Characterization of histamine H₃ receptors in mouse brain using the H₃ antagonist [¹²⁵I]iodophenpropit

Abstract.

We have characterized the binding of the histamine H₃ receptor antagonist [¹²⁵I]iodophenpropit to mouse brain. [¹²⁵I]Iodophenpropit saturably bound to mouse brain membranes with a pK _d-value of 9.31±0.04 nM and a receptor binding density of 290±8 fmol per mg protein. Saturation binding analysis revealed binding of [¹²⁵I]iodophenpropit to a single class of sites, showing linear Scatchard plots and Hill coefficients not different from unity (n _H=0.98±0.02). At a concentration of 0.25 nM [¹²⁵I]iodophenpropit, specific binding represented about 75% of the total binding. Competition binding curves for H₃ receptor antagonists were fitted best to a one-site model, showing pK _i-values in general accordance with the pA ₂-values obtained in mouse cerebral cortex. Displacement of [¹²⁵I]iodophenpropit by the H₃ receptor agonists (R)-α-methylhistamine, immepip, imetit and histamine were fitted best to a two-site model. Competition binding curves of (R)-α-methylhistamine showed a rightward shift upon incubation with GTPγS (10 µM), indicating the involvement of G-proteins in H₃ agonist binding. In contrast, competition binding curves of the antagonists iodophenpropit, thioperamide and burimamide were not affected by GTPγS (10 µM). Autoradiographic experiments showed that [¹²⁵I]iodophenpropit binding sites were heterogeneously distributed, similarly to the distribution of histamine H₃ receptors reported in rat brain. Highest densities were observed in the cerebral cortex, the striatum, the nucleus accumbens, the globus pallidus and the substantia nigra.

In conclusion, we have demonstrated that in mouse brain, [¹²⁵I]iodophenpropit selectively binds to histamine H₃ receptors. We also observed that the mouse brain H₃ receptors labelled by [¹²⁵I]iodophenpropit displayed binding characteristics and a distribution similar to rat brain.