Low affinity of FMRFamide and four FaRPs (FMRFamide-related peptides), including the mammalian-derived FaRPs F-8-Famide (NPFF) and A-18-Famide, for opioid mu, delta, kappa 1, kappa 2a, or kappa 2b receptors

The binding affinities at opioid receptor subtypes in rat or guinea pig brain membranes were determined for the neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2), the two mammalian-derived FMRFamide-related peptides F-8-Famide (NPFF; Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe- NH2) and A-18-Famide (Ala-Gly-Glu-Gly-Leu-Ser-Ser-Pro-Phe-Trp-Ser-Leu-Ala-Ala-Pro-Gln-Arg-Phe -NH2), and the two other FMRFamide-related peptides Tyr-Phe-Met-Arg-Phe-NH2 (Tyr-FMRFamide) and Pro-Gln-Arg-Phe-NH2 (Pro-Gln-RFamide). The mu and delta sites were labeled in rat brain membranes using tritiated [D-Ala2, N-MePhe4,Gly-ol5] enkephalin ([3H]DAMGO) and [D-Ala2,D-Leu5]enkephalin ([3H]DADLE), respectively. The kappa sites were labeled in guinea pig brain using [3H]U-69,593 after treatment with BIT and FIT for kappa 1 and [3H]bremazocine after pretreatment with BIT and FIT for kappa 1 and [3H]bremazocine after pretreatment with BIT and FIT for kappa 2. The kappa 2a binding sites were assayed using [Leu5]enkephalin to block kappa 2b sites and the kappa 2b sites were assayed using (-)-(1S,2S)-U50,488 to block kappa 2a sites. Neither FMRFamide nor any of the FMRFamide-related peptides (up to 61.0 microM) displayed significant affinity at any of the subtypes of opioid receptor. Hence, the known ability of FMRFamide and FaRPs to interact with the opioid system does not appear to be related to direct binding to these opioid receptors.

Selective labeling of kappa 2 opioid receptors in rat brain by [125I]IOXY: interaction of opioid peptides and other drugs with multiple kappa 2a binding sites

Recent studies from our laboratory resolved two subtypes of the kappa 2 binding site, termed kappa 2a and kappa 2b, using guinea pig, rat, and human brain membranes depleted of mu and delta receptors by pretreatment with the site-directed acylating agents BIT (mu-selective) and FIT (delta-selective). 6 beta-Iodo-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan (IOXY), an opioid antagonist that has high affinity for kappa 2 sites, was radioiodinated to maximum specific activity (2200 Ci/mmol) and purified by high pressure liquid chromatography and used to characterize multiple kappa 2 binding sites. The results indicated that [125I]IOXY, like [3H]bremazocine, selectively labels kappa 2 binding sites in rat brain membranes pretreated with BIT and FIT. Using 100 nM [D-Ala2-MePhe4,Gly-ol5]enkephalin to block [125I]IOXY binding to the kappa 2b site, two subtypes of the kappa 2a binding site were resolved, both in the absence and presence of 50 microM 5'-guanylyimidodiphosphate. Viewed collectively, these results provide further evidence for heterogeneity of the kappa opioid receptor, which may provide new targets for drug design, synthesis, and therapeutics.

Differential binding of opioid peptides and other drugs to two subtypes of opioid delta ncx binding sites in mouse brain: further evidence for delta receptor heterogeneity

Research into the functional role of the opioid delta receptor has intensified with the recent in vivo identification of delta receptor subtypes, termed delta 1 and delta 2, which mediate antinociception in the mouse. A variety of data also support the hypothesis of an opioid receptor complex composed of distinct, yet interacting, mu, delta, and perhaps kappa binding sites. This model postulates two classes of delta binding sites: a delta binding site not associated with the opioid receptor complex, termed the delta ncx site, and a delta site associated with the receptor complex, termed the delta cx site. A major purpose of this study was to clarify the relationship between the delta ncx binding sites and the delta 1 and delta 2 receptors. Mouse brain membranes were depleted of mu sites by pretreatment with the site-directed acylating agent, BIT, and the delta ncx binding sites were labeled with [3H][D-Ala2,D-Leu5]enkephalin. Binding surface analysis readily resolved two binding sites (delta ncx-1 and delta ncx-2) in the absence and presence of 100 mM NaCl. Control experiments with guanine nucleotides and the ligand-selectivity analysis indicated that the two sites were not two states of a single receptor. Pretreatment of membranes with DALCE, but not [Cys4]deltorphin, decreased [3H] [D-Ala2,D-Leu5]enkephalin and [3H][D-Ser2,Thr6]enkephalin binding. Ligand-selectivity analysis of the two binding sites suggested that neither delta ncx binding site had the characteristics expected of the delta 2 receptor, and that the delta ncx-1 site, but not the delta ncx-2 site, was synonymous with the delta 1 receptor. Moreover, our finding that the racemic nonpeptide delta agonist, BW373U86, had high affinity at and selectivity for the delta ncx-2 site suggests that this site may be a novel delta receptor that mediates some of the effects of BW373U86.

