Mu and delta receptors: their role in analgesia in the differential effects of opioid peptides on analgesia

Dynorphin-A-(1-13) antagonizes morphine analgesia in the brain and potentiates morphine analgesia in the spinal cord

Intrathecal injection of subanalgesic doses of morphine (7.5 nmol) and dynorphin-A-(1-13) (1.25 nmol) in combination resulted in a marked analgesic effect as assessed by tail flick latency in the rat. The analgesic effect of the composite dynorphin/morphine was dose-dependent in serial dilutions so that a composition of 1/8 of the analgesic dose of dynorphin and 1/3 that of morphine produced an analgesic effect equipotent to full dose of either drug applied separately. The analgesic effect induced by dynorphin/morphine mixture was not accompanied by motor dysfunction and was easily reversed by a small dose (0.5 mg/kg) of naloxone. Contrary to the augmentatory effect of dynorphin on morphine analgesia in the spinal cord, intracerevroventricular (ICV) injection of 20 nmol of dynorphin-A-(1-13) exhibited a marked antagonistic effect on the analgesia produced by morphine (120 nmol, ICV). The theoretical considerations and practical implications of the differential interactions between dynorphin-A-(1-13) and morphine in the brain versus spinal cord are discussed.

The anticonvulsant effects of DADLE are primarily mediated by activation of delta opioid receptors: interactions between delta and mu receptor antagonists

Dose-response comparisons of the ability of the selective delta antagonist ICI 154,129 (12.5-50 nmol), the nonselective antagonist naloxone (29-290 nmol), and the irreversible selective mu antagonist beta-fNA (1.3-21 nmol) to alter the threshold response to DADLE or etorphine was studied in the rat flurothyl seizure test. DADLE (35 nmol, i.c.v.) and etorphine (122 nmol/kg, s.c.) both caused increases in seizure threshold which were differentially antagonized by pretreatment (i.c.v.) with the respective antagonists. For DADLE, only ICI 154,129 and naloxone produced a dose-related blockade of the increase in seizure threshold, with ICI 154,129 being more potent than naloxone. In contrast, the anticonvulsant action of etorphine was not antagonized by ICI 154,129 (50 nmol), but was blocked by a low dose of naloxone (29 nmol) or beta-fNA (21 nmol). In addition, prior occupancy of mu-sites with beta-fNA (21 nmol) significantly diminished the abilities of either ICI 154, 129 (50 nmol) or naloxone (290 nmol) to antagonize the anticonvulsant action of DADLE. The results of this study demonstrated that the effects of DADLE to increase seizure threshold in the rat were primarily mediated by activation of a delta-opioid receptor system. Furthermore, evidence has been provided for a functional interaction between delta and mu receptors in the opioid regulation of seizure threshold.

The effects of opioid and FMRF-amide peptides on thermal behavior in the snail

Administration of methionine-enkephalin, beta-endorphin or, as previously shown, the opiate agonist, morphine sulfate (0.10-10.0 micrograms per snail), resulted in significant dose-dependent increases in the latency of thermal (40 degrees C hot plate) avoidance behavior of the terrestrial snail, Cepaea nemoralis. The analgesic effects could be blocked by the opiate antagonist, naloxone, as well as by the non-opioid peptides, FMRF-amide and YGG-FMRF-amide. When administered by themselves the FMRF-amide peptides had significant bimodal effects either decreasing (0.10 and 10.0 micrograms) or increasing (1.0 micrograms) the latency of the response to the thermal stimulus. These results indicate that opioid and FMRE-amide peptides may be involved in the determination of thermal behavior in the snail. They also suggest that FMRF-amide peptides may function as endogenous modulators of opioid activity.

Leucine enkephalin noncompetitively inhibits the binding of [3H]naloxone to the opiate mu-recognition site: evidence for delta----mu binding site interactions in vitro

Using quantitative methods, this study examined the hypothesis that delta-ligands are noncompetitive inhibitors at a population of mu-binding sites. Evidence is presented that with the defined set of in vitro assay conditions utilized, [3H]naloxone labels two binding sites: the mu binding site and a second site tentatively identified as a kappa binding site, and that leucine enkephalin is a noncompetitive inhibitor at the mu recognition site.

