Live imaging of brain circuitry in mouse models of PTSD Post-traumatic stress disorder (PTSD) occurs after experiencing a life-threatening event and is predicted to affect many returning veterans, yet a major unmet medical need is for reliable methods of prediction, diagnosis and treatment. People with PTSD display alterations in brain functional anatomy and in brain structures, and many experience relief from selective serotonin reuptake inhibitors, linking anatomical changes in PTSD to the serotonergic system. Genetic studies show an association with polymorphisms in the serotonin transporter (SERT, 5-HTT) gene with risk of affective disorders including PTSD. Evidence that serotonin metabolism underlies stress-activated psychopathologies comes from three lines of research: 1) Rodent models, with genetic alterations of SERT or after fear provocation with predator odor; 2) Genetic dissection of human SERT, SLC6A4, and association of alleles with behavior; and 3) Magnetic resonance imaging (MRI), both functional and structural, of human subjects and mouse models. Our working hypothesis is that fear evokes neural activity that in some individuals continues long after the traumatic event resolves, leading to changes in functional anatomy of the mesolimbic cortical circuit. We further hypothesize that persistent activity occurs when SERT levels are low and/or in adults who experienced trauma early in life. Our specific aims are to: (1) Map and measure the intensity and duration of neural responses to fear in wild-type and SERT mutants; (2) Measure the impact of fear on functional circuitry; and (3) Quantify effects of early life trauma on neural activity and mesocortical limbic circuitry in the adult. We will measure the neural activity and circuitry changes before and after fear with time-lapse, quantitative manganese-enhanced MRI of living mice at 11.7T giving 90m3 voxel resolution, and the effects on anatomy with DTI and histology. Transgenic mice, including the validated mouse model of PTSD (SERT knock-out), and new transgenics in which 50kb of the mouse SERT locus has been replaced with human SERT alleles (long or short variants), will be imaged and compared with wild-type littermates. Predator odor will be used to provoke fear responses. To reveal whether SERT mutants affect a common neural network, cholecystokinin knock-out mice with a behavioral phenotype similar to SERT mutants will be imaged and analyzed in parallel. Datasets of 3D whole brain MR images from cohorts of each genotype will be analyzed statistically on a voxel-wise basis with custom-designed computational software. This study is the first to link genetics, functional and structural MRI with fear responses in this new transgenic mouse model carrying human gene alleles of the serotonin transporter. The successful completion of this proposed preclinical study will provide information necessary to address the unmet need by identifying: a) genetic susceptibilities for prediction of risk; and b) sequence of functional changes in the brain during progression to PTSD for design of stage-specific diagnoses and therapeutic interventions.