International Union of Basic and Clinical Pharmacology. LXXIX. Cannabinoid receptors and their ligands: beyond CB₁ and CB₂

There are at least two types of cannabinoid receptors (CB(1) and CB(2)). Ligands activating these G protein-coupled receptors (GPCRs) include the phytocannabinoid Δ(9)-tetrahydrocannabinol, numerous synthetic compounds, and endogenous compounds known as endocannabinoids. Cannabinoid receptor antagonists have also been developed. Some of these ligands activate or block one type of cannabinoid receptor more potently than the other type. This review summarizes current data indicating the extent to which cannabinoid receptor ligands undergo orthosteric or allosteric interactions with non-CB(1), non-CB(2) established GPCRs, deorphanized receptors such as GPR55, ligand-gated ion channels, transient receptor potential (TRP) channels, and other ion channels or peroxisome proliferator-activated nuclear receptors. From these data, it is clear that some ligands that interact similarly with CB(1) and/or CB(2) receptors are likely to display significantly different pharmacological profiles. The review also lists some criteria that any novel "CB(3)" cannabinoid receptor or channel should fulfil and concludes that these criteria are not currently met by any non-CB(1), non-CB(2) pharmacological receptor or channel. However, it does identify certain pharmacological targets that should be investigated further as potential CB(3) receptors or channels. These include TRP vanilloid 1, which possibly functions as an ionotropic cannabinoid receptor under physiological and/or pathological conditions, and some deorphanized GPCRs. Also discussed are 1) the ability of CB(1) receptors to form heteromeric complexes with certain other GPCRs, 2) phylogenetic relationships that exist between CB(1)/CB(2) receptors and other GPCRs, 3) evidence for the existence of several as-yet-uncharacterized non-CB(1), non-CB(2) cannabinoid receptors; and 4) current cannabinoid receptor nomenclature.

Molecular biology of cannabinoid receptors

To date, two cannabinoid receptors have been isolated by molecular cloning. The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor family. There is also evidence for additional cannabinoid receptor subtypes. The CB1 and CB2 receptors recognize endogenous and exogenous cannabinoid compounds, which fall into five structurally diverse classes. Mutagenesis and molecular modeling studies have identified several key amino acid residues involved in the selective recognition of these ligands. Numerous residues involved in receptor activation have been elucidated. Regions of the CB1 receptor mediating desensitization and internalization have also been discovered. The known genetic structures of the CB1 and CB2 receptors indicate polymorphisms and multiple exons that maybe involved in tissue and species-specific regulation of these genes. The cannabinoid receptors are regulated during chronic agonist exposure, and gene expression is altered in disease states. There is a complex molecular architecture of the cannabinoid receptors that allows a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects.

Activation of the CB1 cannabinoid receptor protects cultured mouse spinal neurons against excitotoxicity

Significant advances are being made towards understanding the genetic basis for spinal neurodegenerative diseases, however, effective pharmacotherapy remains elusive. One of the primary theories underlying neuron vulnerability is susceptibility to excitotoxicity. We present for the first time evidence that the activation of the CB(1) cannabinoid receptor effectively modulates kainate toxicity in primary neuronal cultures prepared from mouse spinal cord. Addition of Delta(9)-tetrahydrocannabinol to the culture medium attenuated the toxicity produced by kainate. The CB(1) receptors were localized to spinal neurons and astrocytes. The neuroprotective effect was blocked with the CB(1) receptor antagonist, SR141716A, indicating a receptor-mediated effect.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

Separation of cannabinoid receptor affinity and efficacy in delta-8-tetrahydrocannabinol side-chain analogues

1. The activities of a number of side-chain analogues of delta-8-tetrahydrocannabinol (Delta(8)-THC) in rat cerebellar membrane preparations were tested. 2. The affinities of each compound for the CB(1) receptor were compared by their respective abilities to displace [(3)H]-SR141716A and their efficacies compared by stimulation of [(35)S]-GTPgammaS binding. 3. It was found that the affinities varied from 0.19+/-0.03 nM for 3-norpentyl-3-[6'-cyano,1',1'-dimethyl]hexyl-Delta(8)-THC to 395+/-66.3 nM for 5'-[N-(4-chlorophenyl)]-1',1'-dimethyl-carboxamido-Delta(8)-THC. 4. The efficacies of these compounds varied greatly, ranging from the very low efficacy exhibited to acetylenic compounds such as 1'-heptyn-Delta(8)-THC and 4'-octyn-Delta(8)-THC to higher efficacy compounds such as 5'-(4-cyanophenoxy)-1',1'-dimethyl-Delta(8)-THC and 5'-[N-(4-aminosulphonylphenyl)]-1',1' dimethyl-carboxamido Delta(8)-THC. All agonist activities were antagonized by the CB(1)-selective antagonist SR141716A. 5. It was found that a ligand's CB(1) affinity and efficacy are differentially altered by modifications in the side-chain. Decreasing the flexibility of the side-chain reduced efficacy but largely did not alter affinity. Additionally, the positioning of electrostatic moieties, such as cyano groups, within the side-chain also has contrasting effects on these two properties. 6. In summary, this report details the characterization of a number of novel Delta(8)-THC analogues in rat cerebellar membranes. It provides the first detailed pharmacological analysis of how the inclusion of electrostatic moieties in the side-chain and also how alteration of the side-chain's flexibility may differentially affect a CB(1) cannabinoid receptor ligand's affinity and efficacy.

Novel cannabinol probes for CB1 and CB2 cannabinoid receptors

The observation that the phenolic hydroxyl of THCs was important for binding to the CB1 receptor but not as critical for binding to the CB2 receptor prompted us to extend this finding to the cannabinol (CBN) series. To study the SAR of CBN analogues, CBN derivatives with substitution at the C-1, C-3, and C-9 positions were chosen since these positions have played a key role in the SAR of THCs. CBN-3-(1',1'-dimethylheptyl) analogues were prepared by sulfur dehydrogenation of Delta(8)-THC-3-(1',1'-dimethylheptyl) analogues. 9-Substituted CBN analogues were prepared by the standard sulfur dehydrogenation of 9-substituted Delta(8)-THC analogues (Scheme 1), which in turn were prepared following our previous procedure using selenium dioxide oxidation of the corresponding Delta(8)-THCs followed by sodium chlorite oxidation to give the 9-carboxy-Delta(8)-THC derivatives. 11-Hydroxy-CBN analogues were prepared from the corresponding 9-carbomethoxy-CBN analogues by reduction with LiAlH(4). Deoxy-CBN analogue 14 was prepared from the corresponding Delta(8)-THC analogue 11 by conversion of the phenolic hydroxyl to the phosphate derivative 12, followed by lithium ammonia reduction to provide the deoxy-Delta(8)-THC analogue 13, which in turn was dehydrogenated with sulfur to provide the deoxy-CBN analogue 14 (Scheme 2). The various analogues were assayed for binding both to the brain and the peripheral cannabinoid receptors (CB1 and CB2). We have found that the binding profile differs widely between the CBN and the THC series. Specifically, in the CBN series the removal of the phenolic hydroxyl decreases binding affinity to both the CB1 and CB2 receptors, whereas in the THC series, CB1 affinity is selectively reduced. Thus, in the CBN series, the selectivity of binding observed with the removal of the hydroxy group is decreased severalfold as compared to what occurs in the THC series. Generally, high affinity for the CB2 receptor was found in analogues when the phenolic hydroxyl was present. The 3-(1', 1'-dimethylheptyl) derivatives were found to have much higher affinities than the CBN analogues, which is in complete agreement with previously reported work by Rhee et al.

Influence of the N-1 alkyl chain length of cannabimimetic indoles upon CB(1) and CB(2) receptor binding

The N-1 alkyl side chain of the aminoalkylindole analogues (AAI) has been implicated as one of a three-point interaction with the cannabinoid CB(1) receptor. In this study, the morpholinoethyl of WIN 55,212-2 was replaced with carbon chains of varying lengths ranging from a methyl to heptyl group. Additional groups were added to the naphthoyl and the C2 positions of the molecule. These structural changes revealed that high affinity binding to the CB(1) and CB(2) receptors requires an alkyl chain length of at least three carbons with optimum binding to both receptors occurring with a five carbon side chain. An alkyl chain of 3-6 carbons is sufficient for high affinity binding; however, extension of the chain to a heptyl group results in a dramatic decrease in binding at both receptors. The unique structure of the cannabimimetic indoles provides a useful tool to define the ligand-receptor interaction at both cannabinoid receptors and to refine proposed pharmacophore models.

