Molluscan Memory of Injury: Evolutionary Insights into Chronic Pain

 Brain Behav Evol 2009; 74: 206-218

DOI: 10.1159/000258667

Edgar T. Walters and Leonid L. Moroz

In this paper, the authors raise an interesting hypothesis that the plasticity mechanisms underlying learning and memory in higher organisms may have evolved from adaptive responses that repair damaged neuronal processes or body parts. Persistent nociceptive sensitization in Aplysia nervous system displays many functional similarities to alterations in mammalian nociceptors associated with the clinical problem of chronic pain. The original responses induced regrowth of damaged axon, increased excitability, enhanced release of transmitter, and reorganization of cellular network. Such compensatory responses would be critical for survival in the early age.

Before discussing about the origin of chronic pain and synaptic plasticity, the authors also described how great Aplysia is. They compared molluscs with other model systems like arthropod and nematode by comparing the number of gene homologues that are shared by mammals, and their evolutionary distances from mammals. Mollucs are closer to mammals because they split earlier. Slower rate of gene evolution provided molluscs more homologous genes associated with human disease than Drosophila or C. elegans. DNA methylation can also be seen in molluscs. Thus, integration of genomics and physiological studies in molluscan neurons offers a powerful comparative approach to address molecular and cellular aspects of selected neurological problems.

Altogether, molluscan preparations should become increasingly useful for comparative studies across phyla that can provide insight into cellular functions of clinically important genes.