Capsaicin, the main pungent ingredient in 'hot' chilli peppers, is a TRPV1 agonist with EC50 of 0.29±0.05 μM in HEK293 cells, which elicits a sensation of burning pain by selectively activating sensory neurons that convey information about noxious stimuli to the central nervous system, it may used as a pain therapy by the long-lasting and inhibitory effects on persistent pain. Capsaicin has antioxidant activity , it is more effective than melatonin in suppressing the formation of lipid hydroperoxides, it also reduces anxiety-like behaviours in rats and may be an admissible drug candidate for treating endometriosis.
Histamine, an organic nitrogenous compound, is involved in local immune responses regulating physiological function in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. It is a potent H1 and H2 receptor agonist. Histamine increases Nav1.8 expression in primary afferent neurons via H2 receptor-mediated pathway and thereby contributes to neuropathic pain, H2 receptor antagonists may potentially be used as analgesics for patients with neuropathic pain.
Tartaric acid, oxalic acid and histamine are the well-recognized pain-inducing agents in the stinging hairs of U. thunbergiana.