title: Homodimerization of adenosine A₁ receptors in brain cortex explains the biphasic effects of caffeine.
abstract: Using bioluminescence resonance energy transfer and proximity ligation assays, we obtained the first direct evidence that adenosine A₁ receptors (A₁Rs) form homomers not only in cell cultures but also in brain cortex. By radioligand binding experiments in the absence or in the presence of the A₁Rs allosteric modulator, adenosine deaminase, and by using the two-state dimer receptor model to fit binding data, we demonstrated that the protomer-protomer interactions in the A₁R homomers account for some of the pharmacological characteristics of agonist and antagonist binding to A₁Rs. These pharmacological properties include the appearance of cooperativity in agonist binding, the change from a biphasic saturation curve to a monophasic curve in self-competition experiments and the molecular cross-talk detected when two different specific molecules bind to the receptor. In this last case, we discovered that caffeine binding to one protomer increases the agonist affinity for the other protomer in the A₁R homomer, a pharmacological characteristic that correlates with the low caffeine concentrations-induced activation of agonist-promoted A₁R signaling. This pharmacological property can explain the biphasic effects reported at low and high concentration of caffeine on locomotor activity.
authors.affiliation: Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED), Faculty of Biology, University of Barcelona, Av. Diagonal 643, 08028 Barcelona, Spain. edugss@gmail.com
authors.forename: Eduard
authors.initials: E
authors.lastname: Gracia
authors.forename: Estefania
authors.initials: E
authors.lastname: Moreno
authors.forename: Antoni
authors.initials: A
authors.lastname: Cortés
authors.forename: Carme
authors.initials: C
authors.lastname: Lluís
authors.forename: Josefa
authors.initials: J
authors.lastname: Mallol
authors.forename: Peter J
authors.initials: PJ
authors.lastname: McCormick
authors.forename: Enric I
authors.initials: EI
authors.lastname: Canela
authors.forename: Vicent
authors.initials: V
authors.lastname: Casadó
mesh_terms: D058907:Adenosine A1 Receptor Agonists / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D058916:Adenosine A1 Receptor Antagonists / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D000243:Adenosine Deaminase / Q000737:chemistry / Q000378:metabolism; D000495:Allosteric Site / Q000187:drug effects; D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002110:Caffeine / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D002417:Cattle; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D000697:Central Nervous System Stimulants / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D019281:Dimerization; D057809:HEK293 Cells; D006801:Humans; D007700:Kinetics; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D009419:Nerve Tissue Proteins / Q000819:agonists / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D043682:Receptor, Adenosine A1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism
chemical_list: D058907:Adenosine A1 Receptor Agonists; D058916:Adenosine A1 Receptor Antagonists; D001426:Bacterial Proteins; D000697:Central Nervous System Stimulants; D008164:Luminescent Proteins; D009419:Nerve Tissue Proteins; D043682:Receptor, Adenosine A1; D011993:Recombinant Fusion Proteins; C054562:yellow fluorescent protein, Bacteria; D002110:Caffeine; D000243:Adenosine Deaminase