The Circadian Clock Modulates Core Steps in Long-Term Memory Formation inAplysia

Abstract

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. In contrast, the circadian clock regulated basal levels of ApC/EBP protein with peak levels at night, antiphase to the rhythm in LTS. Importantly, LTS training during the (subjective) day produced greater increases in P-MAPK and ApC/EBP than training at night. Thus, circadian modulation of LTS occurs, at least in part, by suppressing changes in key proteins at night. Rescue of long-term memory formation at night required both facilitation of MAPK and transcription in conjunction with LTS training, confirming that the circadian clock at night actively suppresses MAPK activation and transcription involved in memory formation.The circadian clock appears to modulate LTS at multiple levels. 5-HT levels are increased more when animals receive LTS training during the (subjective) day compared with the night, suggesting circadian modulation of 5-HT release. Circadian modulation also occurred downstream of 5-HT release because animals treated with 5-HT to induce LTS exhibited significantly greater LTS when treated during the (subjective) day compared with the night. Together, our studies suggest that the circadian clock modulates LTS at multiple steps and locations during the formation of long-term memory.

Acute Sleep Deprivation Blocks Short- and Long-Term Operant Memory inAplysia

“…Unlike rodent models, Aplysia do not exhibit any sleep during their active period, maintaining a strong diurnal pattern of activity. 27 Although long-term memory formation in Aplysia is strongly regulated by the circadian clock with training-induced kinase activation and gene expression dependent on the time of training, 94,95 there appear to be no differences in the molecular pathways required for 24 h memory when animals are trained early in the day compared to later in the day, as long as the training still occurs during the animal's active period. Presumably, sleep deprivation interferes with molecular signaling processes affecting the induction of gene expression and protein synthesis necessary for LTM.…”

Effects of aversive stimuli beyond defensive neural circuits: Reduced excitability in an identified neuron critical for feeding in Aplysia

“…For example, application of 5-HT to the isolated buccal ganglion reduces the activity of an identified element in the feeding circuit, including pattern-initiation neuron B31/32 (Kabotyanski et al 2000). Therefore, we investigated whether an in vivo treatment with 5-HT, which induces LTS (Levenson et al 2000;Lyons et al 2006; see Supplemental Material for details), modulates feeding and B51 excitability in a manner analogous to LTS training.…”

Synchrony and desynchrony in circadian clocks: impacts on learning and memory

“…In the hippocampus, circadian cycling of phospho-ERK with peaks during the subjective day promotes memory formation and persistence of long-term memory (Eckel-Mahan et al 2008;Eckel-Mahan 2012). Although a basal circadian rhythm in phospho-MAPK is not observed in Aplysia ganglia, sensitization training during the (subjective) day induces significantly greater MAPK activity compared with training during the night (Lyons et al 2006). Potentially, second messengerdependent kinase pathways such as the cAMP/PKA pathway may also transduce circadian signals in learning acquisition or memory formation.…”

“…Neurotransmitters can also function in conveyance of temporal information in memory context as suggested for acetylcholine in time-stamping (Hut and Van der Zee 2011) and GABA synthesis in fear memory (Bergado-Acosta et al 2014). An example of circadian regulation of training-induced neurotransmitter release comes from Aplysia, in which sensitization training-induced serotonin release is circadianly regulated with higher levels of serotonin observed in the hemolymph following training during the subjective day (Lyons et al 2006). Circadian regulation of inhibitory pathways may also modulate memory processes.…”

“…More recently, hippocampal circadian rhythms in CREB activation have been shown to be dependent upon the Period1 gene (Rawashdeh et al 2014). In Aplysia, training animals during the subjective day results in significantly increased protein levels of the CREBdependent immediate early gene ApC/EBP (Lyons et al 2006), a transcription factor in nonassociative (Alberini et al 1994), and associative memory formation (Levitan et al 2008). In Drosophila mushroom bodies, a region necessary for many forms of memory, dCREB2 exhibits circadian oscillations in gene expression, nuclear protein levels and activity, likely mediating rhythms in LTM formation (Tanenhaus et al 2012;Fropf et al 2014).…”