British Journal of Pharmacology (2007) 150, 673–675
& 2007 Nature Publishing Group All rights reserved 0007–1188/07 $30.00
www.brjpharmacol.org
COMMENTARY
Inhibitors of monoacylglycerol lipase as novel
analgesics
AG Hohmann
Neuroscience and Behavior Program, Department of Psychology, University of Georgia, Athens, GA, USA
2-Arachidonoylglycerol (2-AG) is an endogenous cannabinoid (endocannabinoid) lipid whose functions remain poorly
understood. Guindon and colleagues report the novel finding that exogenous application of 2-AG induces peripheral
antinociceptive effects that are mediated, at least in part, by actions at peripheral cannabinoid CB2 receptors. URB602, a
recently described inhibitor of monoacylglycerol lipase, an enzyme that catalyzes 2-AG hydrolysis in vivo, also induced
peripheral antinociceptive effects and enhanced the actions of 2-AG. Peripheral analgesic mechanisms represent promising
therapeutic targets for suppressing pain in the absence of unwanted central nervous system side-effects (e.g. psychoactivity)
associated with activation of central CB1 receptors. The therapeutic potential of inhibitors of 2-AG deactivation for the
treatment of inflammatory pain is discussed.
British Journal of Pharmacology (2007) 150, 673–675. doi:10.1038/sj.bjp.0707153; published online 12 February 2007
Keywords: analgesia; 2-AG, anandamide; cannabinoid, CB2; endocannabinoid; hyperalgesia; inflammation; monoacylglycerol
lipase; peripheral
Abbreviations: 2-AG, 2-arachidonoylglycerol; CNS, Central nervous system; DGL, diacylglycerol lipase; FAAH, fatty-acid amide
hydrolase; MGL, monoacylglycerol lipase
Activation of cannabinoid CB1 receptors
receptors that
share the same target as D9-tetrahydrocannabinol, the active
ingredient in cannabis
suppresses nociceptive transmission and pain behavior in rodent subjects (for review see
Walker and Hohmann (2005)). Nonetheless, psychoactive
effects also accompany direct activation of CB1 receptors and
severely constrain the therapeutic potential of direct acting
cannabinoid agonists. Recently, inhibitors of the deactivation of endogenous cannabinoid lipids (endocannabinoids)
such as anandamide and 2-arachidonoylglycerol (2-AG) have
been described. Inhibitors of fatty-acid amide hydrolase
(FAAH), an enzyme which catalyzes anandamide hydrolysis
(Cravatt et al., 1996), have been evaluated for their
therapeutic potential for suppressing pain, anxiety and
stress-related responses in preclinical studies (Kathuria
et al., 2003; Hohmann et al., 2005). FAAH inhibitors produce
their pharmacological effects by increasing levels of anandamide (and other fatty acid amides), thereby indirectly
activating cannabinoid receptors, and produce a more
circumscribed spectrum of biological effects compared to
direct acting CB1 agonists (Cravatt and Lichtman, 2003;
Piomelli, 2005). The development of selective FAAH inhibitors, together with the generation of mutant mice lacking
Correspondence: Dr AG Hohmann, Neuroscience and Behavior Program,
Department of Psychology, University of Georgia, Athens, GA 30602-3013, USA.
E-mail: ahohmann@uga.edu
Received 26 September 2006; accepted 22 December 2006; published online
12 February 2007
the FAAH gene, has considerably broadened our understanding of the physiological roles of anandamide in the
nervous system. By contrast, the functional roles of 2-AG
in the nervous system are only beginning to be explored.
Although FAAH has been shown to catalyze hydrolysis
of 2-AG in vitro (Goparaju et al., 1999), a distinct enzyme,
monoacylglycerol lipase (MGL) plays the predominant role
in catalyzing 2-AG hydrolysis in vivo (Dinh et al., 2002, 2004;
Hohmann et al., 2005). Recently, we have described the
development of the first selective pharmacological inhibitor
of MGL, URB602 (Hohmann et al., 2005). URB602 has been
shown to increase levels of 2-AG without altering levels of
anandamide both in vitro and in vivo (Hohmann et al., 2005),
consistent with the selectivity of this compound in inhibiting cytosolic MGL relative to membrane FAAH activity. This
compound induced a CB1-mediated enhancement in endocannabinoid-mediated stress-induced analgesia following
local administration into either the periaqueductal gray
(PAG) or lumbar dorsal horn (Hohmann et al., 2005; Suplita
et al., 2006). The URB602-induced enhancement of stress
antinociception was associated with a profound increase
in levels of 2-AG, but not anandamide, in the PAG. These
studies support the existence of a physiological role for 2-AG
in suppressing pain and raise the possibility that novel
therapeutic interventions may inhibit MGL to suppress pain
by increasing the bioavailability of endogenous 2-AG.
