Chronic Pain, Memory, and Injury: Evolutionary Clues from Snail and Rat Nociceptors

The sensory component of chronic pain is amenable to comparative study and evolutionaryinterpretations. Pain is usually initiated by activation of nociceptors, which detect damaging stimuli.A comparison of rats and a marine snail, Aplysia, shows that nociceptors in each group satisfy thesame functional definition and exhibit similar functional alterations, including persistenthyperexcitability and synaptic potentiation following noxious stimulation. These alterations are alsoassociated with conventional learning and memory. Because of the ancient divergence of theselineages, some similarities probably reflect independent evolution. However, the molecular signalslinked thus far to known forms of long-term neuronal plasticity represent homologous processes thatare found in all metazoan cells. Persistent plasticity mechanisms now used for chronic pain andmemory may have evolved originally in the earliest neurons by selective recruitment of core cellsignaling and effector systems for neuronal repair, sensory compensation, and protective functionsrelated to peripheral injury.