Central delta-opioid receptor interactions and the inhibition of reflex urinary bladder contractions in the rat

The in vivo effects of a number of opioid agonists and antagonists were studied on the spontaneous reflex contractions of the urinary bladder recorded isometrically in the rat anesthetized with urethane. All substances were administered into the central nervous system by the intracereboventricular (i.c.v.) or spinal intrathecal (i.t.) route. The conformationally restricted enkephalin analogues [2-D-penicillamine, 5-L-cysteine] enkephalin (DPLCE), [2-D-penicillamine, 5-L-penicillamine] enkephalin (DPLPE) and [2-D-penicillamine, 5-D-penicillamine] enkephalin (DPDPE) produced dose-related inhibition of reflex bladder contractions when administered by the i.c.v. or i.t. route. Both the novel delta-opioid receptor antagonist ICI 154,129 (200-600 micrograms) [N,N-bisallyl-Tyr-Gly-Gly-Psi-(CH2S)-Phe-Leu-OH) and ICI 174,864 (1-3 micrograms) [N,N-dially-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid] attenuated or abolished the effects of DPLCE, DPLPE and DPDPE when administered by the i.c.v. or i.t. route. The antagonism observed was selective since the equipotent inhibition produced by the mu-opioid receptor agonist [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAGO) was unaffected. Overall, ICI 154,129 was considerably weaker than ICI 174,864 and both antagonists inhibited bladder activity at doses higher than those required to demonstrate delta-receptor antagonism. Further studies of the agonistic effect of ICI 174,864 showed that it was insensitive to low doses of naloxone (2 micrograms, i.c.v. or i.t.) but could be abolished by higher (10-15 micrograms) doses of naloxone. These observations suggested that the agonistic effect of ICI 174,864 was not mediated by mu-opioid receptor. beta-Endorphin (0.2-1.0 micrograms, i.c.v.) inhibited bladder contractions but following recovery from this effect, appeared to prevent the expression of delta-receptor antagonism by ICI 174,864. In addition a previously subthreshold dose of ICI 174,864 now exhibited marked agonistic activity. The inhibitory effect of a submaximal dose of DPDPE was also potentiated by beta-endorphin under these circumstances. These observations suggest that supra-spinal and spinal delta-opioid receptors are involved in the opioid-mediated inhibition of reflex bladder contractions in the rat. Moreover beta-endorphin may be important in regulating central delta-opioid receptors.

Differences in binding properties of mu and delta opioid receptor subtypes from rat brain: kinetic analysis and effects of ions and nucleotides

Differences in binding properties of mu and delta opioid receptors were investigated using DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), which occur, respectively, as the most selective mu and delta radioligands available. At high concentration, each agonist is able to interact with its nonspecific sites. Competition experiments indicated that a two-site competitive model was adequate to explain the interactions of DAGO and DTLET with [3H]DTLET and [3H]DAGO binding sites, respectively. The weak cross-reactivity (congruent to 10%) of DTLET for mu sites was taken into account in these experiments. On the other hand, DAGO and DTLET exhibit differential binding kinetics. Thus, at 35 degrees C, the lifetime of DTLET within its receptor site is about 14 times longer than that of the mu agonist. Sodium and manganese ions decrease the maximal number of high affinity mu and delta sites, but the sensitivity of mu receptors is three times higher towards Na+ and 20-fold higher towards Mn2+ than that of delta receptors. GTP reduces similarly the mu and delta binding whereas only the DAGO binding was modified by the nonhydrolyzable analogue guanylylimidodiphosphate [GMP-P(NH)P]. However, in the presence of Na+ ions, GMP-P(NH)P inhibits the DTLET binding in a concentration-dependent manner. The effects of Na+ and GMP-P(NH)P could be explained by a sequential transformation of delta receptors to low-affinity states. This model predicts that Na+, by lowering the affinity of a fraction of sites, produces a decrease in the maximal number of high-affinity delta receptors and that GMP-P(NH)P enhances the Na+ effect. Moreover, the binding kinetic to this high-affinity state was also modified by Na+ and nucleotides. All of these data support the existence of two independent mu and delta binding sites, the properties of which are differentially regulated by these endogenous effectors.

Restraint stress enhances morphine-induced analgesia in the rat without changing apparent affinity of receptor

Antagonism of morphine analgesia (tail-flick assay) by naloxone was assessed quantitatively by in vivo "apparent" pA2 determination in unstressed rats and in rats subjected to restraint stress. Restrained rats had a higher baseline tail-flick latency than did unstressed (unrestrained) animals, and were more sensitive to the analgesic effect of morphine, as reflected in lower morphine ED50s. There was no significant difference between apparent pA2 values of unstressed and restrained rats using pA2 regression line analysis. This suggests that while stress enhances the analgesic effect of morphine, it does not appreciably alter opiate receptor affinity for naloxone under the conditions of this study.

Reversal of the antinociceptive effects of centrally-administered morphine by the benzodiazepine receptor antagonist Ro 15-1788

The effects of the benzodiazepine receptor antagonist, Ro 15-1788, were examined on analgesia induced by morphine after central (intracerebroventricular, i.c.v., or intrathecal, i.t.) and systemic administration. Analgesia was assessed in squirrel monkeys trained to respond under an electric shock titration procedure and in mice using the radiant heat tail-flick test. Central and systemic administration of morphine produced antinociceptive effects that were antagonized by 0.1 mg/kg of naloxone in both species. Ro 15-1788 antagonized the effects of morphine after central (i.c.v. or i.t.) administration but did not alter the effects of morphine given by the systemic route. This novel interaction suggests that Ro 15-1788 may be useful in pharmacologically separating neural substrates subserving opiate analgesia.

