Differences in ligand binding profiles between cloned rabbit and human 5-HT1D alpha and 5-HT1D beta receptors: ketanserin and methiothepin distinguish rabbit 5-HT1D receptor subtypes.

The study of serotonin receptor function has been complicated by the extreme molecular diversity of serotonin receptor subtypes, the lack of selective agonists and antagonists for many of the subtypes, and divergence in the pharmacological properties of a single receptor subtype across different animal species. An example of this pharmacological diversity between species homologues is provided by the 5-HT1D receptor subfamily. To further advance the ability to characterize and pharmacologically compare functional responses mediated by native 5-HT1D receptors, we have cloned the 5-HT1D alpha and 5-HT1D beta receptor subtypes from the rabbit and evaluated their pharmacological profiles using radioligand binding assays. The deduced amino acid sequences of the rabbit 5-HT1D alpha and 5-HT1D beta receptor genes displayed 60% overall identity [75% transmembrane (TM) identity] to each other and > 90% overall identity (95% TM identity) to their corresponding human homologues. Two compounds were identified in binding assays which discriminated between the closely-related 5-HT1D receptors. Ketanserin exhibited high affinity (pKi = 7.66) and selectivity (> 20-fold) for the 5-HT1D alpha receptor while methiothepin displayed high affinity (pKi = 7.86) and selectivity (16-fold) for the 5-HT1D beta receptor subtype. The rabbit and human recombinant 5-HT1D receptors showed significant intraspecies (rabbit 5-HT1D alpha vs. 5-HT1D beta) and interspecies (i.e. rabbit vs. human 5-HT1D alpha) similarities in their ligand binding profiles. These data suggest that 5-HT1D-mediated responses in rabbit preparations may provide information relevant to the pharmacology of the 5-HT1D receptor subtypes in humans.