piperidines and endomorphin-1

The endogenous N-arachidonoyl-dopamine (NADA) activates both transient receptor potential vanilloid1 (TRPV1) and cannabinoid-1 (CB(1)) receptors. The goal of this study was to characterize the antinociceptive potential of NADA on inflammatory thermal hyperalgesia in rats at spinal level, and to determine its interaction with endomorphin-1 (EM) at the spinal level. The effects of NADA and EM on thermal hyperalgesia were evaluated in rats with a unilateral hind paw carrageenan-induced inflammation. Intrathecal injection of either EM (0.03-10 μg) or NADA (1.5-50 μg) caused dose-dependent antihyperalgesia, but NADA was 5.4 times less potent than EM. The antihyperalgesia caused by 15 μg NADA was inhibited by the TRPV1 antagonist AMG9810, but not by CB(1) antagonist/inverse agonist AM 251, whereas the effect of 50 μg NADA was decreased by both drugs. Co-administration of EM with NADA in 1:15 and 1:50 ratios produced a short-lasting potentiation, but isobolographic analysis for the whole investigated period revealed additive interaction between the two endogenous ligands. The results show that both TRPV1 and CB(1) receptor activation play a substantial role in the antinociceptive effects of NADA at spinal level, while co-administration of NADA with EM did not show potentiation.

Topics: Acrylamides; Analgesics; Animals; Arachidonic Acids; Area Under Curve; Bridged Bicyclo Compounds, Heterocyclic; Dopamine; Dose-Response Relationship, Drug; Drug Synergism; Hot Temperature; Hyperalgesia; Injections, Spinal; Male; Oligopeptides; Pain Measurement; Piperidines; Pyrazoles; Rats; Rats, Wistar; Receptor, Cannabinoid, CB1; Spinal Cord; TRPV Cation Channels

Endokinins are four novel human tachykinins, including endokinins A (EKA), B (EKB), C (EKC), and D (EKD). Endokinin A/B (EKA/B) is the common C-terminal decapeptide in EKA and EKB, while endokinin C/D (EKC/D) is the common C-terminal duodecapeptide in EKC and EKD. In this study, we attempted to investigate the interactions between EKA/B, EKC/D, and endomorphin-1 (EM-1) on the depressor effect at peripheral level. The effects of EKA/B produced a U-shaped curve. The maximal effect was caused by 10 nmol/kg. EKC/D and EM-1 showed a dose-dependent relationship. Co-administration of EKA/B (0.1, 1, 10 nmol/kg) with EM-1 produced effects similar to those of EKA/B alone but slightly lower. Co-injection of EKA/B (100 nmol/kg) with EM-1 caused an effect stronger than any separate injection. Co-administration of EKC/D (10 nmol/kg) with EM-1 (30 nmol/kg) caused a depressor effect, which was one of the tradeoffs of EM-1 and EKC/D. Mechanism studies showed that SR140333B could block the depressor effects of EKA/B, EKC/D, EM-1, EKA/B+EM-1, and EKC/D+EM-1; SR48968C could block EM-1, EKA/B, EKC/D, and EKC/D+EM-1 and partially block EKA/B+EM-1; SR142801 could block EM-1, EKC/D, and EKC/D+EM-1 and partially block EKA/B and EKA/B+EM-1; naloxone could block EM-1, EKC/D, and EKC/D+EM-1 and partially block EKA/B and EKA/B+EM-1. Pretreatment with NG-nitro-l-arginine methyl ester partially decreased depressor intensity and half-recovery time of EKA/B and EKC/D.

Topics: Analgesics; Animals; Antidepressive Agents; Benzamides; Blood Pressure; Dose-Response Relationship, Drug; Drug Combinations; Drug Interactions; Male; Naloxone; Neurokinin-1 Receptor Antagonists; NG-Nitroarginine Methyl Ester; Oligopeptides; Peptide Fragments; Piperidines; Rats; Rats, Wistar; Receptors, Neurokinin-2; Tachykinins; Tropanes

