Cannabinoids produce a plethora of biological effects, including the modulation of neuronal activity through the activation of CB₁ receptors and of immune responses through the activation of CB₂ receptors. The selective targeting of either of these two receptor subtypes has clear therapeutic value. Recent evidence indicates that some of the cannabinomimetic effects previously thought to be produced through CB₁ and/or CB₂ receptors, be they on neuronal activity, on the vasculature tone or immune responses, still persist despite the pharmacological blockade or genetic ablation of CB₁ and/or CB₂ receptors. This suggests that additional cannabinoid and cannabinoid-like receptors exist. Here we will review this evidence in the context of their therapeutic value and discuss their true belonging to the endocannabinoid signaling system.

a.) Novel cannabinoid receptors expressed by endothelial cells

Cardiovascular disease is the leading cause of death in the U.S. and over 80 million adults currently suffer from this devastating illness as quoted by the American Heart Association. With a growing body of evidence supporting the therapeutic effects of cannabinoids acting on the vasculature, targeting the eCBSS clearly constitutes a promising option for the next generation of cardiovascular therapeutics (Pacher et al. 2005; Pacher et al. 2008; Randall et al. 2002). Within this context, there is convincing and very exciting evidence for a novel receptor engaged by cannabinoids to regulate vasodilation (Kunos et al. 2002).

The initial non-CB₁/CB₂ mediated cannabinoid effect shown to modulate the vasculature was ascribed to the “AEA endothelial receptor” (AeR) because of its sensitivity to AEA (Jarai et al. 1999; Offertaler et al. 2003; Wagner et al. 1999). Subsequent studies identified additional compounds targeting this as-yet-uncloned receptor, which helped advance its pharmacological characterization. The phytocannabinoid cannabidiol (CBD) is an important bioactive component of C. sativa that has outstanding therapeutic potential, for it does not produce psychotropic effects and has been shown to act as an inverse agonist at both CB₁and CB₂ receptors (Thomas et al. 2007). More specifically, CBD interacts with CB₁ receptors in an allosteric manner (Thomas et al. 2007). Allosteric modulation of GPCR constitutes an exciting new field of research and compounds that modulate CB₁ receptors by this mechanism hold tremendous therapeutic value (Price et al. 2005). Evidence suggests that CBD may also interact with novel cannabinoid receptors, although the details of this interaction are still unknown. While an initial report showed that CBD antagonize the novel AeR-mediated vasodilation (Jarai et al. 1999), several follow-up reports showed that the CBD analogue O-1918 and the synthetic isomer of CBD, abnormal-cannabidiol (abn-CBD), antagonize and activate the AeR (Ho & Hiley 2003; Mo et al. 2004; Offertaler et al. 2003; Wagner et al. 1999). More specifically, the latter compound causes vasodilation of mesenteric arteries in mice lacking CB₁and CB₂ receptors (Jarai et al. 1999), a reason for some laboratories, including ours, to refer to this receptor as the “abn-CBD receptor” (Walter et al. 2003). Thus, the pharmacological profile of the AeR does not parallel the pharmacology of CB₁ and CB₂ receptors since WIN55,212-2, Δ⁹-THC, and HU-210 lack efficacy at this target (Begg et al. 2003; McCollum et al. 2007; Wagner et al. 1999). Based on these findings, it is now generally accepted that most of the cannabinoid-induced vasodilation is mediated through this distinct target, and in line with this notion, the CB₂ antagonists SR144528, AM281 and AM630, and the CB₁ antagonist AM251 do not affect the AeR-mediated vasodilatory effect (Herradon et al. 2007; Ho & Hiley 2003).

The pharmacological tools shown to target AeR have greatly helped characterize some of the molecular and cellular mechanisms involved in the non-CB₁/ -CB₂mediated modulation of vasodilation. For example, compounds acting through AeR signals in an endothelial-dependent manner because endothelial denudation of the tissue will abolish cannabinoid-induced vasodilation (Ho & Hiley 2003; Wagner et al. 1999). The mechanisms involved in the abn-CBD induced vasodilation might depend on the activation of potassium channels in rat mesenteric arteries that appear to be calcium-sensitive in human umbilical vein endothelial cells (HUVEC) (Begg et al. 2003; Ho & Hiley 2003). In rabbit aortic endothelial cells (RAEC) the PI3K/Akt pathway is activated by methanandamide (mAEA), a metabolically stable analog of AEA. Activation of PI3K/Akt in endothelial cells by mAEA (EC₅₀ = 9.4 nM) results in the phosphorylation of eNOS and subsequent increased NO synthesis, a mechanism likely initiating the vasodilation (McCollum et al. 2007). Accordingly, pharmacological inhibition of PI3K with LY294002 and of G_i/o proteins with PTX decreases the synthesis of NO mediated by mAEA (McCollum et al. 2007). The biological effect of abn-CBD in HUVEC also involves the activation of PI3K/Akt in a G_i/o-protein dependent manner (Offertaler et al. 2003). The dual activation of p42/44 MAP kinase in HUVEC was also observed following abn-CBD treatment, a result not reported in RAEC (Offertaler et al. 2003). Thus, while at first glance the pharmacological profile of AeR and signaling pathways coupled to this receptor may appear consistent across most studies, some interesting inconsistencies also exist. McCollum et al. (2007) reported that mAEA induces vasodilation of RAECs in a SR141716A-insensitive manner whereas Wagner et al. (1999) reported that both the AEA- and mAEA-mediated regulation of vasodilation of rat mesenteric arteries was SR141716A-sensitive (Wagner 1999). A parsimonious explanation for these pharmacological inconsistencies could be that differences in tissue preparations might lead to different pharmacological and cellular responses. Alternatively, and in our opinion more thought-provoking, these discrepancies may reflect the following two possibilities: 1) the presence of yet another receptor in the vasculature different from the AeR or 2) that the AeR is subject to “agonist-induced trafficking” and resulting differential modulation of effector signaling. Be what it may, these two sets of evidence support the existence of non-CB₁/CB₂ receptors that carry tremendous promise for novel therapies aimed to treat and combat cardiovascular disease.

Another player that might also engage AeR is oleamide, an endogenous lipid structurally related to AEA. This ligand exhibits cannabinomimetic effects despite its lack of affinity at either CB₁ or CB₂ receptors, although its relevance remains controversial (Fowler 2004; Leggett et al. 2004). In favor of its action at AeR, oleamide causes vasodilation in rat mesenteric arteries that is partially endothelium-dependent. Transient receptor potential (TRP) channels and potassium-sensitive calcium channels are involved in this oleamide-mediated effect, even though this effect is blocked by SR141716A and O-1918, and not by AM251 (Hoi & Hiley 2006). This effect of oleamide on the vascular tone is PTX-sensitive, suggesting a G_i/o-mediated mechanism (Hoi & Hiley 2006). In a similar study, the novel water soluble cannabinoid-like agonist VSN16 also causes vasodilation in rat mesenteric arteries, an effect that is sensitive to O-1918 and SR141716A, and involves TRP channel activation (Hoi et al. 2007). However, in this case, pre-treatment with PTX did not affect the VSN16 elicited vasodilation (Hoi et al. 2007). We noted that the same tissue was used in both studies, suggesting differential effector coupling induced by these drugs, possibly through PTX-sensitive and PTX-insensitive mechanisms.

An additional study investigated the role of yet another endogenous lipid, N-arachidonoyl-L-serine (ARA-S), also structurally related to AEA. Although ARA-S binds with minimal affinity to CB₁, CB₂, and TRPV1, it produces endothelium-dependent vasodilation in rat isolated mesenteric arteries (K_i = 550nM) and abdominal aorta (K_i = 1.2 μM) (Milman et al. 2006). The effects of ARA-S are PTX-sensitive in the abdominal aorta and in cultured HUVEC, congruent with other studies suggesting that cannabinoid-induced vasodilatory effects are mediated by G_i/o (Milman et al. 2006). However, it should be emphasized that ARA-S also possesses PTX-insensitive effects: ARA-S inhibits lipopolysaccharide (LPS) induced TNFα production in mesenteric arteries in a PTX-insensitive manner, a response inhibited by O-1918 (Milman et al. 2006). Because O-1918 can inhibit this effect, one can postulate that both ligands bind to one target and that the PTX-insensitive component is a unique feature of ARA-S, a result that agrees with differential agonist-induced trafficking at the AeR, similarly to what has been elegantly shown for CB₁ and CB₂ receptors (Mukhopadhyay et al. 2002; Shoemaker et al. 2005). In other words, this evidence points toward an endothelial receptor that couples differentially to signal transduction pathways in a ligand-dependent manner. The integration of these pathways with those coupled to CB₁ receptors may be responsible for the complex cannabinoid-mediated vasodilatory effects.

In summary, it is clear that a novel endothelial CBR mediating the response induced by specific cannabinoid compounds exists. While the reported pharmacological profile remain to be thoroughly understood, it is likely that this receptor couples to distinct effectors in a ligand-dependent manner and thus is able to differentially control vasodilation. However before a specific therapeutic outcome may be unequivocally linked to the targeting of this receptor, it will be necessary to determine its molecular identity. The subsequent pharmacological verification of the cannabinoid-mediated vasodilatory effects in genetically modified animal models lacking the AeR will allow for a more thorough understanding of the molecular mechanisms involved in cannabinoid-mediated regulation of vascular tone.

b.) Novel cannabinoid receptors modulating analgesia

More than 50 million Americans suffer from some form of chronic pain as reported by the NIH pain and research programs. Many cases of chronic pain cannot be relieved by current therapies, highlighting the need for alternative strategies to treat these patients. Furthermore, many analgesics, including opioids and non-steroidal anti-inflammatory drugs (NSAIDs), cause significant side effects associated to their long term use. Because cannnabinoids induce analgesia in both acute and chronic pain models (Guindon & Hohmann 2008), their therapeutic potential as analgesics is being evaluated.