ICI 174864, a putative delta opioid antagonist, reverses endotoxemic hypotension: pretreatment with dynorphin 1-13, A kappa agonist, blocks this action

The putative delta opioid receptor antagonist ICI 174864 (3 mg/kg, i.v.) significantly reversed endotoxic shock hypotension at a dose which lacked significant pressor actions in normotensive, non-endotoxemic rats. In contrast, dynorphin 1-13, when administered either before (0.1 or 1.0 mg/kg, i.v.) or following (1.0 mg/kg, i.v.) injection of endotoxin, failed to alter the course of ensuing circulatory shock. Additionally, pretreatment with dynorphin 1-13 prevented the subsequent reversal of endotoxemic hypotension by ICI 174864. It is concluded that: 1) delta opioid receptors mediate the endogenous opioid component of endotoxic shock hypotension; and 2) functional interactions occur between ligands for mu, delta, and kappa opioid receptor subtypes, which may predict potential interactions with a common macromolecular opioid receptor complex.

Quantitative analysis of the interaction between the agonist and antagonist isomers of picenadol (LY150720) on electric shock titration in the squirrel monkey

The opioid mixed agonist-antagonist picenadol (LY150720) is a racemate whose resolution results in a highly stereospecific separation of opioid agonist and antagonist activity. Attenuation of the antinociceptive effects of the agonist (dextro) isomer LY136596 by the antagonist (levo) isomer LY136595 was evaluated quantitatively in squirrel monkeys responding under a schedule of electric shock titration through the use of a dose-ratio analysis. LY136596 (0.3-3.0 mg/kg) produced a dose-related increase in the intensity at which monkeys maintained the shock. Increases in shock intensity produced by LY136596 were antagonized by LY136595 (0.1-10.0 mg/kg); dose-response curves for LY136596 were shifted to the right in a parallel manner by increasing doses of LY136595. An apparent pA2 (Schild) plot obtained from these data yielded a line with a slope of -0.60 +/- 0.05 and an apparent pA2 value of 5.67 +/- 0.07. These data support previous suggestions that the antinociceptive activity of picenadol (LY150720) resides in the d-isomer (LY136596) and that the l-isomer (LY136595) acts to limit the analgesic efficacy of the racemate.

Opioid control of the ruminant stomach motility: functional importance of mu, kappa and delta receptors

In sheep, the subcutaneous (SC) or intracerebroventricular (ICV) administration of the mu-type opioid agonists, fentanyl and morphine, evokes a blockade of the cyclic contractions of the reticulum. A similar inhibition of forestomach motility was recorded following the administration of the two enkephalin analogs, D-Ala2-Met5-enkephalinamide (DAMA) and D-Ala2-D-Leu5-enkephalin (DADLE) which are mixed mu - delta opioid agonists. In contrast, the reticular contractions were enhanced by the SC or ICV administration of the kappa type agonist, ethylketazocine (EKC) and U - 50 488 H. The proximal duodenum motor activity was transiently increased resulting in the occurrence of a phase III-like activity by these opioid agonists, regardless of the subtypes. The effects of the opioid agonists on reticular motility were prevented by the injection of naloxone but not by the quaternary parent compound methylnaloxone which does not cross the blood-brain barrier. The duodenal motor effects elicited by the opioid agonists were antagonized by both naloxone and methylnaloxone. The results suggest that the inhibition of the ruminant stomach motility is centrally mediated by mu - delta type opioid agonists and are consistent with opposite effects from kappa type opioid agonists. The stimulatory effect of peptide and non-peptide opioid agonists on the duodenum may result in part from direct opioid receptor-mediated actions on smooth muscle.

Opiate receptor supersensitivity produced by chronic naloxone treatment: dissociation of morphine-induced antinociception and conditioned taste aversion

In three separate experiments, rats were used to assess the effects of chronic administration of naloxone on specific binding of 3H-naloxone in various regions of the central nervous system (CNS) and on the efficacy of morphine to produce antinociception and a conditioned taste aversion. Chronic naloxone treatment increased opiate binding in medulla-pons, midbrain, hypothalamus, hippocampus, striatum, and prefrontal cortex, but not in either spinal cord or cerebellum. In those CNS regions exhibiting increased opiate binding, the duration of increased binding following termination of the naloxone treatment differed between regions. In conjunction with the increase in opiate binding, the efficacy of morphine to produce antinociception was potentiated, while the efficacy to produce a conditioned taste aversion was unchanged. Moreover, the administration of naloxone during behavioral testing blocked completely the antinociceptive effect, but not the aversive effect, of morphine. These results indicate that morphine-induced antinociception and conditioned taste aversion may be dissociated neuropharmacologically.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Peripheral opioid receptors influencing heart rate in rats: evidence for endogenous tolerance

