BRAIN CELLS SENSE MORNING CIGARETTE

New York (Reuters) Most smokers are especially fond of the day's first
cigarette. This week in Nature, researchers at Baylor College of Medicine
in Houston suggest the reason lies in the way brain cells respond to the
first nicotine rush of the day.

The study authors examined the brains of nicotinefed rats in order to more
fully understand the chemical mechanisms behind smoking's addictive
pleasures. They believe that nicotine finds its way to a portion of the
brain known as the ventral tegmental area (VTA). Nerve cells in the VTA are
especially receptive to dopamine, a chemical closely linked to the body's
habitforming 'reinforcementreward' response. This response 'rewards'
certain human behaviors with pleasurable sensations.

The researchers believe that during the day's first smoke, an "initial
application of nicotine can increase the activity of the dopamine neurons"
in the brain, thus enhancing "the rewarding aspects of tobacco use."

However, as smoking continues throughout the day, VTA nervecell receptors
specifically 'tuned in' to nicotine become increasingly desensitized due to
their prolonged exposure to nicotine. The Baylor experts say this explains
why the second cigarette of the day "does not elicit the same effects" as
the first.

And since nicotine has a 'halflife' in the body of about two hours, they
believe smokers "maintain a rather steady lowlevel background of nicotine
throughout the day."

A night's sleep, however, temporarily shuts down the smoking habit,
allowing the VTA nervecell receptors to regain their former sensitivity.

The result? Smokers get more pleasure from the morning cigarette, the first
of the day.

But this feeling carries health risks. Tobacco addiction, the Baylor
authors point out, is now "estimated to be the single largest cause of
premature death" in the developed world. SOURCE: Nature (1997;390:401404)

Copyright © 1997 Reuters Limited.