While it is clear that activation of CB₁ and CB₂ receptors induces analgesia, several evidence show that some of the cannabinoid-mediated analgesic responses are not mediated through these two receptor subtypes. A remarkable such example involves a particular endogenous acylethanolamide, palmitoylethanolamide (PEA), which does not activate either CB₁ or CB₂receptors (Showalter et al. 1996), and yet its analgesic property is sensitive to the CB₂ antagonist SR144528 (LoVerme et al. 2005). This may be interpreted in two different ways: PEA and SR144528 bind to a single receptor target, suggesting that SR144528 is non-specific, or PEA binds to a site distinct from SR144528 binding and their subsequent signaling may converge. In either case, PEA is likely acting through a novel receptor target site. This analgesic effect of PEA was first recognized when local administrations of both AEA and PEA were shown to induce classic analgesic paradigms (Calignano et al. 1998). Specifically, AEA inhibits the early phase of formalin-induced pain in a localized manner that is mediated by CB₁ receptors expressed on the peripheral nerve endings of sensory neurons (Calignano et al. 1998; Jaggar et al. 1998), whereas PEA inhibits both the early and late phases of formalin-induced pain independently of CB_1, CB₂and μ-opioid receptors (Calignano et al. 1998). Thus, while the authors convincingly concluded that AEA and PEA act synergistically when co-administered to counteract formalin-induced nociception, the molecular details of PEA’s action remained unknown. This synergistic effect should be considered in light of AEA and PEA being co-released under certain pathophysiological conditions (Di Marzo et al. 1994), which would lead to more robust and relevant analgesia compared to the mere isolated release of AEA (LoVerme et al. 2005;Maccarrone et al. 2002a; Maccarrone et al. 2002b; Mechoulam et al. 1998).

What is the molecular target of this analgesic effect induced by PEA? Both AEA and PEA interact with peroxisome proliferator activated receptor-α (PPAR-α), and thus this subtype of receptor might be involved in part of their actions as analgesics (Lo Verme et al. 2005; Sun et al. 2007). These receptors belong to the nuclear receptor superfamily and are often linked to lipid metabolism and inflammation (reviewed in Burstein 2005; O’Sullivan 2007). There are three subtypes: PPAR-α, PPAR-δ and PPAR-γ (PPAR-γ1, 2 and 3), all of which are expressed in a tissue specific manner. Generally, PPARs heterodimerize with retinoid X receptors (RXRs) and, upon ligand binding and cofactor recruitment, increase the transcription rate of specific genes (Burstein 2005). PEA activates PPAR-α at low micromolar concentrations and thereby is involved in PPAR-α–mediated regulation of gene expression (LoVerme et al. 2005). Accordingly, genetic deletion of PPAR-α abolishes PEA’s inflammatory effects in a CB₂-independent manner (LoVerme et al. 2005). When interpreting this particular study in light of others, one could conclude that there are both CB₂-sensitive and CB₂-insensitive components to PEA’s anti-inflammatory and nociceptive actions (Calignano et al. 1998; LoVerme et al. 2005). However, one should also consider that the ligand-binding domains of PPARs are large and thus may exhibit promiscuous binding to an array of structurally related and unrelated chemicals (Kliewer et al. 1997). Accordingly, 2-AG, AEA, PEA and OEA (Lenman & Fowler 2007; Rockwell et al. 2006), and even some of their metabolites, exhibit comparable activities at PPARs (reviewed in O’Sullivan 2007). Thus, while it is clear that AEA and PEA can promote analgesia in a synergistic manner, an important question remains: does PEA interact with a novel G-protein coupled CBR or a particular subtype of PPAR, or even both?

It is commonly accepted that a bona fide CBR should bind phytocannabinoids, and yet the prototypical phytocannabinoid, Δ⁹-THC, activates PPAR-γ, another PPAR isoform, and increases PPAR-γ-regulated transcription, resulting in adipogenesis and vasorelaxation (O’Sullivan et al. 2006b; O’Sullivan et al. 2005). As a matter of fact, even CBD and the two synthetic cannabinoids WIN55,212-2 and CP55,940 also bind to PPAR-γ and increase transcriptional activity (O’Sullivan et al. 2006a). In regards to Δ⁹-THC, while it has no significant activity at PPAR-α (Sun et al. 2007), it can induce vasorelaxation by producing NO and hydrogen peroxide, signaling molecules that require superoxide dismutase activation (O’Sullivan et al. 2005). Thus, while PEA might promote analgesia through PPAR-α, some phytocannabinoids might also mediate part of their analgesic response through PPAR-γ. Whether some of the PPAR subtypes should be included in the eCBSS and thus would represent novel players in the cannabinoid-mediated analgesia constitutes an intriguing possibility that warrants further investigation.

c.) Novel cannabinoid receptors regulating neurotransmission

The synapse is the fundamental unit of neural communication allowing for the transfer of chemical information from presynaptic terminals to their postsynaptic counterparts. The maintenance and tight regulation of this dynamic unit is crucial, since even small perturbations of this highly structured machinery may lead to dysfunctional neural communication often observed in neurological disease (reviewed in Beck & Yaari 2008). Most active synapses throughout the CNS contains functional elements of the eCBSS, and many laboratories have focused their attention on how the eCBSS regulates the efficacy of GABAergic and glutamatergic neurotransmission (Chevaleyre et al. 2006; Freund et al. 2003; Kreitzer & Regehr 2001; Lutz 2004; Maejima et al. 2001; Wilson et al. 2001). Because impaired eCBSS is implicated in several neurological diseases (Katona & Freund 2008; Kreitzer & Malenka 2007; Lastres-Becker et al. 2002a;Lastres-Becker et al. 2001; Lastres-Becker et al. 2002b; Lastres-Becker et al. 2002c; Pazos et al. 2008; Ramirez et al. 2005), the identification of novel receptors involved in the endocannabinoid (eCB)-mediated modulation of neurotransmission should allow for the development of better tools to understand the intricate role of the eCBSS in neurophysiology and treat neurological disease.

Novel CBR sites were first identified in brain homogenates. While AEA (EC₅₀ = 3.6 μM) and WIN55,212-2 (EC₅₀ = 1.8 μM) act this novel CBR by increasing [³⁵S]-GTPγS binding, this response does not involve CB₁ receptors since it is insensitive to SR141716A and reliably measured in CB₁^−/− mice (Breivogel et al. 2001; Monory et al. 2002). Breivogel et al. (2001) reported that this novel CBR is expressed in brain stem, cortex, hippocampus, midbrain, and spinal cord, while being absent in the cerebellum and basal ganglia (Breivogel et al. 2001). Conversely, Monory et al. (2002) found this binding site in the cerebellum (Monory et al. 2002). However, it should be emphasized that these studies used two different CB₁^−/− strains generated on distinct genetic backgrounds and that such differences could account for some of the discrepancies (Hoffman et al. 2005). While studies have reported CB₂ expression within specific brainstem neurons and in cerebellar granule neurons (Skaper et al. 1996; Van Sickle et al. 2005), this cannabinoid receptor subtype is unlikely to represent the aforementioned AEA- and WIN55,212-2-sensitive binding site since its activation is not blocked by SR144528 and unaffected by typical CB₂ receptor agonists, such as CP55,940, Δ⁹-THC and HU-210 (Breivogel et al. 2001; Monory et al. 2002). In summary, these studies suggest the existence of a binding site sensitive to AEA and WIN55,212-2, resulting in G-protein activation, in a fashion that is clearly distinct from that of CB₁ and CB₂ receptors within the CNS.

Electrophysiological evidence suggests that this novel CBR regulates neurotransmission within the hippocampus, as first described by Hajos et al. by using CB₁^−/− mice (Hajos et al. 2001). Specifically, WIN55,212-2 inhibited EPSCs but not IPSCs at the schaffer collateral synapse in the mouse hippocampal CA1 region (Hajos et al. 2001). This response was also present in rat hippocampal slices, where administration of the CB₁ antagonist AM251 abolishes the WIN55,212-2-mediated inhibition of IPSCs, but not EPSCs, indicating that EPSCs might be regulated by this novel CBR and not by CB₁ receptors (Hajos & Freund 2002). Remarkably, the vanilloid receptor antagonist capsezapine blocked the WIN55,212-2-mediated inhibition of EPSCs but not IPSCs (Hajos & Freund 2002). Considering the sensitivity of EPSCs to vanilloid receptor ligands, one could argue that this response is not mediated by a novel CBR but instead by a vanilloid receptor. While Hajos (2002) did argue against this possibility – citing a study that showed that WIN55,212-2 does not bind to TRP channels (Zygmunt et al. 1999) – more recent studies showed that a number of cannabinoids, including AEA, WIN55,212-2, and SR141716A, do actually bind to TRP channels and modulate TRP channel activity (De Petrocellis et al. 2008;Jeske et al. 2006; Patwardhan et al. 2006). Thus, the existence of a novel CBR (that is not the TRP channel) regulating neurotransmitter release in the hippocampus remains an open question. Note, however, that this situation is further complicated by a different result reported by Yoshida et al. who used juvenile CB₁^−/− mice. In this study neither EPSCs nor IPSCs were inhibited by WIN55,212-2 or endogenously-released cannabinoids (Yoshida et al. 2002), suggesting that this novel CBR is absent in juvenile mice.

In summary, we still require better evidence for the existence of and better understanding for the role played by a novel CBR in regulating neurotransmitter release. This is particularly pertinent to the involvement of the eCBSS in critical neurological function, including learning and memory, and in neurological diseases such as Parkinson’s and Huntington’s disease. Thus, by determining the molecular identity and the fundamental role played by this novel CBR, this field of research will not only help solidify our understanding of how the eCBSS regulates neurotransmission, but also potentially unveil a valuable novel therapeutic target.

a.) GPR55: a promising cannabinoid receptor candidate

The possibility that GPR55 might constitute the target responsible for some of the reported non-CB₁/CB₂ mediated effects has captured an increasing amount of attention. GPR55 was first identified in 1998 by performing homology searches of the amino acid sequences of known GPCRs using BLAST (basic alignment search tool) and publicly available databases (GenBank HighThroughput Genome and expressed sequence tag) (Sawzdargo et al. 1999). Its mRNA is expressed in caudate, putamen, hippocampus, thalamic nuclei, midbrain, spleen, intestine and fetal tissue as shown by Northern blot analysis and in situ hybridization (Sawzdargo et al. 1999). Two patents that followed these initial studies claimed that GPR55 represents a novel CBR, despite possessing only 13.5 % and 14.4% sequence homology to CB₁ and CB₂, respectively (reviewed by Baker et al. 2006). In more recent years, efforts from several independent laboratories aimed to verify if GPR55 indeed represents a novel CBR. These studies showed that cannabinoids, as well as LPI, activate this GPCR, with most studies agreeing that GPR55 activation causes [Ca²⁺]_i release by activation of IP₃ receptors (Henstridge et al. 2008; Lauckner et al. 2008; Oka et al. 2007; Waldeck-Weiermair et al. 2008). An interesting property of GPR55 activation is its potential to regulate neuronal excitability through [Ca²⁺]_i flux (Lauckner et al. 2008). Conversely, one study did not corroborate these results, for it showed that the cannabinoid-induced activation of GPR55 (using FLAG-tagged-GPR55 transiently transfected in HEK293 cells) does not lead to G_q-coupling to elicit the flux of [Ca²⁺]_I; but rather leads to G₁₃-coupling, the latter resulting in RhoA, cdc42 and rac1 activation (Ryberg et al. 2007). One additional factor that could further complicate the comparison of these studies with the other ones is that Ryberg et al. utilized the FLIPR assay to measure [Ca²⁺]_i levels, which may not be as sensitive compared to ratiometric fluorescent dyes tested in single cells (Henstridge et al. 2008; Lauckner et al. 2008; Oka et al. 2007). Furthermore, it is well known that epitope tags can significantly influence the trafficking and downstream effector signaling of GPCRs, and thus native GPR55 receptors might exhibit differential coupling to signaling systems (Brothers et al. 2003).

