Microsomal Opiate Receptors: Characterization of Smooth Microsomal and Synaptic Membrane Opiate Receptors

In continuing studies on smooth microsomal and synaptic membranes from rat forebrain, we compared the binding properties of opiate receptors in these two discrete subcellular populations. Receptors in both preparations were saturable and stereospecific. Scatchard and Hill plots of [3H]naloxone binding to microsomes and synaptic membranes were similar to plots for crude membranes. Both synaptic membranes and smooth microsomes contained similar enrichments of low- and high-affinity [3H]naloxone binding sites. No change in the affinity of the receptors was observed. When [3H]D-ala2-D-leu5-enkephalin was used as ligand, microsomes possessed 60% fewer high-affinity sites than did synaptic membranes, and a large number of low-affinity sites. In competition binding experiments microsomal opiate receptors lacked the sensitivity to (guanyl-5'-yl)imidodiphosphate [Gpp(NH)p] shown by synaptic and crude membrane preparations. In this respect microsomal opiate receptors resembled membranes that were experimentally guanosine triphosphate (GTP)-uncoupled with N-ethylmaleimide (NEM). Agonist binding to microsomal and synaptic membrane opiate receptors was decreased by 100 mM NaCl. Like NEM-treated crude membranes, microsomal receptors were capable of differentiating agonist and antagonists in the presence of 100 mM NaCl. MnCl2 (50-100 microM) reversed the effects of 100 mM NaCl and 50 microM GTP on binding of the mu-specific agonist [3H]dihydromorphine in both membrane populations. Since microsomal receptors are unable to distinguish agonists from antagonists in the presence of Gpp(NH)p, they are a convenient source of guanine nucleotide-uncoupled opiate receptors.     

N-Acylethanolamines in signal transduction of elicitor perception. Attenuation Of alkalinization response and activation of defense gene expression

In a recent study of N-acylphosphatidylethanolamine (NAPE) metabolism in elicitor-treated tobacco (Nicotiana tabacum L.) cells, we identified a rapid release and accumulation of medium-chain N-acylethanolamines (NAEs) (e.g. N-myristoylethanolamine or NAE 14:0) and a compensatory decrease in cellular NAPE (K.D. Chapman, S. Tripathy, B. Venables, A.D. Desouza [1998] Plant Physiol 116: 1163–1168). In the present study, we extend this observation and report a 10- to 50-fold increase in NAE 14:0 content in leaves of tobacco (cv Xanthi) plants treated with xylanase or cryptogein elicitors. Exogenously supplied synthetic NAE species affected characteristic elicitor-induced and short- and long-term defense responses in cell suspensions of tobacco and long-term defense responses in leaves of intact tobacco plants. In general, synthetic NAEs inhibited elicitor-induced medium alkalinization by tobacco cells in a time- and concentration-dependent manner. Exogenous NAE 14:0 induced expression of phenylalanine ammonia lyase in a manner similar to fungal elicitors in both cell suspensions and leaves of tobacco. NAE 14:0, but not myristic acid, activated phenylalanine ammonia lyase expression at submicromolar concentrations, well within the range of NAE 14:0 levels measured in elicitor-treated plants. Collectively, these results suggest that NAPE metabolism, specifically, the accumulation of NAE 14:0, are part of a signal transduction pathway that modulates cellular defense responses following the perception of fungal elicitors.     

Covalent binding of the benzamide RH-4032 to tubulin in suspension-cultured tobacco cells and its application in a cell-based competitive-binding assay

