Joseph S. Davison

Effects of cannabinoid receptor-2 activation on accelerated gastrointestinal transit in lipopolysaccharide-treated rats.

The biological effects of cannabinoids (CB) are mediated by CB1 and CB2 receptors. The role of CB2 receptors in the gastrointestinal tract is uncertain. In this study, we examined whether CB2 receptor activation is involved in the regulation of gastrointestinal transit in rats. Basal and lipopolysaccharide (LPS)‐stimulated gastrointestinal transit was measured after instillation of an Evans blue‐gum Arabic suspension into the stomach, in the presence of specific CB1 and CB2 agonists and antagonists, or after treatment with inhibitors of mediators implicated in the transit process. In control rats a CB1 (ACEA; 1 mg kg−1), but not a CB2 (JWH‐133; 1 mg kg−1), receptor agonist inhibited basal gastrointestinal transit. The effects of the CB1 agonist were reversed by the CB1 antagonist AM‐251, which alone increased basal transit. LPS treatment increased gastrointestinal transit. This increased transit was reduced to control values by the CB2, but not the CB1, agonist. This inhibition by the CB2 agonist was dose dependent and prevented by a selective CB2 antagonist (AM‐630; 1 mg kg−1). By evaluating the inhibition of LPS‐enhanced gastrointestinal transit by different antagonists, the effects of the CB2 agonist (JWH‐133; 1 mg kg−1) were found to act via cyclooxygenase, and to act independently of inducible nitric oxide synthase (NOS) and platelet‐activating factor. Interleukin‐1β and constitutive NOS isoforms may be involved in the accelerated LPS transit. The activation of CB2 receptors in response to LPS is a mechanism for the re‐establishment of normal gastrointestinal transit after an inflammatory stimulus.

Arvanil, anandamide and N‐arachidonoyl‐dopamine (NADA) inhibit emesis through cannabinoid CB1 and vanilloid TRPV1 receptors in the ferret

Cannabinoid (CB) agonists suppress nausea and vomiting (emesis). Similarly, transient receptor potential vanilloid‐1 (TRPV1) receptor agonists are anti‐emetic. Arvanil, N‐(3‐methoxy‐4‐hydroxy‐benzyl)‐arachidonamide, is a synthetic ‘hybrid’ agonist of CB1 and TRPV1 receptors. Anandamide and N‐arachidonoyl‐dopamine (NADA) are endogenous agonists at both these receptors. We investigated if arvanil, NADA and anandamide were anti‐emetic in the ferret and their mechanism of action. All compounds reduced the episodes of emesis in response to morphine 6 glucuronide. These effects were attenuated by AM251, a CB1 antagonist that was pro‐emetic per se, and TRPV1 antagonists iodoresiniferatoxin and AMG 9810, which were without pro‐emetic effects. Similar sensitivity to arvanil and NADA was found for prodromal signs of emesis. We analysed the distribution of TRPV1 receptors in the ferret brainstem and, for comparison, the co‐localization of CB1 and TRPV1 receptors in the mouse brainstem. TRPV1 immunoreactivity was largely restricted to the nucleus of the solitary tract of the ferret, with faint labeling in the dorsal motor nucleus of the vagus and sparse distribution in the area postrema. A similar distribution of TRPV1, and its extensive co‐localization with CB1, was observed in the mouse. Our findings suggest that CB1 and TRPV1 receptors in the brainstem play a major role in the control of emesis by agonists of these two receptors. While there appears to be an endogenous ‘tone’ of CB1 receptors inhibiting emesis, this does not seem to be the case for TRPV1 receptors, indicating that endogenously released endocannabinoids/endovanilloids inhibit emesis preferentially via CB1 receptors.

Review article: endocannabinoids and their receptors in the enteric nervous system.

The therapeutic actions of cannabinoids have been known for centuries. In the last 25 years this area of research has grown exponentially with the discovery of specific cannabinoid receptors and endogenous ligands.

Cannabinoid CB2 receptors in the enteric nervous system modulate gastrointestinal contractility in lipopolysaccharide-treated rats

Enhanced intestinal transit due to lipopolysaccharide (LPS) is reversed by cannabinoid (CB)2 receptor agonists in vivo, but the site and mechanism of action are unknown. We have tested the hypothesis that CB2 receptors are expressed in the enteric nervous system and are activated in pathophysiological conditions. Tissues from either saline- or LPS-treated (2 h; 65 microg/kg ip) rats were processed for RT-PCR, Western blotting, and immunohistochemistry or were mounted in organ baths where electrical field stimulation was applied in the presence or absence of CB receptor agonists. Whereas the CB2 receptor agonist JWH133 did not affect the electrically evoked twitch response of the ileum under basal conditions, in the LPS-treated tissues JWH133 was able to reduce the enhanced contractile response in a concentration-dependent manner. Rat ileum expressed CB2 receptor mRNA and protein under physiological conditions, and this expression was not affected by LPS treatment. In the myenteric plexus, CB2 receptors were expressed on the majority of neurons, although not on those expressing nitric oxide synthase. LPS did not alter the distribution of CB2 receptor expression in the myenteric plexus. In vivo LPS treatment significantly increased Fos expression in both enteric glia and neurons. This enhanced expression was significantly attenuated by JWH133, whose action was reversed by the CB2 receptor antagonist AM630. Taking these facts together, we conclude that activation of CB2 receptors in the enteric nervous system of the gastrointestinal tract dampens endotoxin-induced enhanced intestinal contractility.

