Lipoxin A4 is an allosteric endocannabinoid that strengthens anandamide-induced CB1 receptor activation.

A major advance in the field of cannabinoid research was the discovery of the endocannabinoid system, which is currently thought to consist of two G proteincoupled receptors (cannabinoid CB1 and CB2 receptors) and endogenous compounds such as arachidonoylethanolamide (i.e., anandamide; AEA; Fig. 1) and 2-arachidonoyl glycerol (2-AG) that can activate these receptors and are known as endocannabinoids (1). This system of receptors and endogenous agonists, which is also made up of enzymes that catalyze endocannabinoid biosynthesis or metabolic degradation, and of processes responsible for the cellular uptake of endocannabinoids, is thought to have numerous roles in both health and disease (2, 3). Some of these are “autoprotective” in nature and hence beneficial, with examples including the amelioration of inflammatory pain, multiple sclerosis, and Parkinson disease; whereas a few of its other roles, for example, in obesity, are “autoimpairing,” and therefore unwanted. AEA, 2-AG, and other “direct” cannabinoid receptor agonists are thought to trigger G proteinmediated signaling of CB1 and CB2 receptors by targeting orthosteric sites on these receptors (1). There is evidence, however, that the CB1 receptor also contains one or more “allosteric” sites that can be targeted by allosteric modulators in a manner that can enhance or reduce the efficacy with which direct agonists activate this receptor orthosterically (4–7). Just as the discovery of the CB1 receptor prompted a search for endogenous ligands for this receptor (8), so too the discovery that CB1 receptors contain allosteric sites has prompted a need to look for an endogenous CB1 allosteric modulator. This need has now been met by Pamplona et al. (9), who, in PNAS, present evidence that the endogenous anti-inflammatory ligand lipoxin A4 (LXA4; Fig. 1) can allosterically enhance AEA-induced activation of CB1 receptors within the brain when it is administered exogenously and when it is produced endogenously. This is a ligand that is already known to target the FPR2/ALX receptor as an agonist, mainly outside the brain, and, like AEA and 2-AG, to be an eicosanoid that is formed from arachidonic acid (10–12). In their PNAS paper, Pamplona et al. (9) present data showing that, when administered to mice intracerebroventricularly at doses of 0.1 and 1 pmol, LXA4 can act in an FPR2/ALX receptor-independent manner to produce a set of four effects: hypolocomotion, catalepsy, hypothermia, and antinociception in a hotplate test. These effects of LXA4 all appeared to be CB1 receptor-mediated because they (i) could be prevented by the CB1-selective antagonist/inverse agonist SR141716A, (ii) were not detectable in mice from which the cannabinoid CB1 receptor had been genetically deleted, and (iii) are known to be induced by established CB1 receptor agonists (1). Importantly, the results obtained in this investigation (9) also suggest that LXA4 did not induce this “tetrad” of effects through direct activation of the CB1 receptor, as it did not share the well known ability of established CB1 receptor agonists to produce a complete displacement of [H]SR141716A from specific binding sites in mouse brain membranes, or to inhibit forskolin-induced stimulation of cAMP production by mouse CB1-transfected HEK cells. Instead, it most likely acted by potentiating the activation of CB1 receptors by AEA, as (i) intracerebroventricular injections of doses of AEA and LXA4 that were subeffective by themselves produced catalepsy in mice when they were coadministered, (ii) LXA4 produced a marked leftward shift in the log concentration–response curve of AEA for its inhibition of forskolin-induced stimulation of cAMP production by mouse CB1-HEK cells, and (iii) LXA4 also augmented AEA-induced increases in inward K currents in CB1 receptor-containing Xenopus laevis oocytes. Pamplona et al. (9) also find that, at a concentration at which LXA4 potentiated AEA in vitro (100 nM), it also slows the dissociation of [H]CP55940 from specific binding sites in mouse brain membranes, which is widely accepted to be a strong indication of ligand-induced allosteric modulation (13). Other experiments that Pamplona et al. (9) perform show that LXA4 is present in significant amounts in mouse hippocampus, cortex, and cerebellum. Consequently, they postulate that AEA is potentiated by LXA4 not only when this lipoxin is administered exogenously but also when it has been produced endogenously. This hypothesis is supported by their findings, first, that intracerebroventricularly injected AEA produces much less catalepsy in mice from which the LXA4-synthesizing enzyme 5-lipoxygenase has been genetically deleted than in WT mice, and, second, that this effect of AEA can also be attenuated by the 5-lipoxygenase inhibitor MK-886. Because, when administered alone, LXA4 produces behavioral effects in mice that appear to be mediated by CB1 receptors, it is also likely that it can increase the ability of endogenously released AEA to activate these receptors. It has long been known that AEAinduced activation of CB1 receptors can also be enhanced by drugs that inhibit its metabolism by fatty acid amide hydrolase (FAAH) (3). However, there are three important differences between the ways in which an allosteric enhancer and an FAAH inhibitor increase the activation of the CB1 receptor by endogenously released AEA (Fig. 1). First, an FAAH inhibitor will produce such an increase by elevating the concentration of AEA at the CB1 receptor, whereas an allosteric enhancer will produce it by increasing the potency and/or efficacy with which AEA Fig. 1. Upper: Structures of LXA4 and AEA. Lower: Comparison of how an FAAH inhibitor (FAAH-I) and a CB1 receptor allosteric enhancer could affect the potency, efficacy, and selectivity with which endogenously released AEA targets cannabinoid CB1 receptors in the brain. Further details are provided in the text.