Serotonergic Paracrine Targets in the Intestinal Mucosa.

Serotonin functions as a neurotransmitter in the enteric nervous system. Aside from its neurotransmitter role, serotonin also is a paracrine mediatorial signal in the digestive tract. It is a major paracrine signaling molecule in the integrated physiology of several classes of cells in the intestinal mucosa. Paracrine action can be initiation or suppression of activity in populations of cells that make up divergent phenotypic classes. This underlies phenotypic plasticity in single classes and links single classes to other neighboring phenotypic classes, thereby forming a single and higher-order organization in which different categories of function are integrated to work in harmony as a single homeostatic entity at higher levels of physiological organization. Phenotypic classes of cells that are linked by serotonergic paracrine signaling at upper levels of functional organization in the small intestine are (1) enterochromaffin cells; (2) enteric mast cells; (3) spinal sensory afferents; (4) sympathetic postganglionic neurons; (5) enteric neurons.