Keywords

Introduction

Cannabinoids act simultaneously or synergistically on multiple pain targets within the peripheral and CNS [1,2,3]. Alongside acting on cannabinoid receptors (CB1 and CB2), cannabinoids may modulate pain through interaction with the putative non-CB1/non-CB2 cannabinoid G protein-coupled receptor 55 (GPR55) and GPR18 which is also known as the N-arachidonoyl glycine (NAGly) receptor [4, 5], as well as other well-known G protein-coupled receptors (GPCRs) such as serotonin (5-HT) and opioid receptors [6, 7].

Moreover, cannabinoids can interact with different transient receptor potential ion channel subfamilies (TRPV, TRPA, and TRPM) [2, 3]. TRPV1 is involved with temperature control, pain transmission, and modulation, as well as the integration of diverse painful stimuli [8,9,10].

Cannabinoids have various effects on the cys-loop ligand-gated ion channel superfamily (e.g., nicotinic acetylcholine, glycine, GABAA, GABAA-ρ, 5-HT3 receptors) [11,12,13,14,15,16,17,18,19]. Anandamide, THC, and cannabidiol directly activate glycine receptors, contributing to cannabinoid-induced analgesia in inflammatory and neuropathic pain [12,13,14,15], while 2-arachidonoylglycerol (2-AG) and cannabidiol (CBD) are positive allosteric modulators mainly at the α2-containing GABAA receptor subtypes [16, 17]. On the other hand, cannabinoids (THC) negatively allosterically modulate and inhibit nicotinic and 5-HT3 receptors [11, 18, 19].

The anti-inflammatory action of cannabinoids may contribute to their analgesic effects [20, 21]. Cannabinoid (CBD) action as a CB2 inverse agonist may explain its anti-inflammatory properties [22]. Some cannabinoids modulate and activate different isoforms of the nuclear receptor peroxisome proliferator-activated receptors (PPARα, β, and γ) [23].

Additionally, non-cannabinoid constituents of the cannabis plant (e.g., terpenoids and flavonoids) may contribute to the analgesic and anti-inflammatory effects of cannabis [24, 25].

Endocannabinoids’ Mechanism of Action

Anandamide (AEA)

Anandamide is a partial agonist at both CB1 and CB2 receptors, but a full agonist at the transient receptor potential vanilloid 1 (TRPV1) receptor. Although anandamide is a partial agonist, it has a selectivity and higher affinity to the CB1 receptor than 2-AG [26]. Once actions are carried out at the receptor, anandamide is thought to possibly be taken up by transport proteins on both neurons and glia that mediate endocannabinoid uptake [27]. Anandamide can play a dual role in nociception: antinociceptive at cannabinoid receptors and pronociceptive at the TRPV1 receptor [28]. Anandamide has a noted “tetrad effect” when injected into mice. The tetrad is a combination of inhibition of motor activity, catalepsy, hypothermia, and hypoalgesia [29, 30].

Anandamide also interacts with other neurotransmitter systems that may play a role in nociception. Cannabinoids might directly inhibit 5-HT3 receptors, leading to analgesia and neuroprotection effects [29]. Anandamide exerts part of its CNS effects through the 5-HT3 receptors [29]. In addition, it was shown that micromolar concentrations of anandamide bind to 5-HT1 and 5-HT2 receptors, thus further describing the role of anandamide in other neurotransmitter systems [31].

2-Arachidonoylglycerol (2-AG)

2-AG is a full agonist at CB1 and CB2 receptors. 2-AG may be secreted from the postsynaptic neuron by simple diffusion or through a passive carrier protein [27]. Once bound to CB1, activation leads to inhibition of neurotransmitter release in the presynaptic cell via inhibition of voltage-activated calcium channels and enhancement of inwardly rectifying K+ channels in the cell [27, 32].

Subsequent to neuronal depolarization, the Ca2+-dependent release of glutamate from presynaptic vesicles activates NMDA receptors at the postsynaptic neurons leading to excitatory postsynaptic currents (EPSCs). This variation of membrane excitability quickly triggers the synthesis of 2-AG. Then, 2-AG travels retrograde to stimulate CB1 receptors on presynaptic terminals, which in turn activate K+ channels and inversely inhibit Ca2+ channels, thus inhibiting excitatory neurotransmitter release [32] (Fig. 24.1).

