A cellular model of memory reconsolidation involves reactivation-induced destabilization and restabilization at the sensorimotor synapse in Aplysia

Abstract

The memory reconsolidation hypothesis suggests that a memory trace becomes labile after retrieval and needs to be reconsolidated before it can be stabilized. However, it is unclear from earlier studies whether the same synapses involved in encoding the memory trace are those that are destabilized and restabilized after the synaptic reactivation that accompanies memory retrieval, or whether new and different synapses are recruited. To address this issue, we studied a simple nonassociative form of memory, long-term sensitization of the gill-and siphon-withdrawal reflex in Aplysia, and its cellular analog, long-term facilitation at the sensory-to-motor neuron synapse. We found that after memory retrieval, behavioral long-term sensitization in Aplysia becomes labile via ubiquitin/proteasome-dependent protein degradation and is reconsolidated by means of de novo protein synthesis. In parallel, we found that on the cellular level, longterm facilitation at the sensory-to-motor neuron synapse that mediates long-term sensitization is also destabilized by protein degradation and is restabilized by protein synthesis after synaptic reactivation, a procedure that parallels memory retrieval or retraining evident on the behavioral level. These results provide direct evidence that the same synapses that store the long-term memory trace encoded by changes in the strength of synaptic connections critical for sensitization are disrupted and reconstructed after signal retrieval. memory reorganization | memory recall | 5-HT | local protein synthesis | clasto-lactacystin beta-lactone T he processes of memory reactivation (retrieval) have been the focus of several studies over the last decade. Retrieval is thought to return the memory to an unstable (labile) state, in which de novo protein synthesis-dependent reconsolidation is required to continue maintaining the memory over time (1-4). Memory reconsolidation has been reported for a variety of memory paradigms in a number of different animal models (1, 3, 5, 6); however, how memory reconsolidation works remains unclear.At least two nonmutually exclusive hypotheses have been proposed (7). One hypothesis suggests that reconsolidation is an updating process in which the synapses that encode the preexisting memory are reorganized after memory retrieval so as to recruit new synaptic connections that allow the incorporation of new information (8-10). The second hypothesis suggests a mechanism that is a continuation of the consolidation process at the same set of synaptic connections and that serves to strengthen memory, allowing it to become longer lasting and enduring and thereby preventing forgetting (11). Both of these views of reconsolidation are consistent with retraining or retrieval. In each case, synaptic reactivation could be implicit (e.g., during sleep) or explicit, and both would presumably have the same effect of making the memory stronger, more stable, and more resistant to postretrieval interference.Both types of reconsolidation hypotheses imply that the stored memory become...

Protein degradation by ubiquitin–proteasome system in formation and labilization of contextual conditioning memory

“…The UPS inhibitor MG132 does not have a direct action on memory reconsolidation but in memory labilization. These findings are in keeping with previous studies using inhibitors of protein synthesis as amnesic agents (Lee et al 2008(Lee et al , 2012Jarome et al 2011;Ren et al 2013), and support the interpretation that protein degradation is required for memory to become labile after retrieval induced by a brief re-exposure to the training context. However, another interpretation could be that the balance between synthesis and degradation of positive and negative elements governs neural plasticity, as was found in the case of LTP (Fonseca et al 2006).…”

“…Similar results were reported in sensitization and facilitation in Aplysia (Lee et al 2012). Interestingly however, inhibition of proteasome-dependent protein degradation blocked the amnesic action of protein synthesis inhibitors (Lee et al 2008(Lee et al , 2012Jarome et al 2011;Ren et al 2013). These findings were interpreted as a specific action of the UPS inhibitor on memory labilization, suggesting that the translation blocker was no longer amnesic because memory could not become labile after retrieval.…”

“…In contrast, no effect was observed for fear conditioning upon inhibition in either hippocampus (Lee et al 2008) or amygdala (Jarome et al 2011), or for cocaine-induced conditioned place preference in the nucleus accumbens (Ren et al 2013). Similar results were reported in sensitization and facilitation in Aplysia (Lee et al 2012). Interestingly however, inhibition of proteasome-dependent protein degradation blocked the amnesic action of protein synthesis inhibitors (Lee et al 2008(Lee et al , 2012Jarome et al 2011;Ren et al 2013).…”

“…Inhibition of the UPS during consolidation impairs long-term memory in crab contextual conditioning As outlined above, the results available in the literature about the action of UPS inhibition during memory consolidation are contradictory, with most studies reporting memory impairment, whereas in two cases no effect was found (Lee et al 2008(Lee et al , 2012. To further explore this issue, we initially studied the effect of UPS inhibition on long-term memory in contextual conditioning of the crab Chasmagnathus.…”

“…Inhibition of the UPS has been reported to rescue memory impairment induced by inhibition of translation during reconsolidation (Lee et al 2008(Lee et al , 2012Jarome et al 2011;Ren et al 2013;see above). To test whether this action extends to reconsolidation inhibitors interfering a different molecular mechanism, we focused our attention on the transcription factor nuclear factor kappa B (NF-kB).…”