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Opioid analgesics remain the first choice for the treatment of moderate to severe pain, but they are also notorious for their respiratory depression and addictive effects. This study focused on the pharmacology of a novel opioid receptor mixed agonist DPI-125 and attempted to elucidate the relationship between the δ-, μ- and κ-receptor potency ratio and respiratory depression and abuse liability. Five diarylmethylpiperazine compounds (DPI-125, DPI-3290, DPI-130, KUST202 and KUST13T02) were selected for this study. PKA fluorescence redistribution assays in CHO cells individually expressing δ-, μ- or κ-receptors were used to measure the agonist potency. The respiratory safety profiles were estimated in rats by the ratio of ED50 (pCO2 increase)/ED50 (antinociception). The abuse liability of DPI-125 was evaluated with a self-administration model in rhesus monkeys. The observed agonist potencies of DPI-125 for δ-, μ- and κ-opioid receptors were 4.29±0.36, 11.10±3.04, and 16.57±4.14 nmol/L, respectively. The other four compounds were also mixed agonists with varying potencies. DPI-125 exhibited a high respiratory safety profile, clearly related to its high δ-receptor potency. The ratio of the EC50 potencies for the μ- and δ-receptors was found to be positively correlated with the respiratory safety ratio. DPI-125 has similar potencies for μ- and κ-receptors, which is likely the reason for its reduced abuse potential. Our results demonstrate that the opioid receptor mixed agonist DPI-125 is safer and less addictive than traditional μ-agonist analgesics. These findings suggest that the development of δ>μ∼κ opioid receptor mixed agonists is feasible, and such compounds could represent a promising class of potent analgesics with wider therapeutic windows.

A novel delta-receptor selective compound, ARD-353 [4-((2R,5S)-4-(R)-(4-diethylcarbamoylphenyl)(3-hydroxyphenyl)methyl)-2, 5-dimethylpiperazin-1-ylmethyl)benzoic acid], was evaluated for activity on infarct size in a rat model of acute myocardial infarction. ARD-353 was characterized as having delta receptor selectivity using radioligand binding and had no apparent selectivity between delta receptor subtypes as determined by [(3)H] cyclic [D-Pen(2),D-Pen(5)]enkephalin (delta(1)) and [(3)H]Deltorphin II (delta(2)) competition binding. ARD-353 also showed selective delta receptor agonist activity in mouse-isolated vas deferens. There was no evidence of any seizure-like convulsions when ARD-353 was administered to mice either i.v. or p.o., implying minimal penetration of the blood-brain barrier. ARD-353 decreased infarct size in a left anterior descending coronary artery (LAD) occlusion model of myocardial infarction. In animals pretreated with ARD-353 (i.v.) and then subjected to 30 min of LAD occlusion followed by 90 min of reperfusion, infarct size was reduced in a dose-dependent manner compared with vehicle-treated controls. The effects of ARD-353 on infarct size were blocked by the delta(1)-opioid selective antagonist 7-benzylidenenaltrexone, indicating a significant role for the delta(1)-opioid receptor in the cardioprotective mechanism of ARD-353. ARD-353 (0.3 mg/kg i.v.) produced significant protection when administered 5 min and 12 and 48 h before ischemic insult or when given immediately after the ischemic insult (at the start of reperfusion). Given the lack of central nervous system effects and beneficial efficacy in the rat model of myocardial ischemia, it is felt that ARD-353 is the first nonpeptide delta-receptor agonist with true potential for clinical use before surgically induced ischemia or in an emergency setting.

A tritiated form of the non-peptide delta-opioid receptor agonist (+)-BW373U86 ((+)-4-((alpha-R)-alpha-((2S,5R)-4-allyl-2, 5-dimethyl-l-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide) was synthesized and its binding characteristics studied. [3H](+)-BW373U86 bound with subnanomolar affinity to rat brain membranes and was displaced most effectively by ligands selective for delta-opioid receptors. Naltrindole, naltriben, and 7-benzylidenenaltrexone exhibited apparent inhibition constants of 0.06, 1.54, and 4.49 nM, respectively, while mu- or kappa-selective ligands showed little affinity for this site. [3H](+)-BW373U86 binding was sensitive to the presence of guanine nucleotides; GDP caused a 3-fold decrease and 5'-guanylyl-imidodiphosphate (Gpp[NH]p) caused a 25% increase in binding affinity.

Heparin and related glycosaminoglycans are important modulators of vascular smooth muscle cell growth, and may be involved in pathological processes such as atherosclerosis. Since polyphosphoinositide metabolism is a major mechanism for regulating cellular activities, including proliferation, the effects of glycosaminoglycans and polyanionic compounds on the activities of phosphoinositide kinases were characterized. Heparin and heparan sulphate caused dose-dependent inhibitions of rat brain cytosolic phosphatidylinositol 4-phosphate (PIP) kinase activity, with half-maximal inhibitory concentrations of approx. 0.5 and 5 microM respectively. PIP kinase was also inhibited by several dextran sulphates, but was not sensitive to inhibition by keratin sulphate, chondroitin sulphate or hyaluronic acid. Polynucleotides and acidic polypeptides were only weakly inhibitory. Heparin did not alter either the PIP- or the Mg(2+)-dependence of PIP kinase. Addition of heparin to brain membranes suppressed PIP kinase activity without affecting phosphatidylinositol (PI) kinase activity. Heparin interfered with the ability of a GTP analogue to stimulate PIP kinase activity in these membranes, suggesting that it uncouples the kinase from an activating guanine-nucleotide-binding protein. In cultured A-10 vascular smooth muscle cells, heparin caused dose- and time-dependent inhibition of [3H]thymidine incorporation into DNA. Similar treatments with heparin decreased cellular levels of phosphatidylinositol 4,5-bisphosphate (PIP2) without changing PI and PIP levels. Therefore heparin-mediated inhibition of PIP kinase appears to lead to decreases in PIP2 levels which may attenuate cellular proliferation.

