From the excerpt:
It is now becoming clear that addiction is associated with long-lasting changes in the electrical, morphological, and biochemical functions of neurons and synaptic connections within the medial forebrain pleasure circuit. There are strong suggestions that these changes underlie many of the terrifying aspects of addiction, including tolerance (needing successively larger doses to get high), craving, withdrawal, and relapse. Provocatively, such persistent changes appear to be nearly identical to experience- and learning-driven changes in neural circuitry that are used to store memories in other brain regions. In this way, memory, pleasure, and addiction are closely intertwined.