Pharmacokinetics and pharmacodynamics of 7-nitroindazole, a selective nitric oxide synthase inhibitor, in the rat hippocampus

Purpose. This study was conducted to assess the pharmacokinetics and pharmacodynamics of 7-nitroindazole (7-NI), a selective inhibitor of neuronal nitric oxide synthase (NOS). Methods. Male Sprague-Dawley rats were equipped with peritoneal/venous cannulae and a microdialysis probe in the hippocampal cortex. Rats received 7-NI in peanut oil (25 mg/kg) ip every 2 h for 14 h or peanut oil alone. Blood samples were obtained at timed intervals for serum 7-NI; brain tissue microdialysate for determination of extracellular 7-NI and NO was obtained every 20 min. A pharmacokinetic-pharmacodynamic model was constructed to evaluate the effects of 7-NI on NOS activity. Results. Consistent with previous reports, NOS activity in controls evidenced circadian variation. These cyclic changes in NO production were incorporated into the model of 7-NI effects on NOS. 7-NI produced a rapid (within 2 h) decrease in hippocampal NO. Under the conditions of this experiment, 7-NI produced an ∼50% decrease in hippocampal NO, which was sustained during 7-NI administration. The decrease in NOS activity by 7-NI was concentration-dependent with an apparent IC₅₀ of ∼17 μg/ml. Conclusions. Multiple ip injections of 7-NI result in a predictable, sustained decrease in NO production in the hippocampus. The pharmacokinetic-pharmacodynamic model developed allows design of dosing regimens that can produce designated changes in brain NO content, facilitating use of 7-NI to probe the pharmacological implications of NO in the central nervous system.