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  • Title: Up-regulation of spinal mu-opioid receptor function to activate G-protein by chronic naloxone treatment.
    Author: Narita M, Mizoguchi H, Nagase H, Suzuki T, Tseng LF.
    Journal: Brain Res; 2001 Sep 21; 913(2):170-3. PubMed ID: 11549382.
    Abstract:
    The effects of repeated s.c. administrations of an mu-opioid receptor antagonist naloxone on the G-protein activation induced by mu-opioid receptor agonists [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO), endomorphin-1 and endomorphin-2 in the mouse spinal cord was studied, monitoring guanosine-5'-o-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding. All mu-opioid receptor agonists concentration-dependently increased the [35S]GTPgammaS binding. The increases of [35S]GTPgammaS binding induced by agonists were significantly enhanced in mice pretreated with naloxone. Under the present condition, chronic treatment with naloxone significantly increased the density of [3H]DAMGO binding sites with an increase in K(d) values in spinal cord membranes, indicating an increase in mu-opioid receptors on the membrane surface. These findings suggest that chronic treatment with an mu-opioid receptor antagonist naloxone leads to the supersensitivity to activate G-protein by mu-opioid receptor agonists with an increase in mu-opioid receptors in membranes of the mouse spinal cord.
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