Multiple opiate receptors in peripheral tissue preparations

Modulating effects of opioids, purine compounds, 5-hydroxytryptamine and prostaglandin E2 on cholinergic neurotransmission in a guinea-pig oesophagus preparation

The submucous plexus-longitudinal muscularis mucosae preparation of the guinea-pig oesophagus was used to study the actions of morphine, opioid peptides, purine compounds, 5-hydroxytryptamine (5-HT) and prostaglandin E2 (PGE2) on electrically-induced twitch contractions which are probably mediated by cholinergic nerve stimulation. The twitch contractions were inhibited by morphine (1–100 μM), methionine-enkephalin (1–100 μM) and β-endorphin (0.1–1 μM), but increased by adenosine (1–30 μM), adenosine 5′-triphosphate (1–30 μM), 5-HT (0.01–3 μM) and PGE2 (1–10 nM). The submaximal contraction induced by acetylcholine (12 or 20 nM) which is nearly equivalent to the twitch contractions was unaffected by morphine, methionine-enkephalin, β-endorphin and 5-HT, but augmented by purine compounds and PGE2. It is concluded that cholinergic neurotransmission in the submucous plexus-longitudinal muscularis mucosae of the guinea-pig oesophagus is inhibited by morphine and opioid peptides acting at prejunctional opiate receptors, and facilitated by 5-HT, purine compounds and PGE2 via prejunctional or postjunctional mechanisms.

Differentiation of enantiomers using matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has successfully been used to differentiate pseudo-enantiomeric (isotopically labelled) amino acids by using cyclodextrin as complexing host. By using different pseudo-enantiomeric mixtures (i.e. R(Dn) + S; and R + S(Dn)), it has been demonstrated that the preference of cyclodextrin for S-enantiomers is not due to the size differences caused by the hydrogen/deuterium substitution. It is postulated that this method can be extended to differentiate enantiomers (and determine enantiomeric excess) by using a pair of enantiomeric hosts, as demonstrated previously using other ionization techniques, but with much higher sensitivity.

Stimulation of delta1- and delta2-opioid receptors produces amnesia in mice

The effects of intracerebroventricular administration of delta1- and delta2-selective opioid receptor agonists on spontaneous alternation performance, elevated plus-maze behavior and passive avoidance learning including step-down and step-through types were examined in mice. Although the delta1-selective opioid receptor agonist, [D-Pen2,L-Pen5]enkephalin (DPLPE) (1-10 microg) or the delta2-selective opioid receptor agonist, [D-Ala2]deltorphin II (deltorphin) (1-10 microg) did not markedly affect spontaneous alternation performance or elevated plus-maze behavior, DPLPE (1, 3 and/or 10 microg) and deltorphin (3 and 10 microg) inhibited passive avoidance learning including step-down and step-through types. The delta1-selective opioid receptor antagonist, 7-benzylidenenaltrexone (3.5 ng), and the delta2-selective opioid receptor antagonist, naltriben (19 ng), significantly antagonized the inhibitory effects of DPLPE (3 microg) and deltorphin (3 microg) on passive avoidance learning, respectively. In contrast, DPLPE (3 microg) or deltorphin (3 microg) did not markedly influence behavioral responses induced by electroshocks during training of passive avoidance learning. Moreover, DPLPE (0.3-3 microg) or deltorphin (0.3-3 microg) failed to significantly affect the radiant heat-induced nociceptive responses. These results suggest that stimulation of delta1- and delta2-opioid receptors produces amnesia, depending on the learning tasks used.

Endogenous morphine

This review surveys the discovery of endogenous alkaloids in mammals. The formation of morphine in mammalian brain was assumed in 1970. The existence of morphine was demonstrated by radioimmunoassay. Identification of morphine was performed by spectroscopic methods. The isolation of mammalian morphine raises the question of biosynthesis. Recently, it has been shown that the biosynthetic pathway is similar to that that exists in poppy.

Effects of D-Ala2-D-Leu5-enkephalin, microinjected into the supraoptic and paraventricular nuclei, on urine outflow rate

The effects of D-Ala2-D-Leu5-enkephalin (DADL, a delta-opioid agonist), microinjected directly into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei, on urine outflow rate, urinary osmotic pressure, blood pressure, heart rate, respiratory rate and rectal temperature were investigated in water-loaded and ethanol-anesthetized rats. The microinjection of DADL into both the nuclei decreased urine outflow rate in a dose-dependent manner with an increase in urinary osmotic pressure, but did not change the other recorded parameters. The DADL-induced antidiuretic effect in the SON was inhibited by naloxone, but not by atropine, phenoxybenzamine, timolol nor a vasopressin antagonist, d(CH2)5-D-Tyr(Et)VAVP. The effect in the PVN was inhibited by naloxone, atropine, timolol and d(CH2)5-D-Tyr(Et)VAVP, but not by phenoxybenzamine. These results suggest that DADL causes antidiuretic effects mediated through opioid receptors in both the SON and PVN, and the underlying mechanisms are different between them. Involvement of delta-opioid receptors in the DADL-induced antidiureses was discussed.

DAMGO ([D-Ala2,NMePhe4,Gly-ol]enkephalin), but not DPLPE ([D-Pen2,L- Pen5]enkephalin), specifically inhibits methamphetamine-induced behavioral responses in the mouse

The effects of intracerebroventricular (i.c.v.) injections of mu- and delta-selective opioid agonists on the methamphetamine-induced behavioral alterations in the mouse were determined by using multi-dimensional behavioral analyses. Methamphetamine (1.0 mg/kg) produced a marked increase in linear locomotion, circling, rearing and grooming behavior. Although the mu-selective opioid agonist [D-Ala2,NMePhe4,Gly-ol]enkephalin (DAMGO) (0.003 and 0.01 microgram) itself did not significantly affect different behavioral responses, DAMGO (0.003 and/or 0.01 microgram) antagonized the methamphetamine (1.0 mg/kg)-induced increase in behavioral responses such as linear locomotion, circling, rearing and grooming. Additionally, the effects of DAMGO (0.01 microgram) on the methamphetamine (1.0 mg/kg)-induced behavioral responses were fully reversed by pretreatment with the mu-selective alkylating agent beta-funaltrexamine (beta-FNA) (5.0 micrograms). In contrast, the delta-selective opioid agonist [D-Pen2,L-Pen5]enkephalin (DPLPE) (0.3 or 1.0 microgram) had no marked effects on the methamphetamine (1.0 mg/kg)-induced behavioral responses. These results suggest that the stimulation of mu but not delta opioid receptors plays an inhibitory role in the methamphetamine-induced behavioral responses.

