At the end of the millenium it was clearly shown that the retrieval (reactivation) of consolidated long-term memories can destabilize the memory, making it labile for several hours and exposed to modifications (Nader et al., 2000). The period of lability results either in reconsolidation or erasure of memory. It was proposed that reactivation triggers memory destabilization that is followed by re-stabilization process necessary for successful reconsolidation of the memory trace. Reactivation is usually triggered by a reminder of the initial training conditions (Wideman et al., 2018). Destabilization correlates to synaptic proteins’ degradation through activation of the ubiquitin proteasome system (Kaang and Choi, 2011, Lee et al., 2008, Stiver et al., 2017) and results in a labile state of memory. Consequently, in order to consolidate this labile memory trace, a new protein synthesis during re-stabilization phase is necessary (Nader, 2003). Taking into consideration that the reconsolidation process is responsible for the maintenance of memory and its updating (Finnie and Nader, 2012, Jardine et al., 2022, Lee et al., 2017), it is crucial to understand the mechanisms underlying this process.

Previously several studies demonstrated the existence of reconsolidation in Helix (Gainutdinova et al., 2005). Further, the involvement of the serotonin system in reconsolidation of long-term memories in Helix was shown. Under the conditions of serotonin deficit, caused by administration of either p-chlorophenylalanine (Deryabina et al., 2018) or 5,7-DiHT (Balaban et al., 2016), the reconsolidation in Helix was impaired. On the other hand, an increased release of serotonin (5-HT) caused by the 5-HT precursor 5-hydroxy-L-tryptophan injection during memory reactivation effectively restored the memory impaired during reconsolidation in terrestrial snail Helix (Zuzina et al., 2019).

Despite the fact that the involvement of 5-HT in memory reconsolidation in mollusks (Aplysia) was demonstrated (Cai et al., 2012), to our knowledge, no research has addressed the involvement of serotonin receptors in destabilization and re-stabilization of long-term memories triggered by reminding in Helix. Thus, the current research aimed to improve understanding of serotonin receptors’ involvement in those processes. We used a nonselective inhibitor for serotonin receptors (1, 2, 5, 6 and 7 types) methiothepin (MET). The presence of MET-sensitive 5-HT receptors was previously established in identified premotor interneurons of withdrawal behavior in Helix (Abramova et al., 2006, Pivovarov and Nistratova, 2003). MET has also been used in the study of long-term potentiation mechanisms in Helix (Zuzina & Balaban, 2022). In the current work, we have selected three time intervals for MET administration. To understand whether serotonin receptors are involved in the mechanisms of memory destabilization, MET was injected 40 min before the reminder (group MET + R). MET injection immediately after the reminder was performed to understand whether serotonin receptors contribute to early phase of re-stabilization process (R + METim group). And, finally, MET post-reactivation administration with 2 h delay allowed to estimate the role of serotonin receptors in late re-stabilization mechanisms (R + MET2hrdel group). We showed that serotonin receptors are involved in the reconsolidation of context and food aversive long-term memories in snails. We demonstrated that immediate post-reactivation MET treatment induced an irreversible memory deficit while pre-reactivation MET administration and delayed post-reactivation MET administration caused only slight memory attenuation, but the memory still was well preserved. We also found that pre-reactivation MET administration prevented the well-described for this animal impairing effects of post-reactivation anisomycin (ANI, a protein synthesis inhibitor) injections. Delayed post-reactivation MET administration followed by ANI led to a significant memory impairment. Thus, the serotonin receptors activity may therefore be preferentially involved in the process of destabilization and early phase of re-stabilization while less required for late phase of re-stabilization. The results obtained confirm a complex involvement of serotonin activity in mollusks memory reconsolidation process.