Inhibition of the morphine withdrawal syndrome by a nitric oxide synthase inhibitor,NG-nitro-L-arginine methyl ester

The hypothesis that an arginine-nitric oxide (NO) synthase-NO system mediates the morphine abstinence syndrome was tested in adult male rats implanted subcutaneosly for 3 days with one morphine (75 mg) pellet followed by naloxone-precipitated withdrawal (0.5 mg/kg). Injection with a NO synthase inhibitor, N^G-nitro-L-arginine methyl ester (NAME, 100 mg/kg subcutaneous), shortly before naloxone-induced withdrawal significantly inhibited abstinence signs by 25–80%. Continuous infusion of NAME via subcutaneous osmotic pumps during the development of morphine physical dependence and during naloxone-precipitated withdrawal also inhibited morphine abstinence signs. In addition, treatment with isosorbide dinitrate, a NO donor, induced a quasi morphine-abstinence syndrome (QMAS) that was significantly suppressed by implantation of a morphine pellet 3 days before isosorbide dinitrate treatment. These results indicate that NO mediates part of the expression of the morphine abstinence syndrome.