Synthesis and binding properties of MK-801 isothiocyanates; (+)-3-isothiocyanato-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten- 5,10-imine hydrochloride: a new, potent and selective electrophilic affinity ligand for the NMDA receptor-coupled phencyclidine binding site

Three new site-directed irreversible (wash-resistant) ligands for the high-affinity phencyclidine (PCP) binding site associated with the N-methyl-D-aspartate (NMDA) receptor were synthesized and their binding characteristics were studied. (+)-3- And (+)-2-isothiocyanato-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycl ohepten-5,10 - imine hydrochloride ((+)-8a,b.HCl) were prepared in four steps from the corresponding nitro derivatives (+)-4a,b, which were obtained by nitration of (+)-3 (MK-801). In the same way the optical antipode (-)-8a.HCl was synthesized from (-)-3. At a concentration of 100 nM, the 3-isothiocyanate derivative (+)-8a irreversibly labeled approximately 50% of the (+)-[3H]-3 binding sites, compared to 20 microM needed for its optical antipode (-)-8a and the 2-isothiocyanate (+)-8b. The apparent Ki values for reversible inhibition of (+)-[3H]-3 binding by (+)- and (-)-8a and (+)-8b were 37,838, and 843 nM, respectively. In contrast, metaphit (1b) and etoxadrol m-isothiocyanate (2b), two previously reported irreversible ligands for the PCP binding site, label about 50% of the (+)-[3H]-3 binding sites at 100 microM and 250 nM, respectively, with apparent Ki values for reversible inhibition of 535 and 94 nM. Compound (+)-8a is also a selective affinity ligand, displaying little or no irreversible in vitro affinity at 100 microM for opioid, benzodiazepine, muscarinic, and dopamine receptors. At a 25 microM concentration, (+)-8a caused an irreversible 52% reduction of binding to sigma 1-receptors. Compound (+)-8a is the most potent known electrophilic affinity label for the PCP binding site. Its potency and selectivity should enable it to be a valuable tool for the elucidation of the structure and function of the NMDA receptor-associated PCP binding site in the mammalian central nervous system.

Identification of a GBR12935 homolog, LR1111, which is over 4,000-fold selective for the dopamine transporter, relative to serotonin and norepinephrine transporters

The di-substituted piperazines, GBR12909 (1-[2-[bis(4-fluorophenyl)-methoxy]ethyl]-4-[3- phenylpropyl]piperazine) and GBR12935 (1-[2-(diphenyl-methoxy)-ethyl]-4-(3-phenylpropyl)piperazine), are potent and selective (20-to 100-fold) inhibitors of [3H]dopamine reuptake, relative to [3H]5-HT and [3H]norepinephrine uptake. The GBR12935 analog, 1-(2-(diphenylmethoxy)ethyl)-4-(3-phenylpropyl)homopiperazine (LR1111), was synthesized as part of a systematic structure-activity study of analogs of GBR12935 and GBR12909. LR1111 differs from GBR12935 by the addition of a methylene group into the piperazine ring to yield a compound with a seven-member homopiperazine ring. The IC50 values for LR1111 at the dopamine, norepinephrine, and serotonin transporters were 7.2 nM, 34,072 nM, and greater than 20,000 nM, respectively, whereas the IC50 values of GBR12935 were 3.7 nM, 289 nM, and 1261 nM for these same transporters. This demonstrates that the addition of a single methylene group in the piperazine ring results in a compound with similar affinity but significantly higher selectivity for the dopamine transporter. LR1111 increased motoric activity in rats after intravenous administration. These indicate that LR1111 is a potent and highly selective inhibitor of the dopamine transporter.

