Marijuana smokers report a diverse collection of sensations, and researchers now suspect that is because the drug's main active ingredient -- delta-9-trans-tetrahydrocannabinol, or THC -- is so similar in shape to the endocannabinoids, which are involved in many body and brain functions.

Dr. Rachel Wilson, a researcher at Caltech, discovered when she was at the University of California at San Francisco that endocannabinoids played an important role in the hippocampus, a part of the brain involved in learning and memory, according to a report published this spring in Nature.

No one has figured out exactly how endocannabinoids are used in the hippocampus, but based on the abundance of cannabinoid receptors in this part of the brain and on the experiences of marijuana users, Dr. Wilson suspects that these molecules help lay down new memories by strengthening the connections between nerve cells.

But when the brain is flooded with cannabinoids through marijuana use, forgetfulness results, Dr. Wilson said. It is probably a case of too much of a good thing, she added. When cannabinoids are abundant, every experience becomes strongly linked in our minds, she believes. But when everything is marked for memory, the system is overwhelmed and nothing is remembered.

Dr. Wilson and others also reported last year on another important role played by cannabinoids. They appear to tone down the production of certain neurotransmitters, acting like the brakes of a car when the system is racing too fast.

Another study published this last year in Nature may explain why marijuana users get a case of the munchies. An international group of researchers found that mice that were genetically engineered to be deficient in cannabinoid receptors ate 40 percent less than normal mice. And in a separate experiment, the researchers showed that an interplay existed between the cannabinoids and leptin, a hormone that produces satiety.

In the study, obese rats that were genetically modified to have low levels of leptin produced higher amounts of endocannabinoids. When the rats were given leptin, the endocannabinoid levels dropped.