Discovery of naturally occurring splice variants of the rat histamine H3 receptor that act as dominant-negative isoforms

We described previously the cDNA cloning of three functional rat histamine H-3 receptor (rH(3)R) isoforms as well as the differential brain expression patterns of their corresponding mRNAs and signaling properties of the resulting rH(3A), rH(3B), and rH(3C) receptor isoforms (Mol Pharmacol 59:1-8). In the current report, we describe the cDNA cloning, mRNA localization in the rat central nervous system, and pharmacological characterization of three additional rH(3)R splice variants (rH(3D), rH(3E), and rH(3F)) 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-rH(3) 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-rH(3) isoforms. In contrast to the rH(3A) receptor (rH(3A)R), detection of the rH(3D) isoform using hemagglutinin antibodies revealed that the rH3D 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. Coexpression of the rH(3A)R and the rH(3)D isoforms resulted in the intracellular retention of the rH(3A)R and reduced rH(3A)R functionality. Finally, we show that in rat brain, the H3R mRNA expression levels are modulated upon treatment with the convulsant pentylenetetrazole, suggesting that the rH(3)R isoforms described herein thus represent a novel physiological mechanism for controlling the activity of the histaminergic system.