Constitutive activity of histamine H-3 receptors stably expressed in SK-N-MC cells: Display of agonism and inverse agonism by H-3 antagonists

Agonist-independent activity of G-protein-coupled receptor, also referred t o as constitutive activity, is a well-documented phenomenon and has been re ported recently for both the histamine H-1 and H-2 receptors. Using SK-N-MC cell lines stably expressing the human and rat H-3 receptors at physiologi cal receptor densities (500-600 fmol/mg of protein), we show that both the rat and human H-3 receptors show a high degree of constitutive activity. Th e forskolin-mediated cAMP production in SK-N-MC cells is inhibited strongly upon expression of the G(i)-coupled H-3 receptor. The cAMP production can be further inhibited upon agonist stimulation of the H-3 receptor and can b e enhanced by a variety of H-3 antagonists acting as inverse agonists at th e H-3 receptor. Thioperamide, clobenpropit, and iodophenpropit raise the cA MP levels in SK-N-MC cells with potencies that match their receptor binding affinities. Surprisingly, impentamine and burimamide act as effective H-3 agonists. Modification of the amine group of impentamine dramatically affec ted the pharmacological activity of the ligand. Receptor affinity was reduc ed slightly for most impentamine analogs, but the functional activity of th e ligands varied from agonist to neutral antagonist and inverse agonist, in dicating that subtle changes in the chemical structures of impentamine anal ogs have major impact on the (de)activation steps of the H3 receptor. In co nclusion, upon stable expression of the rat and human H-3 receptor in SK-N- MC cells constitutive receptor activity is detected. In this experimental s ystem, H-3 receptors ligands, previously identified as H-3 antagonists, cov er the whole spectrum of pharmacological activities, ranging from full inve rse agonists to agonists.