Of note concerning all these studies, is the limited number of cannabinoids tested on GPR55, leaving its overall pharmacological profile relatively unexplored. More specifically, Oka et al (2007) states that GPR55 is an LPI receptor because low micromolar concentrations of cannabinoids do not bind (Oka et al. 2007), while other studies do show that cannabinoids, such as Δ⁹-THC, do activate GPR55 albeit at higher concentrations (Lauckner et al. 2008). Although Henstridge et al. (2008) tested AEA and 2-AG (3–30 μM), as well as the synthetic cannabinoid CP55,940 (3 μM), and found no effect on [Ca²⁺]_i mobilization, this panel of cannabinoids remains limited in variety and dose. Moreover, since AEA and 2-AG are subject to hydrolysis at varying degrees depending on cell type and experimental conditions (Giuffrida et al. 2001), it might be important to include inhibitors of eCB hydrolysis so that the full efficacy of these labile lipids is preserved when testing their activity at GPR55.

Atypical cannabinoids, known to induce vasodilation, bind to GPR55 in low nanomolar concentrations (Ryberg et al. 2007), thus suggesting the possibility that GPR55 may constitute the long sought after endothelial receptor. Abn-CBD and O-1602, another atypical cannabinoid, both stimulate [³⁵S]-GTPγS binding in GPR55-expressing HEK293T cells. More importantly, however, the vasodilatory effect of abn-CBD is unchanged in mice lacking GPR55 expression when compared to wild-type control mice (Johns et al. 2007). Thus, it is likely that GPR55 is distinct from the endothelial receptor mediating the cannabinoid-induced vasodilatory effect.

Parallel studies investigated the intriguing notion that GPR55 may constitute the novel receptor responsible for some of the cannabinoid-mediated analgesic effects. As such, genetically modified mice lacking GPR55 were used to investigate the role of GPR55 in hyperalgesia associated with both neuropathic and inflammatory pain (Staton et al. 2007). Results indicate the GPR55 is involved in the regulation of various cytokine levels likely resulting in blunted inflammatory mechanical hyperalgesic responses in addition to the lack of mechanical hyperalgesic responses (Staton et al. 2007). This suggests that targeting GPR55 may be of therapeutic benefit in the regulation of pain, but it is likely that GPR55 will not be identified as the novel CBR mediating analgesia that we outlined above due to clear pharmacological discrepancies. Indeed, as reviewed in see section IIIb of this review, PEA is a unique compound that induces SR144528-sensitive analgesia, yet does not activate CB₂ receptors (LoVerme et al. 2005; Showalter et al. 1996). Interestingly, PEA is a high affinity ligand that activates [³⁵S]-GTPγS binding in GPR55-expressing HEK293 cells (Ryberg et al. 2007). However, SR144528 does not appear to influence GPR55 activation (Lauckner et al. 2008). Thus, GPR55 is a distinct receptor from the novel CBR mediating cannabinoin-induced analgesia.

The classification of GPR55 as either an LPI receptor (LPIR) or an additional CBR brings about three noteworthy speculations. First, it is possible that cannabinoids and lysophospholipids interact with the same GPCR, which would reiterate observations for sphingosine-1-phosphate (S1P) at CB₁ (discussed in a following section of this review). Second, if one assumes that the deorphanization of a receptor depends on the identification of high affinity ligands, as many pharmaceutical companies do, then GPR55 is more likely to constitute a CBR since both patents and one peer-reviewed study reported that several cannabinoids activate GPR55-mediated GTPγS at low nanomolar concentrations (Ryberg et al. 2007). As mentioned previously, a novel CBR is generally defined as a receptor that binds phytocannabinoids; and GPR55 would fulfill this criterion since it is activated by Δ⁹-THC, albeit at high concentrations (Lauckner et al. 2008). Third, it is interesting to consider the disparity between the pharmacological similarity that might exist between GPR55 and CBRs and the lack of amino acid sequence similarities, since GPR55 shares only 14% similarity with CB₁ and CB₂. This argument is further supported by results obtained when analyzing phylogenetic divergences of this family of GPCRs, a concept developed in a recent review by Brown (2007). Specifically, it is quite possible that although GPR55 lacks high sequence similarity to the CBRs, it may still express key amino acids that will allow its interaction with cannabinoid ligands. This logic was recently applied successfully with the past orphan receptors GPR23 and GPR92, now LPA₄ and LPA₅, in that although they are not closely phylogenetically related to the other LPARs, they still bind LPA with high affinity (Brown 2007). This suggests that although phylogenetic analysis might sometimes be a reliable tool to identify novel receptor subtypes, it is not a definitive method and pharmacology will often have the last word.

To conclude, while several studies have reported the pharmacology of some cannabinoids and lipids at GPR55, one of the most pressing and important question still remains open: does this receptor truly belong to the eCBSS? More specific compounds and reliable genetic tools will help the field answer this important question.

b.) S1P and its receptors: A clear link with the eCBSS

As stated above, identification of novel CBRs can be achieved either by using pharmacological tools or by comparing the amino acid sequences of CBRs to phylogenetically similar receptors, or sometimes by the combination of both approaches. While the beginning of this review focused on the pharmacological identification of novel CBRs, the following section discusses phylogenetic evidence supporting the existence of yet additional receptors that are engaged by cannabinoid compounds and thus could be considered novel CBRs. This evidence is based on sequence similarities between the CBRs and other lipid receptors, in particular the lysophospholipid receptors. The majority of our knowledge on the bioactivity of lysophospholipids is restricted to four main players: lysophosphatidic acid (LPA), lysophosphatidylcholine (LPC), sphingosylphosphorylcholine (SPC) and S1P. Here, we will focus on a possible link between CBRs and S1P receptors, the many subtypes of which are expressed throughout the body. A comparison of the literature available on the eCBSS and the S1P signaling systems indicates that these systems possess striking similarities among their respective receptors, ligands and transduction mechanisms, suggesting that they may converge or perhaps even superimpose.

Originally, sphingolipids were thought to only represent structural elements of cell membranes important for their stability and fluidity. However, several landmark studies had uncovered the signaling potential of S1P and this lipid is now recognized as a bona fide mediator of specific physiological functions, some of which have also been implicated in CBR signaling. Like other signaling lipids, including eCBs, the levels of S1P are tightly regulated by the balance between its synthesis and degradation. Sphingosine kinase (SPHK) is responsible for its de novo synthesis, while its degradation is controlled in a reversible manner by S1P-phosphatases (SPPs) and in an irreversible manner by S1P-lyase (Jo et al. 2008). There are two isoforms of SPHK, SPHK1 and SPHK 2, which are differentially expressed throughout the body and possess different levels of activity (Kohama et al. 1998; Liu et al. 2000; Olivera et al. 1999). While low nanomolar concentrations of S1P are found in the intracellular space, much higher concentrations of S1P are found in the serum bound to albumin and other lipoproteins (Okajima 2002).

At the cell surface, S1P binds to and activates G-protein-coupled S1P receptors to elicit or regulate a wide range of biological functions, such as angiogenesis and immune functions (Jo et al. 2008; Lee et al. 1999b; Skoura et al. 2007), as well as cell proliferation and motility (Durand et al. 2006; LaMontagne et al. 2006; Lee et al. 1999a; Park et al. 2007). S1P receptors (S1P_1-5), formerly known as the endothelial differentiation gene (EDG) receptors (EDG_1,3,5,6,8), encompasses a class of GPCRs activated by the major sphingolipid metabolite S1P (Zondag et al. 1998). Interestingly, one report suggests that S1P might also constitute an endogenous ligand for GPR3, GPR6 and GPR12 (Uhlenbrock et al. 2002), but this report has not been followed up by other laboratories. All S1P receptors couple to G_i/o and G_12/13, except S1P_1, which only couples to G_i/o. S1P₂ and S1P₃ receptors can also couple to G_q and G_s proteins while S1P₄ receptors can couple to G_s. Upon activation, all receptors activate MAPK, except S1P₅, which is associated with decreased MAPK phosphorylation (Ishii et al. 2004). Although older studies suggested that S1P might also be a second messenger that mobilizes calcium and regulates cell proliferation (Spiegel 1999; Zhang et al. 1991), its intracellular target remains to be identified and this concept remains to be confirmed.

An important study in the context of this review indicated that S1P analogs interact with CB₁ receptors (Paugh et al. 2006), which raises the exciting possibility that these two signaling systems might interact in ways that warrant further evaluation. Three lines of evidence favor this possibility. Phylogenetic analysis shows that S1P receptors share approximately 30% amino acid identity with CBRs (Figure 2). GPR3, GPR6, and GPR12 also exhibit high levels of homology with CBRs, averaging 28% amino acid identity (Table 2). Both S1PRs and CBRs are activated by endogenous lipid modulators that also share chemical and structural similarities (Figure 3). The last line of evidence – and in our opinion the most exciting – suggested a possible direct association and/or cross-talk between these signaling systems. Specifically, radioligand competition experiments that target CB₁ receptors stably expressed in CHO cells and HEK293 cells, as well as endogenously expressed CB₁ receptors expressed in mouse cerebellar homogenates, were performed. The results showed that low micromolar concentrations of the high affinity non-selective S1P receptor agonist, FTY720, and the endogenous lipid sphingosine, clearly competed for [³H]-CP55,940 specific binding at CB₁ receptors (Paugh et al. 2006). Conversely, this effect was not observed when using CB₂ receptors, highlighting the specificity of these data. Two important findings result from this study. First, there is a clear pharmacological interaction between S1P receptor ligands and CB₁ receptors. Second, sphingosine is the first identified endogenous antagonist for CB₁receptors as shown by performing GTPγS binding experiments (Paugh et al. 2006). In order to fully grasp the depth and relevance of these results, many basic questions still need to be answered, including whether the reverse scenario is true: do eCBs interact with S1P receptors? To our knowledge, an answer to this question has not been reported.

