Previously we reported that pretreatment of rats with the substance P (SP) antagonist CP-96,345 inhibits the enterotoxic responses following administration of toxin A from Clostridium difficile into ileal loops, indicating that SP participates in the intestinal responses to this toxin. We now report that injection of toxin A into rat ileum causes a rapid increase in SP content in lumbar dorsal root ganglia (DRG) and mucosal scrapings 30-60 min after toxin A administration. Toxin A-mediated fluid secretion, mannitol permeability, and ileal histologic damage is significantly increased only after 2 hr. Toxin A also causes an increase in the abundance of SP mRNA in lumbar DRG and ileal mucosa as measured by reverse transcription-PCR. Lamina propria macrophages (LPMs) obtained from toxin A-injected loops release greater amounts of tumor necrosis factor alpha (TNFalpha) and SP as compared with LPMs isolated from buffer-injected loops (P < 0.01). Pretreatment of rats with the SP antagonist CP-96,345 inhibits toxin A-mediated TNFalpha release from isolated LPMs, whereas an inactive enantiomer (CP-96,344) of the SP antagonist has no effect. LPMs obtained from toxin A-injected ileal loops incubated in vitro with SP (10(-8) to 10(-9) M) show enhanced TNFalpha secretion, whereas LPMs isolated from buffer-injected loops do not respond to SP. In addition, LPMs obtained from toxin A-injected ileal loops incubated in vitro with CP-96,345 showed a diminished TNFalpha release. Our results indicate that activated LPMs secrete SP during toxin A enteritis that can lead to secretion of cytokines, suggesting an autocrine/paracrine regulation of cytokine secretion by SP from LPMs during intestinal inflammation.