The effects of capsaicin are first excitation, followed by desensitization and destruction of several populations of neuropeptide containing sensory nerve ending. Beside the tachykinin (TK) peptides substance P (SP) and neurokinin A (NKA), their nerves also contain calcitoningene related peptide, (CGRP), pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), somatostatin, galanin, secretoneurin, glutamate, nitric oxide synthase (NOS) and acetylcholinesterase. The selectivity of capsaicin for sensory neurons has made this neurotoxin an useful too for evaluating physiological and pathological function.