NEW YORK (Reuters) -- Researchers have discovered "striking" new biochemical evidence explaining why so many alcoholics are also heavy smokers.

Their report in the journal Neuroscience Letters says both alcohol and nicotine target the same cell sites in the nervous system, which may explain an underlying mechanism involved in both addictions.

According to Toshio Narahashi, professor of pharmacology at Northwestern University Medical School in Chicago, nicotine stimulates and then deadens or desensitizes the nicotinic acetylcholine receptor site on nerve cells in the brain.

"The acetylcholine receptor of the nicotine addict may be slightly desensitized, meaning that higher doses of alcohol are required to stimulate it," Narahashi said. He and his colleagues point to research showing that about 10% of the U.S. population are heavy smokers, but among alcoholics, 70% to 90% are heavy smokers.

The Chicago researchers say both nicotine and alcohol affect the same receptors at minute concentrations. Narahashi says the potent action of alcohol on acetylcholine receptors suggest that both substances interact at this site on nerve cells. "Our findings may also shed new light on the molecular mechanism of alcohol," he said.

Acetylcholine is a neurotransmitter, or chemical messenger, that affects a number of body systems, including the cardiovascular, gastrointestinal, urinary, and respiratory systems. Receptors are usually protein molecules on or within a cell that bind with neurotransmitters or hormones to stimulate a particular process in the cell.

Narahashi points out that one question still unanswered is how the effects on receptor sites in the early stages of alcohol intoxication differ from effects at later stages. He says alcohol at higher blood concentrations acts on several targets in the nervous system, but the new study suggests that the nicotinic acetylcholine receptors may be its prime targets, especially in the early stages of intoxication.