Modulation of mu-mediated antinociception by delta agonists: characterization with antagonists

Multifunctional Opioid Ligands

The opioid receptor system plays a major role in the regulation of mood, reward, and pain. The opioid receptors therefore make attractive targets for the treatment of many different conditions, including pain, depression, and addiction. However, stimulation or blockade of any one opioid receptor type often leads to on-target adverse effects that limit the clinical utility of a selective opioid agonist or antagonist. Literature precedent suggests that the opioid receptors do not act in isolation and that interactions among the opioid receptors and between the opioid receptors and other proteins may produce clinically useful targets. Multifunctional ligands have the potential to elicit desired outcomes with reduced adverse effects by allowing for the activation of specific receptor conformations and/or signaling pathways promoted as a result of receptor oligomerization or crosstalk. In this chapter, we describe several classes of multifunctional ligands that interact with at least one opioid receptor. These ligands have been designed for biochemical exploration and the treatment of a wide variety of conditions, including multiple kinds of pain, depression, anxiety, addiction, and gastrointestinal disorders. The structures, pharmacological utility, and therapeutic drawbacks of these classes of ligands are discussed.

Synthesis and Investigation of Mixed μ-Opioid and δ-Opioid Agonists as Possible Bivalent Ligands for Treatment of Pain

Several studies have suggested functional association between μ-opioid and δ-opioid receptors and showed that μ-activity could be modulated by δ-ligands. The general conclusion is that agonists for the δ-receptor can enhance the analgesic potency and efficacy of μ-agonists. Our preliminary investigations demonstrate that new bivalent ligands constructed from the μ-agonist fentanyl and the δ-agonist enkephalin-like peptides are promising entities for creation of new analgesics with reduced side effects for treatment of neuropathic pain. A new superposition of the mentioned pharmacophores led to novel μ-bivalent/δ-bivalent compounds that demonstrate both μ-opioid and δ-opioid receptor agonist activity and high efficacy in anti-inflammatory and neuropathic pain models with the potential of reduced unwanted side effects.

Fentanyl-related compounds and derivatives: current status and future prospects for pharmaceutical applications

Fentanyl and its analogs have been mainstays for the treatment of severe to moderate pain for many years. In this review, we outline the structural and corresponding synthetic strategies that have been used to understand the structure-biological activity relationship in fentanyl-related compounds and derivatives and their biological activity profiles. We discuss how changes in the scaffold structure can change biological and pharmacological activities. Finally, recent efforts to design and synthesize novel multivalent ligands that act as mu and delta opioid receptors and NK-1 receptors are discussed.

Involvement of D1/D2 dopamine receptors within the nucleus accumbens and ventral tegmental area in the development of sensitization to antinociceptive effect of morphine

The nucleus accumbens (NAc) and the ventral tegmental area (VTA) are two major areas for the mesolimbic dopaminergic system which are strongly involved in the development of behavioral sensitization. In the present study, we investigated the role of D1/D2 dopaminergic receptors within the NAc or VTA in response to sensitization to morphine by the tail-flick test as a model of acute pain. Sensitization was induced by subcutaneous (SC) injection of morphine (5 mg/kg), once daily for three days followed by 5 days free of drug. After the sensitization period, antinociceptive responses induced by an ineffective dose of morphine (1 mg/kg; SC) were obtained by the tail-flick test, and represented as maximal possible effect (%MPE). In experimental groups, D1 and D2 receptor antagonists, SCH-23390 and sulpiride (0.25, 1 and 4 μg/rat), were separately microinjected into the NAc or VTA, 10 min before morphine administration during the sensitization period, respectively. Results showed that injection of morphine during the sensitization period (development of sensitization) increased %MPE of the ineffective dose of morphine from 2.43±1.4% in naive to 47.75±4.01% in sensitized animals (P<0.001). Unilateral microinjections of different doses of the D1/D2 receptor antagonists, SCH-23390 and sulpiride, into the NAc dose-dependently decreased %MPEs in morphine-sensitized animals. Nonetheless, %MPEs were only affected by intra-VTA administration of SCH-23390 in morphine-sensitized animals (P<0.05). Our findings suggest that both the D1/D2 dopamine receptors in the NAc and the D1 receptors in the VTA may be of more important in the development of sensitization to morphine in rats.

Translation of structure-activity relationships from cyclic mixed efficacy opioid peptides to linear analogues

Most opioid analgesics used in the treatment of pain are mu opioid receptor (MOR) agonists. While effective, there are significant drawbacks to opioid use, including the development of tolerance and dependence. However, the coadministration of a MOR agonist with a delta opioid receptor (DOR) antagonist slows the development of MOR-related side effects, while maintaining analgesia. We have previously reported a series of cyclic mixed efficacy MOR agonist/DOR antagonist ligands. Here we describe the transfer of key features from these cyclic analogs to linear sequences. Using the linear MOR/DOR agonist, Tyr-DThr-Gly-Phe-Leu-Ser-NH2 (DTLES), as a lead scaffold, we replaced Phe(4) with bulkier and/or constrained aromatic residues shown to confer DOR antagonism in our cyclic ligands. These replacements failed to confer DOR antagonism in the DTLES analogs, presumably because the more flexible linear ligands can adopt binding poses that will fit in the narrow binding pocket of the active conformations of both MOR and DOR. Nonetheless, the pharmacological profile observed in this series, high affinity and efficacy for MOR and DOR with selectivity relative to KOR, has also been shown to reduce the development of unwanted side effects. We further modified our lead MOR/DOR agonist with a C-terminal glucoserine to improve bioavailability. The resulting ligand displayed high efficacy and potency at both MOR and DOR and no efficacy at KOR.

Pharmacological Profiles of Oligomerized μ-Opioid Receptors

Opioids are widely prescribed pain relievers with multiple side effects and potential complications. They produce analgesia via G-protein-protein coupled receptors: μ-, δ-, κ-opioid and opioid receptor-like 1 receptors. Bivalent ligands targeted to the oligomerized opioid receptors might be the key to developing analgesics without undesired side effects and obtaining effective treatment for opioid addicts. In this review we will update the biological effects of μ-opioids on homo- or hetero-oligomerized μ-opioid receptor and discuss potential mechanisms through which bivalent ligands exert beneficial effects, including adenylate cyclase regulation and receptor-mediated signaling pathways.

Sensitivity to μ-opioid receptor-mediated anti-nociception is determined by cross-regulation between μ- and δ-opioid receptors at supraspinal level

BACKGROUND AND PURPOSE

The perception of pain and its inhibition varies considerably between individuals, and this variability is still unexplained. The aim of the present study is to determine whether functional interactions between opioid receptors are involved in the inter-individual variability in the sensitivity to μ-opioid receptor agonists.

EXPERIMENTAL APPROACH

Anti-nociceptive tests, radioligand binding, stimulation of [(35) S]GTP-γ-S binding, inhibition of cAMP production and co-immunoprecipitation experiments were performed in two strains of rat (Sprague-Dawley bred at our university - SDU - and Wistar) that differ in their sensitivity to opioids.

KEY RESULTS

The increased anti-nociceptive potency of µ-opioid receptor agonists in SDU rats was reversed by the δ-opioid receptor antagonist, naltrindole. Inhibition of the binding of [(3) H] naltrindole by µ-opioid receptor agonists was different in brain membranes from SDU and Wistar rats. Differences were also evident in the effect of δ-opioid receptor ligands on the binding of [(35) S]GTP-γ-S stimulated by µ-opioid receptors agonists. No strain-related differences were detected in spinal cord membranes. The potency of morphine to inhibit cAMP production in brain membranes varied between the strains, in the presence of deltorphin II and naltrindole. Co-immunoprecipitation experiments demonstrated that δ-opioid receptors were associated with μ-opioid receptors to a higher extent in brain synaptosomal fractions from SDU than in those from Wistar rats.

CONCLUSIONS AND IMPLICATIONS

There was increased supraspinal cross-talk between μ and δ-opioid receptors in SDU, as compared with Wistar rats. This was related to an enhanced sensitivity to anti-nociception induced by µ-opioid receptor agonists.

Effects of the delta-opioid agonist SNC80 on the abuse liability of methadone in rhesus monkeys: a behavioral economic analysis

RATIONALE

Delta-opioid agonists enhance the antinociceptive efficacy of methadone and other mu-opioid agonists. However, relatively little is known about the degree to which delta agonists might enhance the abuse-related effects of mu agonists.

OBJECTIVE

This study used a behavioral economic approach to examine effects of the delta agonist SNC80 [(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide] on the reinforcing effects of methadone in a drug self-administration assay. Interactions between SNC80 and cocaine were also examined for comparison.

