Selective Mu-opioid Receptor Research Articles

Mechanisms of central antitussives

This paper provides an overview of our current understanding of the central mechanisms of cough and antitussives. Systemic administration of 8-OH-DPAT at doses of 0.1 and 0.3 mg/kg, i.p. markedly reduced the number of coughs in rats in a dose-dependent manner. The antitussive effect of 8-OH-DPAT, dihydrocodeine and dextromethorphan significantly was reduced by pretreatment with methysergide, but not ketanserin. Therefore, it is possible to speculate that the 5-HT1 receptors, in particular the 5-HT1A receptors, may be more important than others with respect to the effect of antitussive drugs. DAMGO, a selective mu-opioid receptor agonist, and U-50,488H, a highly selective kappa-opioid receptor agonist, have potent antitussive effects when administered either i.c. or i.p. However, we did not observe a cough-depressant effect of DPDPE, a selective delta-opioid receptor agonist. These results indicate that the antitussive effects of opioids are mediated predominantly by mu- and kappa-opioid receptors. On the other hand, naloxonazine, a selective mu 1-opioid receptor antagonist, had no effect on the antitussive effects associated with i.c.v. DAMGO. These results indicate that mu 2-rather than mu 1-opioid receptors are involved in mu-opioid receptor-induced antitussive effects. Antitussive effects of dextromethorphan and noscapine were significantly and dose-dependently reduced by pretreatment with rimcazole, a specific antagonist of sigma sites. However, rimcazole did not have a significant effect on the antitussive effect of morphine. These results suggest that sigma sites may be involved in the antitussive mechanism of non-narcotic antitussive drugs.

A new selective antagonist of the nociceptin receptor.

[Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 has been tested in the electrically stimulated guinea pig ileum and mouse vas deferens, two nociceptin sensitive preparations. The new compound showed per se little or no effect in the two tissues, but it displaced to the right the concentration-response curves of nociceptin in a concentration-dependent manner. Schild analyses of the data indicated a competitive type of antagonism and pA2 values of 7.02 and 6.75 in the guinea-pig ileum and the mouse vas deferens, respectively. At 10 microM [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 does not modify either the inhibitory effect of deltorphin I (the selective delta opioid receptor agonist) in the mouse vas deferens or that of dermorphine (the selective mu opioid receptor agonist) in the guinea-pig ileum. The present findings indicate that [Phe1psi(CH2-NH)Gly2]NC(1-13)NH2 is a selective antagonist of the nociceptin receptor.

Central opioid receptors mediate glucoprivic inhibition of pituitary LH secretion.

The present studies investigated the significance of glucoprivic metabolic signals, particularly those of central origin, to the regulation of pituitary luteinizing hormone (LH). Groups of gonadectomized (GDX) adult male rats were treated with 2-deoxy-D-glucose (2-DG), an inhibitor of glycolysis, by either intravenous (50, 100, or 200 mg/kg) or intracerebroventricular (5, 20, or 100 microg/rat) administration. Systemic drug treatment caused a significant decrease in mean plasma LH levels compared with saline-treated controls. Intracerebroventricular administration of 2-DG was also efficacious in suppressing circulating LH; animals treated with either of the two highest doses of the drug exhibited a significant reduction in plasma LH. In vitro studies examined direct effects of 2-DG on pituitary gonadotrope secretory activity. Exposure of anterior pituitary tissue to 2-DG during short-term perfusion had no significant impact upon either basal or gonadotropin-releasing hormone-stimulated LH release. Finally, groups of GDX rats were pretreated by intracerebroventricular administration of either the nonselective opioid receptor antagonist, naltrexone, or the selective mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), before intravenous injection of 2-DG. Both receptor antagonists were observed to attenuate the suppressive effects of 2-DG on circulating LH in these animals. In summary, treatment of GDX rats with the glucose antimetabolite, 2-DG, decreased plasma LH, suggesting that metabolic signaling of cellular glucose oxidation is of physiological importance to the regulation of pituitary hormone secretion. Findings that plasma LH was diminished in animals treated intracerebroventricularly with 2-DG implicate central glucoprivic receptors in neuroendocrine mechanisms governing the reproductive endocrine axis. Attenuation of 2-DG-induced decreases in circulating LH by opioid receptor antagonists suggests that these receptors, particularly the mu-subtype, mediate central effects of glucoprivation on circulating LH.