There is a wealth of information from animal models and clinical opioid-analgesic use that indicates a significant role for opioid receptors in the modulation of bladder activity. The novel benzhydrylpiperazine compound DPI-221 [4-((alpha-S)-alpha-((2S,5R)-2,5-dimethyl-4-(3-fluorobenzyl)-1-piperazinyl)benzyl)-N,N-diethylbenzamide] was characterized as having delta receptor selectivity using radioligand binding (K(i) = 2.0 +/- 0.7 nM, delta receptor; 1800 +/- 360 nM, mu receptor; and 2300 +/- 680 nM, kappa receptor), and agonist activity was demonstrated in the mouse isolated vas deferens where DPI-221 inhibited electrically induced contractions with an IC(50) value of 88 +/- 7.5 nM. In the guinea pig isolated ileum, DPI-221 had no effect on electrically induced contractions at concentrations as high as 1 microM. Sterile saline was infused (7 ml/h) into the bladder of Sprague-Dawley rats, via a transmural catheter; DPI-221 (1.0 to 20 mg/kg p.o.) significantly increased the interval between micturition events, whereas peak void pressure was not significantly decreased by any dose of DPI-221. The micturition effects of 10 mg/kg p.o. DPI-221 were blocked by naltrindole, indicating a delta receptor mechanism of action. In isolated rat bladder strips, DPI-221 was ineffective at relaxing detrusor muscle precontracted with carbachol. The most crucial safety aspect of delta agonist administration is the incidence of seizure-like convulsions in rodents. DPI-221 produced no convulsions at doses up to 100 mg/kg p.o. in mice, although rapid bolus i.v. injection of 5 mg/kg produced convulsions in 3% of mice tested. These findings indicate a good safety profile for DPI-221 administered orally, with potent efficacy in modifying bladder activity.

Compound (+)-3-((alpha-R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide (DPI-3290), is one of a series of novel centrally acting agents with potent antinociceptive activity that binds specifically and with high affinity to opioid receptors. In saturation equilibrium binding studies performed at 25 degrees C using membranes from rat brain or guinea pig cerebellum, the Ki values measured for DPI-3290 at delta-, mu-, and kappa-opioid receptors were 0.18 +/- 0.02, 0.46 +/- 0.05, and 0.62 +/- 0.09 nM, respectively. In vas deferens isolated from laboratory mice, DPI-3290 decreased electrically induced tension development in a concentration-dependent manner with corresponding IC50 values of 1.0 +/- 0.3, 6.2 +/- 2.0, and 25.0 +/- 3.3 nM at delta-, mu-, and kappa-receptors, respectively. The activity of DPI-3290 in isolated vas deferens tissue was approximately 20,000, 175.8, and 1500 times more efficacious than morphine, and 492, 2.5, and 35 times more efficacious than fentanyl at delta-, mu-, and kappa-receptors, respectively. In ileal strips isolated from guinea pigs, DPI-3290 inhibited tension development with a corresponding IC50 value of 3.4 +/- 1.6 nM at mu-opioid receptors and 6.7 +/- 1.6 nM at kappa-opioid receptors. Intravenous administration of 0.05 +/- 0.007 mg/kg DPI-3290 produced a 50% antinociceptive response in rats. The antinociceptive properties of DPI-3290 were blocked by naloxone (0.5 mg/kg s.c.). Compared with morphine, this study demonstrated that DPI-3290 is more potent and elicited a similar magnitude of antinociceptive activity in the rat, actions mediated by its mixed opioid receptor agonist activity. The marked antinociceptive activity of DPI-3290 will likely provide a means for relieving severe pain in patients that require analgesic treatment.

Allyl-2,5-dimethyl-1-piperazines have been of interest as analgesic agents for the management of moderate-to-severe pain. In this study, we compared the antinociceptive properties and respiratory depressant activity of one such agent, (+)-3-((alpha-R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(3-fluorophenyl)-N-methylbenzamide (DPI-3290), with those of established narcotic analgesics, morphine and fentanyl. Intravenous administration of DPI-3290 in conscious laboratory rats increased antinociception in a dose-dependent manner with a corresponding ED(50) value of 0.05 +/- 0.0072 mg/kg. Simultaneous measurement of arterial blood gas in animals treated with DPI-3290 demonstrated dose-dependent increases in pCO2 with an ED(50) value of 0.91 +/- 0.22 mg/kg. In comparison, morphine and fentanyl increased antinociception in rats with ED(50) values of 2.01 +/- 0.0005 and 0.0034 +/- 0.00024 mg/kg, respectively, and the ED(50) value for morphine-induced changes in pCO2 was 4.23 +/- 0.72 mg/kg, whereas the ED(50) value for fentanyl-induced changes in pCO2 was 0.0127 +/- 0.0035 mg/kg. A separate series of experiments were designed to examine the effects of DPI-3290 on mu-opioid receptor induced antinociception and hypercapnia. Intravenous bolus doses of DPI-3290 that ranged from 0.2 to 1.0 mg/kg had no effect on antinociception mediated by alfentanil (2 microg/kg/min i.v.) but reduced hypercapnia by approximately 50%. Results from these studies demonstrate the equivalent antinociceptive efficacy of DPI-3290, morphine, and fentanyl but dramatic differences in the hypercapnia that antinociceptive doses of these drugs produce. When measured simultaneously, DPI-3290 had an 18.2-fold difference in the ratio comparing the ED(50) value for antinociception with the ED(50) value for changes in pCO2; this ratio was 2.1 for morphine and 3.7 for fentanyl. Furthermore, DPI-3290 reduced the alfentanil-mediated hypercapnia without any effect on antinociception. Together, the balanced opioid agonist activity of DPI-3290 may provide a means of powerful analgesia while mitigating the mu-opioid receptor-mediated hypercapnia.

