This article in today's San Jose Mercury news (it was in the print edition, which is where I found it) covers a discovery by researchers that some brain cells (of course, being research, it was rat brain cells) produce cannabinoids, making them part of the natural brain chemistry.
For a long time, scientists thought that marijuana altered the mind in a messy and random way that didn't involve any particular part of brain chemistry.
Now they've identified an elegant modus operandi. It adds to a growing body of research that explains the mechanism behind getting "high." Marijuana mimics the cannabinoids made naturally by our brain -- chemicals that influence a smorgasbord of body functions including movement, thought and perception.