Functional and neurochemical profile of place learning after L-nitro-arginine in the rat

Rats in which near-total inhibition of the nitric oxide synthesizing enzyme (NOS) had been obtained by a 5-day pretreatment during which two daily injections of L-nitro-arginine (50 mg/kg per injection) had been administered were subjected to five sessions of training on a place learning task (one session per day). NOS inhibition was associated with significantly impaired task acquisition but on the fifth training session normal task proficiency was achieved. Subsequent pharmacological and behavioral challenges established that dissimilar neural substrates mediated the task in NOS-inhibited and normal animals. In NOS-inhibited rats the neural substrate of task mediation depended less than normally on cholinergic mechanisms while potentially relying more on catecholaminergic mediation. In a separate experiment rats that had acquired the place learning task to asymptotic quality of performance were subjected to a L-nitro-arginine treatment similar to the pretreatment of the first experiment. Such a NOS inhibition did not impair subsequent task performance. It is concluded that nitric oxide participates in processes subserving acquisition rather than performance of at least some forms of place learning.