Figure 2

Phylogenetic tree analysis of GPCRs closely related to CB₁

Figure 3

Structures of endogenous and synthetic lipids

Table 2

Compared sequence identities of lysophospholipid receptors and cannabinoid receptors.

Additional studies focusing on either S1P or CBR signaling indicate further parallels between these systems. For example, several studies have shown therapeutic potential for cannabinoids to control neoangiogenesis and tumor vascularization (Blazquez et al. 2003; Casanova et al. 2003; Galve-Roperh et al. 2000; Sanchez et al. 2001) and similar therapeutic effects that has been shown for S1P (LaMontagne et al. 2006; Schmid et al. 2007). Another intriguing link was reported by Lee et al, (2000), since they showed that micromolar concentrations of S1P increases the cell motility of HUVEC in a PTX-sensitive manner (Lee et al. 2000). While this study directly concluded that this response was mediated by S1P receptors, the authors did not consider the involvement of CB₁ receptors, even thought the latter are abundantly expressed by HUVEC (McCollum et al. 2007).

GPR3, GPR6 and GPR12, all three of which are engaged by S1P, share high amino acid similarity with CBRs (Table 2). These three GPCRs are thought to be constitutively active, stably increasing basal cAMP levels (Eggerickx et al. 1995;Kostenis 2004; Uhlenbrock et al. 2002). Note that GPR12 is grouped in this category of S1P lipid receptors despite its higher affinity toward another lysophospholipid, namely SPC (Uhlenbrock et al. 2002). GPR3, 6 and 12 are expressed at relatively high levels in the periphery and the CNS (Eidne et al. 1991; Heiber et al. 1995; Uhlenbrock et al. 2003), but subtle differences are found when considering their temporal and region specific expression patterns. For example, GPR3 is highly expressed in cerebral granule neurons to regulate neurite outgrowth (Tanaka et al. 2007), whereas GPR6 is expressed in striatopallidal medium spiny neurons where it is involved in instrumental learning (Lobo et al. 2007). With regards to GPR12, this receptor facilitates axonal regeneration by activating PKA and inhibiting Rho activation (Tanaka et al. 2007), a mechanism of likely importance for neural injury and development. Thus, here too, an intriguing link exists between S1P candidate receptors and CBRs.

In summary, evidence indicates that CBRs and S1P receptors share a high degree of sequence homology, are activated by chemically and structurally similar lipids and are implicated in the same biological processes, suggesting the possibility that these signaling systems may converge or even overlap at the receptor or signal transduction levels. This receptor duo is likely to dynamically and efficiently regulate key biological processes, and thus may represent a novel venue for therapeutic approaches.

This work was supported by our NIH grant DA021285. The authors wish to thank Eiron Cudaback, Eric Horne, William Marrs, Aaron Miller and Michelle Sexton, members of the Stella laboratory and Anatol Kreitzer (Gladstone Institute of Neurological Disease and Department of Physiology and Neurology University of California, San Francisco) for their critical reading and insights during the preparation of this manuscript.