Synthesis and evaluation of imidazo[1,5-a][1,4]benzodiazepine esters with high affinities and selectivities at "diazepam-insensitive" benzodiazepine receptors

A series of imidazo[1,5-a][1,4]benzodiazepine esters have been synthesized with varying ester side chains and 8-position substituents. The affinities of these compounds were evaluated at both "diazepam-insensitive" (DI) and diazepam-sensitive (DS) subtypes of the benzodiazepine receptor (BZR). A profound steric effect of the 3-position ester side chain moiety was observed on ligand affinity at DI. In contrast, ester size had a less robust effect on ligand affinity at DS. The tert-butyl ester compound 8 displayed the highest affinity (Ki = 1.7 nM) for DI within a series of 8-chloro esters. Furthermore, halogens at the 8-position resulted in an enhancement of both ligand affinity and selectivity at DI among the series of tert-butyl esters examined. The 8-nitro derivative 23 and 8-isothiocyanato congener 25 had high affinities for both DI and DS but exhibited little subtype selectivity (10.8 and 2.7 nM at DI versus 14 and 3.7 nM at DS, respectively). The 8-azido tert-butyl ester 29 exhibited a significantly higher affinity (Ki = 0.43 nM) and selectivity (DI/DS ratio of 0.2) than the corresponding ethyl ester, the prototypic DI ligand 1 (Ro 15-4513). Among the compounds synthesized, 29 is the highest affinity ligand for DI described to date while its 8-bromo analog 18 is the most selective ligand (DI/DS ratio of 0.17) for this novel BZR subtype.

Cannabinoid structure-activity relationships: correlation of receptor binding and in vivo activities

Although a receptor exists for cannabinoid drugs, it is uncertain which pharmacological actions this receptor mediates. This structure-activity relationship investigation was initiated to determine which effects might correspond to binding affinity for the cannabinoid receptor, as well as to explore the binding requirements of this site. The ability of nearly 60 cannabinoids to displace [3H]CP-55,940 [(-)-3-[2-hydroxy-4-(1,1-dimethylheptyl) phenyl]-4-[3-hydroxy propyl] cyclohexan-1-ol] was determined before establishing correlations between receptor affinity and in vivo pharmacological potency. Analysis of [3H]CP-55,940 binding indicated a Hill coefficient of 0.97, a Bmax of 499 pM (3.3 pmol/mg of protein) and an apparent Kd of 924 pM. Closer inspection indicated the binding assay exhibited "zone B" characteristics, and use of correction equations indicated a true Kd for CP-55,940 of 675 pM. The structure-activity relationship indicated the importance of side chain structure to high-affinity binding, with the most potent analogs (K1 < 10 nM) possessing either a dimethylheptyl side-chain, a similarly complex branched side chain or a halogen substituent at the 5' position. Comparative analysis of K1 values to in vivo potency in a mouse model indicated a high degree of correlation between parameters for the depression of spontaneous locomotor activity (r = 0.91) and for the production of antinociception (r = 0.90), hypothermia (r = 0.89) and catalepsy (r = 0.85). Similarly high correlations were demonstrated between binding affinity and in vivo potency in both the rat drug discrimination model (r = 0.81) and for psychotomimetic activity in humans (r = 0.88).(ABSTRACT TRUNCATED AT 250 WORDS)

Metaphit-induced audiogenic seizures in mice: II. Studies on N-methyl-D-aspartic acid, GABA, and sodium channel receptors and on the disposition of metaphit in the brain

We previously demonstrated that metaphit (a phencyclidine analogue with an acylating isothiocyanate group) induces occurrence of audiogenic seizures in mice exposed to audio stimulation 24 h after metaphit administration. We have studied various receptor systems associated with excitatory and inhibitory networks: sites for competitive and noncompetitive antagonists of the N-methyl D-aspartic acid (NMDA) receptor complex, for [3H]muscimol on the gamma-aminobutyric acid (GABA) receptor complex, and for [3H]batrachotoxinin A20-alpha-benzoate on the voltage-dependent sodium channel. Mice were examined for neurochemical changes at 24 h after pretreatment with metaphit, when susceptibility to audiogenic seizures is greatest. Ex vivo receptor binding studies detected no changes; in vivo labeling of the phencyclidine site in the NMDA receptor complex was reduced by 20% in cortical and midbrain regions. A separate group of experiments was aimed at measuring brain levels of metaphit. One minute after retroorbital administration of [3H]metaphit at a dose sufficient to produce susceptibility to audiogenic seizures 24 h later, the brain level of [3H]metaphit (determined by high-performance liquid chromatography, HPLC) was 49 pmol/mg tissue; at 1, 4, and 24 h, the level was 12, 6, and 1.4 pmol/mg tissue or microM if metaphit was evenly distributed throughout the brain. Although the observed metaphit concentrations during the first 4 h are high enough to acylate receptors, no firm evidence for acylation was found for most of the examined receptors. Finally, the time course of the brain level of metaphit showing a continuous decrease is entirely different from that of development of the seizure susceptibility, which peaks at 18-24 h.

