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The H4R (histamine H4 receptor) is the latest identified member of the histamine receptor subfamily of GPCRs (G-protein-coupled receptors) with potential functional implications in inflammatory diseases and cancer. The H4R is primarily expressed in eosinophils and mast cells and has the highest homology with the H3R. The occurrence of at least twenty different hH3R (human H3R) isoforms led us to investigate the possible existence of H4R splice variants. In the present paper, we report on the cloning of the first two alternatively spliced H4R isoforms from CD34+ cord blood-cell-derived eosinophils and mast cells. These H4R splice variants are localized predominantly intracellularly when expressed recombinantly in mammalian cells. We failed to detect any ligand binding, H4R–ligand induced signalling or constitutive activity for these H4R splice variants. However, when co-expressed with full-length H4R [H4R(390) (H4R isoform of 390 amino acids)], the H4R splice variants have a dominant negative effect on the surface expression of H4R(390). We detected H4R(390)–H4R splice varianthetero-oligomers by employing both biochemical (immunoprecipitation and cell-surface labelling) and biophysical [time-resolved FRET (fluorescence resonance energy transfer)] techniques. mRNAs encoding the H4R splice variants were detected in various cell types and expressed at similar levels to the full-length H4R(390) mRNA in, for example, pre-monocytes. We conclude that the H4R splice variants described here have a dominant negative effect on H4R(390) functionality, as they are able to retain H4R(390) intracellularly and inactivate a population of H4R(390), presumably via hetero-oligomerization. [ABSTRACT FROM AUTHOR]

A dual radioligand binding and electrophysiological study, focusing on a range of ligand-gated ion channels, was performed with a chemically-validated essential oil derived from Melissa officinalis (MO), which has shown clinical benefit in treating agitation. MO inhibited binding of [35S] t-butylbicyclophosphorothionate (TBPS) to the rat forebrain gamma-aminobutyric acid (GABA)A receptor channel (apparent IC50 0.040±0.001 mg mL−1), but had no effect on N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropianate (AMPA) or nicotinic acetylcholine receptors. Electrophysiological analyses with primary cultures of rat cortical neurons demonstrated that MO reversibly inhibited GABA-induced currents in a concentration-dependent manner (0.01–1 mg mL−1), whereas no inhibition of NMDA- or AMPA-induced currents was noted. Interestingly, MO elicited a significant dose-dependent reduction in both inhibitory and excitatory transmission, with a net depressant effect on neurotransmission (in contrast to the classical GABAA antagonist picrotoxinin which evoked profound epileptiform burst firing in these cells). The anti-agitation effects in patients and the depressant effects of MO in in-vitro we report in neural membranes are unlikely to reflect a sedative interaction with any of the ionotropic receptors examined here. [ABSTRACT FROM AUTHOR]