Methylene blue inhibits the increase of inducible nitric oxide synthase activity induced by stress and lipopolysaccharide in the medial basal hypothalamus of rats

In infection bacterial products such as lipopolysaccharides (LPS) induce in ducible nitric oxide synthase (iNOS) that produces large quantities of NO t oxic to the invading organisms, but also often has toxic effects on host ce lls. Therefore, inhibition of iNOS activity might be beneficial in combatti ng these adverse effects. To determine if methylene blue (MB), an oxidizing agent that inactivates iNOS, would reduce the iNOS levels in the medial ba sal hypothalami (MBH) of conscious male rats, LPS (5 mg/kg) was injected in travenously (i.v.), and after 3 h they were injected i.v. with either MB (3 mg/kg) or saline and the effects on iNOS in the MBH determined. iNOS was m easured by conversion of labeled arginine into citrulline by incubating MBH in the absence of calcium (Ca2+) since iNOS does not require Ca2+ for acti vation. The results indicate that iNOS was induced by the injection of sali ne, but the induction by LPS was much greater, an increase of 10-fold above that of control sham-operated animals. Both the induction of INOS from the stress of saline injections and LPS were completely eliminated by MB indic ating that MB might be beneficial in preventing injury to brain tissue foll owing LPS injection. There was no effect of either LPS or MB on the Ca2+-de pendent constitutive NOS activity, Copyright (C) 2000 S. Karger AG, Basel.