Cellular accumulation of anandamide: consensus and controversy

Abstract

The endocannabinoids N-arachidonylethanolamine (AEA or anandamide) and 2-arachidonylglycerol (2-AG) are hypothesized to function in the brain as interneuronal signaling molecules. Prevailing models of the actions of these molecules require that they traverse cellular plasma membranes twice; first, following cellular synthesis and second, prior to enzymatic hydrolysis. The transmembrane movement of AEA has been studied in multiple laboratories with a primary focus on its cellular accumulation following extracellular administration. Although there are areas of consensus among laboratories regarding AEA accumulation, several aspects are very unclear. In particular, there is a lack of consensus in the literature regarding the importance of AEA hydrolysis by fatty acid amide hydrolase in maintaining the driving force for accumulation. Furthermore, evidence for and against a transmembrane carrier protein has been published. We have reviewed the available literature and present a working model of the processes that are involved in the cellular accumulation of AEA. It is our hypothesis that transmembrane movement of AEA is regulated by concentration gradient between extracellular and intracellular free AEA. Furthermore, it is our view that a significant portion of the intracellular AEA in most cells is sequestered either by a protein or lipid compartment and that AEA sequestered in this manner does not equilibrate directly with the extracellular pool. Finally, we discuss the available data that have been presented in support of a transmembrane carrier protein for AEA.

Figure 1

Model of AEA accumulation in cells. Free AEA concentrations are equal on both sides of the plasma membrane because AEA either freely diffuses across plasma membranes (a) or is translocated by a transporter in a process of facilitated diffusion. Once inside the cell, it is likely that a significant portion of the internalized AEA is sequestered (AEA_seq) and this sequestered pool is in equilibrium with AEA_free/in. The amount of AEA_free/in is also influenced by de novo synthesis of AEA (b) and FAAH-mediated hydrolysis (c). AA denotes arachidonic acid; EA denotes ethanolamine.