Visceral sensory neurons that innervate both uterus and colon express nociceptive TRPv1 and P2X3 receptors in rats.

In women, clinical studies suggest that functional pain syndromes such as irritable bowel syndrome, interstitial cystitis, and fibromyalgia, are co-morbid with endometriosis, chronic pelvic pain, and others diseases. One of the possible explanations for this phenomenon is visceral cross-sensitization in which increased nociceptive input from inflamed reproductive system organs sensitize neurons that receive convergent input from an unaffected visceral organ to the same dorsal root ganglion (DRG). The purpose of this study was to determine whether primary sensory neurons that innervate both visceral organs--the uterus and the colon--express nociceptive ATP-sensitive purinergic (P2X3) and capsaicin-sensitive vanilloid (TRPV1) receptors. To test this hypothesis, cell bodies of colonic and uterine DRG were retrogradely labeled with fluorescent tracer dyes micro-injected into the colon/rectum and uterus of rats. Ganglia were harvested, cryo-protected, and cut in 20-microm slices for fluorescent microscopy to identify positively stained cells. Up to 5% neurons were colon-specific or uterus-specific, and 10%-15% of labeled DRG neurons innervate both viscera in the lumbosacral neurons (L1-S3 levels). We found that viscerally labeled DRGs express nociceptive P2X3 and TRPV1 receptors. Our results suggest a novel form of visceral sensory integration in the DRG that may underlie co-morbidity of many functional pain syndromes.