Abstract

The recent cloning and characterization of the human histamine H₃ receptor cDNA marked a significant step toward a greater understanding of the role of this receptor in the central nervous system. We now report the cloning of the rat histamine H₃receptor cDNA and demonstrate distinct pharmacological species differences. The rat cDNA clone encodes a putative 445-amino acid protein with 93% identity to the human H₃ receptor. Northern blot analysis revealed a major single entity of 2.7-kb in length expressed only in brain. Transfection of the rat receptor cDNA into SK-N-MC cells conferred an ability to inhibit forskolin-stimulated cAMP formation in response to histamine and other H₃agonists. N-[³H]methylhistamine saturably bound to transfected cells with high affinity (K_d = 0.8 nM). All agonists tested had low or subnanomolar K_i values similar to that for the human recombinant receptor. The antagonists thioperamide and clobenpropit also bound with high affinity (K_i = 4 and 0.4 nM, respectively). This is in contrast to the antagonist profile obtained for the human recombinant receptor that showed K_i values of 58 and 0.6 nM for thioperamide and clobenpropit, respectively. These data suggest that the low affinity of thioperamide for the human H₃ receptor represents a species difference in pharmacology and not a unique pharmacological subtype. It also was found that chloroproxyfan behaved as a full agonist at the rat recombinant receptor. These findings highlight the significance of validating the pharmacology of experimental compounds at both the rat and human H₃ receptors.