METHODS

Rhesus monkeys (n = 4), surgically implanted with indwelling intravenous catheters, were tested in two phases. In phase 1, drug self-administration dose-effect curves for methadone (0.0032-0.1 mg/kg/injection (inj)) and cocaine (0.0032-0.32 mg/kg/inj) alone were determined under a fixed-ratio 10 (FR 10) schedule of reinforcement. In phase 2, FR values were increased every 3 days (FR 1-FR 1800) during availability of methadone alone (0.032 mg/kg/inj) and in combination with varying proportions of SNC80 (0.1:1, 0.3:1, and 0.9:1 SNC80/methadone) or of cocaine alone (0.032 mg/kg/inj) and in combination with varying proportions of SNC80 (0.33:1, 1:1, and 3:1 SNC80/cocaine). Demand curves related drug intake to FR price, and measures of reinforcement were derived.

RESULTS

Methadone and cocaine alone each functioned as a reinforcer. SNC80 did not alter measures of reinforcement for either methadone or cocaine.

CONCLUSIONS

SNC80 at proportions previously shown to enhance methadone-induced antinociception did not enhance the abuse-related effects of methadone. These results support the proposition that delta agonists may selectively enhance mu agonist analgesic effects without enhancing mu agonist abuse liability.

Selective enhancement of fentanyl-induced antinociception by the delta agonist SNC162 but not by ketamine in rhesus monkeys: Further evidence supportive of delta agonists as candidate adjuncts to mu opioid analgesics

Mu-opioid receptor agonists such as fentanyl are effective analgesics, but their clinical use is limited by untoward effects. Adjunct medications may improve the effectiveness and/or safety of opioid analgesics. This study compared interactions between fentanyl and either the noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist ketamine or the delta-opioid receptor agonist SNC162 [(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-phenyl)methyl]-N,N-diethylbenzamide] in two behavioral assays in rhesus monkeys. An assay of thermal nociception evaluated tail-withdrawal latencies from water heated to 50 and 54°C. An assay of schedule-controlled responding evaluated response rates maintained under a fixed-ratio 30 schedule of food presentation. Effects of each drug alone and of three mixtures of ketamine+fentanyl (22:1, 65:1, 195:1 ketamine/fentanyl) or SNC162+fentanyl (59:1, 176:1, 528:1 SNC162/fentanyl) were evaluated in each assay. All drugs and mixtures dose-dependently decreased rates of food-maintained responding, and drug proportions in the mixtures were based on relative potencies in this assay. Ketamine and SNC162 were inactive in the assay of thermal antinociception, but fentanyl and all mixtures produced dose-dependent antinociception. Drug interactions were evaluated using dose-addition and dose-ratio analysis. Dose-addition analysis revealed that interactions for all ketamine/fentanyl mixtures were additive in both assays. SNC162/fentanyl interactions were usually additive, but one mixture (176:1) produced synergistic antinociception at 50°C. Dose-ratio analysis indicated that ketamine failed to improve the relative potency of fentanyl to produce antinociception vs. rate suppression, whereas two SNC162/fentanyl mixtures (59:1 and 176:1) increased the relative potency of fentanyl to produce antinociception. These results suggest that delta agonists may produce more selective enhancement than ketamine of mu agonist-induced antinociception.

Reversible suppression of food reward behavior by chronic mu-opioid receptor antagonism in the nucleus accumbens

Overindulgence in easily available energy-dense palatable foods is thought to be an important factor in the current obesity epidemic but the underlying neural mechanisms are not well understood. Here we demonstrate that mu-opioid receptor signaling in the nucleus accumbens may be important. Protracted suppression of endogenous mu-opioid receptor signaling focused on the nucleus accumbens shell for several days by means of microinjected beta-funaltrexamine (BFNA) diminished both "liking" of sucrose, as indicated by fewer positive hedonic orofacial responses, and the incentive reinforcement value ("wanting") of a food reward, as indicated by lower completion speed and increased time being distracted in the incentive runway. BFNA-treatment also decreased responding to sucrose and corn oil in the brief access lick paradigm, a test measuring a combination of mainly taste-guided "liking" and low-effort "wanting", as well as 4 h intake of sucrose solution. These effects were not due to nonspecific permanent neuronal changes, as they were fully reversible. We conclude that endogenous mu-opioid signaling in the nucleus accumbens is necessary for the full display of palatable food-induced hyperphagia through mechanisms including hedonic, motivational, and reinforcement processes. Development of obesity could be the result of predisposing innate differences in these mechanisms or overstimulation of these mechanisms by external factors.

Inhibition of the development of morphine tolerance by a potent dual mu-delta-opioid antagonist, H-Dmt-Tic-Lys-NH-CH2-Ph

Three analogues of the dual mu-/delta-antagonist, H-Dmt-Tic-R-NH-CH2-Ph (R = 1, Lys-Z; 2, Lys-Ac; 3, Lys) were examined in vivo: 1 and 2 exhibited weak bioactivity, while 3 injected intracerebroventricularly was a potent dual antagonist for morphine- and deltorphin C-induced antinociception comparable to naltrindole (delta-antagonist), but 93% as effective as naloxone (nonspecific opioid receptor antagonist) and 4% as active as CTOP, a mu antagonist. Subcutaneous or oral administration of 3 antagonized morphine-induced antinociception indicating passage across epithelial and blood-brain barriers. Mice pretreated with 3 before morphine did not develop morphine tolerance indicative of a potential clinical role to inhibit development of drug tolerance.

Role of delta opioid efficacy as a determinant of mu/delta opioid interactions in rhesus monkeys

Delta opioid agonists can selectively enhance the antinociceptive effects of mu opioid agonists without enhancing some other, potentially undesirable mu agonist effects. However, the degree of delta receptor efficacy required to produce this profile of interactions is unknown. To address this issue, the present study examined interactions produced by the mu agonist fentanyl and the intermediate-efficacy delta opioid MSF61 in rhesus monkeys. For comparison, interactions were also examined between fentanyl and the relatively high-efficacy delta agonist SNC243A and the delta antagonist naltrindole, which has negligible efficacy at delta receptors. Two different behavioral procedures were used: (a) a warm-water tail-withdrawal assay of thermal nociception, and (b) an assay of schedule-controlled responding for food reinforcement. Drug interactions within each procedure were evaluated using dose-addition analysis to compare experimental results with expected additivity. Drug interactions across procedures were evaluated using dose-ratio analysis to assess relative potencies to produce antinociception vs. response-rate suppression. As expected, dose-addition analysis found that fentanyl/SNC243A interactions were superadditive in the assay of antinociception but additive in the assay of schedule-controlled responding. Conversely, fentanyl/MSF61 interactions were generally additive in both procedures, and fentanyl/naltrindole interactions were additive or subadditive in both procedures. Dose-ratio analysis found that fentanyl alone produced antinociception and rate suppression with similar potencies. Some fentanyl/SNC243A mixtures produced antinociception with up to 4-fold greater potency than rate-suppression. However, fentanyl/MSF61 and fentanyl/naltrindole mixtures produced antinociception with lower potency than rate suppression. These results suggest that relatively high delta receptor efficacy is required for mu/delta antinociceptive synergy.

Evidence for a mu-delta opioid receptor complex in CHO cells co-expressing mu and delta opioid peptide receptors

Based on non-competitive binding interactions we suggested that mu and delta receptors associate as a mu/delta receptor complex in rat brain. We hypothesized that the same non-competitive binding interactions observed in rat brain will be seen in CHO cells that co-express mu and delta receptors, but not in cells that express just mu or delta receptors. We used CHO cells expressing the cloned human mu receptor, cloned human delta receptor, or cloned mouse delta/human mu ("dimer cell"). Cell membranes were prepared from intact cells pretreated with 100nM SUPERFIT. [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding assays followed published procedures. SUPERFIT, a delta-selective irreversible ligand, decreased [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to delta receptors by approximately 75% and to mu receptors by approximately 50% in dimer cells. SUPERFIT treatment did not decrease [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to mu cells. The IC(50) values observed in SUPERFIT-treated dimer cells were: [d-Pen(2),d-Pen(5)]enkephalin (1820nM) and morphine (171nM). Saturation binding experiments with SUPERFIT-treated dimer cells showed that [d-Pen(2),d-Pen(5)]enkephalin (5000nM) was a competitive inhibitor. In contrast, morphine (1000nM) lowered the B(max) from 1944fmol/mg to 1276fmol/mg protein (35% decrease). Both [d-Pen(2),d-Pen(5)]enkephalin and morphine competitively inhibited [(3)H][d-Ala(2),d-Leu(5)]enkephalin binding to SUPERFIT-treated mu cells. The results indicate that the mu-delta opioid receptor complex defined on the basis of non-competitive binding interactions in rat brain over 20 years ago likely occurs as a consequence of the formation of mu-delta heterodimers. SUPERFIT-treated dimer cells may provide a useful model to study the properties of mu-delta heterodimers.