The benzamide, RH-4032, was found to be a potent antimicrotubule agent in tobacco (Nicotiana tabacum) cells. It strongly inhibited root growth and produced swollen club-shaped roots, an accumulation of cells in arrested metaphase, and loss of microtubules. RH-4032 inhibited the in vitro assembly of bovine tubulin into microtubules, with inhibition requiring a relatively long incubation period. Treatment of tobacco suspension-cultured cells or isolated bovine tubulin with [(14)C]RH-4032, and analysis of radiolabeled protein revealed a highly specific covalent attachment to beta-tubulin. Binding of [(3)H]RH-4032 in tobacco suspension-cultured cells was shown to be saturable and to be influenced by pre-incubation of the cells with various antimicrotubule agents: Binding of [(3)H]RH-4032 was inhibited by the benzamides, pronamide and zarilamide, the N-phenylcarbamate, chlorpropham, and the microtubule-stabilizing drug, paclitaxel, whereas trifluralin and amiprophosmethyl were not inhibitory. A common characteristic of agents that cause microtubule disassembly was a slight enhancement of [(3)H]RH-4032 binding at low concentrations, which did not occur with the microtubule-stabilizing agent paclitaxel. For structural analogs of RH-4032 and various N-phenylcarbamates, it was shown that the ability to inhibit binding of [(3)H]RH-4032 was correlated with the ability to inhibit tobacco root elongation. The results suggest a common binding site on beta-tubulin for RH-4032, pronamide, zarilamide, and chlorpropham, which is distinct from the binding site(s) for trifluralin and amiprophosmethyl. RH-4032 provides a unique approach to studying effects of antimicrotubule agents on plant cells by allowing competitive tubulin binding assays to be conducted in whole cells.     

Myelin Basic Protein Is an Endogenous Inhibitor of the High-Affinity Cannabinoid Binding Site in Brain

Radioligand binding studies with the water-soluble cannabinoid [3H]5'-trimethylammonium delta 8-tetrahydrocannabinol ([3H]TMA) have revealed a saturable high-affinity site in brain that is specific for cannabinoids. To determine whether endogenous compounds of brain might act upon the site physiologically, we sought inhibitors in extracts of brain. An endogenous inhibitor has been purified to homogeneity by acid extraction of rat brain followed by adsorption to a reverse-phase matrix and gel filtration chromatography. The purified inhibitor has a subunit molecular mass of 14,500 daltons by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Inhibition of [3H]TMA binding by the purified inhibitor occurs with a Ki of about 4 nM in a noncompetitive manner. The molecular weight, abundance, and extraction properties are the same as a species of myelin basic protein (MBP). The MBPs of rat, rabbit, pig, and cow also inhibit [3H]TMA binding noncompetitively with similar potencies. The purified inhibitor comigrates with rat MBP-small form on SDS-PAGE, has a similar amino acid composition, and is recognized by antibody directed against MBP. Studies of fragments of rabbit MBP suggest that the determinants of affinity for the [3H]TMA site are contained primarily within the C-terminal half of the rabbit MBP. Synthetic polycationic peptides such as polylysine and polyarginine mimic the effects of MBP, suggesting that the high-affinity cannabinoid binding site recognizes large polycations. The identification of the endogenous inhibitor of [3H]TMA binding as MBP suggests that MBP interacts physiologically with the high-affinity cannabinoid site.     

Characterization of specific binding sites for corticosterone in mouse liver plasma membrane

The specific binding of [3H]corticosterone to mouse liver purified plasma membrane fractions is a saturable, reversible, and temperature-dependent process. Only one type of independent and equivalent binding sites has been determined in plasma membrane (Kd = 4.1 nM and Bmax = 3368 fmol/mg). As can be deduced from displacement data obtained in plasma membrane, the high-affinity binding site is different from nuclear glucocorticoid, nuclear progesterone, and Na+, K(+)-ATPase digitalis receptors. Probably this corticosterone binding site or receptor is the same one determined previously for [3H]cortisol in mouse liver plasma membrane. Such beta- and alpha-adrenergic antagonists as propranolol and phentolamine did not affect [3H]corticosterone binding to plasma membranes; therefore, this binding site is independent of these receptors. The binding sites in plasma membranes are not exclusive for corticosterone, but other steroids are also bound with very different affinities.     

