Differential mechanisms mediating beta-endorphin- and morphine-induced analgesia in mice

• Published in: European journal of pharmacology Vol. 168; no. 1; p. 61
• Main Authors: Suh, H H, Fujimoto, J M, Tseng, L L
• Format: Journal Article
• Language: English
• Published: Netherlands 01.09.1989
• Subjects: Analgesia; Animals; beta-Endorphin - administration & dosage; beta-Endorphin - pharmacology; Injections, Intraventricular; Injections, Spinal; Male; Methysergide - pharmacology; Mice; Mice, Inbred ICR; Morphine - administration & dosage; Morphine - pharmacology; Naloxone - administration & dosage; Naloxone - pharmacology; Reaction Time - drug effects; Yohimbine - pharmacology
• ISSN: 0014-2999
• DOI: 10.1016/0014-2999(89)90633-X

Effects of yohimbine, methysergide and naloxone given intrathecally (i.t.) and naloxone given intracerebroventricularly (i.c.v.) on inhibition of the tail-flick and hot-plate response induced by beta-endorphin and morphine given i.c.v. were studied in male ICR mice. Yohimbine (1.5 and 15 micrograms) and methysergide (1.5 and 15 micrograms) injected i.t. antagonized inhibition of the tail-flick response induced by morphine but not beta-endorphin administered i.c.v. On the other hand, naloxone (20 ng) injected i.t. antagonized inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin but not morphine. Yohimbine and methysergide given i.t. did not antagonize inhibition of the hot-plate response induced by morphine nor did naloxone given i.t. antagonized i.c.v. beta-endorphin-induced inhibition of the hot-plate response. Naloxone given i.c.v. was more effective in antagonizing morphine-induced inhibition of the tail-flick and hot-plate response than inhibition induced by beta-endorphin given i.c.v. Naloxone at doses (0.1 and 1 microgram) which effectively reversed inhibition of the tail-flick response to i.c.v. morphine was not effective in reversing the i.c.v. beta-endorphin-induced inhibition of the tail-flick response. Our results indicate that beta-endorphin and morphine produce analgesia by stimulating separate types of opioid receptors, epsilon- for for beta-endorphin and mu- for morphine, and activate separate descending pain modulatory control systems. The supraspinal epsilon system stimulated by beta-endorphin is mediated by activation of spinal opioid receptors whereas the supraspinal mu system stimulated by morphine is mediated by activation of spinal alpha 2-adrenoceptors and serotonin receptors for the production of analgesia.