structure of most cannabinoids is similar to THC, the main psychoactive ingredient in marijuana. Although researchers have identified several variants of THC, only the most abundant form, delta-9-THC, has been studied extensively (unless stated otherwise, we use the term THC to refer to this compound). The active ingredient in the prescription medicine Marinol is synthetic THC, which is also known by its generic name, dronabinol. Marinol is used to treat chemotherapy-induced nausea and vomiting as well as AIDS wasting syndrome.

Marijuana plants make THC through a multistep process, much as chemists do when they synthesize THC in the laboratory. A series of assembly steps combine several simple molecules to form a cannabinoid compound called cannabigerol. Cannabigerol may be subsequently converted to THC or to another cannabinoid called cannabidiol, which may then be modified to produce THC. It in turn may undergo chemical reactions that convert it to yet another cannabinoid, cannabinol. Unlike THC, neither cannabigerol, cannabidiol, nor cannabinol is psychoactive. Live marijuana plants and dried plant parts contain all of these cannabinoids as well as others that represent either precursors of THC or modified versions of the THC molecule.

CANNABINOIDS AND THE CELL

Although it has long been observed that marijuana alters thinking and behavior, scientists have only recently begun to learn how chemicals in marijuana act on individual cells, both in the brain and elsewhere in the body. That knowledge is crucial to determining exactly how marijuana and its constituent chemicals affect users.

Recent studies indicate that cannabinoids produce most of their effects by binding to proteins, called receptors, on the surfaces of certain types of cells. Many different types of receptor proteins stud the exterior membranes of the cells throughout the human body. Each receptor recognizes only a few specific molecules, known collectively as ligands. When the appropriate ligand binds to its receptor, it typically sets off a chain of biochemical reactions inside the cell. Many drugs, as well as hor-



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