Transgene-mediated and elicitor-induced perturbation of metabolic channeling at the entry point into the phenylpropanoid pathway

3H-l-Phenylalanine is incorporated into a range of phenylpropanoid compounds when fed to tobacco cell cultures. A significant proportion of (3)H-trans-cinnamic acid formed from (3)H-l-phenylalanine did not equilibrate with exogenous trans-cinnamic acid and therefore may be rapidly channeled through the cinnamate 4-hydroxylase (C4H) reaction to 4-coumaric acid. Such compartmentalization of trans-cinnamic acid was not observed after elicitation or in cell cultures constitutively expressing a bean phenylalanine ammonia-lyase (PAL) transgene. Channeling between PAL and C4H was confirmed in vitro in isolated microsomes from tobacco stems or cell suspension cultures. This channeling was strongly reduced in microsomes from stems or cell cultures of transgenic PAL-overexpressing plants or after elicitation of wild-type cell cultures. Protein gel blot analysis showed that tobacco PAL1 and bean PAL were localized in both soluble and microsomal fractions, whereas tobacco PAL2 was found only in the soluble fraction. We propose that metabolic channeling of trans-cinnamic acid requires the close association of specific forms of PAL with C4H on microsomal membranes.     

[3H]MK-801 Labels a Site on the N-Methyl-D-Aspartate Receptor Channel Complex in Rat Brain Membranes

The potent noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist [3H]MK-801 bound with nanomolar affinity to rat brain membranes in a reversible, saturable, and stereospecific manner. The affinity of [3H]MK-801 was considerably higher in 5 mM Tris-HCl (pH 7.4) than in previous studies using Krebs-Henseleit buffer. [3H]MK-801 labels a homogeneous population of sites in rat cerebral cortical membranes with KD of 6.3 nM and Bmax of 2.37 pmol/mg of protein. This binding was unevenly distributed among brain regions, with hippocampus greater than cortex greater than olfactory bulb = striatum greater than medulla-pons, and the cerebellum failing to show significant binding. Detailed pharmacological characterization indicated [3H]MK-801 binding to a site which was competitively and potently inhibited by known noncompetitive NMDA receptor antagonists, such as phencyclidine, thienylcyclohexylpiperidine (TCP), ketamine, N-allylnormetazocine (SKF 10,047), cyclazocine, and etoxadrol, a specificity similar to sites labelled by [3H]TCP. These sites were distinct from the high-affinity sites labelled by the sigma receptor ligand (+)-[3H]SKF 10,047. [3H]MK-801 binding was allosterically modulated by the endogenous NMDA receptor antagonist Mg2+ and by other active divalent cations. These data suggest that [3H]MK-801 labels a high-affinity site on the NMDA receptor channel complex, distinct from the NMDA recognition site, which is responsible for the blocking action of MK-801 and other noncompetitive NMDA receptor antagonists.     

Reconstitution of the entry point of plant phenylpropanoid metabolism in yeast (Saccharomyces cerevisiae): implications for control of metabolic flux into the phenylpropanoid pathway

Phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase (C4H), and the C4H redox partner cytochrome p450 reductase (CPR) are important in allocating significant amounts of carbon from phenylalanine into phenylpropanoid biosynthesis in plants. It has been proposed that multienzyme complexes (MECs) containing PAL and C4H are functionally important at this entry point into phenylpropanoid metabolism. To evaluate the MEC model, two poplar PAL isoforms presumed to be involved in either flavonoid (PAL2) or in lignin biosynthesis (PAL4) were independently expressed together with C4H and CPR in Saccharomyces cerevisiae, creating two yeast strains expressing either PAL2, C4H and CPR or PAL4, C4H and CPR. When [(3)H]Phe was fed, the majority of metabolized [(3)H]Phe was incorporated into p-[(3)H]coumarate, and Phe metabolism was highly reduced by inhibiting C4H activity. PAL alone expressers metabolized very little phenylalanine into cinnamic acid. To test for intermediate channeling between PAL and C4H, we fed [(3)H]Phe and [(14)C]cinnamate simultaneously to the triple expressers, but found no evidence for channeling of the endogenously synthesized [(3)H]cinnamate into p-coumarate. Therefore, efficient carbon flux from Phe to p-coumarate via reactions catalyzed by PAL and C4H does not appear to require channeling through a MEC in yeast, and instead biochemical coupling of PAL and C4H is sufficient to drive carbon flux into the phenylpropanoid pathway. This may be the primary mechanism by which carbon allocation into phenylpropanoid metabolism is controlled in plants.     

