How Marijuana Disrupts Memory Formation In Rats

The reason why your stoner friends can't remember all too well begins to become clear by looking at the effects that marijuana compound tetrahydrocannabinoid (THC) has on the brains of rats.

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.

The stoners ought to try to remember the details of this research to think about it next time they take a toke.

The THC caused hippocampus nerve signal firings to fall out of sync and to fire less powerfully. The rats had been trained to alternate their routes through a maze and the rats on THC did a far worse job of remembering which route to take next based on which route they took previously.

Normal rats accurately alternate their routes about 90% of the time. But rats given THC, which caused asynchronous nerve firing, chose a random direction on each run, and so chose the correct route 50% of the time. The disruptive effect of THC wore off within a few hours. Robbe says he hopes to find out whether chronic exposure to the drug causes lasting effects on the hippocampus in rats. Scientists studying people have found that long-term marijuana users gradually become worse at learning and remembering things (see Pot-smoking your way to memory loss).

Neurons that spend a lot of time firing in some different way in response to a drug probably reconfigure somehow in response to the different pattern of firing. Brains strengthen and weaken connections in response to stimuli, whether those stimuli come from the environment or from drugs or an interaction of the two.

What I'd like to know: What does the THC do to change the development of a fetal hippocampus?