Publisher’s Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

• Abadji V, Lin S, Taha G, Griffin G, Stevenson LA, Pertwee RG, Makriyannis A. (R)-methanandamide: a chiral novel anandamide possessing higher potency and metabolic stability. J Med Chem.1994;37:1889–1893. [PubMed]
• Adams IB, Compton DR, Martin BR. Assessment of anandamide interaction with the cannabinoid brain receptor: SR 141716A antagonism studies in mice and autoradiographic analysis of receptor binding in rat brain. J Pharmacol Exp Ther. 1998;284:1209–17. [PubMed]
• Adams IB, Ryan W, Singer M, Thomas BF, Compton DR, Razdan RK, Martin BR. Evaluation of cannabinoid receptor binding and in vivo activities for anandamide analogs. J Pharmacol Exp Ther.1995;273:1172–81. [PubMed]
• Baker D, Pryce G, Davies WL, Hiley CR. In silico patent searching reveals a new cannabinoid receptor. Trends Pharmacol Sci. 2006;27:1–4.[PubMed]
• Bayewitch M, Rhee MH, Avidor-Reiss T, Breuer A, Mechoulam R, Vogel Z. (-)-Delta9-tetrahydrocannabinol antagonizes the peripheral cannabinoid receptor-mediated inhibition of adenylyl cyclase. J Biol Chem. 1996;271:9902–5. [PubMed]
• Beck H, Yaari Y. Plasticity of intrinsic neuronal properties in CNS disorders. Nat Rev Neurosci. 2008;9:357–69. [PubMed]
• Begg M, Mo FM, Offertaler L, Batkai S, Pacher P, Razdan RK, Lovinger DM, Kunos G. G protein-coupled endothelial receptor for atypical cannabinoid ligands modulates a Ca2+-dependent K+ current. J Biol Chem. 2003;278:46188–94. [PubMed]
• Begg M, Pacher P, Batkai S, Osei-Hyiaman D, Offertaler L, Mo FM, Liu J, Kunos G. Evidence for novel cannabinoid receptors. Pharmacol Ther.2005;106:133–45. [PubMed]
• Belenko SR. Drugs and Drug Policy in America. Greenwood Press; Wesport: 2000.
• Ben-Shabat S, Fride E, Sheskin T, Tamiri T, Rhee MH, Vogel Z, Bisogno T, De Petrocellis L, Di Marzo V, Mechoulam R. An entourage effect: inactive endogenous fatty acid glycerol esters enhance 2-arachidonoyl-glycerol cannabinoid activity. Eur J Pharmacol. 1998;353:23–31.[PubMed]
• Berglund BA, Boring DL, Wilken GH, Makriyannis A, Howlett AC, Lin S. Structural requirements for arachidonylethanolamide interaction with CB1 and CB2 cannabinoid receptors: pharmacology of the carbonyl and ethanolamide groups. Prostaglandins Leukot Essent Fatty Acids.1998;59:111–8. [PubMed]
• Blazquez C, Casanova ML, Planas A, Gomez Del Pulgar T, Villanueva C, Fernandez-Acenero MJ, Aragones J, Huffman JW, Jorcano JL, Guzman M. Inhibition of tumor angiogenesis by cannabinoids. FASEB J.2003;17:529–31. [PubMed]
• Bouaboula M, Poinot-Chazel C, Bourrie B, Canat X, Calandra B, Rinaldi-Carmona M, Le Fur G, Casellas P. Activation of mitogen-activated protein kinases by stimulation of the central cannabinoid receptor CB1. Biochem J. 1995;312(Pt 2):637–41. [PMC free article][PubMed]
• Breivogel CS, Griffin G, Di Marzo V, Martin BR. Evidence for a new G protein-coupled cannabinoid receptor in mouse brain. Mol Pharmacol.2001;60:155–63. [PubMed]
• Brothers SP, Janovick JA, Conn PM. Unexpected effects of epitope and chimeric tags on gonadotropin-releasing hormone receptors: implications for understanding the molecular etiology of hypogonadotropic hypogonadism. J Clin Endocrinol Metab.2003;88:6107–12. [PubMed]
• Brown AJ. Novel cannabinoid receptors. Br J Pharmacol.2007;152:567–75. [PMC free article] [PubMed]
• Burkey TH, Quock RM, Consroe P, Roeske WR, Yamamura HI. delta 9-Tetrahydrocannabinol is a partial agonist of cannabinoid receptors in mouse brain. Eur J Pharmacol. 1997;323:R3–4. [PubMed]
• Burstein S. PPAR-gamma: a nuclear receptor with affinity for cannabinoids. Life Sci. 2005;77:1674–84. [PubMed]
• Calignano A, La Rana G, Giuffrida A, Piomelli D. Control of pain initiation by endogenous cannabinoids. Nature. 1998;394:277–81.[PubMed]
• Casanova ML, Blazquez C, Martinez-Palacio J, Villanueva C, Fernandez-Acenero MJ, Huffman JW, Jorcano JL, Guzman M. Inhibition of skin tumor growth and angiogenesis in vivo by activation of cannabinoid receptors. J Clin Invest. 2003;111:43–50. [PMC free article][PubMed]
• Chambers AP, V, Vemuri K, Peng Y, Wood JT, Olszewska T, Pittman QJ, Makriyannis A, Sharkey KA. A neutral CB1 receptor antagonist reduces weight gain in rat. Am J Physiol Regul Integr Comp Physiol.2007;293:R2185–93. [PubMed]
• Chevaleyre V, Takahashi KA, Castillo PE. Endocannabinoid-Mediated Synaptic Plasticity in the CNS. Annu Rev Neurosci. 2006 [PubMed]
• Compton DR, Rice KC, De Costa BR, Razdan RK, Melvin LS, Johnson MR, Martin BR. Cannabinoid structure-activity relationships: correlation of receptor binding and in vivo activities. J Pharmacol Exp Ther. 1993;265:218–26. [PubMed]
• De Petrocellis L, Vellani V, Schiano-Moriello A, Marini P, Magherini PC, Orlando P, Di Marzo V. Plant-derived cannabinoids modulate the activity of transient receptor potential channels of ankyrin type-1 and melastatin type-8. J Pharmacol Exp Ther. 2008;325:1007–15. [PubMed]
• Devane WA, Dysarz FA, 3rd, Johnson MR, Melvin LS, Howlett AC. Determination and characterization of a cannabinoid receptor in rat brain. Mol Pharmacol. 1988;34:605–13. [PubMed]
• Dewey WL. Cannabinoid pharmacology. Pharmacol Rev. 1986;38:151–78. [PubMed]
• Di Marzo V, Fontana A, Cadas H, Schinelli S, Cimino G, Schwartz J, Piomelli D. Formation and inactivation of endogenous cannabinoid anandamide in central neurons. Nature. 1994;372:686–691. [PubMed]
• Di Marzo V, Breivogel CS, Tao Q, Bridgen DT, Razdan RK, Zimmer AM, Zimmer A, Martin $BR. Levels, metabolism, and pharmacological activity of anandamide in CB(1) cannabinoi receptor knockout mice: evidence for non-CB(1), non-CB(2) receptor-mediated actions of anandamide in mouse brain. J Neurochem. 2000;75:2434–44.[PubMed]
• Dill JA, Howlett AC. Regulation of adenylate cyclase by chronic exposure to cannabimimetic drugs. J Pharmacol Exp Ther. 1988;244:1157–63.[PubMed]
• Durand CA, Westendorf J, Tse KW, Gold MR. The Rap GTPases mediate CXCL13- and sphingosine1-phosphate-induced chemotaxis, adhesion, and Pyk2 tyrosine phosphorylation in B lymphocytes. Eur J Immunol.2006;36:2235–49. [PubMed]
• Eggerickx D, Denef JF, Labbe O, Hayashi Y, Refetoff S, Vassart G, Parmentier M, Libert F. Molecular cloning of an orphan G-protein-coupled receptor that constitutively activates adenylate cyclase. Biochem J. 1995;309(Pt 3):837–43. [PMC free article] [PubMed]
• Eidne KA, Zabavnik J, Peters T, Yoshida S, Anderson L, Taylor PL. Cloning, sequencing and tissue distribution of a candidate G protein-coupled receptor from rat pituitary gland. FEBS Lett. 1991;292:243–8.[PubMed]
• Facci L, Dal Toso R, Romanello S, Buriani A, Skaper SD, Leon A. Mast cells express a peripheral cannabinoid receptor with differential sensitivity to anandamide and palmitoylethanolamide. Proc Natl Acad Sci U S A. 1995;92:3376–80. [PMC free article] [PubMed]
• Felder CC, Glass M. Cannabinoid receptors and their endogenous agonists. Annu Rev Pharmacol Toxicol. 1998;38:179–200. [PubMed]
• Felder CC, Joyce KE, Briley EM, Mansouri J, Mackie K, Blond O, Lai Y, Ma AL, Mitchell RL. Comparison of the pharmacology and signal transduction of the human cannabinoid CB1 and CB2 receptors. Mol Pharmacol. 1995;48:443–50. [PubMed]
• Fowler CJ. Oleamide. a member of the endocannabinoid family? Br J Pharmacol. 2004;141:195–6. [PMC free article] [PubMed]
• Freund TF, Katona I, Piomelli D. Role of endogenous cannabinoids in synaptic signaling. Physiol Rev. 2003;83:1017–66. [PubMed]
• Gatley SJ, Gifford AN, Volkow ND, Lan R, Makriyannis A. 123I-labeled AM251: a radioiodinated ligand which binds in vivo to mouse brain cannabinoid CB1 receptors. Eur J Pharmacol. 1996;307:331–8.[PubMed]
• Galve-Roperh I, Sanchez C, Cortes ML, Gomez del Pulgar T, Izquierdo M, Guzman M. Anti-tumoral action of cannabinoids: involvement of sustained ceramide accumulation and extracellular signal-regulated kinase activation. Nat Med. 2000;6:313–9. [PubMed]
• Gebremedhin D, Lange AR, Campbell WB, Hillard CJ, Harder DR. Cannabinoid CB1 receptor of cat cerebral arterial muscle functions to inhibit L-type Ca2+ channel current. Am J Physiol. 1999;276:H2085–93.[PubMed]
• Giuffrida A, Beltramo M, Piomelli D. Mechanisms of endocannabinoid inactivation: biochemistry and pharmacology. J Pharmacol Exp Ther.2001;298:7–14. [PubMed]
• Gomez del Pulgar T, Velasco G, Guzman M. The CB1 cannabinoid receptor is coupled to the activation of protein kinase B/Akt. Biochem J.2000;347:369–73. [PMC free article] [PubMed]
• Goutopoulos A, Fan P, Khanolkar AD, Xie XQ, Lin S, Makriyannis A. Stereochemical selectivity of methanandamides for the CB1 and CB2 cannabinoid receptors and their metabolic stability. Bioorg Med Chem.2001;9:1673–84. [PubMed]
• Guindon J, Hohmann AG. Cannabinoid CB2 receptors: a therapeutic target for the treatment of inflammatory and neuropathic pain. Br J Pharmacol. 2008;153:319–34. [PMC free article] [PubMed]
• Griffin G, Atkinson PJ, Showalter VM, Martin BR, Abood ME. Evaluation of cannabinoid receptor agonists and antagonists using the guanosine-5′-O-(3-[35S]thio)-triphosphate binding assay in rat cerebellar membranes. J Pharmacol Exp Ther. 1998;285:553–60.[PubMed]
• Gonsiorek W, Lunn C, Fan X, Narula S, Lundell D, Hipkin RW. Endocannabinoid 2-arachidonyl glycerol is a full agonist through human type 2 cannabinoid receptor: antagonism by anandamide. Mol Pharmacol. 2000;57:1045–50. [PubMed]
• Hajos N, Freund TF. Pharmacological separation of cannabinoid sensitive receptors on hippocampal excitatory and inhibitory fibers.Neuropharmacology. 2002;43:503–10. [PubMed]