Multidimensional behavioral analyses show dynorphin A-(1–13) modulation of methamphetamine-induced behaviors in mice

The effects of intracerebroventricular (i.c.v.) injection of dynorphin A-(1-13) on methamphetamine-induced behavioral alterations in mice were determined by using multidimensional behavioral analyses. Methamphetamine (0.3, 1.0 and 3.0 mg/kg s.c.) produced a marked increase in linear locomotion, circling, rearing and/or grooming behaviors. The behavioral effects of methamphetamine (1.0 mg/kg s.c.) were almost completely antagonized by pretreatment with the dopamine D2 receptor antagonist, S(-)-sulpiride (3.0 and/or 10.0 mg/kg i.p.), but not with the dopamine D1 receptor antagonist, SCH 23390 (0.01 or 0.03 mg/kg i.p.). Although dynorphin A-(1-13) (3.0 or 12.5 micrograms i.c.v.) alone did not produce any significant effects on behavior, the methamphetamine (1.0 mg/kg s.c.)-induced increase in circling ipsilateral to the injection side was markedly enhanced by dynorphin A-(1-13) (12.5 micrograms i.c.v.). In contrast, the peptide (12.5 micrograms i.c.v.) inhibited the methamphetamine (1.0 mg/kg s.c.)-induced increase in rearing, whilst the increase in grooming remained unchanged. The effects of dynorphin A-(1-13) (12.5 micrograms i.c.v.) were fully reversed by the opioid antagonist, Mr 2266 (5.6 mg/kg s.c.). These results suggest that the unilateral administration (i.c.v.) of dynorphin A-(1-13) inhibits the activity of dopamine-elicited neurotransmission, resulting in an increase in ipsilateral circling and in a decrease in rearing.

The biphasic effect of morphine on odor conditioning in neonatal rats

Three experiments examined the dose-dependent biphasic effect of morphine on odor conditioning in neonatal rats. In Experiment 1, a single pairing of an odor and a low dose of morphine (0.5 mg/kg) in 5-day-old rats produced an odor preference, relative to an unpaired control group. In Experiment 2, pairing an odor with a high dose of morphine (2.0 mg/kg) produced an odor aversion, relative to an unpaired control group. A third experiment compared performance of a group given odor and morphine (2.0 mg/kg) paired to that of two unpaired groups: one given morphine 24 hr prior to and the other 24 hr after odor exposure. The paired group showed an odor aversion relative to both of the unpaired groups, which did not differ. The latter finding suggests that even if morphine metabolism is incomplete after 24 hr, behavior is unaffected. These results are discussed in reference to the functional development of the opioid system in rats.

Abnormal adrenal gland metabolism in opioid addicts: implications for clinical treatment

Adrenal gland metabolism is markedly altered in heroin addicts. During daytime hours, the addict may suffer corticoid deficiency of the addisonian type, and in the evening, an excess of the cushingoid type. The high plasma levels of cortisol that are found in the evening in addicts antagonize endogenous opioids in a manner similar to naloxone. In the present study, 72% of the heroin addicts who sought treatment demonstrated reduced adrenal cortisol reserve. Effective immune and stress responses are dependent on adrenal cortisol reserve. This finding provides an explanation for the heroin addict's vulnerability to AIDS and other infectious diseases. One of methadone's greatest attributes is that it helps normalize adrenal metabolism. Clinical methods to at least partially correct adrenal metabolism may enhance current opioid addiction treatment modalities.

Proenkephalin A-derived peptide E and its fragments alter opioid contractility in the small intestine

The human and canine small intestine exhibit increased contractility when exposed to exogenous or endogenous opioid peptides. The response of the canine small intestine to the proenkephalin A-derived peptide, peptide E and related processing fragments [Met5]enkephalin, BAM-12P, BAM-18P and BAM-22P was investigated by administering each peptide to isolated, small intestine segments which causes a significant increase in intraluminal pressure. Concentration-response curves from intraarterial bolus administration of peptide E, [Met5]enkephalin, BAM-12P, BAM-18P and BAM-22P showed decreasing efficacy with decreasing amino acid chain length while naloxone (305 nM) significantly antagonized the response. Results using the classical guinea pig ileum/myenteric plexus longitudinal muscle and mouse vas deferens bioassays with specific opioid receptor antagonists provide evidence that peptide E and BAM-18P are relatively specific to the mu opioid receptor, [Met5]enkephalin is more delta specific, BAM-22P is both mu and kappa specific and BAM-12P is kappa opioid receptor specific. These studies demonstrate that locally released (and possibly circulating) peptide E and related processing fragments increase contractility in the small intestine and may be active through more than a single receptor mechanism, particularly the mu receptor.