Cloning and pharmacological characterization of the rat CB(2) cannabinoid receptor

Many of the pharmacological effects of Delta(9)-tetrahydrocannabinol are mediated through CB(1) and CB(2) cannabinoid receptors. However, with the discovery of endogenous cannabinoids, some discrepancies have arisen. Furthermore, unlike the CB(1) receptor, the sequences of the mouse and human CB(2) receptor are divergent, raising the possibility of species specificity. The gene for the rat CB(2) receptor was cloned, expressed, and its properties compared with those of mouse and human CB(2) receptors. Sequence analysis of the coding region of the rat CB(2) genomic clone indicates 90% nucleic acid identity (93% amino acid identity) between rat and mouse and 81% nucleic acid identity (81% amino acid identity) between rat and human. The rat CB(2) receptor was stably expressed in human embryonic kidney-293 cells to examine its pharmacology. The rat CB(2) showed low affinity for anandamide, an endogenous ligand shown to act at the CB(1) receptor. In contrast, high-affinity binding for SR144528 (CB(2)-selective antagonist) as well as several cannabinoid receptor agonists was observed. Coupling to adenylate cyclase was observed. Aspects of the pharmacology of palmitoylethanolamide were also examined. It bound to CB(1) and CB(2) receptors with low affinity and stimulated GTPgammaS binding in the cerebellum and CB(2)-expressing cell lines with low potency. The data in this study suggest that the discrepancies in affinities between rat and human may represent species differences. The rat CB(2) receptor genomic clone will be a useful tool for studying the function and regulation of CB(2) in rats.

3-(1',1'-Dimethylbutyl)-1-deoxy-delta8-THC and related compounds: synthesis of selective ligands for the CB2 receptor

The synthesis and pharmacology of 15 1-deoxy-delta8-THC analogues, several of which have high affinity for the CB2 receptor, are described. The deoxy cannabinoids include 1-deoxy-11-hydroxy-delta8-THC (5), 1-deoxy-delta8-THC (6), 1-deoxy-3-butyl-delta8-THC (7), 1-deoxy-3-hexyl-delta8-THC (8) and a series of 3-(1',1'-dimethylalkyl)-1-deoxy-delta8-THC analogues (2, n = 0-4, 6, 7, where n = the number of carbon atoms in the side chain-2). Three derivatives (17-19) of deoxynabilone (16) were also prepared. The affinities of each compound for the CB1 and CB2 receptors were determined employing previously described procedures. Five of the 3-(1',1'-dimethylalkyl)-1-deoxy-delta8-THC analogues (2, n = 1-5) have high affinity (Ki = < 20 nM) for the CB2 receptor. Four of them (2, n = 1-4) also have little affinity for the CB1 receptor (Ki = > 295 nM). 3-(1',1'-Dimethylbutyl)-1-deoxy-delta8-THC (2, n = 2) has very high affinity for the CB2 receptor (Ki = 3.4 +/- 1.0 nM) and little affinity for the CB1 receptor (Ki = 677 +/- 132 nM).

Cannabinoid receptors can activate and inhibit G protein-coupled inwardly rectifying potassium channels in a xenopus oocyte expression system

In this study, we focused on the pharmacological characterization of cannabinoid receptor coupling to G protein-gated inwardly rectifying potassium (GIRK) channels. Cannabinoids were tested on Xenopus laevis oocytes coexpressing the CB(1) receptor and GIRK1 and GIRK4 channels (CB(1)/GIRK1/4) or the CB(2) receptor and GIRK1/4 channels (CB(2)/GIRK1/4). WIN 55,212-2 enhanced currents carried by GIRK channels in the CB(1)/GIRK1/4 and CB(2)/GIRK1/4 system; however, the CB(2) receptor did not couple efficiently to GIRK1/4 channels. In the CB(1)/GIRK1/4 system, WIN 55,212-2 was the most efficacious compound tested. CP 55,940 and anandamide acted as partial agonists. The rank order of potency was CP 55,940 > WIN 55,212-2 = anandamide. The CB(1)-selective antagonist SR141716A alone acted as a inverse agonist by inhibiting GIRK currents in oocytes expressing CB(1)/GIRK1/4, suggesting the CB(1) receptor is constitutively activated. A conserved aspartate residue, which was previously shown to be critical for G protein coupling in cannabinoid receptors, was mutated (to asparagine, D163N) and analyzed. Oocytes coexpressing CB(1)/GIRK1/4 or D163N/GIRK1/4 were compared. The potency of WIN 55, 212-2 at the mutant receptor was similar to wild type, but its efficacy was substantially reduced. CP 55,940 did not elicit currents in oocytes expressing D163N/GIRK1/4. In summary, it appears the CB(1) and CB(2) receptors couple differently to GIRK1/4 channels. In the CB(1)/GIRK1/4 system, cannabinoids evaluated demonstrated the ability to enhance or inhibit GIRK currents. Furthermore, a conserved aspartate residue in the CB(1) receptor is required for normal communication with GIRK channels in oocytes demonstrating the interaction between receptor and channels is G protein dependent.

Cannabinoid agonists and antagonists discriminated by receptor binding in rat cerebellum

1. The effect of allosteric regulators on the binding affinity of a number of cannabinoid receptor ligands of varying efficacy in the rat cerebellum was investigated. 2. Radioligand ([3H]-SR141716A) competition curves were constructed in the presence or absence of sodium ions, magnesium ions and guanine nucleotides. 3. It was found that the presence of these allosteric regulators did not affect the affinity of the two antagonists used but did cause a significant decrease in the affinity of full and partial agonists. 4. This reduction in affinity ranged from a 3.67 fold rightward shift of the displacement curve of a mixed agonist/antagonist (3-(6-cyano-2-hexynyl)-delta-8-tetrahydrocannabinol-O-823) to a 38 fold rightward shift for 3-(1, 1-dimethyl-6-dimethylcarboxamide)-delta-8-tetrahydrocannabinol (O-1125), a full agonist. 5. In summary, the results of this study suggest a simple method for the inference of functional data using the classical radioligand binding assay.

Evaluation of the cannabinoid CB2 receptor-selective antagonist, SR144528: further evidence for cannabinoid CB2 receptor absence in the rat central nervous system

The aim of this study was to characterize the activity of the cannabinoid CB2 receptor selective antagonist, N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazo le-3-carboxamide] (SR144528) in a number of biochemical assays and to look for evidence of cannabinoid CB2 receptors in the rat central nervous system. SR144528 displaced [3H]CP 55,940 ((-)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)-phenyl]-4-[3-hydroxyprop yl]cyclohexan-1-ol) from binding sites in CB2- and CB1-transfected cells (Ki = 0.67+/-0.30 and 33.0+/-5.09 nM) and from rat cerebellum and whole brain membrane homogenates (Ki = 54.7+/-9.70 and 54.8+/-7.86 nM). In the GTPgammaS binding assay, SR144528 antagonized a number of cannabinoid receptor agonists (K(B) values ranging from 26.3 to 76.6 nM) in rat cerebellar membranes and in rat whole brain membranes (K(B) = 50.8 nM). SR144528 also antagonized CP 55,940-stimulated GTPgammaS binding in a CB2-expressing cell line (K(B) = 6.34 nM). In Xenopus oocytes co-expressing the CB1 receptor and G-protein coupled inwardly rectifying K+ channels (GIRK 1/4), SR144528 antagonized WIN 55212-2((R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrolo [1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone) -stimulated K+ currents (K(B) = 558 nM). In summary, this report characterizes the cannabinoid CB2 receptor-selective cannabinoid antagonist, SR144528, and additionally suggests an absence of cannabinoid CB2 receptors in the rat central nervous system, an observation confirmed by Northern blot.

