The Institute of Medicine published in its Mar. 1999 report titled "Marijuana and Medicine: Assessing the Science Base":
"Delta-9-tetrahydrocannabinol (Delta-9-THC) is the primary psychoactive ingredient [in marijuana]; depending on the particular plant, either THC or cannabidiol is the most abundant cannabinoid in marijuana. [...]
[T]he effects of THC are included among the effects of marijuana, but not all the effects of marijuana are necessarily due to THC. [...]
Cannabinoids are produced in epidermal glands on the leaves (especially the upper ones), stems, and the bracts that support the flowers of the marijuana plant. Although the flower itself has no epidermal glands, it has the highest cannabinoid content anywhere on the plant, probably because of the accumulation of resin secreted by the supporting bracteole (the small leaf-like part below the flower). [...]
Delta-9-THC and Delta-8-THC are the only compounds in the marijuana plant that produce all the psychoactive effects of marijuana. Because Delta-9-THC is much more abundant than Delta-8-THC, the psychoactivity of marijuana has been attributed largely to the effects of Delta-9-THC. [...]
THC molecule, U.S. Institute of Medicine, March, 1999 Marijuana and Medicine: Assessing the Science Base
All recent studies have indicated that the behavioral effects of THC are receptor mediated. Neurons in the brain are activated when a compound binds to its receptor, which is a protein typically located on the cell surface. Thus, THC will exert its effects only after binding to its receptor. [...] Binding to a receptor triggers an event or a series of events in the cell that results in a change in the cell's activity, its gene regulation, or the signals that it sends to neighboring cells."
Laurence O. McKinney, Managing Partner of McKinney & Company, wrote the following in a Sep. 18, 2007 email to ProCon.org:
"THC is created in nature when a plant-produced precursor, a cannabinolic acid, loses a carboxyl group due to heat (the sun, cooking, burning, etc.) and becomes tetrahydrocannabinol (THC) – a cannabis plant’s natural defense against predators and disease. At maturity, the THC-rich brachs (seed coverings) fall off, and birds eat the hard seeds inside. Some seeds inevitably pass through the birds’ bodies, thus propagating the plant.
Many neurological effects are ascribed to THC, but a major aspect appears to be rapid release and uptake of the neurotransmitter norepinephrine. From a cognitive standpoint, this “magnifies” any thought or sensation by engaging a larger population of neurons for any activity at hand, incidentally triggering the adrenalin release responsible for mild euphoria. A temporary norepinephrine deficit and reaction to the adrenalin then leads to a physical and mental drowsiness until normal levels are restored.
Pure THC, a greasy yellow substance, was first identified and characterized by Hebrew University chemist Raphael Mechoulam in 1964. It was later synthesized by Czech chemist Theodor Petrzilka who reacted olivitol with paramenthadyanol and isolated THC using a chromatography column. In 1985, synthetic THC called Marinol was approved as a medicine by the FDA. NORAC Laboratories of Azusa, California is now the sole manufacturer of THC in the United States."
The National Institute on Drug Abuse (NIDA) stated in its section on marijuana on its website "NIDA For Teens" (accessed Dec. 29, 2006):
"THC is up to no good in the brain. THC finds brain cells, or neurons, with specific kinds of receptors called cannabinoid receptors. Then, it binds to these receptors.
When it attaches to a neuron, THC interferes with normal communication between neurons. Think of it as a disruption in the phone service, caused perhaps by too many users all at once. Let's say Neuron #1 needs to tell Neuron #2 to create a new memory. If THC is in the mix, this communication is likely to fail.
Certain parts of the brain have high concentrations of cannabinoid receptors. These areas are: the hippocampus, the cerebellum, the basal ganglia, and the cerebral cortex. [...]
When THC finds its way into the cerebellum, it makes scoring a goal in soccer or hitting a home run pretty tough. THC also does a number on the basal ganglia, another part of the brain that's involved in movement control. [...]
THC, the main active ingredient in marijuana, produces effects that potentially can be useful for treating a variety of medical conditions. It is the main ingredient in a pill that is currently used to treat nausea in cancer chemotherapy patients and to stimulate appetite in patients with wasting due to AIDS. Scientists are continuing to investigate other potential medical uses for cannabinoids.
However, smoking marijuana is difficult to justify medically because the amount of THC in marijuana is not always consistent. It would be difficult—if not impossible—to come up with a safe and effective use of the drug because you could never be sure how much THC you were getting. Moreover, the negative effects of marijuana smoke on the lungs will offset the helpfulness of smoked marijuana for some patients."