Researchers at the University of Chicago has discovered that rats most likely to self-administer addictive drugs had a particular receptor in the brain that is more responsive than the same receptor in rats least likely to self-administer addictive drugs.
This receptor, known as the nicotinic acetylcholine receptor (nAChR), increases excitability within in the brain's reward centres. In the animals that were more likely to take addictive drugs, the effects of these receptors were much stronger, leading to more profound excitation of the cells and pathways associated with reward.
"Stress, and the associated increases in stress hormones, will promote drug-taking behaviour regardless of whether an animal is more or less susceptible," said the researchers.
They showed that stress also increases the responses of nAChRs within the brain's reward areas.
"We tested the exploratory behaviour of rats in an unfamiliar cage. Rats that explore a new environment for a prolonged period of time were more interested in addictive drugs," said Daniel McGehee, PhD, associate professor and lead researcher on this study.
"Those rats also had stronger nAChR responses, meaning their brains responded differently to the drugs. We measured receptor activity in the brain's reward centres that are known to be activated by addictive drugs."
"This study provides valuable insight into the mechanism of addiction. It raises the possibility that nicotinic receptors may be important targets for the treatment of multiple addictions, not just nicotine. Unfortunately, blocking these receptors may also interfere with healthy behaviours that depend upon the same brain circuitry. Precisely where these findings will lead drug treatment strategies is unclear, but this work provides insight into the role of nicotinic receptors in the vulnerability to multiple classes of addictive drugs," said McGehee.
The study is published in the August 15, 2007 issue of the Journal of Neuroscience.