Children born to mothers who use marijuana during pregnancy may suffer a host of lasting mental defects, suggests a new study in rats. Marijuana is the most widely used illegal drug among women of reproductive age.

The offspring of pregnant rats given a low dose of cannabinoid were found to perform poorly in learning tests throughout their lives, compared to rats that were not exposed.

The Italian research team found that long-term learning in the rats was damaged by the cannabinoids irreversibly disrupting chemical and electrical processes in the brain during gestation. The exposed rats were also more hyperactive as infants, although this effect wore off as the rats reached adulthood.

Vincenzo Cuomo, at the La Sapienza University in Rome, and colleagues suggest that similar brain effects could explain learning problems in children born to mothers who use the soft drug during pregnancy.

"This is absolutely relevant," says Peter Fried, a psychologist at Carleton University in Ottawa, Canada, who has done similar work in humans. "What they have found is very consistent with what we have found in humans."

Confounding factors

The possible effects of maternal marijuana use on the unborn child and the child's later behaviour are controversial, say Cuomo and colleagues. They argue that rat studies can be very useful in assessing human effects, because studies of people can be hampered by complex confounding factors. These could include cigarette smoking, wealth or urban living.

In the study, pregnant rats were injected with a low dose of an artificial cannabinoid. Offspring exposed to the drug during gestation showed hyperactivity during infancy and adolescence, as measured by how many times they broke infrared beams crisscrossing their cages.

This stopped when they reached adulthood, but was replaced by problems with memory retention. The researchers showed reduced levels of a messenger chemical called glutamate in the hippocampus, part of the brain associated with learning and visual ability. They also found disruption of electrical processes associated with learning in this region.

Fried told New Scientist that as well as affecting memory and learning, exposure to marijuana during pregnancy has a strong effect on visual mapping and analysis in human children.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0537849100)