Metaphit-induced audiogenic seizures in mice: I. Pharmacologic characterization

Metaphit [an analogue of phencyclidine (PCP) with an acylating isothiocyanate group] induced audiogenic clonic to clonic-tonic seizures in mice exposed to audio stimulation 24 h after metaphit administration. The incidence of seizures was reduced by treatment 30 min before audio stimulation with specific PCP-like compounds [5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801), and PCP itself], competitive N-methyl-D-aspartate antagonists 2-amino-5-phosphonopentanoic acid (AP-5 and NPC-12626), antiepileptic drugs [phenobarbital (PB), phenytoin (PHT)], and gamma-aminobutyric acid (GABA) agonists (muscimol and diazepam). In contrast, when given in conjunction with metaphit, most of these drugs were ineffective in protecting animals from audiogenic seizures 24 h later. Only compounds with long half-lives (t1/2) such as MK-801, PB, and PHT had a protective effect. High-performance liquid chromatography (HPLC) determination of [3H]MK-801 showed its long-term presence in the brain after intraperitoneal (i.p.) administration of [3H]MK-801. Audiogenic seizures observed 24 h after metaphit administration were potentiated by administration of the GABA antagonist picrotoxin 15 min before audio stimulation, and picrotoxin-induced spontaneous seizures were enhanced by pretreatment (24 h earlier) with a dose of metaphit that in itself did not produce spontaneous seizures at the time of the picrotoxin test. Similar observations were made with N-methyl D-aspartic acid (NMDA) instead of picrotoxin. Thus, an interplay exists between excitatory glutaminergic and inhibitory GABAergic circuitries in the metaphit seizure model.

Phenylpiperidine opioid antagonists that promote weight loss in rats have high affinity for the kappa 2B (enkephalin-sensitive) binding site

Certain opioid antagonists of the phenylpiperidine series (PPAs), such as LY255582, seem uniquely efficacious at producing weight loss in lean and meal-fed obese Zucker rats. Comparison of the pharmacological and receptor binding profile of PPAs that promote marked weight loss with those that do not has failed to find any obvious differences between these two groups of narcotic antagonists, which might explain the differences in their biological activities. The potent stimulatory effect of dynorphin, and other kappa agonists, on feeding behavior suggests that the antagonists that promote weight loss might have high affinity for kappa receptors. The recent demonstration by several laboratories of kappa receptor heterogeneity prompted us to test the hypothesis that the antagonists that promote weight loss might have high affinity for a subtype of kappa binding sites. In the present study, therefore, we determined the Ki values of five PPAs, naloxone, and naltrexone at mu, delta, kappa 1, kappa 2a, and kappa 2b binding sites. The data indicate that antagonists have subnanomolar Ki values and high selectivity for the kappa 2b binding site (relative to the kappa 2a binding site) are efficacious at promoting weight loss.

Comparison of bolus and infusion methods for receptor quantitation: application to [18F]cyclofoxy and positron emission tomography

Positron emission tomography studies with the opiate antagonist [18F]cyclofoxy ([18F]CF) were performed in baboons. Bolus injection studies demonstrated initial uptake dependent on blood flow. The late uptake showed highest binding in caudate nuclei, amygdala, thalamus, and brainstem and the least accumulation in cerebellum. By 60 min postinjection, regional brain radioactivity cleared at the same rate as metabolite-corrected plasma, i.e., transient equilibrium was achieved. Compartmental modeling methods were applied to time-activity curves from brain and metabolite-corrected plasma. Individual rate constants were estimated with poor precision. The model estimate of the total volume of distribution (VT), representing the ratio of tissue radioactivity to metabolite-corrected plasma at equilibrium, was reliably determined. The apparent volume of distribution (Va), the concentration ratio of tissue to metabolite-corrected plasma during transient equilibrium, was compared with the fitted VT values to determine if single-scan methods could provide accurate receptor measurements. Va significantly overestimated VT and produced artificially high image contrast. These differences were predicted by compartment model theory and were caused by a plasma clearance rate that was close to the slowest tissue clearance rate. To develop a simple method to measure VT, an infusion protocol consisting of bolus plus continuous infusion (B/I) of CF was designed and applied in a separate set of studies. The Va values from the B/I studies agreed with the VT values from both B/I and bolus studies. This infusion approach can produce accurate receptor measurements and has the potential to shorten scan time and simplify the acquisition and processing of scan and blood data.

