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A‐349821 is a selective histamine H3 receptor antagonist/inverse agonist. Herein, binding of the novel non‐imidazole H3 receptor radioligand [3H]A‐349821 to membranes expressing native or recombinant H3 receptors from rat or human sources was characterized and compared with the binding of the agonist [3H]N‐α‐methylhistamine ([3H]NαMH). [3H]A‐349821 bound with high affinity and specificity to an apparent single class of saturable sites and recognized human H3 receptors with 10‐fold higher affinity compared to rat H3 receptors. [3H]A‐349821 detected larger populations of receptors compared to [3H]NαMH. Displacement of [3H]A‐349821 binding by H3 receptor antagonists/inverse agonists was monophasic, suggesting recognition of a single binding site, while that of H3 receptor agonists was biphasic, suggesting recognition of both high‐ and low‐affinity H3 receptor sites. pKi values of high‐affinity binding sites for H3 receptor competitors utilizing [3H]A‐349821 were highly correlated with pKi values obtained with [3H]NαMH, consistent with labelling of H3 receptors by [3H]A‐349821. Unlike assays utilizing [3H]NαMH, addition of GDP had no effect on saturation parameters measured with [3H]A‐349821, while displacement of [3H]A‐349821 binding by the H3 receptor agonist histamine was sensitive to GDP. In conclusion, [3H]A‐349821 labels interconvertible high‐ and low‐affinity states of the H3 receptor, and displays improved selectivity over imidazole‐containing H3 receptor antagonist radioligands. [3H]A‐349821 competition studies showed significant differences in the proportions and potencies of high‐ and low‐affinity sites across species, providing new information about the fundamental pharmacological nature of H3 receptors. British Journal of Pharmacology (2006) 148, 657–670. doi:

No abstract

The precise regulation of cell-cell communication by numerous signal-transduction pathways is fundamental for many different processes during embryonic development. One important signalling pathway is the evolutionary conserved fibroblast-growth-factor (FGF)-pathway that controls processes like cell migration, axis specification and mesoderm formation in vertebrate and invertebrate animals. In the model insect Drosophila, the FGF ligand / receptor combinations of FGF8 (Pyramus and Thisbe) / Heartless (Htl) and Branchless (Bnl) / Breathless (Btl) are required for the migration of mesodermal cells and for the formation of the tracheal network respectively with both the receptors functioning independently of each other. However, only a single fgf-receptor gene (Tc-fgfr) has been identified in the genome of the beetle Tribolium. We therefore asked whether both the ligands Fgf8 and Bnl could transduce their signal through a common FGF-receptor in Tribolium. Indeed, we found that the function of the single Tc-fgfr gene is essential for mesoderm differentiation as well as for the formation of the tracheal network during early development. Ligand specific RNAi for Tc-fgf8 and Tc-bnl resulted in two distinct non-overlapping phenotypes of impaired mesoderm differentiation and abnormal formation of the tracheal network in Tc-fgf8- and Tc-bnl(RNAi) embryos respectively. We further show that the single Tc-fgfr gene encodes at least two different receptor isoforms that are generated through alternative splicing. We in addition demonstrate through exon-specific RNAi their distinct tissue-specific functions. Finally, we discuss the structure of the fgf-receptor gene from an evolutionary perspective.

Fibroblast growth factor (FGF) signalling plays a key role in early embryonic development and cell migration in vertebrates and in invertebrates. To gain novel insights into FGF signalling in an arthropod, we characterized the fgf1b ortholog in the beetle Tribolium that is not represented in the Drosophila genome. We found that FGF1b dependent signalling organizes the anterior to posterior axis of the early embryo. The loss of Tc-fgf1b function in Tribolium by RNA interference resulted in the reduction of the anteriormost extraembryonic fate, in an anterior shift of embryonic fate and in the loss or malformation of anterior embryonic structures. Without intact extraembryonic membranes the serosa and the amnion, Tc-fgf1b(RNAi) embryos did not undergo morphogenetic movements and remained posteriorly localized throughout embryogenesis. Only weakly affected embryos developed into a cuticle that show dorsally curved bodies with head defects and a dorsal opening. Except for the posterior dorsal amnion, the overall topology of the dorsal-ventral axis seemed unaffected. Moreover, FGF signalling was not required for the onset of mesoderm formation but for fine-tuning this tissue during later development. We also show that in affected embryos the dorsal epidermis was expanded and expressed Tc-dpp at a higher level. We conclude that in the Tribolium blastoderm embryo, FGF1-signalling organizes patterning along the AP-axis and also balances the expression level of Dpp in the dorsal epidermis, a tissue critically involved in dorsal closure.