Unique analogues of anandamide: arachidonyl ethers and carbamates and norarachidonyl carbamates and ureas

To examine the effect of changing the amide bond of anandamide (5, AN) to a less hydrolyzable moiety, analogues 1a-1l, 2a-2c, 3a-3c, and 4a-4h were synthesized from commercially available arachidonyl alcohol or arachidonic acid and tested for their pharmacological activity. Arachidonyl ethers 1a-1k were obtained through the coupling of the arachidonyl mesylate (6) (generated from the mesylation of arachidonyl alcohol) with the appropriate alcohol in potassium hydroxide. Arachidonyl ether 1l was obtained through the phase-transfer coupling of arachidonyl alcohol with 2-(2-iodoethoxy)tetrahydropyran (which was generated from its bromide) followed by cleavage of the tetrahydropyran group with Dowex resin. Arachidonyl carbamates 2a-2c were obtained through the coupling of arachidonyl alcohol with the appropriate isocyanates. Norarachidonyl carbamates 3a-3c and ureas 4a-4h were obtained through the coupling of the norarachidonyl isocyanate (generated from arachidonic acid using diphenyl phosphorazidate and triethylamine upon heating) with the appropriate alcohols and amines, respectively. AN analogues 1-3 have shown poor binding affinities to the CB1 receptor and fail to produce significant pharmacological effect at doses up to 30 mg/kg. Several ether analogues 1 were also evaluated in the CB2 binding assay and were found to be of low affinity. However, norarachidonyl urea analogues 4 have shown generally good binding affinities to the CB1 receptor (Ki = 55-746 nM) and pharmacological activity with AN-like profiles. The most potent analogue of this series is the 2-fluoroethyl analogue 4f which binds 2 times better than AN and was more active in several mouse behavioral assays. It was also observed that urea analogues 4a and 4g, which have weak binding affinities to the CB1 receptor (Ki = 436 and 347 nM, respectively), produced surprisingly potent pharmacological activity. These urea analogues have also shown hydrolytic stability toward the amidase enzymes, responsible for the primary degradation pathway of anandamide, in binding affinity assays in the absence of the enzyme inhibitor PMSF.

An investigation into the structural determinants of cannabinoid receptor ligand efficacy

1. A number of side-chain analogues of delta8-THC were tested in GTPgammaS binding assay in rat cerebellar membranes. O-1125, a saturated side-chain compound stimulated GTPgammaS binding with an Emax of 165.0%, and an EC50 of 17.4 nM. 2. O-1236, O-1237 and O-1238, three-enyl derivatives containing a cis carbon-carbon double bond in the side-chain, stimulated GTPgammaS binding, acting as partial agonists with Emax values ranging from 51.3-87.5% and EC50 values between 4.4 and 29.7 nM. 3. The stimulatory effects of O-1125, O-1236, O-1237 and O-1238 on GTPgammaS binding were antagonized by the CB1 receptor antagonist SR 141716A. The K(B) values obtained ranged from 0.11-0.21 mM, suggesting an action at CB1 receptors. 4. Five-ynyl derivatives (O-584, O-806, O-823, O-1176 and O-1184), each containing a carbon-carbon triple bond in the side-chain, did not stimulate GTPgammaS binding and were tested as potential cannabinoid receptor antagonists. 5. Each -ynyl compound antagonized the stimulatory effects of four cannabinoid receptor agonists on GTPgammaS binding. The K(B) values obtained, all found to be in the nanomolar range, did not differ between agonists or from cerebellar binding affinity. 6. In conclusion, alterations of the side-chain of the classical cannabinoid structure may exert a large influence on affinity and efficacy at the CB1 receptor. 7. Furthermore, this study confirms the ability of the GTPgammaS binding assay to assess discrete differences in ligand efficacies which potentially may not be observed using alternative functional assays, thus providing a unique tool for the assessment of the molecular mechanisms underlying ligand efficacies.

Role of a conserved lysine residue in the peripheral cannabinoid receptor (CB2): evidence for subtype specificity

The human cannabinoid receptors, central cannabinoid receptor (CB1) and peripheral cannabinoid receptor (CB2), share only 44% amino acid identity overall, yet most ligands do not discriminate between receptor subtypes. Site-directed mutagenesis was employed as a means of mapping the ligand recognition site for the human CB2 cannabinoid receptor. A lysine residue in the third transmembrane domain of the CB2 receptor (K109), which is conserved between the CB1 and CB2 receptors, was mutated to alanine or arginine to determine the role of this charged amino acid in receptor function. The analogous mutation in the CB1 receptor (K192A) was found to be crucial for recognition of several cannabinoid compounds excluding (R)-(+)-[2, 3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1, 4-benzoxazin-6-yl](1-naphthalenyl)methanone (WIN 55,212-2). In contrast, in human embryonic kidney (HEK)-293 cells expressing the mutant or wild-type CB2 receptors, we found no significant differences in either the binding profile of several cannabinoid ligands nor in inhibition of cAMP accumulation. We identified a high-affinity site for (-)-3-[2-hydroxyl-4-(1, 1-dimethylheptyl)phenyl]-4-[3-hydroxyl propyl] cyclohexan-1-ol (CP-55,940) in the region of helices 3, 6, and 7, with S3.31(112), T3.35(116), and N7.49(295) in the K109A mutant using molecular modeling. The serine residue, unique to the CB2 receptor, was then mutated to glycine in the K109A mutant. This double mutant, K109AS112G, retains the ability to bind aminoalkylindoles but loses affinity for classical cannabinoids, as predicted by the molecular model. Distinct cellular localization of the mutant receptors observed with immunofluorescence also suggests differences in receptor function. In summary, we identified amino acid residues in the CB2 receptor that could lead to subtype specificity.

Stereoselective mu- and delta-opioid receptor-related antinociception and binding with (+)-thebaine

In vivo and in vitro binding studies with natural thebaine and its enantiomer, (+)-thebaine were conducted to elucidate further their interactions with the opioid system. (-)-Thebaine a key intermediate in the biosynthesis of morphine in the poppy plant (Papaver somniferum) and in mammalian tissue, was poorly effective antinociceptively in mice at doses to 30 mg/kg. Its principal behavioral manifestation was lethal convulsions. Naltrindole, at doses of 1 and 10 mg/kg did not block either the convulsions or lethal effects, suggesting that the delta-opioid receptor system was not involved in this action. Surprisingly, the dextrorotatory isomer exhibited significant antinociceptive activity in the tail-flick [ED50 = 8.9 (3.4-22.1) mg/kg], hot-plate [ED50 = 22.9 (10.9-48.1) mg/kg] and phenylquinone [ED50) = 1.9 (1.6-9.5) mg/kg] assays. Studies with opioid receptor-subtype antagonists, beta-funaltrexamine, nor-binaltorphimine and naltrindole, indicated that antinociception was associated with mu- and delta-opioid receptors. Results of displacement experiments supported the in vivo data. Significant competition for [3H]diprenorphine binding with both isomers for cloned mu- and delta-opioid receptors was observed. However, (-)-thebaine was more effective at the delta-opioid receptor (Ki = 1.02+/-0.01 microM) whereas (+)-thebaine was more effective at the mu-opioid receptor ( Ki = 2.75+/-0.01 microM). Opioid-induced antinociception associated with unnatural thebaine raises the possibility of additional mu- and delta-opioid receptor sites.

