Phylogeny of Endocannabinoid Receptors in the Brain
Biology 342 Fall 06
Phylogeny [fahy-loj-uh-nee] the evolutionary development and history of a physical trait of an organism
The endocannabinoid system comprises the cannabinoid receptors (CB1/CB2), the endocannabinoid ligands (AEA and 2-AG), the precursors for the ligands (e.g. NAPE), and the enzymes involved in biosynthesis (e.g. NAPE-PLD) and inactivation (e.g. FAAH, fatty acid acyl hydrolase) of the ligands. The presence of endocannabinoids appears to be an "evolutionarily ancient phenomenon" possibly dating back to the common ancestor of plants and animals (Elphick and Egertova, 2005). Endocannabinoid ligands are present in many plants and animals as well as the ability to synthesize and break them down. However, the ability to exploit the cannabinoid system with the endocannabinoid-specific receptors CB1 and CB2 present in humans, is not so ancient.
Although various chemicals now known as cannabinoids exist in many different organisms, and have for milions of years, the ability of organisms to "accept", and utilize these chemicals is, evolutionarily speaking, a relatively new trait.
The protosomes and deuterosomes, along with a few smaller phyla, form the bilateria, which is mostly comprised of animals with bilateral symmetry and three germ layers. Potostomes appear to have endocannabinoids present, but no receptor with which to exploit the endocanabinoids. Deuterostomes, in contrast appear to have at least one cannabinoid receptor. Amongst other metazoa, the endocannabinoids AEA and 2-AG have been identified in: the cnidarian H. vulgaris (De Petrocellis et al. 1999), protostomia such as molluscs (Sepe et al. 1998), the annelid H. medicinalis (Matias et al. 2001), the insects Drosophila melanogaster and Apis mellifera (McPartland et al. 2001), and the tick Amblyomma americanum (Fezza et al. 2003).
The endocannabinoid system appers to have arisen at the phylogenetic split illustrated above, as urochordata and all other organisms on the branches below (which evolved later in time) have cannabinoid receptors, while protostomes (like calcichordata) lack the recpetors.
An orthologue of human CB1 or CB2 receptors have only been found in deuterostomia. The tunicate's endocannabinoid system includes a single cannabinoid receptor (CiCBR) that is an intermediate of CB1 and CB2. The CB1/CB2 orthologue CiCBR shows 28% homology with mammalian CB1 and 24% homology with the human CB2 receptor. The receptor CiCBR probably underwent a duplication event that gave rise to the separate CB1 and CB2 receptors. The tunicate C. Intestinalis (Urochordata) therefore has a complete endocannabinoid system (Matias, Mcpartland, and Di Marzo). In C. Intestinalis, CiCBR is expressed in the branchial pharynx, heart, cerebral ganglion, and testes. it is also expressed in lesser quantities in the intestines, ovaries and stomach. Everywhere CiCBr is present, so too are the precursors to cannabinoid ligands and their metabolizing enzymes.
The CiCBR receptor, which is the evolutionary ancestor of the CB1 and CB2 receptors, seems to have arisen soon after the split between protostomes (like mollusks, flatworms, or roundworms) and deuterostomes (which include many organisms like humans, starfish, and sea urchins). The most "primitive" organism with a known complete endocannabinoid system is the tunicate (shown below).
Tunicate images (left to right respctively) from http://www.geocities.com/panuwatsuppakul/Purple_Tunicate.jpg and http://library.thinkquest.org/C006669/media/Biol/img/tunicate.gif
Behavioral Effects of Cannabinoids on Tunicates
The behavioral effects of this system were tested on siphon opening, which is involved in defence, respiration and feeding processes. To test for an affect on this well-known reflex response, the endocannabinoid system was stimulated by injecting a mixed CB1/CB2 agonist. This stimulation resulted in a delayed mouth opening starting 120 minutes after injection of the agonist. The endocannabinoid system was also treated with Hu-210, a CB1 antagonist, which also resulted in delayed siphon opening. Thus, an imbalance in the cannabinoid system whether through over-stimulation (agonist) or under-stimulation (antagonist), resulted in an attenuated reflex response in the model organism C. Intestinalis.
When cannabinoids are injected into tunicates (the most "primitive" organism with cannabinoid receptors) its reflexes are delayed. Specifically the basic action of siphon opening, which is necessary for survival, slows down.