This project pursues three major aims: 1) to characterize the neuroarchitecture and function of the dopaminergic substantia nigra pars compacta (SNc) and noradrenergic locus coeruleus (LC) projections onto periaqueductal gray (PAG) defense circuits; 2) to validate a cell-type and projection-specific viral model of synucleinopathy within SNc/LC/PAG circuits, and to understand its effects on motor- and anxiety-related behavior; 3) to probe the effects of acute and chronic optogenetic subthalamic nucleus stimulation on the characterized function of SNc/LC/PAG circuitry.