The bioactive conformation of aminoalkylindoles at the cannabinoid CB1 and CB2 receptors: insights gained from (E)- and (Z)-naphthylidene indenes

The aminoalkylindoles (AAIs) are agonists at both the cannabinoid CB1 and CB2 receptors. To determine whether the s-trans or s-cis form of AAIs is their receptor-appropriate conformation, two pairs of rigid AAI analogues were studied. These rigid analogues are naphthylidene-substituted aminoalkylindenes that lack the carbonyl oxygen of the AAIs. Two pairs of (E)- and (Z)-naphthylidene indenes (C-2 H and C-2 Me) were considered. In each pair, the E geometric isomer is intended to mimic the s-trans form of the AAIs, while the Z geometric isomer is intended to mimic the s-cis form. Complete conformational analyses of two AAIs, pravadoline (2) and WIN-55, 212-2 (1), and of each indene were performed using the semiempirical method AM1. S-trans and s-cis conformations of 1 and 2 were identified. AM1 single-point energy calculations revealed that when 1 and each indene were overlayed at their corresponding indole/indene rings, the (E)- and (Z)-indenes were able to overlay naphthyl rings with the corresponding s-trans or s-cis conformer of 1 with an energy expense of 1.13/0.69 kcal/mol for the C-2 H (E/Z)-indenes and 0.82/0.74 kcal/mol for the C-2 Me (E/Z)-indenes. On the basis of the hypothesis that aromatic stacking is the predominant interaction of AAIs such as 1 at the CB receptors and on the demonstration that the C-2 H (E/Z)- and C-2 Me (E/Z)-indene isomers can mimic the positions of the aromatic systems in the s-trans and s-cis conformers of 1, the modeling results support the previously established use of indenes as rigid analogues of the AAIs. A synthesis of the naphthylidene indenes was developed using Horner-Wittig chemistry that afforded the Z isomer in the C-2 H series, which was not produced in significant amounts from an earlier reported indene/aldehyde condensation reaction. This approach was extended to the C-2 Me series as well. Photochemical interconversions in both the C-2 H and C-2 Me series were also successful in obtaining the less favored isomer. Thus, the photochemical process can be used to provide quantities of the minor isomers C-2 H/Z and C-2 Me/E. The CB1 and CB2 affinities as well as the activity of each compound in the twitch response of the guinea pig ileum (GPI) assay were assessed. The E isomer in each series was found to have the higher affinity for both the CB1 and CB2 receptors. In the rat brain membrane assay versus [3H]CP-55,940, the Ki's for the C-2 H/C-2 Me series were 2.72/2.89 nM (E isomer) and 148/1945 nM (Z isomer). In membrane assays versus [3H]SR141716A, a two-site model was indicated for the C-2 H/C-2 Me (E isomers) with Ki's of 10. 8/9.44 nM for the higher-affinity site and 611/602 nM for the lower-affinity site. For the Z isomers, a one-site model was indicated with Ki's of 928/2178 nM obtained for the C2 H/C-2 Me analogues, respectively. For the C-2 H/C-2 Me series, the CB2 Ki's obtained using a cloned cell line were 2.72/2.05 nM (E isomer) and 132/658 nM (Z isomer). In the GPI assay, the relative order of potency was C-2 H E > C-2 Me E > C-2 H Z > C-2 Me Z. The C-2 H E isomer was found to be equipotent with 1, while the C-2 Me Z isomer was inactive at concentrations up to 3.16 microM. Thus, results indicate that the E geometric isomer in each pair of analogues is the isomer with the higher CB1 and CB2 affinities and the higher pharmacological potency. Taken together, results reported here support the hypothesis that the s-trans conformation of AAIs such as 1 is the preferred conformation for interaction at both the CB1 and CB2 receptors and that aromatic stacking may be an important interaction for AAIs at these receptors.

High-level expression of the human CB2 cannabinoid receptor using a baculovirus system

A human CB2 recombinant baculovirus (AcNPV-hCB2) was generated by site-specific transposition and employed to express the human CB2 cannabinoid receptor. Northern analysis of total RNA from Spodoptera frugiperda (Sf9) insect cells infected with AcNPV-hCB2 revealed novel expression of a unique 2.3 kb transcript when probed with hCB2 cDNA. This transcript corresponded to the size expected for hCB2 generated from the recombinant virus construct. Western immunoblot analysis of whole cell homogenates of recombinant baculovirus-infected Sf9 cells, using affinity-purified antibody to a human CB2 carboxy terminal domain (anti-hCB2.CV), revealed the presence of novel immunoreactive protein. In addition, when anti-hCB2.CV was employed in immunofluorescence staining, an intense signal was observed within AcNPV-hCB2-infected cells but not within uninfected cells or cells infected with a control beta-galactosidase recombinant baculovirus. The pattern of immunofluorescence at early periods post-infection was in a perinuclear arrangement with a "signet-ring" appearance, suggestive of glycosylation of the expressed recombinant protein. Transmission electron microscopy revealed regions of intranuclear recombinant virus assembly and the presence of numerous intracytoplasmic proteinaceous vesicular inclusions consistent with hyperproduction of hCB2. Scatchard-Rosenthal analysis of [3H]-(-)3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-[3-hydroxypro pyl]cyclohexan-1-ol ([3H]CP 55,940) receptor binding indicated a Kd of 2.24 nM and a Bmax equal to 5.24 pmol/mg of protein. The lack of [3H]CP 55,940 displacement with N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-met hyl-1H-pyrazole-3-carboxamidehydrochloride (SR 141716A), the CB1-selective antagonist, confirmed the identity of the receptor as CB2. These data indicate that AcNPV-hCB2 expresses high levels of the human CB2, which retains properties of the native receptor. Thus, this recombinant virus may prove suitable for hyperproduction of receptor for basic biochemical and biophysical characterization studies.

Evaluation of cannabinoid receptor agonists and antagonists using the guanosine-5'-O-(3-[35S]thio)-triphosphate binding assay in rat cerebellar membranes

Cannabinoid receptors are members of the superfamily of G protein-coupled receptors. Their activation has previously been shown to stimulate guanosine 5'-O-(3-[35S]thio)-triphosphate ([35S]GTP gamma S) binding in a range of brain regions using both membrane preparations and autoradiography. This study evaluates the activities of structurally diverse cannabinoid receptor ligands in the GTP gamma S binding assay, comparing the relationship between receptor binding and activation and also examining efficacy differences between compounds. Using rat cerebellar membrane preparations, the effects of GDP concentration on GTP gamma S binding and the activities of a range of cannabinoid receptor ligands, including the CB1 selective antagonist SR141716A, were investigated. GDP concentration was found to have differing effects on cannabinoid-stimulated [35S]GTP gamma S binding depending on the nature of the agonist used. The stimulation produced by high efficacy compounds, such as CP 55,940 and WIN 55212-2, was increased by raising the GDP concentration, but that of a low efficacy agonist, (-)-delta-tetrahydrocannabinol, was decreased. Of the cannabinoid compounds tested, a wide range of potencies (EC50) and levels of maximal stimulation (Emax) were observed. These ranged from CP 55,244 (Emax of 165, 148-183%, and an EC50 of 0.47, 0.22-0.96, nM) through (-)-delta-tetrahydrocannabinol, cannabinol and anandamide, which produced no concentration-dependent stimulation of [35S]GTP gamma S binding under the same conditions. SR141716A competitively antagonized all the agonists against which it was tested, providing equilibrium dissociation constants (Kd values) in the sub-nanomolar range (0.06-0.40 nM), implicating a CB1 receptor mediated response. These results provide a more detailed characterization of the cannabinoid-stimulated [35S]GTP gamma S binding assay than has previously been reported.

Mutation of a highly conserved aspartate residue in the second transmembrane domain of the cannabinoid receptors, CB1 and CB2, disrupts G-protein coupling

The cannabinoid receptors, CB1 and CB2, are members of the G-protein coupled receptor family and share many of this family's structural features. A highly conserved aspartic acid residue in the second transmembrane domain of G-protein coupled receptors has been shown for many of these receptors to be functionally important for agonist binding and/or G-protein coupling. To determine whether this residue is involved in cannabinoid receptor function, we used site-directed mutagenesis of receptor cDNA followed by expression of the mutant receptor in HEK 293 cells. Aspartate 163 (in CB1) and aspartate 80 (in CB2) were substituted with either asparagine or glutamate. Stably transfected cell lines were tested for radioligand binding and inhibition of cAMP accumulation. Binding of the cannabinoid receptor agonist [3H]CP-55,940 was not affected by either mutation in either the CB1 or CB2 receptor, nor were the affinities of anandamide or (-)-delta 9-tetrahydrocannabinol. Binding of the CB1-selective receptor antagonist SR141716A also was unaltered. However, the affinity of WIN 55,212-2 was attenuated significantly in the CB1, but not the CB2, mutant receptors. Studies examining inhibition of cAMP accumulation showed reduced effects of cannabinoid agonists in the mutated receptors. Our data suggest that this aspartate residue is not generally important for ligand recognition in the cannabinoid receptors; however, it is required for communication with G proteins and signal transduction.

