Isoform dependent differences in histamine H3 receptor constitutive activity revealed by human H3(445) and H3(365) receptors.

Abstract

The histamine H3 receptor (H3R) exists as multiple splice isoforms. Using GTPγS binding and MAPK phosphorylation assays, we compared constitutive signaling by human H3(445) and H3(365) receptor isoforms expressed in C6 rat glioma cells. The basal rate of GTPγS binding was ~50% greater for H3(365) versus H3(445) membranes. H3R inverse agonists inhibited basal H3(365) GTPγS binding and MAPK phosphorylation by up to 35%, whereas maximal GTPγS binding inhibition was ≤ 20% for H3(445). Maximal stimulation of GTPγS binding and MAPK phosphorylation by H3R agonists was ≤ 120% basal for H3(365) while ≥ 2-fold basal levels for H3(445). Pertussis toxin inhibited basal (~20%) and agonist-stimulated MAPK phosphorylation by H3(445) and inhibited H3(365) basal MAPK phosphorylation by ~50%. These results indicate that H3(365) is more constitutively active than H3(445). Additionally, H3R agonists exhibited slightly (~ 3-fold) greater potency with H3(365) in GTPγS binding assays, while H3R inverse agonists exhibited slightly greater potency with H3(445). In [3H]N-α-methylhistamine binding assays, H3R agonists exhibit slightly higher affinity for H3(365) while inverse agonists did not display isoform selectivity. Thus, differing constitutive activities by H3(445) and H3(365) isoforms may be important in the physiologic and potential therapeutic roles of H3R. (Supported by Abbott Laboratories)