The histaminergic system in the rat brain. Histamine-synthesizing neurons are located in the hypothalamus tuberomamillary nucleus, and these neurons send projections to the CNS through three major pathways, two ascending bundles that innervate the forebrain structures, and one descending bundle reaching the spinal cord. Thal, thalamus; Str, striatum.

Alignment of the amino acid sequence of the human, monkey, guinea pig, rat, and mouse H₃Rs. Sequences were obtained from Uniprot (Washington, DC) and correspond to the full-length H₃R (445 aa). Alignment was performed with the Clustal Omega server from the European Bioinformatic Institute (Cambridge, UK). The highly conserved sequences in the seven transmembrane domains are shown in gray. The DRF and NPVLY motifs and the glycosylation site (Asn11) are well conserved among species (green). Residues in blue are responsible for the interspecies differences in H₃R pharmacology. Residues in red are likely to be involved in the receptor high constitutive activity on the basis of their identity with a mutated β₂-adrenoceptor. The third intracellular loop (ICL3) possesses a region (residues 236–300; italics) that is highly variable among species, followed by a highly conserved sequence. The hH₃R has ≥93% sequence identity with the other species, and the same identity can be found in guinea pig, mouse, and rat sequences, with 99% identity between mouse and rat, and among the primate proteins. *, conserved residues; :, conservative mutations; •, nonconservative mutations.

Structure of the human H₃R. As a GPCR, the H₃R contains seven spanning transmembrane domains, an extracellular amino terminus (NT), an intracellular carboxyl terminus (CT), three extracellular (ECL), and three intracellular (ICL) loops, with a long ICL3 (142 aa). Potential residues for phosphorylation are shown in green. The conserved DRF and NPVLY motifs are shown in yellow. The naturally occurring mutations D18E and A280V are depicted in blue. The glycosylation site (Asn11) is shown in red, the palmitoylation site (C228) in gray. Residues Thr119 and Ala122 (purple) are responsible for the interspecies pharmacologic differences. Residues indicated in black in ECL1 and ECL2 correspond to the cysteins forming a disulfide bond important for receptor trafficking. Residues important for agonist recognition are colored in pink.

H₃R signaling pathways. H₃R activation triggers or modulates several pathways through the Gα subunits and G_βγ complexes of Gα_i/o proteins. AA, arachidonic acid; AC, adenylyl cyclases; cAMP, 3′,5′-cyclic adenosine monophosphate; MAPK, mitogen-activated protein kinases; NHE, Na⁺/H⁺ exchanger; PI3K, phosphatidylinositol 3-kinase; PKA, protein kinase A; PLA₂, phosholipase A₂; PLC, phosholipase C.

Binding sites of agonists and antagonists at the human H₃R of 445 aa (hH₃R₄₄₅). (A) Molecular docking of the endogenous agonist histamine in a model of the hH₃R₄₄₅ in the active state. Residues Glu206 and Asp114 are pivotal for agonist binding. (B) Planar view of the histamine binding pocket in the hH₃R₄₄₅. (C) Proposed binding site of a protean ligand (proxyfan) in the inactive hH₃R₄₄₅ as observed by molecular docking, depicting the three functional groups of the ligand important for binding (nitrogen-containing ring, central electronegative group and aromatic ring). (D) Planar view of the suggested binding residues for an antagonist at the hH₃R₄₄₅.

Structure of H₃R agonists.