Importance of the C-1 substituent in classical cannabinoids to CB2 receptor selectivity: synthesis and characterization of a series of O,2-propano-delta 8-tetrahydrocannabinol analogs

The separation of the mood-altering effects of cannabinoids from their therapeutic effects has been long sought. Results reported here for a series of C-9 analogs of the cyclic ether O,2-propano-delta 8-tetrahydrocannabinol (O,2-propano-delta 8-THC) point to the C-1 position in classical cannabinoids as a position for which CB2 subtype selectivity occurs within the cannabinoid receptors. O,2-Propano-11-delta 8-THC, O,2-propano delta 9,11-THC, O,2-propano-9-oxo-11-nor-hexahydrocannabinol (O,2-propano-9-oxo-11-nor-HHC), and O,2-propano-9 alpha- and O,2-propano-9 beta-OH-11-nor-HHC were synthesized and evaluated in radioligand displacement assays for affinity at the CB1 and CB2 receptors and in the mouse vas deferens in vitro assay and the mouse tetrad in vivo assay for cannabinoid activity. Evaluation of binding affinity at the CB1 and CB2 receptors revealed that each compound possesses a modest increased affinity for the CB2 receptor. Analogs which contained an oxygen attached to C-9 (i.e., oxo and hydroxy derivatives) showed the highest affinity and selectivity for CB2 (for O,2-propano-9-oxo-11-nor-HHC, Ki(CB1) = 90 nM, Ki(CB2) = 23 nM, selectivity ratio 3.9; for O,2-propano-9 beta-OH-11-nor-HHC, Ki(CB1) = 26 nM, Ki(CB2) = 5.8 nM, selectivity ratio 4.5). Each compound was found to produce a dose-dependent inhibition of electrically-evoked contractions of the mouse isolated vas deferens when administered at submicromolar concentrations. This inhibition could readily be prevented by the selective CB1 cannabinoid receptor antagonist SR-141716A. The analogs exhibited unique in vivo profiles with O,2-propano-delta 9,11-THC exhibiting antinociception with reduced activity in three other in vivo measures and O,2-propano-9 beta-OH-HHC exhibiting lack of dose responsiveness in all measures. The CB2 selectivities in the O,2-propano analogs may be due to differences in solvation/desolvation that occur when the ligands enter the CB1 vs CB2 binding site. Alternatively, the CB2 selectivities may be a results of an amino acid change from a hydrogen bond-accepting residue in CB1 to a hydrogen bond-donating residue in CB2.

Isolation and expression of a mouse CB1 cannabinoid receptor gene. Comparison of binding properties with those of native CB1 receptors in mouse brain and N18TG2 neuroblastoma cells

The predominant animal model in which the pharmacology of cannabinoids is studied is the mouse. Nonetheless, the structure and functional expression of the mouse cannabinoid receptor (CB1) gene have not been reported. We have cloned and expressed the gene for the mouse CB1 receptor and compared its properties with those of native mouse CB1 receptors in brain and N18TG2 neuroblastoma cells. The mouse CB1 gene was isolated from a mouse 129 strain genomic library. Sequence analysis of a 6-kb BamHI fragment of the mouse CB1 genomic clone indicates 95% nucleic acid identity between mouse and rat (99.5% amino acid identity) and 90% nucleic acid identity (97% amino acid identity) between mouse and human. Examination of the 5' untranslated sequence of the mouse CB1 genomic clone revealed a splice junction site approximately 60 bp upstream from the translation start site, indicating the possibility of splice variants of the CB1 receptors. The coding region of the mouse CB1 receptor was stably expressed in 293 cells, and binding by [3H]SR 141716A and [3H]CP-55,940 was determined. The Bmax and Kd values obtained with [3H]SR 141716A (921 +/- 58 fmol/mg and 0.73 +/- 0.13 nM, respectively) were similar to those of native mouse CB1 receptors in brain (Bmax of 1.81 +/- 0.44 pmol/mg, Kd of 0.16 +/- 0.01 nM) and N18TG2 cells (Bmax of 197 +/- 29 fmol/mg, Kd of 0.182 +/- 0.08 nM). The mouse CB1 receptor genomic clone will be a useful tool for studying the function and regulation of the CB1 receptor in mice.

Evaluation of binding in a transfected cell line expressing a peripheral cannabinoid receptor (CB2): identification of cannabinoid receptor subtype selective ligands

Two cannabinoid receptors have been identified to date; one is located predominantly in the central nervous system (CB1), whereas the other is located exclusively in the periphery (CB2). The purposes of this study were to explore further the binding requirements of the CB2 receptor and to search for compounds displaying distinct affinities for either cannabinoid receptor. The binding affinities of a series of cannabinoids tested previously at the CB1 receptor were determined at cloned human CB1 and CB2 receptors using a filtration assay. In addition, possible allosteric regulation of the CB2 receptor was examined. Sodium and a GTP analog elicited a concentration-dependent decrease in specific binding to the CB2 receptor. The affinity of cannabinol for CB2 receptors (Ki = 96.3 +/- 14 nM) was confirmed to be in approximately the same range as that of delta 9-THC (Ki = 36.4 +/- 10 nM). Affinities at cloned CB1 and CB2 receptors were compared with affinities determined in the brain. Although most of the chosen compounds did not discriminate between CB1 and CB2, several ligands were identified that showed selectivity. Affinity ratios demonstrated that two 2'-fluoro analogs of anandamide were over 23-fold selective for the CB1 receptor and confirmed the CB1 selectivity of SR141716A {N- (piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4- methyl-1H-pyrazole-3-carboxamidehydrochloride}. In addition, WIN-55, 212-2 {(R)-(+)-[2, 3-dihydro-5-methyl-3-[(4-morpholinyl) methyl] pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl) methanone} and a closely related propyl indole analog were shown to be 6.75- and 27.5- fold selective, respectively, for the CB2 receptor. These ligands can now serve as a basis for the design of compounds with even greater selectivity.

Inhibition of exocytotic noradrenaline release by presynaptic cannabinoid CB1 receptors on peripheral sympathetic nerves

1. Activation of CB1 receptors by plant cannabinoids or the endogenous ligand, anandamide, causes hypotension via a sympathoinhibitory action in anaesthetized rats. In mouse isolated vas deferens, activation of CB1 receptors inhibits the electrically evoked twitch response. To determine if these effects are related to presynaptic inhibition of noradrenaline (NA) release, we examined the effects of delta 9-tetrahydrocannabinol (delta 9-THC), anandamide and the CB1 antagonist, SR141716A, on exocytotic NA release in rat isolated atria and vasa deferentia. 2. In isolated atria and vasa deferentia preloaded with [3H]-NA, electrical field stimulation caused [3H]-NA release, which was abolished by tetrodotoxin 0.5 microM and concentration-dependently inhibited by delta 9-THC or anandamide, 0.3-10 microM. The inhibitory effect of delta 9-THC and anandamide was competitively antagonized by SR 141716A, 1-10 microM. 3. Tyramine, 1 microM, also induced [3H]-NA release, which was unaffected by tetrodotoxin, delta 9-THC or anandamide in either atria or vasa deferentia. 4. CB1 receptor mRNA is present in the superior cervical ganglion, as well as in whole brain, cerebellum, hypothalamus, spleen, and vas deferens and absent in medulla oblongata and atria, as demonstrated by reverse transcription-polymerase chain reaction. There was no evidence of the presence of CB1A receptor mRNA in ganglia, brain, or cerebellum. These results suggest that activation of presynaptic CB1 receptors located on peripheral sympathetic nerve terminals mediate sympathoinhibitory effects in vitro and in vivo.

Molecular neurobiology of the cannabinoid receptor

Marijuana is currently the most widely abused street drug. However, the functional significance of the cannabinoid receptor system in health and disease includes the use of cannabinoids as analgesics, antiemetics in cancer patients, anticonvulsants for epilepsy, and as antiglaucoma agents as well as immunomodulatory agents. Our knowledge of the mechanisms of action of cannabinoids has increased greatly in the past several years. Two cannabinoid receptors have been identified to date: one is located predominantly in the central nervous system (CBI), whereas the other is expressed in peripheral tissues (CB2). Both are members of the G-protein-coupled receptor family and couple to inhibition of adenylyl cyclase (as well as additional second messenger systems), in transfected cells expressing these receptors, and in the nervous system. An endogenous ligand has been isolated for the CBI receptor; it is arachidonic acid ethanolamide, or anandamide. Candidate endogenous ligands for the CB2 receptor have also been described. Another development is the discovery of a selective antagonist for the CBI receptor. The distribution of the cannabinoid receptor subtypes has been mapped by receptor autoradiography, RT-PCR and in situ hybridization. These new research tools will aid in the elucidation of the physiological role of the endogenous cannabinoid system.

