Endomorphin-1 and endomorphin-2 activate mu-opioid receptors in myenteric neurons of the guinea-pig small intestine.
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.
Tonini
M
M
Department of Internal Medicine and Therapeutics, University of Pavia, Italy. tonini@ipv36.unipv.it
Fiori
E
E
Balestra
B
B
Spelta
V
V
D'Agostino
G
G
Di Nucci
A
A
Brecha
N C
NC
Sternini
C
C
D000109:Acetylcholine / Q000494:pharmacology; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005260:Female; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects; D007421:Intestine, Small / Q000187:drug effects; D008297:Male; D009197:Myenteric Plexus / Q000187:drug effects*; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000187:drug effects*; D012044:Regression Analysis; D013004:Somatostatin / Q000031:analogs & derivatives / Q000494:pharmacology
D000701:Analgesics, Opioid; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; C049352:phenylalanyl-cyclo(cysteinyltyrosyl-tryptophyl-ornithyl-threonyl-penicillamine)threoninamide; C051844:norbinaltorphimine; C105294:endomorphin 2; D013004:Somatostatin; D009271:Naltrexone; C055382:naltrindole; D000109:Acetylcholine