Studies on opioid receptor selectivity of beta-endorphin antagonists

Inhibition of spinal opioid antinociception by intrathecal beta-endorphin1-27 in the rat

1. The effects of intrathecal (i.t.) administration of beta-endorphin and two shorter fragments, human and ovine beta-endorphin1-27, were examined for antinociceptive activity in the tail-flick and paw-pressure tests in the rat. Additionally, the ability of ovine beta-endorphin1-27 to influence the action of i.t. beta-endorphin, morphine and [D-Pen2-D-Pen5]enkephalin (DPDPE) was also examined in these tests. 2. After i.t. injection, beta-endorphin produced potent dose-related antinociception in the tail-flick and paw-pressure tests. Shorter endorphins produced much weaker effects. The order of antinociceptive efficacy was beta-endorphin > human beta-endorphin1-27 > ovine beta-endorphin1-27. 3. Administration of ovine beta-endorphin1-27 (0.72, 1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of beta-endorphin (2.88 nmol, i.t.) in the tail-flick and paw-pressure tests. 4. Both i.t. morphine and DPDPE produced dose-related antinociception in the tail-flick and paw-pressure tests. The potency of DPDPE was lower than that of morphine in both tests; however, the effect of DPDPE was weaker in the paw-pressure test. 5. Administration of ovine beta-endorphin1-27 (1.44 nmol, i.t.) significantly attenuated the antinociceptive effect of morphine (14.9 nmol, i.t.) in both tests and the effect of DPDPE (38.7 nmol) in the tail-flick test. 6. The results show that beta-endorphin1-27 acts as an opioid antagonist at the spinal level in the rat. Its ability to inhibit the action of morphine and DPDPE suggests that it may attenuate beta-endorphin action by an interaction with mu- and/or delta-opioid receptors.

Synthesis and receptor binding activity of elephant beta-endorphin, a beta-endorphin homolog with highly potent analgesic activity

Elephant beta-endorphin and its analog, elephant beta-endorphin(6-31) were synthesized by standard solid phase method. Receptor binding activity showed that elephant beta-endorphin was five to six times more potent than human beta-endorphin in its ability to bind to opiate receptors on rat brain membrane. In a previous study (Wong, C.-L., Wai, M.-K., Cheng, H.-C., Chung, D. & Yamashiro, D (1990) Clinical and Experimental Pharmacology and Physiology 16, 33-37), tail flick test for intracerebroventricularly administered beta-endorphin showed that the antinociceptive potency of elephant beta-endorphin was seven to eight times higher than that of human beta-endorphin in mice. Results from both studies suggest that elephant beta-endorphin was a much more potent antinociceptive agent than human beta-endorphin in tail flick test and its higher analgesic activity might be due to its higher affinity for opiate receptors in the brain.