Evidence for triacylglycerol synthesis in the lumen of microsomes via a lipolysis-esterification pathway involving carnitine acyltransferases

In this study a pathway for the synthesis of triacylglycerol (TAG) within the lumen of the endoplasmic reticulum has been identified, using microsomes that had been preconditioned by depleting their endogenous substrates and then fusing them with biotinylated phosphatidylserine liposomes containing CoASH and Mg(2+). Incubating these fused microsomes with tri[(3)H] oleoylglycerol and [(14)C]oleoyl-CoA yielded microsome-associated triacylglycerol, which resisted extensive washing and had a [(3)H]:[(14)C] ratio close to 2:1. The data suggest that the precursor tri[(3)H]oleoylglycerol was hydrolyzed by microsomal lipase to membrane-bound di[(3)H]oleoylglycerol and subsequently re-esterified with luminal [(14)C]oleoyl-CoA. The accumulation of TAG within the microsomes, even when overt diacylglycerol acyltransferase (DGAT I) was inactive, is consistent with the existence of a latent diacylglycerol acyltransferase (DGAT II) within the microsomal lumen. Moreover, because luminal synthesis of TAG was carnitine-dependent and markedly reduced by glybenclamide, a potent carnitine acyltransferase inhibitor, microsomal carnitine acyltransferase appears to be essential for trafficking the [(14)C]oleoyl-CoA into the microsomal lumen for subsequent incorporation into newly synthesized TAG. This study thus provides the first direct demonstration of an enzymatic process leading to the synthesis of luminal triacylglycerol, which is a major component of very low density lipoproteins.     

High-Affinity Binding of [3H]Desipramine to Rat Brain: A Presynaptic Marker for Noradrenergic Uptake Sites

Abstract: High-affinity binding sites (apparent K _D= 1.5 nM) for [³H]desipramine have been demonstrated and characterized in membranes prepared from rat brain. The binding of [3H]desipramine was found to be saturable, reversible, heat-sensitive, sodium-dependent, and regionally distributed among various regions of the brain. High concentrations of [³H]desipramine binding sites were found in the septum, cerebral cortex, and hypothalamus, whereas lower concentrations were found in the medulla, cerebellum, and corpus striatum. A very good correlation ( r = 0.81, P < 0.001) was observed between the potencies of a series of drugs in inhibiting high-affinity [³H]desipramine binding and their capacity to block norepinephrine uptake into synaptosomes. In 6-hydroxydopamine-lesioned rats there was a marked decrease in [³H]norepinephrine uptake and [3H]desipramine binding with no significant alterations in either [³H]serotonin uptake or [³H]imipramine binding. These results suggest that the high-affinity binding of [³HJdesipramine to rat brain membranes is pharmacologically and biochemically distinct from the high-affinity binding of [3H]imipramine, and that there is a close relationship between the high-affinity binding site for [³H]desipramine and the uptake site for norepinephrine.     

CHAPS solubilization of a G-protein sensitive 5-HT1A receptor from bovine hippocampus

The binding of [3H] 8-OH-DPAT to membrane-bound 5-HT1A receptors from bovine hippocampus was saturable and corresponded to a single high-affinity state. Solubilization of the bovine hippocampal membranes with 10 mM CHAPS containing 200 mM NaCl, renders a preparation which binds [3H] 8-OH-DPAT with high-affinity (Kd = 1.9 nM) and is guanine nucleotide sensitive and ketanserin insensitive. 50% of [3H] 8-OH-DPAT binding activity is solubilized. The presence of GMP-P(NH)P promotes a low-affinity (Kd = 9.6 nM) state which is characteristic of receptors coupled to G-proteins. GMP-P(NH)P markedly accelerates the dissociation [3H] 8-OH-DPAT from solubilized membranes while having negligible effects on association. Thus, the agonist can activate the terniary complex rather than to promote its formation. 8-OH DPAT, WB 4101 and 5-carboxamidotryptamine dose responsively inhibit soluble [3H] 8-OH-DPAT binding with IC(50) values of 16.1, 15.6 and 1.3 nM, respectively. The CHAPS solubilized membrane preparation retains many of the [3H] 8-OH-DPAT binding characteristics of the membrane bound form.     