Ontogeny of mu-, delta- and kappa-opioid receptors mediating inhibition of neurotransmitter release and adenylate cyclase activity in rat brain

The ontogeny was examined of functional opioid receptors mediating presynaptic inhibition of neurotransmitter release and inhibition of dopamine (DA)-sensitive adenylate cyclase in the rat brain, using highly selective agonists for mu-, delta- and kappa-receptors. On gestational day 17 (E17) strong inhibitory effects of the selective mu-agonist DAGO on the electrically evoked release of [3H]noradrenaline from cortical slices and of the selective kappa-agonist U-50,488 on the electrically evoked release of [3H]DA from striatal slices were found. Electrically evoked release of [3H]acetylcholine from striatal slices was not detectable before postnatal day 7 (P7), but on that day it was already strongly inhibited by the selective delta-agonist DPDPE. Although mu- and delta-opioid receptors coupled to DA-sensitive adenylate cyclase in the striatum are likely to be physically associated in an opioid receptor complex in the adult, they were found to develop asynchronously. Whereas selective activation of mu-receptors with DAGO resulted in an inhibition of D1 dopamine receptor-stimulated adenylate cyclase activity on E17, activation of delta-receptors with DPDPE was not effective until P14. This study confirms the early appearance of mu- and kappa-opioid receptors and the relatively late development of delta-opioid receptors in the rat brain. Most importantly, it shows that in an early stage of development opioids are already able to mediate modulation of noradrenergic (via activation of mu-receptors) and dopaminergic (via activation of mu- and kappa-receptors) neurotransmission processes. Therefore, these opioid receptor types could play a role in brain development and/or developmental disturbances.

Morphine and opioid peptides selectively inhibit the non-cholinergically mediated neurogenic contraction of guinea-pig isolated bronchial muscle

The experiments examine the actions of morphine and opioid peptides on the responses evoked by electrical field stimulation or by acetylcholine (ACh) and substance P (SP) in guinea-pig bronchial strip chain. Electrical field stimulation evoked a biphasic contraction, consisting of a cholinergically mediated fast contraction followed by a non-cholinergically mediated slow contraction. Morphine and opioid peptides caused a concentration-dependent inhibition in the height of the non-cholinergic contraction. The order of inhibitory activity was BW443C greater than dynorphin greater than morphine greater than beta-endorphin greater than leucine-enkephalin greater than methionine-enkephalin. Cholinergically mediated contractions were less potently inhibited by these opioids. Submaximal contractions of bronchial muscle evoked by exogenous ACh (2 microM) or SP (0.2 microM) were not inhibited by morphine (100 microM) or opioid peptides (3-10 microM), rather, they were augmented. The results indicate that in guinea-pig isolated bronchial muscle, morphine and opioid peptides can selectively inhibit excitatory non-cholinergic neurotransmission via prejunctional opioid receptors.

Tolerance to the respiratory actions of opiates: withdrawal tolerance and asymmetrical cross-tolerance

Cross-tolerance to the respiratory depression induced by i.c.v. [D-Ala2,D-Leu5]enkephalin (DADLE) and by s.c., i.c.v. and i.v. morphine was studied in anesthetized rats that had been rendered tolerant to s.c. sufentanil (4 micrograms/h per 7 days). Tolerance induced by i.c.v. sufentanil was also compared during withdrawal and its absence. Tolerance was evaluated by estimation of the changes in both the maximum attained reduction of respiratory frequency (Emax) and the area under the time-course curve for the effects (AUC). In contrast to the failure to develop tolerance, as estimated with the Emax, after i.c.v. sufentanil in the absence of abstinence (tolerance index: 1.1), rats displayed significant degrees of cross-tolerance to i.c.v. morphine (3.6), i.c.v. DADLE (4.3) and s.c. morphine (6.6). No tolerance, however, was obtained to i.v. morphine. Tolerance to i.c.v. sufentanil in non-abstinent rats was not present when the Emax was considered but was when AUC was considered. During withdrawal animals showed tolerance as estimated with both the Emax and AUC. These results complement previous findings and show that the appearance of dissimilar degrees of cross-tolerance to different agonists depends not only on their intrinsic activities but also on their pharmacokinetic properties. These observations, along with the critical influence of withdrawal upon the manifestation of tolerance, support the need to distinguish between the development of the tolerance state and the degree of tolerance that can be expressed under specific conditions.

The effect of etorphine on nicotine- and muscarine-induced catecholamine secretion from perfused rat adrenal glands

The effect of etorphine on nicotine and muscarine-mediated catecholamine (CA) release from isolated perfused rat adrenal glands was investigated. Nicotine increased CA secretion at the low concentration of 0.5 micrograms while higher concentrations of muscarine (5 micrograms) were required. Moreover, muscarine released primarily epinephrine (EP) from rat adrenal glands while nicotine released norepinephrine (NE) and Ep. Etorphine inhibited NE and EP release evoked by nicotine to the same extent, whereas, muscarine-mediated release of NE and EP was not affected. Mecamylamine and verapamil inhibited nicotine but not muscarine-induced CA secretion. Our results suggest that etorphine preferentially interacts with nicotinic receptors on rat adrenal chromaffin cell membranes.

Activation of delta-type opioid receptors modulates the responses of cat terminal ileum to field electrical stimulation

1. The effects of (D-Ala2, D-Leu5) enkephalin amide (DADLE) on the responses of the cat terminal ileum to field electrical stimulation (pulse duration of 0.5 msec, train duration of 10 sec, 30 V) were evaluated by the changes in the contractile or the relaxatory responses of longitudinal and circular strips to electrical stimuli with a frequency of 2, 10 or 30 Hz. 2. Stimulation with a frequency of 2, 10 or 30 Hz elicited contractile responses from the longitudinal strips while in the circular strips 2 Hz stimulation induced contractions and 10 or 30 Hz stimulation caused relaxation. Tetrodotoxin (TTX) (0.1 mumol/l) abolished the electrically-induced responses in both longitudinal and circular strips. 3. DADLE (1 nmol/l) significantly inhibited the cholinergic contractile responses of the longitudinal strips to 2, 10 or 30 Hz stimulation and the contractile responses of the circular strips to 2 Hz stimulation. The relaxatory responses of the circular strips to 10 or 30 Hz stimulation were insignificantly increased by DADLE. 4. On the background of guanetidine (10 mumol/l) and atropine (3 mumol/l) DADLE significantly decreased the nonadrenergic, noncholinergic relaxatory responses of the circular strips to 2, 10 or 30 Hz stimulation. 5. DADLE did not change the maximum effects and the EC50 values of acetylcholine and noradrenaline in both longitudinal and circular strips. 6. It is suggested that in the cat terminal ileum activation of delta-type opioid receptors modulates the mechanical activity suppressing the cholinergic responses in the longitudinal and circular layers as well as the adrenergic and nonadrenergic, noncholinergic responses in the circular layer.

