Received for publication September 29, 2005.
Revised December 14, 2005.
Accepted for publication December 21, 2005.

Discovery of naturally occurring splice variants of the rat histamine H₃ receptor that act as dominant negatives

Abstract

We previously described the cDNA cloning of three functional rat histamine H₃ receptor (rH₃R) isoforms as well as the differential brain expression patterns of their corresponding mRNAs and signalling properties of the resulting rH_3A, rH_3B, and rH_3C receptor isoforms (Drutel et al. (2001) Mol Pharmacol. 59:1-8). Herein we describe the cDNA cloning, mRNA localisation in the rat CNS and a pharmacological characterisation of three additional rH₃R splice variants (rH_3D-F) that differ from the previously published isoforms in that they result from an additional alternative-splicing event. These new H₃R isoforms lack the seventh transmembrane (TM) helix and contain an alternative, putatively extracellular, C-terminus (6TM-rH₃ isoforms). After heterologous expression in COS-7 cells, radioligand binding or functional responses upon the application of various H₃R ligands could not be detected for the 6TM-rH₃ isoforms. In contrast to the rH_3A receptor (rH_3AR), detection of the rH_3D isoform using HA-antibodies revealed that the rH_3D isoform remains mainly intracellular. The expression of the rH_3D-F splice variants, however, modulates the cell surface expression-levels and subsequent functional responses of the 7TM-H₃R isoforms. Co-expression of the rH_3AR and the rH_3D isoforms resulted in the intracellular retention of the rH_3AR and reduced rH_3AR functionality. Finally, we show that in rat brain the H₃R mRNA expression levels are modulated upon treatment with the convulsant pentylenetetrazole, suggesting that the herein described rH₃R isoforms thus represent a novel physiological mechanism for controlling the activity of the histaminergic system.

Key words: Histamine, Gi family, Alternative splicing/RNA editing, Receptor synthesis/trafficking

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