A PET scan showing the areas of the brain that contain the mu opoid receptors.
Credit: Caryn Lerman, University of Pennsylvania
CARDIFF: A variation of a brain protein may make it harder for some individuals to quit smoking than others, according to new findings that may lead to the development of treatments that increase a quitter's chance of success.
The study, published in the current issue of Proceedings of the National Academy of Science, suggests that the difficulties some people experience when trying to quit may be due to the amount of 'mu opioid' receptors in his or her brain. A commonly occurring form of a gene for the receptor is tied to the severity of nicotine dependence, withdrawal, and relapse.
"For the first time we've identified a mechanism that explains why people with a particular genetic background may be more prone to relapse when they try to quit smoking," said senior author Caryn Lerman of the University of Pennsylvania in the U.S. "We've connected the dots between the genes, the brain, and the behaviour."
Connecting the dots
Although multiple mechanisms in the brain have already been implicated in addiction, a growing body of evidence points to the endogenous opioid system and the mu-opioid receptor (MOR) in particular. These act to mediate the reinforcing effects of addictive drugs, including nicotine.
The researchers found that a variation in an individual's genetic makeup can result in an increase in the amount of receptors that are associated with rewards and emotions.
Specifically, smokers who had two copies of the common 'wild-type' version of the mu opioid receptor gene had significantly more receptors available, compared to smokers who inherited just one variant of the mu opioid receptor gene.
"These smokers have a greater number of mu opioid receptors that respond to brain chemicals such as beta-endorphin which are released by nicotine. And having more available receptors of this type appears to be related to finding nicotine more rewarding," said Lerman.
Increased Levels and Increased Reward
Lerman and colleagues used a brain imaging technique, called positron emission tomography (PET), to measure the numbers of mu opioid brain receptors in the study participants.
When the participants reported the degree of satisfaction gained from either a normal cigarette or one that lacked nicotine, no difference was found between them. However, there was an association between greater reported reward and more receptor expression in the participants who carried at least one copy of the genetic variant.
"Although the precise mechanisms are not yet clear, the proposed mechanism is that nicotine increases levels of brain chemicals such as beta-endorphin that bind to mu opioid receptors in the brain. These influence levels of other brain chemicals such as dopamine that contribute to nicotine's rewarding effects" said Lerman.
In patients where more receptors are present, this can have a stronger effect making the addictive effect stronger and harder to give up.
hmm
it's important to note that genes express a predisposition to a given behavior, but not a predetermination. Genes can actually be activated and lie dormant according to environmental experience.
Quitting smoking may be down to our genes
After 40 years of smoking 2 packs a day I quit 3 years ago after about 20 intents, I just decided, and convinced myself, it had been enough. Feel better than I've ever felt before at 62. Just do it.