The Role of the Endocannabinoid System in the Brain-Gut Axis

Abstract

The actions of cannabis are mediated by receptors that are part of an endogenous cannabinoid system. The endocannabinoid system (ECS) consists of the naturally occurring ligands N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol (2-AG), their biosynthetic and degradative enzymes, and the cannabinoid (CB) receptors CB1 and CB2. The ECS is a widely distributed transmitter system that controls gut functions peripherally and centrally. It is an important physiologic regulator of gastrointestinal motility. Polymorphisms in the gene encoding CB1 (CNR1) have been associated with some forms of irritable bowel syndrome. The ECS is involved in the control of nausea and vomiting and visceral sensation. The homeostatic role of the ECS also extends to the control of intestinal inflammation. We review the mechanisms by which the ECS links stress and visceral pain. CB1 in sensory ganglia controls visceral sensation, and transcription of CNR1 is modified through epigenetic processes under conditions of chronic stress. These processes might link stress with abdominal pain. The ECS is also involved centrally in the manifestation of stress, and endocannabinoid signaling reduces the activity of hypothalamic-pituitary-adrenal pathways via actions in specific brain regions, notably the prefrontal cortex, amygdala, and hypothalamus. Agents that modulate the ECS are in early stages of development for treatment of gastrointestinal diseases. Increasing our understanding of the ECS will greatly advance our knowledge of interactions between the brain and gut and could lead to new treatments for gastrointestinal disorders.

Keywords: 2-AG; Anandamide; CB1 Receptor; CB2 Receptor; Enteric Nervous System; Epigenetics; Fatty Acid Amide Hydrolase; HPA Axis; Nausea; Stress; Visceral Pain.

Figure 1. Distribution of enzymes involved in the biosynthesis and degradation of endocannabinoids in the nervous system

The role of 2-AG as a dominant retrograde synaptic transmitter and putative roles of anandamide (AEA) as an anterograde and intracellular transmitter. Anandamide is synthesized from NAPE by a specific phospholipase D (NAPE-PLD) and is degraded by FAAH. 2-AG is synthesized by DAGL-α and degraded by MAGL and ABHD-6 and ABHD-12. Anandamide and 2-AG can also be metabolized by COX2, which is shown in post-synaptic neurons but can also occur presynaptically. AA, arachidonic acid; AGs, 2-acylglycerols; DAGs, diacylglycerols; ER, endoplasmic reticulum; MAPK, mitogen-activated protein kinases; PIP₂, phosphoinositide bisphosphate; PKA, protein kinase A; PPARs, peroxisome proliferator-activated receptors; TRPs, transient receptor potential channels; VGCCs, voltage-gated calcium channels. Dashed lines denote inactivation. Adapted from Di Marzo et al.

Figure 2. Effects of chronic stress on peripheral endocannabinoid pathways in visceral primary afferent neurons

Under conditions of chronic stress, levels of 2-AG and AEA increase and endocannabinoid degradation enzymes COX-2 and FAAH are decreased in nociceptive DRG neurons that innervate the colon and pelvis. Along with this, levels of CB1 are reduced and there is an increase in TRPV1 expression and phosphorylation in nociceptive primary afferent neurons. These effects are mediated by corticosteroids from the HPA pathway.

Figure 3. Effects of chronic stress on epigenetic regulation of the gene encoding CB1 (CNR1)

Chromosomes are located in chromatin-bound territories in the nucleus. Euchromatin is characterized by DNase 1 hypersensitivity and specific combinations of histone marks that define active genomic regulatory elements, such as promoters H3K27ac + H3K4me3, and enhancers H3K27ac + H3K4me1. An enhancer can either increase or decrease transcription. Recent research demonstrates that, in brain, the DNA sequence CAC is a common site of methylation, in contrast to other tissues where CpG is most often methylated. Additionally, in brain, 5-hydroxymethylcytosine (5hmC), a reactive species, is methylated. In contrast, in the periphery, methylcytosine (hmC) is a common site for methylation. DNA methylation is catalyzed by DNMTs. Chronic stress is associated with increased levels of DNMT1-mediated methylation of CRN1, resulting in reduced expression. H3K4me3, histone H3 trimethyl Lys4; H3K27ac, histone H3 Lys27 acetylation; H3Kme1, histone H3 monomethyl Lys4. Adapted from Wiley et al.

Figure 4. Effects of chronic stress on the endocannabinoid system in the brain

Chronic stress is characterized by a sustained reduction in levels of AEA, associated with increases in the degradative enzyme FAAH. These effects are mediated by CRH via the CRH 1 receptor (CRHR1). Corticosterone (in rodents; corticosterol in humans; CORT) appear to mediate the upregulation of CRH. The increased level of CORT are associated with increases in 2-AG. Chronic stress is associated with reduced levels of CB1 and the glucocorticoid receptor (GR) in the brain. Adapted from Morena et al. and Krugers et al.

Figure 5. Role of endocannabinoids in control of nausea and vomiting

In the brainstem regions involved in the control of vomiting (area postrema and nucleus of the solitary tract), activation of CB1 and possibly CB2 attenuates the emetic reflex by reducing the release of excitatory transmitters. In the forebrain, release of 2-AG in the visceral insular cortex inhibits, in a retrograde manner, release of neurotransmitters including serotonin (5-HT), which acts at 5-HT3 receptors to produce nausea. Note that to date this model has been studied in the rat and that in humans the CB1 agonist Δ-tetrahydrocannabinol reduces nausea. 5-HT₃ receptor antagonists are excellent antiemetics but are less effective at reducing nausea. Adapted from Sticht et al.

Figure 6. Effects of endocannabinoids in the GI tract are mediated by CB1 and CB2

Cannabinoid receptors are widely distributed in the GI tract. CB1 is expressed by all classes of cholinergic enteric neurons in the myenteric and submucosal plexuses (primary afferent neurons (blue), interneurons (purple), secretomotor (yellow), and excitatory motor neurons (green), but not on inhibitory motor neurons (red). CB1 is also found on some enteroendocrine cells and in the epithelium. Extrinsic vagal and spinal primary afferent neurons express CB1, which is regulated by feeding (vagal) and stress (spinal), respectively. CB2 is expressed by enteric neurons and immune cells in the GI tract. Under conditions of inflammation and injury, CB2 is upregulated in the epithelium and its function on enteric neurons increases. The classes of enteric neurons that express CB2 have not been determined. The effects of activating cannabinoid receptors in the GI tract include a reduction in motility, reduced inflammation, and reduced immune activation. Adapted from Lomax et al.