Opioid peptides injected into the circulation of rats evoke a vagally mediated bradycardia. The intravenous ED50 of morphine for producing a greater than or equal to 10% fall in heart rate was determined in urethane-anesthetized rats. Hypophysectomy, or adrenalectomy plus treatment with dexamethasone (0.5 microgram/h, s.c., 1 day), procedures that remove endogenous sources of opioid peptides, increased the sensitivity of the animal to morphine bradycardia by 6-10-fold. Conversely, stressing the animals by exposure to cold (4-6 degrees C for two days) elevated the ED50 for morphine sulfate and for beta h-endorphin by about 5-fold. Dexamethasone infusions prevented the cold-induced desensitization to morphine. Intravenous administration of rat corticotropin-releasing factor (CRF) also desensitized the animals to morphine. CRF alone produced a fall in blood pressure and heart rate. The bradycardia was prevented by pretreatment with naloxone. These results indicate that the sensitivity of vagal opioid chemoreceptors is influenced by endogenous sources of opioid peptides. This phenomenon can be called 'endogenous tolerance'.

Concomitant increase in nociceptive flexion reflex threshold and plasma opioids following transcutaneous nerve stimulation

In order to evaluate the role of endogenous opioids in sustaining analgesia induced by transcutaneous nerve stimulation (TNS), we measured plasma beta-lipotropin (BLPH), beta-endorphin (BEP), ACTH and cortisol changes concomitantly with nociceptive flexion reflex (RIII) threshold after TNS (80 microseconds rectangular waves at 85 Hz) in a group of healthy volunteers (A). The same protocol was carried out in another group of volunteers using placebo stimulation (0.5 Hz) (B). RIII threshold significantly increased 0.5 h after TNS in group A and no changes were recorded in group B. Similarly, both BLPH and BEP plasma levels increased at the end of TNS only in group A. ACTH and cortisol concentrations show only random variations after both high and low frequency TNS. A positive linear correlation was found between the maximum percentage increase of RIII threshold after high frequency TNS and the maximum percentage increase of BLPH plasma levels occurring 20 min beforehand (r = 0.856, P less than 0.001). A less positive correlation was found between RIII and BEP levels (r = 0.574, P less than 0.05). These data indicate that the so-called post-stimulation analgesia could be supported by the enhancement of the endogenous opioid system.

The mu rather than the delta subtype of opioid receptors appears to be involved in enkephalin-induced analgesia

The analgesic activity of some opioid peptides which display a relative selectivity for either the mu-receptor subtype, [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGO) or the delta-receptor subtype. [D-Ala2, D-Leu5]enkephalin (DADLE), [D-Ser2, Leu5]enkephalyl-Thr (DSLET) and [D-Thr2, Leu5]enkephalyl-Thr (DTLET) is highly correlated with their affinity for central or peripheral mu- but not delta-receptors. Moreover their analgesic effects as well as those elicited by degrading enzyme inhibitors (bestatin + thiorphan) of endogenous enkephalins were easily antagonized by naloxone with similar pA2 values but not by the delta-antagonist ICI 154,129. Therefore the analgesia produced by opioid peptides including endogenous enkephalins is likely connected to mu-receptor stimulation. Finally, there was no obvious potentiation by delta-agonists of the analgesia resulting from either administration of the mu-agonist morphine or endogenous enkephalins. This suggested that in the hot plate test, there is no modulation of the effect resulting from mu-receptor stimulation by a delta-receptor interaction. Likewise, enkephalinergic activity such as that due to thiorphan + bestatin does not appear to be regulated through mu- or delta-receptor stimulation.

Multiple opioid receptors in endotoxic shock: evidence for delta involvement and mu-delta interactions in vivo

The use of selective delta and mu opioid antagonists has provided evidence that delta opioid receptors within the brain mediate the endogenous opioid component of endotoxic shock hypotension. The selectivity of these delta and mu antagonists was demonstrated by their differing effects upon morphine analgesia and endotoxic hypotension. The mu antagonist beta-funaltrexamine, at doses that antagonized morphine analgesia, failed to alter shock, whereas the delta antagonist M 154,129: [N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH] (ICI) reversed shock at doses that failed to block morphine analgesia. Therefore, selective delta antagonists may have therapeutic value in reversing circulatory shock without altering the analgesic actions of endogenous or exogenous opioids. Additional data revealed that prior occupancy of mu binding sites by irreversible opioid antagonists may allosterically attenuate the actions of antagonists with selectivity for delta binding sites. For endogenous opioid systems, this observation provides an opportunity to link in vivo physiological responses with receptor-level biochemical interactions.