In vivo activation of a mutant mu-opioid receptor by naltrexone produces a potent analgesic effect but no tolerance: role of mu-receptor activation and delta-receptor blockade in morphine tolerance

Opioid analgesics are the standard therapeutic agents for the treatment of pain, but their prolonged use is limited because of the development of tolerance and dependence. Recently, we reported the development of a mu-opioid receptor knock-in (KI) mouse in which the mu-opioid receptor was replaced by a mutant receptor (S196A) using a homologous recombination gene-targeting strategy. In these animals, the opioid antagonist naltrexone elicited antinociceptive effects similar to those of partial agonists acting in wild-type (WT) mice; however, development of tolerance and physical dependence were greatly reduced. In this study, we test the hypothesis that the failure of naltrexone to produce tolerance in these KI mice is attributable to its simultaneous inhibition of delta-opioid receptors and activation of mu-opioid receptors. Simultaneous implantation of a morphine pellet and continuous infusion of the delta-opioid receptor antagonist naltrindole prevented tolerance development to morphine in both WT and KI animals. Moreover, administration of SNC-80 [(+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide], a delta agonist, in the naltrexone-pelleted KI animals resulted in a dose-dependent induction in tolerance development to both morphine- and naltrexone-induced analgesia. We conclude that although simultaneous activation of both mu- and delta-opioid receptors results in tolerance development, mu-opioid receptor activation in conjunction with delta-opioid receptor blockade significantly attenuates the development of tolerance.

Opioid receptor interactions: local and nonlocal, symmetric and asymmetric, physical and functional

The pharmacological effects of opioid drugs and endogenous opioid peptides are mediated by several kinds of receptors, the major ones being mu, delta and kappa. Though classically it has been thought that a particular effect mediated by a drug or other ligand results from its interaction with a single type of receptor, accumulating evidence demonstrates that interactions between receptors play a major role in opioid actions. These interactions may be either local, involving receptors within the same tissue, or nonlocal, between receptors located in different tissues. Nonlocal interactions always involve intercellular mechanisms, whereas local interactions may involve either intercellular or intracellular interactions, the latter including physical association of receptors. Both local and nonlocal interactions, moreover, may be either symmetric, with ligand interaction at one receptor affecting interaction at the other, or asymmetric; and either potentiating or inhibitory. In this article we discuss major examples of these kinds of interactions, and their role in the acute and chronic effects of opioids. Knowledge of these interactions may have important implications for the design of opioids with more specific actions, and for eliminating the addictive liability of these drugs.

Regulation of delta-opioid receptor trafficking via mu-opioid receptor stimulation: evidence from mu-opioid receptor knock-out mice

We recently demonstrated that prolonged treatment with morphine increases the antinociceptive potency of the delta-opioid receptor (deltaOR) agonist deltorphin and promotes cell surface targeting of deltaORs in neurons of the dorsal horn of the rat spinal cord (Cahill et al., 2001b). In the present study we examined whether these effects were mediated selectively via muOR. Using the same intermittent treatment regimen as for morphine, we found that methadone and etorphine, but not fentanyl, enhanced [D-Ala2]-deltorphin-mediated antinociception. However, continuous delivery of fentanyl for 48 hr resulted in augmented deltaOR-mediated antinociception when compared with saline-infused animals. Time course studies confirmed that a 48 hr treatment with morphine was necessary for the establishment of enhanced deltaOR-mediated antinociception. The observed increases in deltaOR agonist potency and deltaOR plasma membrane density were reversed fully 48 hr after discontinuation of morphine injections. Wild-type C57BL/6 mice pretreated with morphine for 48 hr similarly displayed enhanced deltaOR-mediated antinociception in a tonic pain paradigm. Accordingly, the percentage of plasma membrane-associated deltaOR in the dorsal horn of the spinal cord, as assessed by immunogold electron microscopy, increased from 6.6% in naive to 12.4% in morphine-treated mice. In contrast, morphine treatment of muOR gene knock-out (KO) mice did not produce any change in deltaOR plasma membrane density. These results demonstrate that selective activation of muOR is critical for morphine-induced targeting of deltaOR to neuronal membranes, but not for basal targeting of this receptor to the cell surface.

3-(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-alkyl-N-arylbenzamides: potent, non-peptidic agonists of both the micro and delta opioid receptors

Opioid analgesics with both micro and delta opioid receptor activation represent a new approach to the treatment of severe pain with an improved safety profile. Compounds with this profile may exhibit strong analgesic properties due to micro agonism, with a reduced side effect profile resulting from delta agonism. Replacing the p-diethylamide of the known potent delta opioid receptor selective agonist BW373U86 with a m-diethylamide resulted in a compound with agonist activity at both the micro and delta opioid receptors. Modifying the amide to an N-methyl-N-phenylamide increased agonist potency at both receptors. A series of 3-(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-alkyl-N-arylbenzamides have been made to explore the structure-activity relationship (SAR) around the N-methyl-N-phenylamide. Several potent agonists of both the micro and delta opioid receptors have been identified, including (+)-3-((alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N-(4-fluorophenyl)-N-methylbenzamide (23), which has EC50 values of 0.67 and 1.1 nM at the micro (guinea pig ileum assay) and delta (mouse vas deferens assay) opioid receptors, respectively.

Delta-opioid receptor antagonists as a new concept for central acting antitussive drugs

Our recent findings indicated that mu- and kappa-opioid receptors enhance each other's antitussive processes. However, delta-opioid receptors played an inhibitory role in antitussive processes mediated by the mu- and kappa-opioid receptors. We also concluded that delta(1)-opioid receptors may play an inhibitory role, whereas delta(2)-opioid receptors may play a synergistic role, in antitussive processes mediated by mu-opioid receptors. Furthermore, we clearly demonstrated that delta-opioid receptor antagonists, such as naltrindole and 7-benzylidenenaltrexone, produced potent antitussive effects. These delta-opioid receptor-mediated antitussive effects may be mediated by the antagonism of delta(1)-, but not delta(2)-opioid receptors. In this review, we study the possibility of the delta-opioid receptor antagonist as a new concept for central acting antitussive drugs.

Altered nucleus accumbens circuitry mediates pain-induced antinociception in morphine-tolerant rats

We investigated the effect of chronic administration of morphine on noxious stimulus-induced antinociception (NSIA) produced by intraplantar capsaicin injection. In the untreated (naive) rat, we previously found that NSIA depends on activation of dopamine, nicotinic acetylcholine, and mu- and delta-opioid receptors in nucleus accumbens. Rats chronically implanted with subcutaneous morphine pellets demonstrated tolerance to the antinociceptive effects of acute systemic morphine administration but did not show cross-tolerance to NSIA. Morphine pretreatment, however, significantly reduced NSIA dependence on intra-accumbens opioid receptors but not on dopamine or nicotinic acetylcholine receptors. As observed in naive rats, intra-accumbens microinjection of either the dopamine receptor antagonist flupentixol or the nicotinic receptor antagonist mecamylamine blocked NSIA in rats tolerant to the antinociceptive effects of morphine, but, in contrast to naive rats, intra-accumbens microinjection of either the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7 amide or the delta-receptor antagonist naltrindole failed to block NSIA. These findings suggest that although NSIA is dependent on nucleus accumbens opioid receptors in the naive state, this dependence disappears in rats tolerant to the antinociceptive effects of morphine, which may account for the lack of NSIA cross-tolerance. In separate experiments, intra-accumbens extracellular dopamine levels were measured using microdialysis. Dopamine levels increased after either capsaicin or systemic morphine administration in naive rats but only after capsaicin administration in morphine pretreated rats. Thus, intra-accumbens dopamine release paralleled antinociceptive responses in naive and morphine pretreated rats.