• Hajos N, Ledent C, Freund TF. Novel cannabinoid-sensitive receptor mediates inhibition of glutamatergic synaptic transmission in the hippocampus. Neuroscience. 2001;106:1–4. [PubMed]
• Heiber M, Docherty JM, Shah G, Nguyen T, Cheng R, Heng HH, Marchese A, Tsui LC, Shi X, George SR, et al. Isolation of three novel human genes encoding G protein-coupled receptors. DNA Cell Biol.1995;14:25–35. [PubMed]
• Henstridge CM, Balenga NA, Ford LA, Ross RA, Waldhoer M, Irving AJ. The GPR55 ligand L-{alpha}-lysophosphatidylinositol promotes RhoA-dependent Ca2+ signaling and NFAT activation. FASEB J. 2008[PubMed]
• Herradon E, Martin MI, Lopez-Miranda V. Characterization of the vasorelaxant mechanisms of the endocannabinoid anandamide in rat aorta. Br J Pharmacol. 2007;152:699–708. [PMC free article] [PubMed]
• Hillard CJ, Manna S, Greenberg MJ, DiCamelli R, Ross RA, Stevenson LA, Murphy V, Pertwee RG, Campbell WB. Synthesis and characterization of potent and selective agonists of the neuronal cannabinoid receptor (CB1) J Pharmacol Exp Ther. 1999;289:1427–33.[PubMed]
• Ho WS, Hiley CR. Vasodilator actions of abnormal-cannabidiol in rat isolated small mesenteric artery. Br J Pharmacol. 2003;138:1320–32.[PMC free article] [PubMed]
• Hoffman AF, Macgill AM, Smith D, Oz M, Lupica CR. Species and strain differences in the expression of a novel glutamate-modulating cannabinoid receptor in the rodent hippocampus. Eur J Neurosci.2005;22:2387–91. [PMC free article] [PubMed]
• Hoi PM, Visintin C, Okuyama M, Gardiner SM, Kaup SS, Bennett T, Baker D, Selwood DL, Hiley CR. Vascular pharmacology of a novel cannabinoid-like compound, 3-(5-dimethylcarbamoyl-pent-1-enyl)-N-(2-hydroxy-1-methyl-ethyl)benzamide (VSN16) in the rat. Br J Pharmacol. 2007;152:751–64. [PMC free article] [PubMed]
• Hoi PM, Hiley CR. Vasorelaxant effects of oleamide in rat small mesenteric artery indicate action at a novel cannabinoid receptor. Br J Pharmacol. 2006;147:560–8. [PMC free article] [PubMed]
• Hoi PM, Visintin C, Okuyama M, Gardiner SM, Kaup SS, Bennett T, Baker D, Selwood DL, Hiley CR. Vascular pharmacology of a novel cannabinoid-like compound, 3-(5-dimethylcarbamoyl-pent-1-enyl)-N-(2-hydroxy-1-methyl-ethyl)benzamide (VSN16) in the rat. Br J Pharmacol. 2007;152:751–64. [PMC free article] [PubMed]
• Houston DB, Howlett AC. Differential receptor-G-protein coupling evoked by dissimilar cannabinoid receptor agonists. Cell Signal.1998;10:667–74. [PubMed]
• Howlett AC, Mukhopadhyay S. Cellular signal transduction by anandamide and 2-arachidonoylglycerol. Chem Phys Lipids.2000;108:53–70. [PubMed]
• Ishii I, Fukushima N, Ye X, Chun J. Lysophospholipid receptors: signaling and biology. Annu Rev Biochem. 2004;73:321–54. [PubMed]
• Iwamura H, Suzuki H, Ueda Y, Kaya T, Inaba T. In Vitro and in Vivo Pharmacological Characterization of JTE-907, a Novel Selective Ligand for Cannabinoid CB2 Receptor. J Pharmacol Exp Ther. 2001;296:420–425. [PubMed]
• Jaggar SI, Hasnie FS, Sellaturay S, Rice AS. The anti-hyperalgesic actions of the cannabinoid anandamide and the putative CB2 receptor agonist palmitoylethanolamide in visceral and somatic inflammatory pain. Pain. 1998;76:189–99. [PubMed]
• Jarai Z, Wagner JA, Varga K, Lake KD, Compton DR, Martin BR, Zimmer AM, Bonner TI, Buckley NE, Mezey E, Razdan RK, Zimmer A, Kunos G. Cannabinoid-induced mesenteric vasodilation through an endothelial site distinct from CB1 or CB2 receptors. Proc Natl Acad Sci U S A. 1999;96:14136–41. [PMC free article] [PubMed]
• Jeske NA, Patwardhan AM, Gamper N, Price TJ, Akopian AN, Hargreaves KM. Cannabinoid WIN 55,212–2 regulates TRPV1 phosphorylation in sensory neurons. J Biol Chem. 2006;281:32879–90.[PMC free article] [PubMed]
• Jo SK, Bajwa A, Awad AS, Lynch KR, Okusa MD. Sphingosine-1-phosphate receptors: biology and therapeutic potential in kidney disease. Kidney Int. 2008;73:1220–30. [PMC free article] [PubMed]
• Johns DG, Behm DJ, Walker DJ, Ao Z, Shapland EM, Daniels DA, Riddick M, Dowell S, Staton PC, Green P, Shabon U, Bao W, Aiyar N, Yue TL, Brown AJ, Morrison AD, Douglas SA. The novel endocannabinoid receptor GPR55 is activated by atypical cannabinoids but does not mediate their vasodilator effects. Br J Pharmacol.2007;152:825–31. [PMC free article] [PubMed]
• Katona I, Freund TF. Endocannabinoid signaling as a synaptic circuit breaker in neurological disease. Nat Med. 2008;14:923–30. [PubMed]
• Khanolkar AD, Abadji V, Lin S, Hill WA, Taha G, Abouzid K, Meng Z, Fan P, Makriyannis A. Head group analogs of arachidonylethanolamide, the endogenous cannabinoid ligand. J Med Chem. 1996;39:4515–9.[PubMed]
• Kliewer SA, Sundseth SS, Jones SA, Brown PJ, Wisely GB, Koble CS, Devchand P, Wahli W, Willson TM, Lenhard JM, Lehmann JM. Fatty acids and eicosanoids regulate gene expression through direct interactions with peroxisome proliferator-activated receptors alpha and gamma. Proc Natl Acad Sci U S A. 1997;94:4318–23. [PMC free article][PubMed]
• Kohama T, Olivera A, Edsall L, Nagiec MM, Dickson R, Spiegel S. Molecular cloning and functional characterization of murine sphingosine kinase. J Biol Chem. 1998;273:23722–8. [PubMed]
• Kostenis E. Novel clusters of receptors for sphingosine-1-phosphate, sphingosylphosphorylcholine, and (lyso)-phosphatidic acid: new receptors for “old” ligands. J Cell Biochem. 2004;92:923–36. [PubMed]
• Kreitzer AC, Malenka RC. Endocannabinoid-mediated rescue of striatal LTD and motor deficits in Parkinson’s disease models. Nature.2007;445:643–7. [PubMed]
• Kreitzer AC, Regehr WG. Retrograde inhibition of presynaptic calcium influx by endogenous cannabinoids at excitatory synapses onto Purkinje cells. Neuron. 2001;29:717–27. [PubMed]
• Kunos G, Batkai S, Offertaler L, Mo F, Liu J, Karcher J, Harvey-White J. The quest for a vascular endothelial cannabinoid receptor. Chem Phys Lipids. 2002;121:45–56. [PubMed]
• LaMontagne K, Littlewood-Evans A, Schnell C, O’Reilly T, Wyder L, Sanchez T, Probst B, Butler J, Wood A, Liau G, Billy E, Theuer A, Hla T, Wood J. Antagonism of sphingosine-1-phosphate receptors by FTY720 inhibits angiogenesis and tumor vascularization. Cancer Res.2006;66:221–31. [PubMed]
• Lan R, Liu Q, Fan P, Lin S, Fernando SR, McCallion D, Pertwee R, Makriyannis A. Structure-activity relationships of pyrazole derivatives as cannabinoid receptor antagonists. J Med Chem. 1999;42:769–76.[PubMed]
• Landsman RS, Burkey TH, Consroe P, Roeske WR, Yamamura HI. SR141716A is an inverse agonist at the human cannabinoid CB1 receptor.Eur J Pharmacol. 1997;334:R1–2. [PubMed]
• Lastres-Becker I, Berrendero F, Lucas JJ, Martin-Aparicio E, Yamamoto A, Ramos JA, Fernandez-Ruiz JJ. Loss of mRNA levels, binding and activation of GTP-binding proteins for cannabinoid CB1 receptors in the basal ganglia of a transgenic model of Huntington’s disease. Brain Res.2002a;929:236–42. [PubMed]
• Lastres-Becker I, Fezza F, Cebeira M, Bisogno T, Ramos JA, Milone A, Fernandez-Ruiz J, Di Marzo V. Changes in endocannabinoid transmission in the basal ganglia in a rat model of Huntington’s disease.Neuroreport. 2001;12:2125–9. [PubMed]
• Lastres-Becker I, Gomez M, De Miguel R, Ramos JA, Fernandez-Ruiz J. Loss of cannabinoid CB(1) receptors in the basal ganglia in the late akinetic phase of rats with experimental Huntington’s disease. Neurotox Res. 2002b;4:601–608. [PubMed]
• Lastres-Becker I, Hansen HH, Berrendero F, De Miguel R, Perez-Rosado A, Manzanares J, Ramos JA, Fernandez-Ruiz J. Alleviation of motor hyperactivity and neurochemical deficits by endocannabinoid uptake inhibition in a rat model of Huntington’s disease. Synapse. 2002c;44:23–35. [PubMed]
• Lauckner JE, Jensen JB, Chen HY, Lu HC, Hille B, Mackie K. GPR55 is a cannabinoid receptor that increases intracellular calcium and inhibits M current. Proc Natl Acad Sci U S A. 2008;105:2699–704.[PMC free article] [PubMed]
• Lee MJ, Thangada S, Claffey KP, Ancellin N, Liu CH, Kluk M, Volpi M, Sha’afi RI, Hla T. Vascular endothelial cell adherens junction assembly and morphogenesis induced by sphingosine-1-phosphate. Cell.1999a;99:301–12. [PubMed]
• Lee OH, Kim YM, Lee YM, Moon EJ, Lee DJ, Kim JH, Kim KW, Kwon YG. Sphingosine 1-phosphate induces angiogenesis: its angiogenic action and signaling mechanism in human umbilical vein endothelial cells.Biochem Biophys Res Commun. 1999b;264:743–50. [PubMed]
• Lee OH, Lee DJ, Kim YM, Kim YS, Kwon HJ, Kim KW, Kwon YG. Sphingosine 1-phosphate stimulates tyrosine phosphorylation of focal adhesion kinase and chemotactic motility of endothelial cells via the G(i) protein-linked phospholipase C pathway. Biochem Biophys Res Commun. 2000;268:47–53. [PubMed]
• Leggett JD, Aspley S, Beckett SR, D’Antona AM, Kendall DA. Oleamide is a selective endogenous agonist of rat and human CB1 cannabinoid receptors. Br J Pharmacol. 2004;141:253–62. [PMC free article][PubMed]
• Lenman A, Fowler CJ. Interaction of ligands for the peroxisome proliferator-activated receptor gamma with the endocannabinoid system. Br J Pharmacol. 2007;151:1343–51. [PMC free article] [PubMed]
• Lin S, Khanolkar AD, Fan P, Goutopoulos A, Qin C, Papahadjis D, Makriyannis A. Novel analogues of arachidonylethanolamide (anandamide): affinities for the CB1 and CB2 cannabinoid receptors and metabolic stability. J Med Chem. 1998;41:5353–61. [PubMed]
• Liu H, Sugiura M, Nava VE, Edsall LC, Kono K, Poulton S, Milstien S, Kohama T, Spiegel S. Molecular cloning and functional characterization of a novel mammalian sphingosine kinase type 2 isoform. J Biol Chem.2000;275:19513–20. [PubMed]
