G protein activation by endomorphins in the mouse periaqueductal gray matter.
The 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.
Narita
M
M
Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisc. 53226, USA.
Mizoguchi
H
H
Narita
M
M
Dun
N J
NJ
Hwang
B H
BH
Endoh
T
T
Suzuki
T
T
Nagase
H
H
Suzuki
T
T
Tseng
L F
LF
D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D001345:Autoradiography; D002462:Cell Membrane / Q000378:metabolism; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000494:pharmacology; D015636:Magnesium Chloride / Q000494:pharmacology; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology*; D010487:Periaqueductal Gray / Q000737:chemistry* / Q000648:ultrastructure; D017450:Receptors, Opioid, mu / Q000819:agonists; D012965:Sodium Chloride / Q000494:pharmacology; D013004:Somatostatin / Q000031:analogs & derivatives / Q000494:pharmacology; D013462:Sulfur Radioisotopes; D014018:Tissue Distribution
D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D009842:Oligopeptides; D017450:Receptors, Opioid, mu; D013462:Sulfur Radioisotopes; C105293:endomorphin 1; D015636:Magnesium Chloride; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; C049352:phenylalanyl-cyclo(cysteinyltyrosyl-tryptophyl-ornithyl-threonyl-penicillamine)threoninamide; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C105294:endomorphin 2; D012965:Sodium Chloride; D013004:Somatostatin; D009271:Naltrexone; C024524:beta-funaltrexamine; D019204:GTP-Binding Proteins