Adapting rodent cued threat conditioning to planarians: Memory acquisition, consolidation, and reconsolidation

The processes of memory consolidation and reconsolidation are important in understanding the dynamics of memory storage and modification. Models to condition aversive memory have been characterised in rodents yet have been less explored in invertebrates such as planaria. Planaria are a potentially important animal model for the study of threat learning and memory as they can learn conditioned responses and may show conserved neural mechanisms underpinning these responses compared to mammals. However, no model for the study of memory in classical threat conditioning exists for planaria. The aim of this study is to replicate a standardised cued threat conditioning model for memory acquisition and reconsolidation in mice, in brown planaria (Girardia dorotocephala), using shock as the unconditioned stimulus (US) and light as the conditioned stimulus (CS). Defensive behaviours (C-shaped movements and scrunching) were assessed to evaluate learning. Planaria underwent memory acquisition training for a total of 18 learning trials. In the consolidation model, memory retention was tested 24 h after the final training session with a CS exposure. In the reconsolidation model, memory stability was assessed using a recall test 24 h after a reactivation session. In both tests, the paired shock-light group exhibited a significant increase in defensive behaviours, compared to all control groups, suggesting that the pairing of both shock and light was essential for forming a lasting memory. Furthermore, planaria exhibited a gradual extinction of defensive responses to the CS. These findings highlight the utility of planaria for studying associative learning and memory, including memory reconsolidation.