Potentiation of delta 9-tetrahydrocannabinol-induced analgesia by morphine in mice: involvement of mu- and kappa-opioid receptors.

The antinociceptive effect of peripheral delta 9-tetrahydrocannabinol was examined in mice previously treated with an inactive dose of morphine. The ED50 of delta 9-tetrahydrocannabinol was significantly reduced by morphine, both in the tail-flick test (0.85 vs. 2.10 mg/kg) and in the hot-plate test (1.51 vs. 4.71 mg/kg and 0.73 vs. 2.47 mg/kg in jumping and paw-lick responses, respectively). The synergistic effect between morphine and delta 9-tetrahydrocannabinol was partially blocked by the cannabinoid receptor antagonist, SR-141,716 A [(N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichorophenyl)-4-methyl-3 -pyrazolecarboxamide, hydrochloride)], at a dose of 2 mg/kg (i.p.) as well as by the opioid receptor antagonist naloxone, at the dose of 1 mg/kg (s.c.). Such an effect was also blocked by i.t. nor-binaltorphimine (a kappa-selective opioid receptor antagonist) given at 20 micrograms/mouse as well as by beta-funaltrexamine (a mu-selective opioid receptor antagonist) at a dose of 2 nmol/mouse (i.c.v., 24 h before the test). Accordingly, the mu-opioid receptor agonist DAMGO ([D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin) potentiated the effect of delta 9-tetrahydrocannabinol. These data show that the synergism between morphine and delta 9-tetrahydrocannabinol appears to involve cannabinoid as well as mu-supraspinal and kappa-spinal opioid receptors.

Opioids modulate the cholinergic contraction but not the nonadrenergic relaxation in guinea-pig airways in vitro.

(D-ALa2, NMePhe4, Gly-ol5) encephalin (DAMGO), a selective mu-opioid receptor agonist, has previously been demonstrated to inhibit the cholinergic and the noncholinergic contraction in guinea-pig airways. In contrast, opioids had no inhibitory effect on cholinergic neurotransmission in the upper trachea when stimulated at 8 Hz. We investigated whether DAMGO, a selective mu-opioid receptor agonist, [D-Pen 2,5] encephalin (DPDPE), a selective delta-opioid receptor agonist, and U-69593, a selective kappa-opioid receptor agonist could modulate the cholinergic contraction in the upper trachea at different frequencies of stimulation. Moreover, we have investigated whether DAMGO, DPDPE and U-69593 could also modulate the iNANC relaxation. DAMGO (1-100 microM) inhibited the cholinergic contraction in the upper trachea with a maximum inhibition of 57+/-15% at 1 Hz (n=4; p<0.05). On the other hand, DPDPE (10 microM) and U69593 (10 microM) did not produce any significant inhibition of the cholinergic contraction. Naloxone, an opioid receptor antagonist (100 microM), was able to antagonize the inhibitory effect of DAMGO (n=5; p<0.01) on the cholinergic contraction at a frequency of 2 Hz. DAMGO (10 microM) did not displace the cumulative concentration-response relationship to acetylcholine (10 nM-10 mM), (n=4; NS). This provides evidence that prejunctional mu-opioid receptors (and not delta-opioid or kappa-opioid receptors) modulate cholinergic contraction in the upper trachea. In contrast, DAMGO (10 microM) had no significant inhibitory effect on the nonadrenergic relaxation (n=4; NS) in the upper trachea. Neither DPDPE nor U-69593 had any effect on the nonadrenergic relaxation. These findings suggest that DAMGO directly inhibits the cholinergic contraction and that the opioid receptor involved in the inhibition of the cholinergic contraction in the upper trachea is of the mu-opioid type. The finding that opioids inhibit cholinergic contraction without altering NANC relaxations suggests that distinct populations of nerves mediate these two effects.

