Cloning and Functional Expression of the Human Histamine H₃Receptor

TLDR

Histamine modulates neurotransmitter release via presynaptic H3 receptors, yet the receptor’s molecular identity remained unknown despite pharmacological evidence for 15 years. The study aimed to identify and clone the human H3 receptor to provide cDNA for developing reagents and advancing understanding of histamine’s role in brain function. Using homology searches of expressed sequence tag databases, a partial GPCR97 clone was isolated and then screened against a human thalamus library to obtain a full‑length G protein‑coupled receptor. The full‑length receptor showed homology to M2 muscarinic receptors, inhibited forskolin‑stimulated cAMP in response to histamine, possessed a pharmacological profile indistinguishable from the H3 receptor, and was widely expressed in rat brain neuronal systems, confirming it as the human H3 receptor.

Abstract

Histamine regulates neurotransmitter release in the central and peripheral nervous systems through H₃ presynaptic receptors. The existence of the histamine H₃ receptor was demonstrated pharmacologically 15 years ago, yet despite intensive efforts, its molecular identity has remained elusive. As part of a directed effort to discover novel G protein-coupled receptors through homology searching of expressed sequence tag databases, we identified a partial clone (GPCR97) that had significant homology to biogenic amine receptors. The GPCR97 clone was used to probe a human thalamus library, which resulted in the isolation of a full-length clone encoding a putative G protein-coupled receptor. Homology analysis showed the highest similarity to M2 muscarinic acetylcholine receptors and overall low homology to all other biogenic amine receptors. Transfection of GPCR97 into a variety of cell lines conferred an ability to inhibit forskolin-stimulated cAMP formation in response to histamine, but not to acetylcholine or any other biogenic amine. Subsequent analysis revealed a pharmacological profile practically indistinguishable from that for the histamine H₃ receptor. In situ hybridization in rat brain revealed high levels of mRNA in all neuronal systems (such as the cerebral cortex, the thalamus, and the caudate nucleus) previously associated with H₃ receptor function. Its widespread and abundant neuronal expression in the brain highlights the significance of histamine as a general neurotransmitter modulator. The availability of the human H₃ receptor cDNA should greatly aid in the development of chemical and biological reagents, allowing a greater appreciation of the role of histamine in brain function.