TRPV1 and colitis: Because it has previously been shown that toxin A stimulates release of the proinflammatory neurotransmitter SP in the rat ileum via activation of TRPV1 [14, 17], we reasoned that if 5-ASA inhibits toxin A-induced colitis by blocking the generation of an endogenous TRPV1 agonist such as LTB4, then 5-ASA and sulfasalazine, but not sulfapyridine, should inhibit toxin A-induced SP release in the colon.