Studies of the biogenic amine transporters. II. A brief study on the use of [3H]DA-uptake-inhibition to transporter-binding-inhibition ratios for the in vitro evaluation of putative cocaine antagonists

The cocaine receptor on the dopamine transporter is a logical target binding site for the design and synthesis of novel agents for evaluation as possible cocaine antagonists. Although there is no widely accepted and validated assay for detecting a cocaine antagonist, one commonly accepted strategy is to compare the IC50 value of a test agent for inhibition of [3H]dopamine uptake and its IC50 value for inhibition of the binding of a transporter ligand such as [125I]RTI-55. The goal of such a comparison is to guide the synthesis of agents which have high "uptake-to-binding ratios", i.e. agents which are much more potent in the binding assay than they are in the uptake assay. In the present study we tested the hypothesis that ratios different from unity can result from the fact that the two assays are conducted under markedly different conditions. The results showed that conducting the uptake and binding assays under identical conditions reduced the GBR12935 uptake-to-binding ratio of 6.20 (under standard assay conditions) to 0.36. These data indicate that uptake-to-binding ratios must be interpreted with caution, and emphasizes the need for simpler and less expensive methods than cocaine self-administration paradigms to screen compounds as modulators of cocaine reinforcement.

Cocaine and GBR12909 produce equivalent motoric responses at different occupancy of the dopamine transporter

The motoric-stimulating effect of dopamine (DA) reuptake blockers is thought to result from the increase in synaptic dopamine levels, which occurs as a consequence of blockade of DA reuptake. The present study tested measured occupancy of the DA transporter in vivo produced by behaviorally equivalent doses of the DA reuptake blockers GBR12909 (20 mg/kg), cocaine (20 mg/kg), WIN35-065-2 (1 mg/kg), and nomifensine (5 mg/kg). Two methods were used to measure in vivo occupancy of the DA transporter: a) an ex vivo method, in which the ability of whole brain supernatants, prepared from rats administered the test drugs, were tested for their ability to inhibit the reuptake of [3H]DA by striatal synaptosomes; and b) an in vivo binding assay using [3H]N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) to label the striatal DA transporter in vivo. Considerable data support the notion that this measurement is predictive of transporter occupancy in the nucleus accumbens. Similar results were obtained with both methods: The order of potency for apparent transporter occupancy was GBR12909 >> nomifensine > WIN35-065-2 = cocaine. These data indicate that it takes greater occupancy of the DA transporter by GBR12909 to produce behavioral effects equivalent to those produced by cocaine at lower transporter occupancy. The data of the present study suggest, therefore, that studies relating the effects of DA reuptake inhibitors on DA-mediated motoric behaviors to DA transporter occupancy might facilitate the identification of novel compounds potentially useful for the pharmacotherapy of cocaine abuse.

Evidence for mutually exclusive binding of cocaine, BTCP, GBR 12935, and dopamine to the dopamine transporter

The present study addressed the possibility that there are distinct but allosterically interacting populations of binding sites for dopamine/cocaine and BTCP/GBR (N-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine/1-(2-diphenylmethox y) - ethyl]-4-(3-phenylpropyl)piperazine) (selective dopamine uptake blockers) on the dopamine transporter in the rat striatum. Dopamine uptake sites were labeled in vitro with the cocaine analog [3H]CFT (2 beta-carbomethoxy-3 beta-(4-fluorophenyl)-tropane), and the inhibition of binding by CFT or cocaine was measured. A graphic method was adopted for studying shifts in inhibitory potency resulting from the addition of a second compound. Under the conditions used, the co-presence of dopamine, GBR 12935, or BTCP decreased the inhibitory potency of CFT or cocaine to the extent predicted by a model in which all compounds bind to the same site or the binding of all compounds is mutually exclusive. No evidence for negative allosteric interactions between CFT and BTCP was found in experiments comparing inhibition of [3H]CFT binding by BTCP at a low and high concentration of [3H]CFT.

[3H] cocaine labels a binding site associated with the serotonin transporter in guinea pig brain: allosteric modulation by paroxetine

We studied the characteristics of [3H]cocaine binding to membranes prepared from whole guinea pig brain. Cocaine binding was specific and saturable. A one-site binding model fit the data adequately: the Kd value of [3H]cocaine was 44 nM with a Bmax value of 280 fmol/mg protein. The rank order of potency for the [3H]cocaine binding site was paroxetine > clomipramine > (-)-cocaine > fluoxetine > mazindol > desipramine > GBR12909 > phencyclidine > benztropine > GBR12935 > (+)-cocaine. The IC50 values of these drugs for inhibition of [3H]cocaine binding were highly correlated with their IC50 values for inhibition of [3H]5-HT uptake into synaptosomes prepared from whole guinea pig brain. High affinity 5-HT uptake inhibitors produced dose-dependent wash-resistant (pseudoirreversible) inhibition of [3H]cocaine binding. The wash-resistant inhibition produced by paroxetine was due to an increase in the Kd of [3H]cocaine binding sites, and was accompanied by an increase in the dissociation rate, consistent with an allosteric mechanism. These studies suggest that, using membranes prepared from whole guinea pig brain, [3H]cocaine labels a binding site associated with serotonin transporter and that paroxetine and cocaine bind to different sites on the serotonin transporter.

