Second generation H1 antihistamines such as desloratadine are classified as inverse agonists at the H1 receptor. This means they:

• A Accelerate receptor internalization, producing functional antagonism over time
• B Bind the receptor but produce effects opposite to those of endogenous histamine through a separate effector pathway
• C Block the constitutively active conformation of the receptor, reducing basal signaling below normal levels ✓
• D Act only on receptors already occupied by histamine, displacing it allosterically

Explanation

Many GPCRs show constitutive activity even without ligand. An inverse agonist preferentially stabilizes the inactive conformation and lowers basal signaling below the unstimulated level, which distinguishes it from a neutral antagonist that simply blocks agonists without changing baseline activity. Option B describes functional antagonism, not inverse agonism, and the effect occurs at unoccupied as well as occupied receptors. Most classical antihistamines are now recognized as inverse agonists rather than simple antagonists.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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