Functional effects of systemically administered agonists and antagonists of mu, delta, and kappa opioid receptor subtypes on body temperature in mice

We have investigated the roles of peripheral and central mu, delta, and kappa opioid receptors and their subtypes in opioid-induced hypothermia in mice. Measuring rectal temperature after i.p. injection, opioid agonists [morphine, fentanyl, SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)3-methoybenzyl]-N,N-diethylbenzamide), U50,488H ((trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), and loperamide)] were tested alone or with opioid antagonists [naloxone, beta-funaltrexamine, naloxonazine, naltrindole, 7-benzylidenenaltrexone (BNTX), naltriben, nor-binaltorphimine, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA), and methyl-naltrexone] given 15 min after the agonist. All agonists produced dose-related hypothermia, although at low doses, morphine and U50,488H produced hyperthermia. The effects of morphine and fentanyl were antagonized by naloxone and by the mu(1) antagonist naloxonazine. The delta(2) antagonist naltriben potentiated the hypothermic effect of mu agonists. SNC80-induced hypothermia was blocked by the delta antagonist naltrindole but not by the delta(1) antagonist BNTX. Depending on the dose, the delta(2) antagonist naltriben produced either a potentiation or an attenuation of the effect of SNC80. U50,488H-induced hypothermia was antagonized by the kappa antagonist nor-binaltorphimine but not by acute treatment with the irreversible kappa antagonist DIPPA. The peripherally acting opioid loperamide produced hypothermia that could be blocked by several mu-, delta-, or kappa-selective antagonists as well as the peripherally acting antagonist methyl-naltrexone. Methyl-naltrexone produced a weak potentiation of morphine-, fentanyl-, and U50,488H-induced hypothermia, whereas a significant attenuation of SNC80-induced hypothermia was observed. In conclusion, at high doses, morphine- and fentanyl-induced hypothermia may involve composite action on mu, kappa, and possibly delta opioid receptors after initial activation. In the mediation of delta opioid-induced hypothermia, no clear selectivity between the delta(1) and delta(2) subtypes was defined. The studies provide new evidence that maintenance of the initial effects of agonist/receptor activation vary with the agonist and the receptor. The existence of both central and peripheral components of opioid-induced hypothermia is also emphasized.

mu/delta Cooperativity and opposing kappa-opioid effects in nucleus accumbens-mediated antinociception in the rat

We previously demonstrated that noxious peripheral stimulation (e.g. subdermal capsaicin injection in the hind paw) produces antinociception that is mediated by opioid receptors in nucleus accumbens. The current study used the trigeminal jaw-opening nociceptive reflex responses in the rat to assess the role of intra-accumbens mu-, delta- and kappa-opioid receptors in the antinociceptive effect of noxious stimulation and intra-accumbens opioid agonism. Whilst intra-accumbens injection of either the mu-receptor-selective antagonist Cys2,Tyr3,Orn5,Pen7amide (CTOP) or the delta-receptor-selective antagonist naltrindole blocked capsaicin-induced antinociception, neither the selective mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO; 150 or 300 ng) nor the selective delta-agonist D-Pen2,5-enkephalin (DPDPE; 150 or 300 ng) alone induced antinociception. Simultaneous injection of DAMGO and DPDPE (150 ng each), however, produced significant antinociception. Capsaicin-induced antinociception was not blocked by the selective kappa-receptor antagonist nor-binaltorphimine, but was blocked by the kappa-agonist U69,593. U69,593 also antagonized the antinociceptive effect of the DAMGO/DPDPE combination. Thus, in nucleus accumbens, mu- and delta- but not kappa-opioid receptors contributed to capsaicin-induced antinociception; selective activation of individual receptor subtypes was insufficient, but coactivation of mu- and delta-opioid receptors induced antinociception, and kappa-receptors appeared to play an antianalgesic role in nucleus accumbens.

Mu and Delta opioid receptors activate the same G proteins in human neuroblastoma SH-SY5Y cells

1. There is evidence for interactions between mu and delta opioid systems both in vitro and in vivo. This work examines the hypothesis that interaction between these two receptors can occur intracellularly at the level of G protein in human neuroblastoma SH-SY5Y cells. 2. The [(35)S]GTP gamma S binding assay was used to measure G protein activation following agonist occupation of opioid receptors. The agonists DAMGO (EC(50), 45 nM) and SNC80 (EC(50), 32 nM) were found to be completely selective for stimulation of [(35)S]-GTP gamma S binding through mu and delta opioid receptors respectively. Maximal stimulation of [(35)S]-GTP gamma S binding produced by SNC80 was 57% of that seen with DAMGO. When combined with a maximally effective concentration of DAMGO, SNC80 caused no additional [(35)S]-GTP gamma S binding. This effect was also seen when measured at the level of adenylyl cyclase. 3. Receptor activation increased the dissociation of pre-bound [(35)S]-GTP gamma S. In addition, the delta agonist SNC80 promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using the mu agonist DAMGO. Conversely, DAMGO promoted the dissociation of [(35)S]-GTP gamma S from G proteins initially labelled using SNC80. 4. Tolerance to DAMGO and SNC80 in membranes from cells exposed to agonist for 18 h was homologous and there was no evidence for alteration in G protein activity. 5. The findings support the hypothesis that mu- and delta-opioid receptors share a common G protein pool, possibly through a close organization of the two receptors and G protein at the plasma membrane.

Spinal delta-opioid receptors mediate suppression of systemic SNC80 on excitability of the flexor reflex in normal and inflamed rat

Due to low central nervous system (CNS) bioavailability of delta-opioid peptides, little is known about the effect of systemic administration of delta-opioid receptor ligands. The present study examined the effect of non-peptidergic delta-opioid receptor agonists, (+)-4-[(alphaR)-alpha-((2R,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) and (-)dibenzoyl-L-tartaric acid salt (SNC86), on the activity of alpha-motoneurons in decerebrate-spinal rats. The flexor reflex was facilitated by C-afferent conditioning inputs, shown by a decrease in mechanical threshold and increase in touch- and pinch-evoked responses. Systemic administration of SNC80 (10 micromol/kg) prevented and reversed the neuronal hyperactivity. We further examined the effect of this agonist on the hypersensitivity of the flexor reflex induced by intraplantar injection of Freund's adjuvant. SNC80 dose-dependently (1, 3, 5 and 10 micromol/kg) increased the mechanical threshold and decreased touch-, pinch- and Abeta-afferent inputs-evoked responses. Similar effects were seen with SNC86 (5 micromol/kg). Pretreatment with either naloxone (20 micromol/kg, i.p.) or (Cyclopropylmethyl)-6,7-dehydro-4,5alpha-epoxy-14beta-ethoxy-5beta-methylindolo [2',3':6',7']morphinan-3-ol hydrochloride (SH378; 5 micromol/kg, intraarterially (i.a.)), a novel selective delta-opioid receptor antagonist, completely abolished the anti-hypersensitivity effect of SNC80. The effect of SNC80 remained following intrathecal administration of mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP; 1.5 nmol). These results indicate that systemic injection of SNC80 exerted antihypersensitivity in models of both acute and tonic nociception and these effects are mediated mainly through a spinal delta-opioid mechanism.

Delta opioid receptor mediated actions in the rostral ventromedial medulla on tail flick latency and nociceptive modulatory neurons

The rostral ventromedial medulla (RVM) is critical for the modulation of dorsal horn nociceptive transmission. Three classes of RVM neurons (ON, OFF, and NEUTRAL) have been described that have distinct responses to noxious stimuli and mu opioid receptor (MOR) agonists. The present study in barbiturate anesthetized rats investigated the effects of the delta 2 opioid receptor (DOR2) agonist, [D-Ala2]deltorphin II (DELT), microinfused into the RVM on the tail flick reflex and activity of RVM neurons. Tail flick latencies increased dose-dependently after administration of DELT (0.6 nmol and 1.2 nmol). Furthermore, DELT inhibited the tail flick related increase in ON cell activity and shortened the tail flick related pause in OFF cell activity. The activity of NEUTRAL cells was not affected. The antinociceptive effects and corresponding changes in ON and OFF cell activity produced by DELT were antagonized by the DOR2 antagonist, naltriben methanesulfonate, administered at the same site. These DOR2 mediated effects on noxious stimulation-evoked changes in RVM neuronal activity are similar to those reported for MOR agonists and suggest that both DOR2 and MOR produce analgesia through activation of OFF cells.

The effects of RB101, a mixed inhibitor of enkephalin-catabolizing enzymes, on carrageenin-induced spinal c-Fos expression are completely blocked by beta-funaltrexamine, a selective mu-opioid receptor antagonist

We have demonstrated that pre-administered RB101 (40 mg/kg, i.v.), a mixed inhibitor of enkephalin-catabolizing enzymes, decreased spinal c-Fos expression induced 1 h and 30 min after intraplantar (i.pl.) carrageenin (41% reduction, p<0.01). These effects were completely blocked by pre-administered beta-funaltrexamine (10 mg/kg, i.v., 24 h prior to stimulation), a selective long-lasting mu-opioid receptor antagonist. In conclusion, these results clearly demonstrate that the effects of endogenous enkephalins on noxiously evoked spinal c-Fos expression are essentially mediated via mu-opioid receptors.

