Pharmacological effects of naltriben as a ligand for opioid mu and kappa receptors in rat cerebral cortex

Naltriben (NTB) has been used to differentiate the subtypes of delta opioid receptors, delta1 and delta2. However, there is considerable evidence suggesting that NTB may act on other types of opioid receptors too. We examined the effects of NTB on the specific binding of radiolabeled ligands for opioid mu and kappa2 receptors, and the effects on the release of [3H]norepinephrine ([3H]NE) in rat cerebral cortex slices. NTB displaced the specific binding of [3H]DAMGO with Ki value of 19.79 +/- 1.12 nM in rat cortex membranes. Specific binding of [3H]diprenorphine ([3H]DIP) was inhibited by NTB with Ki value of 82.75 +/- 6.32 nM in the presence of DAMGO and DPDPE. High K+ (15 mM)-stimulated release of [3H]NE was attenuated by DAMGO in rat cerebral cortex slices. NTB (30 nM) shifted the dose-response curve of DAMGO to the right and attenuated the maximal effect. In the meantime, NTB inhibited high K+-stimulated [3H]NE release at concentrations above 100 nM. The inhibitory effect of NTB was not attenuated by CTAP (10 nM) and naloxone (3 nM) but by higher concentration of naloxone (30 nM), nor-BNI (300 nM) and bremazocine (3 nM). These results indicate that NTB, depending on the dosage, could acts not only as an antagonist at delta but also as a noncompetitive antagonist for mu receptors, and as an agonist for kappa2 receptors in rat cerebral cortex.     

14 beta-(Bromoacetamido)morphine irreversibly labels mu opioid receptors in rat brain membranes

The binding properties of 14 beta-(bromoacetamido)morphine (BAM) and the ability of BAM to irreversibly inhibit opioid binding to rat brain membranes were examined to characterize the affinity and selectivity of BAM as an irreversible affinity ligand for opioid receptors. BAM had the same receptor selectivity as morphine, with a 3-5-fold decrease in affinity for the different types of opioid receptors. When brain membranes were incubated with BAM, followed by extensive washing, opioid binding was restored to control levels. However, when membranes were incubated with dithiothreitol (DTT), followed by BAM, and subsequently washed, 90% of the 0.25 nM [3H] [D-Ala2,(Me)Phe4,Gly(ol)5]enkephalin (DAGO) binding was irreversibly inhibited as a result of the specific alkylation of a sulfhydryl group at the mu binding site. This inhibition was dependent on the concentrations of both DTT and BAM. The mu receptor specificity of BAM alkylation was demonstrated by the ability of BAM alkylated membranes to still bind the delta-selective peptide [3H] [D-penicillamine2,D-penicillamine5]enkephalin (DPDPE) and (-)-[3H]bremazocine in the presence of mu and delta blockers, selective for kappa binding sites. Under conditions where 90% of the 0.25 nM [3H]DAGO binding sites were blocked, 80% of the 0.8 nM [3H]naloxone binding and 50% of the 0.25 nM 125I-labeled beta h-endorphin binding were inhibited by BAM alkylation. Morphine and naloxone partially protected the binding site from alkylation with BAM, while ligands that did not bind to the mu site did not afford protection.2+hese studies have demonstrated that when a disulfide bond     

Substance P modulation of DAMGO binding in the brain of CXBK and Swiss-Webster mice.

