Why Marijuana Impairs Memory

Scientists may have just found out why marijuana impairs memory and why the brain's natural versions of the drug might help against epilepsy.

The active ingredient of marijuana, THC, is known to impair memory and to bind to areas of the brain linked to memory, such as the hippocampus. Still, the exact mechanisms by which marijuana impairs memory remain unclear.

Neuroscientists David Robbe and Gyorgy Buzsaki at Rutgers University and their colleagues recorded hippocampus activity in rats. Normally brain cells in this region often synchronize their electrical activity.

When the researchers injected rats with THC or a related synthetic drug, they found the normally synchronized workings of the hippocampus became disrupted. While the cells did not change how often they fired nerve impulses, their timing became erratic.

Imagine an orchestra where the musicians are deafened and perhaps blindfolded, Buzsaki said.

"They could still play their own pieces, but without any feedback from the other instruments played by other musicians or the conductor, depending on the nature and the length of the music played, it could be just a bit worse or outright disastrous, even though every note has been played. What is missing is the temporal coordination," Buzsaki explained. This is similar to what the researchers think goes on "in the hippocampal circuits under the influence of marijuana."

The neuroscientists put rats through a standard test of memory, where the animals had to learn to alternate which direction they went in a maze in order to get water. As rat hippocampal brain cell activity grew less synchronized under the influence of drugs, the rodents made more mistakes. This suggests synchronized brain cell activity is crucial for memory formation and hints that THC impairs memory by disrupting this synchronized activity.

The scientists suggest the way THC disrupts synchronized brain cell activity might help fight seizures. During seizures, brain activity becomes abnormally synchronized. Prior research suggested the brain's natural versions of THC, known as endocannabinoids, helped prevent seizures.

Whether or not marijuana is good against epilepsy remains unclear. Anecdotal reports going back centuries say it can help control seizures, but animal studies reveal it could also provoke fits.

The neuroscientists report their findings in the December issue of the journal Nature Neuroscience.