Differential homologous desensitization of the human histamine H3 receptors of 445 and 365 amino acids expressed in CHO-K1 cells
Histamine H3 receptors (H3Rs) signal through Gαi/o proteins and are found in neuronal cells as auto- and hetero-receptors. Alternative splicing of the human H3R (hH3R) originates 20 isoforms, and the mRNAs of two receptors of 445 and 365 amino acids (hH3R445 and hH3R365) are widely expressed in the human brain. We previously showed that the hH3R445 stably expressed in CHO-K1 cells experiences homologous desensitization. The hH3R365 lacks 80 residues in the third intracellular loop, and in this work we therefore studied whether this isoform also experiences homologous desensitization and the possible differences with the hH3R445. In clones of CHO-K1 cells stably expressing similar receptor levels (211 ± 12 and 199 ± 16 fmol/mg protein for hH3R445 and hH3R365, respectively), there were no differences in receptor affinity for selective H3R ligands or for agonist-induced [35S]-GTPγS binding to membranes and inhibition of forskolin-stimulated cAMP accumulation in intact cells. For both cell clones, pre-incubation with the H3R agonist RAMH (1 μM) resulted in functional receptor desensitization, as indicated by cAMP accumulation assays, and loss of receptors from the cell surface and reduced affinity for the agonist immepip in cell membranes, evaluated by radioligand binding. However, functional desensitization differed in the maximal extent (96 ± 15% and 58 ± 8% for hH3R445 and hH3R365, respectively) and the length of pre-exposure required to reach the maximum desensitization (60 and 30 min, respectively). Furthermore, the isoforms differed in their recovery from desensitization. These results indicate that the hH3R365 experiences homologous desensitization, but that the process differs between the isoforms in time-course, magnitude and re-sensitization.