Validation of high-affinity binding sites for succinic acid through distinguishable binding of gamma-hydroxybutyric acid receptor-specific NCS 382 antipodes

Gamma-hydroxybutyric acid (GHB) binding to multiple sites for the tricarboxylic acid cycle intermediate succinic acid (SUC) has been disclosed recently. In order to better characterize these targets, distinguishable binding of GHB receptor-specific NCS 382 antipodes to [(3)H]-SUC or [(3)H]-GHB labelled sites in rat brain synaptic membranes was explored. Eutomer binding parameters suggest identity of the high-affinity target for SUC with a synaptic GHB receptor subtype.     

High-Affinity Cannabinoid Binding Site: Regulation by Ions, Ascorbic Acid, and Nucleotides

The high-affinity cannabinoid site in rat brain is an integral component of brain membranes that recognizes cannabinoids with inhibitory constants (Ki) in the nanomolar range. To clarify its physiological role, we studied the regulation of [3H]5'-trimethylammonium delta 8-tetrahydrocannabinol ([3H]TMA) binding. The site is inhibited by heavy metal ions, such as La3+, at low micromolar concentrations; divalent cations, such as Ca2+ and Mg2+, inhibit [3H]TMA binding, though at somewhat higher concentrations. In contrast, [3H]TMA binding is stimulated by Fe2+, Cu2+, and Hg2+ ions. Ascorbic acid and its analogs are also stimulators of cannabinoid binding at low micromolar concentrations. Stimulation of [3H]TMA binding by ascorbate or ions is dependent upon molecular oxygen, but is not inhibited by metabolic poisons. Metabolically stable nucleoside triphosphate analogs enhance [3H]TMA binding by different mechanisms, with hydrolysis of a high-energy phosphate bond apparently requisite for these influences. These results suggest that the cannabinoid binding site is associated with a nucleotide-utilizing protein possessing multiple regulatory subsites.     

Low-affinity binding sites for 1,4-dihydropyridines in skeletal muscle transverse tubule membranes revealed by changes in the fluorescence of felodipine.

The fluorescence changes accompanying the binding of the fluorescent calcium channel antagonist, felodipine, to transverse tubule membranes from rabbit skeletal muscle have been used to characterize low-affinity binding sites for 1,4-dihydropyridine derivatives in these preparations. In competition experiments, felodipine inhibited the high-affinity binding of (+)-[3H]PN200-110 to transverse tubule membranes with an apparent Ki of 5 +/- 2 nM. Binding of felodipine to additional low-affinity sites resulted in a large, saturable (Kd = 6 +/- 2 microM) increase in its fluorescence which could be excited either directly (380 nm) or indirectly via energy transfer from membrane protein (290 nm). The observed fluorescence enhancement was competitively inhibited by other 1,4-dihydropyridines with inhibition constants of 3-21 microM but was unaffected by the structurally unrelated calcium channel antagonists, diltiazem and verapamil, or by Ca2+, Cd2+, and La3+. Both high- and low-affinity binding sites appear to be localized in the transverse tubular system, since the magnitude of the observed fluorescence enhancement was higher in these membranes than in microsomal preparations and was directly proportional to the density of high-affinity sites for (+)-[3H]PN200-110. Furthermore, both high- and low-affinity sites appear to be conformationally coupled since, over the same concentration range that the fluorescence changes were observed, felodipine accelerated the rate of dissociation of [3H]PN200-110 previously bound to its high-affinity sites. Similar behavior has previously been reported for other 1,4-dihydropyridines [Dunn, S. M. J., & Bladen, C. (1991) Biochemistry 30, 5716-5721].(ABSTRACT TRUNCATED AT 250 WORDS)     

