The substances have already been used to repair damaged memory ability in rats.

However, anti-smoking campaign group ASH (Action on Smoking and Health) has warned that any benefits nicotine may offer are far outweighed by the potential harm of smoking.

And a leading doctor said people must not link the benefits of nicotine directly with smoking cigarettes.

The scientists also warned that no-one should light up to prevent the onset of Alzheimer's.

Brain lesions

Alzheimer's is a degenerative disease of the brain that destroys memory and eventually all other mental functions.

While smoking is thought to double the risk of the disease for most people, in some cases it is thought to prevent the onset of the symptoms.

It was this finding that led scientists to look at the relationship between nicotine - the addictive chemical in cigarettes - and dementia.

Dr Edward Levin, of Duke University in North Carolina, presented the findings at the Society of Neuroscience annual meeting in Los Angeles in the US.

He said his team gave AR-R 17779 - a compound similar to nicotine - to rats with brain lesions.

The brain lesions are similar to those in humans with Alzheimer's disease.

Those who had the drug performed significantly better in learning and memory tests.

AR-R 17779 is one of several nicotine-like compounds currently in development.

The nicotine is thought to combat the effects of Alzheimer's by attaching to receptors for acetylcholine, a chemical important in learning and memory.

Nicotine, or compounds that can trick the brain into thinking they are nicotine, can activate these receptors, which enhances the brain's ability to learn and remember.

Dr Levin is also carrying out trials on human patients with Alzheimer's disease, using nicotine skin patches.

Smoking harm

A spokeswoman for ASH said it was a useful finding but warned that the study did not justify smoking.

Smoking offers nicotine with harmful pollutants

"We've known for some time that nicotine could be an aid to prevent Alzheimer's and other related mental disorders - some people have suggested that this is a beneficial effect of smoking," she said.

"We would caution against that because the research is showing that it's the nicotine itself, not smoking.

"But if nicotine can be extracted and used in a beneficial way to alleviate Alzheimer's and other diseases, then that's a very good step forward."

She said that it was understandable that people, especially the elderly, might want to continue smoking to guard against Alzheimer's.

"It's a very unpleasant disorder and people are genuinely very frightened of getting it," she said.

"But don't use that as a reason not to give up - it's always worth giving up, no matter what your age."

Positive effects

Dr Godfrey Fowler, Emeritus Professor of General Practice at Oxford University, said the findings were "certainly plausible" but added that it was a far cry from finding something in rats to it working in humans.

He said that long-term studies had shown nicotine to be beneficial in the treatment of Alzheimer's disease and ulcerative colitis, a bowel disease.

But he added: "Using nicotine as nicotine patches or nicotine gum is a very different matter from cigarette smoking.

"It is all the other things in cigarette smoke that are harmful, not the nicotine."

Dr Levin said: "I wouldn't want people to buy a pack of cigarettes or a patch or (nicotine) chewing gum."

However, he said he was encouraged that medications to capitalise on the benefits of nicotine without cardiovascular and other side-effects, can be developed.

"These are exciting avenues for drug development, but we're not there yet," Levin said.

Professor Esther Sabban, a biochemistry and molecular biology professor at New York Medical College in Valhalla, New York is also carrying out research into the effects of nicotine.

She said: "The relationship between lung cancer and smoking is clear and it's not the way you want to prevent the onset of Alzheimer's disease."