naltrindole and endomorphin-1

Neuropeptide FF (NPFF) is known to be an endogenous opioid-modulating peptide. Nevertheless, very few researches focused on the interaction between NPFF and endogenous opioid peptides. In the present study, we have investigated the effects of NPFF system on the supraspinal antinociceptive effects induced by the endogenous µ-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2). In the mouse tail-flick assay, intracerebroventricular injection of EM-1 induced antinociception via µ-opioid receptor while the antinociception of intracerebroventricular injected EM-2 was mediated by both µ- and κ-opioid receptors. In addition, central administration of NPFF significantly reduced EM-1-induced central antinociception, but enhanced EM-2-induced central antinociception. The results using the selective NPFF1 and NPFF2 receptor agonists indicated that the EM-1-modulating action of NPFF was mainly mediated by NPFF2 receptor, while NPFF potentiated EM-2-induecd antinociception via both NPFF1 and NPFF2 receptors. To further investigate the roles of µ- and κ-opioid systems in the opposite effects of NPFF on central antinociception of endomprphins, the µ- and κ-opioid receptors selective agonists DAMGO and U69593, respectively, were used. Our results showed that NPFF could reduce the central antinociception of DAMGO via NPFF2 receptor and enhance the central antinociception of U69593 via both NPFF1 and NPFF2 receptors. Taken together, our data demonstrate that NPFF exerts opposite effects on central antinociception of endomorphins and provide the first evidence that NPFF potentiate antinociception of EM-2, which might result from the interaction between NPFF and κ-opioid systems.

Topics: Adamantane; Animals; Benzeneacetamides; Dipeptides; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Injections, Intraventricular; Male; Mice; Naltrexone; Nociception; Oligopeptides; Pyrrolidines

Beta-endorphin, deltorphin II, [D-Ala2, Phe4, Gly5-ol-enkephalin (DAGO) as well as endomorphin-1 and endomorphin-2 injected intracerebroventricularly (i.c.v.) induced gastroprotective action. It has been raised that endogenous opioids may have a central role in maintaining gastric mucosal integrity. Therefore we aimed to study the role of endogenous opioid system in the gastroprotective action induced by activation of alpha 2-adrenoceptors, nociceptin- and cannabinoid-receptors. Our results suggest that the non-selective opioid receptor antagonist naloxone (27 nmol i.c.v.) and the delta-opioid receptor antagonist naltrindole (5 nmol i.c.v.) abolished the mucosal protective effect of alpha 2-adrenoceptor agonists clonidine (470 pmol i.c.v.) and rilmenidine (45 pmol i.c.v.), nociceptin (1 nmol i.c.v.) and the cannabinoid receptor agonist anandamide (110 nmol i.c.v.). Based on our findings it can be raised that opioid system besides its well known regulatory functions might be involved in maintenance of gastric mucosal integrity.

Topics: Animals; Arachidonic Acids; beta-Endorphin; Clonidine; Endocannabinoids; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Ethanol; Excitatory Amino Acids; Gastric Mucosa; Injections, Intraventricular; Male; Naloxone; Naltrexone; Narcotic Antagonists; Neurotransmitter Agents; Nociceptin; Oligopeptides; Opioid Peptides; Oxazoles; Polyunsaturated Alkamides; Rats; Rats, Wistar; Receptors, Opioid; Rilmenidine; Stomach Ulcer

Our previous study showed that YGGFMKKKFMRFamide (YFa), a chimeric peptide of Met-enkephalin, and Phe-Met-Arg-Phe-NH2 induced naloxone-reversible antinociception and attenuated the development of tolerance to morphine analgesia. In continuation, the present study investigated which specific opioid receptors-mu, delta or kappa-mediate the observed YFa antinociception pharmacologically using specific antagonists and whether chronic administration of YFa at 26.01 micromol/kg per day induces tolerance and its effect on the expression of mu and kappa opioid receptors from day 4 to day 6, with endomorphine-1 (EM-1) and saline taken as positive and negative controls, respectively. Quantitative differential expression analysis was carried out by real-time reverse-transcriptase polymerase chain reaction, and the corresponding changes in protein levels were assessed by Western blot. A pharmacological investigation revealed that nor-binaltorphimine, a specific kappa opioid receptor-1 (KOR1) antagonist, completely antagonized the antinociception induced by 39.01 micromol/kg of YFa. Importantly, its chronic intraperitoneal administration did not result in significant tolerance over 6 days, whereas EM-1 induced significant tolerance after day 4. Differential expression analysis revealed that EM-1 caused up-regulation of mu opioid receptor-1 on day 4, followed by down-regulation on later days. Interestingly, YFa treatment caused a decrease on day 4, followed by an increase in the expression of KOR1 from day 5 onward. In conclusion, YFa induces kappa-specific antinociception, with no development of tolerance during 6 days of chronic treatment, which further articulates new directions for improved designing of peptide-based analgesics that may be devoid of adverse effects like tolerance.