The effects of substance P (SP) on the binding of the selective mu opioid agonist [3H]DAMGO to brain membranes of CXBK and Swiss-Webster (SW) mice were compared. We have previously shown that subnanomolar concentrations of SP and N-terminal fragments of SP modulate DAMGO binding in SW brain membranes and hypothesized that modulation occurs via SP interaction with mu 1 sites. In the present study, binding assays using CXBK mice, a strain deficient in mu receptors including mu 1 sites, were performed to assess the effect of mu receptor deficiency on SP-induced modulation of DAMGO binding. Whereas the addition of 0.1 nM SP to the binding mixtures produced up to 30% increase in the values of Kd and maximum binding capacity (R) for the SW strain, SP produced little or no change in the case of CXBK strain. Maximum binding capacity for DAMGO was 43% less in the brain of CXBK mice than in SW mice. No difference was observed in the estimated binding parameters of the spinal cord for the two strains. Whereas pretreatment of brain membranes of SW mice using beta-funaltrexamine (beta-FNA) increased from 2- to 10-fold the modulatory effect of SP, CXBK brain membranes pretreated with beta-FNA remained nearly insensitive to modulation by SP. The effect of SP on the affinity of DAMGO binding in SW mice, but not in CXBK mice, was reversed by the addition of GTP. It is concluded that mu receptor deficiency can markedly influence SP-induced modulation of DAMGO binding.     

Decreased spinal morphine/clonidine antinociceptive synergism in morphine-tolerant mice

The antinociceptive interactions between spinally administered opioids and the alpha₂ agonist clonidine were examined in placebo and morphine pellet-implanted mice using the tail flick test. In placebo pellet-implanted animals, coadministered morphine and clonidine produced a synergistic antinociceptive effect. In mice implanted with morphine pellets, the synergism decreased to an additive interaction. The interactions between clonidine and the mu agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO), the delta agonist D-Pen²-D-Pen⁵-Enkephalin (DPDPE), and the kappa agonist U50-488H were also synergistic in placebo animals. In morphine pellet treated mice the DPDPE/clonidine interaction decreased to an antagonistic interaction, the DAMGO/clonidine remained synergistic and the U50-488H/clonidine interaction decreased to additive. These results support the proposal that the morphine spinal/supraspinal synergism depends upon the interaction between spinal opioid and alpha₂ receptors and a decrease in this interaction is a mechanism involved in development of tolerance to morphine. In addition, delta and kappa receptors appeared to be more involved in the morphine/clonidine decreased interaction than did mu opioid receptors.     

Biphasic competition between opiates and enkephalins: does it indicate the existence of a common high affinity ("mu-1") binding site?

Displacement from brain membranes of labeled opiates by low concentrations of enkephalins and of labeled enkephalins by low concentrations of opiates has been previously explained by the existence of a common high affinity site termed mu-1. An alternative interpretation of the same results is that the trough seen in the low concentration zone of the displacement curves represents cross binding of mu and delta opioid ligands to delta and mu receptors, respectively. In three sets of experiments with brain membranes, the size of the trough is shown to be dependent on the labeled ligand used: The ratio between the size of troughs seen with [3H]D-Ala, D-Leu enkephalin and with [3H]morphine varies with experimental conditions (storage of membranes at 4 degrees C for 72 h), with ratio of mu:delta receptors (e.g. in thalamus and cortex which are enriched in mu and delta sites, respectively) and with pretreatment of membranes with naloxonazine. These results can not be explained by a common high affinity site, but rather by binding of [3H]D-Ala, D-Leu enkephalin to mu and of [3H]morphine to delta opioid receptors.     

Affinity labeling of delta opioid receptors by an enkephalin-derivative alkylating agent, DSLET-Mal

Opioid binding properties of Tyr-D-Ser-Gly-Phe-Leu-Thr-NH-NH-Gly-Mal (DSLET-Mal), a novel enkephalin-framed affinity label, was determined in rat brain membranes. In competition studies the ligand showed high affinity for the delta opioid sites, labelled by [(3)H][Ile(5,6)]deltorphin II (K(i) = 8 nM), whereas its binding to the mu ([(3)H]DAMGO) and kappa ([(3)H]EKC) sites was weaker. Preincubation of the rat brain membranes with DSLET-Mal at micromolar concentrations resulted in a wash-resistant and dose-dependent inhibition of the [(3)H][Ile(5,6)]deltorphin II binding sites (96% blocking at 10 microM concentration). Intracerebroventricular (ICV) administration of DSLET-Mal reduced the density of delta opioid receptors and had no effect on mu and kappa receptors, as determined by saturation binding studies. [Ile(5, 6)]deltorphin II-stimulated [(35)S]GTPgammaS binding was determined in membrane preparations of different brain areas of the ICV-treated animals. In both frontal cortex and hippocampus DSLET-Mal significantly decreased G protein activation by the delta agonist, having no effect on DAMGO stimulated [(35)S]GTPgammaS binding. DSLET-Mal had qualitatively similar effects on both receptor binding and G protein activation. These characteristics of the compound studied suggest that DSLET-Mal can serve as an affinity label for further studies of the delta-opioid receptors.     

