In Aplysia, long-term facilitation (LTF) of sensory neuron synapses requires activation of both protein kinase A (PKA) and mitogen-activated protein kinase (MAPK). We find that 5-HT through activation of PKA regulates secretion of the sensory neuron-specific neuropeptide sensorin, which binds autoreceptors to activate MAPK. Anti-sensorin antibody blocked LTF and MAPK activation produced by 5-HT and LTF produced by medium containing sensorin that was secreted from sensory neurons after 5-HT treatment. A single application of 5-HT followed by a 2 hr incubation with sensorin produced protein synthesis-dependent LTF, growth of new presynaptic varicosities, and activation of MAPK and its translocation into sensory neuron nuclei. Inhibiting PKA during 5-HT applications and inhibiting receptor tyrosine kinase or MAPK during sensorin application blocked both LTF and MAPK activation and translocation. Thus, long-term synaptic plasticity is produced when stimuli activate kinases in a specific sequence by regulating the secretion and autocrine action of a neuropeptide.

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Activation of several signaling pathways contributes to long-term synaptic plasticity, but how brief stimuli produce coordinated activation of these pathways is not understood. In Aplysia, the long-term facilitation (LTF) of sensory neuron synapses by 5-hydroxytryptamine (serotonin; 5-HT) requires the activation of several kinases, including mitogen-activated protein kinase (MAPK). The 5-HT-enhanced secretion of the sensory neuron-specific neuropeptide sensorin mediates the activation of MAPK. We find that stimulus-induced activation of two signaling pathways, phosphoinositide 3-kinase (PI3K) and type II protein kinase A (PKA), regulate sensorin secretion and responses. Treatment with 5-HT produces a rapid increase in sensorin synthesis, especially at varicosities, which precedes the secretion of sensorin. PI3K inhibitor and rapamycin block LTF and the rapid synthesis of sensorin at varicosities even in the absence of sensory neuron cell bodies. Secretion of the newly synthesized sensorin from the varicosities and activation of the autocrine responses of sensorin to produce LTF require type II PKA interaction with AKAPs (A-kinase anchoring proteins). Thus, long-term synaptic plasticity is produced when multiple signaling pathways that are important for regulating distinct cellular functions are activated in a specific sequence and recruit the secretion of a neuropeptide to activate additional critical pathways.

SUMMARY Generalization of fear responses to non-threatening stimuli is a feature of anxiety disorders. It has been challenging to target maladaptive generalized memories without affecting adaptive memories. Synapse-specific long-term plasticity underlying memory involves the targeting of plasticity-related proteins (PRPs) to activated synapses. If distinct tags and PRPs are used for different forms of plasticity, one could selectively remove distinct forms of memory. Using a stimulation paradigm in which associative long-term facilitation (LTF) occurs at one input and non-associative LTF at another input to the same postsynaptic neuron in an Aplysia sensorimotor preparation, we found that each form of LTF is reversed by inhibiting distinct isoforms of protein kinase M (PKM), putative PRPs, in the postsynaptic neuron. A dominant negative atypical PKM selectively reversed associative LTF, while a dominant negative classical PKM selectively reversed non-associative LTF. While both PKMs are formed from calpain-mediated cleavage of PKCs, each form of LTF is sensitive to a distinct dominant negative calpain expressed in the postsynaptic neuron. Associative LTF is blocked by dominant negative classical calpain, while non-associative LTF is blocked by dominant negative small optic lobe (SOL) calpain. Interfering with a putative synaptic tag, the adaptor protein KIBRA, which protects the atypical PKM from degradation, selectively erases associative LTF. Thus, the activity of distinct PRPs and tags in a postsynaptic neuron contribute to the maintenance of different forms of synaptic plasticity at separate inputs allowing for selective reversal of synaptic plasticity and providing a cellular basis for developing therapeutic strategies for selectively reversing maladaptive memories.

Several factors regulate export of mRNAs from neuronal cell bodies. Using in situ hybridization and RT-PCR, we examined how target interaction influences the distribution of mRNAs expressed in sensory neurons (SNs) of Aplysia maintained in cell culture. Interaction with a synaptic target has two effects on the distribution of mRNA encoding an SN-specific peptide, sensorin: the target affects the accumulation of sensorin mRNA at the axon hillock and the stability of sensorin mRNA exported to distal sites. Synapse formation with motor neuron L7 results in the accumulation of high levels of sensorin mRNA in the axon hillock of the SN and in SN neurites contacting L7. SNs cultured alone or in contact with motor neuron L11, with which no synapses form, show a more uniform distribution of sensorin mRNA in the cytoplasm of the SN cell body, with little expression in neurites. Contact with L7 or L11 had little or no effect on the distribution of two other mRNAs in the cytoplasm of SN cell bodies. Sensorin mRNA exported to SN neurites after 1 d in culture is more stable when the SN contacts L7 compared with SN neurites that contact L11. After removal of the SN cell body, the amounts of sensorin mRNA already exported to the neurites are greater when neurites contact L7 compared with neurites in contact with L11. The results indicate that target interaction and synapse formation regulate both the accumulations of specific mRNAs destined for export and their stability at distant sites.