What happens in the brain when we experience addiction? Do some people just have more of an addictive personality than others? We talk with Bret S. Stetka, MD, Medscape's Neurology and Psychiatry editorial director, about different chemical messengers, neurotransmitter networks, our brains' complex reward systems, and the role anticipation plays in addiction.   WebMD is saddened to share that our colleague and guest in this episode, Bret S. Stetka, MD, Medscape editorial director for psychiatry and neurology, died shortly after this episode aired. In addition to his book, A History of the Human Brain, Bret was a frequent contributor to Scientific American and NPR. He will be missed.