Effects of some autonomic drugs and neuropeptides on the mechanical activity of longitudinal and circular muscle strips isolated from the carp intestinal bulb (Cyprinus carpio)

1. The mechanical responses to some autonomic drugs and neuropeptides of longitudinal muscle (LM) and circular muscle (CM) strips isolated from the carp intestinal bulb were investigated in vitro. 2. Acetylcholine and carbamylcholine caused concentration-dependent transient contraction of both LM and CM strips. Tetrodotoxin had no effect, but atropine selectively decreased the contractile responses to acetylcholine and carbamylcholine. 3. Excitatory alpha-2 and inhibitory beta adrenoceptors were present in both LM and CM strips. 4. 5-Hydroxytryptamine (5-HT) caused concentration-dependent contraction of both LM and CM strips. Tetrodotoxin, atropine and methysergide decreased the contractile responses to 5-HT. 5. Some neuropeptides (angiotensin I, angiotensin II, bombesin, bradykinin, neurotensin, somatostatin and vasoactive intestinal polypeptide) did not cause any mechanical response (contraction or relaxation) in either smooth muscle strip. 6. Substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) caused contraction of both LM and CM strips. However, the time course of the contraction in LM was different from that in CM. The order of potency was NKA greater than SP greater than NKB in LM strips and NKA greater than SP much greater than NKB in CM strips. In LM strips, the contractile responses to tachykinins were unaffected by spantide and methysergide, but partly decreased by tetrodotoxin and atropine. On the other hand, the contractile responses of CM strips were unaffected by tetrodotoxin, atropine, methysergide and spantide. 7. Dynorphin (1-13) (DYN), leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk) caused concentration-dependent contraction of both LM and CM strips. The order of potency was DYN greater than M-Enk greater than L-Enk. Naloxone selectively decreased the responses to opiate peptides. 8. The present results indicate that acetylcholine, carbamylcholine, catecholamines, 5-HT, tachykinins (SP, NKA and NKB) and opiate peptides (DYN, L-Enk and M-Enk) affect the mechanical activity of LM and CM strips isolated from the carp intestinal bulb through their specific receptors.

Selective effects of [D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6 and [D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6 (O-t-butyl), two new enkephalin analogues, on neurotransmitter release and adenylate cyclase in rat brain slices

The selectivity and potency of two new enkephalin-derived delta-opioid receptor agonists, DSTBULET ([D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6) and BUBU ([D-Ser2(O-t-butyl),Leu5]enkephalyl-Thr6(O-t-butyl] were determined with functional tests in vitro of mu-, delta- and kappa-opioid receptor activation in the rat brain. Both peptides concentration dependently (1 nM-1 microM) inhibited the release of radiolabeled acetylcholine (ACh) from striatal slices (pD2 7.6-7.9), an effect exclusively mediated by delta-opioid receptor activation. Fentanyl isothiocyanate (FIT), an irreversible delta-antagonist, completely blocked the inhibitory effects of DSTBULET and BUBU. Up to a concentration of 1 microM, the peptides did not affect striatal [3H]dopamine (DA) release nor cortical [3H]noradrenaline (NA) release, processes which are known to be inhibited by opioids activating kappa and mu-receptors, respectively. Furthermore, both DSTBULET and BUBU caused a strong inhibition (pD2 8.2-8.3) of D-1 dopamine receptor-stimulated cyclic AMP efflux from striatal slices, an effect known to be mediated by mu- and/or delta-opioid receptor activation. However, the peptides were without effect when D-1 and D-2 dopamine receptors were stimulated simultaneously, a situation in which only mu-agonists are able to inhibit the resulting cAMP efflux. In conclusion, DSTBULET and BUBU appear to display a high selectivity and potency toward functional delta-opioid receptors in the brain.

[3H]diprenorphine binding to kappa-sites in guinea-pig and rat brain: evidence for apparent heterogeneity

The binding of the unselective opioid antagonist [3H]diprenorphine to homogenates prepared from rat brain and from guinea-pig brain and cerebellum has been studied in HEPES buffer containing 10 mM Mg2+ ions. Sequential displacement of bound [3H]diprenorphine by ligands with selectivity for mu-, delta-, and kappa-opioid receptors uncovers the multiple components of binding. In the presence of cold ligands that occupy all mu-, delta-, and kappa-sites, opioid binding still remains. This binding represents 20% of total specific sites and is displaced by naloxone. The nature of these undefined opioid binding sites is discussed.

Indirect evidence for a role of prostaglandins as second messengers of the prejunctional effect of opioids in guinea-pig ventricular preparations

The cardiac response to adrenergic nerve stimulation was dose dependently reduced in a statistically significant manner by 1-10 microM dynorphin-(1-13) in isolated atria, and by 0.1-1 microM dynorphin-(1-13) in guinea-pig ventricular preparations. The inhibitory effect of dynorphin was maintained in atria that had been pretreated with two cyclooxygenase inhibitors at concentrations that induce an 80% inhibition of the enzyme, namely indomethacin 3 microM and acetylsalicylic acid 200 microM. The inhibitory effect of dynorphin disappeared in similarly pretreated ventricular preparations. These results suggest that, whilst the mediation of the effect of dynorphin is carried out mainly by specific opioid receptors in the atrial section, in the ventricular tissue it occurs through the endogenous prostanoid system.

