ET-1-INDUCED ATTENUATION OF MOR ACTIVITY DIFFERS BETWEEN MORPHINE AND FENTANYL
Authors: Ohshima *Kaori , Nonaka Miki , Kuroda Yui , Miyano Kanako , Takayanagi Hiroshi , Uezono Yasuhito 
Published: February 2025

Abstract

Background
Endothelin-1 (ET-1), known as a vasoconstrictor, induces pain signals through its specific receptor ETA (ETAR). We previously reported that the novel selective ETAR antagonist compound E (provided by Eisai Co., Ltd.) restored the ET-1-induced attenuation of morphine-induced analgesia with HEK293 cells stably expressing both ETAR and mu opioid receptor (ETAR-MOR cells) and with mice pain model (Kuroda et al., 2021). Further, among several opioids including morphine, we found that ET-1 almost completely suppressed the MOR activity induced by fentanyl (attenuated to 4.8%) compared to that of morphine (attenuated to 37%), indicating that analgesic effects induced by fentanyl could be more influenced by endogenous ET-1, since ET-1 was increased in chronic pain status.

Aims & Objectives
In the present study, we investigated and compared the inhibitory effects of ET-1 on MOR activity by morphine or fentanyl.

Method
MOR activity was assessed by cAMP assay. ETAR-MOR co-expressing cells were pretreated with ET-1 with or without compound E for each time period (0, 20, 55 min), followed by evaluation of MOR activity induced by morphine or fentanyl.

Results
At treatment time for 0 min (simultaneous treatment with opioids), ET-1 suppressed the morphine- or fentanyl-induced MOR activity, but no recovery of the attenuated MOR activity by compound E was observed. At treatment times of 20 or 55 min, attenuated MOR activity by ET-1 was recovered by compound E for both opioids, indicating substantial period was required for the inhibition by compound E. ET-1-induced attenuation of MOR activity by morphine was partial since attenuated MOR activities were 29% at 20 min and 37% at 55 min. On the other hand, the attenuated MOR activity by fentanyl was 37% at 20 min and 4.5% at 55 min, indicating almost complete inhibition was observed in the case of fentanyl.

Discussion & Conclusion
Prolonged ET-1 exposure attenuated the MOR activity of fentanyl more effectively than that of morphine. The difference in the response of ET-1 to the MOR activity by both opioids is currently unknown. Fentanyl binding site to MOR has been reported to be different from that of morphine, and it is plausible that such differences may be involved in the distinct attenuation of MOR activity by ET-1. The recovery effect of compound E on the attenuated analgesic effect of ET-1 suggests that pretreatment with compound E is important for maintaining the analgesic effect of opioids, especially when using fentanyl as an analgesic. We are underway to clarify the mechanisms of action of different attenuated MOR activities of ET-1 by morphine and fentanyl.

References
Y Kuroda, M Nonaka, Y Kamikubo, H Ogawa, T Murayama, N Kurebayashi, H Sakairi, K Miyano, A Komatsu, T Dodo, K Nakano-Ito, K Yamaguchi, T Sakurai, M Iseki, M Hayashida, Y Uezono. “Inhibition of endothelin A receptor by a novel, selective receptor antagonist enhances morphineinduced analgesia: Possible functional interaction of dimerized endothelin A and μ-opioid receptors.” Biomed Pharmacother. 141:111800 (2021).