Chronic pain is associated with plasticity in the brain limbic system composed mainly of the amygdala, hippocampus, ventral striatum, and cingulate cortex (ACC) /medial prefrontal cortex (mPFC). These brain areas, especially the ventral striatum, receive dopaminergic input from the ventral-tegmental area (VTA). Although there is a significant literature now showing that limbic brain tracks chronic pain intensity and predicts the risk of transition from sub-acute to chronic pain, the role of dopaminergic input to the limbic brain and the change thereof which occurs in chronic pain, is still not clear. Given the role of dopamine in motivational control and the loss of motivation associated with chronic pain understanding how dopaminergic transmission is altered in the limbic brain of chronic pain patients is critical to the understanding of the pathophysiology of chronic pain. Therefore, the overall aim of this project is to use brain imaging to study how dopaminergic transmission through the oral administration of pro-dopaminergic medications carbidopa/levodopa (CD/LD) and methylphenidate will modulate the brain signature of chronic pain. Chronic pain subjects will be scanned at baseline (no drug administration) and three times after treatment with the two drugs or placebo. The protocol will follow a randomized double-blind approach.