The Role of Lumbar Splanchnic Innervations in Visceral Nociception and Pain

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Project Summary/Abstract Chronic visceral pain is the cardinal symptom of patients with irritable bowel syndrome (IBS) affecting up to 15% of the U.S. population. Efficacious and reliable therapeutic intervention is still unavailable despite the tremendous economic burden imposed by visceral pain. Drugs to treat visceral pain impact both the peripheral and central nervous systems (PNS, CNS) due to similar ion channel/modulator composition, and CNS-related side effects usually outweigh analgesic benefits. Visceral pain differs significantly from other types of pain in the `adequacy' of nociceptive stimuli, defined first by Sherrington as triggering painful and noxious reactions. Noxious cutaneous stimuli (e.g., cutting, pinching, burning) are not reliably nociceptive when applied to hollow visceral organs, whereas mechanical visceral organ distension (stretch/tension) is `adequately' nociceptive. In addition to previous studies that reveal the role of pelvic nerve (PN) afferents in encoding colorectal distension and contributing to prolonged colorectal hypersensitivity, we reveal, for the first time, a more significant participation of afferents in the lumbar splanchnic nerves (LSN) in encoding colorectal distension than previously assumed: ~40% of LSN afferents encode axial colorectal stretch, which is also produced by colorectal distension. We also found that: 1) the colorectal region with dense LSN innervation (next to the mesentery) is more compliant mechanically than the adjacent region, and 2) the colorectal submucosa has a rich network of load-bearing collagen fibers. Our new neural and mechanical data suggest an underappreciated role for LSN afferents in encoding colorectal distension, an `adequate,' noxious stimulus that evokes visceral pain in IBS patients. Accordingly, the objective of this proposal is to reveal lumbar splanchnic afferent neural encoding of colorectal distension and nociception at macro- and micro-mechanical, and molecular levels. Three specific aims are proposed. Aim 1 will quantify lumbar splanchnic afferent neural encoding of colorectal distension and colorectal nociception in prolonged colorectal hypersensitivity. Aim 2 will quantify macro- and micro-mechanics of differential mechanical neural encoding of colorectal afferent endings in the lumbar splanchnic pathway. Aim 3 will define the molecular profiles relevant to colorectal mechanosensitivity of different lumbar splanchnic afferent classes in prolonged colorectal hypersensitivity. The proposed study of the biomechanical factors in colorectal mechanosensitivity and hypersensitivity will complement existing neurophysiological approaches to synergistically advance our mechanistic understanding of colorectal afferent neural encoding and nociception, especially in the lumbar splanchnic pathway. Through this proposed research, we will establish the influence of biomechanics in colorectal mechanosensitivity and nociception in prolonged colorectal hypersensitivity. This work will provide a rationale to identify novel biomechanical and potential `drugable' targets for managing chronic IBS pain while minimizing off-target CNS effects.