Full Length Research Paper

Morphine is an exogenous ligand for MrgX2, a G protein-coupled receptor for cortistatin

Natsuyo Akuzawa¹, Hideru Obinata², Takashi Izumi², Shigeki Takeda¹*

¹Department of Nano-Material Systems, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan.
²Department of Molecular Biochemistry, Gunma University Graduate School of Medicine, Maebashi, Gunma 371-8511, Japan.
 

*Corresponding author. E-mail: stakeda@chem-bio.gunma-u.ac.jp

Accepted 14 December 2007

MrgX2 is reported to be expressed in a specific subset of dorsal root ganglion sensory neurons, and it is activated by the endogenous peptides, cortistatin and proadrenomedullin N-terminal peptide. Here, we show that morphine but not opioid peptides stimulated a dose-dependent increase of the intracellular calcium ion level in MrgX2-expressing cells. Naloxone, an opioid receptor antagonist, did not influence these reactions. It was previously reported that the m-opioid receptor did not show morphine dependent desensitization and internalization. In contrast, we detected morphine induced MrgX2 desensitization and internalization. Since MrgX2 gave a similar EC₅₀ value for morphine with the δ- and κ- opioid receptors, we consider that MrgX2 could be the physiological morphine receptor. It was reported that some morphine effects (e.g., morphine-induced hyperalgesia) were observed even in the presence of an opioid receptor antagonist. Therefore, we propose the importance of studying MrgX2 together with the classical opioid receptors for the investigation of morphine effects.

Key words: morphine, G protein-coupled receptor, MrgX2, desensitization, internalization.