A novel mode of sympathetic reflex activation mediated by the enteric nervous system

Abstract

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex. Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with viscerofugal firing at 2Hz that aligned with 2Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after 4-5 days organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

SIGNIFICANCE STATEMENT Significant interest exists in how the gut can control other body systems. Enteric viscerofugal neurons uniquely project axons out the gut wall forming circuits with prevertebral sympathetic neurons. Long considered principally transmitting mechanosensory information, a new mechanism is demonstrated here whereby a synchronized ENS-generated firing pattern underlying natural gut motor behaviour is also relayed through populations of viscerofugal neurons. Remarkably, this caused parallel firing in sympathetic neurons in the pattern generated by the ENS. This did not require dynamic mechanical activity. The identification of this mechanism revises the current concept of sympathetic reflexes being simply distension reflexes.