Morphine-induced desensitization and down-regulation at mu-receptors in 7315C pituitary tumor cells

Pituitary 7315c tumor cells maintained in culture were treated with varying concentrations of morphine from 10 nM to 300 microM, for periods of five or forty-eight hours. The ability of the mu-opioid receptor agonist, DAMGO, to inhibit forskolin-stimulated adenylyl cyclase in washed membrane preparations from the treated cells was compared with its activity in membranes from cells incubated in the absence of added morphine. In the same membrane preparations, the number and affinity of mu-opioid receptors was estimated by measurements of [3H]diprenorphine binding. After 5 hr of treatment with morphine concentrations of 100 nM or higher, a significant reduction in inhibition of adenylyl cyclase by DAMGO was observed. Little further loss of agonist activity was observed when the incubations were extended to 48 hr. After 5 hr of morphine treatment, there was no change in either the number of receptors, or their affinity for [3H]diprenorphine. However, after 48 hr of morphine treatment, greater than 25% reductions in receptor number were apparent with morphine pretreatment concentrations of 10 microM or higher. These results suggest that opioid tolerance in this system is primarily associated with a reduced ability of agonist-occupied receptor to activate the effector system. Receptor down-regulation was not necessary for loss of agonist response, although a reduction in receptor number occurred after exposure to high concentrations of morphine for periods longer than 5 hr.     

Functional role of the spatial proximity of Asp114(2.50) in TMH 2 and Asn332(7.49) in TMH 7 of the mu opioid receptor

We examined whether a proposed spatial proximity between Asp114(2.50) and Asn332(7.49) affected the functional properties of the mu opioid receptor. The D114(2.50)N mutant had reduced binding affinities for morphine, DAMGO and CTAP, but not for naloxone and [3H]diprenorphine; this mutation also abolished agonist-induced increase in [35S]GTPgammaS binding. The N332(7.49)D mutation eliminated detectable binding of either [3H]diprenorphine or [3H]DAMGO. The combined D114(2.50)N-N332(7.49)D mutation restored high affinity binding for [3H]diprenorphine, CTAP and naloxone, and restored partially the binding affinities, potencies and efficacies of morphine and DAMGO. Thus, reciprocal mutations of Asp114(2.50) and Asn332(7.49) compensate for the detrimental effects of the single mutations, indicating that the residues are adjacent in space and that their chemical functionalities are important for ligand binding and receptor activation.     

Characterization of [125I]AR-M100613, a high-affinity radioligand for delta opioid receptors

AR-M100613 ([I]-Dmt-c[-D-Orn-2-Nal-D-Pro-D-Ala-]) is the iodinated analog of a cyclic casomorphin previously shown to be a potent antagonist at the delta opioid receptor. Specific [125I]AR-M100613 binding to rat whole brain membranes was saturable, reversible, and best fit to a one-site model (Kd = 0.080 +/- 0.008 nM, Bmax = 45.2 +/- 4.4 fmol/mg protein). [125I]AR-M100613 binding was displaced with high affinity by the delta opioid receptor ligands SNC-80, Deltorphin II and DPDPE but not the mu or kappa-selective receptor ligands DAMGO and U69593. Residual non-selective binding of [125I]AR-M 100613 to mu opioid receptors is blocked by the addition of CTOP to the assay buffer. [35S]GTPgammaS binding assays indicate that AR-M100613 is a potent, selective, and reversible antagonist for delta opioid receptors in rat brain membranes. The high-affinity, high specific activity, low nonspecific binding and antagonist profile of [125I]AR-M100613 favor its use as a radiochemical probe for delta opioid receptors.     