Neuroendocrine regulation of inflammation and tissue repair by submandibular gland factors

Interactions between the immune, nervous and endocrine systems are important in inflammation and tissue repair. One neuroendocrine pathway involves polypeptide factors derived from the submandibular glands, whose synthesis and release are controlled by cervical sympathetic nerves. This novel pathway of immune-neuroendocrine communication is the cervical sympathetic trunk-submandibular gland (CST-SMG) axis. Here, Ronald Mathison, Joseph Davison and Dean Befus discuss the contributions of this axis to the neuroendocrine regulation of inflammation and tissue repair.

Evidence for substance P as a functional neurotransmitter in guinea pig small intestinal mucosa

We showed previously that electrical transmural stimulation (TS) of guinea pig jejunal mucosa in vitro released neurotransmitters from submucosal plexus neurons which caused alterations in ion transport. The present studies were performed to obtain information regarding the identity of the neurotransmitters. The addition of exogenous substance P (SP) to the serosal side of the tissue caused a transient increase in short-circuit current (Isc) which closely mimicked the response to TS. Both TS and SP caused net secretion of Cl- ions by stimulating movement toward the luminal side. Tetrodotoxin abolished the response to TS, inhibited approximately 70% of the response to SP but did not affect the response to urecholine, a cholinergic muscarinic agonist. In the presence of the muscarinic antagonist, atropine, Isc responses to both TS and SP were reduced suggesting that a portion of both responses was due to action on enteric nerves causing release of acetylcholine. Following desensitization of the tissue with supramaximal doses of SP the response to TS was significantly reduced but the response to urecholine was unchanged. In the presence of atropine, SP desensitization reduced the nerve-stimulated response by approximately 65%; treatment of tissue with SP antibodies reduced the response by approximately 55%. Under the same conditions Isc responses to histamine were unaltered. Our results suggest that both SP (or a structurally analogous neurotransmitter) and acetylcholine as well as additional unidentified neurotransmitter(s) are functionally important in the regulation of intestinal ion transport in guinea pig jejunum.

Cannabinoid CB2 Receptors in Health and Disease

Marijuana has been used for thousands of years to affect human health. Dissecting the peripheral effects from the central psychotropic effects has revealed a complex interplay between cannabinoids, endocannabinoids and their receptors. This review examines recent advances in understanding the expression, regulation and utilization of the CB2 receptor. Here we highlight the molecular aspects of the CB2 receptor, CB2 receptor signaling and new ligands for this receptor. We focus in the rest of the review on recent findings in the immune system, the gastrointestinal tract and liver, the brain and the cardiovascular system and airways as examples of areas where new developments in our understanding of the CB2 receptor have occurred. Early studies focused on expression of this receptor under baseline physiologic conditions; however, perturbations such as those that occur during inflammation, ischemia/reperfusion injury and cancer are revealing a critical role for the CB2 receptor in regulating these disease processes amongst others. As a result, the CB2 receptor is an appealing therapeutic target as well as a useful tool for shedding new light on physiological regulatory processes throughout the body.

The cholecystokinin antagonist, proglumide, increases food intake in the rat

Cholecystokinin, secreted in response to ingested food entering the duodenum, may play a role in limiting food intake. Inhibition of cholecystokinin should therefore induce an increase in food intake. Proglumide, a specific antagonist of cholecystokinin was used to block the satiety effect of a food preload in rats. A significant increase in food intake was obtained following proglumide injection, thus supporting the hypothesis that cholecystokinin, released by food in the duodenum, acts as a short-term satiety factor.

Involvement of L-arginine-nitric oxide pathways in neural relaxation of the sphincter of Oddi.

To evaluate if L-arginine-nitric oxide-pathways are involved in the neural relaxation of the sphincter of Oddi, we studied the effect of nitric oxide synthase inhibition on electrical field stimulation-induced relaxation of the sphincter of Oddi in the guinea pig in vitro. After incubation with atropine (1 microM), phentolamine (1 microM) and propranolol (1 microM), histamine (50 microM) and cholecystokinin-octapeptide (25 nM) produced similar increases in sphincter tone. Subsequent field stimulation induced sphincteric relaxation, that was significantly greater when the initial tone had been raised by cholecystokinin (5 Hz, 59 +/- 9%; 10 Hz, 79 +/- 9%) compared to histamine (5 Hz, 27 +/- 3%; 10 Hz, 40 +/- 7%). N-omega-Nitro-L-arginine methyl ester (L-NAME, 100 microM), which competitively inhibits nitric oxide synthase, markedly suppressed this relaxation. The subsequent addition of L-arginine (1 mM), but not D-arginine (1 mM), restored the relaxation. Hexamethonium (100 microM) did not affect the relaxation, but tetrodotoxin (1 microM) completely abolished it. Sodium nitroprusside caused a dose-dependent relaxation of the sphincter (ED50 13 nM), which was unaffected by L-NAME. In conclusion, endogenous nitric oxide synthase products represent a major transmitter of non-adrenergic non-cholinergic relaxation of the sphincter of Oddi in the guinea pig. This relaxation is partially facilitated by cholecystokinin.

Injection of neuropeptide Y into the paraventricular nucleus of the hypothalamus inhibits gastric acid secretion in the rat

Neuropeptide Y (NPY) was injected into the paraventricular nucleus (PVN) of the hypothalamus of anesthetized rats in order to assess its effect on gastric acid secretion. NPY evoked a dose-dependent decrease of interdigestive gastric acid output when injected directly into the PVN or immediately ventral to it. Intracerebroventricular NPY and saline injections did not alter acid output. Injection of NPY into adjacent non-PVN hypothalamic areas resulted in either an elevated acid output or had no effect depending on the site of injection. Mean arterial blood pressure and heart rate were not consistently affected by NPY. These results show that injection of NPY into the PVN of anesthetized rats inhibits interdigestive gastric acid output in a dose-dependent manner.