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The Japanese Journal of Pharmacology
Vol. 89 (2002) No. 3 P 229-234



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It is well documented that the μ-opioid receptor (MOP-R) is expressed by neurons in several central nervous system regions. Its occupancy with agonist drugs modulate a variety of physiological processes including pain, reward, stress, immune responses, neuroendocrine functions, and cardiovascular control. Based on the receptor binding assay, endomorphin-1 and endomorphin-2 have the highest specificity and affinity for the MOP-R of any endogenous substance so far described in the mammalian nervous system. In contrast, β-endorphin exhibits the strongest actions among endogenous opioid peptides mainly through the MOP-R; however, it also shows the distinct pharmacological actions. Recent cloning and expression studies have indicated that MOP-Rs are seven-transmembrane domain receptors whose actions are mediated through activation of heterotrimeric guanine nucleotide binding proteins (G-proteins). The activation of G-proteins by MOP-Rs can be measured by assessing agonist-induced stimulation of membrane binding of guanosine-5'-o-(3-[35S]thio)triphosphate ([35S]GTPγS). The subject of the present review is to focus on the differential mechanism underlying G-protein activation induced by these μ-opioid peptides using the [35S]GTPγS binding assay.

Copyright © The Japanese Pharmacological Society 2002

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