Ionic conditions differentially affect 3H-DADL binding to type-I and type-II opiate delta receptors in vitro

It is widely accepted that the prototypic delta agonist DADL (D-ala2-D-leu5-enkephalin) labels two binding sites in vitro. Using the site directed, receptor selective alkylating agents, BIT and FIT (Rice et al.. Science 220:314-316, 1983), we recently described (Rothman et al, Neuropeptides, in press) the preparation of membranes possessing only lower affinity 3H-DADL binding sites (FIT-treated membranes, type-I delta sites) as well as membranes greatly enriched with higher affinity binding sites (BIT-treated membranes, type-II delta sites). In this paper we report that ionic conditions differentially affect the binding of 3H-DADL to FIT- and BIT-treated membranes, supporting the notion that 3-H-DADL labels two distinct delta binding sites.

Preparation of rat brain membranes greatly enriched with either type-I-delta or type-II-delta opiate binding sites using site directed alkylating agents: evidence for a two-site allosteric model

Although it is widely accepted that radiolabeled prototypic delta receptor agonists label two binding sites in vitro, the mechanism by which mu ligands inhibit peptide binding as well as the identity of the binding sites remains unsettled (Rothman and Westfall, Mol. Pharmacol. 21:538-547, 1982 ; Bowen et al., Proc. Natl. Acad. Sci. U.S.A. 78:4818-4822, 1981). Using the site directed, receptor selective alkylating agents, BIT and FIT (Rice et al., Science 220:314-316, 1983), we describe the preparation of membranes devoid of high affinity binding sites and demonstrate that the mu agonist oxymorphone noncompetitively inhibits the binding of [3H]DADL to the residual lower affinity binding sites.

Effect of intrahypothalamic injection of [D-Ala2,D-Leu5]enkephalin on feeding and temperature in the rat

The effectiveness of the delta-opioid agonist [D-Ala2,D-Leu5]enkephalin (DADLE) in eliciting alterations in feeding and core temperature in rats was compared to morphine. When injected into the ventromedial hypothalamus this peptide caused a dose-related increase in feeding which was rapid in onset and of short duration, and a short-lived increase in temperature. Neither of these effects was blocked by the intrahypothalamic injection of naloxone. The alpha-adrenoreceptor antagonist phentolamine significantly reduced DADLE-stimulated feeding, although it did not counteract the hyperthermia. Since the delta-opioid agonist DADLE is more potent than the mu-agonist, morphine, and produces its effect more rapidly, the delta-opiate receptor may have an influence on the regulation of feeding.

Chronic naltrexone increases opiate binding in brain and produces supersensitivity to morphine in the locus coeruleus of the rat

Rats were implanted subcutaneously for 2-4 weeks with slow-release pellets of naltrexone (10 mg) or placebo and then the pellets were removed. One day after removal of the pellet, animals were either (1) sacrificed and various CNS regions examined for specific binding of [3H]naloxone, [3H]etorphine or [3H]rauwolscine or (2) they were anesthetized and prepared acutely for assessing morphine-induced changes in the spontaneous activity of neurons in the locus coeruleus (LC). Naltrexone treatment significantly increased the number of specific binding sites for opiates, but not for alpha 2-adrenergic antagonists, in spinal cord, hypothalamus, striatum and cortex. Specific binding of [3H]naloxone was also increased in the LC. The spontaneous activity of neurons in the LC was reduced by the chronic naltrexone treatment, suggesting that these neurons became supersensitive to the tonic inhibitory effect of endogenous opioid peptides. Moreover, neurons in the LC of chronic naltrexone-treated rats exhibited an enhanced response to the inhibitory effects of morphine administered systemically. These results demonstrate that chronic opiate receptor blockade increases the number of receptor sites for morphine and that this increase in receptors is accompanied by a neuronal supersensitivity in the LC to morphine which can be assessed electrophysiologically.

Evaluation of the interactions of mu and delta selective ligands with [3H]D-Ala2-D-Leu5-enkephalin binding to mouse brain membranes

The interactions of putative mu and delta selective ligands with [3H]D-ala2-D-leu5 enkephalin (DADLE) binding to mouse brain membranes were investigated. Computerized curve fitting of displacement curves performed at three different concentrations of 3H-DADLE indicated that a one site competitive model was sufficient to explain the interactions of leu-enkephalin (LE) and D-ser2-thr6-leucine enkephalin with 3H-DADLE binding. Similar experiments with morphine and morphiceptin were unique in that the multiple displacement curves crossed over one another. A two-site competitive model was required to adequately describe the interactions of these mu selective ligands with 3H-DADLE. This two-site model was one in which the inhibitor had higher affinity for the site labeled with lower affinity by 3H-DADLE. However, this two site model did not correctly predict the interaction of LE with 3H-DADLE in the presence of morphiceptin. These data indicate that: 1) putative mu and delta selective ligands do not bind to a common high affinity site; 2) mu selective ligands are not simple mixed inhibitors of a single site labeled by 3H-DADLE; and 3) competitive binding models may not explain the interaction of mu ligands with 3H-DADLE binding.

