You've probably heard about science experiments in which animals are either rewarded with food (positive) or given a small shock (negative... obviously) simultaneously with some small stimulus like a bell sounding. Then of course, the test animals quickly associate that stimulus with reward or punishment and experience an associated positive or negative emotional response. Side-stepping the whole animal experimentation issue, here is an interesting article about some recent groundbreaking research into how those polar opposite emotional responses have turned out to be controlled by the same brain circuits.
Perhaps the most interesting result of this new research for us anxious folks is the fact that researchers were able to use optogenetics - using light pulses to control brain circuits - to block the fear center circuits and reduce fear-based response learning and vice versa with the reward center. According to the research team, this result could be an important milestone in the development of new ways to address anxiety, addiction, and depression, as they are often based on inappropriate emotional processing.