Two types of cannabinoid receptor have been discovered so far, CB(1) (2.1: CBD:1:CB1:), cloned in 1990, and CB(2) (2.1:CBD:2:CB2:), cloned in 1993. Distinction between these receptors is based on differences in their predicted amino acid sequence, signaling mechanisms, tissue distribution, and sensitivity to certain potent agonists and antagonists that show marked selectivity for one or the other receptor type. Cannabinoid receptors CB(1) and CB(2) exhibit 48% amino acid sequence identity. Both receptor types are coupled through G proteins to adenylyl cyclase and mitogen-activated protein kinase. CB(1) receptors are also coupled through G proteins to several types of calcium and potassium channels. These receptors exist primarily on central and peripheral neurons, one of their functions being to inhibit neurotransmitter release. Indeed, endogenous CB(1) agonists probably serve as retrograde synaptic messengers. CB(2) receptors are present mainly on immune cells. Such cells also express CB(1) receptors, albeit to a lesser extent, with both receptor types exerting a broad spectrum of immune effects that includes modulation of cytokine release. Of several endogenous agonists for cannabinoid receptors identified thus far, the most notable are arachidonoylethanolamide, 2-arachidonoylglycerol, and 2-arachidonylglyceryl ether. It is unclear whether these eicosanoid molecules are the only, or primary, endogenous agonists. Hence, we consider it premature to rename cannabinoid receptors after an endogenous agonist as is recommended by the International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. Although pharmacological evidence for the existence of additional types of cannabinoid receptor is emerging, other kinds of supporting evidence are still lacking.

https://pharmrev.aspetjournals.org/content/pharmrev/54/2/161.full.pdf

International Union of Pharmacology. XXVII. Classification of Cannabinoid Receptors

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2894%2900773-X

SR141716A, a potent and selective antagonist of the brain cannabinoid receptor

Research of cannabinoid actions was boosted in the 1990s by remarkable discoveries including identification of endogenous compounds with cannabimimetic activity (endocannabinoids) and the cloning of their molecular targets, the CB1 and CB2 receptors. Although the existence of an endogenous cannabinoid signaling system has been established for a decade, its physiological roles have just begun to unfold. In addition, the behavioral effects of exogenous cannabinoids such as delta-9-tetrahydrocannabinol, the major active compound of hashish and marijuana, await explanation at the cellular and network levels. Recent physiological, pharmacological, and high-resolution anatomical studies provided evidence that the major physiological effect of cannabinoids is the regulation of neurotransmitter release via activation of presynaptic CB1 receptors located on distinct types of axon terminals throughout the brain. Subsequent discoveries shed light on the functional consequences of this localization by demonstrating the involvement of endocannabinoids in retrograde signaling at GABAergic and glutamatergic synapses. In this review, we aim to synthesize recent progress in our understanding of the physiological roles of endocannabinoids in the brain. First, the synthetic pathways of endocannabinoids are discussed, along with the putative mechanisms of their release, uptake, and degradation. The fine-grain anatomical distribution of the neuronal cannabinoid receptor CB1 is described in most brain areas, emphasizing its general presynaptic localization and role in controlling neurotransmitter release. Finally, the possible functions of endocannabinoids as retrograde synaptic signal molecules are discussed in relation to synaptic plasticity and network activity patterns.

/pdf/role-of-endogenous-cannabinoids-in-synaptic-signaling-115q7g965r.pdf

Role of Endogenous Cannabinoids in Synaptic Signaling

To understand the functional significance and mechanisms of action in the CNS of endogenous and exogenous cannabinoids, it is crucial to identify the neural elements that serve as the structural substrate of these actions We used a recently developed antibody against the CB1 cannabinoid receptor to study this question in hippocampal networks Interneurons with features typical of basket cells showed a selective, intense staining for CB1 in all hippocampal subfields and layers Most of them (856%) contained cholecystokinin (CCK), which corresponded to 969% of all CCK-positive interneurons, whereas only 46% of the parvalbumin (PV)-containing basket cells expressed CB1 Accordingly, electron microscopy revealed that CB1-immunoreactive axon terminals of CCK-containing basket cells surrounded the somata and proximal dendrites of pyramidal neurons, whereas PV-positive basket cell terminals in similar locations were negative for CB1 The synthetic cannabinoid agonist WIN 55,212-2 (001–3 μm) reduced dose-dependently the electrical field stimulation-induced [ 3 H]GABA release from superfused hippocampal slices, with an EC 50 value of 0041 μm Inhibition of GABA release by WIN 55,212-2 was not mediated by inhibition of glutamatergic transmission because the WIN 55,212-2 effect was not reduced by the glutamate blockers AP5 and CNQX In contrast, the CB1 cannabinoid receptor antagonist SR 141716A (1 μm) prevented this effect, whereas by itself it did not change the outflow of [ 3 H]GABA These results suggest that cannabinoid-mediated modulation of hippocampal interneuron networks operate largely via presynaptic receptors on CCK-immunoreactive basket cell terminals Reduction of GABA release from these terminals is the likely mechanism by which both endogenous and exogenous CB1 ligands interfere with hippocampal network oscillations and associated cognitive functions

https://www.jneurosci.org/content/jneuro/19/11/4544.full.pdf

Presynaptically Located CB1 Cannabinoid Receptors Regulate GABA Release from Axon Terminals of Specific Hippocampal Interneurons

