NITRIC-OXIDE SYNTHASE INHIBITORS - PRECLINICAL STUDIES OF POTENTIAL USE FOR TREATMENT OF OPIOID WITHDRAWAL

Four inhibitors of nitric oxide synthase (NOS), administered as acute pretreatments, attenuated several signs of naloxone-precipitated opioid withdrawal in morphine-dependent rats. Profiles of these drugs for inhibiting the expression of withdrawal were similar to that of clonidine, a drug used clinically to treat opioid withdrawal. The nonselective NOS inhibitors, N-G-nitro-L-arginine and N-G-nitro-L-arginine methyl ester, and N(5)-(1-iminoethyl)-L-ornithine, a selective inhibitor of endothelial NOS, increased blood pressure in awake, morphine-naive and morphine-dependent rats not undergoing withdrawal. 7-Nitroindazole, a selective inhibitor of neuronal NOS, did not elevate blood pressure. Insofar as hypertension is a component of opioid withdrawal in humans, the ability of 7-nitroindazole to attenuate morphine withdrawal in rats without eliciting a vasopressor response suggests that 7-nitroindazole may have human therapeutic potential. Research directions for the continued development of 7-nitroindazole as a therapeutic modality are discussed with respect to issues of physical dependence, tolerance, and safety.