Neuronal nitric oxide modulates morphine antinociceptive tolerance by enhancing constitutive activity of the μ-opioid receptor

NO is a key mediator of morphine antinociceptive tolerance. This work was conducted to evaluate the specific effects of NO on μ-opioid receptor activity. To investigate the effects of morphine- and l-arginine (the NO precursor)-induced increases in NO, five groups of rats were treated with saline, l-arginine (100-, 300-, or 500-mg/kg/h), or morphine 3-mg/kg/h for 8 h on Day 1; brain tissue was collected on Day 2. To evaluate the effects of additional increases in NO on morphine-induced alterations of the μ-opioid receptor, six groups of rats were treated with 8-h intravenous infusions for two consecutive days as per the following scheme (Day 1:Day 2): saline:saline (control); saline:morphine 3-mg/kg/h (tolerant); l-arginine 500-mg/kg/h:saline (NO control); l-arginine 100-mg/kg/h:morphine 3-mg/kg/h; l-arginine 300-mg/kg/h:morphine 3-mg/kg/h; and l-arginine 500-mg/kg/h:morphine 3-mg/kg/h (supertolerant). Brain tissue was collected at the end of Day 2. The time course of effects on morphine-induced receptor alterations due to increased NO also was evaluated. Brain tissue was analyzed for changes in radioligand (agonist and antagonist) binding and [³⁵S]GTPγS binding (agonist and antagonist). In the absence of agonist exposure, NO produced an alteration in the μ-opioid receptor that increased receptor activity. In the presence of agonist, NO increased constitutive activation of the μ-opioid receptor and reduced the ability of a selective μ-opioid agonist to activate the μ-opioid G-protein-coupled receptor; these molecular effects occurred in a time course consistent with the development of antinociceptive tolerance. This work establishes important NO-induced alterations in μ-opioid receptor functionality, which directly lead to the development of opioid antinociceptive tolerance.