Analgesic Properties of Next-Generation Modulators of Endocannabinoid Signaling: Leveraging Modern Tools for the Development of Novel Therapeutics

Abstract

Targeting the endocannabinoid (eCB) signaling system for pain relief is an important treatment option that is only now beginning to be mechanistically explored. In this review, we focus on two recently appreciated cannabinoid-based targeting strategies, treatments with cannabidiol (CBD) and α/β-hydrolase domain containing 6 (ABHD6) inhibitors, which have the exciting potential to produce pain relief through distinct mechanisms of action and without intoxication. We review evidence on plant-derived cannabinoids for pain, with an emphasis on CBD and its multiple molecular targets expressed in pain pathways. We also discuss the function of eCB signaling in regulating pain responses and the therapeutic promises of inhibitors targeting ABHD6, a 2-arachidonoylglycerol (2-AG)-hydrolyzing enzyme. Finally, we discuss how the novel cannabinoid biosensor GRAB_eCB2.0 may be leveraged to enable the discovery of targets modulated by cannabinoids at a circuit-specific level. SIGNIFICANCE STATEMENT: Cannabis has been used by humans as an effective medicine for millennia, including for pain management. Recent evidence emphasizes the therapeutic potential of compounds that modulate endocannabinoid signaling. Specifically, cannabidiol and inhibitors of the enzyme ABHD6 represent promising strategies to achieve pain relief by modulating endocannabinoid signaling in pain pathways via distinct, nonintoxicating mechanisms of action.

Fig. 1.

Anatomical pain pathway. Ascending (red) and descending (blue) pathways involved in pain responses are depicted. Painful stimuli information enters the spinal cord dorsal horn through primary afferent fibers in the dorsal root ganglion (DRG) that synapse onto transmission neurons located in the dorsal horn (DH). Ascending information travels to the thalamus, where it is relayed to the amygdala (Amyg) and cortical regions. Information then travels through the descending pathway, which includes brain regions such as the thalamus, PAG, and rostral ventromedial medulla (RVM). The RVM sends projections back to the DH in the spinal cord. Projections throughout this pathway modulate pain inputs and can be targeted for pain relief.

Fig. 2.

Receptor targets of CBD that produce analgesia. CBD is an antagonist at G protein-coupled receptor 55 (GPR55) and orexin 1 receptors type 1 (OX1R); a negative allosteric modulator of μ-opioid receptors (MOR) and of δ-opioid receptors (DOR); a blocker of the equilibrative nucleoside transporter-1 (ENT-1) transporters; an agonist at transient receptor potential vanilloid type 1 (TRPV1), serotonin receptor 1 A (5-HT1A), and α1 and 1β GlyRs; and a positive allosteric modulator of α3 GlyR.

Fig. 3.

Mechanisms regulating 2-AG signaling between neurons and glia. 2-AG signaling in regions associated with pain processing, such as the dorsal horn, can be divided into three processes: 1) 2-AG is synthesized by neurons and glia in an activity dependent manner, which relies on increases in cytosolic calcium either by opening of ionotropic receptors, such as P2X₇ receptors (P2X₇R) in astrocytes and microglia, or through activation of metabotropic receptors, such as group I metabotropic glutamate receptors (mGluR_1/5). 2) 2-AG activates multiple targets (i.e., CB₁R, CB₂R, and TRPV1) expressed by both neurons and glia to modulate neurotransmission and neuroinflammation. 3) 2-AG signaling is terminated by hydrolases (i.e., ABHD6 and MAGL), which hydrolyze 2-AG to produce arachidonic acid (AA) and glycerol (not shown). ABHD6 and MAGL have distinct subcellular localizations, with ABHD6 predominantly expressed in postsynaptic neurons and MAGL predominantly expressed in presynaptic terminals.

Fig. 4.

Methods for measuring changes in 2-AG signaling. Diagram depicts an example of a therapeutic approach that increases 2-AG signaling, here the ABHD6 inhibitor KT-182. Increased 2-AG signaling induces multiple physiological effects, including (A) changes in synaptic transmission and plasticity that can be detected using electrophysiological techniques and (B) modulation of pain responses in animals that can be detected using behavioral assays, such as the von Frey test. Changes in 2-AG levels can also be directly measured by quantifying total 2-AG amounts in bulk tissue using (C) mass spectrometry or by measuring changes in 2-AG levels in specific cells and compartments using a (D) biosensor, such as the genetically encoded eCB sensor GRAB_eCB2.0. These approaches differ in their sensitivity, spatiotemporal resolution, experimental throughput, and the model systems they can be applied in. This combination of tools, when used in concert, represents an innovative platform that can help evaluate the efficacy and molecular mechanism of potential new analgesics that target the eCB system through alternative mechanisms of action.