Identification and mechanosensitivity of viscerofugal neurons

Enteric viscerofugal neurons are interneurons with cell bodies in the gut wall; they project to prevertebral ganglia where they provide excitatory synaptic drive to sympathetic neurons which control intestinal motility and secretion. Here, we studied the mechanosensitivity and firing of single, identified viscerofugal neurons in guinea-pig distal colon. Flat sheet preparations of gut were set up in vitro and conventional extracellular recordings made from colonic nerve trunks. The nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (1mM), was locally pressure ejected onto individual myenteric ganglia. In a few ganglia, DMPP promptly evoked firing in colonic nerves. Biotinamide filling of colonic nerves revealed that DMPP-responsive sites corresponded to viscerofugal nerve cell bodies. This provides a robust means to positively identify viscerofugal neuron firing. Of 15 single units identified in this way, none responded to locally-applied capsaicin (1μM). Probing with von Frey hairs at DMPP-responsive sites reliably evoked firing in all identified viscerofugal neurons (18/18 units tested; 0.8-5mN). Circumferential stretch of the preparation increased firing in all 14/14 units (1-5g, p<0.05). Both stretch and von Frey hair responses persisted in Ca²⁺-free solution (6mMMg²⁺, 1mM EDTA), indicating that viscerofugal neurons are directly mechanosensitive. To investigate their adequate stimulus, circular muscle tension and length were independently modulated (BAY K8644, 1μM and 10μM, respectively). Increases in intramural tension without changes in length did not affect firing. However, contraction-evoked shortening, under constant load, significantly decreased firing (p<0.001). In conclusion, viscerofugal neuron action potentials contribute to recordings from colonic nerve trunks, in vitro. They provide a significant primary afferent output from the colon, encoding circumferential length, largely independent of muscle tension. All viscerofugal neurons are directly mechanosensitive, although they have been reported to receive synaptic inputs. In short, viscerofugal neurons combine interneuronal function with length-sensitive mechanosensitivity.