Topics: Analgesics, Opioid; Analysis of Variance; Animals; Behavior, Animal; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Interactions; Enkephalin, Methionine; FMRFamide; Gene Expression Regulation; Male; Naltrexone; Narcotic Antagonists; Oligopeptides; Pain Measurement; Rats; Rats, Wistar; Reaction Time; Receptors, Opioid, kappa; Receptors, Opioid, mu; Time Factors

The current study investigated the roles of various subtypes of opioid receptors expressed in the thalamic nucleus submedius (Sm) in inhibition of mirror-image allodynia induced by L5/L6 spinal nerve ligation in rats. Morphine was microinjected into the Sm, which produced a dose-dependent inhibition of mirror-image allodynia; this effect was antagonized by pretreatment with non-selective opioid receptor antagonist naloxone. Microinjections of endomorphin-1 (mu-receptor agonist), or [D-Ala(2), D-Leu(5)]-enkephalin (DADLE, delta-/mu-receptor agonist), also inhibited mirror-image allodynia, and these effects were blocked by the selective mu-receptor antagonist, beta-funaltrexamine hydrochloride. The DADLE-induced inhibition, however, was not influenced by the delta-receptor antagonist naltrindole. The kappa-receptor agonist, spiradoline mesylate salt, failed to alter the mirror-image allodynia. These results suggest that Sm opioid receptor signaling is involved in inhibition of mirror-image allodynia; this effect is mediated by mu- (but not delta- and kappa-) opioid receptors in the rat model of neuropathic pain.

Topics: Animals; Behavior, Animal; Disease Models, Animal; Enkephalin, Leucine-2-Alanine; Hyperalgesia; Ligation; Male; Morphine; Naloxone; Naltrexone; Narcotic Antagonists; Neuralgia; Oligopeptides; Pain Measurement; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Spinal Nerves; Thalamic Nuclei

Endomorphin-1 and endomorphin-2 are endogenous peptides that are highly selective for mu-opioid receptors. However, studies of their functional efficacy and selectivity are controversial. In this study, we systematically compared the effects of intrathecal (i.t.) administration of endomorphin-1 and -2 on nociception assays and G protein activation with those of [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), a highly effective peptidic mu-opioid receptor agonist.. Male Sprague-Dawley rats were used. Acute and inflammatory pain models were used to compare the duration and magnitude of antinociception. Agonist-stimulated [(35)S]GTP gamma S binding was used to observe the functional activity at the level of the receptor-G protein in both spinal cord and thalamic membranes. In addition, antagonists selective for each receptor type were used to verify the functional selectivity of endomorphins in the rat spinal cord.. After i.t. administration, endomorphin-1 and -2 produced less antinociceptive effects than DAMGO in the model of acute pain. Concentration-response curves for DAMGO-, endomorphin-1-, and endomorphin-2-stimulated [(35)S]GTP gamma S binding revealed that both endomorphin-1 and -2 produced less G protein activation (i.e., approximately 50%-60%) than DAMGO did in the membranes of spinal cord and thalamus. In addition, i.t. endomorphin-induced antinociception was blocked by mu-opioid receptor selective dose of naltrexone (P < 0.05), but not by delta- and kappa-opioid receptor antagonists, naltrindole and nor-binaltorphimine (P > 0.05).. Endomorphins are partial agonists for G protein activation at spinal and thalamic mu-opioid receptors. Both in vivo and in vitro measurements together suggest that DAMGO is more effective than endomorphins. Spinal endomorphins' antinociceptive efficacy may range between 53% and 84% depending on the intensity and modality of the nociceptive stimulus.