Probing the opioid receptor complex with (+)-trans-SUPERFIT. II. Evidence that mu ligands are noncompetitive inhibitors of the delta cx opioid peptide binding site

Previous studies delineated two classes of delta binding sites; a delta binding site not associated with the opioid receptor complex, termed the delta ncx site, and a delta site associated with the opioid receptor complex, termed the delta cx site. The delta ncx site has high affinity for [D-Pen2,D-Pen5]enkephalin, and is synonymous with what is now identified as the delta 1 binding site. Pretreatment of membranes with the delta-selective acylating agents FIT, or (+)-trans-SUPERFIT, deplete membranes of the delta ncx binding site, which permits the selective labeling of the delta cx binding site with [3H][D-Ala2,Leu5]enkephalin. The present study compared the properties of the delta cx binding site present in brain membranes pretreated with (+)-trans-SUPERFIT with the properties of the delta cx site present in untreated membranes. The major findings are: 1) pretreatment of membranes with (+)-trans-SUPERFIT decreased the IC50 values of delta-preferring drugs, and increased the IC50 values of mu-preferring drugs, for the delta cx binding site; 2) the degree of delta selectivity was highly correlated with the magnitude of the (+)-trans-SUPERFIT-induced shift in the IC50 values; 3) the ligand-selectivity patterns of the mu and delta cx sites present in (+)-trans-SUPERFIT-pretreated membranes were poorly correlated; 4) whereas mu-preferring drugs were noncompetitive inhibitors of [3H][D-Ala2,Leu5]enkephalin binding to the delta cx site, delta-preferring drugs were competitive inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of opioid peptides and other drugs with multiple delta ncx binding sites in rat brain: further evidence for heterogeneity

Recent pharmacological data strongly support the hypothesis of delta receptor subtypes as mediators of both supraspinal and spinal antinociception (delta 1 and delta 2 receptors). In vitro ligand binding data, which are fully supportive of the in vivo data, are still lacking. A previous study indicated that [3H][D-Ala2,D-Leu5]enkephalin labels two binding sites in membranes depleted of mu binding sites by pretreatment with the site-directed acylating agent, 2-(p-ethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimid azole-HCI (BIT). The main goal of the present study was to develop a ligand-selectivity profile of the two delta ncx binding sites. The data indicated that naltrindole and oxymorphindole were relatively selective for site 1 (20-fold). [D-Ser2,Thr6]Enkephalin and deltorphin-II were only 2.7-fold and 2.2-fold selective for site 1. [D-Pen2,D-Pen5]Enkephalin and deltorphin-I were 80-fold and 38-fold selective for site 2. 3-Iodo-Tyr-D-Ala-Gly-Phe-D-Leu was 52-fold selective for site 1. Morphine had moderate affinity for site 1 (Ki = 16 nM), and was about 11-fold selective for site 1. Thus, of the 10 drugs studied, only DPDPE and DELT-I were selective for site 2. Viewed collectively with other data, it is likely that the delta 1 receptor and the delta ncx binding site are synonymous.

Interaction of opioid peptides and other drugs with multiple kappa receptors in rat and human brain. Evidence for species differences

Previous experiments resolved four kappa binding sites in guinea pig brain termed kappa 1a, kappa 1b, and kappa 2b. The present study was undertaken to examine the occurrence of kappa receptor subtypes in rat and human brain. [3H]U69,593 and [3H]bremazocine were used to label kappa 1 and kappa 2 binding sites, respectively, present in brain membranes depleted of mu and delta binding sites by pretreatment with the irreversible ligands, BIT and FIT. Low levels of [3H]U69,593 binding precluded a detailed quantitative study of kappa 1 binding sites in these species. Quantitative examination of [3H]bremazocine binding resolved two kappa 2 binding sites in both rat and human brain whose ligand selectivity patterns differed from that of the guinea pig. These observations suggest that there may be considerable variation in the ligand recognition site of kappa receptor subtypes among mammalian species.

