||It is still not fully understood how THC generates the psychological effects that it does, however more is being discovered every day. Your brain has its own cannabinoid receptors, and even generates its own ("endogenous") chemical, anandamide, that binds to that receptor. Interestingly, compared to other neurotransmitter systems, it was a non-natural agonist (activator), cannabis, that spurred the discovery of the cannabinoid receptors and then the endogenous chemical was found (usually the endogenous chemical is found and the receptor is then found). |
There are several types of cannabinoid receptors, which collectively are the most prevalent type of receptor in your brain. One aspect of both cannabinoid and anandamide action is as a neuromodulator. It is believed that one reason neurons release anandamide is to reduce the output of the neurons activating them. This could be a major contributor to the overall sedation and CNS depressant effects felt by cannabis ingestion. The pharmacology is a little more complex than that, especially because the cannabinoid receptors are all over the brain and body. It is a very interesting field however, and probably one of the freshest and most exciting in medicinal chemistry and molecular pharmacology today.
There are a few files on Erowid which have additional info about Cannabis pharmacology. Also consider searches on "Cannabis" and "Pharmacology":