Inhibition of morphine-induced analgesia and locomotor activity in strains of mice: a comparison of long-acting opiate antagonists

The long-acting opiate antagonistic potency of naloxazone (NXZ), beta-chlornaltrexamine (beta-CNA) and beta-funaltrexamine (beta-FNA) was compared using three inbred strains of mice, in which morphine induces either analgesia (DBA/2), locomotion (C57BL/6), or both responses (C3H/He). The antagonists were applied SC 24-120 hr before morphine (10 or 20 mg/kg, IP), followed by the tests after 30 min. The minimal dose which completely antagonized morphine-induced analgesia in DBA and locomotion in C57 mice during 24 hr were: for NXZ 50 and 100 mg/kg, for beta-CNA 0.8 and 6.2 mg/kg, for beta-FNA 1.6 and 12.5 mg/kg, respectively. beta-FNA and beta-CNA more potently blocked morphine-induced analgesia in DBA mice than the activity response in the C57 strain. In contrast, beta-FNA prevented morphine-induced locomotion at a lower dose (6.2 mg/kg) than analgesia (greater than 50 mg/kg) in C3H mice, while beta-CNA was equipotent (1.6 mg/kg). In general, beta-CNA turned out to be the most reactive compound, antagonizing morphine effects in low doses up to 120 hr. beta-FNA selectively antagonized either morphine-induced analgesia or locomotion, depending on the strain used. This suggests that a given morphine response might be caused by a genetically determined multiplicity of opiate receptor types and their mutual interactions.

The involvement of opioid delta-receptors in stress induced antinociception in mice

The selective opioid delta-receptor antagonist, ICI 154,129, attenuated the antinociception, assessed by prolongation of reaction time on the hot-plate, of mice which had swum for 3 min at 20 degrees C. This stress-induced antinociception was also sensitive to naloxone suggesting the involvement of both delta- and mu-receptors. A swim of 0.5 min at 30 degrees C did not produce antinociception on the hot plate but the writhing response to i.p. acetic acid was blocked by a non-opioid mechanism.

Naloxone does not alter the effect of gamma aminobutyric acid derivative, baclofen, on GH release in man

To evaluate the interaction between opioid peptides and GABAergic system in regulating GH secretion we administered 5 mg of baclofen, a GABA derivative, to eight normal male subjects. The results were compared to those obtained in the same subjects treated with naloxone (10 mg/2 h) plus baclofen. GH levels increased significantly above basal value either after baclofen and naloxone plus baclofen without any significant difference between GH responses during the two tests. It is suggested that the two substances do not act at the level of the same receptor site. The evaluation of a possible interaction between opioid peptides and GABAergic system on GH release requires further investigation.

Further evidence for an opioid receptor complex

We recently presented evidence that distinct morphine and enkephalin receptors coexist in an opioid receptor complex. These studies used membranes prepared from whole rat brain. In this paper the receptor complex is demonstrated to occur in membranes prepared from rat striatum, cortex, and pooled nonstriatal-noncortical regions of the brain. The observation that morphine masks enkephalin receptors is confirmed using 3H-methionine enkephalin to label the enkephalin receptor. These data further support the hypothesis that populations of morphine and enkephalin receptors coexist in an opioid receptor complex.

The effect of opioid drugs on the release of dopamine and 5-hydroxytryptamine from rat striatum following activation of nicotinic-cholinergic receptors

The effect of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin, (Leu5)enkephalin, (D-Ala2,D-Met5)enkephalin and morphine on the release of [3H]dopamine, endogenous dopamine and [3H]5-hydroxytryptamine produced by the nicotinic-cholinergic agonist, dimethylphenyl piperazinium iodide (DMPP), was examined in rat striatal slices. The DMPP-induced release of [3H]dopamine and endogenous dopamine was reduced by the presence of (Met5)enkephalin, (D-Ala2,D-Met5)enkephalin (1-10 microM) or morphine (10 microM) but not by (Leu5)enkephalin or (D-Ala2,D-Leu5)enkephalin. The DMPP-induced release of [3H]5-hydroxytryptamine was reduced by (Leu5)enkephalin, (D-Ala2,D-Leu5)enkephalin, (Met5)enkephalin, (D-Ala2,D-Leu5)enkephalin (1-10 microM), and morphine (10 microM). All three opioids failed to alter the release of [3H]dopamine induced by field stimulation or potassium depolarization (30 microM). The inhibitory effects of opioid peptides and morphine demonstrated in the present study appear to be due to an initial interaction with nicotinic-cholinergic receptors in the striatum.

Interaction of leucine enkephalin with (3H)naloxone binding in rat brain: evidence for an opioid receptor complex

We recently presented evidence that distinct morphine and enkephalin receptors coexist in an opioid receptor complex (Mol. Pharmacol. 21:548-557, 1982). In this paper, we present data which demonstrate that in the presence of sodium leucine enkephalin noncompetitively inhibits the binding of [3H]naloxone to a crude particulate fraction of rat brain. Since the binding site labeled by [3H]naloxone in the presence of sodium may be an alternate conformation of the morphine receptor, these data provide further evidence that morphine and enkephalin receptors are allosterically coupled.