Topics: Analgesics; Analgesics, Opioid; Animals; Behavior, Animal; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Partial Agonism; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Guanosine 5'-O-(3-Thiotriphosphate); Injections, Spinal; Male; Naltrexone; Narcotic Antagonists; Oligopeptides; Pain; Pain Measurement; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Spinal Cord; Sulfur Radioisotopes; Thalamus; Time Factors

[Dmt1]Endomorphin-1 is a novel analogue of the potent mu-opioid agonist endomorphin-1. Given the physiological role of endomorphin-1 in vivo, this compound was investigated to determine if the antinociception occurred through systemic, supraspinal or in a combination of both neuronal pathways. This compound exhibited a potent dose-dependent effect intracerebroventricularly in both spinal and supraspinal regions, and was blocked by opioid antagonist naloxone, which verified the involvement of opioid receptors. Specific opioid antagonists characterized the apparent receptor type: beta-funaltrexamine (mu1/mu2-irreversible antagonist) equally inhibited spinal- and central-mediated antinociception; on the other hand, naloxonazine (mu1-subtype) was ineffective in both neural pathways and naltrindole (delta-selective antagonist) partially (26%), though not significantly, blocked only the spinal-mediated antinociception. Therefore, spinal antinociception was primarily triggered by mu2-subtypes without involvement of mu1-opioid receptors; however, although a slight enhancement of antinociception by delta-receptors cannot be completely ruled out since functional bioactivity indicated mixed mu-agonism/delta-antagonism. In terms of the CNS action, [Dmt1]endomorphin-1 appears to act through mu2-opioid receptor subtypes.

Topics: Analgesia; Animals; Brain; Guinea Pigs; Ileum; Injections, Intraventricular; Male; Mice; Naloxone; Naltrexone; Oligopeptides; Pain; Pain Measurement; Receptors, Opioid, delta; Receptors, Opioid, mu; Spinal Cord; Tail; Vas Deferens

The effects of intracerebroventricular administration of endomorphin-1 and endomorphin-2, endogenous mu-opioid receptor agonists, on passive avoidance learning associated with long-term memory were investigated in mice. Endomorphin-1 (10 and 17.5 microg) and endomorphin-2 (17.5 microg) produced a significant decrease in step-down latency in a passive avoidance learning task. beta-Funaltrexamine (5 microg) almost completely reversed the endomorphin-1 (17.5 microg)- and endomorphin-2 (17.5 microg)-induced shortening of step-down latency, although neither naltrindole (4 ng) nor nor-binaltorphimine (4 microg) produced any significant effects on the effects of endomorphins 1 and 2. These results suggest that endomorphins 1 and 2 impair long-term memory through the mediation of mu-opioid receptors in the brain.

Topics: Analgesics, Opioid; Animals; Avoidance Learning; Behavior, Animal; Dose-Response Relationship, Drug; Male; Mice; Naltrexone; Narcotic Antagonists; Oligopeptides; Receptors, Opioid, mu

The novel opioid tetrapeptides, endomorphin-1 and endomorphin-2, recently isolated from bovine and human brain bind with high affinity and selectivity to central mu-opioid receptors. In the digestive tract, a comprehensive pharmacological analysis of the receptors involved in endomorphin action has not been reported. In this study, we analyzed the effects of endomorphin-1 and endomorphin-2 on longitudinal muscle-myenteric plexus preparations (LMMPs) from the guinea-pig ileum. Both peptides (30 pM - 1 microM) inhibited (-log EC50 values: 8.61 and 8.59, respectively) the amplitude of electrically-induced twitch contractions in a concentration-dependent fashion, up to its abolition. Conversely, in unstimulated LMMPs, they failed to affect contractions to applied acetylcholine (100 nM). In stimulated LMMPs, the highly selective mu-opioid receptor antagonist, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), caused a concentration-dependent (30 nM-1 microM), parallel rightward shift of endomorphin-1 and endomorphin-2 inhibitory curves, without depression of their maximum. Following Schild analysis, calculated pA2 values were 7.81 and 7.85, respectively, with slopes not different from unity. Concentration-response curves to both peptides were not affected by 30 nM naltrindole (a selective delta-receptor antagonist) or 30 nM nor-binaltorphimine (a selective kappa-receptor antagonist). These results demonstrate that endomorphins selectively activate mu-opioid receptors located on excitatory myenteric plexus neurons, and that they act as full agonists.

Topics: Acetylcholine; Analgesics, Opioid; Animals; Dose-Response Relationship, Drug; Electric Stimulation; Female; Guinea Pigs; Ileum; Intestine, Small; Male; Myenteric Plexus; Naltrexone; Oligopeptides; Receptors, Opioid, mu; Regression Analysis; Somatostatin