Biexponential Kinetics of (R)‐α‐[³H]Methylhistamine Binding to the Rat Brain H₃ Histamine Receptor

Abstract

The H3 histamine receptor is a high-affinity receptor reported to mediate inhibition of CNS histidine decarboxylase activity and depolarization-induced histamine release. We have used (R)-alpha-[3H]methylhistamine, a specific, high-affinity agonist, to characterize ligand binding to this receptor. Saturation binding studies with rat brain membranes disclosed a single class of sites (KD = 0.68 nM; Bmax = 78 fmol/mg of protein). Competition binding assays also yielded an apparently single class of sites with a rank order of potency for ligands characteristic of an H3 histamine receptor: N alpha-methylhistamine, (R)-alpha-methylhistamine greater than histamine, thioperamide greater than impromidine greater than burimamide greater than dimaprit. In contrast, kinetic studies disclosed two classes of sites, one with fast, the other with slow on-and-off rates. Density of (R)-alpha-[3H]methylhistamine binding followed the order: caudate, midbrain (thalamus and hippocampus), cortex greater than hypothalamus greater than brainstem greater than cerebellum. These data are consistent with an H3 histamine receptor, distinct from H1 and H2 receptors, that occurs in two conformations with respect to agonist association and dissociation or with multiple H3 receptor subtypes that are at present pharmacologically undifferentiated.

Characterization of histamine‐H3 receptors controlling non‐adrenergic non‐cholinergic contractions of the guinea‐pig isolated ileum

“…In this study we Leff, 1988;Leff et al, 1990 (Arrang et al, 1987;West et al, 1990;Kilpatrick & Michel, 1991 In summary, this study demonstrates that activation of H3 receptors within the guinea-pig ileum, can inhibit NANC contractions elicited by electric field stimulation: the NANC contraction is probably mediated by substance P acting at neurokinin NK1 receptors. In addition, we have identified an irreversible antagonist at H3 receptors within this preparation and as such this compound should become a useful tool for both in vivo and in vitro study of H3 receptors.…”

Brain histamine H3 receptors in rats with portacaval anastomosis: in vitro and in vivo studies

“…The possibility that the lack of changes in H 3 receptors was due to a non-specific labeling by [ 3 H]-R-α-methylhistamine was ruled out since previous studies in rat brain using [ 3 H]-R-α-methylhistamine have shown that this radioligand binds with high affinity to a specific binding site, considered as the histamine H 3 receptor (Arrang et al 1987West et al 1990a). Furthermore, in the present study, the specificity of [ 3 H]-R-α-methylhistamine binding in rat brain was confirmed by both the saturation analysis and the autoradiography assay.…”

“…Membranes were prepared by the method of West et al (1990a) with slight modifications. Briefly, each individual brain sample was homogenized in 30 volumes (wt/vol) of ice-cold 50 mM Tris-HCl buffer (TRIZMA hydrochloride; Sigma Chemical Co., St. Louis, Mo., USA; pH 7.5), with five up-and-down strokes of the glass-Teflon pestle.…”

Guanine nucleotides and pertussis toxin reduce the affinity of histamine H3 receptors on AtT-20 cells

“…When used at nanomolar concentrations, essentially all of the specific binding of this ligand is to H3 receptors (KD =0.7 nM). Two subpopulations of H3 receptors exist in brain tissue membranes [5,6]. Based on the rank order of potencies for several antagonists, these were designated H3A and H3B.…”