Probes for narcotic receptor mediated phenomena. 18. Epimeric 6 alpha- and 6 beta-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinans as potential ligands for opioid receptor single photon emission computed tomography: synthesis, evaluation, and radiochemistry of [125I]-6 beta-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinan

The epimeric 6 beta- and 6 alpha-iodo-3,14-dihydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinans (1, ioxy) and (2, epioxy), respectively, were each synthesized in five steps starting with naltrexone. The configuration of the 6-iodo group of 1 was unequivocally determined to be beta-based on single crystal X-ray analysis of its precursor 3-acetoxy-6 beta-iodo-14-hydroxy-17-(cyclopropylmethyl)-4,5 alpha-epoxymorphinan (10). Both 1 and 2 as well as their corresponding 3-O-acetates 10 and 11 were found to readily cross the blood-brain barrier and completely reverse the analgesic effects of a 10 mg/kg intraperitoneal dose of morphine sulfate as determined by the paw withdrawal latency test. Compounds 1 and 2 were found to bind with high affinity to mu, delta, and kappa receptors in vitro. In general, 1 and 2 exhibited higher affinity for mu and kappa receptors than naltrexone while the 6 beta-iodo epimer 1 (ioxy) was more potent than its epimer 2. In a comparison of the 6 beta-halogen substituent on binding affinity across opioid receptor subtypes, it was generally found that I greater than Br greater than F. On the basis of the results of in vitro and in vivo testing, 1 was selected as a target for radioiodination and evaluation as a potential single photon emission computed tomography imaging agent for opioid receptors. Carrier-free [125I]-1 was synthesized in near quantitative yield by the sequence of reaction of excess 3-acetoxy-6 alpha-[[(trifluoromethyl)sulfonyl]oxy]-14-hydroxy-17- (cyclopropylmethyl)-4,5 alpha-epoxymorphinan (8) with anhydrous Na125I in dry acetonitrile for 90 min at 76 degrees C followed by deacetylation of the product with 1:1 aqueous ammonia/acetonitrile at 25 degrees C. The potential of [125I]-1 as an in vivo imaging agent for opioid receptors is evaluated and discussed.

Characterization of the enantiomers of cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1- pyrrolidinyl)cyclohexylamine (BD737 and BD738): novel compounds with high affinity, selectivity and biological efficacy at sigma receptors

A novel class of compounds with very high affinity and selectivity for sigma receptors has been discovered. BD614 [(+/-)-cis-N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1- pyrrolidinyl)cyclohexylamine] and its optically pure 1S,2R-(-)-[BD737] and 1R,2S-(+)-[BD738]enantiomers bound to sigma receptors of guinea pig brain with Ki = 2.0 +/- 0.4, 1.3 +/- 0.3 and 6 +/- 3 nM, respectively. These compounds exhibited little or no affinity for dopamine-D2, kappa opiate or phencyclidine receptors and displayed high biological efficacy in assays of sigma receptor function, ability to produce alterations in motor behavior and inhibition of the muscarinic cholinergic phosphoinositide response. Microinjection of BD614 into the rat red nucleus or substantia nigra produced a dose-dependent alteration in head position and contralateral circling, respectively. BD614, BD737 and BD738 inhibited stimulation of inositol phosphate production by carbachol or oxotremorine-M in a dose-dependent manner. Thus, N-substituted cis-2-(1-pyrrolidinyl)cyclohexylamines may prove useful in studies of sigma receptor structure and function.

A study of the effect of the irreversible delta receptor antagonist [D-Ala2,Leu5,Cys6]-enkephalin on delta cx and delta ncx opioid binding sites in vitro and in vivo

Several lines of data support the existence of two classes of delta receptors: the delta cx binding site, which is the delta binding site of the mu-delta opioid receptor complex, and the delta ncx, which is the noncomplexed delta receptor. [D-Ala2,Leu5,Cys6]Enkephalin (DALCE) is an extended analog of [Leu5]enkephalin, which has been shown to bind irreversibly to delta receptors via the terminal cysteine by formation of a disulfide bond with the receptor. In vivo studies have shown that DALCE produces short-lived antinociceptive actions, followed by long-term antagonism of delta receptor-mediated antinociception. The major goal of the present study was to examine the effect of DALCE on the delta cx and delta ncx binding sites in vitro and in vivo. Intracerebroventricular administration of 40 micrograms DALCE failed to decrease [3H][D-Ala2,D-Leu5]enkephalin binding to the delta cx and delta ncx binding sites. Pretreatment of membranes with DALCE in vitro greatly reduced the Bmax of the delta ncx binding site, without significantly altering the Bmax of the delta cx binding site. These findings suggest that when administered in vivo, DALCE fails to distribute uniformly throughout the brain, and that it therefore binds covalently to opioid receptors mostly in the periventricular regions. Viewed collectively, these data support the hypothesis that DALCE acts as a selective delta ncx antagonist, and that the delta ncx binding site, which is sensitive to DALCE, is most likely synonymous with the recently described delta 1 receptor.