Opioids modulate N-methyl-D-aspartic acid (NMDA)-evoked responses of trigeminothalamic neurons.

1. The present study investigated opioid-mediated modulation of N-methyl-D-aspartic acid (NMDA)-evoked responses of trigeminothalamic neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in rats anesthetized with urethane. 2. Microiontophoretic application of NMDA activated 18/19 trigeminothalamic neurons. Administration of [D-Ala2, N-Me-Phe4,Gly5-ol]-Enkephalin, a selective mu-opioid receptor agonist, reduced the NMDA-evoked responses in 77% of trigeminothalamic neurons. [D-Pen2,5]-Enkephalin, a selective delta-opioid receptor agonist, produced inhibition of NMDA-evoked responses in 36% of neurons. 3. We suggest that 1) NMDA-receptor activation excites trigeminothalamic nociceptive neurons and may, therefore, mediate nociceptive transmission in the medullary dorsal horn; and 2) the predominantly inhibitory modulation of NMDA-receptor-mediated responses of nociceptive trigeminothalamic neurons by activation of mu- and delta-opioid receptors may provide a neural mechanism for the antinociceptive actions of opioids.

High-threshold Ca2+ currents in rat hippocampal interneurones and their selective inhibition by activation of GABA(B) receptors.

1. Whole-cell calcium currents were recorded from visually identified inhibitory interneurones located in stratum radiatum (near the border with stratum lacunosum-moleculare) of area. CA1 in rat hippocampal slices. Current-voltage (I-V) relationships in relatively well-clamped neurones showed that inward current activated between -50 and -40 mV (holding potential, -80 mV) and was maximal near -10 mV. Currents showed little inactivation over the course of 85 ms steps, and were completely blocked by removal of Ca2+ or addition of Cd2+. Prominent low-threshold currents were not observed under these conditions. 2. The calcium channels contributing to whole-cell currents in interneurones were examined using selective channel antagonists. The selective N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX-GVIA; 10 microM) irreversibly blocked 23.2 +/- 2.8% of whole-cell currents. The P/Q-type antagonist omega-agatoxin IVA (omega-Aga-IVA; 1-5 microM) blocked 10.4 +/- 3.3% of whole-cell currents. Block by omega-Aga-IVA was highly variable, ranging from 0 to 30%. The less selective conotoxin, omega-conotoxin MVIIC (omega-CTX-MVIIC; 5 microM) blocked 31.0 +/- 2.7% of whole-cell currents. The selective L-type channel antagonist nifedipine (20 microM) blocked 27.5 +/- 3.5% of whole-cell currents. 3. Whole-cell calcium currents were reversibly inhibited by the selective GABA(B) receptor agonists (+/-)-baclofen or CGP 27492 (1-3 microM; 18.9 +/- 1.4%). This inhibition was reversed or prevented by the selective GABAB receptor antagonist CGP 55845A (1 microM). Inhibition of inward current activated by voltage ramps was voltage dependent, being greatest near -10 mV, and less pronounced at more positive or negative potentials. Inhibition of calcium currents by GABAB receptor agonists was accompanied by an apparent change in the kinetics of whole-cell currents consistent with a slowing of the rate of activation. CGP 27492 depressed calcium currents by 16.1 +/- 1.9% before application of omega-CgTX-GVIA, and by 3.9 +/- 2.0% after application of omega-CgTX-GVIA in the same cells (P < 0.005), consistent with preferential block of N-type calcium channels. 4. Neither adenosine (200 microM) nor the selective mu-opioid receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol (DAMGO; 2 microM) inhibited calcium currents. Similarly, CGP 27492, but not adenosine or DAMGO, induced an outward current (at - 70 mV) consistent with activation of inwardly rectifying potassium channels. 5. These results indicate that hippocampal inhibitory neurones located in stratum radiatum possess multiple calcium channel subtypes, including N-type, L-type, and at least two other types of high-threshold channels. Activation of GABAB receptors (but not adenosine or mu-opioid receptors) preferentially inhibits N-type channels in these neurones. Similar inhibition occurring in the terminals of interneurones could contribute to depression of inhibitory synaptic transmission by activation of GABAB autoreceptors.

