Substance P gene expression in acute experimental colitis

We have previously shown (Gastroenterology, 101 (1991) 1211-1219), that substance P (SP) decreases early in the colon muscle layer after induction of colitis in rabbits. Since the SP content in the colonic muscle layer was unchanged by sensory denervation with capsaicin, we assume that SP is located predominantly in intrinsic neurons of the colon, and that the decrease of SP during inflammation reflects changes in intrinsic SP content. However, damage of SP neurons by inflammation would be another likely explanation for the decrease in SP content. The aim of this study was to determine SP gene expression in intrinsic (colonic muscle layer) and extrinsic (dorsal root ganglia, DRG) neurons to prove that SP transcription is preserved during colitis as an indicator of neuronal integrity. Colitis was induced in adult white New Zealand rabbits by formalin enemas (4 ml of 0.4% formalin) followed by 0.85 ml immune complex i.v. 2 h later. The animals were sacrificed 48 h after induction of colitis, and SP content and gene expression were determined by radioimmunoassay (RIA) and Northern blot analysis, respectively. Immunoreactive SP was reduced by 40% in the colon muscle layer and by 60% in the dorsal root ganglia (L4-S4) in the animals with colitis compared to controls without colitis. In contrast to the protein data, SP gene expression was not significantly altered in the colon muscle layer and the dorsal root ganglia 48 h after induction of inflammation compared to the control tissue. The preserved SP gene expression suggests that the intrinsic and extrinsic SP neurons are viable in this inflammatory model. The decreases of immunoreactive SP in the colonic extracts and the DRG after induction of colitis suggest that SP is released from extrinsic and intrinsic neurons during inflammation.