Effects and interactions of opioids on plasma exudation induced by cigarette smoke in guinea pig bronchi

The effects of opioids on cigarette smoke-induced plasma exudation were investigated in vivo in the main bronchi of anesthetized guinea pigs, with Evans blue dye as a plasma marker. Acute inhalation of cigarette smoke increased plasma exudation by 216% above air control values. Morphine, 0.1-10 mg/kg but not 30 mg/kg, inhibited the exudation but had no significant effect on substance P-induced exudation. Both 10 and 30 mg/kg of morphine increased exudation in air control animals, an effect inhibited by antihistamines but not by a tachykinin neurokinin type 1-receptor antagonist. Naloxone inhibited all morphine responses. Cigarette smoke-induced plasma exudation was inhibited by a mu-opioid-receptor agonist (DAMGO) but not by agonists at delta (DPDPE)- or kappa (U-50488H)-receptors. None of these agonists affected exudation in air control animals. DPDPE prevented the inhibition by DAMGO of cigarette smoke-induced plasma exudation, and the combination of DAMGO and DPDPE increased exudation in air control animals. Prevention of inhibition and the combination-induced increase were inhibited by antihistamines or the mast cell-stabilizing drug sodium cromoglycate. U-50488H did not alter the response to either DAMGO or DPDPE. We conclude that, in guinea pig main bronchi in vivo, mu-opioid-receptor agonists inhibit cigarette smoke-induced plasma exudation via a prejunctional mechanism. Plasma exudation induced by mu- and delta-receptor interactions is due to endogenous histamine release from mast cells.

Analgesic activity of Nepeta italica L

A screening study was performed on/by essential oils of Nepeta viscida Boiss and Nepeta italica L. using tail-flick and tail immersion (52.5 degrees C) methods. N. italica samples were collected from three different localities of Turkey. Surprisingly, only one of the essential oils showed significant activity, which was blocked by naloxone, indicating the involvement of opioid receptors. This was seen only with the mechanical but not the thermal algesic stimulus, suggesting a specific activity on opioid receptors, excluding mu receptors. The same, active essential oil also exhibited a non-competitive inhibition of acetylcholine contractions of isolated rat ileum but it was inactive on the isolated rat aorta. Furthermore, a correlation between the analgesic activity and the amount of 1,8-cineole was noticed.

Effects of beta-funaltrexamine on dose-effect curves for heroin self-administration in rats: comparison with alteration of [3H]DAMGO binding to rat brain sections

These studies were undertaken to determine the effects of mu-opioid receptor depletion through irreversible alkylation on the dose-effect curve for heroin self-administration. Heroin maintained responding in rats with an inverted U-shaped dose-effect curve and administration of 10 nmol of beta-funaltrexamine i.c.v. (beta-FNA) significantly increased the ED50 on the ascending limb from 1.9 to 5.3 micrograms/infusion, and from 24.3 to 211.8 micrograms/infusion on the descending limb. Administration of saline i.c.v. produced no effect on heroin self-administration. Administration of 40 nmol of beta-FNA increased the ED50S from 5.1 to 33.9 and from 14.4 to 502.8 micrograms/infusion on the ascending and descending portions of heroin's dose-effect curve, respectively. beta-FNA (40 nmol, i.c.v.) had no effect on cocaine self-administration. [3H]DAMGO binding density was decreased in the caudate and nucleus accumbens by 29 or 54% 24 h after administration of 10 or 40 nmol of beta-FNA i.c.v., respectively. The effects of beta-FNA on heroin self-administration were completely overcome by increasing the dose of heroin however, as the shape and slope of the self-administration dose-effect curve was not different when higher doses of heroin were made available for self-administration compared to control data or saline administration. Therefore, there appear to be spare mu-opioid receptors for heroin for the production of its reinforcing effects in rats. Furthermore, the self-administration dose-effect curves returned to control values prior to the return of [3H]DAMGO binding, further suggesting that the full complement of mu-opioid receptors is not necessary for heroin to produce its reinforcing effects. These findings support the existence of spare mu-opioid receptors for heroin in maintaining self-administration in rats.

Nepetalactone: a new opioid analgesic from Nepeta caesarea Boiss

The essential oils of Nepeta species including Nepeta phyllochlamys P. H. Davis, N. nuda L. ssp. nuda, and N. caesarea Boiss. have been screened by use of the tail-flick and tail immersion (52.5 degrees C) methods. Of the species studied, only N. caesarea showed significant analgesic activity, besides marked sedation, which was also blocked by naloxone, indicating involvement of opioid receptors. Moreover, it was only active on mechanical, not thermal, algesic response which suggests specificity for specific opioid receptor subtypes, excluding mu-opioid receptors. Because 4a alpha,7alpha,7a alpha-nepetalactone is the main component of the essential oil of N. caesarea, and is present at very high levels (92-95%), it is concluded that 4a alpha,7alpha,7a alpha-nepetalactone is the active principle and has a specific opioid receptor subtype agonistic activity.

Dual ultrastructural immunocytochemical labeling of mu and delta opioid receptors in the superficial layers of the rat cervical spinal cord

The delta opioid receptor (DOR) and mu opioid receptor (MOR) are abundantly distributed in the dorsal horn of the spinal cord. Simultaneous activation of each receptor by selective opiate agonists has been shown to result in synergistic analgesic effects. To determine the cellular basis for these functional associations, we examined the electron microscopic immunocytochemical localization of DOR and MOR in single sections through the superficial layers of the dorsal horn in the adult rat spinal cord (C2-C4). From a total of 270 DOR-labeled profiles, 49% were soma and dendrites, 46% were axon terminals and small unmyelinated axons, and 5% were glial processes. 6% of the DOR-labeled soma and dendrites, and < 1% of the glial processes also showed MOR-like immunoreactivity (MOR-LI). Of 339 MOR-labeled profiles, 87% were axon terminals and small unmyelinated axons, 12% were soma and dendrites, and 2% were glial processes. 21% of the MOR-labeled soma and dendrites, but none of the axon terminals also contain DOR-LI. The subcellular distributions of MOR and DOR were distinct in axon terminals. In axon terminals, both DOR-LI and MOR-LI were detected along the plasmalemma, but only DOR-LI was associated with large dense core vesicles. DOR-labeled terminals formed synapses with dendrites containing MOR and conversely, MOR-labeled terminals formed synapses with DOR-labeled dendrites. These results suggest that the synergistic actions of selective MOR- and DOR-agonists may be attributed to dual modulation of the same or synaptically linked neurons in the superficial layers of the spinal cord.

Quantitative autoradiographic mapping of mu-, delta- and kappa-opioid receptors in knockout mice lacking the mu-opioid receptor gene

Mice lacking the mu-opioid receptor (MOR) gene have been successfully developed by homologous recombination and these animals show complete loss of analgesic responses to morphine as well as loss of place-preference activity and physical dependence on this opioid. We report here quantitative autoradiographic mapping of opioid receptor subtypes in the brains of wild-type, heterozygous and homozygous mutant mice to demonstrate the deletion of the MOR gene, to investigate the possible existence of any mu-receptor subtypes derived from a different gene and to determine any modification in the expression of other opioid receptors. Mu-, delta-, kappa1- and total kappa-receptors, in adjacent coronal sections in fore- and midbrain and in sagittal sections, were labelled with [3H]DAMGO (D-Ala2-MePhe4-Gly-ol5 enkephalin), [3H]DELTI (D-Ala2 deltorphinI), [3H]CI-977 and [3H]bremazocine (in the presence of DAMGO and DPDPE) respectively. In heterozygous mice, deficient in one copy of the MOR gene, mu-receptors were detectable throughout the brain at about 50% compared to wild-type. In brains from mu-knockout mice there were no detectable mu-receptors in any brain regions and no evidence for mu-receptors derived from another gene. Delta-, kappa1- and total kappa-receptor binding was present in all brain regions in mutant mice where binding was detected in wild-type animals. There were no major quantitative differences in kappa- or delta-binding in mutant mice although there were some small regional decreases. The results indicate only subtle changes in delta- and kappa-receptors throughout the brains of animals deficient in mu-receptors.