Fig. 24.1
figure 1

Synthesis of 2-AG and the retrograde signaling. (a) The variation of postsynaptic membrane excitability triggers the synthesis of 2-AG. (b) 2-AG travels retrograde to stimulate CB1 receptors on presynaptic terminals, which in turn activate K+ channels and inversely inhibit Ca2+ channels, thus inhibiting excitatory neurotransmitter release. 2-AG is metabolized in the presynaptic neuron with MAGL into arachidonic acid and glycerol. 2-AG, 2-arachidonoylglycerol; CB1, cannabinoid receptor 1; CB2, cannabinoid receptor 2; COX2, cyclooxygenase 2; DAG, diacylglycerol; DAGL-α and DAGL-β, diacylglycerol lipase-α or diacylglycerol lipase-β; MAGL, monoacylglycerol lipase; PA, phosphatidic acid; PLCβ, phospholipase Cβ; PLD, phospholipase D; PIP2, sn-2-arachidonoyl-phosphatidylinositol-4,5-bisphosphate. (Used with permission from ©Samer Narouze, MD, PhD)

Full size image

Endocannabinoids and Pain Modulation

Endocannabinoids are sensitized on demand. When noxious stimuli occur, there is an increase in endocannabinoid release, thus leading to pain modulation effects [30]. Animal studies show endocannabinoids to have analgesic actions in the periphery, spinal, and supraspinal pain pathways [30] (Table 24.1).

Table 24.1 Cannabinoid multimodal analgesic mechanisms of action
Full size table

Peripheral Mechanisms

Models of inflammatory pain show elevated concentrations of anandamide and 2-AG in peripheral tissues [28]. The cannabinoid receptor, CB2, in the periphery plays a vital role in analgesia. 2-AG has been studied to show multiple mechanisms leading to pain modulation which include inhibiting production and release of reactive oxygen species and cytokines, and in addition 2-AG will release peripheral endogenous opioids [28]. There is more research describing the anti-inflammatory and antinociceptive mediated actions of 2-AG compared to anandamide. There are also CB1 receptors in the periphery that localize on sensory afferent terminals where endocannabinoids act to gate the transduction of pain from noxious stimuli [28].

Spinal Mechanisms

Endocannabinoids have antinociceptive effects at the dorsal horn in the spinal cord due to high expression of CB1 receptors. At this level, 2-AG inhibits the release of pronociceptive neurotransmitters from primary afferent terminals mediated by CB1 receptors [28]. In contrast, anandamide was shown to have effects on acute and chronic pain via mediation of CB2 receptors expressed on inhibitory interneurons and glial cells [28]. In a surgical incision model, it was shown that hours after a peripheral surgical incision, there was a marked decrease in anandamide concentrations, whereas there were no changes in 2-AG concentration [28]. Anandamide concentrations returned to baseline as nociceptive behavior subsides. 2-AG concentrations increased later in conjunction with glial cell activation, CB2 receptor upregulation, and resolution of the pain state [28]. Endocannabinoids have different effects on pain modulation. Anandamide exerts its action at the onset of pain, whereas 2-AG plays a role in the resolution of pain.

Supraspinal Mechanisms

Endocannabinoids modulate ascending pain signals in the thalamus, descending signals in the brain stem, and pain sensation in the frontal-limbic circuits [28]. Anandamide has a biphasic effect on the supraspinal level of pain modulation. Anandamide is released due to stimulation of the periaqueductal gray (PAG) or peripheral inflammatory insult [27]. In acute pain, anandamide that is released causes antinociceptive actions. When high concentrations of anandamide occur due to prolonged stimulation, anandamide modulates pronociceptive responses via TRPV1 binding [27].

Anandamide and 2-AG Synergistic Effect

Anandamide and 2-AG have synergistic yet different roles in pain modulation at the spinal and supraspinal levels. Stress-induced analgesia exhibits a synergistic effect of anandamide and 2-AG through induction of descending inhibitory GABAergic signaling to the spinal cord, thus mediating stress-induced analgesia [27]. In a prolonged foot shock modulation study, both endocannabinoids were found to be released in the ipsilateral lumbar V dorsal root ganglion upon stimulation [33]. The CB1 receptors at the dorsal root ganglion and CB2 receptors at the periphery involve a synergistic interplay between anandamide and 2-AG [33]. Both endocannabinoids levels were enhanced after 3 and 7 days of chronic constriction injury at the sciatic nerve of a rat [33]. After the 3-day mark, endocannabinoid levels were increased only at the spinal cord and PAG. However, after 7 days, elevated concentrations were detected in the rostral ventral medulla as well [33]. This study provides evidence of endocannabinoid cooperation regarding synergistic involvement in the regulation of pain.

