Marijuana--and its active ingredient, Δ9-tetrahydrocannabinol (THC)--has muddled memories for millennia. But how exactly the wacky weed interferes with remembrance of things past--as well as attention span and speech, among other things--has never been clear. Now neuroscientists have discovered that cannabinoids diminish the brain waves of rats--and disrupt the symphony of synchronous brain cell firing that may be essential for memory.

Neuroscientist David Robbe of Rutgers University and his colleagues tested the impact of THC and a synthetic cannabinoid on rats that had their heads restrained. The drugs affected certain brain waves: the theta (four to 12 hertz) and fast ripple (100 to 200 hertz) waves diminished significantly, whereas the drug had a slightly lesser impact on gamma (30 to 80 hertz) waves. Because theta and gamma oscillations are thought to play a critical role in creating and storing short-term memories--and fast ripple oscillations may allow such short-term memories to be moved into long-term storage--this suppression could mean missing memories for the rats.

In fact, rats that had been trained to follow a specific series of turns to get water--and did fine on the test before being intravenously injected with the drug--found themselves wandering in a daze under its influence. And when the researchers injected the synthetic cannabinoid directly into three rats' brains, it completely disrupted the otherwise synchronized pattern of the firing of their neurons: they fired as much as before, but in a more random pattern. And other types of brain cells, such as interneurons and pyramidal cells, fell out of step as well, although, interestingly, their overall activity actually increased (perhaps an explanation for the random nature of thoughts generated by use of the drug).

The finding suggests that this disruption of synchronized brain cell firing might be responsible for marijuana's memory distortions. "Overall, our findings indicate that under the influence of cannabinoids, neurons are liberated from population control," the researchers write in the paper presenting the finding published in the December issue of Nature Neuroscience. This, they argue, is the direct cause of memory impairment. But the research also reveals that at the highest doses of synthetic cannabinoid, the rats failed to discover the right sequence of turns altogether. In other words, there may be a threshold level of the drug that entirely prohibits learning, and that is something worth remembering very clearly.