The Role of Protease-Activated Receptor-1 in Stress-Induced Neuronal Activity and Behavioural Responses

Serine proteases and their target molecules have been implicated in numerous aspects of CNS function, inducing various forms of learning, memory and emotional responses. This work sought to elucidate the role of the protease-activated receptor-1 (PAR-1) in stress-induced neuronal plasticity and behavioural responses. PAR-1 gene and protein are present in most regions of the mouse brain with high levels in the thalamus, the hippocampus and the amygdala. Due to its localization in limbic areas, PAR-1 regulation in the amygdala was examined following restraint stress or fear conditioning (FC). The finding that PAR-1 protein but not gene levels are downregulated led to the hypothesis that PAR-1 must be cleaved by stress-related proteases. Indeed my results indicate that a limbic-serine protease neuropsin cleaves the receptor in an indirect manner. This led us to investigate behavioural phenotypes of PAR-1־/־ and neuropsin־/־ mice during fear, anxiety, and learning-related tests. PAR-1־/־ and neuropsin־/־ mice are characterised by an anxiogenic and an anxiolytic behaviour, respectively, both profiles being related to amygdala dysfunction. Indeed amygdala-related neuronal activity following FC appeared elevated based on abnormally elevated c-Fos and P-CREB levels in fear conditioned PAR-1־/־ mice. This may underpin a higher susceptibility of these mice towards anxiety and suggest an inhibitory role of PAR-1־/־ during fear learning. Further electrophysiology data suggest that neuronal excitability and plasticity after FC are affected by PAR-1 through modulation of AMPA receptors. A role of the transmembrane AMPA receptor regulatoty protein Stargazin in this process is excluded as its gene levels are not affected by FC. Following fear learning PAR-1־/־ mice display differential phosphorylation levels ofGluR2 subunit. Furthermore dissimilar regulation of AMPAR splice variant between PAR-1־/־ and wild-type mice could account for PAR-I-related behaviour and electrophysiological characteristics. The results presented here reveal a previous uncharacterized inhibitory role of PAR-1 in the central nervous system