Multidimensional analysis of ligand binding data: application to opioid receptors

The existence of distinct mu and delta opioid receptors is now well accepted. Most investigators favor the hypothesis that these receptors are physically distinct and that the enkephalins are only 2-10 fold selective for the delta receptor. Rothman and Westfall (Mol. Pharmacol. 21:548-557) recently challenged this hypothesis, proposing that at least some population of mu and delta receptors coexist in an opioid receptor complex and that the enkephalins are at least 100 fold selective for the delta receptor. In this paper we describe a generally applicable method we have used to design and analyze ligand binding experiments which distinguish between the two different models.

Naloxazone lacks therapeutic effects in endotoxic shock yet blocks the effects of naloxone

Cardiovascular effects of the high affinity, irreversible opiate antagonist, naloxazone, were investigated in conscious rats subjected to endotoxic shock. Unlike other less selective opiate antagonists such as naloxone, naloxazone failed to block or reverse the hemodynamic effects of endotoxemia. However, naloxazone pretreatment prevented the usual therapeutic effects of naloxone in endotoxic shock. Results indicate that high affinity (mu 1-site) opiate binding is not involved in the pathophysiological actions of endogenous opiates in shock, and suggest that interactions among opioid receptor subtypes may occur in vivo.

Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr: a new highly potent and fully specific agonist for opiate delta-receptors

Deltakephalin, Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) was rationally designed as pure delta-probe from proposed models of mu and delta opiate receptors. On peripheral organs, deltakephalin displays a 3000 times higher inhibitory potency on the electrically stimulated mouse vas deferens (IC50 = 0.15 nM) as on the guinea pig ileum (IC50 = 460 nM). As expected [3H]deltakephalin interacts at 35 degrees C in rat brain tissue to a single class of binding sites (delta) (Bmax = 0.115 pmole/mg protein) with a high affinity: KD = 1.35 nM from equilibrium measurements and KD = 0.43 nM from kinetic determinations. Deltakephalin occurs as the most specific ligand for delta-binding sites as shown by the following discrimination ratios KI(mu)/KI(delta): 0.31 for D-Ala2-D-Leu5-enkephalin; 0.15 for D-Ser2-Thr6-Leu-enkephalin and 0.05 for deltakephalin.

Pain control by endogenous enkephalins is mediated by mu opioid receptors

The analgesic effects of bestatin and thiorphan, two enzymatic inhibitors protecting endogenous enkephalins from their degradation, and those of DAGO and deltakephalin, respectively mu and delta opioid peptides, are assessed on the electrical stimulation test of the mouse tail. The relative analgesic potency of DAGO and deltakephalin is in good agreement with their relative potency on mu pharmacological assays: inhibition of electrically-induced contractions of guinea-pig ileum, displacement of 3H DAGO on rat brain. Finally, the analgesic effects of DAGO, deltakephalin and bestatin + thiorphan, are antagonized by the mu antagonist naloxone with similar pA2, and they are not modified by the delta antagonist ICI 154, 129. We conclude that only mu and not delta receptors are involved in the analgesic effects of enkephalins.

Comparison of analgesic potencies of mu, delta and kappa agonists locally applied to various CNS regions relevant to analgesia in rats

Analgesic potencies of relatively selective agonists for mu, delta and kappa subtypes of opioid receptors, morphine, [D-Ala2,D-Leu5]-enkephalin (DADL) and ethylketocyclazocine (EKC), respectively, were examined with the tail-pinch test in the rat, when microinjected into the brain stem regions relevant to analgesia such as the nucleus reticularis paragigantocellularis (NRPG), nucleus raphe magnus (NRM) and periaqueductal gray matter (PAG), and into the lumbar subarachnoid space (LSS). Morphine, DADL and EKC produced dose-dependent analgesic effects at the NRPG, NRM, PAG and LSS. The ED50 values indicated that morphine was more potent than DADL at the NRPG and LSS but less at the NRM and PAG. EKC was the weakest at all the injection sites. Further, naloxone (0.1 mg/kg, s.c.) significantly antagonized the analgesic effect of morphine but not that of DADL or EKC at the NRPG. These findings suggest that mu, delta and kappa subtypes of opioid receptors mediate analgesia, and that the extent of contribution of each subtype to the production of analgesia differs among the CNS sites examined in this study.

Multiple opioid receptors: evidence for mu-delta binding site interactions in endotoxic shock

Using antagonists with selectivity for the delta (ICI 154,129) and mu (beta-funaltrexamine) binding sites, evidence was obtained to indicate that delta receptors within the brain mediate the endogenous opioid component of endotoxic hypotension. The therapeutic actions of intravenous ICI 154,129 were dose related, with effective doses between 15-60 mg/kg. Evidence for a functional interaction between mu and delta binding sites was obtained: prior occupancy of the mu binding site by beta-funaltrexamine prevented the usual therapeutic response to the delta antagonist ICI 154,129 in endotoxemic rats. These data indicate that mu and delta binding sites may be a part of the same macromolecular complex which interact through allosteric coupling.

