Functional coupling between adenosine A1 receptors and G-proteins in rat and postmortem human brain membranes determined with conventional guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding or [35S]GTPγS/immunoprecipitation assay

Receptor-mediated Gi-3 activation in mammalian and human brain membranes: Reestablishment method and its application to nociceptin/orphanin FQ opioid peptide (NOP) receptor/Gi-3 interaction

Functional activation of heterotrimeric guanine nucleotide-binding proteins (G-proteins) via G-protein-coupled receptors (GPCRs) has been extensively explored using guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assay. However, the conventional method is primarily applicable to Gi/o family without discrimination among G-protein subtypes. Therefore, this study aims to reestablish a novel method termed "[35S]GTPγS binding/immunoprecipitation assay" by identifying a most suitable anti-Gαi-3 antibody instead of the previously utilized, now withdrawn antibody. In the initial screening of commercially available anti-Gαi-3 antibodies, two were identified and one was selected for further investigations based on efficacy with adenosine-the most potent agonist in our previous research. After optimizing experimental conditions with rat and postmortem human brain membranes, the stimulatory effects of various agonists were evaluated. Some agonists, including nociceptin, exhibited sufficient stimulatory effects for further pharmacological characterization. Nociceptin increased [35S]GTPγS binding to Gαi-3 in a concentration-dependent manner, response that was insensitive to naloxone but potently inhibited using (±)-J-113397. The method described in this study provides a valuable strategy for determining the intrinsic efficacy of ligands at various GPCRs. This includes nociceptin/orphanin FQ opioid peptide (NOP) receptor selectively coupled to Gαi-3, providing insights into the pharmacological concept of "functional selectivity."

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.