Anatomical and Functional Characterization of Gastrointestinal to Spinal Cord Circuits

Abstract

Viscerosensory functions such as those governing the gastrointestinal (GI) tract are essential for maintaining homeostasis, regulating appetite, and generating the sensory experiences that shape daily life. Sensory circuits linking the colon to dorsal root ganglia (DRG) neurons and spinal cord are required to detect and transmit mechanical and chemical signals from the gut to the central nervous system, enabling appropriate physiological and behavioral responses. Despite their importance, these colon-DRG-spinal cord pathways remain remarkably understudied, leaving fundamental questions about how internal sensory signals are encoded unanswered. Here, we characterized the anatomical and functional organization of colon-DRG-spinal cord circuits and investigated how spinal cord neurons process signals from the colon compared to the skin in normal and disease states. Using genetic labeling strategies targeting colon-innervating DRG subtypes, we identified four distinct central morphologies that include basket-like, simple, peri-central, and midline-crossing arbor subtypes, which are distinct from the more tufted arbor morphology of skin-innervating DRG neurons. In addition, we discovered a unique subset of Calca+ colon-innervating neurons that project through the dorsal columns and terminate in and around the area postrema and nucleus of the tractus solitarius, iii revealing a previously unknown direct pathway for colon-derived sensory information to the brainstem. To assess how sensory information from the colon is processed in the spinal cord, we developed an in vivo electrophysiology recording preparation to assess spinal cord neuron responses to colon or skin stimulation. Surprisingly, the majority of colon- distension-responsive spinal neurons also responded to innocuous skin stroking (‘convergent neurons’). Convergent neurons displayed graded responses to innocuous and noxious colon distension, while colon-stimulus-responsive-only neurons were activated primarily by noxious distension stimuli. Optogenetic activation of colon- and skin-innervating DRG neuron subtypes revealed that convergent neurons exhibited higher peak firing rates and higher response correlations to skin-directed than colon- directed stimuli, suggesting differential integration of visceral versus somatic inputs. Finally, in a mouse model of colitis, we find that convergent neurons exhibited heightened responses to both colon and skin stimulation, including prolonged excitation following innocuous skin touch, while colon-stimulus-responsive-only and skin-stimulus- responsive-only neurons were unaffected by DSS treatment. These data indicate that convergent spinal cord neurons may contribute to cross-sensitization and persistent pain following GI inflammation. Taken together, our findings reveal distinct morphological and functional features of colon-innervating DRG neurons and highlight the extensive convergence of visceral and somatic inputs in the spinal cord, with implications for understanding chronic pain in GI disorders.