Mechanisms of actions of opioids and non-steroidal anti-inflammatory drugs

Opioids and non-steroidal anti-inflammatory drugs (NSAIDs) are the commonest drugs used to treat pain. Opioids mimic the actions of endogenous opioid peptides by interacting with mu, delta or kappa opioid receptors. The opioid receptors are coupled to G1 proteins and the actions of the opioids are mainly inhibitory. They close N-type voltage-operated calcium channels and open calcium-dependent inwardly-rectifying potassium channels. This results in hyperpolarization and a reduction in neuronal excitability. They also decrease intracellular cAMP which modulates the release of nociceptive neurotransmitters (e.g. substance P). Inhibition of prostaglandin synthesis by cyclooxygenase is the principal mode of the analgesic and anti-inflammatory actions of NSAIDs. Cyclo-oxygenase is inhibited irreversibly by aspirin and reversibly by other NSAIDs. The widespread inhibition of cyclo-oxygenase is responsible for many of the adverse effects of these drugs. NSAIDs also reduce prostaglandin production within the CNS. This is the main action of paracetamol.

Opposite role of delta 1- and delta 2-opioid receptors activated by endogenous or exogenous opioid agonists on the endogenous cholecystokinin system: further evidence for delta-opioid receptor heterogeneity

Using the mouse caudate-putamen, where delta-opioid receptor subtypes have been shown to regulate adenylyl cyclase activity, we show in this study that endogenous enkephalins inhibit enzyme activity through activation of delta 1- and delta 2-opioid receptors. Thus, naltriben or 7-benzylidenenaltrexone as well as the delta-selective antagonist naltrindole (mixed delta 1 and delta 2 antagonist) antagonized inhibition of adenylyl cyclase activity induced by methionine- or leucine-enkephalin, while the micro-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) was without effect. Furthermore, we have previously shown that activation of delta-opioid receptors increases cholecystokinin release in the central nervous system, resulting in a potentiation of micro-opioid antinociceptive responses, and the respective role of delta 1- and delta 2-opioid receptors in this facilitatory effect has now been evaluated. Activation of delta 2-opioid receptors, either by endogenous enkephalins protected from catabolism by the complete enkephalin-degrading enzyme inhibitor N-((R,S)-2-benzyl-3((S)(2-amino-4-methyl-thio) butyldithio)-1-oxopropyl)-L-phenyl-alanine benzyl ester (RB 101), or by the delta 2-selective agonist Tyr-D-Ser(O-tert-butyl)-Gly-Phe-Leu-Thr(O-tert-butyl) (BUBU), potentiated micro-opioid antinociceptive responses in the hot-plate test in mice. This effect was antagonized by a selective cholecystokinin-A antagonist. Activation of delta 1-opioid receptors by endogenous opioid peptides decreased the micro-opioid responses. These results suggest that stimulation of delta 2-opioid receptors potentiates micro-opioid analgesia in the hot-plate test in mice through an increase in endogenous cholecystokinin release, while activation of delta 1-opioid receptors could decrease it. Thus, the pre-existing physiological balance between opioid and cholecystokinin systems seems to be modulated in opposite directions depending on whether delta 1- or delta 2-opioid receptors are selectively activated. This is the first demonstration that endogenous enkephalins, methionine- and leucine-enkephalin, are the natural ligands of delta-opioid receptor subtypes, and that delta 2-opioid receptor activation may facilitate the endogenous cholecystokinin-related modulation of micro-opioid analgesia, while the delta 1-opioid receptors may have an inhibitory role. These results could have important applications for the characterization of opioid delta 1 and delta 2 as subtypes or subsites and in pain alleviation.

Synergy between mu/delta-opioid receptors mediates adenosine release from spinal cord synaptosomes

Morphine releases adenosine from the spinal cord and this contributes to spinal antinociception. The present study examined possible interactions between mu- and subclasses of delta-opioid receptors in the release of adenosine. Nanomolar (10(-8), 10(-9) M) concentrations of morphine release adenosine from spinal cord synaptosomes under conditions of partial depolarization with elevated K+, and this component of release is mediated by activation of mu-opioid receptors. Subnanomolar (10(-10), 10(-11) M) concentrations of the mu-opioid receptor agonists morphine, [N-MePhe3,D-Pro4]morphiceptin, and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) have minimal effects on the release of adenosine from the spinal cord. However, [D-Pen2,D-Pen5]enkephalin (DPDPE), a delta 1-opioid receptor agonist, and [D-Ala2,Cys4]deltorphin, a delta 2-opioid receptor agonist, at doses which exhibit no intrinsic effects (10(-8) and 10(-7) M), shifted the dose-response curve for mu-opioid receptor-evoked adenosine release to the left in a dose-dependent manner. DPDPE was more potent than [D-Ala2,Cys4]deltorphin when combined with the highly selective mu-opioid receptor agonist [N-MePhe3,D-Pro4]morphiceptin, but these agents showed similar activity with the less selective agonists DAMGO and morphine. Simultaneous activation of mu- and delta-opioid receptors generates a synergistic release of adenosine from spinal cord synaptosomes. Although agonists for both delta 1- and delta 2-opioid receptor subtypes produce this response, the delta 1-opioid receptor agonist is more potent at eliciting this effect when the most selective mu-opioid receptor ligand is used.

Delta opioid receptors: reflexive, defensive and vocal affective responses in female rats

Ultrasonic vocalizations may be an expression of the affective pain response in laboratory animals. The present experiment compares the effects of morphine to the delta agonist, DPDPE (D-Pen2,D-Pen5 enkephalin) on a range of reflexive, behavioral and affective responses during an aggressive interaction. In experiment 1, naive female Long-Evans rats received morphine (0, 1, 3, 6, 10 micrograms ICV), or DPDPE (0, 30, 60, 100 micrograms ICV). In experiment 2, female rats were treated with naltrindole (1.0 mg/kg IP) 20 min before DPDPE (0, 60, 100 micrograms ICV). The following endpoints were measured: (1) latency to tail flick in response to heat stimuli; (2) high (33-65 kHz) and low (20-32 kHz) frequency ultrasonic and audible vocalizations; (3) defensive behavior; and (4) motoric activity. Following a brief exposure to attack, rats were threatened by the aggressor but protected from further attack by a large, wire mesh cage, thereby allowing for continued behavioral and vocal measurement without the risk of physical injury; video and audio recordings were made during the attack and then during a portion of the protected encounter (2 min). Morphine suppressed pain reactions varying in complexity from a spinal reflex, to an organized escape reaction, to an affective vocal response. The delta agonist, DPDPE, attenuated high frequency ultrasonic calling and tail flick responding. Defensive behaviors were also modulated by DPDPE at doses that had no effect on walking or rearing, indicating behavioral specificity. By contrast, doses of morphine that decreased defensive upright and escape also decreased motor activity. In female rats, morphine and DPDPE share a common profile of effects on a range of functional end-points, but DPDPE appears to modulate more selectively the reactions related to aversiveness without exerting sedative effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a highly selective nonpeptide delta opioid receptor agonist, TAN-67, on morphine-induced antinociception in mice

The effects of a potent and highly selective nonpeptide delta opioid receptor agonist, 2- methyl-4a alpha-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a alpha- octahydroquinolino [2,3,3,-g] isoquinoline (TAN-67), on morphine-induced antinociception were examined using the warm-plate (51 degrees C) method. When a peptide delta 1 opioid receptor agonist, [D-Pen2, Pen5]enkephalin (DPDPE), was co-administered with i.c.v. morphine, low-dose morphine-induced antinociception was significantly increased. In contrast, i.c.v. co-administration of a peptide delta 2 opioid receptor agonist, [D-Ala2]deltorphin II (DELT), with morphine did not affect the morphine-induced antinociception. When morphine and TAN-67 were co-administered i.c.v., low-dose morphine-induced antinociception was significantly increased. Moreover, when TAN-67 and morphine were co-administered s.c., the morphine dose-response curve shifted to the left and the ED50 value of morphine decreased. These effects DPDPE and TAN-67 were antagonized by the delta opioid receptor antagonist naltrindole (NTI) and the delta 1 opioid receptor antagonist 7-benzylidenenaltrexone (BNTX) not by the delta 2 opioid receptor antagonist naltriben (NTB). Moreover, the mu opioid receptor antagonist beta-FNA also antagonized the effects of DPDPE and TAN-67. These results suggest that the effect of TAN-67 may result from the activation of central delta 1 opioid receptors, since the effect of TAN-67 was antagonized by NTI and BNTX, but not NTB. Furthermore, since pretreatment with beta-FNA also antagonized the effects of both DPDPE and TAN-67, a beta-FNA-sensitive site, i.e. a mu-delta complex site, may play an important role in the modulation of morphine-induced antinociception.