Differential development of acute tolerance to analgesia, respiratory depression, gastrointestinal transit and hormone release in a morphine infusion model

To determine whether the differences in development of acute tolerance to several morphine actions correlate with the mu receptor subtype mediating them, we have examined the appearance of acute tolerance to analgesia, respiratory depression, gastrointestinal transit, and hormone release in an intravenous morphine infusion model. Analgesia, a naloxonazine-sensitive mu1 action, peaked at 2 hr after initiation of the infusions. The log dose-response relationship of the infusion rate to peak tailflick latency was linear from 10 to 50 micrograms/kg/min. By 8 hr, the tailflick latencies declined nearly to baseline levels, implying the rapid development of tolerance. Tolerance to morphine-induced prolactin release, another mu1 action, also developed rapidly over 8 hr. In contrast two mu2 actions, respiratory depression measured with arterial blood gas, determinations and gastrointestinal transit, showed no significant tolerance over a similar 8 hr infusion. We also observed no tolerance to morphine-induced growth hormone release, a non-mu1 action, over the same period. Thus, these results demonstrate that mu1 actions develop tolerance in an infusion model far more rapidly than a number of naloxonazine-insensitive (non-mu1) ones and may help explain differences in the rate of tolerance development to morphine actions.

Anticonvulsant effects of mu (DAGO) and delta (DPDPE) enkephalins in rats

The effects of highly selective mu and delta opioid peptide agonists were determined in two rat models of experimentally-induced convulsions, the flurothyl threshold test and the maximal electroshock test. Intracerebroventricular injections of the mu selective enkephalin DAGO (0.3-2.2 nmol) resulted in a dose-related protection in both seizure models. Pretreatment with a low dose of naloxone (29 nmol) or the irreversible mu antagonist beta-FNA (21 nmol), but not the delta opioid antagonist ICI 154,129 (50 nmol), antagonized the anticonvulsant actions of DAGO. Intracerebroventricular injections of the delta selective enkephalin DPDPE (70-140 nmol) also resulted in seizure protection. These effects were selectively antagonized by the delta antagonist ICI 174,864 (2.8 nmol), but not by pretreatment with beta-FNA. Thus, using agonists and antagonists highly selective for mu and delta opioid receptors, anticonvulsant actions of enkephalin have been described against chemically- and electrically-induced convulsions in rats.

Analgesic and tolerance-inducing effects of the highly selective delta opioid agonist [D-Pen2,D-Pen5]enkephalin in mice

The novel and highly selective, conformationally restricted enkephalin analogue for delta-opioid receptors, [D-Pen2,D-Pen5]enkephalin (DPDPE; Pen = penicillamine), was studied in various in vivo tests for analgesia, tolerance and physical dependence. Intracerebroventricular (i.c.v.) administration of DPDPE caused a dose-dependent, naloxone-reversible antinociception, measured with the heat-irradiant (tail-flick) method. Acute tolerance developed to the antinociceptive effect of DPDPE. DPDPE also caused mild signs of physical dependence (withdrawal hypothermia and body weight loss) after repeated peptide treatment. Severe signs of morphine withdrawal (e.g. withdrawal jumping) on the other hand, could not be reversed by the administration of DPDPE. It is concluded that the activation of central delta-opioid receptors may play a role in controlling pain mechanisms, and that this activation is followed by the rapid development of a tolerance to this action.

Action of opiates on gastrointestinal function

Opioid peptides and opioid receptors are distributed along the gastrointestinal (GI) tract, indicating endogenous opiates released peripherally may modulate GI motor and secretory functions. Animal studies have revealed that the effects of opiates on gut motility depend on the nature of the subclasses of receptor involved, the species and the part of bowel. Most opiates that have a selective or predominant mu agonist activity inhibit gastric motility and delay gastric emptying by acting centrally; delta and kappa agonist are inactive when injected systemically. The effect of opiates in delaying intestinal transit observed in man, rat and other species is related to an inhibition (rat) or a stimulation (dog and man) of intestinal contractions as premature phase III-like sequences. The constipating effects of morphine probably result mainly from its action on colonic motility. Morphine stimulates colonic motility in humans by action on both central and peripheral sites. This increase in colonic motility and the delay in colonic transit is associated with a reinforcement of tonic contractions and reduced propulsive waves. Opioid peptides have been shown to participate in the colonic motor response to eating in man and animals. Both delta and mu receptors are involved in the stimulatory effects of opiates on colonic motility, while kappa receptors inhibit colonic contractions, mainly by acting centrally. The effects of opiates on gastric acid secretion are still controversial but it has been well demonstrated that opiates act centrally to reduce pancreatic secretion in rats. Opiates also inhibit intestinal secretions via an action on the enteric nervous system as well as in the CNS. All these results reinforce the hypothesis that opioid peptides have a major physiological role in the control of gut motility and secretions, and these actions explain most of the pharmacological effects of opiate substances on the digestive tract.

Characterization of rabbit ear artery opioid receptors using a delta-selective agonist and antagonist

In order to characterize pre-junctional delta opioid receptors in the rabbit ear artery, the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE), and delta-selective antagonist, ICI 174,864, were used. The selectivity of these compounds for the delta receptor was confirmed using a standard opioid sensitive preparation, the mouse was deferens. Relative potencies of a series of opioid agonists were similar in the rabbit ear artery and the mouse vas deferens. Furthermore, Ke values for IC 174,864 were not different in the two tissues. These findings demonstrate that delta opioid receptors in the rabbit ear artery and mouse vas deferens have similar properties.

