Selective Inflammatory Pain Insensitivity in the African Naked Mole-Rat (Heterocephalus glaber)
(A) Paw withdrawal to radiant heat measured before and after administration of SP peptide to the lumbar spinal cord via an intrathecal cannula in lightly anesthetized animals (IT SP). Note that very low doses of SP lead to thermal hyperalgesia in mice (1 μM). Naked mole-rats also display hyperalgesia following intrathecal SP injection albeit only at higher doses (100 μM, n = 6 animals per dose).
(B) Paw withdrawal to radiant heat was measured before and 1 wk after infection of one paw with transgenic herpes virus carrying the preprotachykinin gene (n = 4 naked mole-rats [NMR]). The virus treatment alone did not alter thermal thresholds (first three data points), but in contrast to naive animals, topical capsaicin leads to thermal hyperalgesia (shortened latencies) in the virus-treated paw.
(C) Pain behavior following injection of capsaicin but not acid is observed following intrathecal administration of SP. Mice and naked mole-rats were lightly anesthetized via inhalation and given an intrathecal injection of 100 μM SP. After recovery from the anesthetic (∼5–10 min) the animals received a paw injection of capsaicin (n = 6 mice, 5 NMR) or acidic (pH 3.5) saline solution (n = 6 NMR) into one foot pad, and paw licking time was measured. Note that SP slightly increased licking behaviors in mice compared to capsaicin injection alone (Pre). Naked mole-rats showed significant licking behavior after intrathecal SP which was virtually absent in untreated animals. No change in the behavioral response to a pH 3.5 solution was observed.