Lateral Septal Regulation of Anxiety

Abstract

The lateral septum is heavily implicated in anxiety regulation, with lesions or pharmacological inhibition of this region suppressing rats' defensive responses in various rat models of anxiety. My first objective was to explore the functional relationship between the lateral septum and its major afferent structure, the ventral hippocampus. Although these structures are extensively connected, it was not clear if they work in concert to regulate anxiety-like behaviours. This idea was tested using a pharmacological disconnection technique, whereby communication between these two structures was disabled by infusing the GABAA agonist muscimol into one side of the lateral septum and the contralateral side of the ventral hippocampus. Increases in open-arm exploration were evident when muscimol was co-infused into one side of the lateral septum and the contralateral ventral hippocampus. By contrast, open arm exploration was not altered when muscimol was co-infused into one side of the lateral septum and the ipsilateral ventral hippocampus. These results support the contention that the ventral hippocampus and the lateral septum regulate rats' open arm exploration in a serial fashion, and that this involves ipsilateral projections from the former to the latter site. My second objective was to further characterize the neuropharmacological aspects of lateral septal regulation of behavioural defence. The lateral septum contains high levels of NPY Y1 and Y2 receptor binding sites in the brain, yet little is known about their contribution in anxiety regulation at this site. Therefore, the second aim of my thesis was to characterize the contribution of NPY and its Y1 and Y2 receptor subtypes in the lateral septal regulation of anxiety in the elevated plus maze, novelty-induced suppression of feeding, and shock-probe burying tests. I determined that distinct NPY receptors differentially contribute to NPY-mediated anxiolysis in a test specific manner, with the Y1 receptor mediating NPY-induced anxiolysis in the novelty-induced suppression of feeding test, and the Y2 receptor mediating NPY13-36-induced anxiolysis in the plus-maze test. Taken together, the results from these studies reinforce the view that the regulation of anxiety involves a variety of different, yet overlapping neural processes.