Neurochemical Studies of the Mesolimbic Dopaminergic Pathway: [3H]Spiperone Labels Two Binding Sites in Homogenates of the Nucleus Accumbens of Rat Brain

The binding of [3H]spiperone to membranes of the nucleus accumbens of the rat brain was studied in vitro and found to be of high affinity, rapid, saturable, reversible and stereospecific. Dissociation and saturation experiments indicated the presence of two specific binding sites with apparent dissociation constants of 70 pM and greater than 1 nM. Specific binding with 25 pM [3H]spiperone represented greater than 90% of total binding and was displaced by dopaminergic agonists, neuroleptic drugs and ergot derivatives. The rank order of potency for the ergot derivatives was bromocryptine greater than pergolide greater than lergotrile, and that for D-2 antagonists was domperidone greater than sulpiride greater than molindone greater than metoclopramide. Noradrenergic, histaminergic and serotonergic components of the binding were not detected. [3H]Spiperone binds to high-affinity sites in homogenates of nucleus accumbens, which are likely to be D-2 receptors.     

α2-Adrenergic Receptors in Neuronal and Glial Cultures: Characterization and Comparison

Membranes prepared from either neuronal or glial cultures contain alpha 2-adrenergic receptors as determined by the characteristics of [3H]yohimbine [( 3H]YOH) binding. The binding was rapid, reversible, saturable, dependent on the protein concentration used, and reached equilibrium by 5 min in membranes from both neuronal and glial cultures. Scatchard analyses of saturation isotherms revealed similar KD values of 13.7 +/- 1.35 nM (n = 10) for neuronal cultures and 18.42 +/- 2.34 nM (n = 10) for glial cultures. Glial cultures contained many more binding sites for [3H]YOH than neuronal cultures, having a Bmax of 1.6 +/- 0.33 pmol/mg protein (n = 10) compared with 0.143 +/- 0.018 pmol/mg protein (n = 10) in neurons. Drugs selective for alpha 2-adrenergic receptors were the most effective displacers of [3H]YOH binding in both neuronal and glial cultures, i.e., the alpha 2-adrenergic antagonists rauwolscine and yohimbine were better displacers than the other catecholamine antagonists prazosin, corynanthine, or propranolol. The agonists showed the same pattern with the alpha 2-selective drugs clonidine and naphazoline being the most effective competitors for the [3H]YOH site. GTP and its nonhydrolyzable analog. 5'-guanylyl-imidodiphosphate, were able to lower the affinity of the alpha 2-receptors for agonists but not antagonists in membranes from both neuronal and glial cultures, suggesting that the receptors are linked to a G protein in both cell types. The presence of alpha 2-adrenergic receptors in neuronal cultures was also substantiated by light microscopic autoradiography of [3H]YOH binding. In summary, we have demonstrated that both neuronal and glial cultures contain alpha 2-adrenoceptors.     

Characterization of beta-adrenergic receptors in the rat vas deferens using [3H]-dihydroalprenolol binding

The beta-adrenergic antagonist, [3H]-dihydroalprenolol ([3H] DHA), binds to membranes prepared from the rat vas deferens in a specific and saturable manner. Scatchard and Hill plot analysis indicates a single class of binding sites with no evidence of cooperative interactions. The specific binding sites have a high affinity (Kd = 0.3 nM) and a maximal occupancy estimated to be 460 fmoles [3H]-DHA bound/g wet tissue weight. Beta-adrenergic agonists and/or antagonists inhibit [3H]-DHA binding to rat vas deferens membranes in a stereospecific manner and with a relative order of potency expected for beta-adrenergic receptors of the beta2 subtype. The receptor affinities of various beta-adrenergic antagonists in the rat vas deferens determined using inhibition of [3H]-DHA binding correlated with their receptor affinities determined physiologically using antagonism of isoproterenol-induced inhibition of neurogenic contractions in-vitro.     

