Purinergic antagonists include clonidine, a potent P 1 antagonist that is also an α 2 and
H 2 agonist (see section 4.3.6), facilitating purine release. Methylxanthines, especially
caffeine (4.241), are potent P 1 antagonists. When one considers the enormous amount
of caffeine consumed in the world, this discovery is significant for understanding the
symptoms of caffeine addiction. Many classes of A 1 receptor antagonists have been
described; the majority of these have traditionally been xanthine analogs. Likewise, a
number of A2Areceptor antagonists based on xanthines and related heterocyclic core
structures have been described. Finally, a structurally diverse set of A 3 receptor antag-
onists has recently been identified.
At the time of writing, many compounds based on the adenosine receptor are in pre-
clinical or early clinical development.
Selected References
Overview of Relevant Neuroanatomy and Neurophysiology
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Acetylcholine and Cholinergic Receptors
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W. H. Bunnelle, M. J. Dart, M. R. Schrimpf (2004). Design of ligands for the nicotinic acetyl-
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