Neuronal nitric-oxide synthase deficiency impairs the long-term memory of olfactory fear learning and increases odor generalization.

Experience-induced changes associated with odor learning are mediated by a number of signaling molecules, including nitric oxide (NO), which is predominantly synthesized by neuronal nitric oxide synthase (nNOS) in the brain. In the current study, we investigated the role of nNOS in the acquisition and retention of conditioned olfactory fear. Mice lacking nNOS received six training trials, each consisting of an odor-CS co-terminating with a foot shock-US. Mice showed reduced freezing responses to the trained odor 24 h and 7 d after training, compared to wild-type mice. Pretraining systemic injections of the NO donor, molsidomine, rescued fear retention in nNOS knockout mice. In wild-type mice, pretraining systemic injections of L-NAME, a nonspecific nNOS blocker, disrupted odor-CS fear retention in a dose-dependent manner. To evaluate whether NO signaling is involved in generalization of fear memories, nNOS knockout mice and wild-type mice receiving L-NAME were trained to one odor and tested with a series of similar odors. In both cases, we found increased generalization, as measured by increased freezing to similar, unpaired odors. Despite the impairment in fear memory retention and generalization, neither mice receiving injections of L-NAME nor nNOS knockout mice showed any deficits in either novel odor investigation time or odor habituation, suggesting intact olfactory perception and short-term memory olfactory learning. These results support a necessary role for neuronal NO signaling in the normal expression and generalization of olfactory conditioned fear.