Transient receptor potential (TRP) channels are a group of membrane proteins involved in the transduction of a plethora of chemical and physical stimuli. These channels modulate ion entry, mediating a variety of neural signaling processes implicated in the sensation of temperature, pressure, and pH, as well as smell, taste, vision, and pain perception. Many diseases involve TRP channel dysfunction, including neuropathic pain, inflammation, and respiratory disorders. In the pursuit of new treatments for these disorders, it was discovered that cannabinoids can modulate a certain subset of TRP channels. The TRP vanilloid (TRPV), TRP ankyrin (TRPA), and TRP melastatin (TRPM) subfamilies were all found to contain channels that can be modulated by several endogenous, phytogenic, and synthetic cannabinoids. To date, six TRP channels from the three subfamilies mentioned above have been reported to mediate cannabinoid activity: TRPV1, TRPV2, TRPV3, TRPV4, TRPA1, and TRPM8. The increasing data regarding cannabinoid interactions with these receptors has prompted some researchers to consider these TRP channels to be “ionotropic cannabinoid receptors.” Although CB1 and CB2 are considered to be the canonical cannabinoid receptors, there is significant overlap between cannabinoids and ligands of TRP receptors. The first endogenous agonist of TRPV1 to be discovered was the endocannabinoid, anandamide (AEA). Similarly, N-arachidonyl dopamine (NADA) and AEA were the first endogenous TRPM8 antagonists discovered. Additionally, Δ9-tetrahydrocannabinol (Δ9-THC), the most abundant psychotropic compound in cannabis, acts most potently at TRPV2, moderately modulates TRPV3, TRPV4, TRPA1, and TRPM8, though Δ9-THC is not reported to modulate TRPV1. Moreover, TRP receptors may modulate effects of synthetic cannabinoids used in research. One common research tool is WIN55,212-2, a CB1 agonist that also exerts analgesic effects by desensitizing TRPA1 and TRPV1. In this review article, we aim to provide an overview and classification of the cannabinoid ligands that have been reported to modulate TRP channels and their therapeutic potential.

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The cannabinoid CB(1) receptor transmembrane helix (TMH) 3-4-5-6 region includes an aromatic microdomain comprised of residues F3.25, F3.36, W4.64, Y5.39, W5.43, and W6.48. In previous work, we have demonstrated that aromaticity at position 5.39 in CB(1) is crucial for proper function of CB(1). Modeling studies reported here suggest that in the inactive state of CB(1), the binding site of the CB(1) inverse agonist/antagonist SR141716A is within the TMH3-4-5-6 aromatic microdomain and involves direct aromatic stacking interactions with F3.36, Y5.39, and W5.43, as well as hydrogen bonding with K3.28. Further, modeling studies suggest that in the active state of CB(1), the CB agonist WIN55,212-2 binds in this same aromatic microdomain, with direct aromatic stacking interactions with F3.36, W5.43, and W6.48. In contrast, in the binding pocket model, the CB agonist anandamide binds in the TMH2-3-6-7 region in which hydrogen bonding and C-H.pi interactions appear to be important. Only one TMH3 aromatic residue, F3.25, was found to be part of the anandamide binding pocket. To probe the importance of the TMH3-4-5-6 aromatic microdomain to ligand binding, stable transfected cell lines were created for single-point mutations of each aromatic microdomain residue to alanine. Improper cellular expression of the W4.64A was observed and precluded further characterization of this mutation. The affinity of the cannabinoid agonist CP55,940 was unaffected by the F3.25A, F3.36A, W5.43A, or W6.48A mutations, making CP55,940 an appropriate choice as the radioligand for binding studies. The binding of SR141716A and WIN55,212-2 were found to be affected by the F3.36A, W5.43A, and W6.48A mutations, suggesting that these residues are part of the binding site for these two ligands. Only the F3.25A mutation was found to affect the binding of anandamide, suggesting a divergence in binding site regions for anandamide from WIN55,212-2, as well as SR141716A. Taken together, these results support modeling studies that identify the TMH3-4-5-6 aromatic microdomain as the binding region of SR141716A and WIN55,212-2, but not of anandamide.

