Exposure to a marijuana-like substance in the womb makes young rats hyperactive and forgetful, researchers have found. The drug damages the hippocampus, a part of the brain important for learning and memory.
Some studies have suggested that pregnant women who smoke marijuana put their unborn children at risk. These kids tend to be smaller and restless, and they have a hard time organizing their thoughts. But although researchers have looked for memory problems, they haven't consistently found them.
To help clear the air, Vincenzo Cuomo, a pharmacologist at the University La Sapienza in Rome, and his colleagues injected pregnant rats with a synthetic form of the drug's psychoactive ingredient. The dose corresponded to moderate consumption of marijuana in humans.
The rats' progeny were agitated early in life, pacing their cages relentlessly. They also performed worse in a memory test: Compared with untreated rats, they were quick to enter a dark compartment where they'd received an electric shock 24 hours earlier. The memory impairment is due to damage in the hippocampus, the team reports online this week in the Proceedings of the National Academy of Sciences. Neurons in the hippocampus of drugged rats released less of the neurotransmitter glutamate, which disrupted a physiological process called long-term potentiation that is thought to be a critical mechanism for storing new information.
The findings suggest that researchers need to look more carefully for memory deficits in children exposed to marijuana prenatally, says Nancy Day, an epidemiologist at the University of Pittsburgh. Psychologist Peter Fried of Carleton University in Ottawa agrees, adding that the research fits with his lab's finding that kids exposed to marijuana have difficulty in spatial tasks that involve the hippocampus.