Muscarinic acetylcholine receptor-mediated activation of G(q) in rat brain membranes determined by guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding using an anti-G protein scintillation proximity assay

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.

Dopamine-induced functional activation of Gαq mediated by dopamine D1-like receptor in rat cerebral cortical membranes

Although multiple roles of dopamine through D₁-like (D₁ and D₅) and D₂-like (D₂, D₃, and D₄) receptors are initiated primarily through stimulation or inhibition of adenylyl cyclase via G_s/olf or G_i/o, respectively, there have been many reports indicating diverse signaling mechanisms that involve alternative G protein coupling. In this study, dopamine-induced Gα_q activation in rat brain membranes was investigated. Agonist-induced Gα_q activation was assessed by increase in guanosine-5'-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding to Gα_q determined by [³⁵S]GTPγS binding/immunoprecipitation assay in rat brain membranes. Dopamine-stimulated Gα_q functionality was highest in cortex as compared to hippocampus or striatum. In cerebral cortical membranes, this effect was mimicked by benzazepine derivatives with agonist properties at dopamine D₁-like receptors, that is, SKF83959, SKF83822, R(+)-SKF81297, R(+)-SKF38393, and SKF82958, but not by the compounds with dopamine D₂-like receptor agonist properties except for aripiprazole. Against expectation, stimulatory effects were also induced by SKF83566, R(+)-SCH23390, and pergolide. The pharmacological profiling by using a series of antagonists indicated that dopamine-induced response was mediated through dopamine D₁-like receptor, which was distinct from the receptor involved in 5-HT-induced response (5-HT_2A receptor). Conversely, the responses induced by SKF83566, R(+)-SCH23390, and pergolide were most likely mediated by 5-HT_2A receptor, but not by dopamine D₁-like receptor. Caution should be paid when interpreting the experimental data, especially in behavioral pharmacological research, in which SKF83566 or R(+)-SCH23390 is used as a standard selective dopamine D₁-like receptor antagonist. Also, possible clinical implications of the agonistic effects of pergolide on 5-HT_2A receptor has been mentioned.

Functional activation of Gαq/11 protein via α1 -adrenoceptor in rat cerebral cortical membranes

Although it is recognized that α₁ -adrenoceptors are coupled to diverse intracellular signalling pathways, its primary transduction mechanisms are evoked by activating phospholipase C in the cell membrane through Gα_q/11 , resulting in production of inositol 1,4,5-trisphosphate and diacylglycerol. However, there have been few studies that indicate directly the involvement of Gα_q/11 proteins in this signalling pathway in the central nervous system. In the current study, we tried to pharmacologically characterize (-)-adrenaline-stimulated [³⁵ S]GTPγS binding to Gα_q/11 in rat brain membranes. Functional activation of Gα_q/11 coupled to α₁ -adrenoceptor was investigated by using [³⁵ S]GTPγS binding/immunoprecipitation assay in the membranes prepared from rat cerebral cortex, hippocampus, and striatum. The specific [³⁵ S]GTPγS binding to Gα_q/11 was stimulated by (-)-adrenaline in a concentration-dependent and saturable manner in rat cerebral cortical membranes. In hippocampal or striatal membranes, the stimulatory effects of (-)-adrenaline were scarce. The effect of (-)-adrenaline was potently inhibited by prazosin, a potent and selective α₁ -adrenoceptor antagonist, but not by yohimbine, a selective α₂ -adrenoceptor antagonist. The response was mimicked by cirazoline, but not by R(-)-phenylephrine. Although oxymetazoline also stimulated the specific [³⁵ S]GTPγS binding to Gα_q/11 as an apparent "super-agonist", detailed pharmacological characterization revealed that its agonistic properties in this experimental system were derived from off-target effects on 5-HT_2A receptors, but not via α₁ -adrenoceptors. In conclusion, functional coupling of α₁ -adrenoceptors to Gα_q/11 proteins are detectable in rat brain membranes by means of [³⁵ S]GTPγS binding/immunoprecipitation assay. It is necessary to interpret the experimental data with caution when oxymetazoline is included as an agonist at α₁ -adrenoceptors.

Comparative analysis of pharmacological properties of xanomeline and N-desmethylclozapine in rat brain membranes

BACKGROUND

3(3-Hexyloxy-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine (xanomeline) and N-desmethylclozapine are of special interest as promising antipsychotics with better efficacy, especially for negative symptoms and/or cognitive/affective impairment.

METHODS

The guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding experiments were performed using (1) conventional filtration technique, (2) antibody-capture scintillation proximity assay, and (3) immunoprecipitation method, in brain membranes prepared from rat cerebral cortex, hippocampus, and striatum.

RESULTS

Xanomeline had agonistic activity at the M1 muscarinic acetylcholine receptor (mAChR) in all brain regions, as well as at the 5-HT1A receptor in the cerebral cortex and hippocampus. On the other hand, N-desmethylclozapine exhibited slight agonistic effects on the M1 mAChR, and agonistic properties at the 5-HT1A receptor in the cerebral cortex and hippocampus. This compound also behaved as an agonist at the δ-opioid receptor in the cerebral cortex and striatum. In addition, the stimulatory effects of N-desmethylclozapine on [(35)S]GTPγS binding to Gαi/o were partially mediated through mAChRs (most likely M4 mAChR subtype), at least in striatum.

