Identification of a Novel Retrieval-dependent Memory Process in the Crab Neohelice granulata

Memory persistence: from fundamental mechanisms to translational opportunities

Memory persistence is a double edge sword. Persistence of adaptive memories is essential for survival and even determines who we are. Neurodegenerative conditions with significant memory loss such as Alzheimer's disease, testify how defects of memory persistence have severe and irreversible effects on personality, among other symptoms. Yet, maintenance of overly strong maladaptive memories underlies highly debilitating psychiatric conditions including post-traumatic stress disorder, specific phobia, substance dependence and binge eating disorder. Here we review the neurobiological mechanisms supporting memory formation, persistence, inhibition and forgetting. We then shift the focus to how such mechanisms have been exploited to alter the persistence of laboratory-generated memories in human healthy volunteers as a proof of concept. Finally, we review the effect of behavioural and pharmacological interventions in anxiety and addiction disorder patients, highlighting key findings, gaps, and future directions for basic and translational research.

Contextual memory reactivation modulates Ca2+-activity network state in a mushroom body-like center of the crab N. granulata

High-order brain centers play key roles in sensory integration and cognition. In arthropods, much is known about the insect high-order centers that support associative memory processes, the mushroom bodies. The hypothesis that crustaceans possess structures equivalent to the mushroom bodies -traditionally called hemiellipsoid body- has been receiving neuroanatomical endorsement. The recent functional support is limited to the short term: in a structure of the true crab Neohelice granulata that has many insect-like mushroom bodies traits, the plastic learning changes express the context attribute of an associative memory trace. Here, we used in vivo calcium imaging to test whether neuronal activity in this structure is associated with memory reactivation in the long-term (i.e., 24 h after training). Long-term training effects were tested by presenting the training-context alone, a reminder known to trigger memory reconsolidation. We found similar spontaneous activity between trained and naïve animals. However, after training-context presentation, trained animals showed increased calcium events rate, suggesting that memory reactivation induced a change in the underlying physiological state of this center. Reflecting the change in the escape response observed in the paradigm, animals trained with a visual danger stimulus showed significantly lower calcium-evoked transients in the insect-like mushroom body. Protein synthesis inhibitor cycloheximide administered during consolidation prevented calcium mediated changes. Moreover, we found the presence of distinct calcium activity spatial patterns. Results suggest that intrinsic neurons of this crustacean mushroom body-like center are involved in contextual associative long-term memory processes.

Over the Edge: Extending the duration of a reconsolidation intervention for spider fear

Pharmacologically disrupting fear memory reconsolidation dramatically reduces fear behaviour. For example, 2-3 min of tarantula exposure followed by 40 mg of propranolol HCl (i.e., a reconsolidation intervention) abruptly decreased spider avoidance, an effect that persisted one year later. However, the success of reconsolidation interventions is not guaranteed: Pavlovian fear-conditioning research shows that the window to target memory reconsolidation is small and easy to miss. If exposure is too long to trigger reconsolidation, but too short for extinction learning, an inactive transitional limbo state occurs, rendering the fear memory unchanged and insensitive to amnesic agents. In this pre-registered study, we aimed to find this behaviourally-controlled boundary condition. Spider-fearful participants underwent a ~3 min (n = 23) or ~14 min (n = 20) exposure to a tarantula, intended to trigger reconsolidation or the limbo state respectively, followed by 40 mg of propranolol. We expected greater spider fear reduction after 3 than 14 min of exposure. Unexpectedly, there were no group differences on any outcome measures. In both groups, Bayesian analysis revealed a marked reduction in fear behaviour towards a generalisation stimulus (a house spider) accompanied by lower self-reported distress, with a sharp decline in spider fear scores two days after treatment that persisted one year later. Possible explanations include that the boundary conditions of reconsolidation are wider in older and stronger memories than experimentally-induced fears, or that alternative processes caused the treatment effects. Although the mechanism is unclear, these results carry a tentative promising message for the potential of brief reconsolidation-targeting interventions to mitigate irrational fears.

Retrieval-Dependent Mechanisms Affecting Emotional Memory Persistence: Reconsolidation, Extinction, and the Space in Between

Maladaptive emotional memories contribute to the persistence of many mental health disorders, and therefore the prospect of disrupting these memories to produce long-term reductions in relapse is of great clinical appeal. Reducing the impact of maladaptive emotional memories on behaviour could be achieved by two retrieval-dependent manipulations that engage separate mnemonic processes: "reconsolidation disruption" and "extinction enhancement." Extinction occurs during a prolonged re-exposure session in the absence of the expected emotional outcome and is widely accepted as reflecting the formation of a new, inhibitory memory that prevents behavioural expression of the original trace. Reconsolidation, by contrast, involves the destabilisation of the original memory, allowing for subsequent updating and restabilisation in specific brain regions, unless the re-stabilization process is prevented through specific pharmacological or behavioural interventions. Both destabilisation of the original memory and memory extinction require that re-exposure induces prediction error-a mismatch between what is expected and what actually occurs-but the parameters that allow reconsolidation and extinction to occur, and control the transition between them, have not been well-characterised. Here, we review what is known about the induction of memory destabilisation and extinction, and the transition period that separates these mnemonic processes, drawing on preclinical and clinical examples. A deeper understanding of the processes that determine the alternative routes to memory persistence or inhibition is critical for designing new and more reliable clinical treatments targeting maladaptive emotional memories.