Phencyclidine binds to blood platelets with high affinity and specifically inhibits their activation by adrenaline

The ion channel probe phencyclidine [1-(1-phenylcyclohexyl)piperidine; PCP] selectively inhibited aggregation, secretion and ultrastructural changes in platelets induced by adrenaline, but did not affect activation induced by other common platelet agonists such as alpha-thrombin, ADP, collagen or ionophore A23187. [3H]PCP bound to platelets with high affinity (Kd 134 +/- 33 nM; 3600 +/- 1020 sites/platelet), as did the thienyl analogue [3H]TCP (1-[1-(2-thienyl)cyclohexyl]piperidine). PCP binding to platelets was increased 3-4-fold in N-methylglucamine buffer in the absence of Na+ ions. Binding was unaffected by haloperidol and was only weakly inhibited (EC50 10-20 microM), without significant stereoselectivity by the two sets of stereoselective ligands, dexoxadrol/levoxadrol and (+)MK801/(-)MK801. Binding of PCP was not competed for by adrenaline or yohimbine. Only the high-affinity binding of [3H]PCP to platelets was blocked by prior treatment of the platelets with the covalent affinity probe Metaphit, and these platelets no longer aggregated in response to adrenaline although they responded normally to alpha-thrombin, ADP and collagen. These results suggest that platelets contain high-affinity receptors for PCP that can modulate adrenaline-induced platelet activation.

Fourphit: a selective probe for the methylphenidate binding site on the dopamine transporter

Fourphit, a phencyclidine derivative containing an isothiocyanate substitution at the 4-position of the piperidine ring, inhibits the binding of the radiolabeled psychomotor stimulant, [3H]methylphenidate, to sites on the dopamine transport complex in membranes prepared from the crude synaptosomal fraction of rat striatal tissue with an IC50 of 7.1 microM. The inhibition caused by Fourphit is irreversible and is associated with a decrease in the Bmax, but not the KD, of [3H]methylphenidate binding. Pretreatment with saturating concentrations of unlabeled methylphenidate effected a modest (but statistically significant) protection of the stimulant binding site from inactivation by Fourphit, indicating that the acylating phencyclidine derivative may act directly at this site. Preincubation with Fourphit rather than vehicle did not alter the dissociation rate of [3H]methylphenidate when measured in the presence of excess amfonelic acid, nor was any difference detected in the off-rate of [3H]methylphenidate when excess Fourphit was substituted for excess unlabeled methylphenidate as the displacing agent. This lack of effect on the dissociation kinetics of [3H]methylphenidate provides further evidence that Fourphit does not act allosterically at the methylphenidate binding site. Unlike Metaphit (an isomer of Fourphit containing the isothiocyanate moiety at the meta position of the aromatic ring), Fourphit can discriminate between the methylphenidate binding site and the phencyclidine binding site associated with the N-methyl-D-aspartate receptor: Metaphit irreversibly inactivates both binding sites, whereas Fourphit binds reversibly to the phencyclidine binding site. The data suggest that Fourphit may be useful as a relatively selective affinity label for the site on the dopamine transport complex recognized by methylphenidate and other psychomotor stimulants.

MPTP lesions of the nigrostriatal dopaminergic projection decrease [3H]1-[1-(2-thienyl)cyclohexyl]piperidine binding to PCP site 2: further evidence that PCP site 2 is associated with the biogenic amine reuptake complex

Our previous studies have demonstrated that, using membranes of guinea pig brain, [3H]1-[1-(2-thienyl)cyclohexyl]piperidine ([3H]TCP) labels not only the phencyclidine binding site associated with the NMDA receptor (PCP site 1), but also a second high affinity binding site which is associated with the biogenic amine reuptake carrier (termed PCP site 2). To test this hypothesis, the binding of [3H]GBR12935 to the dopamine transporter, and [3H]TCP binding to PCP sites 1 and 2 were measured in caudates harvested from control, MPTP-treated and reserpine-treated dogs. MPTP treatment decreased dopamine levels by over 99%, decreased [3H]GBR12935 binding by over 90%, decreased [3H]TCP binding to PCP site 2 by about 50%, and had no significant effect on [3H]TCP binding to PCP site 1. These data are consistent with the hypothesis that a portion of PCP site 2 is associated with dopaminergic nerve terminals in dog caudate.