Chronic pain enhances the endocannabinoid signaling effects of both anandamide and 2-AG. An upregulation of CB2 receptors found in such pain states would benefit from endocannabinoid agonism [27]. 2-AG signaling cascades from microglial cells mediate effects in persistent pain [27].

Endocannabinoid Receptors

CB1 Receptors

Central CB1 Receptors

CB1 receptor is the most abundant GPCR in the mammalian brain; thus it is referred to as the “brain cannabinoid receptor” [34]. CB1 receptors are expressed centrally in all brain structures and in decreasing density from the olfactory bulb, cerebellum, hippocampus, basal ganglia, cortex, and amygdala to the hypothalamus, thalamus, and brain stem [35].

They are expressed in most brain areas on presynaptic terminals of both glutamatergic and gamma aminobutyric acid (GABA)-ergic neurons [36]. Moreover, CB1 receptors can also be expressed postsynaptically where it can form heterodimers in association with other GPCRs including the dopamine D2, adenosine A2, or orexin type-1 receptors [37,38,39].

The intracellular region of CB1 is most regularly coupled to Gi/o proteins. Consequently, the activation of CB1 receptors inhibits adenylate cyclase activity with subsequent reduction of intracellular cyclic adenosine monophosphate (cAMP) level or promotes mitogen-activated protein kinase (MAPK) activity [34] (Fig. 24.2). Decreased cAMP level leads to activation of voltage-gated K+ and inhibition of Ca2+ channels, thus inhibiting neurotransmitter release [40,41,42]. In neurons, CB1 activation of Gi/o can also directly inhibit voltage-activated Ca2+ channels [32].

Fig. 24.2
figure 2

CB1 receptor activation. The intracellular region of CB1 is most regularly coupled to Gi/o proteins. The activation of CB1 receptors by binding to a ligand (2-AG) inhibits adenylate cyclase activity with subsequent reduction of intracellular cyclic adenosine monophosphate (cAMP) level or enhances mitogen-activated protein kinase (MAPK) activity. Decreased cAMP level leads to activation of voltage-gated K+ and inhibition of Ca2+ channels, thus inhibiting neurotransmitter release. (Used with permission from ©Samer Narouze, MD, PhD)

Full size image

Neuronal depolarization rapidly triggers the synthesis of endocannabinoids, particularly 2-AG, at postsynaptic neurons. Subsequently, 2-AG travel backward to stimulate CB1 receptors on presynaptic terminals, and then after it is inactivated by hydrolytic enzymes. This “on-demand” synthesis of endocannabinoids leads to CB1-mediated activation of K+ and inhibition of Ca2+ channels, thus controlling both excitatory and inhibitory neurotransmitter releases, which eventually tunes the duration of synaptic activity and synaptic plasticity [43, 44].

CB1 is also found in non-neuronal cells of the brain, predominately in astrocytes, where its activation stimulates the release of neurotransmitters. Unexpectedly, astroglial CB1 receptor activation seems to induce intracellular Ca2+ levels, triggering the release of glutamate and the subsequent activation of presynaptic metabotropic glutamate receptors [45,46,47] (Table 24.2).

Table 24.2 Cannabinoids mechanism of action in chronic pain
Full size table

Peripheral CB1 Receptors

CB1 receptors are also expressed in the peripheral nervous system and in almost all mammal tissues and organs including the adrenal glands, smooth and skeletal muscle, heart, lung, gastrointestinal tract, liver, male and female reproductive systems, bone, adipose tissue, and skin [32]. The CB1 receptors play a vital role in the maintenance of homeostasis and regulating adrenal, cardiovascular, lung, gastrointestinal, and reproduction functions, among others.

Peripheral CB1 receptors are mainly localized on sensory afferent terminals where endocannabinoids act to gate the transduction of pain from noxious stimuli [3], thus playing an important role in peripheral pain sensitization.

Central CB2 Receptors

The role of CB2 in the brain is still controversial. In contrast to CB1, CB2 receptors in the brain are limited, and its expression is restricted to specific neuronal cells and becomes abundant in activated microglia and astrocytes [45, 48].

Like CB1, CB2 receptor is a GPCR and is coupled to Gi/Go α proteins. Thus, its stimulation inhibits adenylate cyclase activity and activates MAPK [32].

