I dunno if you knew this (probably) but I was just reading an article about the Medical research in to cannabis- ( [www.alternet.org] ) and I came across this quote-
"What THC is doing is impacting on -- or hijacking, if you like -- a natural system that's there physiologically for some reason that we don't really understand," says Iversen. Opiate drugs like heroin likewise have been found to mimic naturally occurring equivalents. "It's an exactly parallel story," says Iversen. "We start by studying a psychoactive plant-derived drug and discover a whole regulatory system in the brain that we didn't know existed."
The first known of these natural cannabislike compounds is called "anandamide," from the Sanskrit word "ananda," meaning bliss. In animal studies, Iversen says, anandamide "has essentially all of the pharmacological and behavioral actions of THC."