• Lo Verme JL, Fu J, Astarita G, La Rana G, Russo R, Calignano A, Piomelli D. The nuclear receptor peroxisome proliferator-activated receptor-alpha mediates the anti-inflammatory actions of palmitoylethanolamide. Molecular Pharmacology. 2005;67:15–19.[PubMed]
• Lobo MK, Cui Y, Ostlund SB, Balleine BW, Yang XW. Genetic control of instrumental conditioning by striatopallidal neuron-specific S1P receptor Gpr6. Nat Neurosci. 2007;10:1395–7. [PubMed]
• LoVerme J, La Rana G, Russo R, Calignano A, Piomelli D. The search for the palmitoylethanolamide receptor. Life Sci. 2005;77:1685–98.[PubMed]
• Lunn CA, Fine JS, Rojas-Triana A, Jackson JV, Fan X, Kung TT, Gonsiorek W, Schwarz MA, Lavey B, Kozlowski JA, Narula SK, Lundell DJ, Hipkin RW, Bober LA. A novel cannabinoid peripheral cannabinoid receptor-selective inverse agonist blocks leukocyte recruitment in vivo. J Pharmacol Exp Ther. 2006;316:780–8. [PubMed]
• Lutz B. On-demand activation of the endocannabinoid system in the control of neuronal excitability and epileptiform seizures. Biochem Pharmacol. 2004;68:1691–8. [PubMed]
• Maccarrone M, Cartoni A, Parolaro D, Margonelli A, Massi P, Bari M, Battista N, Finazzi-Agro A. Cannabimimetic activity, binding, and degradation of stearoylethanolamide within the mouse central nervous system. Mol Cell Neurosci. 2002a;21:126–40. [PubMed]
• Maccarrone M, Pauselli R, Di Rienzo M, Finazzi-Agro A. Binding, degradation and apoptotic activity of stearoylethanolamide in rat C6 glioma cells. Biochem J. 2002b;366:137–44. [PMC free article][PubMed]
• MacLennan SJ, Reynen PH, Kwan J, Bonhaus DW. Evidence for inverse agonism of SR141716A at human recombinant cannabinoid CB1 and CB2 receptors. Br J Pharmacol. 1998;124:619–22. [PMC free article][PubMed]
• Mackie K, Hille B. Cannabinoids inhibit N-type calcium channels in neuroblastoma-glioma cells. Proc Natl Acad Sci U S A. 1992;89:3825–9.[PMC free article] [PubMed]
• Mackie K, Lai Y, Westenbroek R, Mitchell R. Cannabinoids activate an inwardly rectifying potassium conductance and inhibit Q-type calcium currents in AtT20 cells transfected with rat brain cannabinoid receptor.J Neurosci. 1995;15:6552–61. [PubMed]
• Mackie K, Stella N. Cannabinoid receptors and endocannabinoids: evidence for new players. Aaps J. 2006;8:E298–306. [PMC free article][PubMed]
• Maejima T, Ohno-Shosaku T, Kano M. Endogenous cannabinoid as a retrograde messenger from depolarized postsynaptic neurons to presynaptic terminals. Neurosci Res. 2001;40:205–10. [PubMed]
• Maingret F, Patel AJ, Lazdunski M, Honore E. The endocannabinoid anandamide is a direct and selective blocker of the background K(+) channel TASK-1. EMBO J. 2001;20:47–54. [PMC free article] [PubMed]
• Matsuda LA, Lolait SJ, Brownstein MJ, Young AC, Bonner TI. Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature. 1990;346:561–4. [PubMed]
• McAllister SD, Griffin G, Satin LS, Abood ME. Cannabinoid receptors can activate and inhibit G protein-coupled inwardly rectifying potassium channels in a xenopus oocyte expression system. J Pharmacol Exp Ther.1999;291:618–26. [PubMed]
• McCollum L, Howlett AC, Mukhopadhyay S. Anandamide-mediated CB1/CB2 cannabinoid receptor–independent nitric oxide production in rabbit aortic endothelial cells. J Pharmacol Exp Ther. 2007;321:930–7.[PubMed]
• Mechoulam R, Fride E, Di Marzo V. Endocannabinoids. Eur J Pharmacol. 1998;359:1–18. [PubMed]
• Mechoulam R, Ben-Shabat S, Hanus L, Ligumsky M, Kaminski NE, Schatz AR, Gopher A, Almog S, Martin BR, Compton DR, et al. Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochem Pharmacol. 1995;50:83–90. [PubMed]
• Mechoulam R. The pharmacohistory of Cannabis sativa. In: Mechoulam R, editor. Cannabinoids as Therapeutic Agents. CRC Press; Bocan Raton: 1986. pp. 1–19.
• Mechoulam R, Gaoni Y. The absolute configuration of delta-1-tetrahydrocannabinol, the major active constituent of hashish.Tetrahedron Lett. 1967;12:1109–11. [PubMed]
• Miller AM, Stella N. CB2 receptor-mediated migration of immune cells: it can go either way. Br J Pharmacol. 2008;153:299–308.[PMC free article] [PubMed]
• Milman G, Maor Y, Abu-Lafi S, Horowitz M, Gallily R, Batkai S, Mo FM, Offertaler L, Pacher P, Kunos G, Mechoulam R. N-arachidonoyl L-serine, an endocannabinoid-like brain constituent with vasodilatory properties. Proc Natl Acad Sci U S A. 2006;103:2428–33.[PMC free article] [PubMed]
• Mo FM, Offertaler L, Kunos G. Atypical cannabinoid stimulates endothelial cell migration via a Gi/Go-coupled receptor distinct from CB1, CB2 or EDG-1. Eur J Pharmacol. 2004;489:21–7. [PubMed]
• Molina-Holgado F, Molina-Holgado E, Guaza C, Rothwell NJ. Role of CB1 and CB2 receptors in the inhibitory effects of cannabinoids on lipopolysaccharide-induced nitric oxide release in astrocyte cultures. J Neurosci Res. 2002;67:829–36. [PubMed]
• Monory K, Tzavara ET, Lexime J, Ledent C, Parmentier M, Borsodi A, Hanoune J. Novel, not adenylyl cyclase-coupled cannabinoid binding site in cerebellum of mice. Biochem Biophys Res Commun.2002;292:231–5. [PubMed]
• Mukhopadhyay S, Shim JY, Assi AA, Norford D, Howlett AC. CB(1) cannabinoid receptor-G protein association: a possible mechanism for differential signaling. Chem Phys Lipids. 2002;121:91–109. [PubMed]
• Munro S, Thomas KL, Abu-Shaar M. Molecular characterization of a peripheral receptor for cannabinoids. Nature. 1993;365:61–5. [PubMed]
• Navarrete M, Araque A. Endocannabinoids mediate neuron-astrocyte communication. Neuron. 2008;57:883–93. [PubMed]
• O’Sullivan SE. Cannabinoids go nuclear: evidence for activation of peroxisome proliferator-activated receptors. Br J Pharmacol.2007;152:576–82. [PMC free article] [PubMed]
• O’Sullivan SE, Bennet AJ, Kendall DA, Randall MD. Cannabinoids and peroxisome proliferator-activated receptor gamma (PPARgamma) Proc Intl Cannbinoid Res Soc (abstract) 2006a
• O’Sullivan SE, Kendall DA, Randall MD. Further characterization of the time-dependent vascular effects of delta9-tetrahydrocannabinol. J Pharmacol Exp Ther. 2006b;317:428–38. [PubMed]
• O’Sullivan SE, Tarling EJ, Bennett AJ, Kendall DA, Randall MD. Novel time-dependent vascular actions of Delta9-tetrahydrocannabinol mediated by peroxisome proliferator-activated receptor gamma.Biochem Biophys Res Commun. 2005;337:824–31. [PubMed]
• Offertaler L, Mo FM, Batkai S, Liu J, Begg M, Razdan RK, Martin BR, Bukoski RD, Kunos G. Selective ligands and cellular effectors of a G protein-coupled endothelial cannabinoid receptor. Mol Pharmacol.2003;63:699–705. [PubMed]
• Oka S, Nakajima K, Yamashita A, Kishimoto S, Sugiura T. Identification of GPR55 as a lysophosphatidylinositol receptor. Biochem Biophys Res Commun. 2007;362:928–34. [PubMed]
• Okajima F. Plasma lipoproteins behave as carriers of extracellular sphingosine 1-phosphate: is this an atherogenic mediator or an anti-atherogenic mediator? Biochim Biophys Acta. 2002;1582:132–7.[PubMed]
• Olivera A, Kohama T, Edsall L, Nava V, Cuvillier O, Poulton S, Spiegel S. Sphingosine kinase expression increases intracellular sphingosine-1-phosphate and promotes cell growth and survival. J Cell Biol.1999;147:545–58. [PMC free article] [PubMed]
• Oz M. Receptor-independent effects of endocannabinoids on ion channels. Curr Pharm Des. 2006;12:227–39. [PubMed]
• Pinto JC, Potie F, Rice KC, Boring D, Johnson MR, Evans DM, Wilken GH, Cantrell CH, Howlett AC. Cannabinoid receptor binding and agonist activity of amides and esters of arachidonic acid. Mol Pharmacol. 1994;46:516–22. [PubMed]
• Pacher P, Batkai S, Kunos G. Cardiovascular pharmacology of cannabinoids. Handb Exp Pharmacol. 2005:599–625. [PMC free article][PubMed]
• Pacher P, Mukhopadhyay P, Mohanraj R, Godlewski G, Batkai S, Kunos G. Modulation of the endocannabinoid system in cardiovascular disease: therapeutic potential and limitations. Hypertension. 2008;52:601–7.[PMC free article] [PubMed]
• Park KS, Kim MK, Lee HY, Kim SD, Lee SY, Kim JM, Ryu SH, Bae YS. S1P stimulates chemotactic migration and invasion in OVCAR3 ovarian cancer cells. Biochem Biophys Res Commun. 2007;356:239–44.[PubMed]
• Patwardhan AM, Jeske NA, Price TJ, Gamper N, Akopian AN, Hargreaves KM. The cannabinoid WIN 55,212–2 inhibits transient receptor potential vanilloid 1 (TRPV1) and evokes peripheral antihyperalgesia via calcineurin. Proc Natl Acad Sci U S A.2006;103:11393–8. [PMC free article] [PubMed]
• Paugh SW, Cassidy MP, He H, Milstien S, Sim-Selley LJ, Spiegel S, Selley DE. Sphingosine and its analog, the immunosuppressant 2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol, interact with the CB1 cannabinoid receptor. Mol Pharmacol. 2006;70:41–50. [PubMed]
• Pazos MR, Sagredo O, Fernandez-Ruiz J. The endocannabinoid system in Huntington’s disease. Curr Pharm Des. 2008;14:2317–25. [PubMed]
• Price MR, Baillie GL, Thomas A, Stevenson LA, Easson M, Goodwin R, McLean A, McIntosh L, Goodwin G, Walker G, Westwood P, Marrs J, Thomson F, Cowley P, Christopoulos A, Pertwee RG, Ross RA. Allosteric modulation of the cannabinoid CB₁ receptor. Mol Pharm.2005;65:1484–1495. [PubMed]
• Qureshi J, Saady M, Cardounal A, Kalimi M. Identification and characterization of a novel synthetic cannabinoid CP55,940 binder in rat brain cytosol. Mol Cell Biochem. 1998;181:21–7. [PubMed]
• Ramirez BG, Blazquez C, Gomez del Pulgar T, Guzman M, de Ceballos ML. Prevention of Alzheimer’s disease pathology by cannabinoids: neuroprotection mediated by blockade of microglial activation. J Neurosci. 2005;25:1904–13. [PubMed]
• Randall MD, Harris D, Kendall DA, Ralevic V. Cardiovascular effects of cannabinoids. Pharmacol Ther. 2002;95:191–202. [PubMed]
