidate functional differences between the MS and LS, we recorded single unit activity in the MS or LS and electroencephalogram (EEG) in the hippocampus simultaneously while the rats performed the following 2 spatial tasks in an open-field chamber. In task 1, the rat received rewarding intracranial electrical stimulation (ICES) when it entered in a reward place that was set randomly in the open field in each trial. In task 2, the rat received rewarding ICES when it alternately visited two fixed reward places in the open field. Unit activity was analyzed in relation to the pattern of hippocampal EEG, and rat's location, locomotion direction and locomotion speed in the spatial tasks. A total of 47 neurons were recorded in the septal complex (MS, 19; LS, 28). The majority of neurons with activity correlated with hippocampal EEG were found in the MS (14/19). All of the neurons with place-related activity (an increase in unit activity when the rat was in a specific location in the open field) were found in the LS (n = 15). The majority of neurons with direction-related activity were found in the LS (18/23). Twenty-one neurons displayed speed-related activity (MS, 9; LS, 12). The present results indicate that (1) the MS is directly involved in the formation and control of hippocampal EEG patterns, and (2) the LS is important for the processing and integration of spatial information in the environment.