Identification, localization and classification of muscarinic receptor subtypes in the gut.

Muscarinic receptors in the gastrointestinal tract are present on enteric neurons, presynaptic and prejunctional axonal endings, intramural endocrine cells as well as directly on effector cells such as smooth muscle and glandular and epithelial cells. Neural M1 stimulatory receptors are present on myenteric and submucous neurons, while neural M2 inhibitory receptors are present on their axonal endings. Muscle M2 and glandular M2 receptors are stimulatory. Functional and ligand binding studies show that there is heterogeneity among different muscarinic receptors in the gastrointestinal tract. The neural M1 muscle M2 and glandular M2 receptors are distinct from each other, but presynaptic and prejunctional M2 receptors appear to be similar to muscle M2 receptors. The relationship of the gut muscarinic receptors to the structurally-defined muscarinic receptors in the brain is unclear. However, they appear to be different from cardiac M2 and brain M2 receptors.