Evidence for Glucocorticoid Mediated Regulation of Substance P Expression in the Amygdala: A Possible Mechanistic Link Between Chronic Stress and Anxiety

Controlling the expression of the TAC1 gene in the amygdala is crucial to controlling fear, anxiety and may have consequences in depression. TAC1 codes for the protein Substance P; a small neuropeptide that is involved in modulating anxiety and depressive states. Mouse knockout models, lacking either the Substance P receptor (NK1) or the TAC1 gene that are both expressed in the amygdala, demonstrate reduced fear and anxiety responses and more aggression than wild type mice. Significantly, it has been shown that increasing SP levels in the amygdala increases the incidence of anxiety related behaviours. Moreover, stress increases the expression of TAC1 markedly within the amygdala compared to wild type animals. Despite an understanding of the role of Substance P in the amygdala, almost nothing is understood about the way this gene is controlled both in a tissue specific manner and in response to various stimuli, such as stress or pain. To address this we have identified a remote enhancer to the TAC1 gene, termed ECR1 that is able to drive expression of a LacZ reporter construct in transgenic mice, and is only active in the amygdala. ECR1 contains numerous evolutionarily conserved transcription factor binding sites. These sites show conservation over 300 million years of evolution. One of the most interesting and exciting putative binding sites found within the ECR1 enhancer is that of the glucocorticoid receptor (GR). GR has been implicated in stress responses of animals and appears to show increased levels of activity within the amygdala in animals that are stressed or have increased levels of the steroid hormone corticosteroid. It was therefore postulated that there might be a functional link between GR activity and TAC1 gene expression mediated through the ECR1 enhancer. Using a unique combination of bioinformatics, transgenics, organotypic culture and protein-DNA interaction techniques we have begun to establish a direct interaction of GR activity with the ECR1 enhancer. The linking of GR activity to the amygdala specific activity of the ECR1 enhancer is the first direct evidence of a mechanistic link between physiological stress and the amygdala specific expression of a known modulator of anxiety and depression. We believe that establishment and further study of this mechanistic linkage will accelerate our understanding of stress induced depression and will facilitate in the development of future anti-depressive therapies.