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Endomorphin-stimulated [35S]GTPgammaS binding in rat brain: evidence for partial agonist activity at mu-opioid
1Department of Physiology and Pharmacology, Center for Investigative Neuroscience, Bowman Gray School of Medicine, Wake Forest University, Winston-Salem, North Carolina 27157, USA.
Abstract:
Endomorphin-1 is a peptide whose binding selectivity suggests a role as an endogenous ligand at mu-opioid receptors. In the present study, the effect of endomorphin-1 on mu receptor-coupled G proteins was compared with that of the mu agonist DAMGO by using agonist-stimulated [35S]GTPgammaS binding in rat brain. [35S]GTPgammaS autoradiography revealed a similar localization of endomorphin-1- and DAMGO-stimulated [35S]GTPgammaS binding in areas including thalamus, caudate-putamen, amygdala, periaqueductal gray, parabrachial nucleus, and nucleus tractus solitarius. Naloxone blocked endomorphin-1-stimulated labeling in all regions examined. Although the distribution of endomorphin-1-stimulated [35S]GTPgammaS binding resembled that of DAMGO, the magnitude of endomorphin-1-stimulated binding was significantly lower than that produced by DAMGO. Concentration-effect curves of endomorphin-1 and DAMGO in thalamic membranes confirmed that endomorphin-1 produced only 70% of DAMGO-stimulated [35S]GTPgammaS binding. Differences in maximal stimulation of [35S]GTPgammaS binding between DAMGO and endomorphin-1 were magnified by increasing GDP concentrations, and saturation analysis of net endomorphin-1-stimulated [35S]GTPgammaS binding revealed a lower apparent Bmax value than that obtained with DAMGO. Endomorphin-1 also partially antagonized DAMGO stimulation of [35S]GTPgammaS binding. These results demonstrate that endomorphin-1 is a partial agonist for G protein activation at the mu-opioid receptor in brain.
Insights
Endomorphin-1 acts as a partial agonist at mu-opioid receptors, showing reduced G protein activation compared to DAMGO. This peptide partially antagonizes DAMGO
Area of Science:
- Neuroscience
- Pharmacology
Background:
- Endomorphin-1 is a peptide with potential endogenous ligand activity at mu-opioid receptors.
- Understanding its precise role requires investigating its interaction with G proteins.
Purpose of the Study:
- To compare the effects of endomorphin-1 and DAMGO on mu receptor-coupled G protein activation.
- To characterize endomorphin-1's efficacy and intrinsic activity at the mu-opioid receptor.
Main Methods:
- Agonist-stimulated [35S]GTPgammaS binding assays in rat brain tissue.
- [35S]GTPgammaS autoradiography to map receptor activation.
- Concentration-effect curves and saturation analysis to determine binding parameters.
Main Results:
- Endomorphin-1 and DAMGO showed similar distribution patterns of G protein activation in the rat brain.
- Endomorphin-1 produced significantly lower maximal [35S]GTPgammaS binding than DAMGO (70% efficacy).
- Endomorphin-1 acted as a partial agonist and partially antagonized DAMGO-induced G protein activation.
Conclusions:
- Endomorphin-1 functions as a partial agonist at the mu-opioid receptor, with lower intrinsic efficacy for G protein activation compared to DAMGO.
- These findings clarify endomorphin-1's pharmacological profile and its role in opioid signaling.