Inhibition of mu and delta but not kappa opioid binding to membranes by Fab fragments from a monoclonal antibody directed against the opioid receptor

Fab fragments from a monoclonal antibody, OR-689.2.4, directed against the opioid receptor, selectively inhibited opioid binding to rat and guinea pig neural membranes. In a titratable manner, the Fab fragments noncompetitively inhibited the binding of the mu selective peptide [D-Ala2,(Me)Phe4,Gly(OH)5][3H] enkephalin and the delta selective peptide [D-Pen2,D-Pen5] [3H]enkephalin (where Pen represents penicillamine) to neural membranes. In contrast, kappa opioid binding, as measured by the binding of [3H]bremazocine to rat neural membranes and guinea pig cerebellum in the presence of mu and delta blockers, was not significantly altered by the Fab fragments. In addition to blocking the binding of mu and delta ligands, the Fab fragments displaced bound opioids from the membranes. When mu sites were blocked with [D-Ala2,(Me)Phe4,Gly(OH)5]enkephalin, the Fab fragments suppressed the binding of [D-Pen2,D-Pen5][3H]enkephalin to the same degree as when the mu binding site was not blocked. The Fab fragments also inhibited binding to the mu site regardless of whether or not the delta site was blocked with [D-Pen2,D-Pen5]enkephalin. This monoclonal antibody is directed against a 35,000-dalton protein. Since the antibody is able to inhibit mu and delta binding but not kappa opioid binding, it appears that this 35,000-dalton protein is an integral component of mu and delta opioid receptors but not kappa receptors.     

Multiple opioid receptors: An examination of the dissociation of [3H]leucine enkephalin from rat brain membranes

The experiments reported in this paper address the hypothesis that [3H]leucine enkephalin labels both mu and delta receptors. As reported by other workers, this peptide dissociates from rat brain membranes in a biphasic manner. This is consistent with a two site binding model which hypothesizes that the peptide labels both opioid mu and delta receptors from which it dissociates at different rates. To test this hypothesis, we determined the dissociation of bound ligand from rat brain membranes incubated to equilibrium with [3H]leucine enkephalin in the absence and presence of 100 nM morphine. The data were not significantly different. We conclude that the biphasic off-kinetics of [3H]leucine enkephalin is not evidence for a two-site binding model.     

Chronic intracerebroventricular infusion of the antiopioid peptide, Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (NPFF), downregulates mu opioid binding sites in rat brain

Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2 (NPFF), an endogenous mammalian antiopioid peptide, has been shown by other laboratories to attenuate the acute antinociceptive effects of morphine, the development of morphine tolerance, and naloxone-induced withdrawal in morphine-dependent rats. The present study determined the effect of chronic NPFF on mu opioid receptors and mRNA for the endogenous opioids dynorphin and enkephalin. Rats received ICV infusions of either saline or NPFF (5 μg/h) for 13 days via Alzet 2002 osmotic minipumps. Homogenate binding studies, which used whole brain membranes, demonstrated that NPFF decreased the B_max of mu binding sites (labeled by [³H][ -Ala²-MePhe⁴,Gly-ol⁵]enkephalin) from 262 ± 12 to 192 ± 12 fmolmg protein, and increased the K_d from 1.1 to 2.3 nM. Quantitative receptor autoradiography and in situ hybridization experiments were conducted with sections collected at the level of the striatum. The density of mu opioid binding sites labeled by [³H][ -Ala²-MePhe⁴,Gly-ol⁵]enkephalin was decreased in all brain areas measured except the corpus callosum, and there was no change in dynorphin mRNA or enkephalin mRNA in the caudate, the nucleus accumbens, or the ventral pallidum. Rats chronically administered ICV morphine sulfate (20 μg/h) for 14 days developed tolerance to morphine and a low degree of dependence, as measured by naloxone-precipitated withdrawal. Chronic administration of NPFF concurrently with morphine sulfate did not significantly alter naloxone-induced withdrawal signs or the development of morphine tolerance. Viewed collectively with previous findings that chronic ICV infusion of anti-NPFF IgG upregulates mu receptors, these data provide additional evidence that the density of CNS mu receptors is tonically regulated by NPFF in the extracellular fluid. The action of NPFF to decrease mu receptors is consistent with an antiopioid role for this peptide; however, the fact that NPFF (administered into the lateral ventricle) did not appreciably alter expression of morphine tolerance and dependence contrasts with previous findings and reinforces the view that this effect is most reliably seen after third ventricle administration.     

