Selective Inflammatory Pain Insensitivity in the African Naked Mole-Rat (Heterocephalus glaber)
(A) Response latency to tail clip (450 g pressure, n = 25 mice, 25 naked mole-rats [NMR]), and paw withdrawal latency to radiant heat (n = 12 mice, 12 NMR).
(B) Behavioral responses to foot pad injection of capsaicin (n = 10 mice, 10 NMR) and acidic saline solution (pH 3.25, n = 5 mice, 5 NMR), measured as time spent licking the affected paw. Note virtually no behavioral response in terms of paw licking was found for either stimulus in naked mole-rats.
(C) Paw withdrawal thresholds to mechanical stimulation with von Frey hairs before and during inflammation from an injection of CFA; both mice and naked mole-rats show significant sensitization during inflammation (n = 10 mice, 10 NMR).
(D) Paw withdrawal to radiant heat before and 72 h after injection of CFA; mice show a significant sensitization but naked mole-rats show no sensitization (n = 6 mice, 6 NMR).
(E) Paw withdrawal to radiant heat before and after topical application of 1 mM capsaicin (Cap) ( = 6 mice, 12 NMR); mice show a significant sensitization (shortened latency) after capsaicin, naked mole-rats show no sensitization.
(F) Paw withdrawal to radiant heat before and after a systemic injection of nerve growth factor (NGF 1μg/g); mice show a significant sensitization (shortened latency) at 1 h, and this is maintained at 36 h post injection; naked mole-rats show no sensitization at either time point. For panels A-F *p < 0.05; **p < 0.01 Student's t-test.
(G) Frequency of pain behaviors to injection of 1% formalin; pain behaviors are significantly attenuated in naked mole-rats compared to mice in both phase I (0–5 min, p < 0.0001) and phase II (>5 min, p < .01) of the formalin test (n = 6 mice, 6 NMR).