Our laboratory has previously shown that intracerebroventricular (i.c.v.) administration of beta-endorphin suppresses brain and liver ornithine decarboxylase activity (ODC; a growth regulatory enzyme) in preweanling rats. This investigation examined, in 6-day-old rats, the relative participation of brain mu-, delta- and epsilon-opioid receptors in beta-endorphin's ODC effects, by comparing tissue ODC responses to beta-endorphin given alone i.c.v. and in the presence of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; mu-opioid receptor antagonist), N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174,864; delta-opioid receptor antagonist) or beta-endorphin-(1-27) (epsilon-opioid receptor antagonist). Administration of 0.5 microgram of beta-endorphin alone significantly decreased brain and liver ODC activity 4 h after injection, and the effect was completely blocked by coinjection of CTOP (0.075 micrograms) but not by ICI-174,864 (0.75 or 3.75 micrograms) or beta-endorphin-(1-27) (3.75 or 7.5 micrograms). The blockade of endogenous opioid:opioid receptor interactions by either CTOP (at doses > 0.075 microgram) or ICI-174,864 alone was accompanied by increased levels of basal ODC activity. The results obtained demonstrate that i.c.v. beta-endorphin downregulates ODC expression in central as well as in peripheral tissues by interacting with brain mu-opioid receptors, but not with delta- or epsilon-opioid receptors or mu/delta-opioid receptor complexes. Also, they indicate that endogenous opioid systems have a tonic inhibitory influence on ODC activity which is mediated, at least in part, by mu- and delta-opioid receptors.

The effect of pertussis toxin on opioid antinociception was studied in rats. Intrathecal injection of a single dose of pertussis toxin reduced the antinociceptive effect of PL017, a highly selective mu-opioid agonist, in a dose- and time-dependent manner. The maximal effective dose of pertussis toxin was 1 microgram, and the maximal effect was seen at day 3. The effect of the toxin lasted for 2 weeks, and the antinociceptive response recovered partially at the third week. The dose-response curves of the antinociceptive effect of PL017 were shifted to the right with increasing doses of pertussis toxin. Three days after pertussis injection, receptor-binding activity of membranes in the lumbosacral segment of spinal cords decreased to 70% of control as assayed by 125I-FK33824, a highly selective mu-receptor agonist. In experiments using [3H]naloxone as the radiolabeled ligand, displacement curves of FK33824 were shifted to the right after pertussis toxin treatment. The shift also was dose and time dependent. Scatchard analysis of binding data showed that, after pertussis toxin treatment, there was no significant change in the total number of binding sites, but a class of low-affinity binding sites appeared in addition to the high-affinity sites. When spinal membranes were washed in Na+ (100 mM) and guanosine diphosphate (100 microM) and binding was assayed in the presence of Mg2+ (5 mM), all the mu-receptors were in the high-affinity state in control membranes. After the pertussis toxin treatment, the ratio of low-affinity sites to high-affinity sites increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid receptor was solubilized from rat brain membranes with a mixture of the detergents CHAPS and digitonin in the presence of protease inhibitors and 1 M NaCl. The solubilized receptor bound mu-opioid agonists and antagonists with affinities similar to those of native membrane receptor. The affinity of solubilized receptor for the agonist PL017 was greatly reduced by GTP gamma S, suggesting the receptor is still associated with G-protein. The solubilized material was passed through an opioid antagonist (10cd) affinity column and a wheat germ agglutinin column, set up in series, to obtain a partially purified receptor preparation. This partially purified material bound mu-agonist with low affinity and the binding affinity was no longer affected by GTP gamma S. The partially purified receptor was further purified by repeating the affinity and lectin chromatography with smaller size column. Binding of opioid antagonist [3H]diprenorphine to the partially or purified receptors was dependent upon the presence of sodium ions. The purified receptor showed saturable and stereospecific binding for opioid ligands, was predominantly of the mu-type, and exhibited as a diffuse band with a medium molecular mass of 62 kD upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The average specific binding activity of the purified receptor was 18.8 +/- 2.3 pmol/micrograms protein, a value close to the theoretical estimation.

The postnatal levels of the alpha- (Mr 50,000) and beta- (Mr 58,000/60,000) subunits of type II calmodulin-dependent protein kinase were investigated in cytosolic, cytoskeletal and nuclear subfractions of rat brain. Substantial amounts of the beta-subunit were present neonatally, especially in the cytoplasm; both subunits increased markedly during the subsequent three weeks with alpha-preponderance in the cortex and midbrain. The development of the alpha-subunit was attenuated in the cerebellum and brain stem, resulting in a relative excess of the beta-subunit. Primary neuronal cultures from fetal brain initially displayed low enzyme content, followed by an increase in the levels of the beta- but not alpha-subunit. Regulation of the cellular content of type II calmodulin-dependent protein kinase may be relevant to neuronal differentiation.

