Drug receptors consist of recognition sites coupled to transducer and cellular amplifier mechanisms. Slight differences in receptor recognition sites can be exploited pharmacologically to provide drugs with a high degree of selectivity for activating or blocking individual receptor subtypes. For example, it may now be possible to block, selectively, subtypes of muscarinic cholinergic receptors in the gastrointestinal tract with pirenzepine and other drugs that discriminate between subtypes of muscarinic cholinergic receptors. The recognition of subtypes of adrenergic receptors may allow highly selective pharmacological activation and blockage of gastrointestinal neural and smooth muscle receptors. The development of nonpeptide receptor antagonists of gastrointestinal hormones and peptide neurotransmitters also offers promise for improved therapy of digestive diseases. Tremendous progress has occurred in recent years in defining multiple types of opioid receptors that alter gastrointestinal secretory, absorptive and motility functions. These receptors are located in the mucosa, nerves and muscle of the intestine and in the brain and spinal cord.
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