The increase in morphine antinociceptive potency produced by carrageenan-induced hindpaw inflammation is blocked by naltrindole, a selective delta-opioid antagonist

Carrageenan-induced inflammation of the rat hindpaw has been used as a model for persistent pain of inflammatory origin. The induction of inflammation resulting from carrageenan injection in the rat hindpaw has been shown to elicit an increase in the antinociceptive potency of morphine, an effect postulated to be related to reduced levels of spinal cholecystokinin (CCK). Recent findings have related the anti-opioid effect of CCK to a decrease in activation of delta-opioid receptors. For this reason, we have examined the effects of the delta-opioid antagonist naltrindole (NTI) on the modulation of morphine antinociceptive potency resulting from carrageenan-induced inflammation. Rats with carrageenan-induced hindpaw inflammation received several doses of morphine in the absence or presence of NTI and were tested in the hot plate (HP) and tail flick (TF) tests. These results were compared to those of non-carrageenan injected rats. Morphine was significantly more potent in inflamed, than in control, rats in both tests. While NTI did not affect morphine antinociceptive potency in control rats in either test, this opioid delta antagonist blocked the increase in morphine potency resulting from carrageenan inflammation in nearly every case. The blockade of the enhancement of morphine potency was such that the effect of a given dose of morphine was similar in control rats and carrageenan-injected rats with NTI. We suggest that carrageenan-induced inflammation may alter endogenous enkephalin levels, perhaps by a decrease in CCK availability. The enhancement of morphine antinociceptive potency may result from the well-established synergism seen following the activation of opioid delta receptors by enkephalins.

Opioid effects on spinal [3H]5-hydroxytryptamine release are not related to their antinociceptive action

Several opioid compounds were evaluated for an ability to modulate the K(+)-stimulated release of [3H]serotonin ([3H]5-hydroxytryptamine, [3H]5-HT) from rat spinal cord synaptosomal and tissue slice preparations. Selective kappa-opioid receptor agonists depressed K(+)-stimulated release of the radiolabelled transmitter from both tissue preparations, an effect which was reversed by norbinaltorphimine. Conversely, the selective mu- and delta-opioid receptor agonists [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO) and [D-Pen2,D-Pen5]enkephalin (DPDPE), respectively, enhanced the K(+)-stimulated release of [3H]5-HT. This effect was only seen using the tissue slice preparation. When used at concentrations near its reported Kd for mu-opioid receptors, the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) blocked the action of DAMGO, but had no effect on the action of DPDPE. However, higher concentrations of CTOP, as well as all effective concentrations of selective delta-opioid receptor antagonists, blocked the action of both DAMGO and DPDPE. All agonist effects on spinal 5-HT release, regardless of the tissue preparation, were only seen at high (microM) concentrations. Moreover, effects of the opioid agonists were not consistent with the reported involvement of spinal 5-HT neurotransmission in the mediation of their antinociceptive action. Thus, the ability of opioids to modulate spinal 5-HT release appears to be of minimal physiological significance.

Naltrindole, an opioid delta antagonist, blocks the enhancement of morphine-antinociception induced by a CCKB antagonist in the rat

CCK has been shown to inhibit morphine antinociception, while antagonists of CCK receptors enhance morphine antinociceptive potency. These observations have led to the suggestion that CCK may function as an endogenous anti-opioid. Here, the involvement of the CCKB receptor in modulating the antinociceptive effects of morphine has been investigated by examination of the effects of a CCKB antagonist in the absence or presence of naltrindole, an opioid delta receptor antagonist. Intrathecal (i.th.) or subcutaneous (s.c.) L365,260 (a CCKB antagonist) did not produce any antinociceptive actions alone in either the rat tail-flick or hot-plate tests. L365,260 pretreatment enhanced the morphine antinociceptive response after either i.th. or s.c. administration. Naltrindole did not produce any antinociceptive effect alone and did not antagonize the antinociceptive actions of morphine after either i.th. or s.c. administration. However, naltrindole blocked the enhancement of morphine antinociception produced by L365,260 when evaluated by either route. These data suggest a tonic inhibition of enkephalin release by CCK via CCKB receptors. The subsequent enhancement of morphine antinociceptive potency may reflect the well-known modulation of morphine by enkephalins acting at opioid delta receptors.

Beta-funaltrexamine blockade of opioid-induced inhibition of somatostatin secretion from rat stomach

Opioid peptides are potent inhibitors of gastric somatostatin secretion. In the current investigation the effect of mu-opioid receptor blockade on responses to [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) was studied. Gastric inhibitory polypeptide (GIP; 1 nM) -stimulated secretion of immunoreactive somatostatin was almost completely inhibited by DAGO (1 microM). The mu-receptor antagonists, beta-funaltrexamine and naloxonazine, blocked the effect of DAGO. Pretreatment of rats with beta-funaltrexamine, 24 h prior to perfusion, reduced the percentage inhibition by DAGO from 88.6 +/- 5.2% to 50.7 +/- 9.3%. These studies support the involvement of mu-opioid inhibitory receptors in the regulation of gastric somatostatin secretion.

Interaction of [D-Pen2,D-Pen5]enkephalin and [D-Ala2,Glu4]deltorphin with delta-opioid receptor subtypes in vivo

The interaction of [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Ala2,Glu4]deltorphin with delta-opioid receptor subtypes was investigated. Pretreatment of mice with the delta 1-opioid receptor antagonist, [D-Ala2,Leu5,Cys6]enkephalin (DALCE), produced a virtually complete antagonism of the antinociceptive actions of DPDPE, but had no effect on those of [D-Ala2,Glu4]deltorphin. In DALCE pretreated mice (i.e., delta 1-opioid receptors blocked), DPDPE was able to significantly antagonize the antinociceptive effects of [D-Ala2,Glu4]deltorphin. Pretreatment of mice with the delta 2-opioid receptor antagonist, naltrindole-5'-isothiocyanate (5'-NTII) produced a virtually complete antagonism of the antinociceptive effects of [D-Ala2,Glu4]deltorphin, but had no effect on the antinociception produced by DPDPE. In 5'-NTII pretreated mice (i.e., delta 2-opioid receptors blocked), [D-Ala2,Glu4]deltorphin had no effect on the antinociception produced by DPDPE. These data suggest that [D-Ala2,Glu4]deltorphin is highly selective for the delta 2-opioid receptor in vivo, and that neither agonist nor antagonist actions can be demonstrated at delta 1-opioid receptors for this peptide. In contrast, under appropriate conditions, DPDPE can be shown to interact with both delta 1- and delta 2-opioid receptor subtypes; DPDPE may have limited efficacy (i.e., is a partial agonist) at the delta 2-opioid receptor.

Antitussive effects of mu- and kappa-agonists in diabetic rats

We evaluated the antitussive effect of morphine and U-50,488 in diabetic and non-diabetic rats. The antitussive potency of morphine (0.3 mg/kg, i.p.) in diabetic rats was significantly reduced as compared to the results in non-diabetic rats. The antitussive effect of U-50,488, a kappa-agonist, was also significantly lower in diabetic rats than in non-diabetic rats. When naltrindole (0.03 mg/kg, i.p.), a delta-antagonist, was administered 15 min before morphine or U-50,488, there was no difference between the antitussive potencies of these two opioid agonists in non-diabetic rats and in diabetic rats. Furthermore, naltrindole produces a reduction of the number of coughs in diabetic rats, but not in non-diabetic rats. It is possible that the enhancement of the antitussive potency of morphine and U-50,488 in naltrindole-treated diabetic rats is the result of the antitussive synergy produced by these opioid agonists and naltrindole. It seems likely, therefore, that delta-receptor-mediated endogenous inhibitory systems in mu- and kappa-receptor-mediated antitussive processes may be activated under diabetic conditions.

Dose-dependent antagonism of spinal opioid receptor agonists by naloxone and naltrindole: additional evidence for delta-opioid receptor subtypes in the rat

Intrathecally administered mu-opioid (morphine; DAMGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin)) and delta-opioid (DPDPE ([D-Pen2,D-Pen5] enkephalin); DADLE ([D-Ala2,D-Leu5]enkephalin)) receptor preferring agonists were systematically challenged with the competitive opiate antagonists naloxone or naltrindole in the rat. Naloxone produced a dose-dependent reduction in agonist effect with the intrathecal IC50 being similar for all agonists (2.1-5.4 micrograms). In contrast, the naltrindole antagonist profile was (IC50 in micrograms) DPDPE (4.0); morphine (23.5); DADLE (> 30) and DAMGO (> 30). Three points are emphasized: (1) antagonism of DPDPE and not DAMGO by naltrindole suggests two distinct opioid sites; (2) a similar potency for naloxone against these agonists suggests that the agonists may act upon spinal sites for which naloxone has comparable affinity or that they may act upon separate sites which are functionally coupled and that the action of naloxone on one or the other site is responsible for the antagonism; and (3) given the modest cross-tolerance between DADLE and mu agonists, the failure of naltrindole to antagonize DADLE suggests that in the rat this peptide acts through a delta site different from that acted upon by DPDPE.

