Constitutive activity of the histamine H(1) receptor

The histamine H(1) receptor (H(1)R) is a key player in acute inflammatory responses. Antihistamines are widely used to relief the symptoms of allergic rhinitis by antagonizing histamine binding to the H(1)R, without possessing intrinsic activity in classical assays such as guinea pig ileum contraction assays or intracellular Ca(2+) mobilization. Overexpression of H(1)R in heterologous cell lines unmasked the capacity of this receptor to signal in a histamine-independent manner. Moreover, a recent screen of therapeutic and reference antagonists on these H(1)R-overexpressing cells revealed that the majority of these drugs are in fact inverse agonists, as they inhibit basal H(1)R activity. In this chapter, we describe several approaches to study H(1)R constitutive signaling that can be used to identify inverse agonists acting at this blockbuster target.