However, the efficacy of MGL inhibition in suppressing
chronic pain states has yet to be evaluated.
674
Commentary
AG Hohmann
The present report by Guindon and co-workers (Guindon
et al., 2006) demonstrates that exogenous 2-AG induces
peripheral antinociceptive effects in a tissue injury model of
persistent pain, the formalin test. In this study, 2-AG, applied
locally in the rat hind paw, suppressed formalin-evoked pain
behavior during both the early and the late phase of formalin
pain. 2-AG may induce peripheral antinociceptive effects by
specifically suppressing formalin-evoked primary afferent
activation and subsequent central nervous system (CNS)
sensitization. More work is necessary to demonstrate that
endogenous 2-AG is mobilized in the periphery under
physiological conditions to suppress inflammatory nociception. In support of this hypothesis, the MGL inhibitor
URB602, administered locally to the paw, induced antinociception in this model and enhanced the antinociceptive
effects of exogenously applied 2-AG, thereby demonstrating
the efficacy of an MGL inhibitor in suppressing inflammatory nociception. The observed antinociceptive effects were
not secondary to a suppression of formalin-evoked edema.
Mediation by cannabinoid receptors was demonstrated by
the observation that selective antagonists for either CB1
(AM251) or CB2 (AM630) receptors blocked URB602-induced
antinociception. By contrast, exogenous 2-AG produced
peripheral antinociceptive effects that were blocked by the
CB2 but not the CB1 antagonist. It is possible that inhibition
of MGL with URB602 elevated levels of other monoglycerides, which do not activate cannabinoid receptors in
addition to elevating levels of 2-AG. Non-endocannabinoid
monoglycerides would be expected to compete with endogenous 2-AG for MGL-mediated hydrolysis; such ‘entourage
effects’ (Ben-Shabat et al., 1998) could effectively enhance
actions of endocannabinoids at CB1 receptors, producing
a peripheral antinociceptive effect susceptible to blockade
by AM251. Alternatively, a CB1-mediated blockade of locally
administered 2-AG may not have been detected with the
relatively modest sample sizes employed. More work is
necessary to determine whether the doses of URB602 that
suppressed formalin-evoked pain behavior also increased
accumulation of endogenous 2-AG under the conditions of
testing. It remains to be determined whether URB602
produces its pharmacological effects in vivo by targeting the
cloned form of MGL rather than another as yet uncharacterized 2-AG hydrolyzing enzyme. Guindon and co-workers
demonstrate that local administration of either the CB1 or
CB2 antagonist alone failed to induce hyperalgesia, suggesting that endocannabinoids do not act tonically in the
periphery to dampen sensitivity to pain.
The endocannabinoid 2-AG is believed to be synthesized
and released upon demand through a process initiated by
activity-dependent or receptor-stimulated cleavage of membrane phospholipid precursors (for review see Piomelli,
(2003)). In brain, endocannabinoids may function as retrograde messengers to modulate neuronal physiology at
presynaptic CB1 receptors (Wilson and Nicoll, 2001, 2002)
In vitro electrophysiological studies and studies in cell
culture suggest that 2-AG may be formed by the consecutive
activation of two distinct enzymes (Chevaleyre and Castillo,
2003; Jung et al., 2005). First, phospholipase C catalyzes
formation of the 2-AG precursor 1,2-diacylglycerol (DAG)
from membrane phosphoinositides. Second, diacylglycerol
British Journal of Pharmacology (2007) 150 673–675
lipase (DGL) catalyzes hydrolysis of DAG to generate 2-AG.
More work is necessary to demonstrate that these same
mechanisms control 2-AG formation under physiological
conditions and to identify the specific enzyme isoforms and
cell types implicated in this process. A better understanding
of the mechanisms responsible for 2-AG formation and
deactivation may identify novel molecular targets that may
be exploited for the treatment of pain in humans. Work by
Guindon and co-workers suggest, for the first time, that an
MGL inhibitor shows significant therapeutic potential for
the treatment of inflammatory pain.
Acknowledgements
AGH is supported by DA14265, DA14022.
Conflict of Interest
The author is a coinventor on a patent filed by Daniele
Piomelli at the University of California at Irvine.
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