Morphine through mu-opioid receptors and G(i/o) proteins modulates several cellular effector systems; however, the mechanisms involved in the regulation of Na(+),K(+)-ATPase are not well known. We evaluated the effect of two mu-opioid receptor agonists on ouabain-sensitive Na(+),K(+)-ATPase activity in mice forebrain synaptosomes, and examined the modulation of this effect by antagonists of opioid receptors and a blocker of G(i/o) proteins. Incubation of synaptosomes with morphine (10(-9) to 10(-4) M) or buprenorphine (10(-10) to 10(-5) M) concentration-dependently stimulated Na(+),K(+)-ATPase activity, morphine being less potent but more efficacious than buprenorphine. Morphine did not displace [3H]ouabain from its binding site (Na(+),K(+)-ATPase) to forebrain membranes, whereas ouabain did so in a concentration-dependent manner. Naloxone, an opioid antagonist (10(-6) M), added to the synaptosomal medium, antagonized the enhancement of Na(+),K(+)-ATPase activity induced by morphine, producing a parallel shift to the right of the morphine concentration-response curve. Treatment with beta-funaltrexamine, a mu antagonist (2.5 and 10 microg/mouse, i.c.v.) and naloxonazine, a mu1 antagonist (35 mg/kg, s.c.), 24 h before the synaptosomes were obtained, produced a dose-dependent reduction in the E(max) of the morphine-induced increase in Na(+),K(+)-ATPase activity in vitro, but did not significantly modify its EC(50). Pertussis toxin (G(i/o) protein blocker) treatment at a dose of 0.5 microg/mouse, administered i.c.v. 5 days before the synaptosomes were obtained, completely abolished the enhancement of Na(+),K(+)-ATPase activity induced by morphine. A lower dose (0.25 microg/mouse) decreased the E(max) of morphine by 50% but did not significantly affect its EC(50). These results suggest that morphine indirectly enhances Na(+),K(+)-ATPase activity in the brain by activating mu-opioid receptors and G(i/o) proteins.