Abstract
Motivation and the pursuit of reward involves the contribution of many brain regions including the hippocampus and ventral tegmental area (VTA). The lateral septum has been identified as a relay between the hippocampus and VTA, however, its exact influence on motivational and goal-directed behaviours is unclear. This experiment aimed to expand on the understanding of the lateral septum during these behaviours, focusing on lateral septal activity during a navigational reward-approach task. Specifically, the study aimed to determine whether there was anticipatory ramping pattern of the lateral septum activity as an animal approached a reward and whether hippocampal activity was related to this.
Rats were trained to run clockwise and anticlockwise circuits around a track to obtain a food reward at set locations. The two directions and two reward locations were used to help separate goal/approach effects and spatial/location effects. Local field potential (LFP) recordings were made from the CA1 layer of the hippocampus and from the lateral septum. Simultaneously, single-unit recordings were made from tetrodes positioned in the lateral septum. It was hypothesised that LFP activity from the hippocampus and the lateral septum would display coherence. Secondly, it was hypothesised that the hippocampal LFP signal would predominately lead the lateral septal signal, and that this lead will become stronger as the animal approaches the reward location, demonstrating the increasing hippocampal influence during this period. Lastly, it was hypothesised that identified lateral septal cells would increase their cell firing rate, in an anticipatory ramping pattern, as the animal moves toward the reward, potentially linking hippocampal spatial activity to motivational systems such as those in the VTA.
Data revealed coherence between the two LFP signals and a strong hippocampal lead was prominent in the LFP during the initial part of each circuit. Contrary to the hypothesis, however, the lateral septal lead increased as the animal approached the reward location. No evidence of a ramping pattern was found in the firing rate of lateral septal cells, with a more complex and non-linear pattern. Further analyses revealed a population level effect of differential cell firing between the first half of the track and the second half. Taken together, the results suggest that the activity of the lateral septum may be related to goal-directed behaviour, and that this activity is strongly influenced by the hippocampus. It is suggested that lateral septal activity may be used as a value signal to convey relevant information to connecting regions such as the hippocampus and VTA.