Gut Feelings About The Endocannabinoid System: A Recap of Di Marzo’s Work

In the ever-evolving landscape of scientific research, understanding the intricacies of the endocannabinoid system has become increasingly vital. Recently, groundbreaking insights have emerged, shedding light on the profound influence of this intricate system within our gastrointestinal (GI) tract. A remarkable paper authored by Di Marzo and Piscitelli, aptly titled “Gut Feelings About the Endocannabinoid System,” delves into the latest developments in this field. Published in Neurogastroenterology & Motility, this paper unravels a treasure trove of knowledge.

The paper explores the potential for the endocannabinoid system to exert tonic control over intestinal motility, the role of cannabinoid type-1 (CB1) receptors in diverse aspects of gastric function, visceral pain, inflammation, and sepsis, and the emerging significance of cannabinoid type-2 (CB2) receptors within the gut. Additionally, it delves into the pharmacology of endocannabinoid-related compounds and plant cannabinoids, some of which may not necessarily operate through cannabinoid CB1 and CB2 receptors. These revelations highlight the multifaceted nature of endocannabinoid function in the GI tract, laying the foundation for potential therapeutic advancements in the treatment of GI disorders and ushering in intriguing new hypotheses about the role of endocannabinoids in the gutHere is a brief recap of insights from this seminal paper.

CB1 Receptors: From Motility to Pain and Inflammation

Recent advancements in understanding the endocannabinoid system’s role in gastrointestinal (GI) function have unveiled intriguing insights, primarily focusing on CB1 receptors. These receptors, found in the myenteric plexus neurons across various species, including humans, appear to modulate intestinal motility. Activation of CB1 receptors reduces excitatory enteric transmission, particularly cholinergic transmission, leading to motility inhibition. While some debate surrounds whether endocannabinoids tonically activate CB1 receptors under normal conditions, various studies have suggested this possibility. For instance, CB1 antagonists increase motility, paralleling their stimulation of contractions in the guinea pig ileum. Blockade of endocannabinoid catabolism with selective inhibitors also reduces intestinal and colonic motility. These findings collectively support the idea of an endocannabinoid tone controlling GI motility. However, recent research has introduced the concept of “neutral” CB1 antagonists, which behave differently and have raised questions about the precise nature of CB1 receptor modulation.

Pancreatitis, Irritable Bowel Syndrome, and Septic Ileus: The CB1 Question

Studies have shown that the endocannabinoid system, including both endocannabinoid levels and CB1 receptor expression, is upregulated during various types of experimental small intestine and colon inflammation, as well as in human inflammatory bowel diseases. Recent reports have also highlighted the potential involvement of CB2 receptors in taming colonic inflammation and its impact on motility, suggesting the potential use of non-psychotropic CB2 agonists for treating inflammatory bowel diseases. Pancreatitis has been another focus of investigation, with cannabinoid receptor agonists displaying anti-inflammatory effects in both in vitro and in vivo experiments. However, the timing of administration seems critical, as CB1 activation before inflammation may be protective, whereas its activation afterward might worsen the condition. Additionally, the involvement of TRPV1 channels and TRP channels of ankirin-1 type (TRPA1) has been noted in inflammation, raising complex questions about how cannabinoid receptor activation might affect these channels.

In the context of visceral pain, the endocannabinoid system has been linked to the control of irritable bowel syndrome (IBS). Polymorphisms in the Cnr1 gene encoding CB1 receptors have been associated with IBS in certain populations. Studies have explored the impact of CB1 agonists and antagonists/inverse agonists on sensory GI afferents stimulated by serotonin or bradykinin. These experiments have yielded intriguing and somewhat unexpected results, suggesting the complexity of the relationship between CB1 activation and visceral sensitivity. Another GI disorder, ileus, characterized by decreased intestinal motility following injuries or surgeries, has been examined in mice. The data indicate that anandamide levels increase during ileus, and CB1 receptors become overexpressed in myenteric nerves, contributing to reduced transit. Interestingly, antagonizing CB1 receptors with rimonabant alleviated this condition. In septic ileus, both CB1 and CB2 receptors may play roles, influencing upper GI transit and myoelectrical activity. These findings highlight the intricate balance of cannabinoid receptor activation in different GI disorders.

Gastric Motility

Although initially less explored, the role of the endocannabinoid system in controlling gastric motility has garnered attention in recent studies. Evidence suggests the existence of a CB1 “tone” that regulates gastric emptying, with CB1 activation inhibiting this process. However, the inhibition of gastric motility by CB1 activation does not seem to develop tolerance, as observed with other GI transit functions, posing interesting questions for future research. In contrast, recent research suggests that certain CB1 agonists may affect gastric myoelectric function by reducing the frequency of antral pacemaker activity, demonstrating their potential anti-emetic properties. Nonetheless, the involvement of CB1 receptors in emesis is complex, and more studies are needed to understand their precise role. These studies collectively underscore the intricate relationship between the endocannabinoid system and various aspects of GI function, opening avenues for potential therapeutic applications.

