Peripheral antinociceptive effects of the cyclic endomorphin-1 analog c[YpwFG] in a mouse visceral pain model

We previously described a novel cyclic endomorphin-1 analog c[Tyr-D-Pro-D-Trp-Phe-Gly] (c[YpwFG]) acting as a mu-opioid receptor (MOR) agonist This study reports that c[YpwFG] is more lipophilic and resistant to enzymatic hydrolysis than endomorphin-1 and produces preemptive antinociception in a mouse visceral pain model when injected intraperitoneally (i p) or subcutaneously (s c) before 06% acetic acid employed to evoke abdominal writhing (1 p ED50 = 1 24 mg/kg s c ED50 = 2 13 mg/kg) This effect is reversed by the selective MOR antagonist beta-funaltrexamine and by a high dose of the mu(1) -opioid receptor-selective antagonist naloxonazine Conversely the kappa-opioid receptor antagonist nor-binaltorphimine and the delta-opioid receptor antagonist naltrindole are ineffective c[YpwFG] produces antinociception when injected 1 p after acetic acid (ED50 = 4 80 mg/kg) and only at a dose of 20 mg/kg did it elicit a moderate antinociceptive response in the mouse evaluated by the tail flick assay Administration of a lower dose of c[YpwFG] (10 mg/kg 1 p) apparently produces a considerable part of antinociception on acetic acid-Induced writhes through peripheral opioid receptors as this action is fully prevented by i p naloxone methiodide which does not readily cross the blood-brain barrier whereas this opioid antagonist injected intracerebroventricularly (i c v) is not effective Antinociception produced by a higher dose of c[YpwFG] (20 mg/kgi p) is partially reversed by naloxone methiodide i c v administered Thus only at the dose of 20 mg/kg c[YpwFG] can produce antinociception through both peripheral and central opioid receptors In conclusion c[YpwFG] displays sufficient metabolic stability to be effective after peripheral administration and demonstrates the therapeutic potential of endomorphin derivatives as novel analgesic agents to control visceral pain (C) 2010 Elsevier Inc All rights reserved