Serotonin (5-HT) plays a critical role in modulating synaptic plasticity in the marine mollusc Aplysia and in the mammalian nervous system. In Aplysia sensory neurons, 5-HT can activate several signal cascades, including PKA and PKC, presumably via distinct types of G proteincoupled receptors. However, the molecular identities of these receptors have not yet been identified. We here report the cloning and functional characterization of a 5-HT receptor that is positively coupled to adenylyl cyclase in Aplysia neurons. The cloned receptor, 5-HT apAC1, stimulates the production of cAMP in HEK293T cells and in Xenopus oocytes. Moreover, the knockdown of 5-HT apAC1 expression by RNA interference blocked 5-HT-induced cAMP production in Aplysia sensory neurons and blocked synaptic facilitation in nondepressed or partially depressed sensory-to-motor neuron synapses. These data implicate 5-HT apAC1 as a major modulator of learning related synaptic facilitation in the direct sensory to motor neuron pathway of the gill withdrawal reflex.5-HT receptor ͉ memory ͉ cAMP ͉ protein kinase A 5-Hydroxytryptamine (5-HT), or serotonin, is a key neurotransmitter that modulates a variety of behaviors in both invertebrate and vertebrate animals and is involved in the regulation of mood and mood disorders in humans (1). Serotonin also modulates synaptic plasticity in the marine mollusc Aplysia (2, 3). Synaptic facilitation of the connections between sensory and motor neurons of the gill-withdrawal reflex is mediated by 5-HT, and this form of synaptic plasticity has been found to be a critical cellular mechanism of behavioral sensitization (4-6). A number of pharmacological studies have found that, depending on the behavioral history and pattern of sensory stimulation, 5-HT stimulates several downstream signaling pathways, including protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK), suggesting that serotonin acts on more than one receptor type (2,3,7,8). Of these signaling cascades, the adenylyl cyclase-cAMP-PKA cascade has been most extensively investigated because of its important roles in both behavioral sensitization and synaptic facilitation (3, 4, 9, 10). Historically, this was the initially identified second-messenger system involved in the regulation of synaptic plasticity, behavior, and memory storage (4).A single pulse of 5-HT activates PKA, which phosphorylates and inactivates potassium channels (11) and subsequently increases synaptic strength at nondepressed synapses. At depressed synapses, however, PKC becomes the major downstream kinase to be activated by a single pulse of 5-HT (8). In addition, repetitive exposures to 5-HT that induce long-term facilitation result in the activation of additional kinases, including MAPK (12), that translocate to the nucleus to induce gene expression. However, the molecular mechanism for this dynamic coupling specificity of downstream signaling pathways is not known.In vertebrates, seven families of 5-HT receptors have been characterized; six of the...

Consistent evidence from pharmacological and genetic studies shows that cAMP is a critical modulator of synaptic plasticity and memory formation. However, the potential of the cAMP signaling pathway as a target for memory enhancement remains unclear because of contradictory findings from pharmacological and genetic approaches. To address these issues, we have developed a novel conditional genetic system in mice based on the heterologous expression of an Aplysia octopamine receptor, a G-protein-coupled receptor whose activation by its natural ligand octopamine leads to rapid and transient increases in cAMP. We find that activation of this receptor transgenically expressed in mouse forebrain neurons induces a rapid elevation of hippocampal cAMP levels, facilitates hippocampus synaptic plasticity, and enhances the consolidation and retrieval of fear memory. Our findings clearly demonstrate that acute increases in cAMP levels selectively in neurons facilitate synaptic plasticity and memory, and illustrate the potential of this heterologous system to study cAMP-mediated processes in mammalian systems.

Recently, protein kinase M ζ (PKMζ) has emerged as an important player for maintaining memory. It has been reported that PKMζ regulates the trafficking of GluA2 in postsynaptic membranes to maintain memory. However, there has been no study on PKMζ outside the synaptic region regarding memory maintenance. Here, we found that PKMζ is transported to the nucleus in a neural activity-dependent manner. Moreover, we found that PKMζ phosphorylates CREB-binding protein (CBP) at serine residues and that PKMζ inhibition reduces the acetylation of histone H2B and H3. Finally, we showed that the amnesic effect of PKMζ inhibition can be rescued by enhancing histone acetylation level. These results suggest the possibility that nuclear PKMζ has a crucial role in memory maintenance.

The consolidation of long-term memory for sensitization and synaptic facilitation in Aplysia requires synthesis of new mRNA including the immediate early gene Aplysia CCAAT enhancer-binding protein (ApC/EBP). After the rapid induction of ApC/EBP expression in response to repeated treatments of 5-hydroxytryptamine (5-HT), ApC/EBP mRNA is temporarily expressed in sensory neurons of sensory-to-motor synapses. However, the molecular mechanism underlying the rapid degradation of ApC/EBP transcript is not known. Here, we cloned an AU-rich element (ARE)-binding protein, ApAUF1, which functions as a destabilizing factor for ApC/EBP mRNA. ApAUF1 was found to bind to the 3′ UTR of ApC/EBP mRNA that contains AREs and subsequently reduces the expression of ApC/EBP 3′ UTR-containing reporter genes. Moreover, overexpression of ApAUF1 inhibited the induction of ApC/EBP mRNA in sensory neurons and also impaired long-term facilitation of sensory-to-motor synapses by repetitive 5-HT treatments. These results provide evidence for a critical role of the posttranscriptional modification of ApC/EBP mRNA during the consolidation of synaptic plasticity.

Cell-permeable proteins are emerging as unconventional regulators of signal transduction and providing a potential for therapeutic applications. However, only a few of them are identified and studied in detail. We identify a novel cell-permeable protein, mouse LLP homolog (mLLP), and uncover its roles in regulating neural development. We found that mLLP is strongly expressed in developing nervous system and that mLLP knockdown or overexpression during maturation of cultured neurons affected the neuronal growth and synaptic transmission. Interestingly, extracellular addition of mLLP protein enhanced dendritic arborization, demonstrating the non-cell-autonomous effect of mLLP. Moreover, mLLP interacts with CCCTC-binding factor (CTCF) as well as transcriptional machineries and modulates gene expression involved in neuronal growth. Together, these results illustrate the characteristics and roles of previously unknown cell-permeable protein mLLP in modulating neural development.