Peripheral CB2 Receptors

In contrast, CB2 receptors are abundantly expressed in the immune system cells such as monocytes, macrophages, B and T cells, and mast cells. CB2 receptor activation reduces the release of pro-inflammatory cytokines and lymphoangiogenic factors [49,50,51]. Moreover, CB2 receptors are also present in other peripheral organs playing a role in the immune response, including the spleen, tonsils, thymus gland, and keratinocytes, as well as in the gastrointestinal system [32].

Accordingly, CB2 receptors represent key regulators of inflammatory and nociceptive responses and can control the activation and migration of immune cells [52, 53].

Other Putative Endocannabinoid Receptors: TRPV1 and GPR55

TRPV1

The transient receptor potential vanilloid type 1 (TRPV1) channel, also known as the capsaicin receptor, was the first member of the TRPV channel subfamily to be discovered and cloned [54]. TRPV1 channels are activated by capsaicin, endocannabinoids, and phytocannabinoids [55,56,57].

TRPV1 function is heavily dependent on the binding of key regulatory proteins that induce changes in its phosphorylation state. The phosphorylation induced by adenosine triphosphate (ATP), protein kinase A (PKA), PKC, phosphoinositide-binding protein (PIRT), and phosphatidylinositol 4,5-bisphosphate (PIP2) is required for TRPV1 activation and cation gating. TRPV1 activation contributes to pain transmission, neurogenic inflammation, synaptic plasticity, neuronal overexcitability, and neurotoxicity [57,58,59,60].

TRPV1 “desensitization” occurs as the rise of intracellular Ca2+ following TRPV1 stimulation activates proteins (i.e., calmodulin) that stabilize the channel in a closed conformational state or Ca2+-dependent phosphatases (i.e., calcineurin), which dephosphorylate and inactivate TRPV1 [59,60,61,62,63]. This fast process of TRPV1 desensitization and inactivation is thought to underlie the paradoxical analgesic, anti-inflammatory, and anti-convulsant effects of TRPV1 agonists [57, 64, 65].

TRPV1 channels are largely expressed in dorsal root ganglia and sensory nerve fibers (Aδ and C-type) [66]. In sensory neurons, TRPV1 channels work as molecular integrators for multiple types of sensory inputs that contribute to generate and transmit pain. In central neurons, lower amounts of TRPV1 channels are expressed both pre- and postsynaptically, where they act to regulate synaptic strength [66,67,68]. They usually affect pain, anxiety, and depression by inducing effects opposite to those exerted by CB1 receptors in the same context [32].

Moreover, there is intracellular cross talk between TRPV1 and CB1 or CB2 as they are colocalized in peripheral and central neurons (sensory neurons, dorsal root ganglia, spinal cord, brain neurons) [67, 69]. Recently, a multiplicity of interactions between cannabinoid, opioid, and TRPV1 receptors in pain modulation was discovered [70]. This provides a great opportunity for the development of new multiple target ligands for pain control with improved efficacy and side effects profile [71].

GPR55

GPR55 is considered by some experts as the third cannabinoid receptor, CB3. GPR55 belongs to the large family of GPCRs, and its endogenous ligand is lysophosphatidylinositol (LPI) [72, 73].

GPR55 is activated by Δψ-THC while antagonized by cannabidiol (CBD). Conflicting data exist regarding the likelihood that low concentrations of endocannabinoids may activate GPR55 [74, 75]. These controversies might be explained by biased signaling depending on the cell type and condition or the formation of heteromers between GPR55 and CB1 receptors [76, 77]. Activation of GPR55 might play an opposite role to CB1 by enhancing neurotransmitter release [32]. GPR55 may play a role in mechanical hyperalgesia associated with inflammatory and neuropathic pain [78].

Phytocannabinoids (THC and CBD)

THC

Δ9-tetrahydrocannabinol (THC) is an analog to the endocannabinoid, anandamide (AEA). It is responsible for most of the pharmacological actions of cannabis, including the psychoactive, memory, analgesic, anti-inflammatory, antioxidant, antipruritic, bronchodilator, antispasmodic, and muscle relaxant activities [79, 80]. THC acts as a partial agonist at CB1 and CB2 receptors [22]. THC has a very high binding affinity to CB1 receptor which mediates its psychoactive properties. Interestingly, most of the negative effects of THC, psychogenic effects, impaired memory, anxiety, and immunosuppression, can be reversed by other constituents of the cannabis plant (other cannabinoids, CBD, terpenoids, and flavonoids) [24, 80].