Opioid receptor agonists selective for mu and kappa receptors attenuate methamphetamine-induced behavioral sensitization in the mouse.

The effects of intracerebroventricular (i.c.v.) injection of the mu-selective opioid receptor agonist [D-Ala2, N-MePhe4, Gly-ol]enkephalin (DAMGO) and the kappa-selective opioid receptor agonist dynorphin A-(1-13) on the development of methamphetamine-induced behavioral sensitization in the mouse were determined using multidimensional behavioral analyses based upon a capacitance system. Methamphetamine (2 mg/kg, s.c.) was administered to mice on 6 occasions at 3- or 4-d intervals. The methamphetamine-induced increase in linear locomotion and circling was markedly augmented by repeated administrations (3 or more times) of the drug, showing behavioral sensitization. Although repeated administrations of DAMGO (0.003 and 0.01 microgram, i.c.v.) or dynorphin A-(1-13) (3 and 12.5 micrograms, i.c.v.) alone did not produce any significant effects on behavior, repeated administrations of DAMGO (0.003 and 0.01 microgram, i.c.v.) and dynorphin A-(1-13) (3 and 12.5 micrograms, i.c.v.) attenuated the behavioral sensitization induced by methamphetamine (2 mg/kg, s.c.). The attenuating effects of DAMGO (0.003 and 0.01 microgram, i.c.v.) and dynorphin A-(1-13) (3 and 12.5 micrograms, i.c.v.) were fully reversed by withdrawal of these drugs for 3 weeks. Additionally, a single administration of DAMGO (0.003 and 0.01 microgram, i.c.v.) or dynorphin A-(1-13) (3 and 12.5 micrograms, i.c.v.) alone did not produce any significant effects on behavior; DAMGO (0.003 and 0.01 microgram, i.c.v.) and dynorphin A-(1-13) (3 and 12.5 micrograms, i.c.v.) only attenuated the behavioral sensitization which had previously been developed by methamphetamine (2 mg/kg,s.c.). These results suggest that opioid receptor agonists selective for mu and kappa receptors play an inhibitory role in the development of methamphetamine-induced behavioral sensitization.

Studies on mu and delta opioid receptor selectivity utilizing chimeric and site-mutagenized receptors.

Opioid receptors are members of the guanine nucleotide binding protein (G protein)-coupled receptor family. Three types of opioid receptors have been cloned and characterized and are referred to as the delta, kappa and mu types. Analysis of receptor chimeras and site-directed mutant receptors has provided a great deal of information about functionally important amino acid side chains that constitute the ligand-binding domains and G-protein-coupling domains of G-protein-coupled receptors. We have constructed delta/mu opioid receptor chimeras that were express in human embryonic kidney 293 cells in order to define receptor domains that are responsible for receptor type selectivity. All chimeric receptors and wild-type delta and mu opioid receptors displayed high-affinity binding of etorphine (an agonist), naloxone (an antagonist), and bremazocine (a mixed agonist/antagonist). In contrast, chimeras that lacked the putative first extracellular loop of the mu receptor did not bind the mu-selective peptide [D-Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO). Chimeras that lacked the putative third extracellular loop of the delta receptor did not bind the delta-selective peptide, [D-Ser2,D-Leu5]enkephalin-Thr (DSLET). Point mutations in the putative third extracellular loop of the wild-type delta receptor that converted vicinal arginine residues to glutamine abolished DSLET binding while not affecting bremazocine, etorphine, and naltrindole binding. We conclude that amino acids in the putative first extracellular loop of the mu receptor are critical for high-affinity DAMGO binding and that arginine residues in the putative third extracellular loop of the delta receptor are important for high-affinity DSLET binding.