• Rhee MH, Vogel Z, Barg J, Bayewitch M, Levy R, Hanus L, Breuer A, Mechoulam R. Cannabinol derivatives: binding to cannabinoid receptors and inhibition of adenylylcyclase. J Med Chem. 1997;40:3228–33.[PubMed]
• Rinaldi-Carmona M, Barth F, Heaulme M, Shire D, Calandra B, Congy C, Martinez S, Maruani J, Neliat G, Caput D, et al. SR141716A, a potent and selective antagonist of the brain cannabinoid receptor. FEBS Lett.1994;350:240–4. [PubMed]
• Rockwell CE, Snider NT, Thompson JT, Vanden Heuvel JP, Kaminski NE. Interleukin-2 suppression by 2-arachidonyl glycerol is mediated through peroxisome proliferator-activated receptor gamma independently of cannabinoid receptors 1 and 2. Mol Pharmacol.2006;70:101–11. [PubMed]
• Ross RA, Coutts AA, McFarlane SM, Anavi-Goffer S, Irving AJ, Pertwee RG, MacEwan DJ, Scott RH. Actions of cannabinoid receptor ligands on rat cultured sensory neurones: implications for antinociception.Neuropharmacology. 2001;40:221–32. [PubMed]
• Ross RA, Brockie HC, Stevenson LA, Murphy VL, Templeton F, Makriyannis A, Pertwee RG. Agonist-inverse agonist characterization at CB1 and CB2 cannabinoid receptors of L759633, L759656, and AM630.Br J Pharmacol. 1999;126:665–72. [PMC free article] [PubMed]
• Ruiu S, Pinna GA, Marchese G, Mussinu JM, Saba P, Tambaro S, Casti P, Vargiu R, Pani L. Synthesis and characterization of NESS 0327: a novel putative antagonist of the CB1 cannabinoid receptor. J Pharmacol Exp Ther. 2003;306:363–70. [PubMed]
• Ryberg E, Larsson N, Sjogren S, Hjorth S, Hermansson NO, Leonova J, Elebring T, Nilsson K, Drmota T, Greasley PJ. The orphan receptor GPR55 is a novel cannabinoid receptor. Br J Pharmacol.2007;152:1092–101. [PMC free article] [PubMed]
• Sanchez C, de Ceballos ML, Gomez del Pulgar T, Rueda D, Corbacho C, Velasco G, Galve-Roperh I, Huffman JW, Ramon S, Cajal y, Guzman M. Inhibition of glioma growth in vivo by selective activation of the CB(2) cannabinoid receptor. Cancer Res. 2001;61:5784–9. [PubMed]
• Savinainen JR, Saario SM, Niemi R, Jarvinen T, Laitinen JT. An optimized approach to study endocannabinoid signaling: evidence against constitutive activity of rat brain adenosine A1 and cannabinoid CB1 receptors. Br J Pharmacol. 2003;140:1451–9. [PMC free article][PubMed]
• Sawzdargo M, Nguyen T, Lee DK, Lynch KR, Cheng R, Heng HH, George SR, O’Dowd BF. Identification and cloning of three novel human G protein-coupled receptor genes GPR52, PsiGPR53 and GPR55: GPR55 is extensively expressed in human brain. Brain Res Mol Brain Res. 1999;64:193–8. [PubMed]
• Schatz AR, Lee M, Condie RB, Pulaski JT, Kaminski NE. Cannabinoid receptors CB1 and CB2: a characterization of expression and adenylate cyclase modulation within the immune system. Toxicol Appl Pharmacol.1997;142:278–87. [PubMed]
• Schmid G, Guba M, Ischenko I, Papyan A, Joka M, Schrepfer S, Bruns CJ, Jauch KW, Heeschen C, Graeb C. The immunosuppressant FTY720 inhibits tumor angiogenesis via the sphingosine 1-phosphate receptor 1.J Cell Biochem. 2007;101:259–70. [PubMed]
• Selley DE, Stark S, Sim LJ, Childers SR. Cannabinoid receptor stimulation of guanosine-5′-O-(3-[35S]thio)triphosphate binding in rat brain membranes. Life Sci. 1996;59:659–68. [PubMed]
• Sheng WS, Hu S, Min X, Cabral GA, Lokensgard JR, Peterson PK. Synthetic cannabinoid WIN55,212–2 inhibits generation of inflammatory mediators by IL-1beta-stimulated human astrocytes. Glia.2005;49:211–9. [PubMed]
• Shire D, Calandra B, Rinaldi-Carmona M, Oustric D, Pessegue B, Bonnin-Cabanne O, Le Fur G, Caput D, Ferrara P. Molecular cloning, expression and function of the murine CB2 peripheral cannabinoid receptor. Biochim Biophys Acta. 1996;1307:132–6. [PubMed]
• Shoemaker JL, Ruckle MB, Mayeux PR, Prather PL. Agonist-directed trafficking of response by endocannabinoids acting at CB2 receptors. J Pharmacol Exp Ther. 2005;315:828–38. [PubMed]
• Showalter VM, Compton DR, Martin BR, Abood ME. Evaluation of binding in a transfected cell line expressing a peripheral cannabinoid receptor (CB2): identification of cannabinoid receptor subtype selective ligands. J Pharmacol Exp Ther. 1996;278:989–99. [PubMed]
• Skaper SD, Buriani A, Dal Toso R, Petrelli L, Romanello S, Facci L, Leon A. The ALIAmide palmitoylethanolamide and cannabinoids, but not anandamide, are protective in a delayed postglutamate paradigm of excitotoxic death in cerebellar granule neurons. Proc Natl Acad Sci U S A. 1996;93:3984–9. [PMC free article] [PubMed]
• Skoura A, Sanchez T, Claffey K, Mandala SM, Proia RL, Hla T. Essential role of sphingosine 1-phosphate receptor 2 in pathological angiogenesis of the mouse retina. J Clin Invest. 2007;117:2506–16. [PMC free article][PubMed]
• Spiegel S. Sphingosine 1-phosphate: a prototype of a new class of second messengers. J Leukoc Biol. 1999;65:341–4. [PubMed]
• Stander S, Schmelz M, Metze D, Luger T, Rukwied R. Distribution of cannabinoid receptor 1 (CB1) and 2 (CB2) on sensory nerve fibers and adnexal structures in human skin. J Dermatol Sci. 2005;38:177–88.[PubMed]
• Staton PC, Hatcher JP, Walker DJ, Morrison AD, Shapland EM, Hughes JP, Chong E, Mander PK, Green PJ, Billinton A, Fulleylove M, Lancaster HC, Smith JC, Bailey LT, Wise A, Brown AJ, Richardson JC, Chessell IP. The putative cannabinoid receptor GPR55 plays a role in mechanical hyperalgesia associated with inflammatory and neuropathic pain. Pain.2008;139:225–36. [PubMed]
• Steffens M, Zentner J, Honegger J, Feuerstein TJ. Binding affinity and agonist activity of putative endogenous cannabinoids at the human neocortical CB1 receptor. Biochem Pharmacol. 2005;69:169–78.[PubMed]
• Sugiura T, Kondo S, Sukagawa A, Tonegawa T, Nakane S, Yamashita A, Waku K. N-arachidonoylethanolamine (anandamide), an endogenous cannabinoid receptor ligand, and related lipid molecules in the nervous tissues. J Lipid Mediat Cell Signal. 1996;14:51–6. [PubMed]
• Sun Y, Alexander SP, Garle MJ, Gibson CL, Hewitt K, Murphy SP, Kendall DA, Bennett AJ. Cannabinoid activation of PPAR alpha; a novel neuroprotective mechanism. Br J Pharmacol. 2007;152:734–43.[PMC free article] [PubMed]
• Tanaka S, Ishii K, Kasai K, Yoon SO, Saeki Y. Neural expression of G protein-coupled receptors GPR3, GPR6, and GPR12 up-regulates cyclic AMP levels and promotes neurite outgrowth. J Biol Chem.2007;282:10506–15. [PubMed]
• Thomas A, Ross RA, Saha B, Mahadevan A, Razdan RK, Pertwee RG. 6′-azidohex-2′-yne-cannabidiol: a potential neutral, competitive cannabinoid CB₁ receptor antagonist. Eur J Pharmacol. 2004;487:213–221. [PubMed]
• Thomas A, Baillie GL, Phillips AM, Razdan RK, Ross RA, Pertwee RG. Cannabidiol displays unexpectedly high potency as an antagonist of CB1 and CB2 receptor agonists in vitro. Br J Pharmacol. 2007;150:613–23.[PMC free article] [PubMed]
• Uhlenbrock K, Gassenhuber H, Kostenis E. Sphingosine 1-phosphate is a ligand of the human gpr3, gpr6 and gpr12 family of constitutively active G protein-coupled receptors. Cell Signal. 2002;14:941–53. [PubMed]
• Uhlenbrock K, Huber J, Ardati A, Busch AE, Kostenis E. Fluid shear stress differentially regulates gpr3, gpr6, and gpr12 expression in human umbilical vein endothelial cells. Cell Physiol Biochem. 2003;13:75–84.[PubMed]
• Van Sickle MD, Duncan M, Kingsley PJ, Mouihate A, Urbani P, Mackie K, Stella N, Makriyannis A, Piomelli D, Davison JS, Marnett LJ, Di Marzo V, Pittman QJ, Patel KD, Sharkey KA. Identification and functional characterization of brainstem cannabinoid CB2 receptors.Science. 2005;310:329–32. [PubMed]
• Vogel Z, Barg J, Levy R, Saya D, Heldman E, Mechoulam R. Anandamide, a brain endogenous compound, interacts specifically with cannabinoid receptors and inhibits adenylate cyclase. J Neurochem.1993;61:352–5. [PubMed]
• Wagner JA, Varga K, Jarai Z, Kunos G. Mesenteric vasodilation mediated by endothelial anandamide receptors. Hypertension.1999;33:429–34. [PubMed]
• Waldeck-Weiermair M, Zoratti C, Osibow K, Balenga N, Goessnitzer E, Waldhoer M, Malli R, Graier WF. Integrin clustering enables anandamide-induced Ca2+ signaling in endothelial cells via GPR55 by protection against CB1-receptor-triggered repression. J Cell Sci.2008;121:1704–17. [PubMed]
• Walter L, Franklin A, Witting A, Wade C, Xie Y, Kunos G, Mackie K, Stella N. Nonpsychotropic cannabinoid receptors regulate microglial cell migration. J Neurosci. 2003;23:1398–405. [PubMed]
• Wilson RI, Kunos G, Nicoll RA. Presynaptic specificity of endocannabinoid signaling in the hippocampus. Neuron. 2001;31:453–62. [PubMed]
• Wise A, Gearing K, Rees S. Target validation of G-protein coupled receptors. Drug Discov Today. 2002;7:235–46. [PubMed]
• Wotherspoon G, Fox A, McIntyre P, Colley S, Bevan S, Winter J. Peripheral nerve injury induces cannabinoid receptor 2 protein expression in rat sensory neurons. Neuroscience. 2005;135:235–45.[PubMed]
• Yoshida T, Hashimoto K, Zimmer A, Maejima T, Araishi K, Kano M. The cannabinoid CB1 receptor mediates retrograde signals for depolarization-induced suppression of inhibition in cerebellar Purkinje cells. J Neurosci. 2002;22:1690–7. [PubMed]
• Zhang H, Desai NN, Olivera A, Seki T, Brooker G, Spiegel S. Sphingosine-1-phosphate, a novel lipid, involved in cellular proliferation.J Cell Biol. 1991;114:155–67. [PMC free article] [PubMed]
• Zondag GC, Postma FR, Etten IV, Verlaan I, Moolenaar WH. Sphingosine 1-phosphate signalling through the G-protein-coupled receptor Edg-1. Biochem J. 1998;330(Pt 2):605–9. [PMC free article][PubMed]
• Zygmunt PM, Petersson J, Andersson DA, Chuang H, Sorgard M, Di Marzo V, Julius D, Hogestatt ED. Vanilloid receptors on sensory nerves mediate the vasodilator action of anandamide. Nature. 1999;400:452–7.[PubMed]