Intrathecal opioids block a spinal action of substance P in mice: functional importance of both mu- and delta-receptors

Inhibition of intrathecal substance P-elicited behavior by mu-, delta- and kappa-opioids was compared. In both the substance P behavioral test and the tail flick antinociceptive test, intrathecal [D-Ala2, Met5]enkephalinamide and [D-Ala2, D-Leu5]enkephalin (DADL) were equipotent, morphine and ethylketazocine were less potent, but nalorphine was inactive. A long-lasting, highly selective, mu-receptor antagonist, beta-funaltrexamine, blocked the inhibition of substance P behaviors by both morphine and ethylketazocine, but did not block the effect of DADL. These results suggest that spinal postsynaptic modulation of nociception is mediated by both delta-type and mu-type opioid agonists.

mu Receptors and opioid cardiovascular effects in the NTS of rat

In order to assess the potential role of mu (mu) and delta (delta) opiate receptors in the central regulation of the cardiovascular and respiratory systems, the cardiovascular and respiratory effects of the relatively selective mu-opioid agonist D-Ala2, MePhe4, Gly-ol5 enkephalin (DAGO) and relatively selective delta-agonist D-Ala2-D-Leu5 enkephalin (DADL) were compared following microinjection of these compounds into the nucleus tractus solitarius of pentobarbital-anesthetized rats. Both opioid agonists produced dose dependent increases in systolic and diastolic blood pressure as well as heart rate; but DAGO was nearly ten times more potent in eliciting these changes. Respiratory rate was increased by DADL and by lower doses of DAGO, but was depressed by higher doses of DAGO. Tidal volume was depressed by both peptides. These data support the concept that the cardiovascular pressor responses and tachycardia as well as the respiratory effects of opioids in the rat NTS are mediated by mu receptors.

Delta versus mu receptors: cardiovascular and respiratory effects of opiate agonists microinjected into nucleus tractus solitarius of cats

The cardiovascular and respiratory responses to relatively specific mu or delta agonists microinjected (0.5 microliter/kg) into the region of the nucleus of tractus solitarius (NTS) were examined in anesthetized cats. Blood pressure, heart rate, and respiratory rate were monitored for 30 min after the microinjection of opioid compounds or saline vehicle. The delta agonist, (D-Ala2,D-Leu5)-enkephalin (10-100 nmol/kg) elicited dose-dependent decreases in blood pressure, heart rate, and respiratory rate which were naloxone reversible. In contrast the mu agonists, morphine (10-54 nmol/kg) and morphiceptin (100-320 nmol/kg) had no effect on blood pressure or respiratory rate; yet, naloxone elicited pressor responses in animals pretreated with these mu agonists. A receptor-binding assay demonstrated a predominance of mu sites in the NTS. These data show that the delta opiate agonist is more effective than mu agonists in modifying cardiovascular variables in the NTS; we suggest caution in relating specific cardiovascular function to receptor subtypes defined by binding assays.

Cross-tolerance studies distinguish morphine- and metkephamid-induced analgesia

In vitro data demonstrate that metkephamid (LY127623), an analog of methionine enkephalin, has a high affinity for the delta opioid receptor, as well as the mu-receptor. Data generated utilizing two in vivo measures of receptor selectivity, furthermore, indicate that metkephamid's analgesic activity is in part mediated by delta opioid receptors. The analgesic activity of metkephamid was investigated in the mouse writhing assay following chronic treatment with morphine, the prototypic mu agonist. Mice were treated chronically with increasing doses of morphine or saline and the inhibition of writhing measured in response to an acute injection of morphine or metkephamid. The dose response curve for morphine was shifted to the right 3- to 4-fold following chronic administration of morphine. In contrast, no such shift in the dose response curve for metkephamid was observed in these morphine-tolerant mice. In a further series of tests, a 50 mg/kg dose of naloxazone 20 hr prior to the assessment of morphine or metkephamid analgesia in the mouse hot plate test substantially shifted the dose-response curve for morphine to the right, while leaving the dose-response curve for metkephamid unchanged. These results suggest that delta-receptor activation contributes to the analgesia produced by metkephamid.

Differential effects of D-Ala2 analogues of enkephalins on substance P-induced analgesia in rodents

The systemic administration of subanalgesic doses of [D-Ala2, D-Leu5]enkephalin significantly potentiated the analgesia elicited in rats or mice by intraventricular injection of substance P. On the contrary, systemic administration of low doses of [D-Ala2,Met5]enkephalinamide antagonized the substance P-induced analgesia. The results support the notion of different physiological functions for the enkephalins and suggest an integrated role for enkephalins and substance P in the control of pain at supraspinal levels.