Morphine- and ketocyclazocine-induced analgesia in the developing rat: differences due to type of noxious stimulus and body topography

Patterns of morphine- and ketocyclazocine-induced analgesia in limb withdrawal and tail-flick tests of thermal and mechanical nociception were examined in the preweanling rat. In the forepaw test, morphine was more effective than ketocyclazocine with both thermal and mechanical stimuli. Both drugs first induced analgesia between 3 and 5 days of age. In the tail-flick test, ketocyclazocine-induced analgesia preceded morphine's effects against both thermal and mechanical stimuli by several days. Ketocyclazocine produced robust analgesia between 7 and 10 days of age, while the effects of morphine did not peak until day 14. In the hindpaw, morphine was more effective than ketocyclazocine against a higher intensity mechanical stimulus, while ketocyclazocine was more effective against a lower intensity mechanical stimulus. Morphine-induced analgesia was reversed by lower doses of naloxone than was ketocyclazocine-induced analgesia, regardless of body part tested, against all noxious stimuli. These findings demonstrate differences in morphine- and ketocyclazocine-induced analgesia that are dependent upon age, body topography, stimulus type and intensity and imply different physiologic roles of mu- and chi-opioid receptors in analgesia.

Beta-endorphin-sensitive opioid receptors in the rat tail artery

Isolated tail arteries of rats were perfused and field-stimulated every 2 min with 2 pulses at 1 Hz. Different opioid peptides depressed the contractile responses to stimulation; their concentration-response curves showed a maximum at about 40% inhibition. The rank order of potency of the peptides was beta-endorphin (IC50 = 97 nmol/l) approximately equal to BAM-22P greater than FK-33824 greater than DAGO greater than [D-Ala2,D-Leu5]-enkephalin greater than or equal to metorphamide greater than dynorphin A-(1-13) approximately equal to [Met5]enkephalin. All these substances have in common a certain activity at opioid mu-receptors, although the enkephalins are preferential delta-, and the dynorphins preferential kappa-agonists. However, the selective delta-agonist [D-Pen2,L-Pen5]enkephalin was ineffective at up to 10 mumol/l, and the kappa-agonists ethylketocyclazocine and U-50488 acted only at concentrations higher than 3 mumol/l. Whereas the effects of beta-endorphin, DAGO and [D-Ala2,D-Leu5]enkephalin could be reduced by the mu-preferential antagonist naloxone, the effects of ethylketocyclazocine and U-50488 were not changed. The delta-selective antagonist ICI 174864 did not influence the action of [D-Ala2,D-Leu5]enkephalin. Naloxone in a concentration (1 mumol/l) which nearly abolished the effect of DAGO 3 mumol/l, slightly enhanced responses to stimulation. Neither beta-endorphin nor DAGO influenced vasoconstriction evoked by the application of noradrenaline or adenosine triphosphate; U-50488 reduced it. In arteries preincubated with [3H]noradrenaline DAGO depressed, whereas naloxone enhanced the tritium overflow and vasoconstriction evoked by field stimulation (0.4 Hz, 24 pulses every 14 min). In addition, naloxone antagonized the effect of DAGO.(ABSTRACT TRUNCATED AT 250 WORDS)

On the presence of opioid receptors in guinea-pig ventricular tissue

The cardiac response to sympathetic nerve stimulation, induced by trains of field pulses, was studied in isolated guinea-pig ventricular strips. Dynorphin-(1-13) and [D-Ala2, D-Leu5]enkephalinamide, but not morphine, reduced, in a dose-dependent manner, the cardiac sympathetic response. The effect of the two opioid peptides was antagonized by naloxone. The opioid agonists did not affect the response to exogenous noradrenaline. Neither naloxone nor a mixture of peptidase inhibitors modified the cardiac response to sympathetic nerve stimulation.

Autoradiographic localization in rat brain of kappa opiate binding sites labelled by [3H]bremazocine

[3H]Bremazocine, in the presence of saturating concentrations of mu and delta receptor blocking agents, was used to label putative kappa opiate binding sites in rat brain. The binding of [3H]bremazocine under these conditions was completely displaced with high affinity by U-50488H and dynorphin1-17, and the potency of a series of opiate ligands was consistent with an action at kappa receptors. Therefore, [3H]bremazocine, in the presence of mu and delta blockers, was used to localize U-50488H-displaceable kappa binding sites by autoradiography. A distribution different from that of mu and delta receptors was seen, with levels highest in the claustrum, striatum, medial preoptic area, suprachiasmatic nucleus, medial amygdala and superior layer of the superior colliculus. The results show that the U-50488H-displaceable kappa sites have a distinct distribution which is discussed in terms of the possible functional roles of kappa receptors.

Opioid activity and distribution of fentanyl metabolites

Fentanyl, a short-term analgesic frequently used in neuroleptanalgesia, has in a number of cases been reported to cause unexpected, severe postanesthetic respiratory depression which can successfully be treated with naloxone. Several explanations for this rebound effect produced by fentanyl (in combination with other drugs) have been proposed, though so far none has proved completely satisfactory. The possibility that this effect may be due to a secondary accumulation of fentanyl or fentanyl metabolites with opioid activity in the brain has led us to investigate the relative opioid potency of several known or proposed metabolites by measuring their inhibitory action on the contraction of guinea-pig ileum in comparison with that of morphine, pethidine, and fentanyl itself. Two proposed metabolites containing the phenethyl sidechain were found to possess an opioid activity lying between that of morphine and pethidine, whereas metabolites without the side-chain were generally less active than pethidine. Using thin-layer chromatography, it was possible to detect one of these proposed active metabolites in vivo in rats. This result may have some relevance for the understanding of the fentanyl rebound. However, the possibility that multiple doses of fentanyl, such as may be given during neuroleptanalgesia, or interactions with other drugs, e.g. tranquilizers and general anesthetics, may be the cause of fentanyl rebound, remains open.