Based on both binding and functional data, this study introduces SR 144528 as the first, highly potent, selective and orally active antagonist for the CB2 receptor. This compound which displays subnanomolar affinity ( Ki = 0.6 nM) for both the rat spleen and cloned human CB2 receptors has a 700-fold lower affinity ( Ki = 400 nM) for both the rat brain and cloned human CB1 receptors. Furthermore it shows no affinity for any of the more than 70 receptors, ion channels or enzymes investigated (IC50 > 10 μM). In vitro , SR 144528 antagonizes the inhibitory effects of the cannabinoid receptor agonist CP 55,940 on forskolin-stimulated adenylyl cyclase activity in cell lines permanently expressing the h CB2 receptor (EC50 = 10 nM) but not in cells expressing the h CB1 (no effect at 10 μM). Furthermore, SR 144528 is able to selectively block the mitogen-activated protein kinase activity induced by CP 55,940 in cell lines expressing h CB2 (IC50 = 39 nM) whereas in cells expressing h CB1 an IC50 value of more than 1 μM is found. In addition, SR 144528 is shown to antagonize the stimulating effects of CP 55,940 on human tonsillar B-cell activation evoked by cross-linking of surface Igs (IC50 = 20 nM). In vivo , after oral administration SR 144528 totally displaced the ex vivo [3H]-CP 55,940 binding to mouse spleen membranes (ED50 = 0.35 mg/kg) with a long duration of action. In contrast, after the oral route it does not interact with the cannabinoid receptor expressed in the mouse brain (CB1). It is expected that SR 144528 will provide a powerful tool to investigate the in vivo functions of the cannabinoid system in the immune response.

/pdf/sr-144528-the-first-potent-and-selective-antagonist-of-the-58gtabjpgn.pdf

SR 144528, the First Potent and Selective Antagonist of the CB2 Cannabinoid Receptor

Pravadoline (1) is an (aminoalkyl)indole analgesic agent which is an inhibitor of cyclooxygenase and, in contrast to other NSAIDs, inhibits neuronally stimulated contractions in mouse vas deferens (MVD) preparations (IC50 = 0.45 microM). A number of conformationally restrained heterocyclic analogues of pravadoline were synthesized in which the morpholinoethyl side chain was tethered to the indole nucleus. Restraining the morpholine diminished the ability of these pravadoline analogues to inhibit prostaglandin synthesis in vitro. In contrast, mouse vas deferens inhibitory activity was enhanced in [2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl] pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(4-methoxyphenyl)methano ne (20). Only the R enantiomer of 20 was active (IC50 = 0.044 microM). An optimal orientation of the morpholine nitrogen for MVD inhibitory activity within the analogues studied was in the lower right quadrant, below the plane defined by the indole ring. A subseries of analogues of 20 and a radioligand of the most potent analogue, (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo [1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone (21) were prepared. Inhibition of radioligand binding in rat cerebellar membranes was observed to correlate with functional activity in mouse vas deferens preparations. Binding studies with this ligand (Win 55212-2) have helped demonstrate that the (aminoalkyl)indole binding site is functionally equivalent with the CP-55,940 cannabinoid binding site. These compounds represent a new class of cannabinoid receptor agonists.

Conformationally restrained analogues of pravadoline: nanomolar potent, enantioselective, (aminoalkyl)indole agonists of the cannabinoid receptor.

C-Attached aminoalkylindoles: potent cannabinoid mimetics

2--R2 --R4 -substituted-3--R3 --CO--1--[(C-attached-N-heteryl)-(Alk)n ]--1H-indoles useful as analgesic, anti-rheumatic, anti-inflammatory or anti-glaucoma agents.

3-arylcarbonyl-1(C-attached-N-heteryl)-1H-indoles

2- and 3-Aminomethyl-6-arylcarbonyl-2,3-dihydropyrrolo[1,2,3-de]-1,4-benzoxazines, useful as analgesic agents, are prepared either by reaction of a 2- or 3-aminomethyl-2,3-dihydropyrrolo[1,2,3-de]-1,4-benzoxazine with an arylcarboxylic acid halide in the presence of a Lewis acid or by reaction of a 2- or 3-aminomethyl-4-amino-3,4-dihydro-2H-1,4-benzoxazine with a 1-lower-alkyl-3-aryl-1,3-diketone or with a β-aryl-β-ketopropionaldehyde under dehydrating conditions and heating the resulting hydrazone in an acid medium.

Thomas E. D'Ambra

Papers

Conformationally restrained analogues of pravadoline: nanomolar potent, enantioselective, (aminoalkyl)indole agonists of the cannabinoid receptor.

C-Attached aminoalkylindoles: potent cannabinoid mimetics

3-arylcarbonyl-1(C-attached-N-heteryl)-1H-indoles

2- and 3-aminomethyl-6-arylcarbonyl-2,3-dihydropyrrolo[1,2,3-de]-1,4-benzoxazines

2- or 3-aminomethyl-4-amino-3,4-dihydro-2H-1,4-benzoxazine intermediates