Science 321, 1690–1692 (2008)

When a lab rat is learning that it will get a sugar treat shortly after a light flashes, the synapses within the dopamine circuitry in its brain become temporarily more efficient.

Antonello Bonci from the Ernest Gallo Clinic and Research Center at the University of California, San Francisco and his colleagues identified this cellular mechanism involved in predicting the availability of a reward. They used, among other methods, new techniques to measure rapid changes in neurotransmitter levels in vivo while the rats were actually being trained.

Learning to associate an environmental signal with a reward is critical for survival. Addictive drugs short-circuit this system. They similarly increase synaptic strength, but do so for very long periods.