Effects of a stable enkephalin analogue, (D-Met2,Pro5)-enkephalinamide, and naloxone on cortical blood flow and cerebral blood volume in experimental brain ischemia in anesthetized cats

The effects of intracarotid injection of the stable enkephalin analogue (D-Met2,Pro5)-enkephalinamide (ENK) and intravenous administration of naloxone on the cerebrocortical blood flow (dye dilution method) and cerebral blood volume (CBV) (photoelectric method) were investigated during unilateral brain ischemia in anesthetized cats. Both parameters were measured simultaneously in the intact and ischemic (middle cerebral artery occluded) hemispheres. An intracarotid injection of ENK 0.5 mg/kg induced a significant increase in cortical vascular resistance and a -87% decrease in cerebrocortical blood flow from 25 +/- 3 to 4 +/- 3 ml/100 g/min, without CBV alteration in the ischemic hemisphere. Naloxone (1 mg/kg i.v.), on the other hand, induced a marked two-fold increase in cerebrocortical blood flow and a significant elevation of CBV from 5.9 +/- 0.5 to 7.4 +/- 0.7 vol% in the ischemic hemisphere. No change in cerebrocortical blood flow or CBV was observed in the intact hemisphere either after ENK or after naloxone administration. Arterial blood gases and hematocrit remained unchanged. On the basis of the present findings, we conclude that besides other factors, endogenous opioid mechanisms may also participate in ischemic cerebrovascular reactions and the cerebral circulatory effects of naloxone probably reflect its opiate receptor blocking property and not simply its other non-opiate-related actions.

Kappa opioid receptor-mediated analgesia in the developing rat

The prototypic kappa opiate ketocyclazocine produced robust analgesia in 10-day-old rats in the tail-flick nociceptive test. The kappa-opiate behavioral response coincided with the onset of a rapid rise to adult levels in brain kappa receptor site density. In contrast, morphine (prototypic mu opiate) was without marked effect until 14 days of age. The period of rapid mu receptor increase did not take place until days 14-16, which was after kappa receptor levels had already plateaued. Further, there was no or incomplete cross-tolerance between ketocyclazocine and morphine at 14 days of age. The present study, therefore, establishes a role for the kappa binding site in thermal analgesia in the tail flick test and differentiates its ontogenetic pattern from that of the mu receptor.

Presynaptic opioid delta-receptors in the rabbit mesenteric artery

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation were recorded from muscle cells of the rabbit isolated mesenteric artery. At 0.03 Hz the e.j.p. amplitudes were stable. When a train of fifteen pulses was applied at 0.25 Hz or at higher frequencies (0.5, 1 and 2 Hz), e.j.p.s showed an initial facilitation followed by depression. [Met5]enkephalin 0.1 and 1 mumol/l, [D-Ala2,D-Leu5]enkephalin 0.1 and 1, but not 0.01 mumol/l, and [D-Pen2, L-Pen5]enkephalin 3 mumol/l all depressed the e.j.p.s evoked by trains of fifteen pulses at 1 Hz. When more than one concentration was used ([Met5]enkephalin, [D-Ala2,D-Leu5]enkephalin), the inhibition was concentration dependent. It was always greater for the first few e.j.p.s than for the later ones in a train. [Met5]enkephalin 1 mumol/l reduced the first e.j.p. at 1 Hz and the e.j.p.s evoked by 0.03 Hz to a similar extent. The inhibitory effect of [Met5]enkephalin 1 mumol/l on e.j.p.s persisted in the presence of yohimbine 0.3 mumol/l. Naloxone 1 mumol/l did not interfere with the effect of [Met5]enkephalin 1 mumol/l. Naloxone 10 mumol/l depressed some e.j.p.s and prevented the inhibition by [Met5]enkephalin 1 mumol/l. Neither ICI 154129 10 mumol/l nor ICI 174864 0.3 mumol/l had any effect of their own and both compounds antagonized the action of [Met5]enkephalin 1 mumol/l. Normorphine 10 mumol/l, fentanyl 1 mumol/l, ethylketocyclazocine 0.1 mumol/l, and dynorphin A(1-13) 1 mumol/l were all ineffective. Ethylketocyclazocine 1 mumol/l did not change the e.j.p.s either, but antagonized [Met5]enkephalin 1 mumol/l. [Met5]enkephalin 1 mumol/l failed to influence both the resting membrane potential of the muscle cells and the depolarizing effect of noradrenaline 3 and 30 mumol/l. We suggest that the axon terminals of post-ganglionic sympathetic neurones in the rabbit mesenteric artery possess opioid delta-, but not mu- or kappa-receptors. The activation of presynaptic delta-receptors inhibits the release of the neuroeffector transmitter. There is no evidence for any effect of co-released endogenous opioid peptides under our experimental conditions.

The significance of mu- and delta-receptors in rat pancreatic islets for the opioid-mediated insulin release

The binding and the insulinotropic effects of enkephalin analogs and of morphine were investigated in rat pancreatic islets. Binding of [3H]Met-enkephalin was saturable, specific and reversible; the rank order for inhibition competition of [3H]Met-enkephalin binding by various compounds was Met-enkephalin = D-Ala2-MePhe4, Met(0)ol enkephalin) greater than Leu-enkephalin greater than morphine with half-maximal inhibitory constants (IC50) of approx. 0.3, 0.3, 100 and greater than 100 nM, respectively. Both the natural enkephalins exerted their insulinotropic effect only at stimulatory glucose concentrations. They had a dual action; whereas insulin secretion was increased at low enkephalin concentration, this effect was reversed at higher concentrations. However, the various enkephalins exerted this effect at different concentrations; only the EC50 values (half-maximal effective concentrations) of their insulinotropic effect were in the same range as the IC50 values of inhibition of [3H]met-enkephalin binding. Cysteamine pretreatment of rats (depletion of somatostatin containing D-cells and decrease in somatostatin secretion) did not change the Met-enkephalin effect on insulin secretion. In contrast to Met-enkephalin, binding of [3H]morphine to islets was not saturable, and morphine had no effect on insulin secretion unless at unphysiologically high concentrations. The data, therefore, indicate that: mu-receptors (affinity for morphine) do not play a role in rat pancreatic islets; delta-receptors (binding site for Met-enkephalin when mu-receptors are not present) mediate the insulinotropic effect of low Met-enkephalin concentrations; and the insulinotropic action of Met-enkephalin is not mediated indirectly via the paracrine effect of an inhibition of somatostatin secretion.

