Narrator: This is Science Today. What makes an addict an addict? Howard Fields of the University of California, San Francisco is working to find out by studying molecules in the brain called opioids, which are responsible for feelings of pleasure. According to Fields, the answer lies in the interaction between two opposing parts of the brain’s pleasure system: the kappa and the mu opioid systems.

Fields: Endogenous opioids act at several different receptors. Morphine and heroine, they act at the mu receptor. But there’s another receptor called the kappa opioid receptor, and it has effects that are opposite to the mu receptor.

Narrator: Dynorphin, one of the brains natural opioids, acts at the kappa receptors where it is responsible for counteracting the pleasurable effects caused by addictive drugs at the mu receptors. One theory is addicts don’t have the same dynorphin response as non-addicts.

Fields: In most people, maybe there’s more dynorphin than endorphin, so there’s more of an action at the kappa receptor than there is at the mu receptor.

Narrator: For Science Today, I’m Larissa Branin.