This study characterized the pharmacology of the peripherally restricted opioid receptor antagonists, alvimopan, its metabolite, ADL 08-0011, and methylnaltrexone. The activities of the compounds were investigated with respect to human or guinea pig opioid receptor binding and function in recombinant cell lines and mechanical responsiveness of the guinea pig ileum. Alvimopan and ADL 08-0011 had higher binding affinity than methylnaltrexone at human mu opioid receptors (pK (i) values of 9.6, 9.6, and 8.0, respectively). The compounds had different selectivities for the mu receptor over human delta and guinea pig kappa opioid receptors. ADL 08-0011 had the highest mu receptor selectivity. With respect to their mu opioid receptor functional activity ([(35)S]GTPgammaS incorporation), methylnaltrexone had a positive intrinsic activity, consistent with partial agonism, unlike alvimopan and ADL 08-0011, which had negative intrinsic activities. Alvimopan, ADL 08-0011, and methylnaltrexone antagonized inhibitory responses mediated by the mu opioid agonist, endomorphin-1 (pA (2) values of 9.6, 9.4, and 7.6, respectively) and by U69593, a kappa opioid agonist (pA (2) values of 8.4, 7.2, and 6.7, respectively). In morphine-naive guinea pig ileum, methylnaltrexone reduced, while alvimopan and ADL 08-0011 increased, the amplitude of electrically evoked contractions and spontaneous mechanical activity. In tissue from morphine-dependent animals, alvimopan and ADL 08-0011 increased spontaneous activity to a greater degree than methylnaltrexone. The data suggested that alvimopan-induced contractions resulted predominantly from an interaction with kappa opioid receptors. It is concluded that alvimopan, ADL 08-0011, and methylnaltrexone differ in their in vitro pharmacological properties, particularly with respect to opioid receptor subtype selectivity and intrinsic activity. The clinical significance of the data from this study remains to be determined.

Topics: Analgesics, Opioid; Animals; Benzeneacetamides; CHO Cells; Cricetinae; Cricetulus; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Guanosine 5'-O-(3-Thiotriphosphate); Guinea Pigs; Humans; Ileum; In Vitro Techniques; Male; Morphine; Muscle Contraction; Naltrexone; Narcotic Antagonists; Oligopeptides; Piperidines; Pyrrolidines; Quaternary Ammonium Compounds; Receptors, Opioid; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Recombinant Proteins; Transfection

Although both pre- and postsynaptic mechanisms have been implicated in the analgesia produced by mu-opioids at the spinal cord, it is not known under what conditions these different controls come into play. Because the mu-opioid receptor (MOR) can be visualized in individual lamina II excitatory interneurons and internalizes into endosomes on ligand binding, we tested whether MOR internalization could be monitored and used to measure postsynaptic MOR signaling. To test whether endogenous opioids modulate these lamina II interneurons during noxious stimulation, we next assessed the magnitude of postsynaptic MOR internalization under a variety of nociceptive conditions. As observed in other systems, we show that MOR internalization in dorsal horn interneurons is demonstrated readily in response to opioid ligands. The MOR internalization is dose-dependent, with a similar dose-response to that observed for opioid-induced increases in potassium conductance. We demonstrate that MOR internalization in lamina II neurons correlates precisely with the extent of analgesia produced by intrathecal DAMGO. These results suggest that MOR internalization provides a good marker of MOR signaling in the spinal cord and that postsynaptic MORs on lamina II interneurons likely participate in the analgesia that is produced by exogenous opioids. We found, however, that noxious stimuli, under normal or inflammatory conditions, did not induce MOR internalization. Thus, endogenous enkephalins and endomorphins, thought to be released during noxious peripheral stimuli, do not modulate nociceptive messages via postsynaptic MORs on lamina II interneurons. We suggest that any endogenous opioids that are released by noxious stimuli target presynaptic MORs or delta-opioid receptors.

Topics: Analgesics, Opioid; Animals; Dose-Response Relationship, Drug; Drug Implants; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; In Vitro Techniques; Inflammation; Injections, Spinal; Injections, Subcutaneous; Interneurons; Ligands; Morphine; Narcotics; Oligopeptides; Pain Measurement; Piperidines; Posterior Horn Cells; Potassium; Rats; Receptors, Opioid, mu; Remifentanil; Signal Transduction; Substantia Gelatinosa; Synapses