Hyperalgesia as a Pain Memory

Although pain conditioning does not involve central pathways for pain perception and memory, can it — like fear conditioning (NEJM JW Psychiatry Feb 26 2014) — be extinguished during a labile period when a conditioned stimulus is re-presented? Researchers, some with industry funding, conducted studies on hyperalgesia in mice.

Injection of capsaicin into a paw produced hyperalgesia (measured by paw withdrawal from touch) that lasted >6 hours. Hyperalgesia was mediated by protein synthesis in spinal dorsal-horn neurons and by release of glutamate (which acts on AMPA and NMDA receptors) and substance P (which acts on NK-1 receptors) from pain fibers to sensitize second-order neurons in the dorsal horn. A second capsaicin administration given wi…