Electrochemical evidence of increased dopamine transmission in prefrontal cortex and nucleus accumbens elicited by ventral tegmental mu-opioid receptor activation in freely behaving rats.

Chronoamperometry was used in combination with monoamine-selective electrodes to monitor, in nucleus accumbens (NAcc) and prefrontal cortex (PFC) of freely behaving rats, changes in dopamine (DA)-like electrochemical signals elicited by unilateral ventral tegmental microinjections of the selective mu-opioid receptor agonist D-Ala, N-Me-Phe-Gly-Ol-Enkephalin (DAMGO; 0.01, 0.1, and 1.0 nmol). The results show that DAMGO dose-dependently increased electrochemical signals both in Nacc and PFC within a few minutes of injection. While DAMGO elicited signal increases of comparable amplitudes in both regions, the increases recorded in PFC were significantly longer lasting than those in NAcc; at the highest dose tested (1.0 nmol), DAMGO produced signal increases that lasted (mean +/- sem) 129 +/- 7.3 min in PFC and 96 +/- 12.5 min in NAcc. Pretreatment with the opioid receptor antagonist, naloxone (2 mg/kg, sc), significantly attenuated the peak amplitude and reduced the duration of DAMGO-induced (0.1 nmol) signal increases both in PFC and NAcc. In contrast, pretreatment with apomorphine (50 micrograms/kg, sc), a D1/D2 DA receptor agonist, significantly reduced the duration and the rate of rise of the signal increases in both regions but had little effect on the peak increases in signal. Unilateral ventral tegmental DAMGO administration (0.01, 0.1, and 1.0 nmol) also caused dose-dependent increases in contraversive circling the duration of which approximated that of the signal increases recorded in NAcc. However, differences in the time courses of DAMGO-induced contraversive circling and signal increases in NAcc suggest that the behavioral stimulant effect of ventral tegmental mu-opioid receptor activation may not be mediated exclusively by meso-NAcc DA neurons. The results of this study suggest that enkephalins modulate the activity of meso-PFC DA neurons and that behaviorally relevant activation of mu-opioid receptors in the ventral tegmental area increases DA transmission in PFC to a same, if not to a greater extent as in NAcc. These findings are discussed in relation to evidence indicating that the response of meso-NAcc DA neurons to a variety of stimuli, including drugs of abuse, is indirectly regulated by a DA-sensitive neurons in PFC.

The antinociception induced by \-endorphin administered intrathecally is mediated by the activation of ? and ?-opioid receptors in the rat

The antinociception induced by beta-endorphin given supraspinally has been previously demonstrated to be mediated by the stimulation of epsilon-, but not mu-, delta- or kappa-opioid receptors in rats and mice. The present study was designed to determine what types of opioid receptors in the spinal cord are involved in the antinociception induced by intrathecally (i.t.) administered beta-endorphin. Antinociception was assessed by the tail-flick test in male Sprague-Dawley rats. CTOP (0.9-6.6 nmol), a selective mu-opioid receptor antagonist, or nor-BNI(13.6-95.3 nmol), a selective kappa-opioid receptor antagonist, given i.t. dose-dependently reversed i.t. administered beta-endorphin-induced inhibition of the tail-flick response. On the other hand, naltrindole (6.6-44.4 nmol), a selective delta-opioid receptor antagonist, or beta-endorphin (1-27) (1-6.7 nmol), a selective epsilon-opioid receptor antagonist given i.t., did not antagonize the inhibition of the tail-flick response induced by i.t. administered beta-endorphin. The results are consistent with the previous study in mice [Tseng LF and Collins KA (1992) Eur J Pharmacol 214: 59-65] that the antinociception induced by beta-endorphin given i.t. is mediated by the stimulation of mu- and kappa-, but not delta- and epsilon-opioid receptors.

