Nitric oxide synthase inhibitors do not substitute in rats trained to discriminate phencyclidine from saline

Release of nitric oxide occurs as a consequence of glutamate stimulation of NMDA receptors and is dependent upon calcium-calmodulin activation of the enzyme nitric oxide synthase. Since nitric oxide may serve as an intracellular messenger for NMDA glutamatergic neurons, it could be hypothesized that blockade of its synthesis may produce pharmacological effects similar to those of NMDA receptor antagonists. The purpose of the present study was to compare the effects of nitric oxide synthase inhibitors to those of the high affinity NMDA open channel blocker phencyclidine in drug discrimination, a pharmacologically selective procedure in which phencyclidine produces distinctive effects. Rats were trained to discriminate 2 mg/kg phencyclidine from saline in a standard two-lever discrimination task with food reward. Whereas phencyclidine dose-dependently substituted for itself, 7-nitroindazole, L-NAME (N(G)-nitro-L-arginine methyl ester), and L-NOARG (N(G)-nitro-L-arginine) failed to substitute for phencyclidine when administered intraperitoneally. L-NAME and 7-nitroindazole were tested up to doses that disrupted responding, providing evidence that a behaviorally-relevant dosage range was evaluated. Although these results conflict with those of a previous study which found that nitric oxide synthase inhibitors substituted for phencyclidine and produced phencyclidine-like catalepsy in pigeons, they are consistent with research showing that these drugs did not produce phencyclidine-like pharmacological effects in behavioral procedures in rats.