Specific cell surface membrane receptors, labeled by forming a complex with low concentrations (about 10--9 M to 10--10 M) of a highly radioactive (125-I, carrier-free) ligand, can serve as simple, reliable, sensitive, and quantitative markers for plasma membranes in fractionation procedures. 125-I-Labeled insulin, cholera toxin and the plant lictins, wheat germ agglutinin (WGA), and concanavalin A are the receptor ligands used for labeling plasma membranes. Plasma membranes are labeled before homogenization by incubating intact cells briefly at 24 degrees or 4 degrees, or by very brief in situ perfusion of the organ, with the 125-I-Labeled marker. After removing the free 125-I-labeled ligand from the medium by washing (at 4 degrees), the membrane-marker complex remains intact over prolonged (days) periods of time at 4 degrees. Labeling occurs nearly exclusively on the cell surface, the specificity of this plasma membrane reaction is maintained through homogenization and fractionation, and little dissociation of the complex, detectable exchange of label, or aggregation occur even upon prolonged incubation of the homogenates. When desired, the complex can be dissociated deliberately by manipulating experimental conditions such as temperature or by adding specific simple sugars. The most generally suitable marker appears to be WGA. At least in certain tissues (e. g. fat cells) labeling of the plasma membrane with 125-I-WGA and 125-I-isnulin can be performed equally well and selectively in homogenates as in the intact cell. 125-I-Cholera toxin cannot be used in homogenates because of significant binding to nuclei. The use of 125-I-labeled WGA as a specific plasma membrane marker is illustrated in following the course of fractionations, and in quantitating the yield and purity, of plasma membranes from fat cells, lymphocytes, and liver. The results are compared with simultaneous measurements of the plasma membrane enzyme "markers," ATPase, 5-nucleotidase, and basal as well as hormone-stimulated adenylate cyclase activities. The fractionation of liver plasma membranes by aqueous dextran-polyethylene glycol two-phase polymer systems and by conventional differential centrifugation procedures arealso quantitated with the marker, 125I-WGA. Substantial quantities of plasma membrane material are no recovered in the interphase of the two-phase polymer system. Conventional liver fractionation procedures which retain, for further purification, only the readily sedimented pellet (2000 times g, 15 min) discard a very large (at least 70%) questenal hy

The benzomorphan subpopulation of opiate binding sites was labeled by [3H]diprenorphine in the presence of unlabeled ligands selected to quench mu and delta opiate binding sites. The distribution of benzomorphan binding sites was then localized autoradiographically. Areas particularly enriched in these sites were nucleus solitarius, nucleus ambiguus, substantia gelantinosa of the trigeminal nerve, the habenula, and the medial nucleus of the amygdala. Within hippocampal formation, binding was relatively enhanced in the pyramidal and granule cell layers. Within the basal ganglia, binding was greatest in the dorsomedial caudate nucleus and least in the globus pallidus. No 'patches' of increased binding were present in the striatum. The interstitial nucleus of the stria terminalis and the medial preoptic nucleus also showed significant binding. This distribution differs from the distributions of mu, delta and kappa opiate binding and is quite similar to the distribution of beta-endorphin immunoreactivity. These observations support the hypothesis, based on biochemical studies in brain membranes, that benzomorphan binding sites may represent the ligand recognition sites of putative epsilon receptors.

DNA topoisomerase II from Drosophila was phosphorylated effectively by protein kinase C. With a Km of about 100 nM, the reaction was rapid, occurring at 4 degrees C as well as at 30 degrees C and requiring as little as 0.6 ng of the protein kinase per 170 ng of topoisomerase. About 0.85 mol of phosphate could be incorporated per mol of topoisomerase II, with phosphoserine as the only phospho amino acid produced. The reaction was dependent on Ca2+ and phosphatidylserine and was stimulated by phorbol esters. Calmodulin-dependent protein kinase II, but not cyclic AMP-dependent protein kinase, was also able to phosphorylate the topoisomerase. Phosphorylation of topoisomerase II by protein kinase C resulted in appreciable activation of the topoisomerase, suggesting that it may represent a possible target for the regulation of nuclear events by protein kinase C. This possibility is supported by the finding that the phorbol ester-induced differentiation of HL-60 cells was blocked by the topoisomerase II inhibitors novobiocin and 4'-(9-acridinylamino)methanesulfon-m-anisidide(m-AMSA), but not by the inactive analog o-AMSA.

A polyclonal rabbit antibody (9120) against the alpha subunit of the "other" guanine nucleotide-binding protein Go (alpha o) was raised against a synthetic alpha o peptide fragment (Asp-Gly-Ile-Ser-Ala-Ala-Lys-Asp-Val) attached to a branched core system. Antiserum 9120, at a final dilution of 1:400, can detect alpha o in as little as 0.2 microgram of Go on immunoblots, and, at a final dilution of 1:20,000, can detect alpha o in 1 microgram of Go on immunoblots. Antiserum and affinity-purified antibody are specific to alpha o. No cross-reactivity was detected to the alpha subunits of the stimulatory or inhibitory guanine nucleotide-binding regulatory proteins or of transducin (alpha s, alpha i, or alpha T) or to the beta or gamma subunits. Immunoblots revealed a high density of alpha o in rat brain and lung membrane preparations, but other tissues (such as adipose tissue, heart, erythrocytes, and liver) have no detectable alpha o. Developmental studies showed that alpha o in rat brain was low before birth, increased after birth, and reached the full adult level at 4 weeks of age. In contrast, ADP-ribosylation of 40-kDa proteins increased for up to 1 week and then decreased. Immunocytochemistry revealed that alpha o was localized to somatic and synaptic membranes in rat brain, whereas little or no alpha o was detected in the cytoplasm of neuronal cell bodies. Our observations suggest that Go in brain might have a role in membrane signal transduction at synaptic and extrasynaptic sites.

