Differences in ligand binding profiles between cloned rabbit and human 5-HT sub(1D alpha ) and 5-HT sub(1D beta ) receptors: Ketanserin and methiothepin distinguish rabbit 5-HT sub(1D) 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-HT sub(1D) receptor subfamily. To further advance the ability to characterize and pharmacologically compare functional responses mediated by native 5-HT sub(1D) receptors, we have cloned the 5-HT sub(1D alpha ) and 5-HT sub(1D beta ) receptor subtypes from the rabbit and evaluated their pharmacological profiles using radioligand binding assays. The deduced amino acid sequences of the rabbit 5-HT sub(1D alpha ) and 5-HT sub(1D 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-HT sub(1D) receptors. Ketanserin exhibited high affinity (pK sub(i) = 7.66) and selectivity ( > 20-fold) for the 5-HT sub(1D alpha ) receptor while methiothepin displayed high affinity (pK sub(i) = 7.86) and selectivity (16-fold) for the 5-HT sub(1D beta ) receptor subtype. The rabbit and human recombinant 5-HT sub(1D) receptors showed significant intraspecies (rabbit 5-HT sub(1D alpha ) vs. 5-HT sub(1D beta )) and interspecies (i.e. rabbit vs. human 5-HT sub(1D alpha )) similarities in their ligand binding profiles. These data suggest that 5-HT sub(1D)-mediated responses in rabbit preparations may provide information relevant to the pharmacology of the 5-HT sub(1D) receptor subtypes in humans.