Differential modulation of mu-opioid receptor-mediated antitussive activity by delta-opioid receptor agonists in mice

We examined the effect of [D-Ala2]deltorphin II, a selective delta 2-opioid receptor agonist, on the antitussive effect of [D-Ala2, MePhe4,Gly-ol5]enkephalin (DAMGO), a selective mu-opioid receptor agonist. [D-Ala2]deltorphin (3 nmol i.c.v.) had no significant effect on the number of coughs. However, upon i.c.v. pretreatment with [D-Ala2]deltorphin II (3 nmol) the antitussive activity of DAMGO (0.03 nmol) was significantly enhanced. The enhancement of the antitussive activity of DAMGO caused by [D-Ala2]deltorphin II was prevented by a benzofuran derivative of naltrindole (0.1 mg/kg s.c.), a selective delta 2-opioid receptor antagonist. These results suggest that delta 2-opioid receptors may play a synergistic role in antitussive processes that are mediated by mu-opioid receptors.

Unexpected antinociceptive potency of cyclic [D-Tca1]CTAP: potential for a novel mechanism of action

This study tested the hypothesis that compounds which may bind simultaneously to delta and mu receptors may be more potent antinociceptive agents than would be predicted from their binding affinities at individual mu and delta opioid receptors. D-Tca-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 ([D-Tca1]CTAP) (where D-Tca is a cyclic D-tryptophan analogue) was synthesized and evaluated in radioligand competition assays, opioid bioassays, and in an antinociceptive assay (the tail-flick test in mice). Additionally, the metabolic stability of [D-Tca1]CTAP was evaluated in striatal and cerebellar tissue slices. In rat brain in vitro, [D-Tca1]CTAP competed weakly for sites labelled by [3H]D-Phe-Cys-Tyr-D-Trp-Om-Thr-Pen-Thr-NH2 ([3H]CTOP) (mu-ligand), and [3H][D-Pen2,pCl-Phe4,D-Pen5]enkephalin (delta-ligand); [D-Pen2,D-Pen5]enkephalin (DPDPE) (delta-agonist) was 6.5-fold less and 230-fold more potent, respectively, against these ligands. Additionally, in mouse isolated vas deferens and guinea pig isolated ileum smooth muscle preparations, [D-Tca1]CTAP proved to be weak as either a delta (IC50 of approximately 2 microM) or mu (IC50 > 8 microM) receptor agonist. Surprisingly, however, i.c.v. [D-Tca1]CTAP produced antinociception with potency similar to DPDPE. The antinociceptive actions of [D-Tca1]CTAP were apparently not due to a metabolite or the release of endogenous opioids, as this compound proved stable in both striatal and cerebellar tissue slices and its antinociceptive actions were not enhanced by the 'enkephalinase' inhibitor thiorphan. The suggestion that [D-Tca1]CTAP might be acting by binding simultaneously to mu and delta receptors to produce its antinociceptive effect is supported by the demonstrated antagonism resulting from mu receptor blockade with either beta-funaltrexamine (beta-FNA) or naloxonazine, or by delta receptor blockade by ICI 174,864 ([N,N-diallyl-Tyr1,Aib2,3,Leu5] enkephalin). Furthermore, the antinociceptive properties of [D-Tca1]CTAP were antagonized by (naltrindole-5'-isothiocyanate) (5'-NTII), an antagonist at the delta 2 opioid receptor subtype, but not by the delta 1 antagonist [D-Ala2,D-Leu5,Cys6]enkephalin (DALCE). Additionally, no antagonism was produced by nor-binaltorphimine (nor-BNI), a kappa antagonist. From these data, [D-Tca1]CTAP appears to bind to mu, and 5'-NTII-sensitive delta 2, opioid receptors, and may represent the first of a class of compounds which may act at an opioid receptor complex via 'self-potentiation'.

A review of the role of anti-opioid peptides in morphine tolerance and dependence

Studies on the mechanisms of tolerance and dependence have mostly focused on changes at the receptor level. These experiments, conducted with model systems ranging from clonal cell lines to whole animals, have identified a number of important adaptive mechanisms which occur at the receptor level. However, none of these adaptive mechanisms can completely account for the phenomena which serve to define the state of morphine tolerance and dependence, especially the observation that as an animal becomes more tolerant to morphine, less naloxone is required to trigger withdrawal. The data reviewed in this paper provide strong support for the hypothesis that the brain synthesizes and secretes neuropeptides which act as part of a homeostatic system to attenuate the effects of morphine and endogenous opioid peptides. According to this model, administration of morphine releases anti-opioid peptides (AOP), which then attenuate the effects of morphine. As more morphine is given, more AOP are released, thereby producing tolerance to the effects of morphine. Cessation of morphine administration, or administration of naloxone, produces a relative excess of anti-opioid, which is in part responsible for the withdrawal syndrome. Since endogenous and exogenous antagonists might together produce synergistic effects, less naloxone might be required to trigger withdrawal in the presence of higher levels of AOPs. Although the study of AOP is in its infancy, a deeper understanding of the central nervous system (CNS) anti-opioid systems may lead to new treatments for chronic pain, substance abuse, and psychiatric disorders.

Effects of naltrindole and nor-binaltorphimine treatment on antinociception induced by sub-acute selective mu opioid receptor blockade

When administered repeatedly, in conjunction with hot plate testing, naloxone and naltrexone have the paradoxical effect of producing antinociception in rats and mice. Recently, we have found that the sub-acute selective blockade of mu opioid receptors leads to the development of antinociception and an augmentation of kappa receptor-mediated antinociception. In this study, acute delta/kappa antagonist treatment produced a significant decrease in paw lick latency in rats displaying antinociception induced by sub-acute mu blockade, however, the response level of these animals was still significantly above the baseline. In addition, rats receiving sub-acute combined mu and delta antagonist treatment took longer to develop an antinociceptive response than those treated with a mu antagonist alone. Sub-acute selective blockade of kappa or delta opioid receptors had no overall effect on paw lick latency during the course of 5 days of hot plate testing. The results indicate that delta receptor activity may play a role in the antinociception induced by sub-acute mu blockade. However, while delta antagonist treatment effected the expression, it did not completely attenuate the antinociception induced by sub-acute mu blockade suggesting that there is still a significant non-opioid component to this analgesic response. The results of a final experiment, in which acute delta antagonist treatment had no effect on antinociception induced by repeated systemic injections of naloxone, supported this hypothesis.

Precipitation of morphine withdrawal syndrome in rats by administration of mu-, delta- and kappa-selective opioid antagonists

The acute effects of opioid drugs are generally hypothesized to be mediated by multiple receptors, for which three types of binding sites have been established. In order to evaluate the selective participation of each type of opioid receptor in opiate withdrawal, the opiate withdrawal syndrome, precipitated by the intraventricular acute administration of mu-, delta- and kappa-selective opioid antagonists was investigated. After implantation of the cannula into the lateral ventricle, rats were made physically dependent by subcutaneous insertion of two 75-mg pellets of morphine (base). D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) (5-5000 ng), a mu-selective opioid antagonist, naltrindole (62-2000 ng), a delta-selective antagonist or nor-binaltorphimine (nor-BNI) (600-20,000 ng), a kappa-selective antagonist, were administered 72 hr after implantation of the pellets. All three drugs elicited some signs of morphine withdrawal but they differed in both their potency and their efficacy. The most efficacious and the most potent was CTAP, eliciting 8 of the 14 withdrawal signs at doses of 5-5000 ng. Nor-BNI was less efficacious and less potent, eliciting a significant increase in 5 of the 14 withdrawal signs in a dose range of 600-20,000 ng. Naltrindole was the least potent and least efficacious of the three drugs, eliciting a significant increase of only 2 withdrawal signs after intraventricular administration of 2000 ng. In a second experiment, the withdrawal syndrome was precipitated by the combined administration of CTAP+naltrindole or CTAP+nor-BNI. The severity of withdrawal, obtained with these two combinations, was similar to that observed with CTAP alone. These results support the importance of the mu receptor in the expression of central opiate dependence and suggest a minor role for delta and kappa receptors.