Effects of kappa opiate agonists on neurochemical and neuroendocrine indices: evidence for kappa receptor subtypes

Four kappa opiate agonists, U-50488H, MR-2034, EKC and tifluadom, elevated plasma corticosterone and decreased plasma TSH in a dose-dependent manner. These effects were naloxone-reversible. However, WIN 44441-3, a long acting narcotic antagonist, was unable to reverse the effects of U-50488H and MR-2034 upto doses of 5 mg/kg. U-50488H and MR-2034 but not tifluadom or EKC, also increased levels of DOPAC and HVA in the olfactory tubercle. This effect was also naloxone-reversible but not WIN 44441-3 reversible. Tifluadom and EKC did not increase DOPAC and HVA. The differential responses of the tested kappa agonists to WIN 44441-3 antagonism and dopamine metabolism in A10 neurons suggest that the kappa agonists can be separated into two groups. This is the first physiological evidence suggestive of kappa opioid receptor subtypes.

Differential effect of stimulation strength in mouse vas deferens on inhibition of neuroeffector transmission by receptor type selective opioids

In the mouse isolated vas deferens the amplitude of excitatory junction potentials (e.j.p.s) recorded intracellularly from smooth muscle cells varied with the strength of stimulation. Receptor type selective opioids were tested in this preparation. The mu-agonist normorphine (2,000 nmol/l) reduced the amplitude of e.j.p.s and shifted the stimulus-response curve in a parallel way to the right. By contrast, the kappa-agonist U-50488 (1,000 nmol/l) and the delta-agonist [D-Ala2,D-Leu5]-enkephalin (2 nmol/l) caused a non-parallel shift of the curve. In addition, opioids having a lower selectivity for one type of receptor were also used. The preferential kappa-agonists ethylketocyclazocine (40 nmol/l) and dynorphin A1-13 (100 nmol/l) produced parallel and non-parallel shifts, respectively. Thus, normorphine and ethylketocyclazocine were more effective in depressing e.j.p.s evoked by low intensities of stimulation than those evoked by high intensities of stimulation. U-50488, dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin caused an equal depression of e.j.p.s evoked by either intensity of stimulation. The preferential mu- and delta-antagonists naloxone (1,000 nmol/l) and ICI 154129 (10,000 nmol/l), reversed the action of the respective agonists normorphine and [D-Ala2, D-Leu5]-enkephalin. In addition, ICI 154129 (10,000 nmol/l) reversed the action of dynorphin A1-13, as well. The preferential kappa-antagonist MR-2266 (1,000 nmol/l) prevented the effect of both ethylketocyclazocine and U-50488. It is concluded that under the conditions of these experiments normorphine and ethylketocyclazocine acted at mu-, U-50488 at kappa-, and dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin at delta-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

A radiolabeled-ligand-binding technique for the characterization of opioid receptors in the intact mouse vas deferens

The mouse vas deferens has served as a useful bioassay for examining the properties of opiate receptor subtypes. However, recent data indicate that the response of the vas deferens to opiates may be mediated by one or more of the several opiate receptors found in this preparation. Although a number of techniques can be utilized to assess the relative contribution of these receptors to the response of the mouse vas deferens to opiates (e.g., selective tolerance and naloxone antagonism studies), a radiolabeled-binding technique would provide an independent means of more completely characterizing the opiate receptor profiles in this preparation. Up to the present, however, there has been only limited success in developing a binding assay utilizing crude membrane fractions of the mouse vas deferens. To circumvent these problems, we have developed a binding technique utilizing the intact vas deferens. In contrast to results obtained with membrane fractions, we found highly specific (90-95%) and saturable binding of D-[2-3H]alanine, 5-D-leucine enkephalin, a ligand selective for delta opiate receptors, to the intact vas. Scatchard analyses indicated a single class of binding sites with an apparent Kd of 1.5 nM and a Bmax of approximately 12 pmol/2 vas. The selectivity of binding was also examined. Naltrexone was 40 times less potent than unlabeled 2-D-alanine, 5-D-leucine enkephalin in displacing binding, whereas morphine and ethylketocyclazocine were 300 and 500 times less effective, respectively. This technique, coupled with the mouse vas deferens bioassay, should provide a more complete characterization of opioid receptor populations than has heretofore been possible.

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.

Contractor responses of the isolated colon of the mouse to morphine and some opioid peptides

Morphine (1 X 10(-8) - 1 X 10(-4)M), fentanyl (1 X 10(-9) - 1 X 10(-5)M) and alfentanyl (1 X 10(-10) - 1 X 10(-5)M) as well as methionine enkephalin [Met5]enkephalin (1 X 10(-11) - 1 X 10(-8)M), [D-Ala2, Met5]enkephalin (1 X 10(-12) - 1 X 10(-8)M) and dynorphin A(1 - 13) (1 X 10(-9) - 1 X 10(-6)M) caused a contractor response of the longitudinal musculature of the terminal colon of the mouse. These effects were competitively antagonized by naloxone. The pA2 values obtained for naloxone antagonism of morphine and opioid peptides and the high sensitivity of the preparation to enkephalins suggest the presence of delta-opiate receptors in this preparation but mu- and kappa-receptors may also be present. Opiate-induced contractions in the mouse colon were abolished by tetrodotoxin and after incubation with indomethacin. It is concluded that the excitatory actions of the opiates in the mouse colon are mediated via opiate receptors located on nerves which do not release acetylcholine, noradrenaline or 5-hydroxytryptamine. The opiates may produce their action by removing an inhibitory neural influence (the nature of which remains to be elucidated) allowing a prostaglandin-mediated effect to predominate, thereby increasing muscle tone.