University of Chicago researchers Daniel McGehee, Huibert Mansvelder, and Russel Keath revealed a novel effect of nicotine on the brain in a paper published in the March 14 issue of the journal Neuron, shedding light on another aspect of the cause of nicotine addiction. The results of the study show that nicotine blocks the brain's natural ability to inhibit the release of dopamine, a neurotransmitter that encourages certain behavior by rewarding the individual with a pleasurable sensation. This blockage can last for an hour, during which dopamine is continually released, prolonging the pleasurable sensation.
"We suspect that this ability to extend the reward only enhances the drug's ability to reinforce smoking," said McGehee, an assistant professor in the department of anesthesia and critical care, who examined the effects of nicotine on the brains of rats with post-doctoral researchers Keath and Mansvelder.
While it has previously been shown that nicotine initiates a prolonged release of dopamine that lasts several hours, the new finding helps to show how this occurs. Nicotine manipulates dopamine release by targeting what are called nicotinic acetylcholine receptors in a certain area of the brain, which in turn activate dopamine neurons by discharging a chemical called glutamate. What has puzzled researchers is that this release of dopamine continues for approximately one hour, long after the nicotine exposure has subsided.
The recent study finds an explanation for the prolonged dopamine release by pinpointing the effect of nicotine on a different kind of neuron that interferes with dopamine neurons by releasing GABA, a chemical that limits dopamine release. Nicotine blocks the ability of these receptors to release GABA. This disabling of GABA release lasts for approximately an hour.
The combined effects of nicotine in promoting the release of dopamine and suppressing the brain's ability to inhibit that release provide a clearer picture of how nicotine addiction takes root.
In addition to investigating why some individuals are more sensitive to nicotine exposure than others, McGehee plans to continue studying nicotine's effects on the brain, but he will focus on these effects as influenced by repeated nicotine usage, instead of by initial nicotine exposure. He hopes to observe the changes in brain signaling under the influence of repeated nicotine usage. "Once we know what those are, I think it will be easier to identify specific targets for drugs that will intervene," said McGehee.
According to the World Health Organization, tobacco use is responsible for the occurrence of more than 4 million deaths per year. The results of this study expose further information about how nicotine addiction works and pose a new angle from which nicotine addiction may be attacked. The new finding may advance the development of an effective medical antidote.
"I'm also very concerned with the fact that there is an apparent increase in smoking among college students, and the concern is when people start when they are relatively young, the harder it is to quit," said McGehee, further noting how low the success rate is for quitting.
"It's impossible to quite smoking unless you want to, completely," said Anna Sampson, a third-year in the College and a smoker of three years. "I've tried many times, but you just start thinking about how enjoyable the whole thing isand if you can't see the immediate effects on your health, you just go back to it."
According to the American Lung Association, 420 billion cigarettes were consumed in the year 2000 alone among adults 18 years and older. Each adult would have smoked roughly 2504 cigarettes.
"It's difficult to follow through with a plan [to stop smoking] because college life changes so much from week to week," said Matt Treskon, a fourth-year in the College and a smoker of three and a half years. "Someday I'll quit, I know that. I just hope I decide to."