Prostaglandin D2 receptor of mastocytoma P-815 cells--possible regulation by phosphorylation and dephosphorylation

The 3H-labeled prostaglandin D2 [( 3H]PGD2) binding protein in the membrane fraction of mastocytoma P-815 cells was characterized. The specific binding of [3H]PGD2 to the cells or the membranes reached a maximum at pH 5.6, and was saturable, displaceable and of high affinity when incubated at 0 or 37 degrees C. The Bmax values for [3H]PGD2 binding in the two preparations at pH 5.6 were much higher at 0 degrees C than at 37 degrees C, whereas the Kd values were almost equal (85.3 nM for the cells and 80.5 nM for the membranes, respectively). High specific [3H]PGD2 binding activity in the mildly acid-treated cells was still observed when the external pH was raised from 5.6 to 7.2. Furthermore, specific [3H]PGD2 binding to the membranes (at 0 degrees C, pH 5.6) increased on addition of phosphatase inhibitors (NaF and molybdate) in the presence of 10 microM ATP, but practically disappeared on pretreatment of the membranes with phosphatase. On incubation of the membrane with [gamma-32P]ATP and molybdate, the stimulated incorporation of the [32P]phosphate into several peptides, including ones having an Mr of around 100,000-120,000, was observed. These results suggest that [3H]PGD2 binding in the mastocytoma P-815 cell membrane is controlled through phosphorylation-dephosphorylation of the receptor itself.     

Characterization of Ligand Binding to the Cannabinoid Receptor of Rat Brain Membranes Using a Novel Method: Application to Anandamide

Abstract: Ligand binding to the cannabinoid receptor of brain membranes has been characterized using [³H]CP 55,940 and the Multiscreen Filtration System. Binding of [³H]CP 55,940 is saturable and reaches equilibrium by 45 min at room temperature. At a concentration of 10 µg of membrane protein/well, the K _D for [³H]CP 55,940 is 461 p M and the B _max is 860 fmol/mg of protein. The apparent K _D of [³H]CP 55,940 is dependent upon tissue protein concentration, increasing to 2,450 p M at 100 µg of membrane protein. Binding of [³H]CP 55,940 is dependent upon the concentration of bovine serum albumin in the buffer; the highest ratio of specific to nonspecific binding occurs between 0.5 and 1.0 mg/ml. The K _i of anandamide, a putative endogenous ligand of the cannabinoid receptor, is 1.3 µ M in buffer alone and 143 n M in the presence of 0.15 m M phenylmethylsulfonyl fluoride. When [¹⁴C]anandamide is incubated with rat forebrain membranes at room temperature, it is degraded to arachidonic acid; the hydrolysis is inhibited by 0.15 m M phenylmethylsulfonyl fluoride. These results support the hypothesis that anandamide is a high-affinity ligand of the cannabinoid receptor and that it is rapidly degraded by membrane fractions.     

Modulation of cannabinoid agonist binding by 5-HT in the rat cerebellum

Cross-talk between cannabinoid CB1 and serotonin 5-HT receptors in rat cerebellar membranes was investigated using radioligand binding. In competition against the CB1 antagonist, [3 H]SR141716A, the agonist, WIN 55,212-2 yielded a biphasic isotherm. The majority of binding was to a high-affinity state that was significantly reduced by the GTP analogue, Gpp(NH)p. Interestingly, 5-HT enhanced the high-affinity binding constant of WIN 55,212-2 while attenuating the proportion of high-affinity binding. 5-HT also significantly reduced the proportion of high-affinity binding of the cannabinoid agonist, HU 210, but had no effect on the agonist, CP 55,940. The effect of 5-HT on WIN 55,212-2 binding was inhibited by the 5-HT2 receptor antagonist ritanserin as well as Gpp(NH)p, suggesting a dependence on the 5-HT2 receptor and on G protein-receptor interactions, respectively. Subsequent [3 H]WIN 55,212-2 dissociation kinetic experiments revealed that 5-HT promoted a slower-dissociating species of radiolabelled agonist-receptor complex. Our findings support a membrane-delimited cross-talk between two G protein-coupled receptors that are co-localized in certain cells of the central nervous system. Intriguingly, the cannabinoid agonist dependence of the 5-HT modulatory effect suggests that agonist-specific conformations of the CB1 receptor may also be important in determining the extent of this cross-talk.