• M Narita, H Mizoguchi, N J Dun, B H Hwang, T Endoh, T Suzuki, H Nagase, and L F Tseng.
• Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisc. 53226, USA.
• J. Biomed. Sci. 2000 May 1; 7 (3): 221-5.

AbstractThe midbrain periaqueductal gray matter (PAG) is an important brain region for the coordination of mu-opioid-induced pharmacological actions. The present study was designed to determine whether newly isolated mu-opioid peptide endomorphins can activate G proteins through mu-opioid receptors in the PAG by monitoring the binding to membranes of the non-hydrolyzable analog of GTP, guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS). An autoradiographic [(35)S]GTPgammaS binding study showed that both endomorphin-1 and -2 produced similar anatomical distributions of activated G proteins in the mouse midbrain region. In the mouse PAG, endomorphin-1 and -2 at concentrations from 0.001 to 10 microM increased [(35)S]GTPgammaS binding in a concentration-dependent manner and reached a maximal stimulation of 74.6+/-3.8 and 72.3+/-4.0%, respectively, at 10 microM. In contrast, the synthetic selective mu-opioid receptor agonist [D-Ala(2),NHPhe(4), Gly-ol]enkephalin (DAMGO) had a much greater efficacy and produced a 112.6+/-5.1% increase of the maximal stimulation. The receptor specificity of endomorphin-stimulated [(35)S]GTPgammaS binding was verified by coincubating membranes with endomorphins in the presence of specific mu-, delta- or kappa-opioid receptor antagonists. Coincubation with selective mu-opioid receptor antagonists beta-funaltrexamine or D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH(2) (CTOP) blocked both endomorphin-1 and-2-stimulated [(35)S]GTPgammaS binding. In contrast, neither delta- nor kappa-opioid receptor antagonist had any effect on the [(35)S]GTPgammaS binding stimulated by either endomorphin-1 or -2. These findings indicate that both endomorphin-1 and -2 increase [(35)S]GTPgammaS binding by selectively stimulating mu-opioid receptors with intrinsic activity less than that of DAMGO and suggest that these new endogenous ligands might be partial agonists for mu-opioid receptors in the mouse PAG.

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