Scientists have found that marijuana-like substances naturally produced in the brain stimulate appetite. The finding not only offers clues to treating obesity but also explains why people tend to feel hungry after smoking up on pot.

The study suggests endocannabinoids are part of the brain's complex system for controlling when and how much we eat. These chemicals are similar to the active ingredient in marijuana but don't make people high.

To find out how endocannabinoids work, scientists genetically engineered some mice so receptors in their brain couldn't respond to the chemicals. This caused mice to eat less than normal.

Smoking up an appetite

The findings support the assumption that marijuana activates receptors involved in increasing appetite.

Dr. George Kunos is the scientific director of the National Institute on Alcohol Abuse and Alcoholism and led the study.

Kunos said his results suggest endocannabinoids are part of the complex neural circuitry controlled by leptin, a hormone that tells the brain when it is time to lose weight. Leptin also reduces levels of other appetite-enhancing substances.

Scientists don't know how the body creates endocannabinoids or precisely how they work. But this study shows they can work independently of other appetite triggers.

Kunos suggests efforts to control weight gain or loss with a single drug likely won't work because there are many back-up systems to overcome.

The study appears in the April 12 issue of Nature.