CB2 Receptors: Beyond Inflammation

CB2 receptors have emerged as significant players in the gastrointestinal (GI) tract, with research focusing on their roles in inflammation and other functions. CB2 receptors are activated by elevated endocannabinoid levels in various types of experimental colitis. Mice lacking CB2 receptors display heightened sensitivity to the inflammatory effects of substances like trinitrobenzene sulfonic acid. Recent studies suggest that CB2 receptors have a broader role beyond inflammation control. For instance, a study in mice using the CB2-selective agonist AM1241 found that it could block bradykinin-induced elevation of mesenteric afferent nerve activity, a correlate of small intestine sensitivity. This effect, fully antagonized by a CB2 antagonist and absent in CB2-deficient mice, underscores the potential analgesic and anti-inflammatory properties of CB2 agonists.

While CB2 activation effectively counters motility alterations during inflammatory conditions, it typically does not affect motility in healthy animals. However, recent research has questioned this notion, with one study showing that the CB2 inverse agonist SR144528 can potentiate the inhibitory effect of the CB1/CB2 agonist WIN55,212-2 on gastric emptying. Surprisingly, this effect also delayed emptying of the small intestine, cecum, and colon. This raises questions about the role of CB2 receptors in gastric motility and their potential for developing weight loss-inducing drugs. It’s worth noting that CB2-deficient mice resist weight gain on a high-fat diet, suggesting a connection between CB2 receptors and weight regulation.

Endocannabinoid-Related Molecules and Phytocannabinoids: Novel Mechanisms

Beyond CB1 and CB2 receptors, there’s growing interest in endocannabinoid-related molecules and phytocannabinoids’ effects in the GI tract. Oleoylethanolamide (OEA), an anorexigen mediator, inhibits small intestine motility. Recent studies using mice lacking CB1, CB2, or PPAR-α receptors suggest that OEA’s effect is not primarily mediated by these receptors, despite PPAR-α receptor presence in the myenteric plexus. OEA also inhibits gastric transit, potentially contributing to satiety and weight loss, although more research is needed to confirm these effects.

Cannabidiol (CBD), a non-THC cannabinoid from Cannabis, shows potential therapeutic properties in the GI tract. It has low affinity for CB1 and CB2 receptors but indirectly activates CB1 receptors, reproducing some effects of selective inhibitors of anandamide hydrolysis or reuptake. Cannabidiol reverses small intestine hypermotility induced by croton oil but worsens hypomotility induced by LPS. It also produces anti-inflammatory effects in colitis models. Additionally, other plant-derived natural products with similarities to cannabinoids are under investigation for their GI effects, such as salvinorin A (SA), a κ-opioid receptor agonist. SA’s influence on GI transit appears to involve both KOR and CB1 receptors, despite its low affinity for CB1 and CB2 receptors. This suggests functional cross-talk between these receptors, raising intriguing possibilities for future research.

These studies collectively highlight the multifaceted role of CB2 receptors, endocannabinoid-related molecules, and phytocannabinoids in the GI tract, offering potential avenues for therapeutic interventions beyond traditional cannabinoid receptor targets.

Conclusion

The research into the endocannabinoid system’s role in the gastrointestinal tract is shedding light on a complex and multifaceted network of interactions. While CB1 and CB2 receptors play central roles in regulating motility, inflammation, and pain, the exploration of endocannabinoid-related molecules and phytocannabinoids adds intriguing layers to this story.

CB2 receptors, initially known for their involvement in inflammation, are now recognized for their potential impact on pain modulation and sensitivity. Recent findings suggest that CB2 agonists may hold promise as analgesics in conditions like inflammatory bowel disease.

Moreover, endocannabinoid-related molecules like oleoylethanolamide (OEA) and plant-derived compounds like cannabidiol (CBD) demonstrate diverse effects on motility and inflammation. OEA’s potential for weight management and satiety is particularly intriguing. CBD, with its indirect activation of CB1 receptors and anti-inflammatory properties, holds promise for a wide range of GI disorders.

The inclusion of phytocannabinoids like salvinorin A (SA) further underscores the complexity of the endocannabinoid system. SA’s actions on GI transit, involving both KOR and CB1 receptors, hint at exciting possibilities for future research into receptor cross-talk.

In this rapidly evolving field, the potential therapeutic applications of the endocannabinoid system in gastrointestinal health are vast. These discoveries may lead to innovative treatments and interventions for conditions ranging from inflammatory bowel diseases to pain management and weight control. As we continue to unravel the mysteries of the gut’s endocannabinoid system, it is clear that exciting developments lie ahead.