Nicotine's addictive properties are closely tied to its ability to improve memory and learning, new research has shown.
The findings present an obstacle to using the tobacco chemical, or similar artificial drugs, to treat conditions such as Alzheimer's disease.
Previous research has shown that nicotine boosts memory and alertness, both of which are lost by Alzheimer's patients.
But nicotine is difficult to administer other than by smoking, and is highly addictive.
Although not as toxic as other tobacco chemicals, it may have some harmful side effects, especially during pregnancy.
Pharmaceutical companies are keen to develop safe nicotine-like substances for Alzheimer's and other forms of dementia.
The new research presented today at FENS 2008, the Forum of European Neuroscience meeting in Geneva, Switzerland, shows why this goal is so elusive.
Scientists led by Professor Ian Stolerman, from the Institute of Psychiatry at King's College London, studied the underlying mechanisms behind nicotine's effects on the brain.
They identified the role of nicotinic receptors - proteins that respond to nicotine - and several neuro-signalling chemicals in the brain, including dopamine, noradrenaline, glutamate and serotonin.
Dopamine is the "reward" chemical that triggers feelings of pleasure in response to certain stimuli such as food or sex, and is closely associated with addiction.
Prof Stolerman said: "We found several similarities and only small differences between the cognitive mechanisms and those involved in the addictive effects of nicotine.
"The cognitive 'boost' that many smokers experience from nicotine probably contributes to the reason people smoke cigarettes, so it may not be possible to totally prevent addiction. Nevertheless, the potential for abuse of a medicine based on a pure nicotine-like substance is likely to be very small."
The new findings may speed the discovery of agents that are better brain boosters than nicotine, with longer lasting effects, he said.
"This is a promising stage in the years of research that have endeavoured to separate the beneficial from the harmful effects of nicotine," Prof Stolerman added.
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "Nicotine has previously been shown to help treat Alzheimer's disease in animal studies. This new review brings together evidence to explain the processes behind this and which types of nerve cells nicotine affects in the brain.
"Although nicotine has therapeutic qualities, when it is absorbed through smoking the health risks outweigh the benefits. Smoking increases risk of vascular dementia, the second most common form of dementia and is associated with a number of other health risks.
"More research is now needed to find a safe and effective treatment for dementia, with the potential benefits of nicotine, but without the health risks."
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