Evaluation of a series of N-alkyl benzomorphans in cell lines expressing transfected delta- and mu-opioid receptors

Transfection of individual opioid receptors in Chinese hamster ovary (CHO) cells provides a pure, homogeneous population of receptors for screening drug candidates, and an alternative to the use of selective ligands. To evaluate the potential of this system, we chose a series of (-)-5,9 alpha-dimethyl-2-hydroxy-N-substituted-6,7-benzomorphans, for which the receptor selectivity and in vivo activity had been characterized recently, and tested them in CHO cells stably transfected with either the rat delta-opioid receptor or the mouse mu-opioid receptor. [3H]Diprenorphine was used to measure opioid receptors in P2 membrane preparations. A Bmax of 7.58 +/- 0.8 pmol/mg protein and a Kd of 0.42 +/- 0.04 nM was obtained in the mu-opioid receptor expressing cell line used in these studies. In addition, [3H]naltrindole was used to confirm the delta-specificity of the cloned receptor. Both compounds gave a Bmax of 1.2 pmol/mg in the CHO cells expressing the rat delta-opioid receptor. Displacement assays were performed with eleven (-)-N-alkyl-benzomorphans in the absence and presence of 150 mM NaCl, as well as known delta- and mu-selective agonists. Sodium reduced agonist affinity in the transfected cell lines. The benzomorphan compounds displayed a range of affinities in the mu- and delta-opioid receptor expressing cell lines. Good correlations were found between their affinities at the cloned mu- and delta-opioid receptors and those in rat brain and monkey cortex (r2 from 0.73 to 0.89, P < 0.001). Comparative analysis of Ki values with in vivo potency in the mouse tail flick test indicated a high degree of correlation between antinociception and affinity in the mu-opioid receptor cell line (r2 = 0.83, p < 0.0001). Lesser correlations were found between antinociception in the mouse and affinity at the rat mu-opioid receptor (r2 = 0.6610) and at the monkey mu-opioid receptor (r2 = 0.695). In sum, these studies indicate that the cell lines expressing the cloned mu- and delta-opioid receptors are appropriate models for determining the binding affinities of this class of opioid compounds. The diminishing correlations found between species when comparing in vitro and in vivo activity suggest that caution should be taken when extrapolating binding data to pharmacological activity among species.

Characterization of a delta opioid receptor in rat pheochromocytoma cells

In one subclone of PC12 pheochromocytoma cells, PC12h, the levels of delta opioid receptors markedly increase in response to nerve growth factor (NGF). This increase, as assessed by [3H]diprenorphine binding, is found only under specific culture conditions. NGF treatment of PC12h cells also results in the induction of delta opioid receptor (DOR-1) mRNA. The time course for NGF induction of mRNA and protein is similar, although the levels of mRNA increase approximately 5-fold, whereas the levels of receptor increase only 2-fold. Competition studies with selective delta ([D-Pen2,5]-enkephalin, DPDPE), mu ([D-Ala2,N-Me-Phe4,Gly-ol5]- enkephalin, DAMGO) and kappa (trans-[+/-]-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzenacetamide methansulfonate salt, U50,488) opioid agonists to displace [3H]diprenorphine confirm that the delta subtype of opioid receptor is present on PC12h cells. The delta opioid receptor is coupled functionally, as indicated by agonist inhibition of forskolin-stimulated cAMP accumulation in a naloxone-reversible manner. Finally, the delta opioid receptor was cloned from PC12h cells. The sequence was identical with that described previously for the rat clone. The PC12h cell line thus provides a model system in which to study regulation of delta opioid receptors.

Antisense oligodeoxynucleotides to the kappa-1 receptor block the antinociceptive effects of delta 9-THC in the spinal cord

Intrathecal pretreatment of mice with an antisense oligodeoxynucleotide directed against the kappa-1 receptor significantly reduced the antinociceptive effects of the kappa receptor agonist U50,488 as well as delta 9-THC, the major psychoactive ingredient found in cannabis. A mismatched oligodeoxynucleotide which contained four switched bases did not block the antinociception produced by U50,488 or delta 9-THC. Furthermore, kappa-1 antisense did not alter the antinociceptive effects of either the mu receptor-selective opioid DAMGO, or the delta receptor-selective opioid DPDPE. By using kappa-1 antisense, we were able to demonstrate that an interaction occurs between the cannabinoids and opioids in the spinal cord.

Analysis of the antinociceptive actions of the kappa-opioid agonist enadoline (CI-977) in neonatal and adult rats: comparison to kappa-opioid receptor mRNA ontogeny

Earlier reports indicate that kappa-opioid agonists may be especially potent in the formalin test of tonic nociception, and that neonatal rat pups are more sensitive to mu-agonists, when compared to adults. We have assessed the potency of enadoline (CI-977), a novel and selective kappa-opioid agonist, in the formalin and tail-flick nociceptive tests in 3-day-old rat pups and compared their responses to adults in the same tests. In addition, the recent cloning of the kappa-opioid receptor has allowed us to make the first evaluation of the ontogeny of kappa-opioid receptor mRNA in an effort to elucidate a possible mechanism for differences in sensitivity to kappa-opioid agonists. Enadoline was found to be a potent antinociceptive agent in the formalin test; the neonates were eight times more sensitive than the adults. kappa-Opioid receptor mRNA, however, is present in whole brain at adult levels as early as postnatal day 3. Previous studies have shown kappa-opioid receptor levels, as measured by radioligand binding and receptor autoradiography, to be present at postnatal day 3 as well. Consequently, it is unlikely that gross differences in receptor number subserved the modest increase in sensitivity to enadoline observed in the neonates in the formalin test. Enadoline was less potent and less effective in the tail-flick test in the neonates. The adults were similarly insensitive to the antinociceptive effects of enadoline in the tail-flick test. Additional studies indicated that enadoline significantly increased locomotor activity, as assessed by the open-field test, in neonates, while decreasing activity in the adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Isolation and developmental expression of a rat cDNA encoding a cysteine-rich zinc finger protein

A number of cysteine-rich proteins have recently been isolated by homology screening, differential library screens, and association with other proteins. In this report, we describe the isolation of the rat cysteine-rich protein from a rat brain library during a search for clones with homology to the delta-opioid receptor. One of the cDNAs isolated hybridized to a 1.8 kb mRNA abundantly expressed throughout the rat brain as well as in rat liver. In situ hybridization reveals a wide distribution in rat brain; in particular, abundant hybridization was detected in the hippocampus, cerebellum, habenula, reticular thalamic nucleus and interposed nucleus. Nucleotide sequence analysis of a 1403 bp cDNA clone indicated 77% identity with the cDNA for human cysteine-rich protein (hCRP), that translates into a 99% identity at the amino acid level. The predicted amino acid sequence suggests four zinc fingers, two of the C4 class and two of the C2HC class. This structural motif is characteristic of members of the LIM domain protein family. The mRNA is serum-inducible in Balb/c 3T3 cells. Additional study suggests that its expression is not induced by either NGF treatment of PC12h pheochromocytoma cells, or inflammation-induced injury in the spinal cord at up to 60 min after injury. It does appear to be developmentally expressed in rat brain, consistent with a potential role in neuronal development. The rat CRP clone will be useful for studying the function of CRPs in rodent models.

