SEROTONERGIC ACTIVATION OF TONIC-TYPE ENTERIC NEURONS IN GUINEA-PIG SMALL BOWEL

Electrical stimulation of interganglionic fiber tracts of Auerbach's plexus released a chemical substance, which produced a slow EPSP associated with an increase in input resistance in the postsynaptic neuron. This effect of fiber tract stimulation was mimicked by microiontophoretic application of serotonin (5-HT) to the neurons. Both the endogenous transmitter substance and exogenous 5-HT produced membrane depolarization, both increased the input resistance of the neuron, both reduced or abolished hyperpolarizing afterpotentials of the action potentials, and both augmented membrane excitability. During tachyphylaxis to excess 5-HT (1 μM) in the perfusion solution, the response to fiber tract stimulation was also reversibly blocked. Methysergide (30 μM) blocked both the response to fiber tract stimulation and to exogenous 5-HT. Cholinergic agonists and antagonists did not alter the slow EPSP and the response to exogenous 5-HT. The evidence suggests that 5-HT is the neurotransmitter substance that produces the slow EPSP in myenteric neurons.