Pregnenolone is considered the inactive precursor of all steroid hormones and its potential functional effects have been largely neglected. The administration of the main active principle of Cannabis sativa (marijuana) Δ9-tetrahydrocannabinol (THC) substantially increases the synthesis of pregnenolone in the brain via the activation of type-1 cannabinoid (CB1) receptor. Pregnenolone then, acting as a signaling specific inhibitor of the CB1 receptor, reduces several effects of THC. This negative feedback mediated by pregnenolone reveals an unknown paracrine/autocrine loop protecting the brain from CB1 receptor over-activation that could open an unforeseen novel approach for the treatment of cannabis intoxication and addiction.

The cannabinoid receptors belong to the class A (rhodopsin family) of G protein-coupled receptors (GPCRs).2 The second cannabinoid receptor subtype, CB2 (2), is highly expressed throughout the immune system (3, 4) and has been described in the central nervous system under both pathological (5) and physiological conditions (6). All known CB2 ligands are highly lipophilic. In fact, the CB2 endogenous cannabinoid, sn-2-arachidonoylglycerol (2-AG) (7,8), is synthesized on demand from the lipid bilayer itself in a two-step process in which phospholipase C-␤ hydrolyzes phosphatidylinositol 4,5-bisphosphate to generate diacylglycerol, which is then hydrolyzed by diacylglycerol lipase to yield 2-AG (9, 10). After 2-AG interaction with the membraneembedded CB receptor, it is hydrolyzed to arachidonic acid and glycerol by a membrane-associated enzyme, monoacylglycerol lipase (11). As revealed by the crystal structures of rhodopsin (12-15), the ␤ 2 -adrenergic receptor (AR) (16 -18), ␤ 1 -AR (19), and adenosine A2A receptor (20), the general topology of a GPCR includes the following: 1) an extracellular (EC) N terminus; 2) seven transmembrane ␣-helices (TMHs) arranged to form a closed bundle; 3) loops connecting TMHs that extend intra-and extracellularly; and 4) an intracellular (IC) C terminus that begins with a short helical segment (helix 8) oriented parallel to the membrane surface. Agonists bind inside the crevice formed by the TMH bundle and produce conformational changes on the IC face of the receptor that uncover previously masked G protein-binding sites (21), which then lead to G protein coupling. Biophysical studies using a variety of techniques indicate that ligand-induced receptor activation produces the following changes: 1) a conformational change in the W6.48 "toggle switch" within the ligand binding pocket (22); 2) a change in the relative orientations of TMH3 and -6 that breaks an IC "ionic lock" (23-28), with the intracellular end of TMH6 moving away from TMH3 by hinging and moving up toward lipid (27); 3) the uptake of two protons (29); and 4) an influx of water (30).Recent isothiocyanate covalent labeling studies have suggested that a classical cannabinoid, AM841, enters the CB2 receptor via the lipid bilayer (1). However, the sequence of steps involved in such a lipid pathway entry has not yet been elucidated. We report here microsecond time scale unbiased

We have previously identified allosteric modulators of the cannabinoid CB 1 receptor (Org 27569, PSNCBAM-1) that display a contradictory pharmacological profile: increasing the specific binding of the CB 1 receptor agonist [ 3 H]CP55940 but producing a decrease in CB 1 receptor agonist efficacy. Here we investigated the effect one or both compounds in a broad range of signaling endpoints linked to CB 1 receptor activation. We assessed the effect of these compounds on CB 1 receptor agonist-induced [35 S]GTPgS binding, inhibition, and stimulation of forskolin-stimulated cAMP production, phosphorylation of extracellular signal-regulated kinases (ERK), and b-arrestin recruitment. We also investigated the effect of these allosteric modulators on CB 1 agonist binding kinetics. -mediated), and inhibition (Ga i -mediated) of cAMP production and b-arrestin recruitment. In contrast, it acts as an enhancer of agonist-induced ERK phosphorylation. Alone, the compound can act also as an allosteric agonist, increasing cAMP production and ERK phosphorylation. We find that in both saturation and kineticbinding experiments, the Org 27569 and PSNCBAM-1 appeared to influence only orthosteric ligand maximum occupancy rather than affinity. The data indicate that the allosteric modulators share a common mechanism whereby they increase available high-affinity CB 1 agonist binding sites. The receptor conformation stabilized by the allosterics appears to induce signaling and also selectively traffics orthosteric agonist signaling via the ERK phosphorylation pathway.

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In superior cervical ganglion neurons, N-(piperidiny-1-yl)-5-(4-