Involvement of the mesolimbic dopamine system in mediating the aversive effects of opioid antagonists in the rat

An unbiased place preference conditioning procedure was used to examine the neural substrates mediating the aversive effects of opioid antagonists in the rat. Microinjection of the non-selective opioid receptor antagonist naloxone into either the ventral tegmental area (VTA) or the nucleus accumbens (NAc) produced significant aversions for an environment previously paired with its administration. The minimum doses producing these effects were 10.0 and 7.5µg, respectively. Microinjections into either the caudate/putamen or medial prefrontal cortex were without effect. Place aversions of equivalent magnitude were also observed in response to the intra-VTA or intra-NAc administration of the highly selective mu opioid receptor antagonist CTOP. Doses as low as 0.3µg resulted in significant effects. 6-hydroxydopamine lesions of the NAc attenuated the aversive effect of intra-VTA CTOP. Such lesions did not modify the aversive effects of intra-NAc CTOP; they also failed to modify the aversive effects of systemically administered naloxone. These data demonstrate that the blockade of either VTA or NAc mu opioid receptors is sufficient for the expression of the aversive effects of opioid antagonists. Furthermore, they suggest that whereas the aversive effects of intracranially applied opioid antagonists may involve both a mesolimbic DA-dependent (VTA) and independent (NAc) component, the aversive effects produced by systemically administered opioid receptor antagonists are independent of mesolimbic DA neurons.

Unexpected prosecretory action component of loperamide at mu-opioid receptors in the guinea-pig colonic mucosa in vitro.

1. In a voltage clamp setting (Ussing chamber), the antidiarrhoeal drug, loperamide (Lop) slightly augmented prostaglandin E1 (PGE1) plus theophylline-stimulated net chloride secretion above control values at low concentrations (10(-10) and 10(-9) M) but inhibited it at higher concentrations (10(-6) and 10(-5) M). The apparently weak prosecretory action component of Lop was turned into a clear cut antisecretory effect by pretreatment with 2 x 10(-7) M naloxonazine plus 10(-7) M CTOP-NH2 (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2), two selective mu opioid receptor antagonists. This indicates a prosecretory effect of loperamide at mu opioid receptors. The antisecretory effect of low Lop concentrations, uncovered by mu opioid receptor blockade, was prevented by additional blockade of kappa opioid receptors by 5 x 10(-9) M nor-binaltorphimine (nor-BNI). 2. The nonselective opioid antagonist, naloxone, at 10(-6) M did not significantly reduce either PGE1 plus theophylline-stimulated net chloride secretion in Lop-free controls or the antisecretory action of Lop. By contrast, the partial agonist ethylketocyclazocine (EKC), which activates kappa but blocks mu opioid receptors, concentration-dependently inhibited PGE1 plus theophylline-stimulated net chloride secretion without any consistent prosecretory action component. Nor-BNI at 5 x 10(-8) M significantly blocked the antisecretory action of EKC. 3. It is concluded that, in the guinea-pig colonic mucosa under the present conditions, mu opioid receptors mediate enhancement and kappa opioid receptors inhibition of PGE1-stimulated net chloride secretion by low Lop concentrations. The two opposite actions are largely masked by superimposition. An opioid receptor-independent mechanism of action contributes to the antisecretory effect of Lop at high concentrations.

Acute tolerance to the excitatory effects of opioids in the rat hippocampus.

