Tachyphylaxis, capsaicin

Its use leads to desensitization (tachyphylaxis) rather than to irritation, which is the main factor limiting the therapeutic use of capsaicin. 

The evidence for a direct neural link causing ACTH release following chemotoxic stress is not too clear. Makara et al. (M6) using thoracic-2 spinal cord transection in the rat has shown that formalin injection into a limb caused ACTH secretion by a mechanism involving nervous pathways, but capsaicin injections could stimulate ACTH release even when injected to sites distal to the cord transection. They also noted that repeated formalin injection did not produce tachyphylaxis, whereas capsaicin did, but only for ACTH release due to capsaicin. Repeated injections of capsaicin did not affect the ability of subsequent injections of formalin to stimulate ACTH secretion. 

Fujimura M, Kamio Y, Sakamoto S et al. (1992). Tachyphylaxis to capsaicin-induced cough and its reversal by indomethacin, in patients with the sino-bronchial syndrome. Clin Autonomic Res, 2, 397-401. 

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