The identification of opiate-like substances in extracts of the gastrointestinal tract and nervous system of vertebrates suggests that the known endogenous opiate-like peptides [Met 5 ]- and [Leu 5 ]-enkephalin might have a role in neurotransmission. In this study the gastrointestinal tract of guinea-pigs, rats and hamsters was examined by the immunoperoxidase-bridge method using specific antisera raised against [Met 5 ]- and [Leu 5 ]-enkephalin. Immunostained nerve fibers were most numerous in Meissner's plexus of the duodenum and in the circular muscle layer of the stomach and rectum of the guinea-pig. Nerve fibers in the guinea-pig esophagus and cardia of the stomach stained with [Met 5 ]- but not with [Leu 5 ]-enkephalin antiserum. Staining was not observed in any epithelial cells. The regional distribution of these peptides was also examined by radioimmunoassay of extracts of the gut of guinea-pigs and rats. The highest concentrations of [Met 5 ]- and [Leu 5 ]-enkephalin were found in extracts of guinea-pig duodenum at a ratio of 11:1, respectively. These findings provide evidence for an enkephalinergic nervous system in the gastrointestinal tract.

Analgesic activities of morphiceptin, beta-casomorphins, [D-Ala2, D-Leu5]enkephalin and Sandoz peptide, FK 33-824, were examined by intracerebroventricular administration in rats. Their relative potencies in vivo were compared with their receptors binding activities. The receptors binding affinities were determined from the competition curves against [3H]naloxone binding in the absence and presence of sodium ions for morphine (micro) receptors and against 125I-[D-A1A2, D-Leu]enkephalin binding for enkephalin (delta) receptors. A good correlation between analgesic activity and morphine (micro) receptor but not enkephalin (delta) receptor binding affinity was obtained. These data extend the hypothesis that morphine (micro) receptors mediate the major portion of the analgesic activity of opioids.

The simultaneous addition of [D-Ala2, D-Leu5]-enkephalin and morphiceptin at concentrations at which 98% of enkephalin (delta) and morphine (mu) receptors are occupied only partially inhibits the binding of [3H]diprenorphine to rat brain membranes. These conditions, furthermore, do not affect the curves for displacement of [3H]diprenorphine binding by unlabeled diprenorphine. These data suggest that [3H]diprenorphine binds to a third subtype of opiate binding site, which has high affinity for diprenorphine but very low affinity for mu and delta agonists. The [3H]-diprenorphine binding observed in the presence of morphiceptin and [D-Ala2, D-Leu5]enkephalin exhibits high affinity for several benzomorphan drugs in the chemical family of 6,7-benzomorphan (e.g., cyclazocine, ethylketocyclazocine, SKF 10047, UM 1072, oxilorphan, etc). Because of its selectivity for most benzomorphan drugs, this putative receptor site is tentatively referred to as a benzomorphan binding site. Its regional distribution in rat brain is similar to that of morphine (mu) receptors but differs from that for enkephalin (delta) receptors. The content of benzomorphan binding sites in rat brain is only one-half to one-third that of morphine receptors. The relative affinities of various opioids to morphine enkephalin, and benzomorphan binding sites are also described.

The affinities of the oxymorphamine epimers for mu and delta opioid receptors were determined in vitro. The 6 alpha and 6 beta epimers are potent mu-selective ligands with similar receptor-binding profiles. The relatively undramatic effect of C-6 chirality on receptor interactions is intriguing in view of the recently demonstrated profound influence of C-6 chirality on ring-C solution conformations (ring C is a chair conformer in the beta epimer but adopts a twist-boat conformation in the alpha epimer) and prompted the determination of the crystal structure of alpha-oxymorphamine. The crystal structure showed that ring C in this epimer also adopts a twist-boat conformation in the solid state. Molecular modeling of the oxymorphamines in the preferred ring-C conformations (alpha = boat, beta = chair) demonstrated that the 6-amino groups project to spatial loci significantly more proximal (0.35 A) than would be the case (2.2 A) if both epimers adopted chair conformations for ring C. Consequently, although the epimeric oxymorphamines differ in ring-C conformation, the principle potential heteroatomic binding sites in each epimer are oriented similarly, which may be partly responsible for the lack of dramatic differences in receptor binding profiles.

The fluorescent and photo-affinity derivatives of enkephalin, Tyr-D-Ala-Gly-Phe-Leu-Lys-Ne-Rhodamine (II) and Tyr-D-Ala-Gly-Phe-Leu-Lys-Ne-nitro-azidophenyl (III), were prepared by conventional methods followed by chemical modification. The two peptides inhibit the binding of 125I-labeled enkephalin to brain membrane preparations, with apparent IC50 values of 5.9 nM and 5.5 nM for peptides II and III, respectively. The iodinated derivative of peptide III binds specifically to brain membrane preparations with an apparent Kd of about 2.1 × 10−9M.