CBD

Cannabidiol (CBD) is the other important cannabinoid in the cannabis plant. It is the non-psychoactive analog of THC. CBD have significant analgesic, anti-inflammatory, anti-convulsant, and anxiolytic activities without the psychoactive effect of THC [81]. CBD has little binding affinity for either CB1 or CB2 receptors, but it can antagonize them in the presence of THC. CBD behaves as a non-competitive negative allosteric modulator of CB1 receptor, and it reduces the efficacy of THC and AEA [82]. This may explain the “entourage effect” that CBD displays, as it improves the tolerability and safety of THC by reducing the likelihood of psychoactive effects and other adverse effects such as tachycardia, sedation, and anxiety [80, 83].

Mechanisms of Action in Pain Modulation

The phytocannabinoids THC and CBD are lipophilic substances that readily cross the blood-brain barrier and interact with receptors in both the central and peripheral nervous systems, exerting analgesic effects especially in hyperalgesia and inflammatory states [84, 85] (Table 24.3).

Table 24.3 THC and CBD mechanisms of action in pain modulation
Full size table

THC

THC exhibits CB1 receptor-mediated antinociception through activation of supraspinal sites and descending serotonergic and noradrenergic pain modulatory pathways to produce antinociceptive effects via spinal 5-HT7, 5-HT2A, and alpha-2 adrenoceptor activation [86, 87].

The frontal-limbic distribution of cannabinoid receptors explains the central mechanism of THC analgesia as it targets preferentially the affective qualities of pain. Functional magnetic resonance imaging revealed that amygdala activity contributes to the dissociative effect of THC on pain perception related to cutaneous ongoing pain and hyperalgesia that were temporarily induced by capsaicin [88]. THC reduced the reported unpleasantness, but not the intensity of ongoing pain and hyperalgesia. THC also reduced functional connectivity between the amygdala and primary sensorimotor areas during the ongoing pain state. The authors concluded that peripheral mechanisms alone cannot account for the dissociative effects of THC on the pain that was observed and amygdala activity contributes to inter-individual response to cannabinoid analgesia [88].

The analgesic effects of THC are mediated through mechanisms distinct from those responsible for the psychoactive effects. THC has additive analgesic effect with kappa opioid receptor agonists. This effect is blocked by kappa antagonism, but opioid receptor antagonism does not alter the psychoactive effects of THC [89].

Cannabinoids may exert other non-CB1/non-CB2 receptor-mediated antinociceptive effects by interacting with 5HT3 and N-methyl-d-aspartate receptors [89, 90].

CB2 receptors serve an important role in immune function, inflammation, and pain modulation specially in allodynia and hyperalgesia states [91, 92]. The presence of CB2 receptors on microglia within the nervous system may explain the cannabinoids’ role in neuropathic pain modulation by reducing cytokine-mediated neuro-inflammation [91, 92].

CB2 receptor expression has been demonstrated in areas of the peripheral and central nervous system relevant to pain perception and modulation, including the dorsal root ganglion, spinal cord, and microglia. This explains the analgesic effects produced by CB2 agonists [93,94,95,96,97].

CB2-selective agonists suppress neuronal activity in the dorsal horn via reduction in C-fiber activity and wind-up involving wide dynamic range (WDR) neurons [98, 99]. There is increase in peripheral CB2 receptor protein or mRNA expression in inflamed tissues and in the dorsal root ganglion in neuropathic states [100,101,102].

CBD

CBD regulates the perception of pain mainly through non-CB1/non-CB2 mechanisms. CBD interacts with a significant number of other targets, including non-cannabinoid GPCRs (e.g., 5-HT1A), ion channels (TRPV1, TRPA1, TPRM8, GlyR), and PPARs. Moreover, CBD augments anandamide (AEA) effects by inhibiting its uptake as well as its hydrolysis by the enzyme fatty acid amide hydrolase (FAAH) [3, 78, 103].

CBD can act synergistically with THC and contribute to its analgesic effect while providing an “entourage effect,” minimizing the negative psychoactive effects of THC [80]. This depends on the differences in concentration of THC/CBD in the cannabis chemovar. Although CBD as a monotherapy has not been evaluated clinically in the management of pain, its anti-inflammatory and anti-spasmodic effects and good safety profile suggest that it could be a safe and effective analgesic [104, 105].