Characteristics of central binding sites for [3H] DAMGO in spontaneously hypertensive rats

The binding of [3H] DAMGO, a highly selective ligand for mu-opiate receptors, to membranes of discrete brain regions and spinal cord of 10 week old spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats was determined. The brain regions examined were hypothalamus, amygdala, hippocampus, corpus striatum, pons and medulla, midbrain and cortex. [3H] DAMGO bound to membranes of brain regions and spinal cord at a single high affinity site. The receptor density (Bmax value) and apparent dissociation constant (Kd value) of [3H] DAMGO to bind to membranes of hippocampus, corpus striatum, pons and medulla, cortex and spinal cord of WKY and SHR rats did not differ. The Bmax value of [3H] DAMGO in membranes of hypothalamus and midbrain of SHR rats was significantly higher than in WKY rats but the Kd values in the two strains did not differ. On the other hand, the Bmax value of [3H] DAMGO in membranes of amygdala of SHR rats was lower than that of WKY rats but the Kd values in the two strains were similar. It is concluded that SHR rats have higher density of mu-opiate receptors in hypothalamus and midbrain but lower density in amygdala in comparison with WKY rats, and that such differences in the distribution of mu-opiate receptors may be related to the elevated blood pressure in SHR rats.     

Studies on the effects of glucose in vitro and of the glycaemic state in vivo on the binding characteristics of mu, delta and kappa opiate receptors in mouse brain.

The effect of glucose on the binding characteristics of opiate receptor subtypes was investigated in brain membranes from normoglycaemic lean Aston (C57BL/6J) mice using [3H][D-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO), [3H][D-Pen2,D-Pen5]enkephalin (DPDPE) and [3H]U69,593 as selective ligands for mu, delta and kappa opiate receptors respectively. The equilibrium dissociation constants (Kd) and maximal binding capacities (Bmax) of [3H]DAMGO and [3H]DPDPE were unaltered by 20mM glucose in vitro. Similarly, [3H]U69,593 binding was not modified by increasing the concentration of glucose from 0 to 20mM (P between 0.10 and 0.05), or by the presence of 20mM fructose and of 20mM 3-O-me-glucose, a non-metabolisable sugar, in the incubation medium. The nonselective opiate ligand, [3H]diprenorphine, bound with similar affinity and binding capacity to brain membranes prepared from control and streptozotocin-diabetic Swiss (CD1) mice. The addition of 20mM glucose or of 20mM fructose in vitro induced no changes in their binding parameters. The affinity and binding capacity of [3H]U69,593 to STZ-diabetic Swiss mouse brain membranes was not significantly different to that of normoglycaemic controls; 20mM glucose in vitro had no effect on ligand binding to kappa sites in STZ-diabetic mouse brain membranes. We conclude that glucose does not interact directly with the opiate receptor to modfy it in such as way as could explain the altered sensitivity to different opioid agonists seen in obese and hyperglycaemic animal models in vivo.     

