β-Endorphin-(1–27) antagonizes β-endorphin- but not morphine-, D-PEN 2- D-PEN 2-enkephalin- and U50, 488H-induced analgesia in mice

Abstract

β-Endorphin-(1–27), administered intraventricularly has been previously reported to block the analgesia induced by β-endorphin injected intraventricularly. The present study was to determine if the blocking effect of β-endorphin-(1–27) was specific to β-endorphin which stimulates epsilon receptors, but not to other opioids with activity at different opioid receptors. The antagonistic effects of β-endorphin-(1–27) on the analgesia induced by β-endorphin (epsilon-opioid receptor agonist), d-Ala 2-NMePhe 4-Gly-ol-enkephalin (DAGO) and morphine, (mu-opioid receptor agonists), d-Pen 2- d-Pen 5-enkephalin (DPDPE) and d-Ala 2- d-Leu 5-enkephalin(DADLE) (delta-opioid receptor agonists) and U-50, 488H (kappa-opioid receptor agonist) were studied. β-Endorphin-(1–27) injected intraventricularly, at doses which, when injected alone did not produce analgesia, antagonized the analgesia induced by β-endorphin given intraventricularly. However, the analgesia induced by DAGO, morphine, DPDPE, DADLE and U-50,488H given intraventricularly was not antagonized by β-endorphin-(1–27). The data suggest that β-endorphin-(1–27) selectively blocks the analgesia induced by the stimulation of epsilon receptors but not by the stimulation of mu, delta, and kappa receptors. The results support the previously proposed hypothesis that β-endorphin produces its analgesia by stimulating specific epsilon receptors.