Cannabinoid Receptors in the Central Nervous System: Their Signaling and Roles in Disease

Abstract

The identification and cloning of the two major cannabinoid (CB₁ and CB₂) receptors together with the discovery of their endogenous ligands in the late 80s and early 90s, resulted in a major effort aimed at understanding the mechanisms and physiological roles of the endocannabinoid system (ECS). Due to its expression and localization in the central nervous system (CNS), the CB₁ receptor together with its endogenous ligands (endocannabinoids (eCB)) and the enzymes involved in their synthesis and degradation, has been implicated in multiple pathophysiological events ranging from memory deficits to neurodegenerative disorders among others. In this review, we will provide a general overview of the ECS with emphasis on the CB₁ receptor in health and disease. We will describe our current understanding of the complex aspects of receptor signaling and trafficking, including the non-canonical signaling pathways such as those mediated by β-arrestins within the context of functional selectivity and ligand bias. Finally, we will highlight some of the disorders in which CB₁ receptors have been implicated. Significant knowledge has been achieved over the last 30 years. However, much more research is still needed to fully understand the complex roles of the ECS, particularly in vivo and to unlock its true potential as a source of therapeutic targets.

Keywords: CB1 receptors; endocannabinoid system; neuromodulation; signaling; Δ9-THC.

Figure 1

Differential cannabinoid (CB) receptor signaling modalities can impact neuromodulation in health and disease in specific ways. (A) Key enzymes such as diacylglycerol lipase (DGLα) and phospholipase D (PLD) produce the endogenous ligands arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG). These activate the cannabinoid 1 receptor (CB₁) receptor in the central nervous system (CNS). The result can include modulation of adenylate cyclase activity to inhibit cAMP accumulation, voltage-gated calcium channels (VGCC), K+ channels and neurotransmitter release in presynaptic excitatory and inhibitory synapses. (B) Following activation of the CB₁ receptor by ligand binding, signaling via G protein and/or β-arrestin may occur at the plasma membrane, in endocytic pits or in endosomes after internalization of the receptor. G proteins usually bind the unphosphorylated receptor while β-arrestin binds the receptor phosphorylated by G protein receptor kinases.