Secondhand Smoke May Increase Risk for Nicotine Addiction

Megan Brooks

May 10, 2011

May 10, 2011 — Moderate secondhand smoke exposure results in occupancy of brain α4 β2 nicotinic acetylcholine receptors (nAChRs), which may increase vulnerability to smoking and nicotine addiction, new research shows.

Study investigators found that after 1 hour of exposure to secondhand smoke in an enclosed space, a substantial amount of nicotine reaches the brains of nonsmokers and binds to receptors that are normally targeted by direct exposure to tobacco smoke — a finding that has important implications.

"We know that secondhand smoke exposure results in exposed individuals being more likely to become smokers and to have a harder time quitting smoking [if they are already smokers]," first author Arthur L. Brody, MD, from the Greater Los Angeles Veterans Affairs Healthcare System and the University of California, Los Angeles, told Medscape Medical News.

"Our study presents further evidence for the need to limit exposure to secondhand smoke in vulnerable individuals," he added.

The study was published online May 2 in the Archives of General Psychiatry.

Priming the Brain

In laboratory rats, long-term exposure to cigarette smoke leads to nicotine dependence and an upregulation of nAChR levels in the brain. Yet, "brain nAChR occupancy from secondhand smoke exposure has not yet (to our knowledge) been demonstrated," the researchers write.

In the study, 11 moderately dependent cigarette smokers and 12 nonsmokers underwent positron emission tomography with the radioligand 2-(18)F-FA-85380, which permits visualization of brain α4 β2 nAChRs, one of the most abundant nAChR subtypes in the brain.

Each participant underwent 2 scanning sessions during which they sat in the passenger seat of a car for 1 hour and were exposed to moderate secondhand smoke or to no secondhand smoke.

During the smoke exposure condition, the mean air carbon monoxide level was 7.4 parts per million (ppm) — significantly higher (P < .001) than during the control condition (0.5 ppm).

For the entire group (smokers and nonsmokers), moderate secondhand smoke exposure led to an average 19% brain α4 β2 nAChR occupancy, the researchers say.

Evidence Supports Public Smoking Ban

"These results show that even limited secondhand smoke exposure delivers enough nicotine to the brain to alter its function," Nora D. Volkow, MD, director of the National Institute on Drug Abuse, who was not involved in the study, noted in a statement.

"Chronic or severe exposure could result in even higher brain nicotine levels, which may explain why secondhand smoke exposure increases vulnerability to nicotine addiction," Dr. Volkow added.

The researchers also found that smokers had a 23% increase, on average, in craving with secondhand smoke exposure, and they saw a correlation between nAChR occupancy and craving alleviation with subsequent cigarette smoking. This finding suggests that moderate secondhand smoke exposure delivers a priming dose of nicotine to the brain that contributes to continued cigarette use in smokers, they say.

"This study," Dr. Brody said, "gives concrete evidence to support policies that ban smoking in public places, particularly enclosed spaces and around children.''

This study was supported in part by grants from the Tobacco-Related Disease Research Program and the National Institute on Drug Abuse. The authors have disclosed no relevant financial relationships.

Arch Gen Psych. Published online May 2, 2011.

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