The distribution of enkephalin-like immunoreactivity in the hippocampal formation of the rat was analyzed. Two specific projection systems are described. The first emerges from the hilus of the dentate gyrus and appears to terminate with notably large boutons on the proximal apical and, to a lesser extent, basal dendrites of hippocampal regio inferior pyramidal cells. This projection corresponds in source, position, and character to the hippocampal mossy fiber system. The second axonal population enters the temporal hippocampal formation from the medial wall of the subicular complex and follows the hippocampal fissure to occupy stratum lacunosum-moleculare of the hippocampus proper and the distal third of the dentate gyrus molecular layer; this pattern corresponds to the distribution of afferent input from the lateral entorhinal cortex and/or perirhinal area. Lesions of the hilus or retrohippocampal area caused a selective depletion of immunoreactivity in the mossy fiber fields and molecular layers of the dentate gyrus, respectively. Enkephalin-like immunoreactivity was found within the somata of three types of hippocampal neurons: 1) granule cells of the dentate gyrus, 2) occasional pyramidal shaped cells of field CA1 stratum pyramidale, and 3) varied scattered interneurons. Of this last group, two types of interneurons were consistently seen. The first occupy the border between stratum radiatum and stratum lacunosum-moleculare and extend processes at right angles to the long axis of the pyramidal cell dentrites, whereas the second lie within stratum radiatum of field CA1 and extend processes in alignment with the long axis of the pyramidal cell dendrites. Cells containing enkephalin-like immunoreactivity were also observed in the subiculum and retrohippocampal region, most notably including layers II and III of the lateral entorhinal cortex-perirhinal area--the probable source of extrinsic immunoreactive input to the hippocampal formation. Intraventricular colchicine treatment intensified the immunoreactive staining of some hippocampal neurons but did not reveal any cell types not seen to be labeled in untreated rats.

Opiate receptors of the mu type were labeled with [125I]D-Ala2,N-Me-Phe4,Met-(O)5-ol-enkephalin (FK-33824). delta receptors were labeled with [125I]D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of excess (N-Me-Phe3,D-Pro4)-morphiceptin (PL017). Since DADLE binds mu and delta receptor sites, and PL017 blocks mu receptors, this protocol improves specific labeling of delta receptors. Quantitative autoradiography showed that chronic dehydration causes no changes in mu receptor binding in the medulla oblongata of Long-Evans rats. However, there is increased delta receptor binding in the solitary, hypoglossal and gracilis nuclei, and the spinal nucleus of trigeminal system of dehydrated animals, suggesting that delta opiate receptors participate in the physiological response to dehydration.

Scatchard analyses of [125I]D-ala2-D-leu5-enkephalin binding in rat brain membranes are curvilinear, suggesting low and high affinity sites. Treating the membranes with naloxazone abolishes the high affinity binding with slight effect on low affinity binding. Displacement of [125I]-D-ala2-D-leu5-enkephalin binding by morphine in untreated membranes is biphasic. Displacement by morphine in naloxazone-treated tissue is monophasic, with no inhibition by low concentrations of morphine. Naloxazone treatment has little effect on displacements by unlabeled D-ala2-D-leu5-enkephalin. Binding in N4TG1 neuroblastoma cells, which demonstrates a linear Scatchard plot with single affinity constant similar to that of the low affinity binding in brain, is less sensitive to naloxazone's actions. Naloxazone treatment in vivo inhibits D-ala2-D-leu5-enkephalin analgesia.

Image intensified fluorescence microscopy has been very useful in visualising the patterns and mobility of receptors for epidermal growth factor (EGF), insulin and α2-macroglobulin in intact cells1,2. Recently, we have synthesised a bioactive derivative of enkephalin3, Tyr-D-Ala-Gly-Phe-Leu-Lys-rhodamine, and used it for the microscopic visualisation and localisation of opiate (enkephalin) receptors in neuroblastoma cells4. In contrast to the case of polypeptide hormones1,2, where fluorescently labelled receptors are initially distributed uniformly and quickly form patches which are subsequently internalised1,2, the enkephalin-labelled receptor patches in neuroblastoma cells appear only slowly and are not internalised4. Sulphydryl groups are involved in the binding of opiates to brain membranes and may affect differentially the binding of agonists and antagonists5,6. Also, sulphydryl and disulphide groups are involved in the binding sites of adrenergic, cholinergic and muscarinic receptors7–11. These observations prompted us to examine the effects of disulphide and sulphydryl reagents on the clustering of opiate (enkephalin) receptors in N4TG1 neuroblastoma cells. We report here that in these cells there are reactive sulphydryl and disulphide groups which are essential for cluster formation (but not binding), and that a sulphydryl–disulphide exchange reaction may be involved in this process. In addition, the sulphydryl reagents seem to dissociate the two steps of binding and cluster formation, and thus provide a tool for studying the pharmacological importance of receptor clustering.

The synthetic peptide NH2-Tyr-Pro-Phe-Pro-CONH2 (morphiceptin), which is the amide of a fragment of the milk protein beta-casein, has morphinelike activities and is highly specific for morphine (mu) receptors but not for enkephalin (delta) receptors. It is as active as morphine in the guinea pig ileum but much less active in the mouse and rat vas deferens. The discovery of this specific morphine receptor ligand substantiates the hypothesis of multiple opiate receptors. The ligand, which may be of physiological significance since a very similar, or identical, activity can be detected in enzymatic digests of beta-casein, may prove useful for further investigation of the functions of opiate receptor subtypes.

Recent studies have shown that opioid peptide levels are altered in hippocampal formation of kindled animals. We therefore studied the distributions of mu and delta opioid binding sites in hippocampal formation of kindled and control rats using quantitative in vitro autoradiography. Animals received daily stimulations of the amygdala until they experienced 3 class 5 seizures. Paired control animals underwent implantation of electrodes but were not stimulated. Mu binding sites were labeled with 125I-FK-33824. Twenty-four hours after the last kindled seizure, mu binding was decreased by 32% in stratum pyramidale of CA1 and stratum radiatum of CA2 and by 17-27% throughout most of the rest of CA1, CA2, and CA3. Few, if any, differences were seen between kindled and control animals at 7 or 28 days after the last kindled seizure. Delta binding sites were labeled with 125I-[D-Ala2,D-Leu5]enkephalin in the presence of the morphiceptin analog PL-032. Twenty-four hours after the last kindled seizure, delta binding was decreased only in stratum moleculare of the dentate gyrus. Seven days after the last kindled seizure, delta binding was decreased by 11-17% throughout CA1, CA3, and the dentate gyrus. At 28 days after the last seizure, however, no differences were found between kindled and control animals. Since the decreases in mu and delta opioid binding are transient, they are unlikely to be the molecular basis of the permanent kindling phenomenon. Rather, these changes in opioid binding may represent responses to repeated seizures.

