The new research, in rats, has pinpointed a mechanism in the brain that could explain why smoking marijuana during pregnancy can lead to cognitive and memory problems in offspring.
"Exposure to marijuana can produce long-lasting, subtle changes in the memory of rat offspring," says study author Vincenzo Cuomo, a professor of pharmacology at the University La Sapienza Roma in Italy. "Our study is in line with clinical findings in humans."
The study appears in this week's issue of the Proceedings of the National Academy of Sciences.
Marijuana is the most widely used illicit drug among women of reproductive age, according to the study. But research on the effect of marijuana on babies is sparse, Cuomo says. Some limited research has shown exposure to marijuana in utero can lead to long-term problems with memory and learning, even when the babies appear normal at birth.
To get at the mechanism underlying it, researchers injected pregnant rats with a compound called WIN, a synthetic cannabinoid. Cannabinoids are the psychoactive ingredient of marijuana responsible for the "high."
In research, WIN is used rather than marijuana because the cannabinoid doses can be carefully controlled, whereas the level of cannabinoids in marijuana plants can vary significantly, Cuomo says.
The pregnant rats were injected with a dose of WIN that corresponded to low to moderate use of marijuana by a human smoker.
Researchers found the offspring of the rats injected with WIN were more hyperactive than a control group of baby rats. The hyperactivity disappeared as the rats reached adulthood.
But the WIN offspring continued to score lower on learning and memory tests throughout their lives.
Researchers found that WIN disrupted the release of glutamate, a neurotransmitter found in the hippocampus region of the brain that has been shown to be instrumental in learning and memory. WIN also interfered with a process called long-term potentiation, an electrical activity in the brain associated with learning.
"This could be the mechanism for the cognitive impairment," Cuomo says.
While the current research was on rats, it could also explain marijuana's effect on the brains of human babies, he says.
Previous research has linked daily marijuana smoking by pregnant women to hyperactivity, inattentiveness and impulsivity in their children.