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

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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 β-endorphin and morphine given i.c.v. were studied in male ICR mice. Yohimbine (1.5 and 15 μg) and methysergide (1.5 and 15 μg) injected i.t. antagonized inhibition of the tail-flick response induced by morphine but not β-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 β-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. β-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 β-endorphin given i.c.v. Naloxone at doses (0.1 and 1 μg) which effectively reversed inhibition of the tail-flick response to i.c.v. morphine was not effective in reversing the i.c.v. β-endorphin-induced inhibition of the tail-flick response. Our results indicate that β-endorphin and morphine produce analgesia by stimulating separate types of opioid receptors, ϵ- for β-endorphin and μ- for morphine, and activate separate descending pain modulatory control systems. The supraspinal ϵ system stimulated by β-endorphin is mediated by activation of spinal opioid receptors whereas the supraspinal μ system stimulated by morphine is mediated by activation of spinal α 2-adreceptors and serotonin receptors for the production of analgesia.