1. Leucine-enkephalin and some analogues were assayed for activity in vitro on the mouse vas deferens and for binding to opiate receptors from rat brain. 2. The experimental data were analysed in terms of the stringency for glycine, a D-amino acid or an L-amino acid at each position in the peptide. 3. The observed configurational specificity was compared with the stringency that would be predicted to occur if enkephalin adopted certain hydrogen-bonded conformations at the receptor. 4. A small subset of the conformations examined was found to be compatible with the experimental data.

Cultured mouse neuroblastoma cells (N4TG1)1,2 and neuroblastoma–glioma hybrid cells (NG108-15)3–5 contain enkephalin (δ) receptors2. Studies on hybrid cells have indicated that occupation of enkephalin receptors by opiates and opioid peptides leads to a time-dependent inhibition of adeny-late cyclase activity3–5. If exposure of these cells to agonists is continued, the activity of adenylate cyclase gradually returns to normal and increases above normal on removal of opioids, a phenomenon thought to resemble tolerance and dependence in animals. Direct receptor binding studies on neuroblastoma cells have shown no internalization of receptors1. Using rhodamine-conjugated [D-Ala2,Lys6]Leu-enkephalin and image-intensified fluorescence microscopy, enkephalin (δ) receptors in neuroblastoma cells were found slowly to form clusters which do not appear to be internalized6. This is in contrast to many polypeptide receptors7–12 which are internalized after the initial receptor–ligand interactions. This internalization is believed to be related to the so-called down-regulation or desensitization. We now report that after one to several hours exposure of monolayer cell cultures to enkephalin, the number of enkephalin receptors measured subsequently in the extensively washed membrane preparations is greatly reduced. This reduction in receptor number seems to be selective for opioid peptides and is antagonized by a variety of opiate agonists and antagonists. These observations may be important in elucidating the phenomenon of opioid-induced desensitization.

Arg-vasopressin (AVP) stimulates the production of inositol-1,4,5-trisphosphate, inositol-1,4-bisphosphate and inositol-1-phosphate in A10 smooth muscle cell line. The AVP stimulation is rapid, time and dose dependent with an ED 50 value of 5 nM. Protein kinase C activator, phorbol ester blocks the AVP effect on the production of inositol phosphates, suggesting that AVP induced phospholipase C (PLC) activation is under the negative feedback regulation by diacylglycerol production. Prolonged overnight treatment with either pertussis toxin and cholera toxin resulted partial inhibition of AVP-induced production of inositol phosphates. This result suggests that a noval G-protein similar to transducin might be involved in the AVP-induced PLC activation.

The regulation of mu- and delta-opiate receptors by guanine nucleotides and cations was studied by examining the binding of [3H][DAla2, DLeu5]enkephalin to rat brain membranes. The binding to mu-opiate receptors could be suppressed by 1 microM [DPro4]morphiceptin, a highly specific mu-agonist, thus permitting separate assessment of delta-opiate receptor binding. GTP, GDP, and the nonhydrolyzable analogs 5'-guanylyl imidodiphosphate (Gpp[NH]p) and guanosine 5'-O-(2-thiodiphosphate) (GDP-S) effectively decreased the binding to both receptor types. This inhibitory effect was potentiated by Na+. The inhibitory effect of GTP and GDP, but not of the nonhydrolyzable analogs, was reversed by Mg2+. Pretreatment of membranes with GDP or GTP increased substantially the subsequently measured high-affinity binding, and this effect required the presence of Mg2+ in the binding assay. Similar pretreatment with GDP-S resulted in only a partial increase of binding compared to GTP or GDP, and Gpp[NH]p was relatively ineffective. Similar results were observed for both mu- and delta-receptors, although the effects on mu-receptors were quantitatively more profound. These data suggest that guanine nucleotides play a dual function in regulating opiate receptor binding in a manner dependent upon the presence of Na+ or Mg2+.

Radioiodinated derivatives of the metabolically stable enkephalin analogues, [DAla2,Leu5]- and [DAla2,DLeu5]-enkephalin, have been prepared. Such derivatives show sterospecific binding to receptors in brain homogenates and some neuroblastoma cell lines such as NG108-15 and N4TG1. The relative effects of levorphanol and dextrorphan and Na+ and Mn++ ions on enkephalin binding in brain and cells indicate that the iodinated derivatives are interacting with opiate receptors. Levorphanol is considerably more potent in displacing specifically bound enkephalin than dextrorphan. Sodium ions at physiological concentrations decrease enkephalin binding whereas manganese ions enhance it. Unlabelled monoiodo derivatives retain high potency in the guinea-pig ileum, mouse vas deferens and receptor binding assays. Unlabelled diiodo derivatives show far lower potency in these assays. It is concluded that radio-iodinated derivatives containing one iodine per molecule retain high affinity for the opiate receptor but diiodo derivatives do not.

