Review
2-Arachidonoylglycerol and the cannabinoid receptors

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Abstract

2-Arachidonoylglycerol (2-AG) is a unique molecular species of monoacylglycerol isolated from rat brain and canine gut as an endogenous cannabinoid receptor ligand (Sugiura, T., Kondo, S., Sukagawa, A., Nakane, S., Shinoda, A., Itoh, K., Yamashita, A., Waku, K., 1995. 2-Arachidonoylglycerol: a possible endogenous cannabinoid receptor ligand in brain. Biochem. Biophys. Res. Commun. 215, 89–97; Mechoulam, R., Ben-Shabat, S., Hanus, L., Ligumsky, M., Kaminski, N.E., Schatz, A.R., Gopher, A., Almog, S., Martin, B.R., Compton, D.R., Pertwee, R.G., Giffin, G., Bayewitch, M., Brag, J., Vogel, Z., 1995. Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochem. Pharmacol. 50, 83–90). 2-AG binds to the cannabinoid receptors (CB1 and CB2) and exhibits a variety of cannabimimetic activities in vitro and in vivo. Recently, we found that 2-AG induces Ca2+ transients in NG108-15 cells, which express the CB1 receptor, and in HL-60 cells, which express the CB2 receptor, through a cannabinoid receptor- and Gi/Go-dependent mechanism. Based on the results of structure-activity relationship experiments, we concluded that 2-AG but not anandamide is the natural ligand for both the CB1 and the CB2 receptors and both receptors are primarily 2-AG receptors. Evidences are gradually accumulating that 2-AG is a physiologically essential molecule, although further detailed studies appear to be necessary to determine relative importance of 2-AG and anandamide in various animal tissues. In this review, we described mainly our previous and current experimental results, as well as those of others, concerning the tissue levels, bioactions and metabolism of 2-AG.

Section snippets

Cannabinoid receptors

Δ9-Tetrahydrocannabinol (Δ9-THC), a major psychoactive ingredient of marijuana, possesses a variety of pharmacological activities. When administered orally or intravenously, Δ9-THC induces diverse biological responses such as reduced spontaneous motor activity, immobility, analgesia, heightened sensory awareness, euphoria, hypothermia and impairment of short-term memory (Dewey, 1986). Δ9-THC is also known to exert profound effects on several biological systems other than the central nervous

Identification of 2-AG as a cannabinoid receptor ligand

The discovery of the specific receptors for cannabinoids stimulated the search for endogenous ligand(s). Devane et al. (1992) isolated N-arachidonoylethanolamine (anandamide) (Fig. 1) from porcine brain as the first endogenous cannabinoid receptor ligand. They demonstrated that anandamide exhibits several cannabimimetic activities in vitro and in vivo. Anandamide appears to be an important lipid mediator in the nervous system, as well as in other systems (Mechoulam and Fride, 1995, Mechoulam et

Biological activities of 2-AG

About 5 years have passed since the discovery of 2-AG as an endogenous cannabinoid receptor ligand. Nonetheless, available information concerning 2-AG is still limited as compared with the case of anandamide (Di Marzo, 1998a, Di Marzo, 1998b, Di Marzo and Deutsch, 1998, Di Marzo et al., 1998a, Di Marzo et al., 1999a, Mechoulam et al., 1998a, Piomelli et al., 1998, Sugiura et al., 1998b). As mentioned above, 2-AG binds to the central and peripheral cannabinoid receptors (CB1 and CB2, Mechoulam

Structure-activity relationship of 2-AG and related compunds as CB1 receptor agonists

Various cannabinoid receptor ligands including 2-AG induce Ca2+ transients in NG108-15 cells through a cannabinoid receptor-dependent mechanism (Sugiura et al., 1996b). Because such a cellular response can be detected immediately after the addition of the ligands to the cells, and because the method employed is sensitive enough to detect even small responses induced by low concentrations of cannabinoid receptor agonists such as 2-AG, measurement of [Ca2+]i is a useful tool in evaluating the

Structure-activity relationship of 2-AG and related compounds as CB2 receptor agonists

We then investigated the structure–activity relationship of the CB2 receptor ligands employing HL-60 cells, which express the CB2 receptor (Sugiura et al., 2000b). As described before, we found that 2-AG induces rapid transient elevation of [Ca2+]i in HL-60 cells through a cannabinoid CB2 receptor-dependent mechanism. The response was detectable at 1 nM and was augmented with increasing concentration of 2-AG. Two types of positional isomers of 2-AG, 1-AG and 3-AG, also exhibited appreciable

Tissue levels of 2-AG

Not much information was available concerning the tissue levels of 2-AG until recently. Mechoulam et al. (1995) detected 2-AG in canine gut, although they did not estimate its level. We detected 3.25 nmol/g tissue of AG (Sugiura et al., 1995) or 3.36 nmol/g tissue of 2-AG (Kondo et al., 1998), and Stella et al. (1997) detected 4.0 nmol/g tissue of 2-AG in rat brain. Recently, we further estimated the levels of 2-AG in several rat tissues. The levels of 2-AG in rat liver, spleen, lung, kidney

Biosynthesis and degradation of 2-AG

It was 17 years ago that the generation of AG in thrombin-stimulated platelets was mentioned in the literature (Prescott and Majerus, 1983). Later, the generation of AG in platelet-derived growth factor-stimulated Swiss 3T3 cells (Hasegawa-Sasaki, 1985) and in bradykinin-stimulated rat dorsal ganglion neurons (Gammon et al., 1989) was reported. However, at that time, physiological significance of AG or 2-AG as an endogenous cannabinoid receptor ligand was unknown. Stimulus-induced generation of

Possible physiological roles of 2-AG

What, then, is the physiological role(s) of 2-AG in the nervous system? It seems unlikely that the physiological compound 2-AG induces psychedelic reactions such as heightened sensory awareness, dissociation of ideas, errors in judgement of time and space, illusions and hallucinations in living animals, such as are very often observed with Δ9-THC. Δ9-THC is a partial agonist toward the CB1 receptor and is metabolically rather stable, in contrast to 2-AG; Δ9-THC may interfere with the action of

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