Pharmacological characterization of the zebrafish Hrh2a histamine H₂ receptor

The zebrafish, Danio rerio, is a widely adopted in vivo model that conserves organs such as the liver, kidney, stomach, and brain, being, therefore, suitable for studying human diseases, drug discovery and toxicology. The brain aminergic systems are also conserved and the histamine H₁, H₂ and H₃ receptors were previously cloned and identified in the zebrafish brain. Genome studies identified another putative H₂ receptor (Hrh2) with ∼50% sequence identity with H₂ receptor orthologs. In this study, we recombinantly expressed both zebrafish H₂ receptor paralogs (hrh2a and hrh2b) and compared their pharmacology with the human H₂ receptor ortholog. Our results showed that both zebrafish receptors conserve all the class A GPCR motifs. However, in contrast with the Hrh2a paralog, the Hrh2b does not possess all the amino acid residues shown to participate in histamine binding. The zebrafish Hrh2a receptor displays high affinity for [³H]-tiotidine with a binding profile for H₂ receptor ligands similar to that of the human H₂ receptor. The zebrafish Hrh2a receptor couples to Gα_S and Gα_q/11 proteins, resulting in cAMP accumulation and activation of several reporter genes linked to the Gα_q/11 pathway. Additionally, this receptor shows high constitutive activity, with histamine potency in the low nanomolar range for cAMP accumulation and the micromolar range for the activation of the NFAT response element. Moreover, dimaprit and amthamine seem to preferentially activate Gα_S over Gα_q/11 proteins via the zebrafish Hrh2a receptor. These results can contribute to clarifying the functional roles of the H₂ receptor in zebrafish.