Endocannabinoid receptors: CNS localization of the CB₁ cannabinoid receptor

The evolution of plant metabolic pathways to invent compounds which distract predators, and the history of medicine to find treatments for diseases, often share a common logic. An attractive example to illustrate the rationale behind this is the Cannabis sativa plant, which was exploited for its widespread therapeutic effects for several thousand years, but historical "prescriptions" highlighted its distractive behavioral side-effects if abused. This chapter aims to explain the characteristically wide pharmacological and behavioral profile of the Cannabis plant by pointing to the ubiquitous anatomical distribution of CB₁ cannabinoid receptors, its predominant molecular target, throughout the nervous system. However, in contrast to their abundant regional and cellular localization, the subcellular arrangement of CB₁ receptors and the enzymes involved in the metabolism of its main endogenous ligand, 2-arachidonoylglycerol (2-AG), are strikingly polarized on the neuronal surface in the adult brain. Though there are still several unresolved issues, the known pieces of the puzzle outline a picture in which the biosynthetic machinery for 2-AG is accumulated in the somatodendritic compartment of neurons, whereas its receptor and degrading enzyme are both found on axon terminals. This molecular architecture suggests that a main physiological role of endocannabinoid signaling is the retrograde regulation of synaptic transmission, and the present chapter aims to summarize compelling evidence that it is an ancient and fundamental component of several distinct types of synapses throughout the nervous system.