ABSTRACT
As a general rule, when excitatory receptors are located on motor neurons, their activation is associated with contraction, relaxation, or both of gastrointestinal (GI) smooth muscle, depending on the nature of released neurotransmitter(s). Conversely, the relationship between excitatory effects caused by receptors located on interneurons or sensory neurons and final motor responses is less immediate. In the GI tract, 5-HT4 receptors were initially demonstrated in the isolated guinea pig ileum, where they mediate potentiation of electrically induced cholinergic “twitch” contractions. Evidence for a functional role of 5-HT4 in the modulation of gastrointestinal motility is limited to a few findings. The observation that the administration of SDZ 205-557 failed to modify the peristaltic reflex in the isolated guinea pig small intestine can be taken as evidence that 5-HT4 receptors are not the primary target of endogenously released 5-HT during reflex development.