Prolonged iontophoretic administrations of delta- and mu-selective opioid receptor agonists were conducted in the hippocampus of rats, in order to study the possible development of acute tolerance to the excitatory effects of the opioids. Acute tolerance (AT) to the excitatory effects of the delta-selective opioid receptor agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) was observed when the drug was applied locally for 3-5 min in the CA1 hippocampal pyramidal neurons. The acute tolerance was expressed as a decrease in the commissurally evoked spike responsiveness during peptide's administration and led to a long-lasting potentiation of the population spike (PS) upon its withdrawal. In all cases, where AT and spike potentiation were evident, the population excitatory postsynaptic potential (pEPSP) remained unaltered. Pharmacological studies of AT and long-lasting spike potentiation showed the following: (1) the nonselective opioid receptor antagonist, naloxone, while effective in blocking the excitatory effects of DSLET when applied prior and during the application of the latter, failed to exhibit any effect on the long-lasting potentiating effect of the opioid; and (2) during the spike potentiation phase, administration of DSLET exhibited a depressant effect towards baseline values. This depressant effect of the opioid was evident 2-3 min from the beginning of the application and was completely antagonized by naloxone. The above results show that the development of acute tolerance to the excitatory effects of the DSLET led to long-lasting spike potentiation, which manifests a withdrawal phenomenon.

Spinal serotonin IA and IC/2 receptors mediate supraspinal mu opioid-induced analgesia.

Intracerebroventricular (i.c.v.) injection of highly selective mu opioid receptor agonist ohmefentanyl (OMF) to rats produced dose-dependent antinociception as assessed with the tail flick test. This analgesia could be blocked by intrathecal (i.t.) injection of the 5-HT1A receptor antagonist spiperone or the 5-HT1C/2 receptor antagonist mianserin, but not by the 5-HT2 receptor antagonist 1-NP or the 5-HT3 receptor antagonist ICS 205-930. The results suggest that the descending 5-HT system is involved in mediating spinal mu opioid analgesia via spinal 5-HT1A and 5-HT1C/2 receptors.

Participation of opioid receptor types on post-tetanic twitch inhibition in isolated guinea-pig heal longitudinal muscle

The participation of opioid receptor types in the inhibition of the twitch (0.1 Hz, 0.5 msec duration, maximum intensity) following high frequency stimulation (10 Hz, 0.5 msec duration, maximum intensity for 1 min) (post-tetanic twitch inhibition) was investigated in isolated guinea-pig ileal longitudinal muscle using highly selective mu, kappa and delta opioid receptor antagonists. The mu antagonist, β-funaltrexamine (β-FNA), produced a concentration-dependent twitch inhibition, which disappeared with thorough washing. The concentration-response curve of the twitch inhibition for morphine was shifted rightward about 10–20-fold by β-FNA pre-exposure, whereas that for dynorphin 1–13 inhibition remained unchanged. However, after thorough washing of β-FNA the twitch inhibition of morphine continued during the post-tetanic twitch inhibition for as long as it was tested. The post-tetanic twitch inhibition was partially antagonized by β-FNA pretreatment and further antagonized by additional exposure to naloxone (NLX). In the presence of nor-binaltorphimine (nor-BNI), a kappa antagonist, which has no effect on the twitch inhibition of morphine, the concentration-response curve of the twitch inhibition of dynorphin 1–13 was shifted rightward approx 100-fold. The post-tetanic twitch inhibition was antagonized by nor-BNI, and no further antagonism was produced by NLX. (Allyl) 2-Tyr-Aib-Aib-Phe-Leu (ICI 174,864), a delta antagonist, did not affect the post-tetanic twitch inhibition. These results suggest that both mu and kappa opioid receptors participate in post-tetanic twitch inhibition in isolated guinea-pig ileal myenteric plexus-longitudinal muscle (MPLM).

Different aromatic ring separations in enkephalins and nonpeptide opioids may influence selectivity for MU and delta opioid receptors

Different aromatic ring separations in enkephalins and nonpeptide opioids may influence selectivity for MU and delta opioid receptors

Blockade of delta-opioid receptors prevents morphine-induced place preference in mice.

