u-50488 and endomorphin-2

Multiple opioid receptor (OR) types and endogenous opioid peptides exist in the spinal dorsal horn and there may be interactions among these receptor types that involve opioid peptides. In a previous study we observed that antinociceptive effects of the selective κ-opioid receptor (κOR) agonist, U50,488H, was attenuated in μ-opioid receptor (μOR) knockout mice as compared to wild-type mice when administered spinally. This suggests that an interaction between κORs and μORs exits in the spinal cord. The present study was aimed at investigating whether endogenous opioid peptides were involved in such interaction.. We examined whether the presence of antibodies to endogenous opioid peptides, endomorphin-2, met-enkephalin and dynorphin A affected the antinociceptive effects of spinal U50,488H in rats. The tail-flick test was used to assess pain thresholds.. The increase in tail-flick latency after spinal U50,488H was attenuated when the rats were pretreated intrathecally with antiserum against endomorphin-2. Pretreatments with antisera against met-enkephalin and dynorphin A had no effect on U50,488H antinociception. The antisera alone did not affect pain threshold.. The results suggest that endomorphin-2, an endogenous opioid peptide highly selective to the μOR, plays a role in antinociception induced by κOR activation in the spinal cord.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics, Non-Narcotic; Animals; Dynorphins; Enkephalin, Methionine; Male; Oligopeptides; Pain; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; Receptors, Opioid, mu; Spinal Cord

The G-protein activation induced by mu-opioid receptor agonists was determined in spinal cord membranes from two types of mu-opioid receptor knockout mice: mice with a disruption of exon 1 (MOR (Exon 1)-KO) or exons 2 and 3 (MOR (Exons 2 and 3)-KO) of the mu-opioid receptor gene. The G-protein activation induced by the opioid agonists was measured by monitoring the increases of guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTP gamma S) binding. The mu-opioid receptor agonists (D-Ala(2),N-MePhe (4),Gly-ol(5)]enkephalin, endomorphin-1, endomorphin-2, morphine, morphine-6 beta-glucuronide, and fentanyl produced concentration-dependent increases of [(35)S]GTP gamma S binding to spinal cord membranes in wild-type mice, but not in MOR (Exon 1)-KO mice or MOR (Exons 2 and 3)-KO mice. On the other hand, the delta-opioid receptor agonist [D-Pen (2,5)]enkephalin, the kappa-opioid receptor agonist (-)U50,488H, or the ORL1-receptor agonist nociception increased [(35)S]GTP gamma S binding in the spinal cord membranes from both MOR (Exon 1)-KO mice and MOR (Exons 2 and 3)-KO mice to the same extent as in the corresponding wild-type mice. The results provide further information about the important roles of the sequences encoded within exon 1 and exons 2 and 3 of mu-opioid receptor gene for the activation of G-proteins by mu-opioid receptor agonists in the mouse spinal cord.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Animals; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Exons; Fentanyl; GTP-Binding Proteins; Guanosine 5'-O-(3-Thiotriphosphate); In Vitro Techniques; Mice; Mice, Knockout; Morphine Derivatives; Nociceptin; Nociceptin Receptor; Oligopeptides; Opioid Peptides; Radioligand Assay; Receptors, Opioid; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Spinal Cord

1. The ability of mu-opioid receptor agonists to activate G-proteins in the spinal cord of mu-opioid receptor knockout mice was examined by monitoring the binding to membranes of the non-hydrolyzable analogue of GTP, guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS). 2. In the receptor binding study, Scatchard analysis of [3H][D-Ala2,NHPhe4,Gly-ol]enkephalin ([3H]DAMGO; mu-opioid receptor ligand) binding revealed that the heterozygous mu-knockout mice displayed approximately 40% reduction in the number of mu-receptors as compared to the wild-type mice. The homozygous mu-knockout mice showed no detectable mu-binding sites. 3. The newly isolated mu-opioid peptides endomorphin-1 and -2, the synthetic selective mu-opioid receptor agonist DAMGO and the prototype of mu-opioid receptor agonist morphine each produced concentration-dependent increases in [35S]GTPgammaS binding in wild-type mice. This stimulation was reduced by 55-70% of the wild-type level in heterozygous, and virtually eliminated in homozygous knockout mice. 4. No differences in the [35S]GTPgammaS binding stimulated by specific delta1- ([D-Pen2,5]enkephalin), delta2-([D-Ala2]deltorphin II) or kappa1-(U50,488H) opioid receptor agonists were noted in mice of any of the three genotypes. 5. The data clearly indicate that mu-opioid receptor gene products play a key role in G-protein activation by endomorphins, DAMGO and morphine in the mouse spinal cord. They support the idea that mu-opioid receptor densities could be rate-limiting steps in the G-protein activation by mu-opioid receptor agonists in the spinal cord. These thus indicate a limited physiological mu-receptor reserve. Furthermore, little change in delta1-, delta2- or kappa1-opioid receptor-G-protein complex appears to accompany mu-opioid receptor gene deletions in this region.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics, Opioid; Animals; Binding, Competitive; Dose-Response Relationship, Drug; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Enkephalins; GTP-Binding Proteins; Guanosine 5'-O-(3-Thiotriphosphate); Membranes; Mice; Mice, Inbred C57BL; Mice, Inbred Strains; Mice, Knockout; Mice, Transgenic; Morphine; Oligopeptides; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu; Spinal Cord; Sulfur Radioisotopes; Tritium

We examined the effect of endogenous opioid peptides on vagally evoked release of acetylcholine (ACh) from the isolated, vascularly perfused rat stomach. The vagus nerves were electrically stimulated twice at 2.5 Hz for 2 min, and test substances were administered during the second stimulation. beta-Endorphin (10(-7) and 3 x 10(-7) M), an endogenous nonselective agonist of mu-receptors, inhibited the release of ACh. However, [Leu5]-enkephalin, an endogenous nonselective agonist of delta-receptors, and U-50488, a kappa-receptor agonist, had no effect at a higher dose of 10(-6) M. Beta-endorphin-induced inhibition was abolished by naloxone. Endomorphins 1 and 2 (3 x 10(-7) and 10(-6) M), endogenous selective agonists of mu-receptors, also inhibited the release of ACh. These results suggest that the mu-receptor is involved in the endogenous opioid peptide-induced inhibition of the release of ACh from the rat stomach.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Acetylcholine; Analgesics, Non-Narcotic; Analgesics, Opioid; Animals; beta-Endorphin; Electric Stimulation; Enkephalins; Gastric Mucosa; In Vitro Techniques; Male; Naloxone; Narcotic Antagonists; Oligopeptides; Opioid Peptides; Rats; Receptors, Opioid, mu; Stomach; Vagus Nerve