SPARC: Determine the topology and function of DRG neurons innervating mouse colon and rectum.

Abstract

Visceral pain is the major complaint of patients with irritable bowel syndrome (IBS) that affects ~15% of the U.S. population, and satisfactory management of IBS pain remains an unmet clinical challenge. Neuromodulation targeting afferent somata in dorsal root ganglions (DRG) has emerged as a new and effective therapy for many types of chronic pain. To advance neuromodulation for treating IBS‐related visceral pain, new knowledge is needed regarding the topological distribution and neural encoding functions of lumbosacral DRG neurons that innervate distal colon and rectum (colorectum). Here, we implemented GCaMP6f to conduct high‐throughput optical recordings of colorectal afferent activities in lumbosacral DRG. By establishing this optical approach with single‐spike resolution, the current study characterizes the neural encoding from hundreds of colorectal afferents in the DRG, which are sparsely distributed and difficult to study with conventional methods. Using a mouse ex vivo preparation with distal colorectum and L5‐S1 DRG in continuity, we recorded 791 colorectal afferents’ responses to graded colorectal distension (15, 30, 40, 60 mmHg) and/or luminal shear flow (20–30 mL/min), then functionally classified them into four mechanosensitive classes, and determined the topological distribution of their somata in the DRG. Of the 791 colorectal afferents, 90.8% were in the L6 DRG, 8.3% in the S1 DRG, and only 0.9% in the L5 DRG. L6 afferents had all four classes: 29% mucosal, 18.4% muscular‐mucosal, 34% low‐threshold (LT) muscular and 18.2% high‐threshold (HT) muscular afferents. S1 afferents only had three classes: 19.7% mucosal, 34.8% LT muscular and 45.5% HT muscular afferents. All seven L5 afferents were HT muscular. In L6 DRG, somata of HT muscular afferents, i.e., putative colorectal mechano‐nociceptors were clustered in the caudal region whereas somata of the other classes did not cluster in specific regions. Outcomes of this study can directly inform the design and improvement of next‐generation neuromodulation devices that likely implement spatially selective DRG modulation to alleviate visceral pain in IBS patients.Support or Funding InformationSupported by NIH SPARC grant U01 NS113873 (BF and GZ)