574 A.A. Izzo and A.A. Coutts
nal motility and secretion is due to reduced acetylcholine release from enteric
nerves. Conversely, endocannabinoids stimulate intestinal primary sensory neu-
rons via the vanilloid VR1 receptor, resulting in enteritis and enhanced motility.
The endogenous cannabinoid system has been found to be involved in the physi-
ological control of colonic motility and in some pathophysiological states, includ-
ing paralytic ileus, intestinal inflammation and cholera toxin-induced diarrhoea.
Cannabinoids also possess antiemetic effects mediated by activation of central
and peripheral CB 1 receptors. Pharmacological modulation of the endogenous
cannabinoid system could provide a new therapeutic target for the treatment of
a number of gastrointestinal diseases, including nausea and vomiting, gastric ul-
cers, secretory diarrhoea, paralytic ileus, inflammatory bowel disease, colon cancer
and gastro-oesophageal reflux conditions.
KeywordsCannabinoid receptors · Intestinal motility · Intestinal secretion ·
Emesis · Intestinal inflammation · Feeding
1
Introduction
Preparations ofCannabis sativa(Indian hemp) have been used medicinally for the
treatment of a variety of gastrointestinal disorders, including gastrointestinal pain,
flatulence, gastroenteritis, Crohn’s disease, diarrhoea and diabetic gastroparesis
(Di Carlo and Izzo 2003). The main psychotropic constituent ofCannabis sativais
∆^9 -tetrahydrocannabinol (∆^9 -THC), which exerts its biological effects mainly by
activating two G protein-coupled cannabinoid receptors (Pertwee and Ross 2002).
These are CB 1 receptors, present in central and peripheral nerves, including the
enteric nervous system, and CB 2 receptors, expressed mainly in immune cells.
A general feature of CB 1 activation is the reduction of the release of a variety
of neurotransmitters (e.g. acetylcholine from enteric nerves), whereas there is
currently no evidence for a role for CB 2 receptors in the gastrointestinal (GI) tract
(Di Carlo and Izzo 2003). Endogenous ligands for the cannabinoid receptors have
been identified, the best-known being anandamide, 2-arachidonoyl glycerol (2-
AG) (non-selective cannabinoid receptor agonists), noladin ether (CB 1 receptor
agonist) and virodhamine (CB 1 receptor antagonist/CB 2 receptor agonist) (De
Petrocellis et al. 2004). When released, anandamide and 2-AG are removed from
extracellular compartments by a carrier-mediated re-uptake process. Once within
thecell,endocannabinoidsarehydrolysedbytheenzymefattyacidamidehydrolase
(FAAH, also named anandamide amidohydrolase) (Sugiura et al. 2002). Also, 2-
AG has been shown to be degraded by monoglyceride lipase (monoacyl glycerol
lipase). Both FAAH and monoglyceride lipase have been demonstrated in the
intestine (Oleinik 1995; Katayama et al. 1997). In addition to the two cannabinoid
receptors, anandamide and 2-AG can also activate transient receptor potential
vanilloid subtype 1 (VR1, also known as TRPV1) receptors, the molecular target
for the pungent plant compound capsaicin (Zygmunt et al. 1999). Cannabinoid
receptors, their endogenous ligands (endocannabinoids) and the proteins involved