Identification of putative guanosine receptor in rat brain

G-protein-coupled receptors (GPCRs), recognising adenosine and purine and pyrimidine nucleotides as extracellular messengers, have been characterised and classified as purinergic P1 and P2 receptors, respectively. However, some experimental data suggest that other nucleosides as guanosine, inosine and hypoxanthine can also act as signallingmolecules via the activation of specificmembrane receptors [1]. In particular, extracellular guanosine seems to possess many trophic effects, including promotion division and growth of astrocytes and other neuron-like cells [2]. Binding studies, performed on rat brain membranes using [3H]guanosine, have shown that the compound interacts with binding sites, which are distinct from the well-characterised ATP and adenosine receptors. Furthermore, guanosine itself and 6-thioguanosine are equally effective in displacing [3H]guanosine from rat brain membranes [3]. Hence, potent and selective agonists and antagonists are highly needed for the characterisation of the physiopathological profile of the new putative guanosine receptor. Starting from these observations and aimed at developing a new assay that allows evaluating the potency of ligands at the putative guanosine receptor, Eu-GTP assay was performed in rat brain. This technique, using the principle of the [35S]GTPgammaS binding assay, replaces the radioactive material with Eu-GTP and exploits the unique fluorescence properties of Europium lanthanide chelate. New molecules, synthesised in our laboratory, were tested using Eu-GTP assay. The results showed that the new compounds are able to activate the putative guanosine receptor more than guanosine itself. On the other hand, this functional assay seems to demonstrate that a GPCR activated by guanosine is present in rat brain.