Mechanisms of mu opioid receptor/G-protein desensitization in brain by chronic heroin administration

Previous studies have shown that chronic opiate treatment decreases mu opioid-stimulated [35S]GTPgammaS binding in specific brain regions. To extend these findings, the present study investigated DAMGO-stimulated [35S]GTPgammaS binding in membrane homogenates and coronal sections from rats non-contingently administered heroin. Rats were administered saline or increasing doses of heroin i.v. hourly up to 288 mg/kg/day over 40 days. In brain sections, chronic heroin administration decreased DAMGO-stimulated [35S]GTPgammaS binding in medial thalamus and amygdala, with no effect in cingulate cortex or nucleus accumbens. Chronic heroin administration also reduced [35S]GTPgammaS binding stimulated by the principal metabolite of heroin, 6-monoacetylmorphine. In contrast, no significant changes in mu opioid receptor binding were observed in amygdala or thalamus using [3H]DAMGO autoradiography. In membranes from amygdala and thalamus, chronic heroin treatment decreased the maximal effect of DAMGO in stimulating [35S]GTPgammaS binding, with no effect on DAMGO potency. GTPgammaS saturation analysis showed that chronic heroin treatment decreased the Bmax, and increased the K(D), of DAMGO-stimulated [35S]GTPgammaS binding. These data suggest potential mechanisms by which chronic agonist treatment produces opioid receptor/G-protein desensitization in brain.     

Opioid receptors in human neuroblastoma SH-SY5Y cells: evidence for distinct morphine (mu) and enkephalin (delta) binding sites

Human neuroblastoma SH-SY5Y cells exhibited a heterogeneous population of mu and delta types of opioid binding sites. These specific binding sites displayed the characteristic saturability, stereospecificity and reversibility, expected of a receptor. Scatchard analysis of [3H]-D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of 10(-5) M D-Pro4-morphiceptin (to block the mu receptors) and the competitive displacement by various highly selective ligands yielded the binding parameters of delta sites which closely resemble those of the delta receptors in brain and mouse neuroblastoma clones. Similarly, the high affinity binding of [3H]-dihydromorphine, together with the higher potency of morphine analogues to displace [3H]-naloxone binding established the presence of mu sites. Guanine nucleotides and NaCl significantly inhibited the association and increased the dissociation of [3H]-DADLE binding. The observed heterogeneity of opioid receptors in cultured SH-SY5Y cells would serve as an excellent model for the biochemical and pharmacological characterization of brain opiate receptors.     

Modulation of Opioid Receptor Binding by Cis and Trans Fatty Acids

In synaptosomal brain membranes, the addition of oleic acid (cis), elaidic acid (trans), and the cis and trans isomers of vaccenic acid, at a concentration of 0.87 mumol of lipid/mg of protein, strongly reduced the Bmax and, to a lesser degree, the binding affinity of the mu-selective opioid [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([3H]DAMGO). At comparable membrane content, the cis isomers of the fatty acids were more potent than their trans counterparts in inhibiting ligand binding and in decreasing membrane microviscosity, both at the membrane surface and in the core. However, trans-vacenic acid affected opioid receptor binding in spite of just marginally altering membrane microviscosity. If the receptors were uncoupled from guanine nucleotide regulatory protein, an altered inhibition profile was obtained: the impairment of KD by the fatty acids was enhanced and that of Bmax reduced. Receptor interaction of the delta-opioid [3H](D-Pen2,D-Pen5)enkephalin was modulated by lipids to a greater extent than that of [3H]DAMGO: saturable binding was abolished by both oleic and elaidic acids. The binding of [3H]naltrexone was less susceptible to inhibition by the fatty acids, particularly in the presence of sodium. In the absence of this cation, however, cis-vaccenic acid abolished the low-affinity binding component of [3H]naltrexone. These findings support the membrane model of opioid receptor sequestration depicting different ionic environments for the mu- and delta-binding sites. The results of this work show distinct modulation of different types and molecular states of opioid receptor by fatty acids through mechanisms involving membrane fluidity and specific interactions with membrane constituents.     