Expression of a cannabinoid receptor in baculovirus-infected insect cells

A cannabinoid receptor recombinant baculovirus (AcNPV-THCR) has been constructed and employed to express rat neural cannabinoid receptors. Northern analysis of total RNA from Spodoptera frugiperda (Sf9) insect cells infected with AcNPV-THCR revealed novel hyper-production of a 3.3 kb transcript when probed with nick-translated rat cannabinoid receptor cDNA. Optimal viral protein expression was observed in 35S-metabolically labeled AcNPV-THCR-infected Sf9 cells at a multiplicity of infection of 2.5. Transmission electron microscopy of AcNPV-THCR-infected Sf9 cells showed extensive membrane perturbation and electron-dense cytoplasmic perinuclear accumulation, indicative of receptor glycoprotein expression. Immunofluorescence staining using antiserum produced to a fusion protein consisting of the external domain of the cannabinoid receptor and hepatitis B core antigen revealed cannabinoid receptor expression in AcNPV-THCR-infected Sf9 cells. Scatchard-Rosenthal analysis of [3H]CP55,940 receptor binding indicated a Kd of 3.4 nM and a Bmax equal to 3.17 pmol/mg protein. Western immunoblotting performed on AcNPV-THCR-infected Sf9 cell lysates revealed immunoreactive bands with relative molecular weights ranging from 45 to 79 kDa. The predominant species (55 kDa) exhibited a relative molecular weight consistent with that predicted for the translational product obtained from the cannabinoid receptor cDNA coding sequence. In vitro translation using AcNPV-THCR mRNA also yielded a 55 kDa immunoreactive species. These data indicate that the baculovirus expression system is a viable means of expressing relatively large quantities of cannabinoid receptor recombinant protein.

Cannabinoid receptors in developing rats: detection of mRNA and receptor binding

Despite a large body of research directed at assessing the effects of perinatal cannabinoid exposure, little is known about the development of the cannabinoid receptor. Recent advances, including the cloning of the cannabinoid receptor, have afforded us the opportunity to plot the postnatal ontogeny of the cannabinoid receptor and its mRNA in whole brain using the methods of receptor binding and RNA blot hybridization, respectively. Our results indicate that cannabinoid receptor mRNA is present at adult levels as early as postnatal day 3. The Bmax, on the other hand, increases almost fifty percent with increasing postnatal age, while the affinity does not change. The Hill coefficients for all ages studied were approximately 1. These findings suggest the possibility of a developmental progression for cannabinoid receptor development with receptor mRNA appearing first, followed by a period of rapid proliferation of the receptors themselves.

Molecular cloning and expression of a delta-opioid receptor from rat brain

We have isolated and characterized a rat delta-opioid receptor. The deduced amino acid sequence (372 aa) closely resembles the murine delta-opioid receptor, DOR-1. In fact, 97% of the amino acid residues are conserved between the two species, while 93% of the nucleic acid residues are identical. A 6 kb mRNA was detected in rat cortex using rat DOR-1 as a probe. When expressed in COS cells, the clone shows high-affinity opioid binding with selectivity for delta-opioids. The rat delta-opioid receptor cDNA clone will be a useful tool for studying the function of delta-opioid receptor in rats.

Developmental expression of cannabinoid receptor mRNA

Cloning of the cannabinoid receptor affords the opportunity to examine its developmental expression. Other G-protein-coupled receptor systems, those for the opioids for example, exhibit distinct ontogenies. For the initial study, therefore, cannabinoid receptor mRNA expression was assessed in rat pups postnatal days 3, 5, 8, 10, 12, 15, 18 and 21. The brains were grossly dissected into cerebellum/brainstem and forebrain, and total RNA was extracted by a modified acid-extraction method. Expression of the cannabinoid receptor was analyzed by two methods: polymerase chain reaction (PCR) and Northern blot analysis. Oligonucleotide primers based on bp 1-21 and bp 824-843 on the opposite strand were chosen for use in the PCR. The probe used in the Northern blot analysis was a full length cDNA corresponding to the rat cannabinoid receptor and was cloned in our lab based on published sequence information. Our results indicate that by postnatal day 3, cannabinoid receptor mRNA can be detected in the brain. Our results further indicate that cannabinoid mRNA expression steadily increases in the cerebellum/brainstem until postnatal days 18-21, while expression in the forebrain does not change. The findings from the present study indicate that cannabinoid receptor mRNA is present in very young rats. Our data also suggest, however, regional differences in the relative expression of message which may parallel cerebellar proliferation and organization.

Development of behavioral tolerance to delta 9-THC without alteration of cannabinoid receptor binding or mRNA levels in whole brain

The effect of repetitive administration of delta-9-tetrahydrocannabinol (delta 9-THC) in mice on behavioral and biochemical tolerance was determined in this study. Mice were injected twice daily with 10 mg/kg delta 9-THC for 6.5 days. On day 8, spontaneous activity was assessed or whole-brain homogenates were prepared for the cannabinoid receptor binding and mRNA studies. Although a twenty-sevenfold tolerance to delta 9-THC was observed in the behavioral assay, there was no significant alteration in receptor binding or mRNA levels.

Identification of a functionally relevant cannabinoid receptor on mouse spleen cells that is involved in cannabinoid-mediated immune modulation

Extensive behavioral and biochemical characterization of cannabinoid-mediated effects on the central nervous system has revealed at least three lines of evidence supporting the role of a putative guanine nucleotide-binding protein-coupled cannabinoid receptor for cannabimimetic effects, (i) stereoselectivity, (ii) inhibition of the adenylate cyclase/cAMP second messenger system, and (iii) radioligand-binding studies with the synthetic cannabinoid [3H]CP-55,940 indicating a high degree of specific binding to brain tissue preparations. Based on recent findings from our laboratory demonstrating that delta 9-tetrahydrocannabinol markedly inhibited forskolin-stimulated cAMP accumulation in mouse spleen cells, the presence of a guanine nucleotide-binding protein-coupled cannabinoid receptor associated with mouse spleen cells and its functional role in immune modulation were investigated. In the present studies, stereoselective immune modulation was observed with the synthetic bicyclic cannabinoid (-)-CP-55,940 versus (+) CP-56,667 and with 11-OH-delta 8-tetrahydrocannabinol-dimethylheptyl, (-)-HU-210 versus (+)-HU-211. In both cases, the (-)-enantiomer demonstrated greater immunoinhibitory potency than the (+)-isomer, as measured by the in vitro sheep red blood cell antibody-forming cell response. Radioligand binding studies produced a saturation isotherm exhibiting approximately 45-65% specific binding to mouse spleen cells. Scatchard analysis demonstrated a single binding site on spleen cells, possessing a Kd of 910 pM and a Bmax of approximately 1000 receptors/spleen cell. RNA polymerase chain reaction of isolated splenic RNA using specific primers for the cannabinoid receptor resulted in the amplification of a 854-kilobase predicted product that hybridized with cannabinoid receptor cDNA, demonstrating the presence of cannabinoid receptor mRNA in mouse spleen. Together, these findings strongly support the role of a cannabinoid receptor in immune modulation by cannabimimetic agents.

Neurobiology of marijuana abuse

Marijuana has a long history of abuse yet, as described here by Mary Abood and Billy Martin, there is little evidence that animals will self-administer the primary psychoactive constituent, tetrahydrocannabinol, or that marijuana stimulates brain reward pathways. While marked tolerance develops to marijuana, it has been difficult to demonstrate physical dependence, and until recently the mechanisms by which cannabinoids produced their behavioral effects were poorly defined. The development of new synthetic analogs played a critical role in the characterization and cloning of the cannabinoid receptor. Insight into cannabinoid receptors may lead to a better understanding of marijuana abuse in humans and provide new therapeutic strategies for several disorders.

Regulation of both preproenkephalin mRNA and its derived opioids by haloperidol--a method for measurement of peptides and mRNA in the same tissue extract

The goal of this study was to delineate the effects of dopamine antagonists on the regulation of preproenkephalin mRNA and opioid peptides in the rat brain. We have developed a method whereby both mRNA and peptides can be efficiently measured in the same tissue extract, thus reducing the effects of intraspecies variation, differences in dissection and the number of animals required for statistical significance. A sub-chronic dose of haloperidol (3 mg/kg given i.p. in 100 microliters DMSO daily for 5 days) produced a 1.8-fold increase (P less than 0.001) in striatal preproenkephalin mRNA levels when compared to animals injected with vehicle dimethyl sulfoxide (DMSO) employing the same schedule. Total opioid peptides as measured by a radioimmunoassay directed to the N-terminus of enkephalins and endorphins were elevated 1.6 fold (P less than 0.001) in the rat striatum. However in other brain regions examined no increases were observed either in preproenkephalin mRNA or the tissue levels of opioid peptides. Analysis of the opioid-like immunoreactive peptides by reverse-phase HPLC analysis showed no dramatic changes in the ratios of the various opioid peptides between haloperidol and vehicle injected animals. Naive animals showed no statistical differences in opioid peptide levels compared to the haloperidol treated animals. There was a statistically significant decrease (30%) in the opioid peptide content of the animals injected with vehicle daily for 5 days when compared with the animals merely sacrificed, or those given acute injections (either with haloperidol or vehicle) the day of sacrifice.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and regulation of a cDNA clone for rat pancreastatin

Pancreastatin is a recently isolated pancreatic peptide which has been shown to be a potent inhibitor of insulin release in a perfused rat pancreas assay (1). In order to study the regulation of pancreastatin mRNA and to examine the primary structure of its precursor, genomic and cDNA clones corresponding to pancreastatin have been isolated from rat genomic and pancreatic libraries. The number of genes isolated, along with southern blot analyses, suggested that pancreastatin may be part of a gene family. We examined the regulation of pancreastatin expression by insulin and streptozotocin treatment. Pancreastin mRNA levels in rats are altered in response to insulin and streptozotocin.

