The midbrain periaqueductal gray matter (PAG) is an important brain region for the coordination of μ-opioid-induced pharmacological actions. The present study was designed to determine whether newly isolated μ-opioid peptide endomorphins can activate G proteins through μ-opioid receptors in the PAG by monitoring the binding to membranes of the non-hydrolyzable analog of GTP, guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS). An autoradiographic [35S]GTPγS 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 μM increased [35S]GTPγS binding in a concentration-dependent manner and reached a maximal stimulation of 74.6 ± 3.8 and 72.3 ± 4.0%, respectively, at 10 μM. In contrast, the synthetic selective μ-opioid receptor agonist [D-Ala2,NHPhe4,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 [35S]GTPγS binding was verified by coincubating membranes with endomorphins in the presence of specific μ-, δ- or κ-opioid receptor antagonists. Coincubation with selective μ-opioid receptor antagonists β- funaltrexamine or D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Phe-Thr-NH2 (CTOP) blocked both endomorphin-1 and-2-stimulated [35S]GTPγS binding. In contrast, neither δ- nor κ-opioid receptor antagonist had any effect on the [35S]GTPγS binding stimulated by either endomorphin-1 or -2. These findings indicate that both endomorphin-1 and -2 increase [35S]GTPγS binding by selectively stimulating μ-opioid receptors with intrinsic activity less than that of DAMGO and suggest that these new endogenous ligands might be partial agonists for μ-opioid receptors in the mouse PAG.
| Identifer | oai:union.ndltd.org:ETSU/oai:dc.etsu.edu:etsu-works-15878 |
| Date | 01 January 2000 |
| Creators | Narita, Minoru, Mizoguchi, Hirokazu, Narita, Michiko, Dun, Nae J., Hwang, Bang H., Endoh, Takashi, Suzuki, Tomohiko, Nagase, Hiroshi, Suzuki, Tsutomu, Tseng, Leon F. |
| Publisher | Digital Commons @ East Tennessee State University |
| Source Sets | East Tennessee State University |
| Detected Language | English |
| Type | text |
| Source | ETSU Faculty Works |
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