CONCLUSIONS

The agonistic effects on the mAChRs (particularly M1 subtype, and also probably M4 subtype), the 5-HT1A receptor and the δ-opioid receptor expressed in native brain tissues, some of which are common to both compounds and others specific to either, likely shape the unique beneficial effectiveness of both compounds in the treatment for schizophrenic patients. These characteristics provide us with a clue to develop newer antipsychotics, beyond the framework of dopamine D2 receptor antagonism, that are effective not only on positive symptoms but also on negative symptoms and/or cognitive/affective impairment.

Functional activation of Gαq via serotonin2A (5-HT2A) and muscarinic acetylcholine M1 receptors assessed by guanosine-5׳-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding/immunoprecipitation in rat brain membranes

Functional coupling between serotonin2A (5-HT2A) receptors and Gαq proteins in native brain membranes has been sparsely reported thus far. In the present study, the guanosine-5׳-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding assay combined with immunoprecipitation using magnetic beads (Dynabeads Protein A) coated with anti-Gαq antibody was developed. Under experimental conditions optimised for assay constituents (GDP, MgCl2, and NaCl), for contents of membrane protein, anti-Gαq antibody, and Dynabeads Protein A, and for the incubation period, 5-HT stimulated specific [35S]GTPγS binding to Gαq in rat cerebral cortical membranes in a concentration-dependent and saturable manner, with a signal/noise ratio that was sufficiently high for further detailed pharmacological characterisation. This characterisation revealed an involvement of 5-HT2A receptors. Activation of Gαq proteins was also detectable by the addition of carbachol via muscarinic acetylcholine M1 receptors, (-)-epinephrine, and dopamine, but not by L-glutamate or (±)-baclofen. When 5-HT2A receptors and M1 receptors were stimulated simultaneously, there were non-additive effects, indicating that the two receptors were coupled to the same components of Gαq proteins in the rat cerebral cortex. This method will serve as an efficacious strategy for neurobiological investigations aimed at elucidating the physiological and pathological implications of signal transduction systems mediated via Gαq proteins coupled with 5-HT2A receptors and muscarinic acetylcholine M1 receptors.

Activation of Gq proteins coupled with 5-HT2 receptors in rat cerebral cortical membranes assessed by antibody-capture scintillation proximity assay/[S]GTPγS binding

BACKGROUND/AIMS

Functional activation of Gq coupled with 5-HT2 receptors was investigated in rat cerebral cortical membranes.

METHODS

Antibody-capture scintillation proximity assay (SPA)/[(35)S]GTPγS binding with anti-Gαq antibody was performed.

RESULTS

The specific [(35)S]GTPγS binding to Gαq was increased by 5-hydroxytryptamine (5-HT) in a concentration-dependent but unsaturable manner. The increase elicited by micromolar concentrations of 5-HT was inhibited completely by ketanserin, whereas it inhibited the response by submillimolar to millimolar concentrations of 5-HT only partially. Analysis of the concentration-dependent increases by 5-HT in the absence and presence of ketanserin, methiothepin, WAY100635, and pirenzepine clearly indicates that there are two distinct components of 5-HT-stimulated [(35)S]GTPγS binding, one of which is a pharmacologically relevant increase elicited by lower concentrations (-30 μmol/l) of 5-HT mediated through 5-HT2 receptors and the other is pharmacologically undefined stimulation by higher concentrations of 5-HT. When 5-HT and carbachol were added simultaneously, there was apparently lack of additivity.

CONCLUSION

It is concluded that by means of antibody-capture SPA/[(35)S]GTPγS binding it is possible to detect two distinct components of 5-HT-elicited activation of Gq shared by M1 muscarinic receptors, one of which is mediated through 5-HT2 receptors and the other is derived from unknown origin in rat cerebral cortical membranes.

Measuring substrate binding and affinity of purified membrane transport proteins using the scintillation proximity assay

The scintillation proximity assay (SPA) is a rapid radioligand binding assay. Upon binding of radioactively labeled ligands (here L-[(3)H]arginine or D-[(3)H]glucose) to acceptor proteins immobilized on fluoromicrospheres (containing the scintillant), a light signal is stimulated and measured. The application of SPA to purified, detergent-solubilized membrane transport proteins allows substrate-binding properties to be assessed (e.g., substrate specificity and affinity), usually within 1 d. Notably, the SPA makes it possible to study specific transporters without interference from other cellular components, such as endogenous transporters. Reconstitution of the target transporter into proteoliposomes is not required. The SPA procedure allows high sample throughput and simple sample handling without the need for washing or separation steps: components are mixed in one well and the signal is measured directly after incubation. Therefore, the SPA is an excellent tool for high-throughput screening experiments, e.g., to search for substrates and inhibitors, and it has also recently become an attractive tool for drug discovery.