Modification of opioid agonist binding by pertussis toxin

Membrane fractions prepared from rat striate, cortex and midbrain were treated with pertussis toxin, which has been shown to adenosine diphosphate (ADP)-ribosylate the GTP-binding protein Gi, reducing its coupling with receptors. In striatal membranes, treatment with 40 micrograms toxin per mg membrane protein labeled 60% of the Gi present and 70% of another G protein, Go; this treatments reduced binding of the opioid agonist [3H]D-Ala2-D-Leu5-enkephalin ([3H]DADLE) 20-50%, with the decrease largely reflecting a decreased affinity. In cortex, toxin treatment reduced [3H]DADLE binding by 35-70%, corresponding to ADP-ribosylation of 50% of Gi and 40% of Go. In midbrain, [3H]DADLE binding was unaffected by toxin treatment that ADP-ribosylated 86% of the Gi and 72% of the Go. These results provide further evidence that opioid receptors are associated with GTP-binding proteins in striatum and cortex, where they have also been shown to inhibit adenylate cyclase. Despite the presence of Gi and Go in midbrain, however, there appears to be no coupling between them and opioid receptors.

Pertussis toxin treatment modifies opiate action in the rat brain striatum

In this report we present evidence that a guanine nucleotide regulatory protein, Gi, mediates opiate action in the rat brain striatum. Opiates inhibit basal adenylate cyclase activity in rat brain striatum. This effect on adenylate cyclase is dose-dependently attenuated by pretreatment of membranes with pertussis toxin, which ADP-ribosylates a protein with a molecular mass of 41,000 daltons. This protein co-migrates with the GTP-binding subunit of Gi, which mediates inhibition of adenylate cyclase. Several brain regions were compared for the extent of radiolabeling and effects on adenylate cyclase activity. Although Gi was found in each region examined, opiate inhibition of adenylate cyclase is clearly seen only in the striatum.

Functional homology between signal-coupling proteins. Cholera toxin inactivates the GTPase activity of transducin

Both the light-stimulated cGMP phosphodiesterase of retinal rod outer segments (ROS) and hormone-stimulated adenylate cyclase are regulated by guanine nucleotide-binding regulatory proteins (N). Transducin serves as the signal-carrying regulatory protein in ROS, and the N protein (also called G or G/F) performs this role in the adenylate cyclase system. The GTP form of these regulatory proteins activates the corresponding enzyme, whereas the GDP form does not. Both transducin and the N protein possess a GTPase activity that restores the regulatory protein to the unstimulated state. Cholera enterotoxin catalyzes the transfer of ADP-ribose from NAD+ to the N protein, which inhibits its GTPase activity and activates adenylate cyclase. We report here that the toxin also catalyzes ADP-ribosylation of the alpha-subunit of transducin in ROS membranes. This modification of the guanine nucleotide-binding subunit of transducin is markedly enhanced by the bleaching of rhodopsin and by the addition of guanosine-5'-(beta, gamma-imino)triphosphate. In contrast, GDP, GTP, and guanosine-5'-(3-O)thiotriphosphate inhibit the reaction, while GMP and ATP have no effect. Under optimal conditions, toxin catalyzes labeling of 0.7 mol of the alpha-subunit of transducin/mol of bound [3H]guanosine-5'-(beta, gamma-imido)triphosphate and causes 70% inhibition of the light-dependent GTPase activity of transducin in ROS. These results indicate close functional homology between transducin of ROS and the N protein of adenylate cyclase.

Deficient activity of receptor-cyclase coupling protein is transformed lymphoblasts of patients with pseudohypoparathyroidism, type I

Erythrocytes of many patients with pseudohypoparathyroidism, type 1 (PHP-I) exhibit quantitatively reduced activity of the N protein, the guanine nucleotide-binding regulatory component of adenylate cyclase. We have designated this group of patients PHP-Ia to distinguish them from PHP-Ib patients, in whom erythrocyte N activity is quantitatively normal. In virus-transformed lymphoblasts of three normal, three PHP-Ia, and two PHP-Ib subjects, we compared N and adenylate cyclase activities, as well as cAMP accumulation and susceptibility to radiolabeling in the presence of [32P]NAD and cholera toxin. In comparison to normal lymphoblasts, N activities were reduced by approximately 50% in lymphoblasts of the PHP-Ia patients, but were not reduced in lymphoblasts from the PHP-Ib patients. Toxin-catalyzed radiolabeling of the 42,000 molecular weight subunit of the N protein was also reduced in lymphoblasts of the PHP-Ia patients. These results are consistent with the hypothesis that N deficiency is generalized in tissues of PHP-Ia patients. Deficient lymphoblast N activity in PHP-Ia was not associated with decreases in adenylate cyclase activity or cAMP accumulation, probably because these activities involve many potential regulable cellular components in addition to the N protein.

Cholera toxin can catalyze ADP-ribosylation of cytoskeletal proteins

Cholera toxin catalyzes transfer of radiolabel from [32P]NAD+ to several peptides in particulate preparations of human foreskin fibroblasts. Resolution of these peptides by two-dimensional gel electrophoresis allowed identification of two peptides of Mr = 42,000 and 52,000 as peptide subunits of a regulatory component of adenylate cyclase. The radiolabeling of another group of peptides (Mr = 50,000 to 65,000) suggested that cholera toxin could catalyze ADP-ribosylation of cytoskeletal proteins. This suggestion was confirmed by showing that incubation with cholera toxin and [32P]NAD+ caused radiolabeling of purified microtubule and intermediate filament proteins.

Membrane fluidity and fatty acid composition of phospholipids in erythrocyte membranes of patients with Huntington disease

A study was undertaken to determine whether alterations in the configuration of erythrocyte membranes of patients with Huntington disease, reported by other investigators, could be reflected in membrane fluidity-viscosity. Since membrane fluidity appears to be related to the fatty acid composition of membrane phospholipids, these were also examined. Membrane fluidity, which was determined by measuring the intrinsic fluorescence of membranous tryptophan, was found to be normal. Although the range of fatty acid distribution within the various phospholipids tended to vary somewhat, the pattern was not significantly different from that of the controls.

Enhancement of opiate binding by various molecular forms of phosphatidylserine and inhibition by other unsaturated lipids

A study was undertaken on the possible involvement of phospholipids on stereospecific opiate binding to a rat brain membrane fraction comprised mainly of synaptic membranes. The addition of acidic phospholipids such as phosphatidylserine, phosphoinositides, and phosphatidic acid significantly enhanced opiate binding. With the exception of phosphatidylserine, when the acidic phospholipids contained a polyunsaturated acyl group, they were actually inhibitory, along with neutral phospholipids derived from brain. Both the C18:0, C18:1 form (derived from myelin) and the C18:0, C22:6 form of phosphatidylserine (derived from synaptic membranes) produced as much as a 45% enhancement in opiate binding. Unsaturated fatty acids were highly inhibitory, the degree of inhibition being related to the degree of unsaturation. Both phospholipase A and C were inhibitory; and the inhibitory effect of A could not be prevented by albumin or overcome with the addition of phosphatidylserine. With the use of the cross-linking agent, dinitrodifluorobenzene, it could be demonstrated that the phosphatidylserine of synaptic membranes appeared to be preferentially associated with membrane protein. The enhancement of opiate binding by phosphatidylserine diminished with increasing degree of cross-linking.