Inverse Agonist
A molecule that binds to a receptor and produces an effect opposite to that of an agonist, reducing the receptor's baseline constitutive activity below normal levels. Inverse agonists are pharmacologically distinct from simple antagonists, which only block activation without reducing baseline signalling.
Technical Context
Some receptors exhibit constitutive activity — they produce a low-level biological signal even without ligand binding. A neutral antagonist blocks additional activation but leaves constitutive activity intact. An inverse agonist actively suppresses constitutive activity below baseline. The distinction between antagonism and inverse agonism is pharmacologically significant because inverse agonists can produce effects even in the absence of the natural ligand. Many compounds previously classified as antagonists have been reclassified as inverse agonists upon closer investigation. While more commonly discussed for small molecule GPCRs, the concept applies to any constitutively active receptor system.