Opioid Signal Transduction in Intact and Fragmented SH-SY5Y Neural Cells

Parameters of ligand binding, stimulation of low-Km GTPase, and inhibition of adenylate cyclase were determined in intact human neuroblastoma SH-SY5Y cells and in their isolated membranes, both suspended in identical physiological buffer medium. In cells, the mu-selective opioid agonist [3H]Tyr-D-Ala-Gly(Me)Phe-Gly-ol ([3H]DAMGO) bound to two populations of sites with KD values of 3.9 and 160 nM, with less than 10% of the sites in the high-affinity state. Both sites were also detected at 4 degrees C and were displaced by various opioids, including quaternary naltrexone. The opioid antagonist [3H]naltrexone bound to a single population of sites, and in cells treated with pertussis toxin the biphasic displacement of [3H]naltrexone by DAMGO became monophasic with only low-affinity binding present. The toxin specifically reduced high-affinity agonist binding but had no effect on the binding of [3H]naltrexone. In isolated membranes, both agonist and antagonist bound to a single population of receptor sites with affinities similar to that of the high-affinity binding component in cells. Addition of GTP to membranes reduced the Bmax for [3H]DAMGO by 87% and induced a linear ligand binding component; a low-affinity binding site, however, could not be saturated. Compared with results obtained with membranes suspended in Tris buffer, agonist binding, including both receptor density and affinity, in the physiological medium was attenuated. The results suggest that high-affinity opioid agonist binding represents the ligand-receptor-guanine nucleotide binding protein (G protein) complex present in cells at low density due to modulation by endogenous GTP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Upregulation of the opioid receptor complex by the chronic administration of morphine: a biochemical marker related to the development of tolerance and dependence.

Studies conducted after the development of the rapid filtration assay for opiate receptors, and before the recognition of multiple opioid receptors, failed to detect changes in opioid receptors induced by chronic morphine. Recent experiments conducted in our laboratories were designed to examine the hypothesis that only one of several opioid receptor types might be altered by chronic morphine. Using binding surface analysis and irreversible ligands to increase the "resolving power" of the ligand binding assay, the results indicated that chronic morphine increased both the Bmax and Kd of the opioid receptor complex, labeled with either [3H][D-Ala2,D-Leu5]enkephalin, [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin or [3H]6-desoxy-6 beta-fluoronaltreone. In the present study rats were pretreated with drugs known to attenuate the development of tolerance and dependence [the irreversible mu-receptor antagonist, beta-funaltrexamine (beta-FNA), and the inhibitor of tryptophan hydroxylase, para-chlorophenylalanine], prior to subcutaneous implantation of morphine pellets. The results demonstrated that 1) unlike chronic naltrexone, beta-FNA failed to upregulate opioid receptors and 2) both beta-funaltrexamine and PCPA pretreatment attenuated the chronic morphine-induced increase in the Bmax, but not the Kd, of the opioid receptor complex. These results provide evidence that naltrex-one-induced upregulation of the opioid receptor complex might occur indirectly as a consequence of interactions at beta-funaltrexamine-insensitive opioid receptors and that morphine-induced upregulation (increased Bmax) of the opioid receptor complex is a relevant in vitro marker related to the development of tolerance and dependence. These data collectively support the hypothesis that endogenous antiopiate peptides play an important role in the development of tolerance and dependence to morphine.     

Signal transduction efficacy of the highly potent mu opioid agonist 14-methoxymetopon

In search of a truly high-efficacy (i.e., tau > 100) mu opioid analgesic, we determined the efficacy (tau) and apparent in vivo affinity (KA) of the high-potency alkoxymorphinan 14-methoxymetopon. However, in the present study, 14-methoxymetopon's efficacy proved to be only 1.5-fold higher than that of morphine (tau, 19 vs. 12). KA values were 2,900 nmol/kg for 14-methoxymetopon and 46,000 nmol/kg for morphine (Ki for [3H]DAMGO binding, 0.33 vs 3.4 nmol/l). Thus, the 24-fold higher potency of methoxymetopon could be fully accounted for by its 16-fold higher apparent in vivo affinity and its only 1.5-fold higher efficacy. Furthermore, the 10-fold higher affinity of 14-methoxymetopon for the mu opioid receptor - as previously determined in radioligand binding assays - was confirmed in the present behavioral tests of thermal antinociception.