“Before the 1980s, it was often speculated that cannabinoids produced their physiological and behavioral effects via nonspecific interaction with cell membranes, instead of interacting with specific membrane-bound receptors.

The discovery of the first cannabinoid receptors in the 1980s helped to resolve this debate.

These receptors are common in animals, and have been found in mammals, birds, fish, and reptiles.

At present, there are two known types of cannabinoid receptors, termed CB₁ and CB₂, with mounting evidence of more.

Cannabinoid receptor type 1

CB₁ receptors are found primarily in the brain, to be specific in the basal ganglia and in the limbic system, including the hippocampus.

They are also found in the cerebellum and in both male and female reproductive systems. CB₁ receptors are absent in the medulla oblongata, the part of the brain stem responsible for respiratory and cardiovascular functions. Thus, there is not a risk of respiratory or cardiovascular failure as there is with many other drugs. CB₁ receptors appear to be responsible for the euphoric and anticonvulsive effects of cannabis.

Cannabinoid receptor type 2

CB₂ receptors are almost exclusively found in the immune system, with the greatest density in the spleen.

While found only in the peripheral nervous system, a report does indicate that CB₂ is expressed by a subpopulation of microglia in the human cerebellum.

CB₂ receptors appear to be responsible for the anti-inflammatory and possibly other therapeutic effects of cannabis.”

http://www.news-medical.net/health/Cannabinoid-Receptors.aspx