Serotonin mediates a slow excitatory potential in mammalian celiac ganglia

Neuropharmacological and histochemical evidence presented here indicates that serotonin (5-HT) is the transmitter mediating one of the postsynaptic potentials in the guinea pig celiac ganglion. Repetitive nerve stimulation elicited in celiac neurons, in addition to the nicotinic fast excitatory postsynaptic potential (EPSP), a slow EPSP that was resistant to cholinergic antagonists. Application of 5-HT caused a depolarization with membrane characteristics similar to those of the slow EPSP; furthermore, the latter was reversibly suppressed by 5-HT. The slow depolarization evoked by either nerve stimulation or 5-HT was augmented by fluoxetine, a 5-HT reuptake blocker, and depressed by cyproheptadine, a 5-HT receptor blocker; in addition, tryptophan, a precursor of 5-HT, enhanced differentially the slow EPSP. Lastly, histochemical study revealed dense networks of 5-HT immunoreactive nerve fibers encircling many ganglionic neurons.