The nitric oxide (NO)-cGMP signaling system and cAMP system play critical roles in formation of multiple-trial induced, protein synthesis-dependent long-term memory (LTM) in many vertebrates and invertebrates. The relationship between the NO-cGMP system and cAMP system, however, remains controversial. In honey bees, the two systems have been suggested to converge on protein kinase A (PKA), based on the finding in vitro that cGMP activates PKA when sub-optimal dose of cAMP is present. In crickets, however, we have suggested that NO-cGMP pathway operates on PKA via activation of adenylyl cyclase and production of cAMP for LTM formation. To resolve this issue, we compared the effect of multiple-trial conditioning against the effect of an externally applied cGMP analog for LTM formation in crickets, in the presence of sub-optimal dose of cAMP analog and in condition in which adenylyl cyclase was inhibited. The obtained results suggest that an externally applied cGMP analog activates PKA when sub-optimal dose of cAMP analog is present, as is suggested in honey bees, but cGMP produced by multiple-trial conditioning cannot activate PKA even when sub-optimal dose of cAMP analog is present, thus indicating that cGMP produced by multiple-trial conditioning is not accessible to PKA. We conclude that the NO-cGMP system stimulates the cAMP system for LTM formation. We propose that LTM is formed by an interplay of two classes of neurons, namely, NO-producing neurons regulating LTM formation and NO-receptive neurons that are more directly involved in formation of long-term synaptic plasticity underlying LTM formation.3
Main textRecent studies have suggested that many of the molecular mechanisms underlying learning and memory are conserved among vertebrates and invertebrates, the most convincing evidence for which has been obtained by the study of the mechanisms of the formation of LTM [7]. LTM is defined as a protein synthesis-dependent phase of memory lasting for day to a lifetime. It is usually formed by multiple pairing trials but not by a single trial. It has been demonstrated that the cAMP signaling system plays critical roles in producing LTM, or long-term synaptic plasticity considered to underlie LTM formation, in mammals [1], insects [6,19] and mollusks [2]. In all of these animals, production of cAMP stimulates PKA, and this activates the transcription factor CREB (cAMP element-binding protein). Activation of CREB leads to a protein synthesis-dependent long-term synaptic plasticity that underlies LTM formation [7,20].The NO-cGMP system also plays critical roles in the formation of LTM in mammals [9], honey bees [15,16], crickets [14] and mollusks [8]. NO is a membrane-permeable intercellular signaling molecule produced by NO synthase (NOS).NO diffuses into neighboring cells and stimulates soluble guanylyl cyclase, and produced cGMP plays various physiological roles [5], including induction of LTM in many animals.In crickets, we have reported that the NO-cGMP system and cAMP system play major roles in formatio...