The gut-brain axis: spatial relationship between spinal afferent nerves and 5-HT-containing enterochromaffin cells in mucosa of mouse colon.

Cross talk between the gastrointestinal tract and brain is of significant relevance for human health and disease. However, our understanding of how the gut and brain communicate has been limited by a lack of techniques to identify the precise spatial relationship between extrinsic nerve endings and their proximity to specific cell types that line the inner surface of the gastrointestinal tract. We used an in vivo anterograde tracing technique, previously developed in our laboratory, to selectively label single spinal afferent axons and their nerve endings in mouse colonic mucosa. The closest three-dimensional distances between spinal afferent nerve endings and axonal varicosities to enterochromaffin (EC) cells, which contain serotonin (5-hydroxytryptamine; 5-HT), were then measured. The mean distances (± standard deviation) between any varicosity along a spinal afferent axon or its nerve ending, and the nearest EC cell, were 5.7 ± 6.0 μm (median: 3.6 μm) and 26.9 ± 18.6 μm (median: 24.1 μm), respectively. Randomization of the spatial location of EC cells revealed similar results to this actual data. These distances are ∼200-1,000 times greater than those between pre- and postsynaptic membranes (15-25 nm) that underlie synaptic transmission in the vertebrate nervous system. Our findings suggest that colonic 5-HT-containing EC cells release substances to activate centrally projecting spinal afferent nerves likely via diffusion, as such signaling is unlikely to occur with the spatial fidelity of a synapse. We show an absence of close physical contact between spinal afferent nerves and 5-HT-containing EC cells in mouse colonic mucosa. Similar relative distances were observed between randomized EC cells and spinal afferents compared with actual data. This spatial relationship suggests that substances released from colonic 5-HT-containing EC cells are unlikely to act via synaptic transmission to neighboring spinal afferents that relay sensory information from the gut lumen to the brain.