The binding of many opiates and enkephalins to enkephalin (delta) and morphine (mu) receptors was compared by using three different binding assays: (i) 125I-labeled[D-Ala2, D-Leu5]enkephalin or 125I-labeled[D-Ala2,N-Me-Phe4,Met(O)5ol]-enkephalin to brain membranes; (ii) [3H]ethylketocyclazocine to brain membranes; and (iii) [3H]diprenorphine and [3H]naloxone to neuroblastoma cell and brain membranes, respectively. According to their relative binding potencies and the effects of Na+ and GTP on the binding to these two receptors, opiates and enkephalins can be classified into seven classes: (i) morphine-type mu agonists; (ii) enkephalin-type delta agonists; (iii) mixed agonists-antagonists; (iv) putative kappa agonists; (v) putative sigma agonists; (vi) nalorphine-type antagonists; and (vii) opiate antagonists. Studies with [3H]ethylketocyclazocine do not reveal specific kappa receptors distinct from those already described that bind morphine and enkephalins. The benzomorphan analogs ketocyclazocine and ethylketocyclazocine (putative kappa agonists) and N-allylnormetazocine (putative sigma agonist) bind to morphine (mu) and enkephalin (delta) receptors with similarly high affinities. The potency of putative kappa agonists, measured by competition with binding of the 3H-labeled antagonist, is greatly reduced by the presence of Na+ and GTP; the "Na+ and GTP ratios" are similar to those of morphine and enkephalins. However, Na+ and GTP greatly decrease the potency of binding of putative sigma agonists to enkephalin receptors but only slightly decrease the binding to morphine receptors. These data suggest that putative kappa agonists have agonistic activity toward both receptors, whereas putative sigma agonists behave as agonists for enkephalin receptors but have antagonist activity for morphine receptors. Mixed agonist-antagonists also show smaller difference in affinity to both receptors. These findings may have important implications for understanding the differences in the pharmacological effects of these drugs.

In rat hippocampal slices, opioids potentiate the synaptic activation of pyramidal neurons as revealed by the shift to the left in the input-output curve constructed by plotting the population spike as a function of the field EPSP. The peak effect was obtained within 12-25 min with D-Ala2,D-Leu5-enkephalin (DADLE), morphiceptin and morphine. However, the effect of both peptides declined during constant superfusion. About 60% peak effect was lost after 1 hr superfusion with morphiceptin or after 4 hr with DADLE. In contrast, the effect of morphine gradually increased over a 4 hr incubation. Following superfusion of the slices for 4 hr in DADLE or morphine, or 1.5 hr in morphiceptin, the membrane particulate fractions were prepared from the homogenate of slices. Opiate receptor binding activities were measured with 125I-DADLE (delta-receptors) and 125I-FK 33824 (mu-receptors). A significant reduction in delta- but not mu-receptor binding was detected in slices treated with DADLE. This seems to correlate to the development of desensitization to DADLE. Neither mu-receptor nor delta-receptor binding activity was altered by the superfusion of morphine or morphiceptin despite the development of desensitization to morphiceptin. These data suggest that there are differences in the regulation of mu- and delta-receptors in hippocampus. The down-regulation of delta-receptors may result in desensitization to delta-agonists and a different mechanism may be responsible for desensitization to mu-agonists.

[Met 5 ] and [Leu 5 ]-enkephalins inhibit 3 H-naloxone binding to brain opiate receptors much more effectively at 4°C than at 25°C. At 25°C several protease inhibitors potentiate the action of the enkephalins. Bacitracin is the most effective of these. In the presence of 100 μg/ml of bacitracin, the potencies of [Met 5 ]- and [Leu 5 ]-enkephalins are similar to those at 4°C. Some protease inhibitors, such as TLCK and TAME, are effective by themselves in inhibiting the binding of 3 H-naloxone. Enkephalins with D-amino acids in the 2-position are equally effective at 0°C and at 25°C and their action is not potentiated by bacitracin. In particular, [D-Ala 2 ]-enkephalins do not seem to be significantly degraded by the membrane enzymes which destroy the natural enkephalins.

The binding of 3H-noradrenaline to intact cells and microsomes seems not to measure catecholamine receptor interactions, but rather a membrane catechol-binding protein which may be related to the enzyme, catechol-O-methyl transferase. True receptors are probably scarce and detection will require catecholamine derivatives of much higher specific activity than are at present available.

Methionine (met) and leucine (leu)-enkephalin were measured by radioimmunoassay in 16 brain regions removed by microdissection in intact rats. In hypophysectomized rats the regional concentrations of the enkephalins was not altered. Brain enkephalins do not appear to be derived from pituitary endorphins.

Previous studies have shown that morphiceptin is a highly selective mu-receptor agonist. Recently we have obtained a more potent and stable analog, Tyr-Pro-NMePhe-D-Pro-NH2 (PL017). This peptide retains mu-receptor selectivity. beta-Endorphin is known to be a potent but non-selective opioid peptide for mu-, delta- and benzomorphan binding sites. Dynorphin is a putative kappa-agonist with significant affinity to mu-, delta- and benzomorphan binding sites in rat brain membranes. To understand the structural requirement for receptor type selectivity the enkephalin sequence of beta-endorphin and dynorphin was replaced by that of morphiceptin analog. Replacing the Met-enkephalin sequence of beta h-endorphin by PL017 yields a peptide highly selective for mu-binding sites. Substituting the Leu-enkephalin sequence of dynorphin-17 produces a peptide [PL017-dynorphin(6-17)] that retains high affinities for mu- and kappa-binding sites and has very low affinities for delta- and benzomorphan binding sites. These results suggest that a morphiceptin sequence at the amino-terminus of large opioid peptides can dictate mu-receptor selectivity. An enkephalin sequence at the amino-terminus of large opioid peptides seems to be required to retain high affinity for delta- and benzomorphan binding sites. The high affinity of PL017-dynorphin(6-17) for kappa-binding sites but not for benzomorphan binding sites suggests that benzomorphan sites of rat brain are not kappa-sites.