Agonist-independent activity of G-protein-coupled receptor, also referred to as constitutive activity, is a well-documented phenomenon and has been reported 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 physiological 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. The 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 be enhanced by a variety of H(3) antagonists acting as inverse agonists at the H(3) receptor. Thioperamide, clobenpropit, and iodophenpropit raise the cAMP 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 affected the pharmacological activity of the ligand. Receptor affinity was reduced slightly for most impentamine analogs, but the functional activity of the ligands varied from agonist to neutral antagonist and inverse agonist, indicating that subtle changes in the chemical structures of impentamine analogs have major impact on the (de)activation steps of the H(3) receptor. In conclusion, upon stable expression of the rat and human H(3) receptor in SK-N-MC cells constitutive receptor activity is detected. In this experimental system, H(3) receptors ligands, previously identified as H(3) antagonists, cover the whole spectrum of pharmacological activities, ranging from full inverse agonists to agonists.