The Endocannabinoid System
The endocannabinoid system is an evolutionarily conserved neuromodulatory system composed of the endocannabinoid signalling molecules, 2-arachidonyl glycerol (2AG) and anandamide, (AEA) and their G-protein coupled cannabinoid receptors CB1 and CB2 (1).
Endocannabinoid signalling molecules are synthesised as a result of depolarisation in the post-synaptic terminal and work in a paracrine fashion upon pre-synaptic cannabinoid receptors. The primary pathway through which AEA is synthesised involves the Ca2+ dependant transfer of arachidonic acid to the membrane bound phosphatidylethanolamine. The resulting N-arachidonyl phosphatidylethanolamine is then hydrolysed to form AEA (1,2). In most cases 2AG is synthesised through cleavage of the membrane bound phosphatidylinositol by phospholipase Cβ resulting in the formation of 1,2-diacylglycerol which is further catalysed by the either sn-diacylglycerol lipase α or β to 2AG. In the central nervous system, the concentration of 2AG is 200-fold higher than that of AEA (3).
The CB1 receptor is highly expressed in the central nervous system at the terminals of central and peripheral neurons where they regulate neurotransmitter release and psychoactivity (4). CB2 receptor expression is associated with the peripheral immune system, neurons within the brainstem and microglia during inflammation (5,6). The degradation of endocannabinoids is carried out by two enzymes: fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) which act upon AEA and 2AG respectively (7,8).
Exogenous ligands to cannabinoid receptors also exist. Phytocannabinoids such as ∆9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are derived from the Cannabis sativa plant, while synthetic CB1/CB2 agonists and antagonists have been developed in the labratory. Manipulation of the endocannabinoid system has also been carried out by the inhibition of endocannabinoid biosynthesis, membrane transport and degradation (9).
Chemical structures of endocannabinoids and phytocannabinoids. (Adapted with permission from Fagan S. 2016. Pannexin 1 and Cannabinoid-Mediated Neuroprotection)
Downstream Signalling of Cannabinoid Receptors
Stimulation of cannabinoid receptors can have a range of downstream signalling effects. These include, inhibition of adenylyl cyclase, regulation of K+ and Ca2+ ion channels and activation of mitogen activated protein kinases (MAPK) and c-Jun N-terminal kinases which in turn affect nuclear transcription factors (10). Inhibition of adenylyl cyclase is Gi/o protein-sensitive and causes a reduction in cyclic AMP accumulation and dampens protein kinase A activity (11-13). As a result of this, reduced phosphorylation by protein kinase A causes the activation of A-type K+ channels (14). Furthermore, direct coupling of cannabinoid receptor G-proteins and inwardly-rectifying K+ channels regulated K+ influx to the cell (15). One of the primary roles of endocannabinoid as a depolarisation-induced retrograde messenger is the inhibition of Ca2+ channels. Stimulation of cannabinoid receptors inhibits voltage gated, L, N, P and Q Ca2+ channels in a Gi/o-mediated fashion (10). Stimulation of CB1 receptors and the induction of MAPK pathways has been shown to activate a number of second messengers such as extracellular regulating kinase 1 and 2 (ERK1/2), phosphatidylinositol 3-kinase and the transactivation of VEGF receptors through inhibition of Src tyrosine kinases (10, 16-18). It is through these signalling cascades that the endocannabinoid system regulates neuroinflammation, Ca2+ buffering, metabolic activity and neurotransmission.
Adapted from Fagan S. 2016. Pannexin 1 and Cannabinoid-Mediated Neuroprotection.
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