Protein phosphatase-dependent circadian regulation of intermediate-term associative memory.

The endogenous circadian clock is a principal factor modulating memory across species. Determining the processes through which the circadian clock modulates memory formation is a key issue in understanding and identifying mechanisms to improve memory. We used the marine mollusk Aplysia californica to investigate circadian modulation of intermediate-term memory (ITM) and the mechanisms through which the circadian clock phase specifically suppresses memory using the operant learning paradigm, learning that food is inedible.

Massed training-induced intermediate-term operant memory in aplysia requires protein synthesis and multiple persistent kinase cascades.

The Aplysia feeding system with its high degree of plasticity and well characterized neuronal circuitry is well suited for investigations of memory formation. We used an operant paradigm, learning that food is inedible (LFI), to investigate the signaling pathways underlying intermediate-term memory (ITM) in Aplysia. During a single massed training session, the animal associates a specific seaweed with the failure to swallow, generating short-term (30 min) and long-term (24 h) memory.

PKG-mediated MAPK signaling is necessary for long-term operant memory in Aplysia.

Signaling pathways necessary for memory formation, such as the mitogen-activated protein kinase (MAPK) pathway, appear highly conserved across species and paradigms. Learning that food is inedible (LFI) represents a robust form of associative, operant learning that induces short- (STM) and long-term memory (LTM) in Aplysia. We investigated the role of MAPK signaling in LFI memory in vivo.

PKA and PKC are required for long-term but not short-term in vivo operant memory in Aplysia.

We investigated the involvement of PKA and PKC signaling in a negatively reinforced operant learning paradigm in Aplysia, learning that food is inedible (LFI). In vivo injection of PKA or PKC inhibitors blocked long-term LFI memory formation. Moreover, a persistent phase of PKA activity, although not PKC activity, was necessary for long-term memory.

Intermediate-term memory is modulated by the circadian clock.

Sensitization of the tail-siphon withdrawal reflex in Aplysia, a nonassociative form of learning, affords a superb opportunity to investigate the regulation of learning and memory by the circadian clock. The circadian clock has been shown to modulate long-term but not short-term sensitization. However, no previous studies have examined the role of the circadian clock in intermediate-term memory.

Training with inedible food in Aplysia causes expression of C/EBP in the buccal but not cerebral ganglion.

Training with inedible food in Aplysia increased expression of the transcription factor C/EBP in the buccal ganglia, which primarily have a motor function, but not in the cerebral or pleural ganglia. C/EBP mRNA increased immediately after training, as well as 1-2 h later. The increased expression of C/EBP protein lagged the increase in mRNA.

The circadian clock modulates core steps in long-term memory formation in Aplysia.

The circadian clock modulates the induction of long-term sensitization (LTS) in Aplysia such that long-term memory formation is significantly suppressed when animals are trained at night. We investigated whether the circadian clock modulated core molecular processes necessary for memory formation in vivo by analyzing circadian regulation of basal and LTS-induced levels of phosphorylated mitogen-activated protein kinase (P-MAPK) and Aplysia CCAAT/enhancer binding protein (ApC/EBP). No basal circadian regulation occurred for P-MAPK or total MAPK in pleural ganglia.