Participation of NO signaling in formation of long-term memory in salivary conditioning of the cockroach.

The molecular and neural basis of protein synthesis-dependent long-term memory (LTM) has been the subject of extensive studies in vertebrates and invertebrates. In crickets and honey bees, it has been demonstrated that nitric oxide (NO) signaling plays critical roles in LTM formation, but no experimental system appropriate for electrophysiological study of neural mechanisms by which production of NO leads to LTM formation has been established in insects. We have reported that cockroaches, as do dogs and humans, exhibit conditioning of salivation, i.e., they exhibit an increased level of salivation in response to an odor paired with sucrose reward. Salivary conditioning can be monitored by activity changes of salivary neurons in rigidly immobilized animals and thus is useful for the study of brain mechanisms of learning and memory. In the present study, we found that injection of cycloheximide, a protein synthesis inhibitor, into the hemolymph before multiple conditioning trials impairs formation of 1-day memory, but not that of 30-min memory. This indicates that formation of 1-day memory requires protein synthesis but that of earlier memory does not. We also found that injection of l-NAME, an inhibitor of NO synthase, before multiple conditioning impairs formation of 1-day memory but not that of 30-min memory. We thus conclude that NO signaling participates in the formation of protein synthesis-dependent LTM but not that of earlier memory in salivary conditioning. Salivary conditioning in cockroaches should become a pertinent system for the study of neural mechanisms by which activation of NO synthase leads to LTM formation.