Objective TRPV1 is a nonspecific cation channel of the transient receptor potential channel family and may be upregulated during visceral nociceptive processing.The expression of TRPV1 of nerve fibres, mucosa and muscular layers in gastro-intestinal tract was increased in visceral pain.Recent studies demonstrated that TRPV1 is expressed in peripheral blood cells (PBCs).The aim was to investigate whether TRPV1 expression in PBCs is increased in chronic visceral hypersensitivity in adult rats induced by colon irritation during postnatal development.Methods Hypersensitive SD rats were generated by coloreetal distension (CRD) during postnatal days 8 to 14.The sensitivity of the colon to balloon distention in adult was measured by grading their abdominal withdrawal reflex (AWR).The PBCs were obtained by cardiac puncture after distention.Mononuclear cells were isolated from PBCs by density gradient centrifugation.RT-PCR was performed to detect the expression of TRPV1 and GAPDH mRNA.The signal intensity of each TRPV1 band was normalized by GAPDH.The ODs in each sample were expressed as a ratio of TRPV l over GAPDH densitometric intensities.Results The AWR score in response to CRD in adult rats with neonatal colorectal irritation was significantly higher than control with distension pressures of 20 (Mean±-SD, 1.069±0.457 vs 0.1667±0.389, P<0.01), 40 (2.069±0.529 vs 1.25±0.753, P<0.01), 60 (3.137±0.639 vs 2.20±0.63, P<0.01) and 80 mmHg (3.827±0.601 vs 3.20±0.632, P<0.05).The expression of TRPV1 mRNA of mononuclear cells of PBCs in the control group was in lower level (Mean±SD, 0.148±0.084).The TRPV1 expression in neonatal CRD group was increased in adults (Mean±SD, 0.724±0.593, P>0.05).Conclusions The colorectal distension in neonates can result in visceral hypersensitivity to CRD in adult rats compared with controls.The TRPV1 mRNA is expressed in PBCs and its expression may show up-regulation by neonatal colon irritation.The results suggest that the TRPV1 expression in PBCs may be involved in abdominal pain to CRD.