Abstract
The periaqueductal grey (PAG) is classically seen as a region responsible for the control of defensive reactions. This thesis provides a detailed review of anatomical and functional data on the different parts of the PAG together with the dorsal raphe. Based on anatomical features, this thesis proposes a new subdivision of the PAG that accounts for the distinct characteristics of the area. I provide a comprehensive functional view of the PAG, going beyond simple panic and escape to integrate data on fear, anxiety, and depression. Importantly, I conclude that this periaqueductal cluster of nuclei is broadly involved in motivated behavior controlling not only aversive but also appetitive behavior and with some involvement in more complex motivational processes such as approach-avoidance conflict resolution. To test some of these hypotheses, I present experimental evidence showing that the PAG can produce an anxiety-like signal in the forebrain, and compare it to the signal evoked by reticular stimulant. Furthermore, I explore the effect of pharmacological interventions in tegmental areas and how these impact PAG- and RPO-evoked responses. In sum, I propose that the PAG is a highly conserved region surrounding the aqueduct and it appears to be the simplest, foundational, element of controlling integrated motivated goal-directed behaviors of all types.