Viscerofugal neurons recorded from guinea-pig colonic nerves after organ culture

Background: Enteric viscerofugal neurons provide cholinergic synaptic inputs to prevertebral sympathetic neurons, forming reflex circuits that control motility and secretion. Extracellular recordings of identified viscerofugal neurons have not been reported.

Methods: Preparations of guinea pig distal colon were maintained in organotypic culture for 4-6 days (n = 12), before biotinamide tracing, immunohistochemistry, or extracellular electrophysiological recordings from colonic nerves.

Key results: After 4-6 days in organ culture, calcitonin gene-related peptide and tyrosine hydroxylase immunoreactivity in enteric ganglia was depleted, and capsaicin-induced firing (0.4 μmol L(-1) ) was not detected, indicating that extrinsic sympathetic and sensory axons degenerate in organ culture. Neuroanatomical tracing of colonic nerves revealed that viscerofugal neurons persist and increase as a proportion of surviving axons. Extracellular recordings of colonic nerves revealed ongoing action potentials. Interestingly, synchronous bursts of action potentials were seen in 10 of 12 preparations; bursts were abolished by hexamethonium, which also reduced firing rate (400 μmol L(-1) , P < 0.01, n = 7). DMPP (1,1-dimethyl-4-phenylpiperazinium; 10(-4) mol L(-1) ) evoked prolonged action potential discharge. Increased firing preceded both spontaneous and stretch-evoked contractions (χ(2) = 11.8, df = 1, P < 0.001). Firing was also modestly increased during distensions that did not evoke reflex contractions. All single units (11/11) responded to von Frey hairs (100-300 mg) in hexamethonium or Ca(2+) -free solution.

Conclusions & inferences: Action potentials recorded from colonic nerves in organ cultured preparations originated from viscerofugal neurons. They receive nicotinic input, which coordinates ongoing burst firing. Large bursts preceded spontaneous and reflex-evoked contractions, suggesting their synaptic inputs may arise from enteric circuitry that also drives motility. Viscerofugal neurons were directly mechanosensitive to focal compression by von Frey hairs.