Nicotine plays deadly role in infant death.

As a result of studies associating smoking with miscarriage and sudden infant death syndrome (SIDS), pregnant women are usually advised by their doctors to kick the habit (SN: 3/11/95, p.151). A new study adds weight to that advice and explains how smoking may lead to SIDS.

A group of North Carolina researchers found that rats exposed to nicotine as fetuses were born without the ability to adjust to periods of oxygen deprivation, resulting in a rodent syndrome resembling SIDS. That finding could result in pregnant women being advised to forgo the nicotine patch as well as smoking.

"Perhaps pregnant women should be advised to go cold turkey," says study leader Theodore A. Slotkin of Duke University Medical Center in Durham, N.C.

The researchers gave pregnant rats nicotine in dosages representative of either moderate smoking (the equivalent of 10 cigarettes per day) or heavy smoking (40 cigarettes per day). Control animals received only water. The researchers then exposed the newborn pups to low oxygen concentrations similar to what they would experience if they suffered from sleep apnea, the transient cessation of breathing during sleep. One-third of the pups exposed to nicotine before birth died, while all of the control pups survived.

As the team reports in the July Brain Research Bulletin, the nicotine-exposed pups that died failed to produce the stress hormones adrenaline and noradrenaline when faced with oxygen deprivation. Without this response, they couldn't maintain a normal heartbeat.

The results of prenatal exposure to nicotine continue after birth, says Slotkin. In humans, the nervous system develops throughout the first year of life. During that time, the adrenal glands, which produce the stress hormones, aren't fully integrated into the nervous system. In infancy, immature cells in those glands respond to low oxygen by producing a surge of stress hormones. Later, after nerve cells reach the adrenal glands and cause the cells to mature, the nervous system takes over control of hormone output.

Nicotine, by mimicking nervous system chemicals, forces the adrenal cells to mature prematurely, so they cannot secrete stress hormones without the go-ahead from the nervous system. The result, says Slotkin, is a child with "no defenses against low oxygen until nerves innervate his adrenal glands around his first birthday."

Slotkin's results may allow researchers to check adrenal function and identify infants likely to suffer from SIDS, says Marian Willinger of the National Institute of Child Health and Human Development in Bethesda, Md.