Receptors of the enteric nervous system: potential targets for drug therapy.

It is recognised that the enteric nervous system (ENS) has a unique ability to mediate reflex activity independently of input from the brain or spinal cord. This ability implies that the ENS contains sensory receptors, primary aVerent neurones, interneurones, and motor neurones. The events that are controlled, at least in part, by the ENS are multiple and include motor activity, secretion, absorption, blood flow, and interaction with other organs such as the gall bladder or pancreas. The extensive regulatory activities of the ENS are made possible by the presence and abundance of diVerent types of neurones within the wall of the gastrointestinal tract. The ENS contains about 10 neurones, approximately the number of neurones found in the spinal cord. Morphological, electrophysiological, and pharmacological studies have revealed a substantial diversity of neurones within the ENS. Moreover, a surprisingly large number of established or candidate neurotransmitters can be found in enteric neurones. Most neurones contain several of these substances, and distinctive patterns of colocalisation of mediators allow identification of diVerent functional classes of neurones. 3 The presence of receptors on enteric neurones is another expression of their heterogeneity. Myenteric neurones can express receptors for both peptide and nonpeptide (amines, amino acids, purines) neurotransmitters. Generally, expression of a receptor is limited to a subset of myenteric neurones, with probably the only exception being expression of nicotinic cholinergic receptors on all myenteric neurones in the guinea pig stomach. 7 Unfortunately, we know very little about the relationship between the functional role of enteric neurones and their expression of receptors.