Effect of L-NAME, an inhibitor of nitric oxide synthesis, on motor behaviour in rats

We have tested spontaneous motor activity (SMA) and rota-rod motor coordination as indices of motivated behaviour and a forced task performance respectively in rats 5 and 30 min after intraperitoneal administration of the nitric oxide (NO) synthesis inhibitor, N-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg) or the NO precursor, L-arginine (1000 mg/kg). The effects were correlated with changes produced by the compounds on the concentration of NO and nitric oxide synthase (NOS) activity in the cerebrum and cerebellum. L-NAME decreased NOS activity and NO concentration whereas L-arginine-treated animals showed increased NOS activity and NO concentration. However, both compounds inhibited SMA but not motor coordination. These results suggest that a motivation deficit is responsible for an inhibition of SMA in animals treated with L-NAME and L-arginine through different mechanisms and that endogenous NO has a role in the motivated behaviour of rats. A reduction by L-NAME of NO concentration in the brain accounted for its SMA inhibiting action. L-arginine produced the same effect by markedly increasing NO concentration. A feedback inhibition by NO of excitatory mechanism was suggested for this action of L-arginine. We conclude that inhibition of motivated behaviour by L-NAME may be indicative of its anti-anxiety action.