Effects of highly selective delta-opioid receptor antagonists on the morphine-induced place preference in ddY and mu 1-opioid receptor deficient CXBK mice were investigated. Pretreatment with naltrindole (NTI: a non-selective delta-opioid receptor antagonist), 7-benzylidenenaltrexone (BNTX: a selective delta 1-opioid receptor antagonist) or naltriben (NTB: a selective delta 2-opioid receptor antagonist) abolished the morphine-induced place preference in ddY mice in a dose-dependent manner. These findings suggest that the morphine-induced place preference may be mediated by both delta 1- and delta 2-opioid receptors. On the other hand, in mu 1-opioid receptor deficient CXBK mice, pretreatment with these selective delta-opioid receptor antagonists did not affect the morphine-induced place preference, although pretreatment with beta-funaltrexamine (beta-FNA: a selective mu-opioid receptor antagonist) significantly inhibited the morphine-induced place preference. [D-Pen2,D-Pen5]enkephalin (DPDPE: a delta 1-opioid receptor agonist) and [D-Ala2,Glu4]deltorphin (deltorphin II: a delta 2-opioid receptor agonist) induced a significant place preference in ddY mice, but not in CXBK mice. These results suggest that delta 1- and delta 2-opioid receptors in the nucleus accumbens that are related to the DPDPE- and deltorphin II-induced place preference may be dysfunctional and/or poor in CXBK mice. These findings also indicate that delta 1- and delta 2-opioid receptors may be involved in the modulation of the reinforcing effect of morphine.

Cloning and pharmacological characterization of a rat kappa opioid receptor.

A full-length cDNA was isolated from a rat striatal library by using low-stringency screening with two PCR fragments, one spanning transmembrane domains 3-6 of the mouse delta opioid receptor and the other unidentified but homologous to the mouse delta receptor from rat brain. The novel cDNA had a long open reading frame encoding a protein of 380 residues with 59% identity to the mouse delta receptor and topography consistent with a seven-helix guanine nucleotide-binding protein-coupled receptor. COS-1 cells transfected with the coding region of this clone showed high-affinity binding to kappa opioid receptor-selective ligands such as dynorphin A and U-50,488 and also nonselective opioid ligands such as bremazocine, ethylketocyclazocine, and naloxone. Not bound at all (or bound with low affinity) were dynorphin A-(2-13), enantiomers of naloxone and levophanol [i.e., (+)-naloxone and dextrorphan], and selective mu and delta opioid receptor ligands. Activation of the expressed receptor by kappa receptor agonists led to inhibition of cAMP. Finally, in situ hybridization revealed a mRNA distribution in rat brain that corresponded well to the distribution of binding sites labeled with kappa-selective ligands. These observations indicate that we have cloned a cDNA encoding a rat kappa receptor of the kappa 1 subtype.

Delta-opiod receptor-mediated forced swimming stress-induced antinociception in the formalin test.

Forced swimming stress-induced antinociception (FSSIA) was assessed using the formalin test. Male ICR mice, weighing about 30 g, were forced to swim in water at 20 degrees C for 3 min. In unstressed mice, SC injection of formalin (0.5%) to the hindpaw caused a biphasic response: an immediate nociceptive response (first phase) followed by a tonic response (second phase). Although forced swimming stress (FSS) had no effect on the duration of the first-phase response, FSS significantly reduced the duration of the second-phase response. The effect of FSSIA on the second-phase response was blocked by naltrindole (1 mg/kg, SC), a selective delta-opioid receptor antagonist, but not by beta-funaltrexamine (20 mg/kg, SC), a selective mu-opioid receptor antagonist. These results indicate that FSS may selectively reduce the second phase of the formalin-induced nociceptive response, primarily through delta-opioid receptors.