Habitual marijuana smoking seems to decrease the number of receptors in the brain for the drug's active ingredient, according to a study presented at the Society of Nuclear Medicine's annual meeting in San Antonio, Texas on Monday. But the effect appears to be reversible.

As nearly every teenager knows, the active ingredient in cannabis is delta-9-tetrahydrocannabinol, or THC. This substance binds to cannabinoid receptors in the brain and the body. In addition to producing the sought-after high, it also affects various cognitive functions, such as perception, memory and coordination, as well as bodily functions, such as respiration, circulation and the immune system.

Right now, two classes of cannabinoid receptors are known, CB1 and CB2, with CB1 mainly affecting the central nervous system.

With CB1, it's known that in rodents these receptors downregulate after repeated exposure to pot, but then recover once the drug's given up. And it seems something similar is at work in humans.

According to the abstract, in the study, the researchers took PET scans of 30 habitual, male pot smokers the day they were admitted to an inpatient research unit. After they had been off the drug for four weeks, 14 of these men were then re-scanned. Twenty-eight non-smokers - who had smoked a joint fewer than 10 times in their lives - were also scanned once, as a control.

Before each scan, the volunteers were injected with 18F-FMPEP-d2, a radiotracer coupled with a neurotransmitter analog that binds with CB1 receptors.

After the scans, the researchers found that chronic pot users had about 20 percent fewer receptors than the non-drug using controls. But when the 14 subjects who had gone clean for a month were re-scanned, the receptors had increased, suggesting the effect was reversible.

"With this study, we were able to show for the first time that people who abuse cannabis have abnormalities of the cannabinoid receptors in the brain," said Dr. Jussi Hirvonen, lead author of the study, in a statement. "This information may prove critical for the development of novel treatments for cannabis abuse."

The study was a collaboration between the National Institute of Mental Health and the National Institute of Drug Abuse in Bethesda, Md.

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