Receptors in the gastrointestinal tract.
The receptor concept has been recently evolved and a new science was actually created, namely "receptorology". Receptors are now identified by means of different techniques (binding, agonist-antagonist interaction, autoradiography, etc.). The new techniques allowed the investigators to define new receptors and new subtypes of the "classical" ones. In the gastrointestinal (GI) tract a number of receptors have been identified and localized both on the effector organ and in the nerve terminal where they exert an important modulatory function on the neurotransmitter release. Recent biochemical studies have allowed a better understanding of the post-receptor event involving the second or third messenger regulation. Particular changes of receptors were recognized and they allow us to consider receptors not as static entities but as very dynamic components of the plasma membrane capable of different kinds of alterations, like interconversion, internalization, mobility, up- and downregulation, etc. Together with the "classical" receptors (cholinergic, adrenergic, opioid, etc.) also new receptors were identified: different subtypes of receptors for the tachykinins, for prostaglandin of the E type in the gastric parietal cell and the so-called dihydropyridine (DHP) receptor in the calcium channel of different areas of the gut. It is obvious that the precise knowledge of receptors and of their agonists and antagonists will represent the basis for a more specific and efficacious treatment of various gastrointestinal disorders.